explanation blue bibcodes open ADS page with paths to full text
Author name code: krucker
ADS astronomy entries on 2022-09-14
author:"Krucker, Sam"
---------------------------------------------------------
Title: Observations of Thomson Scattering from a Loop-prominence
System
Authors: Martínez Oliveros, Juan Carlos; Guevara Gómez, Juan Camilo;
Saint-Hilaire, Pascal; Hudson, Hugh; Krucker, Säm
2022ApJ...936...56M Altcode: 2022arXiv220806007M
We describe observations of the white-light structures in the low
corona following the X8.2 flare SOL 2017-09-10, as observed in full
Stokes parameters by the Helioseismic and Magnetic Imager (HMI) of
the Solar Dynamics Observatory. These data show both bright loops
and a diffuse emission region above them. We interpret the loops as
the white-light counterpart of a classical loop-prominence system,
intermediate between the hot X-ray loops and coronal rain. The diffuse
emission external to the loops is linearly polarized and has a natural
interpretation in terms of Thomson scattering from the hot plasma seen
prior to its cooling and recombination. The polarimetric data from
HMI enable us to distinguish this contribution of scattering from the
HMI pseudocontinuum measurement, and to make a direct estimation of
the coronal mass in the polarized source. For a snapshot at 16:19 UT,
we estimate a mass 8 × 10<SUP>14</SUP> g. We further conclude that
the volumetric filling factor of this source is near unity.
---------------------------------------------------------
Title: Spatial distribution of jets in solar active regions
Authors: Odermatt, J.; Barczynski, K.; Harra, L. K.; Schwanitz, C.;
Krucker, S.
2022A&A...665A..29O Altcode: 2022arXiv220709923O
Context. Solar active regions are known to have jets. These jets are
associated with heating and the release of particles into the solar
wind. <BR /> Aims: Our aim is to understand the spatial distribution
of coronal jets within active regions to understand if there is a
preferential location for them to occur. <BR /> Methods: We analysed
five active regions using Solar Dynamics Observatory Atmospheric
Imaging Assembly data over a period of 2-3.5 days when the active
regions were close to disk centre. Each active region had a different
age, magnetic field strength, and topology. We developed a methodology
for determining the position and length of the jets. <BR /> Results:
Jets are observed more frequently at the edges of the active regions
and are more densely located around a strong leading sunspot. The
number of coronal jets for our active regions is dependent on the
age of the active region. The older active regions produce more jets
than younger ones. Jets were observed dominantly at the edges of the
active regions, and not as frequently in the centre. The number of
jets is independent of the average unsigned magnetic field and total
flux density in the whole active region. The jets are located around
the edges of the strong leading sunspot.
---------------------------------------------------------
Title: The Lyman-$\alpha$ Emission in a C1.4 Solar Flare Observed
by the Extreme Ultraviolet Imager aboard Solar Orbiter
Authors: Li, Ying; Li, Qiao; Song, De-Chao; Battaglia, Andrea
Francesco; Xiao, Hualin; Krucker, Säm; Schühle, Udo; Li, Hui; Gan,
Weiqun; Ding, M. D.
2022arXiv220806182L Altcode:
The hydrogen Lyman-$\alpha$ (H {\sc i} Ly$\alpha$) emission during solar
flares has rarely been studied in spatially resolved images and its
physical origin has not been fully understood. In this paper, we present
novel Ly$\alpha$ images for a C1.4 solar flare (SOL2021-08-20T22:00)
from the Extreme Ultraviolet Imager aboard Solar Orbiter, together
with multi-waveband and multi-perspective observations from the
Solar Terrestrial Relations Observatory Ahead and the Solar Dynamics
Observatory spacecraft. It is found that the Ly$\alpha$ emission has
a good temporal correlation with the thermal emissions at 1--8 Å and
5--7 keV, indicating that the flaring Ly$\alpha$ is mainly produced by
a thermal process in this small event. However, nonthermal electrons
play a minor role in generating Ly$\alpha$ at flare ribbons during
the rise phase of the flare, as revealed by the hard X-ray imaging and
spectral fitting. Besides originating from flare ribbons, the Ly$\alpha$
emission can come from flare loops, likely caused by plasma heating
and also cooling that happen in different flare phases. It is also
found that the Ly$\alpha$ emission shows fairly similar features with
the He {\sc ii} 304 Å emission in light curve and spatio-temporal
variation along with small differences. These observational results
improve our understanding of the Ly$\alpha$ emission in solar flares
and also provide some insights for investigating the Ly$\alpha$
emission in stellar flares.
---------------------------------------------------------
Title: Detection of weak ubiquitous impulsive nonthermal emissions
from the solar corona
Authors: Sharma, Rohit; Oberoi, Divya; Battaglia, Marina; Krucker, Sam
2022arXiv220807147S Altcode:
A ubiquitous presence of weak energy releases is one of the most
promising hypotheses to explain coronal heating, referred to as the
nanoflare hypothesis. The accelerated electrons associated with such
weak heating events are also expected to give rise to coherent impulsive
emission via plasma instabilities in the meterwave radio band, making
this a promising spectral window to look for their presence. Recently
\citet{Mondal2020b} reported the presence of weak impulsive emissions
from quiet Sun regions which seem to meet the requirements of being
radio counterparts of the hypothesized nanoflares. Detection of such
low-contrast weak emission from the quiet Sun is challenging and, given
their implications, it is important to confirm their presence. In this
work, using data from the Murchison Widefield Array, we explore the use
of an independent robust approach for their detection by separating the
dominant slowly varying component of emission from the weak impulsive
one in the visibility domain. We detect milli-SFU level bursts taking
place all over the Sun and characterize their brightness temperatures,
distributions, morphologies, durations and association with features
seen in EUV images. We also attempt to constraint the energies of
the nonthermal particles using inputs from the FORWARD coronal model
along with some reasonable assumptions and find them to lie in the
sub-pico flare ($\sim 10^{19}-10^{21}$ ergs) range. In the process,
we also discover perhaps the weakest type III radio burst and another
one that shows clear signatures of weakest quasi-periodic pulsations.
---------------------------------------------------------
Title: NuSTAR observations of a quiet Sun minifilament eruption
Authors: Hannah, Iain; Sterling, Alphonse; Grefenstette, Brian;
Glesener, Lindsay; White, Stephen; Smith, David; Cooper, Kristopher;
Krucker, Sam; Paterson, Sarah; Hudson, Hugh
2022cosp...44.2538H Altcode:
We present a unique set of observations of a confined minifilament
eruption from the quiet-Sun during solar minimum. The Nuclear
Spectroscopic Telescope Array (NuSTAR) spotted a tiny, compact hard
X-ray (HXR) flare on 2019 April 26, peaking about 02:06UT for a few
minutes, finding brief emission >5MK. Observations with SDO/AIA
and Hinode/XRT show this HXR emission was due to a tiny flare arcade
underneath a confined minifilament eruption - behaviour similar to those
seen in both major active-region filament eruptions and minifilament
eruptions that lead to coronal jets. Line-of-sight magnetograms from
SDO/HMI show that this eruption is due to opposite polarity flux
moving together and cancelling and not due to flux emergence. This
eruption occurred near disk-centre, so the Earth orbiting observatories
provide a top-down view of the event, but fortuitously a side-on view
is obtained from STEREO-A/SECCHI, giving a clearer sense of eruption
geometry. We also explore the possibility of non-thermal emission
due to accelerated electrons in the NuSTAR HXR observations of this
small-scale phenomena in the quiet Sun.
---------------------------------------------------------
Title: The Spectrometer Telescope for Imaging X-rays (STIX) on
Solar Orbiter
Authors: Hayes, Laura A.; Musset, Sophie; üller, Daniel M; Krucker,
S äm
2022arXiv220702079H Altcode:
The Spectrometer/Telescope for Imaging X-rays (STIX) is one of the
10 instruments on-board the scientific payload of ESA's Solar Orbiter
mission. STIX provides hard X-ray imaging spectroscopy in the 4-150~keV
energy range, observing hard X-ray bremsstrahlung emission from the
Sun. These observations provide diagnostics of the hottest thermal
plasmas ($>$10~MK) and information on the non-thermal energetic
electrons accelerated above 10~keV during solar flares. STIX has a
spectral resolution of 1~keV, and employs the use of in-direct bi-grid
Fourier imaging to spatially locate hard X-ray emission. Given that
STIX provides critical information about accelerated electrons at
the Sun through hard X-ray diagnostics, it is a powerful contribution
to the Solar Orbiter suite and has a significant role to explore the
dynamics of solar inputs to the heliosphere. This chapter describes
the STIX instrument, its design, objectives, first observations and
outlines the new perspectives STIX provides over the mission lifetime
of Solar Orbiter.
---------------------------------------------------------
Title: A statistical analysis of fast time variations in the hard
X-ray emission of solar flares with Solar Orbiter's STIX.
Authors: Collier, Hannah; Krucker, Sam; Battaglia, Andrea; Hayes, Laura
2022cosp...44.2567C Altcode:
The Spectrometer Telescope for Imaging X-rays (STIX) onboard Solar
Orbiter consists of 32 energy channels which detect X-ray emission
from solar flares with energies in the range of 4-150 keV. With STIX,
it is possible to perform imaging spectroscopy for solar flares with a
1 keV energy resolution at an unprecedented high time cadence (0.5 s),
in the hard X-ray range. This work exploits the novel capability of STIX
by focusing on the investigation of fast time, oscillatory variations
in the hard X-ray emission of flares. Gaining an understanding of
the mechanism behind such fast time variations in solar flares is
pertinent for revealing the fundamental processes involved in the
large energy release of flares and ultimately for the generation of
a unified solar flare model. Solar Orbiter will reach its perihelion
of ~0.3 AU in March 2022. In this work a statistical overview of fast
time variations in the hard X-ray emission from flares is given based
on the existing catalogue including tens of M class flares observed by
STIX since the launch of Solar Orbiter plus additional observations
obtained during perihelion. Furthermore, imaging spectroscopy for a
selection of events is presented.
---------------------------------------------------------
Title: Heating during small solar flares with Solar Orbiter STIX,
Hinode and SDO.
Authors: Harra, Louise K.; Reeves, Kathy; Krucker, Sam; Barczynski,
Krzysztof; Battaglia, Andrea; Collier, Hannah
2022cosp...44.2525H Altcode:
One of the key aspects of solar flares is how, where and how fast
plasma is heated. Spectroscopic observations are constrained by the
duration it takes to build up an image and hence the time resolution
is often of the order of 5 minutes during a flare. To obtain higher
cadence spectroscopic information, it is also possible to use the wide
slot (260") - or 'overlappogram' data. An observing campaign (HOP361)
was carried out in October using Hinode EIS, XRT and coordinating
with Solar Orbiter STIX. The campaign was carried out at high time
cadence and was telemetry intensive. SDO is also of key interest to
these observations. On the 8th October, a B2 classfication flare took
place, and was observed by all instruments. The EIS 'overlappogram'
allows high time cadence observations (10 secs) of the whole active
region. During this flare, it is possible to extract information on
the hot plasma from Fe XXIV, STIX and XRT. There was a smaller flare
which wasn't registered by GOES that followed, that provides a contrast
example that doesn't show clear hot emission.
---------------------------------------------------------
Title: The relativistic solar particle event on 28 October 2021:
Evidence of particle acceleration within and escape from the solar
corona
Authors: Klein, Karl-Ludwig; Musset, Sophie; Vilmer, Nicole; Briand,
Carine; Krucker, Säm; Francesco Battaglia, Andrea; Dresing, Nina;
Palmroos, Christian; Gary, Dale E.
2022A&A...663A.173K Altcode:
<BR /> Aims: We analyse particle, radio, and X-ray observations
during the first relativistic proton event of solar cycle 25 detected
on Earth. The aim is to gain insight into the relationship between
relativistic solar particles detected in space and the processes of
acceleration and propagation in solar eruptive events. <BR /> Methods:
To this end, we used ground-based neutron monitor measurements of
relativistic nucleons and space-borne measurements of electrons with
similar speed to determine the arrival times of the first particles at 1
AU and to infer their solar release times. We compared the release times
with the time histories of non-thermal electrons in the solar atmosphere
and their escape to interplanetary space, as traced by radio spectra and
X-ray light curves and images. <BR /> Results: Non-thermal electrons
in the corona are found to be accelerated in different regions. Some
are confined in closed magnetic structures expanding during the course
of the event. Three episodes of electron escape to the interplanetary
space are revealed by groups of decametric-to-kilometric type III
bursts. The first group appears on the low-frequency side of a type
II burst produced by a coronal shock wave. The two latter groups
are accompanied at higher frequencies by bursts with rapid drifts
to both lower and higher frequencies (forward- or reverse-drifting
bursts). They are produced by electron beams that propagate both sunward
and anti-sunward. The first relativistic electrons and nucleons observed
near Earth are released with the third group of type III bursts, more
than ten minutes after the first signatures of non-thermal electrons
and of the formation of the shock wave in the corona. Although the
eruptive active region is near the central meridian, several tens of
degrees east of the footpoint of the nominal Parker spiral to the Earth,
the kilometric spectrum of the type III bursts and the in situ detection
of Langmuir waves demonstrate a direct magnetic connection between the
L1 Lagrange point and the field lines onto which the electron beams
are released at the Sun. <BR /> Conclusions: We interpret the forward-
and reverse-drifting radio bursts as evidence of reconnection between
the closed expanding magnetic structures of an erupting flux rope and
ambient open magnetic field lines. We discuss the origin of relativistic
particles near the Earth across two scenarios: (1) acceleration at the
CME-driven shock as it intercepts interplanetary magnetic field lines
rooted in the western solar hemisphere and (2) an alternative where
the relativistic particles are initially confined in the erupting
magnetic fields and get access to the open field lines to the Earth
through these reconnection events. <P />Movie is available at <A
href="https://www.aanda.org/10.1051/0004-6361/202243903/olm">https://www.aanda.org</A>
---------------------------------------------------------
Title: First Hard X-Ray Imaging Results by Solar Orbiter STIX
Authors: Massa, Paolo; Battaglia, Andrea F.; Volpara, Anna; Collier,
Hannah; Hurford, Gordon J.; Kuhar, Matej; Perracchione, Emma;
Garbarino, Sara; Massone, Anna Maria; Benvenuto, Federico; Schuller,
Frederic; Warmuth, Alexander; Dickson, Ewan C. M.; Xiao, Hualin;
Maloney, Shane A.; Ryan, Daniel F.; Piana, Michele; Krucker, Säm
2022SoPh..297...93M Altcode: 2022arXiv220209334M
The Spectrometer/Telescope for Imaging X-rays (STIX) is one of six
remote sensing instruments on-board Solar Orbiter. The telescope
applies an indirect imaging technique that uses the measurement of 30
visibilities, i.e., angular Fourier components of the solar flare X-ray
source. Hence, the imaging problem for STIX consists of the Fourier
inversion of the data measured by the instrument. In this work, we show
that the visibility amplitude and phase calibration of 24 out of 30
STIX sub-collimators has reached a satisfactory level for scientific
data exploitation and that a set of imaging methods is able to provide
the first hard X-ray images of solar flares from Solar Orbiter. Four
visibility-based image reconstruction methods and one count-based
are applied to calibrated STIX observations of six events with GOES
class between C4 and M4 that occurred in May 2021. The resulting
reconstructions are compared to those provided by an optimization
algorithm used for fitting the amplitudes of STIX visibilities. We
show that the five imaging methods produce results morphologically
consistent with the ones provided by the Atmospheric Imaging Assembly
on board the Solar Dynamic Observatory (SDO/AIA) in UV wavelengths. The
χ<SUP>2</SUP> values and the parameters of the reconstructed sources
are comparable between methods, thus confirming their robustness.
---------------------------------------------------------
Title: On the faintest solar coronal hard X-rays observed with FOXSI
Authors: Buitrago-Casas, Juan Camilo; Glesener, Lindsay; Christe,
Steven; Krucker, Säm; Vievering, Juliana; Athiray, P. S.; Musset,
Sophie; Davis, Lance; Courtade, Sasha; Dalton, Gregory; Turin,
Paul; Turin, Zoe; Ramsey, Brian; Bongiorno, Stephen; Ryan, Daniel;
Takahashi, Tadayuki; Furukawa, Kento; Watanabe, Shin; Narukage,
Noriyuki; Ishikawa, Shin-nosuke; Mitsuishi, Ikuyuki; Hagino, Kouichi;
Shourt, Van; Duncan, Jessie; Zhang, Yixian; Bale, Stuart D.
2022arXiv220504291B Altcode:
Solar nanoflares are small eruptive events releasing magnetic energy in
the quiet corona. If nanoflares follow the same physics as their larger
counterparts, they should emit hard X-rays (HXRs) but with a rather
faint intensity. A copious and continuous presence of nanoflares would
deliver enormous amounts of energy into the solar corona, possibly
accounting for its high temperatures. To date, there has not been
any direct observation of such sustained and persistent HXRs from the
quiescent Sun. However, Hannah et al. in 2010 constrained the quiet
Sun HXR emission using almost 12 days of quiescent solar-off-pointing
observations by RHESSI. These observations set upper limits at
$3.4\times 10^{-2}$ photons$^{-1}$ s$^{-1}$ cm$^{-2}$ keV$^{-1}$
and $9.5\times 10^{-4}$ photons$^{-1}$ s$^{-1}$ cm$^{-2}$ keV$^{-1}$
for the 3-6 keV and 6-12 keV energy ranges, respectively. Observing
feeble HXRs is challenging because it demands high sensitivity
and dynamic range instruments in HXRs. The Focusing Optics X-ray
Solar Imager (FOXSI) sounding rocket experiment excels in these
two attributes. Particularly, FOXSI completed its third successful
flight (FOXSI-3) on September 7th, 2018. During FOXSI-3's flight,
the Sun exhibited a fairly quiet configuration, displaying only one
aged non-flaring active region. Using the entire $\sim$6.5 minutes of
FOXSI-3 data, we constrained the quiet Sun emission in HXRs. We found
$2\sigma$ upper limits in the order of $\sim 10^{-3}$ photons$^{-1}$
s$^{-1}$ cm$^{-2}$ keV$^{-1}$ for the 5-10 keV energy range. FOXSI-3's
upper limit is consistent with what was reported by Hannah et al.,
2010, but FOXSI-3 achieved this result using $\sim$1/2640 less time
than RHESSI. A possible future spacecraft using FOXSI's concept would
allow enough observation time to constrain the current HXR quiet Sun
limits further or perhaps even make direct detections.
---------------------------------------------------------
Title: The Coupling of an EUV Coronal Wave and Ion Acceleration in
a Fermi-LAT Behind-the-Limb Solar Flare
Authors: Pesce-Rollins, Melissa; Omodei, Nicola; Krucker, Säm; Di
Lalla, Niccolò; Wang, Wen; Battaglia, Andrea F.; Warmuth, Alexander;
Veronig, Astrid M.; Baldini, Luca
2022ApJ...929..172P Altcode: 2022arXiv220504760P
We present the Fermi-LAT observations of the behind-the-limb (BTL)
flare of 2021 July 17 and the joint detection of this flare by STIX
on board the Solar Orbiter. The separation between Earth and the
Solar Orbiter was 99.°2 at 05:00 UT, allowing STIX to have a front
view of the flare. The location of the flare was S20E140 in Stonyhurst
heliographic coordinates, making this the most distant behind-the-limb
flare ever detected in >100 MeV gamma-rays. The LAT detection lasted
for ~16 minutes, the peak flux was 3.6 ± 0.8 (10<SUP>-5</SUP>) ph
cm<SUP>-2</SUP> s<SUP>-1</SUP> with a significance >15σ. A coronal
wave was observed from both STEREO-A and SDO in extreme ultraviolet
(EUV), with an onset on the visible disk in coincidence with the LAT
onset. A complex type II radio burst was observed by GLOSS also in
coincidence with the onset of the LAT emission, indicating the presence
of a shock wave. We discuss the relation between the time derivative
of the EUV wave intensity profile at 193 Å as observed by STEREO-A
and the LAT flux to show that the appearance of the coronal wave at the
visible disk and the acceleration of protons as traced by the observed
>100 MeV gamma-ray emission are coupled. We also report how this
coupling is present in the data from three other BTL flares detected by
Fermi-LAT, suggesting that the protons driving the gamma-ray emission
of BTL solar flares and the coronal wave share a common origin.
---------------------------------------------------------
Title: Forward-fitting STIX visibilities
Authors: Volpara, Anna; Massa, Paolo; Perracchione, Emma; Battaglia,
Andrea Francesco; Garbarino, Sara; Benvenuto, Federico; Massone,
Anna Maria; Krucker, Sam; Piana, Michele
2022arXiv220414148V Altcode:
Aima. To determine to what extent the problem of forward fitting
visibilities measured by the Spectrometer/Telescope Imaging X-rays
(STIX) on-board Solar Orbiter is more challenging with respect to the
same problem in the case of previous hard X-ray solar imaging missions;
to identify an effective optimization scheme for parametric imaging
for STIX. Methods. This paper introduces a Particle Swarm Optimization
(PSO) algorithm for forward fitting STIX visibilities and compares its
effectiveness with respect to the standard simplex-based optimization
algorithm used so far for the analysis of visibilities measured by the
Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI). This
comparison is made by considering experimental visibilities measured by
both RHESSI and STIX, and synthetic visibilities generated by accounting
for the STIX signal formation model. Results. We found out that the
parametric imaging approach based on PSO is as reliable as the one based
on the simplex method in the case of RHESSI visibilities. However,
PSO is significantly more robust when applied to STIX simulated
and experimental visibilities. Conclusions. Standard deterministic
optimization is not effective enough for forward-fitting the few
visibilities sampled by STIX in the angular frequency plane. Therefore
a more sofisticated optimization scheme must be introduced for
parametric imaging in the case of the Solar Orbiter X-ray telescope. The
forward-fitting routine based on PSO we introduced in this paper proved
to be significantly robust and reliable, and could be considered as
an effective candidate tool for parametric imaging in the STIX context.
---------------------------------------------------------
Title: Multi-instrument STIX microflare study
Authors: Saqri, Jonas; Veronig, Astrid M.; Warmuth, Alexander; Dickson,
Ewan C. M.; Battaglia, Andrea Francesco; Podladchikova, Tatiana;
Xiao, Hualin; Battaglia, Marina; Hurford, Gordon J.; Krucker, Säm
2022A&A...659A..52S Altcode: 2022arXiv220100712S
Context. During its commissioning phase in 2020, the
Spectrometer/Telescope for Imaging X-rays (STIX) on board the Solar
Orbiter spacecraft observed 69 microflares. The two most significant
events from this set (of GOES class B2 and B6) were observed on-disk
from the spacecraft as well as from Earth and analysed in terms of
the spatial, temporal, and spectral characteristics. <BR /> Aims:
We complement the observations from the STIX instrument with EUV
imagery from SDO/AIA and GOES soft X-ray data by adding imaging
and plasma diagnostics over different temperature ranges for a
detailed microflare case study that is focussed on energy release
and transport. <BR /> Methods: We used data from the GOES for SXR
plasma diagnostics and SDO/AIA for carrying out high-cadence EUV
imaging and reconstruction of differential emission measure (DEM)
maps of the thermal flare plasma. The reconstructed DEM profiles were
used to study the temporal evolution of thermal flare plasma in the
kernels and loops independently. We derived the time evolution of the
flare plasma parameters (EM, T) and thermal energy from STIX, GOES,
and AIA observations. In particular, we studied the STIX spectra to
determine the nonthermal emission from accelerated electrons. <BR />
Results: A spectral fitting of the STIX data shows clear nonthermal
emission for both microflares studied here. For both events, the
plasma temperature and EM derived from STIX and GOES as well as
the reconstructed DEM maps differ in absolute values and timing,
with AIA (which is sensitive to lower plasma temperatures) lagging
behind. The deduced plasma parameters from either method roughly
agree with the values in the literature for microflares as do the
nonthermal fit parameters from STIX. This finding is corroborated
by the Neupert effect exhibited between the time derivative of the
GOES SXR emission and the STIX HXR profiles. For the B6 event, for
which such an analysis was possible, the non-thermal energy deduced
from STIX roughly coincides with the lower estimates of the thermal
energy requirement deduced from the SXR and EUV emissions. <BR />
Conclusions: The observed Neupert effects and impulsive and gradual
phases indicate that both events covered in this study are consistent
with the standard chromospheric evaporation flare scenario. For the
B6 event on 7 June 2020, this interpretation is further supported by
the temporal evolution seen in the DEM profiles of the flare ribbons
and loops. For this event, we also find that accelerated electrons
can roughly account for the required thermal energy. The 6 June 2020
event demonstrates that STIX can detect nonthermal emission for GOES
class B2 events that is nonetheless smaller than the background rate
level. We demonstrate for the first time how detailed multi-instrument
studies of solar flares can be performed with STIX.
---------------------------------------------------------
Title: Heliospheric Distributed In-Situ Constellation (HelioDISC):
A Mission to Untangle the Dynamic Mesoscale Sun-Earth Connection
Authors: Allen, R. C.; Leary, M.; Vievering, J.; Vines, S. K.; Jian,
L.; Wilson, L.; Li, G.; Lepri, S.; Livi, S.; Raines, J. M.; Anderson,
B. J.; Ho, G. C.; Mostafavi, P.; Smith, E.; Wimmer-Schweingruber,
R. F.; Krucker, S.; Malaspina, D.; Borovsky, J.; Whittlesey, P.;
Maruca, B.; Verscharen, D.; Lugaz, N.
2022heli.conf.4015A Altcode:
HelioDISC is a mission concept supported by the NASA Heliophysics
Mission Concept Study (HMCS) program to be considered by the National
Academy of Sciences and Engineering Heliophysics Decadal Committee as
a future Solar Terrestrial Probes mission.
---------------------------------------------------------
Title: Real-Time Solar Flare Predictions for Improved Flare
Observations
Authors: Vievering, J. T.; Athiray, P. S.; Buitrago-Casas, J. C.;
Chamberlin, P.; Glesener, L.; Golub, L.; Knoer, V.; Krucker, S.;
Machol, J.; Pantazides, A.; Peck, C.; Reeves, K.; Savage, S.; Schmit,
D.; Smith, B.; Vigil, G.; Winebarger, A.
2022heli.conf.4038V Altcode:
Improving near-term flare forecasts is important for observatories
targeting flare physics that are restricted in FOV and/or observing
time. We propose a tool that aggregates near-real-time signatures of
flare onset to provide early flare predictions.
---------------------------------------------------------
Title: Solar Flare Observations with STIX in 2021
Authors: Krucker, Sam
2021AGUFMSH21A..04K Altcode:
The hard X-ray imaging spectrometer STIX onboard Solar Orbiter has
been operating almost continuously in 2021 and has recorded over 1000
solar flares in the first half of 2021 alone. In this talk we will give
an overview of initial scientific results with a focus on comparing
Solar Orbiters perspective with observations taken from Earth. Of
particular interest are joint STIX and FERMI/GBM observations, which
provide a first systematic look at hard X-ray directivity measurements
from these two vantage points.
---------------------------------------------------------
Title: Evidence for Energetic Electrons Trapped and Accelerated in
a Magnetic Bottle above a Solar Flare Arcade
Authors: Chen, Bin; Kong, Xiangliang; Shen, Chengcai; Guo, Fan; Yu,
Sijie; Glesener, Lindsay; Zhang, Yixian; Li, Xiaocan; Fleishman,
Gregory; Krucker, Sam
2021AGUFMSH24B..01C Altcode:
Recent studies have shown a local depression of magnetic field
strength right above the post-reconnection solar flare arcade. This
peculiar structure, referred to as a "magnetic bottle," is a natural
consequence of the presence of a large-scale reconnection current sheet
that drives the flare energy release. It has been suggested that the
magnetic bottle may play a crucial role in confining charged particles
and facilitating their repeated acceleration to high energies. However,
testing this scenario requires joint observationalmodeling studies in a
realistic solar flare context, which has been heretofore elusive. Here
we present results by combining microwave, extreme ultraviolet,
and X-ray observations of an eruptive solar flare with macroscopic
magnetohydrodynamics and particle modeling in a standard flare
geometry. We show that the location, morphology, and spectral properties
of the observed nonthermal microwave and hard X-ray sources agree well
with the synthetic emissions from energetic electrons accelerated
and confined in the above-the-flare-arcade magnetic bottle. Such an
agreement in the emission signatures supports the magnetic bottle as a
candidate for the primary site of particle acceleration in solar flares.
---------------------------------------------------------
Title: Energy Spectrum of Solar Energetic Electron Events Over
25 Years
Authors: Wang, Wen; Wang, Linghua; Liu, Zixuan; Krucker, Sam
2021AGUFMSH55F1902W Altcode:
We investigate 427 SEEs with good energy spectrum at energies
from 4.2keV to 108keV observed by WIND/3DP from 1994 Dec through
2019 Dec. The SEE energy spectrum can be classified into three
types: Power-Law (PL), Ellison-Ramaty (ER) and Log-parabola (LP),
respectively. Among the 335 PL cases, 253 are downward DPL cases where
2 > 1 with low energy spectral index 1 = 2.1 0.4, high energy
spectral index 2 = 4.1 1.0 and a double peak distribution of break
energy EB: first peak at 6.3keV and the second peak at 69 keV ; 39
cases are upward DPL where 2 < 1 with 1 = 3.1 0.6, 2 = 2.2 0.3 and
average break energy EB = 7.0keV ; 43 cases are SPL where 2 1 with 1 =
2.70.5, 2 = 3.20.8. 33 cases are ER with low energy spectral index ER =
2.00.4 and average cut-off energy Ec = 47 keV; 59 cases are LP with low
energy spectral index 1 = 1.7 0.5 and high energy spectral index 2 =
3.8 0.8. By comparing HXR-related events with HXR-not-related events,
we find that: no differences for spectral indexes of PL but PL break
energy EB and energy flux of HXR-related events are larger than that of
HXR-not-related events; No differences for spectral indexes and energy
flux of LP. By comparing solar cycle 23 events with solar cycle 24
events, we find that: energy flux of events in solar cycle 23 is larger
than that in cycle 24 for PL, ER and LP events; No significant solar
cycle differences found in spectral indexes for PL, ER and LP events.
---------------------------------------------------------
Title: Modeling Effects of Charge Sharing on the Response of the
FOXSI Sounding Rockets
Authors: Duncan, Jessie; Glesener, Lindsay; Athiray, P. S.;
Buitrago-Casas, Juan Camilo; Christe, Steven; Krucker, Sam; Musset,
Sophie; Nagasawa, Shunsaku; Takahashi, Tadayuki; Vievering, Juliana;
Watanabe, Shin
2021AGUFMSH55B1829D Altcode:
The FOXSI sounding rocket experiments are the first solar-dedicated
direct-focusing hard X-ray (HXR) instruments. FOXSI sounding rockets
use Wolter-1 style concentric-shell hard X-ray (HXR) optics and both
silicon (Si) and cadmium telluride (CdTe) double-sided strip detectors
to observe the Sun. FOXSI images of solar HXR sources are shaped by
the point spread function (PSF) of the optics, the 2D segmentation of
the detector into square strip intersections (with dimensions 60m^2
(CdTe) or 75m^2 (Si)), and noise in the detector readout. FOXSI images
are also affected by charge-sharing in the detector, which occurs when
one incident photon causes signals in multiple adjacent strips. Charge
sharing is more likely to occur near strip boundaries, making it a
sub-strip-position-dependent effect. In this work, a model has been
developed combining the FOXSI optical and detector response. Using
this model, generated sources can be convolved with the FOXSI system
to simulate FOXSI data. Additionally, a corresponding deconvolution
process can be used to extract a reconstructed incident source from
the simulated data, and the original and reconstructed sources can be
compared. The incorporation of charge sharing into the modeled detector
response means that the reconstructed source approximates the original
to a higher degree of spatial resolution than what would result from
using strip-based position knowledge only. This FOXSI response model is
useful to aid in interpreting existing FOXSI data, and also to predict
possible future results from the upcoming FOXSI-4 flare campaign
(March 2024). Additionally, the capacity to model charge sharing in
spatially segmented semiconductor detectors has the potential to be
useful in maximizing the scientific results from future space-based
solar HXR imagers, as well as any other application of such a system
where charge sharing occurs.
---------------------------------------------------------
Title: Hard X-ray upper limits of the quiet Sun with new FOXSI
observations
Authors: Buitrago-Casas, Juan Camilo; Glesener, Lindsay; Christe,
Steven; Krucker, Sam; Vievering, Juliana; Athiray, P. S.; Musset,
Sophie; Davis, Lance; Courtade, Sasha; Dalton, Gregory; Turin,
Paul; Turin, Zoe; Ramsey, Brian; Bongiorno, Stephen; Ryan, Daniel;
Takahashi, Tadayuki; Furukawa, Kento; Watanabe, Shin; Narukage,
Noriyuki; Ishikawa, Shin-nosuke; Mitsuishi, Ikuyuki; Hagino, Kouichi
2021AGUFMSH51A..04B Altcode:
Solar nanoflares are small eruptive events releasing magnetic energy
in the quiet corona. If nanoflares follow the same physics as their
larger counterparts, they should emit hard X-rays (HXRs) but with a
rather faint intensity. A copious and continuous presence of nanoflares
would result in a sustained and persistent emission in HXRs, which in
turn would deliver enormous amounts of energy into the solar corona,
possibly accounting for its high temperatures. To date, there has not
been any direct observation of such sustained and persistent HXRs from
the quiescent Sun. However, Hannah et al. in 2010 constrained the quiet
Sun HXR emission using almost 12 days of quiescent solar-off-pointing
observations by RHESSI. These observations set $2\sigma$ upper limits
at $3.4\times 10^{-2}$ photons$^{-1}$ s$^{-1}$ cm$^{-2}$ keV$^{-1}$
and $9.5\times 10^{-4}$ photons$^{-1}$ s$^{-1}$ cm$^{-2}$ keV$^{-1}$
for the 3-6 keV and 6-12 keV energy ranges, respectively. Observing
feeble HXRs is challenging because it demands high sensitivity and
dynamic range instruments in the HXR energy band. The Focusing Optics
X-ray Solar Imager (FOXSI) sounding rocket experiment excels in these
two attributes when compared with RHESSI. Particularly, FOXSI completed
its third successful flight (FOXSI-3) on September 7th, 2018. During
FOXSI-3s flight, the Sun exhibited a fairly quiet configuration,
displaying only one aged non-flaring active region. Using the entire
$\sim$6.5 minutes of FOXSI-3 data, we constrained the quiet Sun emission
in HXRs. We found $2\sigma$ upper limits in the order of $\sim 10^{-3}$
photons$^{-1}$ s$^{-1}$ cm$^{-2}$ keV$^{-1}$ for the 5-10 keV energy
range. FOXSI-3's upper limit is consistent with what was reported by
Hannah et al., 2010, but FOXSI-3 achieved this result using $\sim$1/2640
less time than RHESSI. A possible future spacecraft using FOXSI's
concept would allow enough observation time to constrain the current
HXR quiet Sun limits further or perhaps even make direct detections.
---------------------------------------------------------
Title: The solar PolArization and Directivity X-Ray Experiment: PADRE
Authors: Martinez Oliveros, Juan Carlos; Christe, Steven;
Saint-Hilaire, Pascal; Krucker, Sam; Hayes, Laura; Caspi, Amir;
Limousin, Olivier; Peretz, Eliad
2021AGUFMSH51A..03M Altcode:
The solar PolArization and Directivity X-Ray Experiment (PADRE) is
a 12U Cubesat observatory that will observe the Sun in hard X-ray
(HXRs) from low earth orbit. PADRE will investigate the accelerated
electron angular distribution in solar flares with two unique
and complementary approaches (1) by making spatially-integrated
spectro-polarimetric x-ray measurements (~10-100 keV) and (2) by
coordinating with Solar Orbiter/STIX to make the first two point
measurements of x-rays and determining their directivity. We present
the PADRE observatory concept, its science objectives and design. We
will discuss the type of observations that will be made by the SHARP
and XDSOX instruments. Spectro-polarimetric observations will provide
unique information of the electron angular distribution. Furthermore,
STIX on board Solar Orbiter will perform X-ray observations of solar
flares from 0.28 AU (at perihelion) to 1.2AU (at aphelion) and up
to inclinations of 25 degrees at heliospheric angles significantly
different from the Earth. This provides a unique opportunity to make
stereoscopic X-ray observations and measure the electron anisotropy
of individual flares confidently for the first time. PADRE has one
science objective: determine the angular distribution of accelerated
electrons from standalone and joint SolO/STIX observations. This will
allow the determination of the angular distribution of flare-accelerated
electrons.
---------------------------------------------------------
Title: The FOXSI-4 Sounding Rocket: High Resolution Focused X-ray
Observations of the Sun
Authors: Glesener, Lindsay; Buitrago-Casas, Juan Camilo; Vievering,
Juliana; Pantazides, Athanasios; Musset, Sophie; Panchapakesan,
Subramania Athiray; Baumgartner, Wayne; Bongiorno, Stephen; Champey,
Patrick; Christe, Steven; Courtade, Sasha; Duncan, Jessie; Ishikawa,
Shin-nosuke; Krucker, Sam; Martinez Oliveros, Juan Carlos; Perez-Piel,
Savannah; Mitsuishi, Ikuyuki; Narukage, Noriyuki; Ryan, Daniel;
Takahashi, Tadayuki; Watanabe, Shin
2021AGUFMSH55B1831G Altcode:
It has been firmly demonstrated that direct-focusing instruments
can transform the way high-energy X-rays from astronomical objects,
including the Sun, are measured. The NuSTAR spacecraft has increased the
sensitivity to faint astrophysical sources by 100 times as compared with
previous, indirect, imagers. The first three flights of the Focusing
Optics X-ray Solar Imager (FOXSI) sounding rocket established the
usefulness and feasibility of a similar method optimized for the Sun,
and showed that in addition to greater sensitivity, a vastly improved
dynamic range can be obtained in this way. This technology stands
ready to revolutionize understanding of solar flares by elucidating
particle acceleration sites in the corona, studying how electrons
propagate and deposit their energies, and how accelerated particles
escape into interplanetary space. While the fundamental building
blocks of solar hard X-ray (HXR) focusing are in place and ready for a
spacecraft mission, concurrent development is required to prepare for
the next generation of high-energy solar explorers, which will require
higher rate capability and higher angular resolution to investigate
finer-scale structure and to better complement instruments at other
wavelengths. FOXSI-4 features technological advances that enable
high angular resolution as well as measurement of bright sources. In
the first category, we will develop high-precision mirror production
methods and finely pixelated Si CMOS sensors, and will demonstrate
substrip/subpixel resolution in fine-pitch CdTe sensors. Secondly,
we will demonstrate rate capability of these sensors sufficient
for flare measurement, and will develop novel pixelated attenuators
that optimize energy coverage even at high rates. The experiment will
demonstrate these technologies in NASAs first-ever solar flare campaign,
flying in tandem with the Hi-C FLARE rocket and in a campaign with the
SNIFS rocket. The campaign will position multiple rocket experiments
awaiting an opportunistic signal and will launch the experiments for
near-simultaneous observation of the flare.This campaign will allow
for direct collaboration with the Parker Solar Probe (PSP) during one
of its perihelia.
---------------------------------------------------------
Title: NuSTAR Observations of a Repeatedly Microflaring Active Region
Authors: Cooper, Kristopher; Hannah, Iain; Grefenstette, Brian;
Glesener, Lindsay; Krucker, Sam; Hudson, Hugh; White, Stephen; Smith,
David; Duncan, Jessie
2021AGUFMSH22B..03C Altcode:
We present observations of microflares from Sep 9-10 2018 in X-rays
with NuSTAR, EUV with SDO/AIA, and photosphere magnetic field with
SDO/HMI. We investigate how the energy released in these small flares
contributes to the heating of the solar atmosphere and the role
flux cancellation/emergence plays during onset and occurrence of the
microflares. 10 microflares were studied from AR12721, all equivalent
to GOES <A1 Class, and we find that many contain plasma heated
to 5-10 MK with energies of 10261028 erg. One particularly small
microflare, equivalent to GOES Class A0.005, demonstrated emission
from 6.7 MK plasma with a thermal energy of 1.11026 erg. Another
microflare, equivalent to GOES Class A0.1, showed clear non-thermal
emission in the X-ray spectra, with non-thermal energy of 1.31027
erg, making it one of the faintest non-thermal X-ray microflares on
record. For 8 of the 10 microflares we can identify areas of magnetic
flux cancellation/emergence at the footpoints, indicating the role
these play in this active region repeatedly producing microflares.
---------------------------------------------------------
Title: Solar Energetic Electron Events Associated with Hard X-ray
Flares
Authors: Wang, Wen; Wang, Linghua; Krucker, Sam; Mason, Glenn; Su,
Yang; Bucik, Radoslav
2021AGUFMSH52B..07W Altcode:
We investigate 16 solar energetic electron (SEE) events measured
by WIND/3DP with a double-power-law spectrum and the associated
western hard X-ray (HXR) flares measured by RHESSI with good count
statistics, from 2002 February to 2016 December. In all the 16 cases,
the presence of an SEE power-law spectrum extending down to 5 keV
at 1 AU implies that the SEE source would be high in the corona, at
heliocentric distance of >1.3 solar radii, while the footpoint or
footpoint-like emissions shown in HXR images suggest that the observed
HXRs are likely produced mainly by thick-target bremsstrahlung processes
very low in the corona. We find that the power-law spectral index of
HXR-producing electrons, estimated under the relativistic thick-target
bremsstrahlung model, is significantly larger than (similar to) and
positively correlated with the observed high-energy spectral index of
SEEs, in 8 cases (8 cases). In addition, the estimated number of SEEs
is only 10-4-10-2 of the estimated number of HXR-producing electrons
at energies above 30 keV, but with a positive correlation between
the two numbers. These results suggest that in these cases, SEEs are
likely formed by upward-traveling electrons from an acceleration source
high in the corona, while their downward-traveling counterparts may
undergo a secondary acceleration before producing HXRs via thick-target
bremsstrahlung processes. In addition, the associated 3He=4He ratio is
positively correlated with the observed high-energy spectral index of
SEEs, indicating a possible relation of the 3He ion acceleration with
high-energy SEEs.
---------------------------------------------------------
Title: The Multiview Observatory for Solar Terrestrial Science (MOST)
Authors: Gopalswamy, Nat; Kucera, Therese; Leake, James; MacDowall,
Robert; Wilson, Lynn; Kanekal, Shrikanth; Shih, Albert; Christe,
Steven; Gong, Qian; Viall, Nicholeen; Tadikonda, Sivakumar; Fung,
Shing; Yashiro, Seiji; Makela, Pertti; Golub, Leon; DeLuca, Edward;
Reeves, Katharine; Seaton, Daniel; Savage, Sabrina; Winebarger, Amy;
DeForest, Craig; Desai, Mihir; Bastian, Tim; Lazio, Joseph; Jensen,
P. E., C. S. P., Elizabeth; Manchester, Ward; Wood, Brian; Kooi,
Jason; Wexler, David; Bale, Stuart; Krucker, Sam; Hurlburt, Neal;
DeRosa, Marc; Pevtsov, Alexei; Tripathy, Sushanta; Jain, Kiran;
Gosain, Sanjay; Petrie, Gordon; Kholikov, Shukirjon; Zhao, Junwei;
Scherrer, Philip; Woods, Thomas; Chamberlin, Philip; Kenny, Megan
2021AGUFMSH12A..07G Altcode:
The Multiview Observatory for Solar Terrestrial Science (MOST) is a
comprehensive mission concept targeting the magnetic coupling between
the solar interior and the heliosphere. The wide-ranging imagery and
time series data from MOST will help understand the solar drivers and
the heliospheric responses as a system, discerning and tracking 3D
magnetic field structures, both transient and quiescent in the inner
heliosphere. MOST will have seven remote-sensing and three in-situ
instruments: (1) Magnetic and Doppler Imager (MaDI) to investigate
surface and subsurface magnetism by exploiting the combination of
helioseismic and magnetic-field measurements in the photosphere; (2)
Inner Coronal Imager in EUV (ICIE) to study large-scale structures
such as active regions, coronal holes and eruptive structures by
capturing the magnetic connection between the photosphere and the
corona to about 3 solar radii; (3) Hard X-ray Imager (HXI) to image
the non-thermal flare structure; (4) White-light Coronagraph (WCOR) to
seamlessly study transient and quiescent large-scale coronal structures
extending from the ICIE field of view (FOV); (5) Faraday Effect
Tracker of Coronal and Heliospheric structures (FETCH), a novel radio
package to determine the magnetic field structure and plasma column
density, and their evolution within 0.5 au; (6) Heliospheric Imager
with Polarization (HIP) to track solar features beyond the WCOR FOV,
study their impact on Earth, and provide important context for FETCH;
(7) Radio and Plasma Wave instrument (M/WAVES) to study electron beams
and shocks propagating into the heliosphere via passive radio emission;
(8) Solar High-energy Ion Velocity Analyzer (SHIVA) to determine spectra
of electrons, and ions from H to Fe at multiple spatial locations
and use energetic particles as tracers of magnetic connectivity; (9)
Solar Wind Magnetometer (MAG) to characterize magnetic structures at
1 au; (10) Solar Wind Plasma Instrument (SWPI) to characterize plasma
structures at 1 au. MOST will have two large spacecraft with identical
payloads deployed at L4 and L5 and two smaller spacecraft ahead of L4
and behind L5 to carry additional FETCH elements. MOST will build upon
SOHO and STEREO achievements to expand the multiview observational
approach into the first half of the 21st Century.
---------------------------------------------------------
Title: Multiple Electron Acceleration Instances during a Series of
Solar Microflares Observed Simultaneously at X-Rays and Microwaves
Authors: Battaglia, Marina; Sharma, Rohit; Luo, Yingjie; Chen, Bin;
Yu, Sijie; Krucker, Säm
2021ApJ...922..134B Altcode: 2021arXiv210912847B
Even small solar flares can display a surprising level of complexity
regarding their morphology and temporal evolution. Many of their
properties, such as energy release and electron acceleration can be
studied using highly complementary observations at X-ray and radio
wavelengths. We present X-ray observations from the Reuven Ramaty High
Energy Solar Spectroscopic Imager and radio observations from the Karl
G. Jansky Very Large Array (VLA) of a series of GOES A3.4-B1.6 class
flares observed on 2013 April 23. The flares, as seen in X-ray and
extreme ultraviolet, originated from multiple locations within active
region NOAA 11726. A veritable zoo of different radio emissions between
1 GHz and 2 GHz was observed cotemporally with the X-ray flares. In
addition to broadband continuum emission, broadband short-lived
bursts and narrowband spikes, indicative of accelerated electrons,
were observed. However, these sources were located up to 150″ away
from the flaring X-ray sources but only some of these emissions could
be explained as signatures of electrons that were accelerated near the
main flare site. For other sources, no obvious magnetic connection to
the main flare site could be found. These emissions likely originate
from secondary acceleration sites triggered by the flare, but may
be due to reconnection and acceleration completely unrelated to the
cotemporally observed flare. Thanks to the extremely high sensitivity
of the VLA, not achieved with current X-ray instrumentation, it is
shown that particle acceleration happens frequently and at multiple
locations within a flaring active region.
---------------------------------------------------------
Title: Detection of weak solar radio transients using residual
radio imaging
Authors: Oberoi, Divya; Sharma, Rohit; Battaglia, Marina; Krucker, Sam
2021AGUFMSH52A..02O Altcode:
Recently very weak radio bursts, down to mSFU levels, have been
detected at low radio frequencies with the Murchison Widefield Array
(Mondal et al., 2020, ApJ, 895, L39). This opens the tantalising
possibility that these bursts are the radio counterparts of the
nanoflares originally hypothesized to explain coronal heating. Here we
use an alternative approach to detect and image very weak transient
solar radio emissions. We estimate the slowly varying component
of the solar emission using a median filter for cross-correlations
(visibilities) measured for every baseline individually and subtract
it. We then image the residual visibilities obtained thus at time
and frequency resolutions of 0.5 s and 2 MHz. About 30 min of data
from a quiet sun period in 8 spectral bands spanning the frequency
range from 108 to 240 MHz are used. Median subtraction is found to
reduce the amplitudes of individual visibilities by 2 to 3 orders of
magnitude, enabling us to reliably probe emissions with brightness
temperatures down to around 500 K and flux densities in the range of
0.5-10 mSFU (1 SFU = 10,000 Jy). The distribution of the number of
reliably detected impulsive features covers the entire solar disc,
including the quiet Sun regions. At the highest two frequencies there
is a clear clustering of these features with locations of active
regions. For the time series of 0.5 s residual maps, we computed
the rms for every pixel (see figure). Both these sets of maps show
smoothly varying trends across frequency. Above about 145 MHz, the
entire solar surface shows very similar values of rms, implying a
uniform distribution of weak impulsive features all over the quiet
Sun. At the highest frequencies the observed rms is much higher at the
locations in the vicinity of the active regions. At frequencies below
145 MHz, a different cluster of high rms value is seen which lies in
a large coronal hole region, but does not seem to have an association
with any other solar feature. These are among the weakest detections
of impulsive nonthermal solar emissions. This paper will present the
details of this work and its implications.
---------------------------------------------------------
Title: Imaging from STIX visibility amplitudes
Authors: Massa, Paolo; Perracchione, Emma; Garbarino, Sara; Battaglia,
Andrea F.; Benvenuto, Federico; Piana, Michele; Hurford, Gordon;
Krucker, Säm
2021A&A...656A..25M Altcode: 2021arXiv210804901M
<BR /> Aims: This study is aimed at providing the first demonstration
of STIX Fourier-transform X-ray imaging using semi-calibrated
(amplitude-only) visibility data acquired during Solar Orbiter's cruise
phase. <BR /> Methods: We used a parametric imaging approach whereby
STIX visibility amplitudes are fitted by means of two non-linear
optimization methods: a fast meta-heuristic technique inspired
by social behavior and a Bayesian Monte Carlo sampling method
that, although slower, provides a better way of quantifying the
uncertainties. <BR /> Results: When applied to a set of solar flare
visibility amplitudes recorded by STIX on November 18, 2020, the two
parametric methods provide very coherent results. The analysis also
demonstrates the ability of STIX to reconstruct data at a high time
resolution and, from a spectral viewpoint, it shows the reliability
of a double-source scenario that is consistent with a thermal versus
nonthermal interpretation. <BR /> Conclusions: In this preliminary
analysis of STIX imaging based only on visibility amplitudes, we
formulate the imaging problem as a non-linear parametric issue addressed
by means of two high-performance optimization techniques that both show
the ability to sample the parametric space in an effective fashion,
thus avoiding any local minima.
---------------------------------------------------------
Title: STIX X-ray microflare observations during the Solar Orbiter
commissioning phase
Authors: Battaglia, Andrea Francesco; Saqri, Jonas; Massa, Paolo;
Perracchione, Emma; Dickson, Ewan C. M.; Xiao, Hualin; Veronig,
Astrid M.; Warmuth, Alexander; Battaglia, Marina; Hurford, Gordon J.;
Meuris, Aline; Limousin, Olivier; Etesi, László; Maloney, Shane A.;
Schwartz, Richard A.; Kuhar, Matej; Schuller, Frederic; Senthamizh
Pavai, Valliappan; Musset, Sophie; Ryan, Daniel F.; Kleint, Lucia;
Piana, Michele; Massone, Anna Maria; Benvenuto, Federico; Sylwester,
Janusz; Litwicka, Michalina; Stȩślicki, Marek; Mrozek, Tomasz;
Vilmer, Nicole; Fárník, František; Kašparová, Jana; Mann,
Gottfried; Gallagher, Peter T.; Dennis, Brian R.; Csillaghy, André;
Benz, Arnold O.; Krucker, Säm
2021A&A...656A...4B Altcode: 2021arXiv210610058B
Context. The Spectrometer/Telescope for Imaging X-rays (STIX) is the
hard X-ray instrument onboard Solar Orbiter designed to observe solar
flares over a broad range of flare sizes. <BR /> Aims: We report
the first STIX observations of solar microflares recorded during
the instrument commissioning phase in order to investigate the STIX
performance at its detection limit. <BR /> Methods: STIX uses hard
X-ray imaging spectroscopy in the range between 4-150 keV to diagnose
the hottest flare plasma and related nonthermal electrons. This first
result paper focuses on the temporal and spectral evolution of STIX
microflares occuring in the Active Region (AR) AR12765 in June 2020,
and compares the STIX measurements with Earth-orbiting observatories
such as the X-ray Sensor of the Geostationary Operational Environmental
Satellite (GOES/XRS), the Atmospheric Imaging Assembly of the Solar
Dynamics Observatory, and the X-ray Telescope of the Hinode mission. <BR
/> Results: For the observed microflares of the GOES A and B class, the
STIX peak time at lowest energies is located in the impulsive phase of
the flares, well before the GOES peak time. Such a behavior can either
be explained by the higher sensitivity of STIX to higher temperatures
compared to GOES, or due to the existence of a nonthermal component
reaching down to low energies. The interpretation is inconclusive
due to limited counting statistics for all but the largest flare
in our sample. For this largest flare, the low-energy peak time is
clearly due to thermal emission, and the nonthermal component seen at
higher energies occurs even earlier. This suggests that the classic
thermal explanation might also be favored for the majority of the
smaller flares. In combination with EUV and soft X-ray observations,
STIX corroborates earlier findings that an isothermal assumption
is of limited validity. Future diagnostic efforts should focus on
multi-wavelength studies to derive differential emission measure
distributions over a wide range of temperatures to accurately describe
the energetics of solar flares. <BR /> Conclusions: Commissioning
observations confirm that STIX is working as designed. As a rule of
thumb, STIX detects flares as small as the GOES A class. For flares
above the GOES B class, detailed spectral and imaging analyses can
be performed.
---------------------------------------------------------
Title: The high-energy Sun - probing the origins of particle
acceleration on our nearest star
Authors: Matthews, S. A.; Reid, H. A. S.; Baker, D.; Bloomfield, D. S.;
Browning, P. K.; Calcines, A.; Del Zanna, G.; Erdelyi, R.; Fletcher,
L.; Hannah, I. G.; Jeffrey, N.; Klein, L.; Krucker, S.; Kontar, E.;
Long, D. M.; MacKinnon, A.; Mann, G.; Mathioudakis, M.; Milligan,
R.; Nakariakov, V. M.; Pesce-Rollins, M.; Shih, A. Y.; Smith, D.;
Veronig, A.; Vilmer, N.
2021ExA...tmp..135M Altcode:
As a frequent and energetic particle accelerator, our Sun provides
us with an excellent astrophysical laboratory for understanding
the fundamental process of particle acceleration. The exploitation
of radiative diagnostics from electrons has shown that acceleration
operates on sub-second time scales in a complex magnetic environment,
where direct electric fields, wave turbulence, and shock waves all
must contribute, although precise details are severely lacking. Ions
were assumed to be accelerated in a similar manner to electrons, but
γ-ray imaging confirmed that emission sources are spatially separated
from X-ray sources, suggesting distinctly different acceleration
mechanisms. Current X-ray and γ-ray spectroscopy provides only a basic
understanding of accelerated particle spectra and the total energy
budgets are therefore poorly constrained. Additionally, the recent
detection of relativistic ion signatures lasting many hours, without
an electron counterpart, is an enigma. We propose a single platform
to directly measure the physical conditions present in the energy
release sites and the environment in which the particles propagate and
deposit their energy. To address this fundamental issue, we set out
a suite of dedicated instruments that will probe both electrons and
ions simultaneously to observe; high (seconds) temporal resolution
photon spectra (4 keV - 150 MeV) with simultaneous imaging (1 keV -
30 MeV), polarization measurements (5-1000 keV) and high spatial and
temporal resolution imaging spectroscopy in the UV/EUV/SXR (soft X-ray)
regimes. These instruments will observe the broad range of radiative
signatures produced in the solar atmosphere by accelerated particles.
---------------------------------------------------------
Title: NuSTAR observations of a repeatedly microflaring active region
Authors: Cooper, Kristopher; Hannah, Iain G.; Grefenstette, Brian W.;
Glesener, Lindsay; Krucker, Säm; Hudson, Hugh S.; White, Stephen M.;
Smith, David M.; Duncan, Jessie
2021MNRAS.507.3936C Altcode: 2021arXiv210900263C; 2021MNRAS.tmp.2159C
We investigate the spatial, temporal, and spectral properties of 10
microflares from AR12721 on 2018 September 9 and 10 observed in X-rays
using the Nuclear Spectroscopic Telescope ARray and the Solar Dynamic
Observatory's Atmospheric Imaging Assembly and Helioseismic and Magnetic
Imager. We find GOES sub-A class equivalent microflare energies of
10<SUP>26</SUP>-10<SUP>28</SUP> erg reaching temperatures up to 10
MK with consistent quiescent or hot active region (AR) core plasma
temperatures of 3-4 MK. One microflare (SOL2018-09-09T10:33), with
an equivalent GOES class of A0.1, has non-thermal hard X-ray emission
during its impulsive phase (of non-thermal power ~7 × 10<SUP>24</SUP>
erg s<SUP>-1</SUP>) making it one of the faintest X-ray microflares
to have direct evidence for accelerated electrons. In 4 of the 10
microflares, we find that the X-ray time profile matches fainter and
more transient sources in the extreme-ultraviolet, highlighting the need
for observations sensitive to only the hottest material that reaches
temperatures higher than those of the AR core (>5 MK). Evidence
for corresponding photospheric magnetic flux cancellation/emergence
present at the footpoints of eight microflares is also observed.
---------------------------------------------------------
Title: The Solar Orbiter Radio and Plasma Waves (RPW) instrument
(Corrigendum)
Authors: Maksimovic, M.; Bale, S. D.; Chust, T.; Khotyaintsev, Y.;
Krasnoselskikh, V.; Kretzschmar, M.; Plettemeier, D.; Rucker, H. O.;
Souček, J.; Steller, M.; Štverák, Š.; Trávníček, P.; Vaivads,
A.; Chaintreuil, S.; Dekkali, M.; Alexandrova, O.; Astier, P. -A.;
Barbary, G.; Bérard, D.; Bonnin, X.; Boughedada, K.; Cecconi,
B.; Chapron, F.; Chariet, M.; Collin, C.; de Conchy, Y.; Dias, D.;
Guéguen, L.; Lamy, L.; Leray, V.; Lion, S.; Malac-Allain, L. R.;
Matteini, L.; Nguyen, Q. N.; Pantellini, F.; Parisot, J.; Plasson,
P.; Thijs, S.; Vecchio, A.; Fratter, I.; Bellouard, E.; Lorfèvre,
E.; Danto, P.; Julien, S.; Guilhem, E.; Fiachetti, C.; Sanisidro,
J.; Laffaye, C.; Gonzalez, F.; Pontet, B.; Quéruel, N.; Jannet,
G.; Fergeau, P.; Brochot, J. -Y.; Cassam-Chenai, G.; Dudok de Wit,
T.; Timofeeva, M.; Vincent, T.; Agrapart, C.; Delory, G. T.; Turin,
P.; Jeandet, A.; Leroy, P.; Pellion, J. -C.; Bouzid, V.; Katra, B.;
Piberne, R.; Recart, W.; Santolík, O.; Kolmašová, I.; Krupař,
V.; Krupařová, O.; Píša, D.; Uhlíř, L.; Lán, R.; Baše, J.;
Ahlèn, L.; André, M.; Bylander, L.; Cripps, V.; Cully, C.; Eriksson,
A.; Jansson, S. -E.; Johansson, E. P. G.; Karlsson, T.; Puccio, W.;
Břínek, J.; Öttacher, H.; Panchenko, M.; Berthomier, M.; Goetz,
K.; Hellinger, P.; Horbury, T. S.; Issautier, K.; Kontar, E.; Krucker,
S.; Le Contel, O.; Louarn, P.; Martinović, M.; Owen, C. J.; Retino,
A.; Rodríguez-Pacheco, J.; Sahraoui, F.; Wimmer-Schweingruber, R. F.;
Zaslavsky, A.; Zouganelis, I.
2021A&A...654C...2M Altcode:
No abstract at ADS
---------------------------------------------------------
Title: FOXSI-4: the high resolution focusing X-ray rocket payload
to observe a solar flare
Authors: Buitrago-Casas, Juan Camilo; Vievering, Juliana; Musset,
Sophie; Glesener, Lindsay; Athiray, P. S.; Baumgartner, Wayne;
Bongiorno, Stephen; Champey, Patrick; Christe, Steven; Courtade,
Sasha; Dalton, Gregory; Duncan, Jessie; Gilchrist, Kelsey; Ishikawa,
Shin-nosuke; Jhabvala, Christine; Kanniainen, Hunter; Krucker,
Säm.; Gregory, Kyle; Martinez Oliveros, Juan Carlos; McCracken,
Jeff; Mitsuishi, Ikuyuki; Narukage, Noriyuki; Pantazides, Athanasios;
Peretz, Eliad; Perez-Piel, Savannah; Ramanayaka, Aruna; Ramsey, Brian;
Ryan, Danny; Savage, Sabrina; Takahashi, Tadayuki; Watanabe, Shin;
Winebarger, Amy; Zhang, Yixian
2021SPIE11821E..0LB Altcode:
The FOXSI-4 sounding rocket will fly a significantly upgraded instrument
in NASA's first solar are campaign. It will deploy direct X-ray focusing
optics which have revolutionized our understanding of astrophysical
phenomena. For example, they have allowed NuSTAR to provide X-ray
imaging and IXPE (scheduled for launch in 2021) to provide X-ray
polarization observations with detectors with higher photon rate
capability and greater sensitivity than their predecessors. The FOXSI
sounding rocket is the first solar dedicated mission using this method
and has demonstrated high sensitivity and improved imaging dynamic range
with its three successful flights. Although the building blocks are
already in place for a FOXSI satellite instrument, further advances are
needed to equip the next generation of solar X-ray explorers. FOXSI-4
will develop and implement higher angular resolution optics/detector
pairs to investigate fine spatial structures (both bright and faint)
in a solar are. FOXSI-4 will use highly polished electroformed Wolter-I
mirrors fabricated at the NASA/Marshall Space Flight Center (MSFC),
together with finely pixelated Si CMOS sensors and fine-pitch CdTe strip
detectors provided by a collaboration with institutes in Japan. FOXSI-4
will also implement a set of novel perforated attenuators that will
enable both the low and high energy spectral components to be observed
simultaneously in each pixel, even at the high rates expected from a
medium (or large) size solar are. The campaign will take place during
one of the Parker Solar Probe (PSP) perihelia, allowing coordination
between this spacecraft and other instruments which observe the Sun
at different wavelengths.
---------------------------------------------------------
Title: ALMA small-scale features in the quiet Sun and active regions
Authors: Brajša, R.; Skokić, I.; Sudar, D.; Benz, A. O.; Krucker,
S.; Ludwig, H. -G.; Saar, S. H.; Selhorst, C. L.
2021A&A...651A...6B Altcode: 2021arXiv210503644B
<BR /> Aims: The main aim of the present analysis is to decipher (i)
the small-scale bright features in solar images of the quiet Sun and
active regions obtained with the Atacama Large Millimeter/submillimeter
Array (ALMA) and (ii) the ALMA correspondence of various known
chromospheric structures visible in the Hα images of the Sun. <BR
/> Methods: Small-scale ALMA bright features in the quiet Sun region
were analyzed using single-dish ALMA observations (1.21 mm, 248 GHz)
and in an active region using interferometric ALMA measurements (3
mm, 100 GHz). With the single-dish observations, a full-disk solar
image is produced, while interferometric measurements enable the
high-resolution reconstruction of part of the solar disk, including
the active region. The selected quiet Sun and active regions are
compared with the Hα (core and wing sum), EUV, and soft X-ray images
and with the magnetograms. <BR /> Results: In the quiet Sun region,
enhanced emission seen in the ALMA is almost always associated with a
strong line-of-sight magnetic field. Four coronal bright points were
identified, while other small-scale ALMA bright features are most likely
associated with magnetic network elements and plages. In the active
region, in 14 small-scale ALMA bright features randomly selected and
compared with other images, we found five good candidates for coronal
bright points, two for plages, and five for fibrils. Two unclear cases
remain: a fibril or a jet, and a coronal bright point or a plage. A
comparison of the Hα core image and the 3 mm ALMA image of the analyzed
active region showed that the sunspot appears dark in both images
(with a local ALMA radiation enhancement in sunspot umbra), the four
plage areas are bright in both images and dark small Hα filaments are
clearly recognized as dark structures of the same shape also in ALMA.
---------------------------------------------------------
Title: Assessing quiet Sun hard X-rays using observations from the
FOXSI Sounding Rockets
Authors: Buitrago-Casas, J.; Glesener, L.; Christe, S.; Krucker,
S.; Vievering, J.; Athiray, P.; Musset, S.; Ryan, D.; Ishikawa, S.;
Narukage, N.; Bongiorno, S.; Furukawa, K.; Ramsey, B.; Courtade, S.;
Dalton, G.; Turin, P.; Takahashi, T.; Watanabe, S.; Mitsuishi, I.;
Hagino, K.; Duncan, J.
2021AAS...23810604B Altcode:
In solar and helio-physics, the coronal heating problem relates to the
question of identifying and explaining the mechanism(s) causing the
corona's temperatures to be a few hundred times hotter than the solar
surface. Among the various plausible hypotheses proposed to explain
this problem, one of the strongest candidates relates to copious low
energy magnetic reconnections (nanoflares) occurring throughout the
solar corona. When examined thoroughly, this mechanism implies heating
that happens impulsively on individual flux tubes (strands). Emission
of hard X-rays (HXRs) should be a consequence of such non-thermal
phenomena, or even of purely thermal transients, if hot enough. In
quiescent solar corona areas, nanoflares should manifest in HXRs via
very faint signatures covering vast regions. Observing feeble HXRs
demands an instrument with high sensitivity and dynamic range for
energies between 4 and 15 keV. FOXSI (which stands for the Focusing
Optics X-ray Solar Imager) is such an instrument. As a payload of
a NASA/LCAS (low-cost access to space program) sounding rocket,
FOXSI has successfully completed three launches. The two most recent
flights (FOXSI-2 and -3) included quiescent areas of the Sun as part
of the targets. For this presentation, we will show a full assessment
of the HXR flux from the quiet Sun observed with FOXSI. We begin by
presenting a thorough characterization of the stray light (ghost rays)
impinging into FOXSI's detectors caused by sources outside of the field
of view. We then identify areas free of ghost rays where the instrument
sensitivity reaches a maximum to quiet Sun HXR detections. Finally,
we implement a Bayesian (known as ON/OFF analysis) to estimate an upper
detectability threshold of quiet Sun HXRs and a probability distribution
for quiet-Sun HXR fluxes when sources are supposed to exist.
---------------------------------------------------------
Title: Solar Energetic Electron Events Associated with Hard X-Ray
Flares
Authors: Wang, Wen; Wang, Linghua; Krucker, Säm; Mason, Glenn M.;
Su, Yang; Bučík, Radoslav
2021ApJ...913...89W Altcode:
We investigate 16 solar energetic electron (SEE) events measured by
WIND/3DP with a double-power-law spectrum and the associated western
hard X-ray (HXR) flares measured by RHESSI with good count statistics,
from 2002 February to 2016 December. In all the 16 cases, the presence
of an SEE power-law spectrum extending down to ≤5 keV at 1 au implies
that the SEE source would be high in the corona, at a heliocentric
distance of ≥1.3 solar radii, while the footpoint or footpoint-like
emissions shown in HXR images suggest that the observed HXRs are
likely produced mainly by HXR-producing electrons via thick-target
bremsstrahlung processes very low in the corona. We find that for all
the 16 cases, the estimated power-law spectral index of HXR-producing
electrons is no less than the observed high-energy spectral index
of SEEs, and it shows a positive correlation with the high-energy
spectral index of SEEs. In addition, the estimated number of SEEs is
only ∼10<SUP>-4</SUP>-10<SUP>-2</SUP> of the estimated number of
HXR-producing electrons at energies above 30 keV, but with a positive
correlation between the two numbers. These results suggest that in these
cases, SEEs are likely formed by upward-traveling electrons from an
acceleration source high in the corona, while their downward-traveling
counterparts may undergo a secondary acceleration before producing HXRs
via thick-target bremsstrahlung processes. In addition, the associated
<SUP>3</SUP>He/<SUP>4</SUP>He ratio is positively correlated with the
observed high-energy spectral index of SEEs, indicating a possible
relation of the <SUP>3</SUP>He ion acceleration with high-energy SEEs.
---------------------------------------------------------
Title: The Solar PolArization and Directivity X-Ray Experiment (PADRE)
Authors: Martinez Oliveros, J.; Christe, S.; Saint-Hilaire, P.;
Krucker, S.; Caspi, A.; Peretz, E.; Hayes, L.; Limousin, O.; Meuris, A.
2021AAS...23831309M Altcode:
The solar PolArization and Directivity X-Ray Experiment (PADRE) is
a 12U Cubesat observatory that will observe the Sun in hard X-ray
(HXRs) from low earth orbit. PADRE will investigate the accelerated
electron angular distribution in solar flares with two unique
and complementary approaches (1) by making spatially-integrated
spectro-polarimetric x-ray measurements (~10-100 keV) and (2) by
coordinating with Solar Orbiter/STIX to make the first two point
measurements of x-rays and determining their directivity. We present
the PADRE observatory concept, its science objectives and design. We
will discuss the type of observations that will be made by the SHARP
and XDSOX instruments. Spectro-polarimetric observations will provide
unique information of the electron angular distribution. Furthermore,
STIX on board Solar Orbiter will perform X-ray observations of solar
flares from 0.28 AU (at perihelion) to 1.2AU (at aphelion) and up to
inclinations of ∼25 degrees at heliospheric angles significantly
different from the Earth. This provides a unique opportunity to make
stereoscopic X-ray observations and measure the electron anisotropy
of individual flares confidently for the first time. PADRE has one
science objective: determine the angular distribution of accelerated
electrons from standalone and joint SolO/STIX observations. This will
allow the determination of the angular distribution of flare-accelerated
electrons.
---------------------------------------------------------
Title: High Resolution FOXSI: The Development Of FOXSI-4
Authors: Glesener, L.; Buitrago-Casas, J.; Duncan, J.; Nagasawa, S.;
Pantazides, A.; Perez-Piel, S.; Zhang, Y.; Vievering, J.; Musset,
S.; Panchapakesan, S.; Baumgartner, W.; Bongiorno, S.; Champey, P.;
Christe, S.; Courtade, S.; Kanniainen, H.; Krucker, S.; Ishikawa,
S.; Martinez Oliveros, J.; Mitsuishi, I.; Narukage, N.; Peretz, E.;
Ryan, D.; Takahashi, T.; Watanabe, S.; Winebarger, A.
2021AAS...23831301G Altcode:
It has been firmly demonstrated that direct-focusing instruments
can transform the way high-energy X-rays from astronomical objects,
including the Sun, are measured. The NuSTAR spacecraft has increased the
sensitivity to faint astrophysical sources by 100 times as compared with
previous, indirect, imagers. The first three flights of the Focusing
Optics X-ray Solar Imager (FOXSI) sounding rocket established the
usefulness and feasibility of a similar method optimized for the Sun,
and showed that in addition to greater sensitivity, a vastly improved
dynamic range can be obtained in this way. This technology stands
ready to revolutionize understanding of solar flares by elucidating
particle acceleration sites in the corona, studying how electrons
propagate and deposit their energies, and how accelerated particles
escape into interplanetary space. While the fundamental building
blocks of solar hard X-ray (HXR) focusing are in place and ready for
a spacecraft mission, concurrent development is required to prepare
for a subsequent generation of high-energy solar explorers, which
will require higher rate capability and higher angular resolution to
investigate finer-scale structure and to better complement instruments
at other wavelengths.The fourth flight of FOXSI (FOXSI-4) features
technological advances that enable high angular resolution as well as
measurement of bright sources. In the first category, we are developing
high-precision mirror production methods and finely pixelated Si CMOS
sensors, and will demonstrate substrip/subpixel resolution in fine-pitch
CdTe sensors. Secondly, we will demonstrate a rate capability of these
sensors sufficient for flare measurement, and are developing novel
pixelated attenuators that optimize energy coverage even at high
rates.The experiment will demonstrate these technologies in NASA's
first-ever solar flare campaign, flying in tandem with the Hi-C FLARE
rocket and on the same campaign as the SNIFS rocket. The campaign will
position multiple rocket experiments awaiting an opportunistic signal
and will launch multiple payloads for near-simultaneous observation
of the flare.This campaign will allow for direct collaboration with
the Parker Solar Probe (PSP) during one of its perihelia.
---------------------------------------------------------
Title: FOXSI-2 Solar Microflares. II. Hard X-ray Imaging Spectroscopy
and Flare Energetics
Authors: Vievering, Juliana T.; Glesener, Lindsay; Athiray, P. S.;
Buitrago-Casas, Juan Camilo; Musset, Sophie; Ryan, Daniel F.; Ishikawa,
Shin-nosuke; Duncan, Jessie; Christe, Steven; Krucker, Säm
2021ApJ...913...15V Altcode: 2020arXiv201104753V
We study the nature of energy release and transfer for two sub-A class
solar microflares observed during the second Focusing Optics X-ray Solar
Imager (FOXSI-2) sounding rocket flight on 2014 December 11. FOXSI is
the first solar-dedicated instrument to utilize focusing optics to
image the Sun in the hard X-ray (HXR) regime, sensitive to energies
of 4-20 keV. Through spectral analysis of the microflares using an
optically thin isothermal plasma model, we find evidence for plasma
heated to ∼10 MK and emission measures down to ∼10<SUP>44</SUP>
cm<SUP>-3</SUP>. Though nonthermal emission was not detected for
the FOXSI-2 microflares, a study of the parameter space for possible
hidden nonthermal components shows that there could be enough energy in
nonthermal electrons to account for the thermal energy in microflare 1,
indicating that this flare is plausibly consistent with the standard
thick-target model. With a solar-optimized design and improvements in
HXR focusing optics, FOXSI-2 offers approximately five times greater
sensitivity at 10 keV than the Nuclear Spectroscopic Telescope Array
for typical microflare observations and allows for the first direct
imaging spectroscopy of solar HXRs with an angular resolution at scales
relevant for microflares. Harnessing these improved capabilities to
study small-scale events, we find evidence for spatial and temporal
complexity during a sub-A class flare. This analysis, combined with
contemporaneous observations by the Atmospheric Imaging Assembly on
board the Solar Dynamics Observatory, indicates that these microflares
are more similar to large flares in their evolution than to the single
burst of energy expected for a nanoflare.
---------------------------------------------------------
Title: Plasma Diagnostics of Microflares observed by STIX and AIA
Authors: Saqri, Jonas; Veronig, Astrid; Dickson, Ewan; Krucker, Säm;
Battaglia, Andrea Francesco; Battaglia, Marina; Xiao, Hualin; Warmuth,
Alexander; the STIX Team
2021EGUGA..23.7966S Altcode:
Solar flares are generally thought to be the impulsive release
of magnetic energy giving rise to a wide range of solar phenomena
that influence the heliosphere and in some cases even conditions of
earth. Part of this liberated energy is used for particle acceleration
and to heat up the solar plasma. The Spectrometer/Telescope for Imaging
X-rays (STIX) instrument onboard the Solar Orbiter mission launched
on February 10th 2020 promises advances in the study of solar flares
of various sizes. It is capable of measuring X-ray spectra from 4
to 150 keV with 1 keV resolution binned into 32 energy bins before
downlinking. With this energy range and sensitivity, STIX is capable
of sampling thermal plasma with temperatures of≳10 MK, and to
diagnose the nonthermal bremsstrahlung emission of flare-accelerated
electrons. During the spacecraft commissioning phase in the first
half of the year 2020, STIX observed 68 microflares. Of this set, 26
events could clearly be identified in at least two energy channels,
all of which originated in an active region that was also visible from
earth. These events provided a great opportunity to combine the STIX
observations with the multi-band EUV imagery from the Atmospheric
Imaging Assembly (AIA) instrument on board the earth orbiting
Solar Dynamics Observatory (SDO). For the microflares that could be
identified in two STIX science energy bands, it was possible to derive
the temperature and emission measure (EM) of the flaring plasma assuming
an isothermal source. For larger events where more detailed spectra
could be derived, a more accurate analysis was performed by fitting
the spectra assuming various thermal and nonthermal sources. These
results are compared to the diagnostics derived from AIA images. To
this aim, the Differential EmissionMeasure (DEM) was reconstructed from
AIA observations to infer plasma temperatures and EM in the flaring
regions. Combined with the the relative timing between the emission
seen by STIX and AIA, this allows us to get deeper insight into the
flare energy release and transport processes.
---------------------------------------------------------
Title: Early results for the STIX image reconstruction problem:
imaging from visibility amplitudes
Authors: piana, michele; massa, paolo; perracchione, emma; battaglia,
andrea francesco; benvenuto, federico; massone, anna maria; hurford,
gordon; krucker, sam
2021EGUGA..23.9422P Altcode:
The Spectrometer/Telescope for Imaging X-rays (STIX) is the instrument
of the Solar Orbiter mission conceived for the observation of the
hard X-ray flaring emission, with the objective of providing insights
on the diagnosis of thermal and non-thermal accelerated electrons at
the Sun. The STIX imaging system is composed of 30 pairs of tungsten
grids, each one placed in front of a four-pixel detector, and produces
as many Fourier components of the angular distribution of the flaring
source, via Moiré pattern modulation. Therefore, the data recorded
by STIX, named visibilities, can be interpreted as a sparse sampling
of the Fourier transform of the X-ray signal and the corresponding
image reconstruction problem requires the inversion of the Fourier
transform from limited data, usually addressed with regularization
techniques. Since the current calibration status of STIX measurements
still prevents the use of visibility phases for imaging purposes, here
we propose a parameter identification process based on forward fitting
just the amplitude of the experimental visibilities. Specifically, we
have parameterized the flaring source by means of pre-assigned source
shapes (e.g., circular and elliptical bi-variate Gaussian functions),
and we relied on several approaches to non-linear optimization in order
to estimating the shape parameters. In particular, we have implemented
a forward-fit method based on deterministic chi-squared minimization,
a stochastic optimization algorithm and a deep neural approach based
on ensemble learning, also equipping them with an ad hoc statistical
technique for uncertainty quantification. The performances of the
three approaches are compared in the case of both microflares and M
class events recorded by STIX during its commissioning phase and the
validation of results is realized also exploiting the EUV information
provided by the Atmospheric Imaging Assembly within the Solar Dynamics
Observatory.
---------------------------------------------------------
Title: First results of the STIX hard X-ray telescope onboard
Solar Orbiter
Authors: Battaglia, Andrea Francesco; Saqri, Jonas; Dickson, Ewan;
Xiao, Hualin; Veronig, Astrid; Warmuth, Alexander; Battaglia, Marina;
Krucker, Säm
2021EGUGA..23.4390B Altcode:
With the launch and commissioning of Solar Orbiter, the
Spectrometer/Telescope for Imaging X-rays (STIX) is the latest hard
X-ray telescope to study solar flares over a large range of flare
sizes. STIX uses hard X-ray imaging spectroscopy in the range from 4 to
150 keV to diagnose the hottest temperature of solar flare plasma and
the related nonthermal accelerated electrons. The unique orbit away
from the Earth-Sun line in combination with the opportunity of joint
observations with other Solar Orbiter instruments, STIX will provide
new inputs into understanding the magnetic energy release and particle
acceleration in solar flares. Commissioning observations showed that
STIX is working as designed and therefore we report on the first solar
microflare observations recorded on June 2020, when the spacecraft
was at 0.52 AU from the Sun. STIX"s measurements are compared with
Earth-orbiting observatories, such as GOES and SDO/AIA, for which
we investigate and interpret the different temporal evolution. The
detected early peak of the STIX profiles relative to GOES is due either
by nonthermal X-ray emission of accelerated particles interacting
with the dense chromosphere or the higher sensitivity of STIX toward
hotter plasma.
---------------------------------------------------------
Title: Solar Energetic Electron Events Associated with Hard X-ray
Flares
Authors: Wang, Wen; Wang, Linghua; Krucker, Sam; Mason, Glenn M.;
Su, Yang; Bucik, Radoslav
2021EGUGA..2310734W Altcode:
We investigate 16 solar energetic electron (SEE) events measured
by WIND/3DP with a double power-law spectrum and the associated
western hard X-ray (HXR) flares measured by RHESSI with good count
statistics, from 2002 February to 2016 December. In all 16 cases,
the presence of an SEE power-law spectrum extending down to 65 keV
at 1 AU implies that the SEE source would be high in the corona, at
a heliocentric distance of >1.3 solar radii, while the footpoint
or footpoint-like emissions shown in HXR images suggest that the
observed HXRs are likely produced mainly by thick target bremsstrahlung
processes very low in the corona. We find that in 8 cases (the other
8 cases), the power-law spectral index of HXR-producing electrons,
estimated under the relativistic thick-target bremsstrahlung model,
is significantly larger than (similar to) the observed high-energy
spectral index of SEEs, with a positive correlation. In addition,
the estimated number of SEEs is only ∼10-4 - 10-2 of the estimated
number of HXRproducing electrons at energies above 30 keV, but also
with a positive correlation. These results suggest that in these
cases, SEEs are likely formed by upward-traveling electrons from an
acceleration source high in the corona, while their downward-traveling
counterparts may undergo a secondary acceleration before producing HXRs
via thick-target bremsstrahlung processes. In addition, the associated
3He=4He ratio is positively correlated with the observed high-energy
spectral index of SEEs, indicating a possible relation of the 3He ion
acceleration with high-energy SEEs
---------------------------------------------------------
Title: Radio Spectral Imaging of an M8.4 Eruptive Solar Flare:
Possible Evidence of a Termination Shock
Authors: Luo, Yingjie; Chen, Bin; Yu, Sijie; Bastian, T. S.; Krucker,
Säm
2021ApJ...911....4L Altcode: 2021arXiv210206259L
Solar flare termination shocks have been suggested as one of the
viable mechanisms for accelerating electrons and ions to high
energies. Observational evidence of such shocks, however, remains
rare. Using radio dynamic spectroscopic imaging of a long-duration
C1.9 flare obtained by the Karl G. Jansky Very Large Array (VLA),
Chen et al. suggested that a type of coherent radio bursts, referred
to as "stochastic spike bursts," were radio signatures of nonthermal
electrons interacting with myriad density fluctuations at the front
of a flare termination shock. Here we report another stochastic spike
burst event recorded during the extended energy release phase of a
long-duration M8.4-class eruptive flare on 2012 March 10. VLA radio
spectroscopic imaging of the spikes in 1.0-1.6 GHz shows that, similar
to the case of Chen et al., the burst centroids form an extended,
∼10″-long structure in the corona. By combining extreme-ultraviolet
imaging observations of the flare from two vantage points with hard
X-ray and ultraviolet observations of the flare ribbon brightenings,
we reconstruct the flare arcade in three dimensions. The results show
that the spike source is located at ∼60 Mm above the flare arcade,
where a diffuse supra-arcade fan and multitudes of plasma downflows are
present. Although the flare arcade and ribbons seen during the impulsive
phase do not allow us to clearly understand how the observed spike
source location is connected to the flare geometry, the cooling flare
arcade observed 2 hr later suggests that the spikes are located in the
above-the-loop-top region, where a termination shock presumably forms.
---------------------------------------------------------
Title: Desaturated EUV Flare Ribbons in an X-class Flare
Authors: Krucker, Säm; Torre, Gabriele; Schwartz, Richard A.
2021ApJ...909...43K Altcode:
This paper studies the energy dissipation of nonthermal electrons in
the chromospheric flare ribbons during the peak time of a Geostationary
Operational Environmental Satellite X-class flare (SOL2011-09-06)
using desaturated Solar Dynamics Observatory/Atmospheric Imaging
Assembly extreme-ultraviolet (EUV) narrow-band images. The temperature
distribution in emission measure, called the differential emission
measure (DEM), derived from the EUV fluxes from the flare ribbons
shows an increase in the emission measure up to a temperature
around 9 × 10<SUP>6</SUP> K, followed by a steep decline at higher
temperatures. In contrast, the flare loop reaches temperatures up to
27 × 10<SUP>6</SUP> K. This result is in agreement with previously
reported single-temperature measurements using soft X-ray filter
images, as well as DEM distributions reported for smaller flares
obtained from EUV line observations. The main difference between
small and large flares appears to be an increased emission measure
in the flare ribbons, while the ribbon peak temperature is similar
for all flares. This is different from the flare loop temperatures,
where the hottest temperatures occur in the largest flares. However,
the physically relevant quantity for energy dissipation, the energy
content of the heated plasma as a function of temperature, does not
need to peak at the same temperature as the DEM. The poorly constrained
source thickness in radial extent of the flare ribbons has a significant
impact on the shape of the differential thermal energy distribution. In
particular, if the highest temperatures occur over a wide radial extent
as "evaporating" plasma starts expending, the largest amount of energy
could potentially be hidden above the peak temperature of the DEM.
---------------------------------------------------------
Title: Energetic Electron Distribution of the Coronal Acceleration
Region: First Results from Joint Microwave and Hard X-Ray Imaging
Spectroscopy
Authors: Chen, Bin; Battaglia, Marina; Krucker, Säm; Reeves, Katharine
K.; Glesener, Lindsay
2021ApJ...908L..55C Altcode: 2021arXiv210205173C
Nonthermal sources located above bright flare arcades, referred to
as the "above-the-loop-top" sources, have been often suggested as the
primary electron acceleration site in major solar flares. The X8.2 limb
flare on 2017 September 10 features such an above-the-loop-top source,
which was observed in both microwaves and hard X-rays (HXRs) by the
Expanded Owens Valley Solar Array and the Reuven Ramaty High Energy
Solar Spectroscopic Imager, respectively. By combining the microwave
and HXR imaging spectroscopy observations with multifilter extreme
ultraviolet and soft X-ray imaging data, we derive the coronal magnetic
field and energetic electron distribution of the source over a broad
energy range from <10 keV up to ∼MeV during the early impulsive
phase of the flare. The source has a strong magnetic field of over
800 G. The best-fit electron distribution consists of a thermal "core"
from ∼25 MK plasma. A nonthermal power-law "tail" joins the thermal
core at ∼16 keV with a spectral index of ∼3.6, which breaks down
at above ∼160 keV to >6.0. Temporally resolved analysis suggests
that the electron distribution above the break energy rapidly hardens
with the spectral index decreasing from >20 to ∼6.0 within 20 s,
or less than ∼10 Alfvén crossing times in the source. These results
provide strong support for the above-the-loop-top source as the primary
site where an ongoing bulk acceleration of energetic electrons is
taking place very early in the flare energy release.
---------------------------------------------------------
Title: NuSTAR Observation of Energy Release in 11 Solar Microflares
Authors: Duncan, Jessie; Glesener, Lindsay; Grefenstette, Brian W.;
Vievering, Juliana; Hannah, Iain G.; Smith, David M.; Krucker, Säm;
White, Stephen M.; Hudson, Hugh
2021ApJ...908...29D Altcode: 2020arXiv201106651D
Solar flares are explosive releases of magnetic energy. Hard X-ray
(HXR) flare emission originates from both hot (millions of Kelvin)
plasma and nonthermal accelerated particles, giving insight into flare
energy release. The Nuclear Spectroscopic Telescope ARray (NuSTAR)
utilizes direct-focusing optics to attain much higher sensitivity
in the HXR range than that of previous indirect imagers. This paper
presents 11 NuSTAR microflares from two active regions (AR 12671 on
2017 August 21 and AR 12712 on 2018 May 29). The temporal, spatial, and
energetic properties of each are discussed in context with previously
published HXR brightenings. They are seen to display several "large
flare" properties, such as impulsive time profiles and earlier peak
times in higher-energy HXRs. For two events where the active region
background could be removed, microflare emission did not display
spatial complexity; differing NuSTAR energy ranges had equivalent
emission centroids. Finally, spectral fitting showed a high-energy
excess over a single thermal model in all events. This excess was
consistent with additional higher-temperature plasma volumes in 10/11
microflares and only with an accelerated particle distribution in the
last. Previous NuSTAR studies focused on one or a few microflares at a
time, making this the first to collectively examine a sizable number of
events. Additionally, this paper introduces an observed variation in
the NuSTAR gain unique to the extremely low livetime (<1%) regime
and establishes a correction method to be used in future NuSTAR solar
spectral analysis.
---------------------------------------------------------
Title: New Methods for Solar Hard X-ray Imaging Analysis with the
FOXSI Sounding Rocket Experiment
Authors: Vievering, J. T.; Glesener, L.; Athiray, P. S.;
Buitrago-Casas, J. C.; Musset, S.; Ryan, D.; Ishikawa, S. N.; Watanabe,
S.; Takahashi, T.; Duncan, J. M.; Christe, S.; Krucker, S.
2020AGUFMSH0480010V Altcode:
Spectroscopic imaging in the hard X-ray (HXR) regime is essential
for investigating the sites of energy release and transfer in solar
flares, as HXRs provide insight into the acceleration of electrons
and plasma heating. The Focusing Optics X-ray Solar Imager ( FOXSI )
sounding rocket experiment is the first solar-dedicated direct imager
for HXRs and offers unprecedented capabilities for high-energy imaging
spectroscopy of flares. Through use of grazing incidence optics,
FOXSI achieves greater sensitivity and imaging dynamic range than its
esteemed predecessor, the Reuven Ramaty High-Energy Solar Spectroscopic
Imager ( RHESSI ), which was limited by the high background inherent
to its indirect imaging technique. Additionally, FOXSI improves upon
the Nuclear Spectroscopic Telescope Array ( NuSTAR ) with an angular
resolution at scales relevant for studying flares and a higher detector
throughput for measuring solar sources. Here we demonstrate the power of
these enhanced capabilities through imaging analyses of two sub-A class
microflares observed by the second FOXSI sounding rocket ( FOXSI-2 )
on December 11, 2014. We apply a custom FOXSI image deconvolution
method using a maximum likelihood procedure to explore the changing
morphology of these small scale-events and compare the results to
features observed in contemporaneous data from the Solar Dynamics
Observatory Atmospheric Imaging Assembly ( SDO /AIA). Additionally,
we perform imaging spectroscopy to study the spatial complexity of
energy release in these microflares and assess whether they are more
similar in evolution to large flares or nanoflares.
---------------------------------------------------------
Title: NuSTAR Observation of Eleven Solar Microflares
Authors: Duncan, J. M.; Glesener, L.; Grefenstette, B.; Vievering,
J. T.; Hannah, I. G.; Smith, D. M.; Krucker, S.; White, S. M.; Hudson,
H. S.
2020AGUFMSH045..07D Altcode:
This work presents eleven microflares observed by the Nuclear
Spectroscopic Telescope ARray (NuSTAR), representing the first
time that a sizable number of these events have been examined
collectively. NuSTAR's direct focusing optics give it a dramatic
increase in sensitivity over indirect imagers in the hard X-ray
(HXR) range. HXR emission in solar flares originates from both hot
(millions of Kelvin) plasma and nonthermal accelerated particles,
both of which are diagnostic of flare energy release. Therefore,
NuSTAR solar observation campaigns can give unique insight into the
energetics of faint microflares, including those that were unobservable
with previous-generation HXR instruments. We discuss the temporal,
spatial, and energetic properties of all eleven microflares in context
with previously published HXR brightenings. They are seen to display
several `large-flare' properties, such as impulsive time profiles
and earlier peaktimes in higher energy HXRs. For two events where
active region background could be removed, microflare emission did
not display spatial complexity: differing NuSTAR energy ranges had
equivalent emission centroids. Finally, spectral fitting showed a high
energy excess over a single thermal model in all events. This excess
was found to most likely originate from additional higher-temperature
plasma volumes in 10/11 microflares, and from an accelerated particle
distribution in the last. Finally, we introduce an observed variation
in the NuSTAR gain unique to the extremely low-livetime (< 1%)
regime, and establish a correction method to be used in future NuSTAR
solar spectral analysis.
---------------------------------------------------------
Title: Updates on the Fundamentals of Impulsive Energy Release in
the Corona Explorer (FIERCE) mission concept
Authors: Shih, A. Y.; Glesener, L.; Krucker, S.; Guidoni, S. E.;
Christe, S.; Reeves, K.; Gburek, S.; Caspi, A.; Alaoui, M.; Allred,
J. C.; Battaglia, M.; Baumgartner, W.; Dennis, B. R.; Drake, J. F.;
Goetz, K.; Golub, L.; Hannah, I. G.; Hayes, L.; Holman, G.; Inglis,
A.; Ireland, J.; Kerr, G. S.; Klimchuk, J. A.; McKenzie, D. E.; Moore,
C. S.; Musset, S.; Reep, J. W.; Ryan, D.; Saint-Hilaire, P.; Savage,
S. L.; Schwartz, R.; Seaton, D. B.; Steslicki, M.; Woods, T. N.
2020AGUFMSH0480012S Altcode:
The Fundamentals of Impulsive Energy Release in the Corona Explorer
( FIERCE ) Medium-Class Explorer (MIDEX) mission concept addresses
the following science questions: <P />What are the physical origins
of space-weather events? <P />How are particles accelerated at the
Sun? <P />How is impulsively released energy transported throughout
the solar atmosphere? <P />How is the solar corona heated? <P />FIERCE
achieves its science objectives through co-optimized X-ray and extreme
ultraviolet (EUV) observations by the following instruments: <P />FOXSI,
a focusing hard X-ray spectroscopic imager that is able to capture the
full range of emission in flares and CMEs (e.g., faint coronal sources
near bright chromospheric sources) <P />THADIS, a high-resolution,
fast-cadence EUV imager that will not saturate for even intense flares
to follow dynamic changes in the configuration of plasma structures
<P />STC, a soft X-ray spectrometer that provides detailed thermal and
elemental composition diagnostics <P />If selected, FIERCE will launch
in 2025, near the peak of the next solar cycle, which is also well timed
with perihelia of Parker Solar Probe and Solar Orbiter . We describe the
status and latest updates of the mission concept since it was proposed
to NASA last year. We also highlight the anticipated science return
from co-observations with other observatories/instruments such as the
Expanded Owens Valley Solar Array (EOVSA) or the STIX instrument on
Solar Orbiter .
---------------------------------------------------------
Title: The Micro Solar Flare Apparatus (MiSolFA)
Authors: Hayes, L. A.; Christe, S.; Ryan, D.; Krucker, S.; Martinez
Oliveros, J. C.; Kontar, E.; Jeffrey, N.; Caspi, A.; Saint-Hilaire,
P.; Limousin, O.; Meuris, A.; Battaglia, M.; Casadei, D.
2020AGUFMSH056..08H Altcode:
During a solar flare the rapid release of magnetic energy drives
extremely efficient particle acceleration through a mechanism which
remains largely unknown. Hard X-ray observations are one of the
most direct signatures of flare accelerated energetic electrons
at and near the acceleration site, and X-ray spectra can provide
key diagnostics to the physical processes occurring in flares. In
particular, a measure of the electron angular distribution (the hard
X-ray directivity) is a prime diagnostic of the unknown acceleration
mechanism. However, to-date, observational constraints of directivity
have not been clear. Looking towards the next solar cycle, stereoscopic
X-ray observations of solar flares will make significant advances at
measuring the hard X-ray directivity. The Micro Solar Flare Apparatus
(MiSolFA) is a CubeSat Observatory that will observe solar flares at
the same time as Solar Orbiter/STIX using cross-calibrated flight-spare
detectors. During the rise of the next solar maximum, STIX on board
Solar Orbiter will perform X-ray observations of solar flares from
0.28 AU (at perihelion) and up to inclinations of ∼25 degrees at
heliospheric angles significantly different than the Earth. These two
instruments working together will be the first pair of cross-calibrated
X-ray spectrometers to observe solar flares from very different
points of view allowing us to measure the anisotropy of the flare
hard x-rays and finally confidently constrain the flare-accelerated
electron directivity in individual flares for the first time.
---------------------------------------------------------
Title: The Above-the-looptop Source of the 2017 September 10 Solar
Flare: Energetic Electron Distribution over a Broad Energy Range
Authors: Chen, B.; Battaglia, M.; Krucker, S.; Reeves, K.; Glesener, L.
2020AGUFMSH0430001C Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Pan-Spectrum Fitting Formula for Suprathermal Particles
Authors: Liu, Zixuan; Wang, Linghua; Wimmer-Schweingruber, Robert F.;
Krucker, Säm.; Mason, Glenn M.
2020JGRA..12528702L Altcode:
We propose a pan-spectrum fitting formula of
suprathermal particles, J=A×E<SUP>-β<SUB>1</SUB></SUP>[1 +
(E/E<SUB>0</SUB>)<SUP>α</SUP>]<SUP>β<SUB>1</SUB>-β<SUB>2</SUB>/α</SUP>,
where J is the particle flux (or intensity), E is the particle energy,
A is the amplitude coefficient, E<SUB>0</SUB> represents the spectral
transition energy, α (>0) describes the sharpness and width of
spectral transition around E<SUB>0</SUB>, and the power-law index
β<SUB>1</SUB> (β<SUB>2</SUB>) gives the spectral shape before
(after) the transition. This formula incorporates many commonly
used spectrum functions as special cases. When α goes to infinity
(zero), this spectral formulabecomes the classical double-power-law
(logarithmic-parabola) function. When both β<SUB>2</SUB> and
E<SUB>0</SUB> approach infinity and α is equal to 1, this formula
can be simplified to the Ellison-Ramaty function. Under some other
specific parameter conditions, this formula can be transformed to the
Kappa or Maxwellian distribution. Considering the uncertainties in both
particle intensity and energy, we improve the fitting method and fit
this pan-spectrum formula well to the representative energy spectra
of various suprathermal particle phenomena including SEPs (electrons,
protons, <SUP>3</SUP>He, and heavier ions), ESPs, bow-shocked electrons,
solar wind suprathermal electrons, anomalous cosmic rays, and hard
X-rays. Therefore, this pan-spectrum fitting formula would help us
comparatively examine the properties of energy spectrum of different
suprathermal particle phenomena typically with a single energy break.
---------------------------------------------------------
Title: The 2017 September 10 loop-prominence system: A polarization
and density analysis
Authors: Martinez Oliveros, J. C.; Guevara Gómez, J. C.;
Saint-Hilaire, P.; Krucker, S.
2020AGUFMSH046..05M Altcode:
Classical loop-prominence systems associated with powerful solar flares
have been observed since the 1930's. Since their initial observations,
loop prominence systems (LPS) were a topic of particular interest
for solar physicists as they seem to have a clear relationship with
solar flares. The time evolution of loop prominence systems is related
to the thermodynamic and radiative properties of the chromospheric
plasma. From observations it is clear that these systems do not grow
from lower to higher altitudes, instead their generation is the product
of higher loops getting more intense, while the lower decay. Here, we
report observations of white-light ejecta in the low corona after the
2017 September 10 flare, using data from the Helioseismic and Magnetic
Imager (HMI) of the Solar Dynamics Observatory. We interpret this as
a classical loop-prominence system, but find it to be brighter than
expected and possibly seen here in the continuum rather than line
emission. We studied the spatial and temporal relation between RHESSI
X-ray and the white-light emissions at altitudes, where the polarization
levels detected match well those expected from pure Thomson scattering
by the coronal material. We also studied the HMI spectroscopic data
to determine the most probable emission mechanism that can explain
the observation of the loop-prominence system.
---------------------------------------------------------
Title: GOES-class Estimation for Behind-the-limb Solar Flares Using
MESSENGER SAX
Authors: Lastufka, Erica; Krucker, Säm
2020ApJ...905..161L Altcode: 2020arXiv201210221L
Mercury mission MESSENGER's Solar Assembly for X-rays (SAX) observed
almost 700 solar flares between 2007 May 28 and 2013 August 19, as
cataloged by Dennis Brian et al. The SAX instrument, part of the X-ray
Spectrometer, operated at 1-10 keV, partially overlapping the energy
range of the GOES X-ray spectrometers. SAX provides viewing angles
different from the Earth-Sun line and can therefore be used as a GOES
proxy for partially or fully occulted flares as seen from Earth. For
flares with GOES classes above C2 seen on-disk for both instruments,
we found an empirical relationship between the soft X-ray (SXR)
fluxes measured by both SAX and GOES. Due to the different energy
response of the two SXR instruments, individual events can deviate
on average by about a factor of 2 from the empirical relationship,
implying that predictions of the GOES class of occulted flares from SAX
data are therefore accurate to within the same factor. The distinctive
GOES energy response in combination with the multithermal nature of
flares makes it difficult for any instrument, even other soft X-ray
spectrometers, to provide a GOES proxy more accurate than a factor of 2.
---------------------------------------------------------
Title: X-ray Microflare Observations by Solar Orbiter STIX
Authors: Krucker, S.
2020AGUFMSH038..06K Altcode:
The Spectrometer Telescope for Imaging X-rays (STIX) on Solar
Orbiter is a hard X-ray imaging spectrometer covering the energy
range from 4 to 150 keV. Through the bremsstrahlung process, STIX
provides diagnostics of the hottest (above ~8 MK) flare plasmas while
simultaneously quantifying the location, spectrum, and energy content
of flare-accelerated non-thermal electrons. This presentation will
highlight scientific results obtained during the STIX commissioning
phase in May and June 2020, when STIX observed a total of 63 solar
microflares at GOES class level A and B. We will present statistical
results of our initial set of observations and compare them to
previously obtained X-ray microflare statistics by RHESSI and NuSTAR.
---------------------------------------------------------
Title: Solar Energetic Electron Events Associated with Hard X-ray
Flares
Authors: Wang, W.; Wang, L.; Krucker, S.; Mason, G. M.; Bucik, R.;
Tian, H.; He, J.; Su, Y.; Bale, S.
2020AGUFMSH045..05W Altcode:
We presented a comparison survey of 16 solar energetic electron (SEE)
events measured by WIND/3DP with associated hard X-ray (HXR) flares
measured by RHESSI with good count statistics, from 2002 February to
2016 December. Among the 16 good cases, all show a double-power-law
shape with a downward break at ~ 68keV in the electron peak flux versus
energy spectrum at ~ 5-200keV; All cases show a clear single-power-law
in the HXR peak flux versus energy spectrum, and a single-power-law in
the flux versus energy spectrum of HXR-producing electrons derived via
bremsstrahlung mechanisms. For SEEs, their acceleration site at the
Sun should occur high (at a heliocentric altitude of ≧1.3 R<SUB>s
</SUB>(the average solar radius)), to remain a power-law spectrum
extending down to ~ 5 keV during their anti-sunward escape into the
interplanetary medium (IPM). For 14 out of 16 cases, the spectral
index above the energy break of SEEs observed at 1AU basically agreed
with the derived spectral index of HXR-producing electrons. However,
the SEE spectral index below the energy break is all smaller than the
HXR-producing electron spectral index. These results suggest that SEEs
and HXR-producing electrons share a common origin and an additional
acceleration could exist between the SEE source and the flare region. In
addition, for cases with <SUP>3</SUP>He/<SUP>4</SUP>He>0.01, the
observed <SUP>3</SUP>He/<SUP>4</SUP>He shows a correlation with the
spectral index above the energy break of SEEs, indicating a possible
relation of the <SUP>3</SUP>He-rich ion acceleration at high corona
with SEEs.
---------------------------------------------------------
Title: The FOXSI-4 Sounding Rocket: High Resolution Focused X-ray
Observations of the Sun
Authors: Glesener, L.; Buitrago-Casas, J. C.; Musset, S.; Vievering,
J. T.; Athiray, P. S.; Baumgartner, W.; Bongiorno, S.; Champey, P. R.;
Christe, S.; Courtade, S.; Duncan, J. M.; Ishikawa, S. N.; Krucker,
S.; Martinez Oliveros, J. C.; Mitsuishi, I.; Narukage, N.; Ryan, D.;
Takahashi, T.; Watanabe, S.; Winebarger, A. R.
2020AGUFMSH0480011G Altcode:
It has been firmly demonstrated that direct-focusing instruments
can transform the way high-energy X-rays from astronomical objects,
including the Sun, are measured. The NuSTAR spacecraft has increased the
sensitivity to faint astrophysical sources by 100 times as compared with
previous, indirect, imagers. The first three flights of the Focusing
Optics X-ray Solar Imager (FOXSI) sounding rocket established the
usefulness and feasibility of a similar method optimized for the Sun,
and showed that in addition to greater sensitivity, a vastly improved
dynamic range can be obtained in this way. This technology stands
ready to revolutionize understanding of solar flares by elucidating
particle acceleration sites in the corona, studying how electrons
propagate and deposit their energies, and how accelerated particles
escape into interplanetary space. While the fundamental building
blocks of solar hard X-ray (HXR) focusing are in place and ready for a
spacecraft mission, concurrent development is required to prepare for
the next generation of high-energy solar explorers, which will require
higher rate capability and higher angular resolution to investigate
finer-scale structure and to better complement instruments at other
wavelengths. <P />FOXSI-4 features technological advances that enable
high angular resolution as well as measurement of bright sources. In
the first category, we will develop high-precision mirror production
methods and finely pixelated Si CMOS sensors, and will demonstrate
substrip/subpixel resolution in fine-pitch CdTe sensors. Secondly,
we will demonstrate rate capability of these sensors sufficient for
flare measurement, and will develop novel pixelated attenuators that
optimize energy coverage even at high rates. <P />The experiment
will demonstrate these technologies in NASA's first-ever solar flare
campaign, flying in tandem with the Hi-C FLARE rocket. The campaign will
position multiple rocket experiments awaiting an opportunistic signal
and will launch the experiments for near-simultaneous observation of
the flare.This campaign will allow for direct collaboration with the
Parker Solar Probe (PSP) during one of its perihelia.
---------------------------------------------------------
Title: Limits on the X-ray emission of the quiet Sun from the FOXSI
sounding rockets
Authors: Buitrago-Casas, J. C.; Glesener, L.; Christe, S.; Krucker,
S.; Vievering, J. T.; Athiray, P. S.; Musset, S.; Ryan, D.; Ishikawa,
S. N.; Narukage, N.; Bongiorno, S.; Furukawa, K.; Ramsey, B.; Davis,
L.; Courtade, S.; Dalton, G.; Turin, P.; Turin, Z.; Takahashi, T.;
Watanabe, S.; Mitsuishi, I.; Hagino, K.; Duncan, J. M.
2020AGUFMSH0430002B Altcode:
Nanoflares are a potential solution for the coronal heating problem. In
the quiet Sun, nanoflares should exhibit hard X-ray (HXR) signatures
manifested to an observer via either i) a single HXR kernel or
ii) HXRs from many flares spread all over the Sun. In both cases,
detecting nanoflare HXRs requires an instrument with superior dynamic
range and sensitivity in the ~4 to 15 keV energy range over previous
solar dedicated HXR telescopes, like RHESSI. The Focusing Optics
X-ray Solar Imager (FOXSI) fulfills these requirements. FOXSI has
successfully flown on three sounding rocket campaigns; the last two
(FOXSI-2 and -3) included the observation of quiet areas of the solar
disk. For FOXSI-3, several techniques were tested and developed to
minimize a background effect unique to hard X-ray focusing optics,
frequently referred to as ghost rays. <P />In this presentation, we
provide, for the first time, a FOXSI sounding rocket assessment of the
HXR flux from the quiet Sun. To fully characterize the sensitivity
of FOXSI, we assessed ghost rays generated by sources outside of
the field of view via a ray-tracing algorithm. This is particularly
important for observations from FOXSI-2 when techniques to minimize
ghost rays were not implemented yet. Using the ray tracing tool, we
identify areas free of ghost rays that contribute to the background
and therefore have maximum sensitivity to quiet Sun HXR emission. We
use a bayesian approach to provide upper thresholds of quiet Sun HXR
emission and probability distributions for the expected flux when a
quiet-Sun HXR source is assumed to exist. We compare this result with
prior constraints such as that made by RHESSI (Hannah et al. 2010).
---------------------------------------------------------
Title: Solar Orbiter: connecting remote sensing and in situ
measurements
Authors: Horbury, T. S.; Auchere, F.; Antonucci, E.; Berghmans, D.;
Bruno, R.; Carlsson, M.; del Toro Iniesta, J. C.; Fludra, A.; Harra,
L.; Hassler, D.; Heinzel, P.; Howard, R. A.; Krucker, S.; Livi, S. A.;
Long, D.; Louarn, P.; Maksimovic, M.; Mueller, D.; Owen, C. J.; Peter,
H.; Rochus, P. L.; Rodriguez-Pacheco, J.; Romoli, M.; Schühle, U.;
Solanki, S. K.; Teriaca, L.; Wimmer-Schweingruber, R. F.; Zouganelis,
Y.; Laker, R.
2020AGUFMSH038..10H Altcode:
A key science goal of the Solar Orbiter mission is to make connections
between phenomena on the Sun and their manifestations in interplanetary
space. To that end, the spacecraft carries a carefully tailored
payload of six remote sensing instruments and four making in situ
measurements. During June 2020, while the spacecraft was around 0.5
AU from the Sun, the remote sensing instruments operated for several
days. While this was primarily an engineering activity, the resulting
observations provided outstanding measurements and represent the ideal
first opportunity to investigate the potential for making connections
between the remote sensing and in situ payloads on Solar Orbiter. <P
/>We present a preliminary analysis of the available remote sensing and
in situ observations, showing how connections can be made, and discuss
the potential for further, more precise mapping to be performed as
the mission progresses.
---------------------------------------------------------
Title: Use of a ray-tracing simulation to characterize ghost rays
in the FOXSI rocket experiment
Authors: Buitrago-Casas, J. C.; Christe, S.; Glesener, L.; Krucker,
S.; Ramsey, B.; Bongiorno, S.; Kilaru, K.; Athiray, P. S.; Narukage,
N.; Ishikawa, S.; Dalton, G.; Courtade, S.; Musset, S.; Vievering,
J.; Ryan, D.; Bale, S.
2020JInst..15P1032B Altcode: 2020arXiv201007049B
Imaging X-rays by direct focusing offers greater sensitivity and
a higher dynamic range compared to techniques based on indirect
imaging. The Focusing Optics X-ray Solar Imager (FOXSI) is a
sounding rocket payload that uses seven sets of nested Wolter-I
figured mirrors to observe the Sun in hard X-rays through direct
focusing. Characterizing the performance of these optics is critical
to optimize their performance and to understand their resulting
data. In this paper, we present a ray-tracing simulation we created and
developed to study Wolter-I X-ray mirrors. We validated the accuracy
of the ray-tracing simulation by modeling the FOXSI rocket optics. We
found satisfactory agreements between the simulation predictions
and laboratory data measured on the optics. We used the ray-tracing
simulation to characterize a background pattern of singly reflected
rays (i.e., ghost rays) generated by photons at certain incident
angles reflecting on only one of a two-segment Wolter-I figure and
still reaching the focal plane. We used the results of the ray-tracing
simulation to understand, and to formulate a set of strategies that
can be used to mitigate, the impact of ghost rays on the FOXSI optical
modules. These strategies include the optimization of aperture plates
placed at the entrance and exit of the smallest Wolter-I mirror used
in FOXSI, a honeycomb type collimator, and a wedge absorber placed
at the telescope aperture. The ray-tracing simulation proved to be
a reliable set of tools to study Wolter-I X-ray optics. It can be
used in many applications, including astrophysics, material sciences,
and medical imaging.
---------------------------------------------------------
Title: Coordination within the remote sensing payload on the Solar
Orbiter mission
Authors: Auchère, F.; Andretta, V.; Antonucci, E.; Bach, N.;
Battaglia, M.; Bemporad, A.; Berghmans, D.; Buchlin, E.; Caminade,
S.; Carlsson, M.; Carlyle, J.; Cerullo, J. J.; Chamberlin, P. C.;
Colaninno, R. C.; Davila, J. M.; De Groof, A.; Etesi, L.; Fahmy,
S.; Fineschi, S.; Fludra, A.; Gilbert, H. R.; Giunta, A.; Grundy,
T.; Haberreiter, M.; Harra, L. K.; Hassler, D. M.; Hirzberger, J.;
Howard, R. A.; Hurford, G.; Kleint, L.; Kolleck, M.; Krucker, S.;
Lagg, A.; Landini, F.; Long, D. M.; Lefort, J.; Lodiot, S.; Mampaey,
B.; Maloney, S.; Marliani, F.; Martinez-Pillet, V.; McMullin, D. R.;
Müller, D.; Nicolini, G.; Orozco Suarez, D.; Pacros, A.; Pancrazzi,
M.; Parenti, S.; Peter, H.; Philippon, A.; Plunkett, S.; Rich, N.;
Rochus, P.; Rouillard, A.; Romoli, M.; Sanchez, L.; Schühle, U.;
Sidher, S.; Solanki, S. K.; Spadaro, D.; St Cyr, O. C.; Straus, T.;
Tanco, I.; Teriaca, L.; Thompson, W. T.; del Toro Iniesta, J. C.;
Verbeeck, C.; Vourlidas, A.; Watson, C.; Wiegelmann, T.; Williams,
D.; Woch, J.; Zhukov, A. N.; Zouganelis, I.
2020A&A...642A...6A Altcode:
Context. To meet the scientific objectives of the mission, the Solar
Orbiter spacecraft carries a suite of in-situ (IS) and remote sensing
(RS) instruments designed for joint operations with inter-instrument
communication capabilities. Indeed, previous missions have shown that
the Sun (imaged by the RS instruments) and the heliosphere (mainly
sampled by the IS instruments) should be considered as an integrated
system rather than separate entities. Many of the advances expected
from Solar Orbiter rely on this synergistic approach between IS and
RS measurements. <BR /> Aims: Many aspects of hardware development,
integration, testing, and operations are common to two or more
RS instruments. In this paper, we describe the coordination effort
initiated from the early mission phases by the Remote Sensing Working
Group. We review the scientific goals and challenges, and give an
overview of the technical solutions devised to successfully operate
these instruments together. <BR /> Methods: A major constraint for the
RS instruments is the limited telemetry (TM) bandwidth of the Solar
Orbiter deep-space mission compared to missions in Earth orbit. Hence,
many of the strategies developed to maximise the scientific return from
these instruments revolve around the optimisation of TM usage, relying
for example on onboard autonomy for data processing, compression,
and selection for downlink. The planning process itself has been
optimised to alleviate the dynamic nature of the targets, and an
inter-instrument communication scheme has been implemented which can
be used to autonomously alter the observing modes. We also outline the
plans for in-flight cross-calibration, which will be essential to the
joint data reduction and analysis. <BR /> Results: The RS instrument
package on Solar Orbiter will carry out comprehensive measurements
from the solar interior to the inner heliosphere. Thanks to the close
coordination between the instrument teams and the European Space
Agency, several challenges specific to the RS suite were identified
and addressed in a timely manner.
---------------------------------------------------------
Title: Models and data analysis tools for the Solar Orbiter mission
Authors: Rouillard, A. P.; Pinto, R. F.; Vourlidas, A.; De Groof, A.;
Thompson, W. T.; Bemporad, A.; Dolei, S.; Indurain, M.; Buchlin, E.;
Sasso, C.; Spadaro, D.; Dalmasse, K.; Hirzberger, J.; Zouganelis, I.;
Strugarek, A.; Brun, A. S.; Alexandre, M.; Berghmans, D.; Raouafi,
N. E.; Wiegelmann, T.; Pagano, P.; Arge, C. N.; Nieves-Chinchilla,
T.; Lavarra, M.; Poirier, N.; Amari, T.; Aran, A.; Andretta, V.;
Antonucci, E.; Anastasiadis, A.; Auchère, F.; Bellot Rubio, L.;
Nicula, B.; Bonnin, X.; Bouchemit, M.; Budnik, E.; Caminade, S.;
Cecconi, B.; Carlyle, J.; Cernuda, I.; Davila, J. M.; Etesi, L.;
Espinosa Lara, F.; Fedorov, A.; Fineschi, S.; Fludra, A.; Génot,
V.; Georgoulis, M. K.; Gilbert, H. R.; Giunta, A.; Gomez-Herrero, R.;
Guest, S.; Haberreiter, M.; Hassler, D.; Henney, C. J.; Howard, R. A.;
Horbury, T. S.; Janvier, M.; Jones, S. I.; Kozarev, K.; Kraaikamp,
E.; Kouloumvakos, A.; Krucker, S.; Lagg, A.; Linker, J.; Lavraud,
B.; Louarn, P.; Maksimovic, M.; Maloney, S.; Mann, G.; Masson, A.;
Müller, D.; Önel, H.; Osuna, P.; Orozco Suarez, D.; Owen, C. J.;
Papaioannou, A.; Pérez-Suárez, D.; Rodriguez-Pacheco, J.; Parenti,
S.; Pariat, E.; Peter, H.; Plunkett, S.; Pomoell, J.; Raines, J. M.;
Riethmüller, T. L.; Rich, N.; Rodriguez, L.; Romoli, M.; Sanchez,
L.; Solanki, S. K.; St Cyr, O. C.; Straus, T.; Susino, R.; Teriaca,
L.; del Toro Iniesta, J. C.; Ventura, R.; Verbeeck, C.; Vilmer, N.;
Warmuth, A.; Walsh, A. P.; Watson, C.; Williams, D.; Wu, Y.; Zhukov,
A. N.
2020A&A...642A...2R Altcode:
Context. The Solar Orbiter spacecraft will be equipped with a wide
range of remote-sensing (RS) and in situ (IS) instruments to record
novel and unprecedented measurements of the solar atmosphere and
the inner heliosphere. To take full advantage of these new datasets,
tools and techniques must be developed to ease multi-instrument and
multi-spacecraft studies. In particular the currently inaccessible
low solar corona below two solar radii can only be observed
remotely. Furthermore techniques must be used to retrieve coronal
plasma properties in time and in three dimensional (3D) space. Solar
Orbiter will run complex observation campaigns that provide interesting
opportunities to maximise the likelihood of linking IS data to their
source region near the Sun. Several RS instruments can be directed
to specific targets situated on the solar disk just days before
data acquisition. To compare IS and RS, data we must improve our
understanding of how heliospheric probes magnetically connect to the
solar disk. <BR /> Aims: The aim of the present paper is to briefly
review how the current modelling of the Sun and its atmosphere
can support Solar Orbiter science. We describe the results of a
community-led effort by European Space Agency's Modelling and Data
Analysis Working Group (MADAWG) to develop different models, tools,
and techniques deemed necessary to test different theories for the
physical processes that may occur in the solar plasma. The focus here
is on the large scales and little is described with regards to kinetic
processes. To exploit future IS and RS data fully, many techniques have
been adapted to model the evolving 3D solar magneto-plasma from the
solar interior to the solar wind. A particular focus in the paper is
placed on techniques that can estimate how Solar Orbiter will connect
magnetically through the complex coronal magnetic fields to various
photospheric and coronal features in support of spacecraft operations
and future scientific studies. <BR /> Methods: Recent missions such as
STEREO, provided great opportunities for RS, IS, and multi-spacecraft
studies. We summarise the achievements and highlight the challenges
faced during these investigations, many of which motivated the Solar
Orbiter mission. We present the new tools and techniques developed
by the MADAWG to support the science operations and the analysis of
the data from the many instruments on Solar Orbiter. <BR /> Results:
This article reviews current modelling and tool developments that ease
the comparison of model results with RS and IS data made available
by current and upcoming missions. It also describes the modelling
strategy to support the science operations and subsequent exploitation
of Solar Orbiter data in order to maximise the scientific output
of the mission. <BR /> Conclusions: The on-going community effort
presented in this paper has provided new models and tools necessary
to support mission operations as well as the science exploitation of
the Solar Orbiter data. The tools and techniques will no doubt evolve
significantly as we refine our procedure and methodology during the
first year of operations of this highly promising mission.
---------------------------------------------------------
Title: The Solar Orbiter Radio and Plasma Waves (RPW) instrument
Authors: Maksimovic, M.; Bale, S. D.; Chust, T.; Khotyaintsev, Y.;
Krasnoselskikh, V.; Kretzschmar, M.; Plettemeier, D.; Rucker, H. O.;
Souček, J.; Steller, M.; Štverák, Š.; Trávníček, P.; Vaivads,
A.; Chaintreuil, S.; Dekkali, M.; Alexandrova, O.; Astier, P. -A.;
Barbary, G.; Bérard, D.; Bonnin, X.; Boughedada, K.; Cecconi,
B.; Chapron, F.; Chariet, M.; Collin, C.; de Conchy, Y.; Dias, D.;
Guéguen, L.; Lamy, L.; Leray, V.; Lion, S.; Malac-Allain, L. R.;
Matteini, L.; Nguyen, Q. N.; Pantellini, F.; Parisot, J.; Plasson,
P.; Thijs, S.; Vecchio, A.; Fratter, I.; Bellouard, E.; Lorfèvre,
E.; Danto, P.; Julien, S.; Guilhem, E.; Fiachetti, C.; Sanisidro,
J.; Laffaye, C.; Gonzalez, F.; Pontet, B.; Quéruel, N.; Jannet,
G.; Fergeau, P.; Brochot, J. -Y.; Cassam-Chenai, G.; Dudok de Wit,
T.; Timofeeva, M.; Vincent, T.; Agrapart, C.; Delory, G. T.; Turin,
P.; Jeandet, A.; Leroy, P.; Pellion, J. -C.; Bouzid, V.; Katra, B.;
Piberne, R.; Recart, W.; Santolík, O.; Kolmašová, I.; Krupař,
V.; Krupařová, O.; Píša, D.; Uhlíř, L.; Lán, R.; Baše, J.;
Ahlèn, L.; André, M.; Bylander, L.; Cripps, V.; Cully, C.; Eriksson,
A.; Jansson, S. -E.; Johansson, E. P. G.; Karlsson, T.; Puccio, W.;
Břínek, J.; Öttacher, H.; Panchenko, M.; Berthomier, M.; Goetz,
K.; Hellinger, P.; Horbury, T. S.; Issautier, K.; Kontar, E.; Krucker,
S.; Le Contel, O.; Louarn, P.; Martinović, M.; Owen, C. J.; Retino,
A.; Rodríguez-Pacheco, J.; Sahraoui, F.; Wimmer-Schweingruber, R. F.;
Zaslavsky, A.; Zouganelis, I.
2020A&A...642A..12M Altcode:
The Radio and Plasma Waves (RPW) instrument on the ESA Solar Orbiter
mission is described in this paper. This instrument is designed
to measure in-situ magnetic and electric fields and waves from the
continuous to a few hundreds of kHz. RPW will also observe solar radio
emissions up to 16 MHz. The RPW instrument is of primary importance
to the Solar Orbiter mission and science requirements since it is
essential to answer three of the four mission overarching science
objectives. In addition RPW will exchange on-board data with the other
in-situ instruments in order to process algorithms for interplanetary
shocks and type III langmuir waves detections.
---------------------------------------------------------
Title: The Energetic Particle Detector. Energetic particle instrument
suite for the Solar Orbiter mission
Authors: Rodríguez-Pacheco, J.; Wimmer-Schweingruber, R. F.; Mason,
G. M.; Ho, G. C.; Sánchez-Prieto, S.; Prieto, M.; Martín, C.;
Seifert, H.; Andrews, G. B.; Kulkarni, S. R.; Panitzsch, L.; Boden,
S.; Böttcher, S. I.; Cernuda, I.; Elftmann, R.; Espinosa Lara, F.;
Gómez-Herrero, R.; Terasa, C.; Almena, J.; Begley, S.; Böhm, E.;
Blanco, J. J.; Boogaerts, W.; Carrasco, A.; Castillo, R.; da Silva
Fariña, A.; de Manuel González, V.; Drews, C.; Dupont, A. R.;
Eldrum, S.; Gordillo, C.; Gutiérrez, O.; Haggerty, D. K.; Hayes,
J. R.; Heber, B.; Hill, M. E.; Jüngling, M.; Kerem, S.; Knierim,
V.; Köhler, J.; Kolbe, S.; Kulemzin, A.; Lario, D.; Lees, W. J.;
Liang, S.; Martínez Hellín, A.; Meziat, D.; Montalvo, A.; Nelson,
K. S.; Parra, P.; Paspirgilis, R.; Ravanbakhsh, A.; Richards, M.;
Rodríguez-Polo, O.; Russu, A.; Sánchez, I.; Schlemm, C. E.; Schuster,
B.; Seimetz, L.; Steinhagen, J.; Tammen, J.; Tyagi, K.; Varela, T.;
Yedla, M.; Yu, J.; Agueda, N.; Aran, A.; Horbury, T. S.; Klecker, B.;
Klein, K. -L.; Kontar, E.; Krucker, S.; Maksimovic, M.; Malandraki,
O.; Owen, C. J.; Pacheco, D.; Sanahuja, B.; Vainio, R.; Connell,
J. J.; Dalla, S.; Dröge, W.; Gevin, O.; Gopalswamy, N.; Kartavykh,
Y. Y.; Kudela, K.; Limousin, O.; Makela, P.; Mann, G.; Önel, H.;
Posner, A.; Ryan, J. M.; Soucek, J.; Hofmeister, S.; Vilmer, N.;
Walsh, A. P.; Wang, L.; Wiedenbeck, M. E.; Wirth, K.; Zong, Q.
2020A&A...642A...7R Altcode:
After decades of observations of solar energetic particles from
space-based observatories, relevant questions on particle injection,
transport, and acceleration remain open. To address these scientific
topics, accurate measurements of the particle properties in the inner
heliosphere are needed. In this paper we describe the Energetic Particle
Detector (EPD), an instrument suite that is part of the scientific
payload aboard the Solar Orbiter mission. Solar Orbiter will approach
the Sun as close as 0.28 au and will provide extra-ecliptic measurements
beyond ∼30° heliographic latitude during the later stages of the
mission. The EPD will measure electrons, protons, and heavy ions with
high temporal resolution over a wide energy range, from suprathermal
energies up to several hundreds of megaelectronvolts/nucleons. For
this purpose, EPD is composed of four units: the SupraThermal
Electrons and Protons (STEP), the Electron Proton Telescope (EPT),
the Suprathermal Ion Spectrograph (SIS), and the High-Energy Telescope
(HET) plus the Instrument Control Unit that serves as power and data
interface with the spacecraft. The low-energy population of electrons
and ions will be covered by STEP and EPT, while the high-energy
range will be measured by HET. Elemental and isotopic ion composition
measurements will be performed by SIS and HET, allowing full particle
identification from a few kiloelectronvolts up to several hundreds
of megaelectronvolts/nucleons. Angular information will be provided
by the separate look directions from different sensor heads, on the
ecliptic plane along the Parker spiral magnetic field both forward
and backwards, and out of the ecliptic plane observing both northern
and southern hemispheres. The unparalleled observations of EPD will
provide key insights into long-open and crucial questions about the
processes that govern energetic particles in the inner heliosphere.
---------------------------------------------------------
Title: The Spectrometer/Telescope for Imaging X-rays (STIX)
Authors: Krucker, Säm; Hurford, G. J.; Grimm, O.; Kögl, S.;
Gröbelbauer, H. -P.; Etesi, L.; Casadei, D.; Csillaghy, A.; Benz,
A. O.; Arnold, N. G.; Molendini, F.; Orleanski, P.; Schori, D.; Xiao,
H.; Kuhar, M.; Hochmuth, N.; Felix, S.; Schramka, F.; Marcin, S.;
Kobler, S.; Iseli, L.; Dreier, M.; Wiehl, H. J.; Kleint, L.; Battaglia,
M.; Lastufka, E.; Sathiapal, H.; Lapadula, K.; Bednarzik, M.; Birrer,
G.; Stutz, St.; Wild, Ch.; Marone, F.; Skup, K. R.; Cichocki, A.; Ber,
K.; Rutkowski, K.; Bujwan, W.; Juchnikowski, G.; Winkler, M.; Darmetko,
M.; Michalska, M.; Seweryn, K.; Białek, A.; Osica, P.; Sylwester, J.;
Kowalinski, M.; Ścisłowski, D.; Siarkowski, M.; Stęślicki, M.;
Mrozek, T.; Podgórski, P.; Meuris, A.; Limousin, O.; Gevin, O.; Le
Mer, I.; Brun, S.; Strugarek, A.; Vilmer, N.; Musset, S.; Maksimović,
M.; Fárník, F.; Kozáček, Z.; Kašparová, J.; Mann, G.; Önel,
H.; Warmuth, A.; Rendtel, J.; Anderson, J.; Bauer, S.; Dionies, F.;
Paschke, J.; Plüschke, D.; Woche, M.; Schuller, F.; Veronig, A. M.;
Dickson, E. C. M.; Gallagher, P. T.; Maloney, S. A.; Bloomfield, D. S.;
Piana, M.; Massone, A. M.; Benvenuto, F.; Massa, P.; Schwartz, R. A.;
Dennis, B. R.; van Beek, H. F.; Rodríguez-Pacheco, J.; Lin, R. P.
2020A&A...642A..15K Altcode:
<BR /> Aims: The Spectrometer Telescope for Imaging X-rays (STIX)
on Solar Orbiter is a hard X-ray imaging spectrometer, which
covers the energy range from 4 to 150 keV. STIX observes hard X-ray
bremsstrahlung emissions from solar flares and therefore provides
diagnostics of the hottest (⪆10 MK) flare plasma while quantifying
the location, spectrum, and energy content of flare-accelerated
nonthermal electrons. <BR /> Methods: To accomplish this, STIX applies
an indirect bigrid Fourier imaging technique using a set of tungsten
grids (at pitches from 0.038 to 1 mm) in front of 32 coarsely pixelated
CdTe detectors to provide information on angular scales from 7 to 180
arcsec with 1 keV energy resolution (at 6 keV). The imaging concept of
STIX has intrinsically low telemetry and it is therefore well-suited
to the limited resources available to the Solar Orbiter payload. To
further reduce the downlinked data volume, STIX data are binned on
board into 32 selectable energy bins and dynamically-adjusted time
bins with a typical duration of 1 s during flares. <BR /> Results:
Through hard X-ray diagnostics, STIX provides critical information
for understanding the acceleration of electrons at the Sun and their
transport into interplanetary space and for determining the magnetic
connection of Solar Orbiter back to the Sun. In this way, STIX serves
to link Solar Orbiter's remote and in-situ measurements.
---------------------------------------------------------
Title: Coordination of the in situ payload of Solar Orbiter
Authors: Walsh, A. P.; Horbury, T. S.; Maksimovic, M.; Owen, C. J.;
Rodríguez-Pacheco, J.; Wimmer-Schweingruber, R. F.; Zouganelis,
I.; Anekallu, C.; Bonnin, X.; Bruno, R.; Carrasco Blázquez, I.;
Cernuda, I.; Chust, T.; De Groof, A.; Espinosa Lara, F.; Fazakerley,
A. N.; Gilbert, H. R.; Gómez-Herrero, R.; Ho, G. C.; Krucker,
S.; Lepri, S. T.; Lewis, G. R.; Livi, S.; Louarn, P.; Müller, D.;
Nieves-Chinchilla, T.; O'Brien, H.; Osuna, P.; Plasson, P.; Raines,
J. M.; Rouillard, A. P.; St Cyr, O. C.; Sánchez, L.; Soucek, J.;
Varsani, A.; Verscharen, D.; Watson, C. J.; Watson, G.; Williams, D. R.
2020A&A...642A...5W Altcode:
Solar Orbiter's in situ coordination working group met frequently
during the development of the mission with the goal of ensuring
that its in situ payload has the necessary level of coordination to
maximise science return. Here we present the results of that work,
namely how the design of each of the in situ instruments (EPD, MAG,
RPW, SWA) was guided by the need for coordination, the importance of
time synchronisation, and how science operations will be conducted
in a coordinated way. We discuss the mechanisms by which instrument
sampling schemes are aligned such that complementary measurements
will be made simultaneously by different instruments, and how burst
modes are scheduled to allow a maximum overlap of burst intervals
between the four instruments (telemetry constraints mean different
instruments can spend different amounts of time in burst mode). We
also explain how onboard autonomy, inter-instrument communication,
and selective data downlink will be used to maximise the number of
transient events that will be studied using high-resolution modes of
all the instruments. Finally, we briefly address coordination between
Solar Orbiter's in situ payload and other missions.
---------------------------------------------------------
Title: The Solar Orbiter Science Activity Plan. Translating solar
and heliospheric physics questions into action
Authors: Zouganelis, I.; De Groof, A.; Walsh, A. P.; Williams, D. R.;
Müller, D.; St Cyr, O. C.; Auchère, F.; Berghmans, D.; Fludra,
A.; Horbury, T. S.; Howard, R. A.; Krucker, S.; Maksimovic, M.;
Owen, C. J.; Rodríguez-Pacheco, J.; Romoli, M.; Solanki, S. K.;
Watson, C.; Sanchez, L.; Lefort, J.; Osuna, P.; Gilbert, H. R.;
Nieves-Chinchilla, T.; Abbo, L.; Alexandrova, O.; Anastasiadis, A.;
Andretta, V.; Antonucci, E.; Appourchaux, T.; Aran, A.; Arge, C. N.;
Aulanier, G.; Baker, D.; Bale, S. D.; Battaglia, M.; Bellot Rubio,
L.; Bemporad, A.; Berthomier, M.; Bocchialini, K.; Bonnin, X.; Brun,
A. S.; Bruno, R.; Buchlin, E.; Büchner, J.; Bucik, R.; Carcaboso,
F.; Carr, R.; Carrasco-Blázquez, I.; Cecconi, B.; Cernuda Cangas, I.;
Chen, C. H. K.; Chitta, L. P.; Chust, T.; Dalmasse, K.; D'Amicis, R.;
Da Deppo, V.; De Marco, R.; Dolei, S.; Dolla, L.; Dudok de Wit, T.;
van Driel-Gesztelyi, L.; Eastwood, J. P.; Espinosa Lara, F.; Etesi,
L.; Fedorov, A.; Félix-Redondo, F.; Fineschi, S.; Fleck, B.; Fontaine,
D.; Fox, N. J.; Gandorfer, A.; Génot, V.; Georgoulis, M. K.; Gissot,
S.; Giunta, A.; Gizon, L.; Gómez-Herrero, R.; Gontikakis, C.; Graham,
G.; Green, L.; Grundy, T.; Haberreiter, M.; Harra, L. K.; Hassler,
D. M.; Hirzberger, J.; Ho, G. C.; Hurford, G.; Innes, D.; Issautier,
K.; James, A. W.; Janitzek, N.; Janvier, M.; Jeffrey, N.; Jenkins,
J.; Khotyaintsev, Y.; Klein, K. -L.; Kontar, E. P.; Kontogiannis,
I.; Krafft, C.; Krasnoselskikh, V.; Kretzschmar, M.; Labrosse, N.;
Lagg, A.; Landini, F.; Lavraud, B.; Leon, I.; Lepri, S. T.; Lewis,
G. R.; Liewer, P.; Linker, J.; Livi, S.; Long, D. M.; Louarn, P.;
Malandraki, O.; Maloney, S.; Martinez-Pillet, V.; Martinovic, M.;
Masson, A.; Matthews, S.; Matteini, L.; Meyer-Vernet, N.; Moraitis,
K.; Morton, R. J.; Musset, S.; Nicolaou, G.; Nindos, A.; O'Brien,
H.; Orozco Suarez, D.; Owens, M.; Pancrazzi, M.; Papaioannou, A.;
Parenti, S.; Pariat, E.; Patsourakos, S.; Perrone, D.; Peter, H.;
Pinto, R. F.; Plainaki, C.; Plettemeier, D.; Plunkett, S. P.; Raines,
J. M.; Raouafi, N.; Reid, H.; Retino, A.; Rezeau, L.; Rochus, P.;
Rodriguez, L.; Rodriguez-Garcia, L.; Roth, M.; Rouillard, A. P.;
Sahraoui, F.; Sasso, C.; Schou, J.; Schühle, U.; Sorriso-Valvo, L.;
Soucek, J.; Spadaro, D.; Stangalini, M.; Stansby, D.; Steller, M.;
Strugarek, A.; Štverák, Š.; Susino, R.; Telloni, D.; Terasa, C.;
Teriaca, L.; Toledo-Redondo, S.; del Toro Iniesta, J. C.; Tsiropoula,
G.; Tsounis, A.; Tziotziou, K.; Valentini, F.; Vaivads, A.; Vecchio,
A.; Velli, M.; Verbeeck, C.; Verdini, A.; Verscharen, D.; Vilmer, N.;
Vourlidas, A.; Wicks, R.; Wimmer-Schweingruber, R. F.; Wiegelmann,
T.; Young, P. R.; Zhukov, A. N.
2020A&A...642A...3Z Altcode: 2020arXiv200910772Z
Solar Orbiter is the first space mission observing the solar plasma
both in situ and remotely, from a close distance, in and out of the
ecliptic. The ultimate goal is to understand how the Sun produces
and controls the heliosphere, filling the Solar System and driving
the planetary environments. With six remote-sensing and four in-situ
instrument suites, the coordination and planning of the operations are
essential to address the following four top-level science questions:
(1) What drives the solar wind and where does the coronal magnetic field
originate?; (2) How do solar transients drive heliospheric variability?;
(3) How do solar eruptions produce energetic particle radiation that
fills the heliosphere?; (4) How does the solar dynamo work and drive
connections between the Sun and the heliosphere? Maximising the
mission's science return requires considering the characteristics
of each orbit, including the relative position of the spacecraft
to Earth (affecting downlink rates), trajectory events (such
as gravitational assist manoeuvres), and the phase of the solar
activity cycle. Furthermore, since each orbit's science telemetry
will be downloaded over the course of the following orbit, science
operations must be planned at mission level, rather than at the level
of individual orbits. It is important to explore the way in which those
science questions are translated into an actual plan of observations
that fits into the mission, thus ensuring that no opportunities are
missed. First, the overarching goals are broken down into specific,
answerable questions along with the required observations and the
so-called Science Activity Plan (SAP) is developed to achieve this. The
SAP groups objectives that require similar observations into Solar
Orbiter Observing Plans, resulting in a strategic, top-level view of
the optimal opportunities for science observations during the mission
lifetime. This allows for all four mission goals to be addressed. In
this paper, we introduce Solar Orbiter's SAP through a series of
examples and the strategy being followed.
---------------------------------------------------------
Title: Understanding the origins of the heliosphere: integrating
observations and measurements from Parker Solar Probe, Solar Orbiter,
and other space- and ground-based observatories
Authors: Velli, M.; Harra, L. K.; Vourlidas, A.; Schwadron,
N.; Panasenco, O.; Liewer, P. C.; Müller, D.; Zouganelis, I.;
St Cyr, O. C.; Gilbert, H.; Nieves-Chinchilla, T.; Auchère, F.;
Berghmans, D.; Fludra, A.; Horbury, T. S.; Howard, R. A.; Krucker,
S.; Maksimovic, M.; Owen, C. J.; Rodríguez-Pacheco, J.; Romoli,
M.; Solanki, S. K.; Wimmer-Schweingruber, R. F.; Bale, S.; Kasper,
J.; McComas, D. J.; Raouafi, N.; Martinez-Pillet, V.; Walsh, A. P.;
De Groof, A.; Williams, D.
2020A&A...642A...4V Altcode:
Context. The launch of Parker Solar Probe (PSP) in 2018, followed
by Solar Orbiter (SO) in February 2020, has opened a new window in
the exploration of solar magnetic activity and the origin of the
heliosphere. These missions, together with other space observatories
dedicated to solar observations, such as the Solar Dynamics Observatory,
Hinode, IRIS, STEREO, and SOHO, with complementary in situ observations
from WIND and ACE, and ground based multi-wavelength observations
including the DKIST observatory that has just seen first light,
promise to revolutionize our understanding of the solar atmosphere
and of solar activity, from the generation and emergence of the Sun's
magnetic field to the creation of the solar wind and the acceleration of
solar energetic particles. <BR /> Aims: Here we describe the scientific
objectives of the PSP and SO missions, and highlight the potential for
discovery arising from synergistic observations. Here we put particular
emphasis on how the combined remote sensing and in situ observations of
SO, that bracket the outer coronal and inner heliospheric observations
by PSP, may provide a reconstruction of the solar wind and magnetic
field expansion from the Sun out to beyond the orbit of Mercury in the
first phases of the mission. In the later, out-of-ecliptic portions of
the SO mission, the solar surface magnetic field measurements from SO
and the multi-point white-light observations from both PSP and SO will
shed light on the dynamic, intermittent solar wind escaping from helmet
streamers, pseudo-streamers, and the confined coronal plasma, and on
solar energetic particle transport. <BR /> Methods: Joint measurements
during PSP-SO alignments, and magnetic connections along the same
flux tube complemented by alignments with Earth, dual PSP-Earth,
and SO-Earth, as well as with STEREO-A, SOHO, and BepiColumbo will
allow a better understanding of the in situ evolution of solar-wind
plasma flows and the full three-dimensional distribution of the
solar wind from a purely observational point of view. Spectroscopic
observations of the corona, and optical and radio observations,
combined with direct in situ observations of the accelerating solar
wind will provide a new foundation for understanding the fundamental
physical processes leading to the energy transformations from solar
photospheric flows and magnetic fields into the hot coronal plasma
and magnetic fields and finally into the bulk kinetic energy of the
solar wind and solar energetic particles. <BR /> Results: We discuss
the initial PSP observations, which already provide a compelling
rationale for new measurement campaigns by SO, along with ground-
and space-based assets within the synergistic context described above.
---------------------------------------------------------
Title: The Solar Orbiter mission. Science overview
Authors: Müller, D.; St. Cyr, O. C.; Zouganelis, I.; Gilbert, H. R.;
Marsden, R.; Nieves-Chinchilla, T.; Antonucci, E.; Auchère, F.;
Berghmans, D.; Horbury, T. S.; Howard, R. A.; Krucker, S.; Maksimovic,
M.; Owen, C. J.; Rochus, P.; Rodriguez-Pacheco, J.; Romoli, M.;
Solanki, S. K.; Bruno, R.; Carlsson, M.; Fludra, A.; Harra, L.;
Hassler, D. M.; Livi, S.; Louarn, P.; Peter, H.; Schühle, U.;
Teriaca, L.; del Toro Iniesta, J. C.; Wimmer-Schweingruber, R. F.;
Marsch, E.; Velli, M.; De Groof, A.; Walsh, A.; Williams, D.
2020A&A...642A...1M Altcode: 2020arXiv200900861M
<BR /> Aims: Solar Orbiter, the first mission of ESA's Cosmic Vision
2015-2025 programme and a mission of international collaboration between
ESA and NASA, will explore the Sun and heliosphere from close up and
out of the ecliptic plane. It was launched on 10 February 2020 04:03
UTC from Cape Canaveral and aims to address key questions of solar and
heliospheric physics pertaining to how the Sun creates and controls
the Heliosphere, and why solar activity changes with time. To answer
these, the mission carries six remote-sensing instruments to observe
the Sun and the solar corona, and four in-situ instruments to measure
the solar wind, energetic particles, and electromagnetic fields. In
this paper, we describe the science objectives of the mission, and how
these will be addressed by the joint observations of the instruments
onboard. <BR /> Methods: The paper first summarises the mission-level
science objectives, followed by an overview of the spacecraft and
payload. We report the observables and performance figures of each
instrument, as well as the trajectory design. This is followed by a
summary of the science operations concept. The paper concludes with a
more detailed description of the science objectives. <BR /> Results:
Solar Orbiter will combine in-situ measurements in the heliosphere
with high-resolution remote-sensing observations of the Sun to address
fundamental questions of solar and heliospheric physics. The performance
of the Solar Orbiter payload meets the requirements derived from the
mission's science objectives. Its science return will be augmented
further by coordinated observations with other space missions and
ground-based observatories. <P />ARRAY(0x207ce98)
---------------------------------------------------------
Title: NuSTAR X-ray Observations of a Minuscule Microflare from a
Repeatedly Microflaring Active Region
Authors: Cooper, K.; Hannah, I.; Grefenstette, B.; Glesener, L.;
Krucker, S.; Hudson, H.
2020SPD....5121101C Altcode:
Highly frequent, small flares are thought to contribute to heating
the Sun's atmosphere, particularly in active regions. This impulsive
energy release would heat plasma to at least 10 MK and accelerate
electrons, producing weak thermal and non-thermal signatures that
could be observed by a very sensitive X-ray telescope. No such solar
telescope exists (yet) so we use Nuclear Spectroscopic Telescope Array
(NuSTAR), an astrophysical X-ray telescope, with focusing optics imaging
spectroscopy providing a unique sensitivity for observing the Sun
above 2.5 keV. We present an overview of several microflares from the
recently emerged active region AR12721 on 2018 September 9-10. Using
NuSTAR's imaging spectroscopy and the Solar Dynamics Observatory's
Atmospheric Imaging Assembly's (SDO/AIA) EUV imaging capabilities we
can analyse the temporal, spatial, and spectral evolution of these
microflares, determining the energy release and associated heating of
the solar atmosphere. All microflares from AR12721 were below GOES A1
equivalent level and the heated coronal loops were all visible in an
Fe XVIII proxy channel derived from SDO/AIA channels. In particular,
we present our recently published analysis of the weakest microflare
from AR12721 (Cooper et al. 2020 ApJL 893 2) finding it to be one
of the smallest active region X-ray flares on record, with material
heated up to 7MK and a thermal energy of just 1e26 erg.
---------------------------------------------------------
Title: NuSTAR Observation of Quiet Sun X-ray Bright Points
Authors: Paterson, S.; Hannah, I.; Grefenstette, B.; Hudson, H.;
Krucker, S.; Glesener, L.
2020SPD....5121013P Altcode:
NuSTAR is a focusing hard X-ray telescope designed for observing
astrophysical sources, but it is also capable of being pointed at
the Sun. NuSTAR's much greater sensitivity compared to RHESSI and the
current minimum of the solar activity cycle provide a unique opportunity
to investigate quiet Sun features that it has previously not been
possible to with X-ray imaging spectroscopy. We present analysis from
NuSTAR quiet Sun full disk mosaics from April 2019. With the absence
of very bright sources, these mosaics show very small and faint
X-ray bright points. We investigate the contribution of these small
events to heating the solar atmosphere. The X-ray spectra of these
features have been fit, allowing for estimates of their temperatures
and emission measures to be obtained. The temperatures were found to
lie in the range 1.2-3.5 MK. The temperatures and emission measures
predicted by the spectral fits can be tested through comparison to
SDO/AIA observations. Using the characteristics of the bright points
found from their spectra, heating processes occurring in these features
will be investigated.
---------------------------------------------------------
Title: The Micro Solar Flare Apparatus (MiSolFA) instrument concept
Authors: Lastufka, Erica; Casadei, Diego; Hurford, Gordon; Kuhar,
Matej; Torre, Gabriele; Krucker, Säm
2020AdSpR..66...10L Altcode:
The Micro Solar-Flare Apparatus (MiSolFA) is a compact X-ray imaging
spectrometer designed for a small 6U micro-satellite. As a relatively
inexpensive yet capable Earth-orbiting instrument, MiSolFA is designed
to image the high-energy regions of solar flares from a different
perspective than that of Solar Orbiter's STIX, operating from a highly
elliptical heliocentric orbit. Two instruments working together in
this way would provide a 3-dimensional view of X-ray emitting regions
and can bypass the dynamic range limitation preventing simultaneous
coronal and chromospheric imaging. Stereoscopic X-ray observations
would also contain valuable information about the anisotropy of the
flare-accelerated electron distribution. To perform these types
of observations, MiSolFA must be capable of imaging sources with
energies between 10 and 100 keV, with 10 arcsec angular resolution. <P
/>MiSolFA's Imager will be the most compact X-ray imaging spectrometer
in space. Scaling down the volume by a factor of ten from previous
instrument designs requires special considerations. Here we present the
design principles of the MiSolFA X-ray optics, discuss the necessary
compromises, and evaluate the performance of the Engineering Model.
---------------------------------------------------------
Title: The STIX Aspect System (SAS): The Optical Aspect System of
the Spectrometer/Telescope for Imaging X-Rays (STIX) on Solar Orbiter
Authors: Warmuth, A.; Önel, H.; Mann, G.; Rendtel, J.; Strassmeier,
K. G.; Denker, C.; Hurford, G. J.; Krucker, S.; Anderson, J.;
Bauer, S. -M.; Bittner, W.; Dionies, F.; Paschke, J.; Plüschke,
D.; Sablowski, D. P.; Schuller, F.; Senthamizh Pavai, V.; Woche, M.;
Casadei, D.; Kögl, S.; Arnold, N. G.; Gröbelbauer, H. -P.; Schori,
D.; Wiehl, H. J.; Csillaghy, A.; Grimm, O.; Orleanski, P.; Skup,
K. R.; Bujwan, W.; Rutkowski, K.; Ber, K.
2020SoPh..295...90W Altcode:
The Spectrometer/Telescope for Imaging X-rays (STIX) is a remote
sensing instrument on Solar Orbiter that observes the hard X-ray
bremsstrahlung emission of solar flares. This paper describes the
STIX Aspect System (SAS), a subunit that measures the pointing of
STIX relative to the Sun with a precision of ±4<SUP>″</SUP>, which
is required to accurately localize the reconstructed X-ray images on
the Sun. The operating principle of the SAS is based on an optical
lens that images the Sun onto a plate that is perforated by small
apertures arranged in a cross-shaped configuration of four radial
arms. The light passing through the apertures of each arm is detected
by a photodiode. Variations of spacecraft pointing and of distance
from the Sun cause the solar image to move over different apertures,
leading to a modulation of the measured lightcurves. These signals are
used by ground analysis to calculate the locations of the solar limb,
and hence the pointing of the telescope.
---------------------------------------------------------
Title: Measurement of magnetic field and relativistic electrons
along a solar flare current sheet
Authors: Chen, B.; Shen, C.; Gary, D.; Reeves, K.; Fleishman, G.;
Yu, S.; Guo, F.; Krucker, S.; Lin, J.; Nita, G.; Kong, X.
2020AAS...23611202C Altcode:
In the standard model of solar flares, a large-scale reconnection
current sheet is postulated as the central engine for powering the flare
energy release and accelerating particles. However, where and how the
energy release and particle acceleration occur remain unclear due to
the lack of measurements for the magnetic properties of the current
sheet. Here we report the first measurement of spatially-resolved
magnetic field and flare-accelerated relativistic electrons along a
current-sheet feature in a solar flare. The measured magnetic field
profile shows a local maximum where the reconnecting field lines
of opposite polarities closely approach each other, known as the
reconnection X point. The measurements also reveal a local minimum near
the bottom of the current sheet above the flare loop-top, referred to
as a "magnetic bottle". This spatial structure agrees with theoretical
predictions and numerical modeling results. A strong reconnection
electric field of ~4000 V/m is inferred near the X point. This location,
however, shows a local depletion of microwave-emitting relativistic
electrons. In contrast, these electrons concentrate at or near the
magnetic bottle structure, where more than 99% of them reside at each
instant. Our observations suggest crucial new input to the current
picture of high energy electron acceleration.
---------------------------------------------------------
Title: Microwave and Hard X-Ray Flare Observations by NoRH/NoRP and
RHESSI: Peak-flux Correlations
Authors: Krucker, Säm; Masuda, Satoshi; White, Stephen M.
2020ApJ...894..158K Altcode:
This paper presents initial results from a statistical study of solar
microwave and hard X-ray flares jointly observed over the past two
solar cycles by the Nobeyama Radio Polarimeters, the Nobeyama Radio
Heliograph, and the Reuven Ramaty High Energy Solar Spectroscopic
Imager. As has been previously demonstrated, the microwave (17 GHz and
34 GHz) peak flux shows a linear correlation with the nonthermal hard
X-ray bremsstrahlung peak emission seen above 50 keV. The correlation
holds for the entire rise phase of each individual burst, while the
decay phases tend to show more extended emission at microwaves than
is generally attributed to particle trapping. While the correlation
is highly significant (coefficient of 0.92) and holds over more than
four orders of magnitude, individual flares can be above or below
the fitted line by an average factor of about 2. By restricting the
flare selection to source morphologies with the radio emission from
the top of the flare loop, the correlation tightens significantly,
with a correlation coefficient increasing to 0.99 and the scatter
reduced to a factor of 1.3. These findings corroborate the assumption
that gyrosynchrotron microwave and hard X-ray bremsstrahlung emissions
are produced by the same flare-accelerated electron population. The
extent of the linear correlation over four orders of magnitude suggests
that magnetic field strengths within nonthermal 17 GHz sources are
surprisingly similar over a wide range of flare sizes.
---------------------------------------------------------
Title: The Energy Spectrum of Solar Energetic Electron Events
Authors: Wang, Linghua; Liu, Zixuan; Fu, Haobo; Krucker, Sam
2020EGUGA..22.1944W Altcode:
Solar energetic electron events (SEEs) are one of the most common
particle acceleration phenomena occurring at the Sun, and their
energy spectrum likely reflects the crucial information on the
acceleration. Here we present a statistical survey of the energy
spectrum of 160 SEEs measured by Wind/3DP with a clear velocity
dispersion at energies of ~1-200 keV from January 1995 through
December 2016, utilizing a general spectrum formula proposed by
Liu et al. (2000). We find that among these 160 SEEs, 144 (90%)
have a power-law (or power-law-like) spectrum bending down at high
energies, including 108 (67.5%) double-power-law events, 24 (15%)
Ellison-Ramaty-like events and 12 (7.5%) log-parabola events, while
16 (10%) have a power-law spectrum extending to high energies. The
average power-law spectral index β1 is 2.1±0.4 for double-power-law
events, 1.7±0.8 for Ellison-Ramaty-like events, and 2.8±0.11 for
single-power-law events. For the 108 double-power-law events, the
spectral break energy E0 ranges from 2 keV to 165 keV, with an average
of 71±79 keV, while the average spectral index β2 at energies above
E0is 4.4±2.3. E0 shows a positive correlation with the electron peak
flux at energies above ~40 keV, while β1 has a negative correlation
with the electron peak flux at energies above ~15 keV.
---------------------------------------------------------
Title: NuSTAR Observation of a Minuscule Microflare in a Solar
Active Region
Authors: Cooper, Kristopher; Hannah, Iain G.; Grefenstette, Brian W.;
Glesener, Lindsay; Krucker, Säm; Hudson, Hugh S.; White, Stephen M.;
Smith, David M.
2020ApJ...893L..40C Altcode: 2020arXiv200411176C
We present X-ray imaging spectroscopy of one of the weakest active
region (AR) microflares ever studied. The microflare occurred
at ∼11:04 UT on 2018 September 9 and we studied it using the
Nuclear Spectroscopic Telescope ARray (NuSTAR) and the Solar Dynamic
Observatory's Atmospheric Imaging Assembly (SDO/AIA). The microflare
is observed clearly in 2.5-7 keV with NuSTAR and in Fe XVIII emission
derived from the hotter component of the 94 Å SDO/AIA channel. We
estimate the event to be three orders of magnitude lower than a GOES
A class microflare with an energy of 1.1 × 10<SUP>26</SUP> erg. It
reaches temperatures of 6.7 MK with an emission measure of 8.0 ×
10<SUP>43</SUP> cm<SUP>-3</SUP>. Non-thermal emission is not detected
but we instead determine upper limits to such emission. We present the
lowest thermal energy estimate for an AR microflare in literature, which
is at the lower limits of what is still considered an X-ray microflare.
---------------------------------------------------------
Title: Accelerated Electrons Observed Down to <7 keV in a NuSTAR
Solar Microflare
Authors: Glesener, Lindsay; Krucker, Säm; Duncan, Jessie; Hannah,
Iain G.; Grefenstette, Brian W.; Chen, Bin; Smith, David M.; White,
Stephen M.; Hudson, Hugh
2020ApJ...891L..34G Altcode: 2020arXiv200312864G
We report the detection of emission from a nonthermal electron
distribution in a small solar microflare (GOES class A5.7) observed
by the Nuclear Spectroscopic Telescope Array, with supporting
observation by the Reuven Ramaty High Energy Solar Spectroscopic Imager
(RHESSI). The flaring plasma is well accounted for by a thick-target
model of accelerated electrons collisionally thermalizing within the
loop, akin to the "coronal thick-target" behavior occasionally observed
in larger flares. This is the first positive detection of nonthermal
hard X-rays from the Sun using a direct imager (as opposed to indirectly
imaging instruments). The accelerated electron distribution has a
spectral index of 6.3 ± 0.7, extends down to at least 6.5 keV, and
deposits energy at a rate of ∼2 × 10<SUP>27</SUP> erg s<SUP>-1</SUP>,
heating the flare loop to at least 10 MK. The existence of dominant
nonthermal emission in X-rays down to <5 keV means that RHESSI
emission is almost entirely nonthermal, contrary to what is usually
assumed in RHESSI spectroscopy. The ratio of nonthermal to thermal
energies is similar to that of large flares, in contrast to what has
been found in previous studies of small RHESSI flares. We suggest
that a coronal thick target may be a common property of many small
microflares based on the average electron energy and collisional mean
free path. Future observations of this kind will enable understanding
of how flare particle acceleration changes across energy scales,
and will aid the push toward the observational regime of nanoflares,
which are a possible source of significant coronal heating.
---------------------------------------------------------
Title: FOXSI-2 Solar Microflares. I. Multi-instrument Differential
Emission Measure Analysis and Thermal Energies
Authors: Athiray, P. S.; Vievering, Juliana; Glesener, Lindsay;
Ishikawa, Shin-nosuke; Narukage, Noriyuki; Buitrago-Casas, Juan
Camilo; Musset, Sophie; Inglis, Andrew; Christe, Steven; Krucker,
Säm; Ryan, Daniel
2020ApJ...891...78A Altcode: 2020arXiv200204200A
In this paper we present the differential emission measures
(DEMs) of two sub-A class microflares observed in hard X-rays
(HXRs) by the FOXSI-2 sounding rocket experiment, on 2014 December
11. The second Focusing Optics X-ray Solar Imager (FOXSI) flight
was coordinated with instruments X-ray Telescope (Hinode/XRT) and
Solar Dynamics Observatory/Atmospheric Imaging Assembly (AIA), which
provided observations in soft X-rays and Extreme Ultraviolet. This
unique data set offers an unprecedented temperature coverage,
useful for characterizing the plasma temperature distribution
of microflares. By combining data from FOXSI-2, XRT, and AIA, we
determined a well-constrained DEM for the microflares. The resulting
DEMs peak around 3 MK and extend beyond 10 MK. The emission measures
determined from FOXSI-2 were lower than 10<SUP>26</SUP> cm<SUP>-5</SUP>
for temperatures higher than 5 MK; faint emission in this range is best
measured in HXRs. The coordinated FOXSI-2 observations produce one of
the few definitive measurements of the distribution and the amount
of plasma above 5 MK in microflares. We utilize the multi-thermal
DEMs to calculate the amount of thermal energy released during both
the microflares as ∼5.0 × 10<SUP>28</SUP> erg for Microflare 1
and ∼1.6 × 10<SUP>28</SUP> erg for Microflare 2. We also show the
multi-thermal DEMs provide more comprehensive thermal energy estimates
than isothermal approximation, which systematically underestimates
the amount of thermal energy released.
---------------------------------------------------------
Title: Focus on the Extremes: Harnessing the Power of Hard X-ray
Focusing Optics to Probe Events Ranging from Solar Microflares to
Stellar Superflares
Authors: Vievering, J.; Glesener, L.; Grefenstette, B.; Smith, D.;
Panchapakesan, S.; Buitrago-Casas, J.; Musset, S.; Ryan, D.; Inglis,
A.; Christe, S.; Krucker, S.
2020AAS...23544203V Altcode:
Sensitive measurements of solar and stellar flares in the hard X-ray
regime are necessary for investigating energy release and transfer
during flaring events, as hard X-rays provide insight into the
acceleration of electrons and emission from hot plasmas. This research
harnesses the powerful capabilities of two instruments that use focusing
optics for hard X-rays, the Focusing Optics X-ray Solar Imager (FOXSI),
flown on three sounding rocket campaigns, and the Nuclear Spectroscopic
Telescope Array (NuSTAR). With the heightened sensitivity of these
instruments, it is finally possible to probe faint events in hard
X-rays that have previously been elusive, ranging from small-scale solar
events to bright X-ray flares on distant stars. In this presentation,
we explore the nature of energy release for flaring events of vastly
different magnitudes, including solar microflares observed by FOXSI-2
and superflares on young stellar objects (YSOs) observed by NuSTAR,
and ask whether these events are linked by the common thread of the
standard flare model. Additionally, we investigate the complexity of
these solar microflares and the impact of ionizing radiation from YSO
flares on protoplanetary disks. In exploring these uncharted regimes,
this work probes some of the most intriguing mysteries of the stars,
from coronal heating to the formation of planetary systems.
---------------------------------------------------------
Title: Measurement of magnetic field and relativistic electrons
along a solar flare current sheet
Authors: Chen, Bin; Shen, Chengcai; Gary, Dale E.; Reeves, Katharine
K.; Fleishman, Gregory D.; Yu, Sijie; Guo, Fan; Krucker, Säm; Lin,
Jun; Nita, Gelu M.; Kong, Xiangliang
2020NatAs...4.1140C Altcode: 2020arXiv200512757C; 2020NatAs.tmp..150C
In the standard model of solar flares, a large-scale reconnection
current sheet is postulated to be the central engine for
powering the flare energy release<SUP>1-3</SUP> and accelerating
particles<SUP>4-6</SUP>. However, where and how the energy release
and particle acceleration occur remain unclear owing to the lack of
measurements of the magnetic properties of the current sheet. Here we
report the measurement of the spatially resolved magnetic field and
flare-accelerated relativistic electrons along a current-sheet feature
in a solar flare. The measured magnetic field profile shows a local
maximum where the reconnecting field lines of opposite polarities
closely approach each other, known as the reconnection X point. The
measurements also reveal a local minimum near the bottom of the current
sheet above the flare loop-top, referred to as a `magnetic bottle'. This
spatial structure agrees with theoretical predictions<SUP>1,7</SUP>
and numerical modelling results. A strong reconnection electric field
of about 4,000 V m<SUP>-1</SUP> is inferred near the X point. This
location, however, shows a local depletion of microwave-emitting
relativistic electrons. These electrons instead concentrate at or near
the magnetic bottle structure, where more than 99% of them reside at
each instant. Our observations suggest that the loop-top magnetic
bottle is probably the primary site for accelerating and confining
the relativistic electrons.
---------------------------------------------------------
Title: NuSTAR observations of the quietest Sun
Authors: Hannah, I. G.; Cooper, K.; Grefenstette, B.; Glesener, L.;
Krucker, S.; Smith, D. M.; Hudson, H. S.; White, S. M.; Kuhar, M.
2019AGUFMSH41F3335H Altcode:
Observing X-rays (above a few keV) from the Sun provides a
direct insight into energy release (heating and/or particle
acceleration) in the solar atmosphere. Targeting the faintest
X-ray emission allows the study of the smallest flares and
eruption, and crucially their contribution to heating the solar
atmosphere. NuSTAR is an astrophysics telescope that uses directly
focusing X-rays optics to detect weak X-rays from the Sun. We have
observed the Sun many times since the start of solar pointings
in Sep 2014 through to our latest observations in 2019. See <A
href="http://ianan.github.io/nsovr/">http://ianan.github.io/nsovr/</A>
for an overview. During the current solar minimum, when the Sun is
devoid of active regions and presenting the very quietest levels of
activity, NuSTAR has targeted the Sun several times. We have detected
X-rays from a variety of sources: large diffuse sources, steady compact
sources, brief flares/brightenings and small eruptions. The NuSTAR
X-ray images of these weak sources are related to features seen at
other wavelengths, such as in softer X-rays with Hinode/XRT and EUV
with SDO/AIA. Crucially, NuSTAR's imaging spectroscopy allows us to
obtain and fit the X-ray spectrum from these small events determining
their thermal (and potentially non-thermal) properties. We will present
some of the latest solar observations with NuSTAR as we go through
the current solar minimum.
---------------------------------------------------------
Title: FOXSI-4: Instrument Upgrades for a Proposed Fourth Focusing
Optics X-Ray Solar Imager Sounding Rocket Experiment
Authors: Vievering, J. T.; Glesener, L.; Buitrago-Casas, J. C.;
Panchapakesan, S. A.; Musset, S.; Duncan, J. M.; Narukage, N.; Ryan,
D.; Inglis, A. R.; Takahashi, T.; Watanabe, S.; Christe, S.; Krucker,
S.; Turin, P.; Ramsey, B.
2019AGUFMSH31C3315V Altcode:
Observations of the Sun in hard X-rays can provide insight into many
solar phenomena, including the mechanisms behind energy release and
transport in flares. The indirect imaging methods used by RHESSI ,
the previous state-of-the-art solar hard X-ray instrument, however,
were fundamentally limited in sensitivity and imaging dynamic range . By
instead using the direct imaging technique of focusing hard X-rays, the
structure and evolution of faint coronal sources, including microflares,
active regions, and flare acceleration sites, can be investigated in
greater depth. FOXSI ( Focusing Optics X-ray Solar Imager ), a hard
X-ray instrument flown on three sounding rocket campaigns to date,
seeks to achieve these improved capabilities by using focusing optics
for solar observations in the 4-20 keV range. A fourth FOXSI sounding
rocket experiment, FOXSI-4, has been proposed for launch in 2023,
and the planned instrument upgrades are presented here. One main
focus of FOXSI-4 will be to improve the angular resolution for solar
hard X-ray instruments through the use of high-resolution optics,
with the goal of reaching a half power diameter (HPD) of ∼5". Higher
resolution will allow for individual footpoints and acceleration sites
to be distinguished during a flare, which would benefit studies of the
chromospheric response and provide constraints on flare acceleration
mechanisms. FOXSI-4 will ideally be part of a flare campaign, in which
multiple solar sounding rockets would be launched one after another
to demonstrate the strength of new solar technologies when utilized
in conjunction for flare observations.
---------------------------------------------------------
Title: Solar flare hard X-ray and gamma-ray imaging
spectro-polarimetry with GRIPS and SAPPHIRE/SHARPIE
Authors: Saint-Hilaire, P.; Jeffrey, N. L. S.; Martinez Oliveros,
J. C.; Shih, A. Y.; Zoglauer, A.; Caspi, A.; Lichtmacher, P.; Boggs,
S.; Hurford, G. J.; Krucker, S.; Sample, J. G.; Tremsin, A.
2019AGUFMSH31C3310S Altcode:
To date, flare hard X-ray (HXR) and gamma-ray polarimetry is a poorly
exploited field of Solar Physics. <P />The Gamma-Ray Imager/Polarimeter
for Solar flares (GRIPS) has already flown once on a high-altitude
balloon payload from Antarctica in January 2016. Using a rotating
monogrid eight meters in front of spatially-differentiated germanium
strip detectors (3D-GeDs), GRIPS images the sun from ~40 keV to ~10
MeV, with a 12.5" resolution and ~1 degree FOV. It can also provide
polarization information above ~150 keV. <P />The SolAr Polarimeter
for Hard x-Rays (SAPPHIRE) is a spatially-integrated polarimeter
module with a CubeSat form factor currently being proposed. It aims to
extract polarization information from solar flare-generated HXRs (~5-100
keV). The SHARPIE (Solar HArd x-ray Polarimer/Imager Experiment) concept
is an imager using several SAPPHIRE modules behind RHESSI-like bigrid
subcollimators. <P />We shall review these instruments or instrument
concepts and their capabilities and the science questions that they
address, with an emphasis on the polarization measurements.
---------------------------------------------------------
Title: Hardware upgrades and science outcomes from the latest flights
of the FOXSI rocket
Authors: Buitrago-Casas, J. C.; Glesener, L.; Courtade, S.; Vievering,
J. T.; Athiray, P. S.; Musset, S.; Ryan, D.; Dalton, G.; Ishikawa,
S. N.; Narukage, N.; Bongiorno, S.; Furukawa, K.; Davis, L.; Turin,
P.; Turin, Z.; Takahashi, T.; Watanabe, S.; Krucker, S.; Christe,
S.; Ramsey, B.
2019AGUFMSH31C3316B Altcode:
FOXSI (which stands for the Focusing Optics X-ray Solar Imager)
is a sounding rocket payload that has completed three successful
flights supported by the Low-Cost Access to Space (LCAS) program of
NASA. FOXSI stands out for being the first telescope optimized to use
direct focusing to perform imaging spectroscopy of solar hard X-rays
(HXRs). We present the latest instrument upgrades incorporated into
the rocket payload for its third flight. We highlight the way these
upgrades substantially improved the telescope performance and we
present the observations obtained. Particularly, we describe the
strategies implemented to reduce stray-light (ghost-ray) background
and the inclusion of a soft X-ray CMOS sensor as well as HXR CdTe
strip detectors to widen the spectral range of the telescope. <P
/>We finalize with an overview of the science results of the FOXSI
flights. We present a differential emission measure (DEM) and thermal
analyses of microflares observed during the second flight. We prove
the relevance of the FOXSI measurements for obtaining unprecedented
constrained microflare DEMs and discuss their implications on the
energetics of these small flaring events. We finish presenting a
discussion on the FOXSI measurements of an aged active region and
quiet-sun areas observed during its latest flight.
---------------------------------------------------------
Title: Combined Next-Generation X-ray and EUV Observations with the
FIERCE Mission Concept
Authors: Shih, A. Y.; Glesener, L.; Christe, S.; Reeves, K.; Gburek,
S.; Alaoui, M.; Allred, J. C.; Baumgartner, W.; Caspi, A.; Dennis,
B. R.; Drake, J. F.; Goetz, K.; Golub, L.; Guidoni, S. E.; Inglis,
A.; Hannah, I. G.; Holman, G.; Hayes, L.; Ireland, J.; Kerr, G. S.;
Klimchuk, J. A.; Krucker, S.; McKenzie, D. E.; Moore, C. S.; Musset,
S.; Reep, J. W.; Ryan, D.; Saint-Hilaire, P.; Savage, S. L.; Seaton,
D. B.; Steslicki, M.; Woods, T. N.
2019AGUFMSH33A..08S Altcode:
While there have been significant advances in our understanding
of impulsive energy release at the Sun through the combination
of RHESSI X-ray observations and SDO/AIA EUV observations, there
is a clear science need for significantly improved X-ray and EUV
observations. These new observations must capture the full range
of emission in flares and CMEs (e.g., faint coronal sources near
bright chromospheric sources), connect the intricate evolution of
energy release with dynamic changes in the configuration of plasma
structures, and identify the signatures of impulsive energy release in
even the quiescent Sun. The Fundamentals of Impulsive Energy Release
in the Corona Explorer ( FIERCE ) MIDEX mission concept makes these
observations by combining the two instruments previously proposed on the
FOXSI SMEX mission concept - a focusing hard X-ray spectroscopic imager
and a soft X-ray spectrometer - with a high-resolution EUV imager that
will not saturate for even intense flares. All instruments observe at
high cadence to capture the initiation of solar transient events and
the fine time structure within events. FIERCE would launch in mid-2025,
near the peak of the next solar cycle, which is also well timed with
perihelions of Parker Solar Probe and Solar Orbiter.
---------------------------------------------------------
Title: Characterization of Charge Sharing in the FOXSI Sounding Rocket
Hard X-ray Detectors Using the Advanced Light Source at Berkeley
Authors: Duncan, J. M.; Panchapakesan, S. A.; Musset, S.; Vievering,
J. T.; MacDowell, A. A.; Glesener, L.; Davis, L.; Buitrago-Casas,
J. C.; O'Brien, C.; Ishikawa, S. N.; Takahashi, T.; Watanabe, S.;
Narukage, N.; Furukawa, K.; Ryan, D.; Hagino, K.; Courtade, S.;
Christe, S.; Krucker, S.
2019AGUFMSH31C3317D Altcode:
The FOXSI sounding rocket experiments represent the first ever
solar-dedicated direct-focusing hard X-ray (HXR) instruments. The
most recent flight (FOXSI-3) occurred in September 2018, utilizing
Wolter-1 style concentric-shell optics and both silicon (Si) and
cadmium telluride (CdTe) double-sided strip detectors. The angular
resolution of the optics (5" FWHM, equivalent to 50um at the detector
bench) is finer than the strip pitch of the Si and CdTe detectors
(75um and 60um, respectively), meaning that the resolution of the
instrument at sounding rocket focal lengths is detector-limited
[1]. In order to further improve characterization of the spatial and
spectral properties of the FOXSI-3 detectors, experiments were recently
completed using Beamline 3.3.2. at the Advanced Light Source (ALS)
at Lawrence Berkeley National Laboratory. Using this beamline, a fine
(2um x 2um or 5um x 5um) monoenergetic x-ray beam was scanned across
detector strips and inter-strip regions in steps much smaller than the
strip pitch. Such scans were performed for both Si and CdTe detectors
at a number of x-ray energies between 5.5-12 keV. These experiments were
particularly motivated by the desire to measure the incidence of charge
sharing, which occurs when a single incident photon causes signals
to be registered in multiple adjacent strips. Precise measurement of
charge sharing behavior is important to optimize characterization of
detector efficiency. Additionally, with a strong understanding of
charge sharing behavior in a detector, spatial resolution could be
improved by sub-strip localization of photon impacts. Here, we present
initial analysis of results from these ALS experiments, including
investigation into charge sharing properties in both the Si and CdTe
FOXSI-3 detectors. We also discuss the implications of these results
for the use of these detectors in the future FOXSI-4 mission, which
will utilize yet-higher-resolution HXR optics. <P />[1] Furukawa,K.,et
al. "Development of 60um pitch CdTe Double-sided Strip Detectors for
the FOXSI-3 sounding rocket experiment." NIMPR, Section A: HSTD11, 2017.
---------------------------------------------------------
Title: Spectrometer for Temperature and Composition for the FIERCE
MIDEX mission concept
Authors: Steslicki, M.; Mrozek, T.; Gburek, S.; Caspi, A.; Podgórski,
P.; Ścisłowski, D.; Bąkała, J.; Kowaliński, M.; Shih, A. Y.;
Christe, S.; Glesener, L.; Krucker, S.; Saint-Hilaire, P.
2019AGUFMSH31C3314S Altcode:
Spectrometer for Temperature and Composition (STC) will be a small
soft x-ray spectrometer and will monitor solar X-ray flux between 1
keV and 15 with 0.125 keV energy resolution. The data collected by STC
will complement two other FIERCE mission instruments and additionally
allowing temperature and emission measure analysis of small flares
as well as a quiet Sun. The instrument will provide fast solar X-ray
spectroscopy in the entire flux intensity range, variable seven order
in intensity form quiet solar conditions to strongest flares. The
exceptionally wide dynamic range of this instrument as well as its high
spectral and temporal resolution and wide plasma temperature coverage
will result in unprecedented measurements, crucial for energy release
processes and space weather monitoring.
---------------------------------------------------------
Title: The magnetic structure and electrodynamics of the emerging
solar wind
Authors: Bale, S. D.; Badman, S. T.; Bonnell, J. W.; Bowen, T. A.;
Burgess, D.; Case, A. W.; Cattell, C. A.; Chandran, B. D. G.;
Chaston, C. C.; Chen, C. H. K.; Drake, J. F.; Dudok de Wit, T.;
Eastwood, J. P.; Webster, J.; Farrell, W. M.; Fong, C.; Goetz, K.;
Goldstein, M. L.; Goodrich, K.; Harvey, P.; Horbury, T. S.; Howes,
G. G.; Kasper, J. C.; Kellogg, P. J.; Klimchuk, J. A.; Korreck,
K. E.; Krasnoselskikh, V.; Krucker, S.; Laker, R.; Larson, D. E.;
MacDowall, R. J.; Maksimovic, M.; Malaspina, D.; Martinez Oliveros,
J. C.; McComas, D. J.; Meyer-Vernet, N.; Moncuquet, M.; Mozer, F.;
Phan, T.; Pulupa, M.; Raouafi, N. E.; Salem, C. S.; Stansby, D.;
Stevens, M. L.; Szabo, A.; Velli, M.; Woolley, T.; Wygant, J. R.
2019AGUFMSH11A..05B Altcode:
Convection and rotation drive the solar dynamo and, ultimately,
provide the mechanical energy flux required to heat the solar corona
and accelerate the solar wind. However, the way in which energy is then
dissipated to heat the corona and wind are not well understood. Some
energization models invoke non-thermal energy flux imparted by plasma
Alfvén waves, while others rely on a carpet of small nano-flares as
energy input, however these models have been unconstrained by direct
measurements of the solar wind near its origin. Here we use in situ
measurements from the FIELDS instrument suite during the first solar
encounter (E1) at 35.7 solar radii (Rs) of the NASA Parker Solar
Probe (PSP) mission to reveal the magnetic structure and kinetics of
slow Alfvénic solar wind emerging from a small, equatorial coronal
hole. Our measurements show that, at solar minimum, the slow wind can
escape from above the low-lying, complex magnetic structures of the
equatorial streamer belt, carrying a magnetic field that is highly
dynamic, exhibiting polarity reversals on timescales from seconds
to hours. These rapidly oscillating field structures are associated
with clustered radial jets of plasma in which the energy flux is
dramatically enhanced and turbulence levels are higher. Time intervals
between groups of jets indicate a solar wind that is steady with a
mostly radial magnetic field and relatively low levels of Alfvénic
turbulent fluctuations. This 'quiet' wind however shows clear signatures
of plasma micro-instabilities associated with ion and electron beams
and velocity-space structure.
---------------------------------------------------------
Title: The Hard X-ray Spectrometer STIX onboard Solar Orbiter
Authors: Krucker, S.
2019AGUFMSH21D3300K Altcode:
We will present the STIX hard X-ray imaging concept and highlight key
science objectives of STIX.
---------------------------------------------------------
Title: A Statistical Study of Magnetic Features of White-light Flares
Authors: Watanabe, K.; Tsuruda, K.; Masuda, S.; Krucker, S.
2019AGUFMSH13D3425W Altcode:
It is believed that the origin of white-light enhancement is accelerated
non-thermal electrons. Previously, we performed a statistical analysis
that search for the difference between white-light flares (WLFs) and
no white-light flares (NWLFs), and found that the relationship between
emission measure and temperature for WLFs indicated the existence
of strong magnetic field around the acceleration site (Watanabe et
al., 2017). Although the field strength at the acceleration site
cannot measure directly, we tried to estimate by using the turn-over
frequency of gyro-synchrotron emission in microwaves (Dulk 1985). We
used the Nobeyama Radio polarimeters (NoRP) for >M3 class flares
during the period from January 2011 to December 2017. We found 29
events which were simultaneously observed with white-light (SDO/HMI
continuum) and microwave (NoRP) which had loop-top microwave source in
Nobeyama Radioheliograph (NoRH) image. However, we couldn't find any
difference in turn-over frequencies for WLF and NWLF. This is because
the turn-over frequency is determined by not only the magnetic field
strength but also the electron density (Dulk 1985). Thus, we have to
think about the variation of electron density for this study. <P />In
order to investigate the relationship between magnetic field strength
and white-light emissions, next we tried to compare field strength and
temperature of white-light emission region. The former/latter can be
derived from SDO/HMI magnetogram/three continuum bands of Hinode/SOT. We
performed a statistical analysis for 26 Hinode white-light events,
we found the field strength of white-light emission region for WLFs
were correlated with the maximum counts of white-light emission. When
we assume the photosperic field strength is correlated with the field
strength at the flare acceleration site, these results support the
existence of strong field strength for WLFs.
---------------------------------------------------------
Title: A new spectrum fitting model: asymptotic double power law
Authors: Liu, Z.; Wang, L.; Fu, H.; Krucker, S.; Wimmer-Schweingruber,
R. F.
2019AGUFMSH23C3344L Altcode:
We propose a new formula of energy spectrum of energetic particles,
J=A(E/E<SUB>0</SUB>)<SUP>-β<SUB>1</SUB></SUP>[1+(E/E0)<SUP>α</SUP>]<SUP>(β<SUB>2</SUB>-β<SUB>1</SUB>)/α</SUP>,
where J is the particle intensity (e.g., flux), E is the particle
energy, A is the amplitude factor,E<SUB>0</SUB> likely represents
the spectral break energy, β1 (β2 ) is equivalent to the power-law
spectral index at energies below (above) E<SUB>0</SUB>, and α
(>0) describes the transition shape of energy spectrum around
E<SUB>0</SUB>. When α >> 10, this spectral formula becomes
a classical double-power-law spectrum, while, when α << 10,
this formula has a curved spectral transition. In addition, when
α = 1, E<SUB>0</SUB> and β2 tend to infinity, t his formula can
be simplified to the Ellison-Ramaty equation. We can utilize this
spectral formula to fit the energy spectrum of energetic electron,
proton, <SUP>3</SUP>He and heavier ions, HXRs, Gamma rays, etc., and
obtain self-consistently all the spectral parameters by including both
uncertainties in particle intensity and energy.
---------------------------------------------------------
Title: FIERCE Science: Expected Results From a High-Energy
Medium-Class Explorer
Authors: Glesener, L.; Shih, A. Y.; Christe, S.; Reeves, K.; Gburek,
S.; Alaoui, M.; Allred, J. C.; Baumgartner, W.; Caspi, A.; Dennis,
B. R.; Drake, J. F.; Golub, L.; Goetz, K.; Guidoni, S. E.; Hannah,
I. G.; Hayes, L.; Holman, G.; Inglis, A.; Ireland, J.; Kerr, G. S.;
Klimchuk, J. A.; Krucker, S.; McKenzie, D. E.; Moore, C. S.; Musset,
S.; Reep, J. W.; Ryan, D.; Saint-Hilaire, P.; Savage, S. L.; Seaton,
D. B.; Steslicki, M.; Woods, T. N.
2019AGUFMSH31C3313G Altcode:
A variety of individual X-ray and EUV instruments have probed
high-energy aspects of the Sun over the decades, each contributing
pieces to the puzzles of the energization, heating, and acceleration of
solar plasma and particles. But fundamental difficulties in sensitivity
and dynamic range impart big challenges in probing the details of
particle acceleration sites, understanding how eruptions and flares are
initiated, and tracking the intricacies of energy transfer as flares
evolve. The Fundamentals of Impulsive Energy Release in the Corona
Explorer ( FIERCE ) mission will make substantial leaps forward in
these scientific ventures by combining a variety of instruments into
one platform, each optimized to have high sensitivity and dynamic
range. FIERCE is a proposed NASA Heliophysics Medium-Class Explorer
that will investigate high-energy solar phenomena across a variety
of spectral and spatial dimensions. It combines hard X-ray imaging
spectroscopy (via focusing, for the first time for a solar-dedicated
spacecraft), spatially integrated soft X-ray spectroscopy, and fast,
high-resolution extreme ultraviolet imaging at coronal and flare
temperatures. FIERCE uses this array of instruments to make important
contributions toward probing the genesis of space weather events,
the acceleration of particles, the transport of flare energy, and the
heating of the corona. Here, we present some of the expected science
outcomes for the FIERCE observatory, concentrating on the ways in which
FIERCE can probe confined and eruptive events, particle acceleration
everywhere it may occur on the Sun, and the connections of solar
high-energy phenomena to the heliosphere.
---------------------------------------------------------
Title: Highly structured slow solar wind emerging from an equatorial
coronal hole
Authors: Bale, S. D.; Badman, S. T.; Bonnell, J. W.; Bowen, T. A.;
Burgess, D.; Case, A. W.; Cattell, C. A.; Chandran, B. D. G.;
Chaston, C. C.; Chen, C. H. K.; Drake, J. F.; de Wit, T. Dudok;
Eastwood, J. P.; Ergun, R. E.; Farrell, W. M.; Fong, C.; Goetz,
K.; Goldstein, M.; Goodrich, K. A.; Harvey, P. R.; Horbury, T. S.;
Howes, G. G.; Kasper, J. C.; Kellogg, P. J.; Klimchuk, J. A.; Korreck,
K. E.; Krasnoselskikh, V. V.; Krucker, S.; Laker, R.; Larson, D. E.;
MacDowall, R. J.; Maksimovic, M.; Malaspina, D. M.; Martinez-Oliveros,
J.; McComas, D. J.; Meyer-Vernet, N.; Moncuquet, M.; Mozer, F. S.;
Phan, T. D.; Pulupa, M.; Raouafi, N. E.; Salem, C.; Stansby, D.;
Stevens, M.; Szabo, A.; Velli, M.; Woolley, T.; Wygant, J. R.
2019Natur.576..237B Altcode:
During the solar minimum, when the Sun is at its least active, the solar
wind<SUP>1,2</SUP> is observed at high latitudes as a predominantly fast
(more than 500 kilometres per second), highly Alfvénic rarefied stream
of plasma originating from deep within coronal holes. Closer to the
ecliptic plane, the solar wind is interspersed with a more variable slow
wind<SUP>3</SUP> of less than 500 kilometres per second. The precise
origins of the slow wind streams are less certain<SUP>4</SUP>; theories
and observations suggest that they may originate at the tips of helmet
streamers<SUP>5,6</SUP>, from interchange reconnection near coronal hole
boundaries<SUP>7,8</SUP>, or within coronal holes with highly diverging
magnetic fields<SUP>9,10</SUP>. The heating mechanism required to
drive the solar wind is also unresolved, although candidate mechanisms
include Alfvén-wave turbulence<SUP>11,12</SUP>, heating by reconnection
in nanoflares<SUP>13</SUP>, ion cyclotron wave heating<SUP>14</SUP>
and acceleration by thermal gradients<SUP>1</SUP>. At a distance of
one astronomical unit, the wind is mixed and evolved, and therefore
much of the diagnostic structure of these sources and processes
has been lost. Here we present observations from the Parker Solar
Probe<SUP>15</SUP> at 36 to 54 solar radii that show evidence of
slow Alfvénic solar wind emerging from a small equatorial coronal
hole. The measured magnetic field exhibits patches of large,
intermittent reversals that are associated with jets of plasma and
enhanced Poynting flux and that are interspersed in a smoother and
less turbulent flow with a near-radial magnetic field. Furthermore,
plasma-wave measurements suggest the existence of electron and ion
velocity-space micro-instabilities<SUP>10,16</SUP> that are associated
with plasma heating and thermalization processes. Our measurements
suggest that there is an impulsive mechanism associated with solar-wind
energization and that micro-instabilities play a part in heating, and
we provide evidence that low-latitude coronal holes are a key source
of the slow solar wind.
---------------------------------------------------------
Title: Joint hard X-ray observations with ASO-S/HXI and SO/STIX
Authors: Krucker, Säm; Hurford, Gordon J.; Su, Yang; Gan, Wei-Qun
2019RAA....19..167K Altcode: 2019RAA....19..167H
This paper discusses the potential of future joint hard X-ray solar
flare observations between the Hard X-ray Imager (HXI) onboard
the Advanced Space-based Solar Observatory (ASO-S) mission and the
Spectrometer/Telescope for Imaging X-rays (STIX) on Solar Orbiter. The
different viewing perspectives of the two telescopes relative to
the Sun will allow us for the first time to systematically study
non-thermal hard X-ray emissions stereoscopically. During the 4-years
of the nominal mission of ASO-S, we expect to jointly observe about
160 flares above GOES M1 class to systematically study hard X-ray
directivity. For about 16 partially limb-occulted STIX flares, we will
have observations of the entire flare by HXI. Such observations will
enable us to simultaneously study the all-important coronal hard X-ray
sources, which are generally lost in the instrument’s individual
imaging dynamic range, in combination with the chromospheric footpoint
emissions. The two different detector systems used in the two telescopes
make the relative calibration between the two instruments a key task
that needs to be addressed before creditable science results can be
published. If an accurate inter-calibration can be achieved using
jointly observed flares on the disk, observations with HXI and STIX
will provide new and essential key diagnostics for solar flare physics.
---------------------------------------------------------
Title: Multiwavelength Stereoscopic Observation of the 2013 May 1
Solar Flare and CME
Authors: Lastufka, Erica; Krucker, Säm; Zimovets, Ivan; Nizamov,
Bulat; White, Stephen; Masuda, Satoshi; Golovin, Dmitriy; Litvak,
Maxim; Mitrofanov, Igor; Sanin, Anton
2019ApJ...886....9L Altcode: 2020arXiv201210179L
A M-class behind-the-limb solar flare on 2013 May 1
(SOL2013-05-01T02:32), accompanied by a (∼400 km s<SUP>-1</SUP>)
coronal mass ejection (CME), was observed by several space-based
observatories with different viewing angles. We investigated the
RHESSI-observed occulted hard X-ray (HXR) emissions that originated
at least 0.1 {R}<SUB>{{S</SUB>}} above the flare site. Emissions
below ∼10 keV revealed a hot, extended (11 MK, >60″) thermal
source from the escaping CME core, with densities around 10<SUP>9</SUP>
cm<SUP>-3</SUP>. In such a tenuous hot plasma, ionization times scales
are several minutes, consistent with the nondetection of the hot
CME core in SDO/AIA’s 131 Å filter. The nonthermal RHESSI source
originated from an even larger area (∼100″) at lower densities
(10<SUP>8</SUP> cm<SUP>-3</SUP>) located above the hot core, but
still behind the CME front. This indicates that the observed part
of the nonthermal electrons are not responsible for heating the CME
core. Possibly the hot core was heated by nonthermal electrons before
it became visible from Earth, meaning that the unocculted part of the
nonthermal emission likely originates from a more tenuous part of the
CME core, where nonthermal electrons survive long enough to became
visible from Earth. Simultaneous HXR spectra from the Mars Odyssey
mission, which viewed the flare on disk, indicated that the number
of nonthermal electrons >20 keV within the high coronal source is
∼0.1%-0.5% compared with the number within the chromospheric flare
ribbons. The detection of high coronal HXR sources in this moderate
size event suggests that such sources are likely a common feature
within solar eruptive events.
---------------------------------------------------------
Title: The Micro Solar Flare Apparutus (MiSolFA) Instrument Concept
Authors: Lastufka, Erica; Casadei, Diego; Hurford, Gordon; Kuhar,
Matej; Torre, Gabriele; Krucker, Säm
2019arXiv191001825L Altcode:
The Micro Solar-Flare Apparatus (MiSolFA) is a compact X-ray imaging
spectrometer designed for a small 6U micro-satellite. As a relatively
inexpensive yet capable Earth-orbiting instrument, MiSolFA is designed
to image the high-energy regions of solar flares from a different
perspective than that of Solar Orbiter's STIX, operating from a highly
elliptical heliocentric orbit. Two instruments working together in
this way would provide a 3-dimensional view of X-ray emitting regions
and can bypass the dynamic range limitation preventing simultaneous
coronal and chromospheric imaging. Stereoscopic X-ray observations
would also contain valuable information about the anisotropy of the
flare-accelerated electron distribution. To perform these types of
observations, MiSolFA must be capable of imaging sources with energies
between 10 and 100 keV, with 10 arcsec angular resolution. MiSolFA's
Imager will be the most compact X-ray imaging spectrometer in
space. Scaling down the volume by a factor of ten from previous
instrument designs requires special considerations. Here we present the
design principles of the MiSolFA X-ray optics, discuss the necessary
compromises, and evaluate the performance of the Engineering Model.
---------------------------------------------------------
Title: Ghost-ray reduction and early results from the third FOXSI
sounding rocket flight
Authors: Musset, Sophie; Buitrago-Casas, Juan Camilo; Glesener,
Lindsay; Bongiorno, Stephen; Courtade, Sasha; Athiray, P. S.;
Vievering, Juliana; Ishikawa, Shin-nosuke; Narukage, Noriyuki;
Furukawa, Kento; Ryan, Daniel; Dalton, Greg; Turin, Zoe; Davis, Lance;
Takahashi, Tadayuki; Watanabe, Shin; Mitsuishi, Ikuyuki; Hagino,
Kouichi; Kawate, Tomoko; Turin, Paul; Christe, Steven; Ramsey, Brian;
Krucker, Säm.
2019SPIE11118E..12M Altcode:
The Focusing Optics X-ray Solar Imager (FOXSI) sounding rocket
experiment demonstrates the technique of focusing hard X-ray (HXR)
optics for the study of fundamental questions about the high-energy
Sun. Solar HXRs provide one of the most direct diagnostics of
accelerated electrons and the impulsive heating of the solar
corona. Previous solar missions have been limited in sensitivity
and dynamic range by the use of indirect imaging, but technological
advances now make direct focusing accessible in the HXR regime, and the
FOXSI rocket experiment optimizes HXR focusing telescopes for the unique
scientific requirements of the Sun. FOXSI has completed three successful
flights between 2012 and 2018. This paper gives a brief overview of
the experiment, focusing on the third flight of the instrument on 2018
Sept. 7. We present the telescope upgrades highlighting our work to
understand and reduce the effects of singly reflected X-rays and show
early science results obtained during FOXSI's third flight.
---------------------------------------------------------
Title: Frequency Agile Solar Radiotelescope
Authors: Bastian, Tim; Bain, H.; Bradley, R.; Chen, B.; Dahlin, J.;
DeLuca, E.; Drake, J.; Fleishman, G.; Gary, D.; Glesener, L.; Guo,
Fan; Hallinan, G.; Hurford, G.; Kasper, J.; Ji, Hantao; Klimchuk,
J.; Kobelski, A.; Krucker, S.; Kuroda, N.; Loncope, D.; Lonsdale,
C.; McTiernan, J.; Nita, G.; Qiu, J.; Reeves, K.; Saint-Hilaire, P.;
Schonfeld, S.; Shen, Chengcai; Tun, S.; Wertheimer, D.; White, S.
2019astro2020U..56B Altcode:
We describe the science objectives and technical requirements for a
re-scoped Frequency Agile Solar Radiotelescope (FASR). FASR fulfills
a long term community need for a ground-based, solar-dedicated, radio
telescope - a next-generation radioheliograph - designed to perform
ultra-broadband imaging spectropolarimetry.
---------------------------------------------------------
Title: Statistical Study of Hard X-Ray Spectral Breaks in Solar Flares
Authors: Alaoui, Meriem; Krucker, Säm; Saint-Hilaire, Pascal
2019SoPh..294..105A Altcode:
The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI)
provides hard X-ray spectral observations with ≈1 keV resolution
to study flare-accelerated (>10 keV) electrons through their
bremsstrahlung emission. Here we report on a statistical study of RHESSI
flares with emission above 150 keV, focusing on the spectral shape at
the hard X-ray peak. Spectral parameters are derived by fitting the
photon spectrum with a broken power law and by the standard thick-target
fit. Consistent with previous studies, the most common spectral shape
of the photon spectrum (52 out of 65 events) is a double power law
with a downward break ("knee"), with ten events showing a single power
law and three events having an upward break ("ankle"). The spectral
breaks occur typically around 55 keV and the difference of the spectral
index above and below the break, γ<SUB>2</SUB> and γ<SUB>1</SUB>, is
typically between 0.3 and 1. We show correlations between the downward
break parameters. The most prominent correlation, with a rank order
coefficient of ρ =0.92 , is between the power-law indices above and
below the break: γ<SUB>1</SUB>=(0.74 ±0.04 )γ<SUB>2</SUB>+(0.34
±0.14 ). Applying a thick target fit to the photon spectrum, a
similar correlation is also found for the flare-accelerated electron
spectra with δ<SUB>1</SUB>=(0.85 ±0.08 )δ<SUB>2</SUB>−(0.3 ±0.3 )
(ρ =0.67 ). Spectral breaks could be a property of the acceleration
mechanism itself or they could be a secondary effect produced by
particle transport or wave-particle interactions. Any theoretical
models should be consistent with these correlations. In addition,
we find that one upward and 23 (49%) downward breaks are consistent
with nonuniform ionization within the thick target.
---------------------------------------------------------
Title: Joint X-Ray, EUV, and UV Observations of a Small Microflare
Authors: Hannah, Iain G.; Kleint, Lucia; Krucker, Säm; Grefenstette,
Brian W.; Glesener, Lindsay; Hudson, Hugh S.; White, Stephen M.;
Smith, David M.
2019ApJ...881..109H Altcode: 2018arXiv181209214H
We present the first joint observation of a small microflare in X-rays
with the Nuclear Spectroscopic Telescope ARray (NuSTAR), in UV with the
Interface Region Imaging Spectrograph (IRIS), and in EUV with the Solar
Dynamics Observatory/Atmospheric Imaging Assembly (SDO/AIA). These
combined observations allow us to study the hot coronal and cooler
chromospheric/transition region emission from the microflare. This small
microflare peaks from 2016 July 26 23:35 to 23:36 UT, in both NuSTAR,
SDO/AIA, and IRIS. Spatially, this corresponds to a small loop visible
in the SDO/AIA Fe XVIII emission, which matches a similar structure
lower in the solar atmosphere seen by IRIS in SJI1330 and 1400 Å. The
NuSTAR emission in both 2.5-4 and 4-6 keV is located in a source at
this loop location. The IRIS slit was over the microflaring loop,
and fits show little change in Mg II but do show intensity increases,
slight width enhancements, and redshifts in Si IV and O IV, indicating
that this microflare had most significance in and above the upper
chromosphere. The NuSTAR microflare spectrum is well fitted by a
thermal component of 5.1 MK and 6.2 × 10<SUP>44</SUP> cm<SUP>-3</SUP>,
which corresponds to a thermal energy of 1.5 × 10<SUP>26</SUP> erg,
making it considerably smaller than previously studied active region
microflares. No non-thermal emission was detected but this could be
due to the limited effective exposure time of the observation. This
observation shows that even ordinary features seen in UV can remarkably
have a higher-energy component that is clear in X-rays.
---------------------------------------------------------
Title: FOXSI-2 Solar Microflares: Hard X-ray Spectroscopy and Flare
Energetics
Authors: Vievering, Juliana; Glesener, Lindsay; Athiray Panchapakesan,
Subramania; Buitrago-Casas, Juan Camilo; Ryan, Daniel; Musset, Sophie;
Inglis, Andrew; Christe, Steven; Krucker, Sam
2019AAS...23420412V Altcode:
Sensitive measurements of solar flares in the hard X-ray regime
are necessary for investigating energy release and transfer during
flaring events, as hard X-rays provide insight into the acceleration
of electrons and emission of high-temperature plasmas. Due to use of
indirect imaging, past solar-dedicated instruments in this energy
range have been fundamentally limited in sensitivity and dynamic
range. By instead using a direct imaging technique, the structure and
evolution of even small flares and active regions can be investigated
in greater depth. The Focusing Optics X-ray Solar Imager (FOXSI),
a hard X-ray instrument flown on three sounding rocket campaigns,
seeks to achieve these improved capabilities by using focusing optics
for solar observations in the 4-20 keV range. During the second
flight, which occurred on 2014 December 11, FOXSI-2 observed two
microflares, estimated as GOES class A0.1 and A0.3. Here we present
imaging and spectral analyses of these microflares, investigating
the nature of energy release and exploring the structure and dynamics
in comparison to larger flares. Through this work, we find evidence
for high-temperature plasma ( 10 MK) as well as spatial and temporal
complexity for microflares that are an order of magnitude fainter than
those observed by previous solar-dedicated instruments in this energy
range, highlighting the benefits of direct imaging.
---------------------------------------------------------
Title: The FOXSI-3 rocket: Overview and early results of its latest
flight
Authors: Buitrago-Casas, Juan Camilo; Glesener, Lindsay; Courtade,
Sasha; Vievering, Juliana; Athiray Panchapakesan, Subramania; Musset,
Sophie; Ryan, Daniel; Dalton, Gregory; Ishikawa, Shin-Nosuke; Narukage,
Noriyuki; Bongiorno, Stephen; Furukawa, Kento; Davis, Lance; Turin,
Paul; Turin, Zoe; Takahashi, Tadayuki; Watanabe, Shin; Krucker, Sam;
Christe, Steven; Ramsey, Brian
2019AAS...23412602B Altcode:
Hard X-rays (HXRs) from the solar corona are closely connected to energy
releases and particle transport in solar flares of all sizes. Expressly,
faint solar HXR emissions are of remarkable interest in understanding
solar flare structure and dynamics. This is because of their connection
with, for instance, loop top emission and small flares. Mainly due to
the indirect imaging methods used by past solar-dedicated HXR imagers,
like RHESSI, faint HXRs observations have been limited by the imaging
dynamic range and sensitivity of the instruments. <P />The Focusing
Optics X-ray Solar Imager (FOXSI) sounding rocket payload is the first
solar-dedicated instrument designed for performing imaging spectroscopy
in the 4-20 keV range by using direct focusing optics. FOXSI has
successfully flown three times from the White Sands Missile Range
in New Mexico. For its latest rocket campaign (FOXSI-3), an enhanced
version of the experiment, which includes optics and detector upgrades,
was implemented for the launch which happened on September 7, 2018. In
this talk, we present an overview of the FOXSI-3 campaign, describing
in detail the improved capabilities of the telescope and how they
allow for better investigation of faint coronal HXRs. We also present
a preliminary analysis of the FOXSI-3 observations.
---------------------------------------------------------
Title: Radio Spectroscopic Imaging of Solar Flare Termination Shocks:
Split-band Feature and A Second Possible Event
Authors: Chen, Bin; Luo, Yingjie; Yu, Sijie; Krucker, Sam; Reeves,
Kathy; Shen, Chengcai; Bastian, Timothy S.
2019AAS...23421003C Altcode:
Solar termination shocks (TSs) can form above the looptop when
reconnection outflows that impinge upon newly reconnected flare
arcades exceed the local fast-mode magnetosonic speed. TSs have been
suggested as one of the promising drivers for particle acceleration in
solar flares, yet observational evidence remains rare. By utilizing
radio dynamic spectral imaging of decimetric stochastic spike bursts
(SSBs) observed during a C1.9 eruptive flare on 2012 March 3, Chen et
al. (2015) found that the bursts were associated with a dynamic TS-like
feature above the looptop. They also showed evidence for the TS as
an electron accelerator. One piece of observational evidence that
strongly supports the TS interpretation is the split-band feature
- a phenomenon well-known in type II radio bursts associated with
CME-driven shocks, one interpretation for which attributes to radio
emission from both the upstream and downstream side of the shock. We
perform detailed spectral imaging analysis of the split-band feature in
the 2012 March 3 SSB event, and find evidence that supports the shock
upstream-downstream interpretation. We also report another SSB event
observed during an M8.4 eruptive flare on 2012 March 10, and show that
the radio centroids of the SSBs form a similar shock-surface-shaped
structure to the earlier event, located above the reconnected flare
arcades and below supra-arcade plasma downflows.
---------------------------------------------------------
Title: The Focusing Optics X-ray Solar Imager (FOXSI)
Authors: Christe, Steven; Shih, Albert Y.; Krucker, Sam; Glesener,
Lindsay; Saint-Hilaire, Pascal; Caspi, Amir; Gburek, Szymon;
Steslicki, Marek; Allred, Joel C.; Battaglia, Marina; Baumgartner,
Wayne H.; Drake, James; Goetz, Keith; Grefenstette, Brian; Hannah,
Iain; Holman, Gordon D.; Inglis, Andrew; Ireland, Jack; Klimchuk,
James A.; Ishikawa, Shin-Nosuke; Kontar, Eduard; Massone, Anna-maria;
Piana, Michele; Ramsey, Brian; Schwartz, Richard A.; Woods, Thomas N.;
Chen, Bin; Gary, Dale E.; Hudson, Hugh S.; Kowalski, Adam; Warmuth,
Alexander; White, Stephen M.; Veronig, Astrid; Vilmer, Nicole
2019AAS...23422501C Altcode:
The Focusing Optics X-ray Solar Imager (FOXSI), a SMEX mission concept
in Phase A, is the first-ever solar-dedicated, direct-imaging, hard
X-ray telescope. FOXSI provides a revolutionary new approach to
viewing explosive magnetic-energy release on the Sun by detecting
signatures of accelerated electrons and hot plasma directly in
and near the energy-release sites of solar eruptive events (e.g.,
solar flares). FOXSI's primary science objective is to understand the
mystery of how impulsive energy release leads to solar eruptions, the
primary drivers of space weather at Earth, and how those eruptions are
energized and evolve. FOXSI addresses three important science questions:
(1) How are particles accelerated at the Sun? (2) How do solar plasmas
get heated to high temperatures? (3) How does magnetic energy released
on the Sun lead to flares and eruptions? These fundamental physics
questions are key to our understanding of phenomena throughout
the Universe from planetary magnetospheres to black hole accretion
disks. FOXSI measures the energy distributions and spatial structure of
accelerated electrons throughout solar eruptive events for the first
time by directly focusing hard X-rays from the Sun. This naturally
enables high imaging dynamic range, while previous instruments have
typically been blinded by bright emission. FOXSI provides 20-100 times
more sensitivity as well as 20 times faster imaging spectroscopy
than previously available, probing physically relevant timescales
(<1 second) never before accessible. FOXSI's launch in July 2022
is aligned with the peak of the 11-year solar cycle, enabling FOXSI
to observe the many large solar eruptions that are expected to take
place throughout its two-year mission.
---------------------------------------------------------
Title: Hard X-ray Spectroscopy of Six NuSTAR Microflares
Authors: Duncan, Jessie McBrayer; Glesener, Lindsay; Hannah, Iain;
Smith, David M.; Krucker, Sam; Hudson, Hugh S.; Grefenstette, Brian
2019AAS...23420404D Altcode:
Hard X-ray (HXR) emission in solar flares can originate from regions
of high temperature plasma, as well as from non-thermal particle
populations. Both of these sources of HXR radiation make solar
observation in this band important for study of flare energetics. NuSTAR
is the first HXR telescope with direct focusing optics, giving it
a dramatic increase in sensitivity over previous indirect imaging
methods. Here we present NuSTAR observation of six microflares from
one solar active region during a period of several hours on May 29th,
2018. Spectral fitting of emission at each flare time shows excess
high energy emission over an isothermal spectral component in all
six flares. The most likely origin of this excess could be either
additional volumes of high-temperature plasma, or non-thermally
accelerated particles. For each event, characterization of this excess
is presented, including determination of upper limits on the non-thermal
emission possible in events where it is not directly observed.
---------------------------------------------------------
Title: FOXSI-2 Solar Microflares : Multi-Instrument Differential
Emission Measure Analysis
Authors: Athiray, P. S.; Glesener, Lindsay; Vievering, Juliana;
Ishikawa, Shin-Nosuke; Inglis, Andrew; Narukage, Noriyuki; Ryan,
Daniel; Buitrago-Casas, Juan Camilo; Christe, Steven; Musset, Sophie;
Krucker, Sam
2019AAS...23422502A Altcode:
The plasma temperature distribution above 5 MK during microflares is
often loosely constrained due to limited high sensitivity hard X-ray
measurements covering the wide range of temperatures observed. The
Focusing Optics X-ray Solar Imager (FOXSI) sounding rocket experiment
performs direct imaging and spectroscopy of the Sun in hard X-rays,
in the energy range 4 to 20 keV. FOXSI offers better sensitivity for
temperatures above 5 MK by using direct focusing grazing incidence X-ray
optics, the first of its kind for dedicated solar observations. The
second FOXSI flight was launched on 2014 December 11 and observed
microflares, quiescent Sun and quiescent active regions. This flight
was coordinated with the X-ray Telescope (XRT) onboard Hinode and
the Atmospheric Imaging Assembly (AIA) onboard SDO which offers
unprecedented temperature coverage for characterizing the plasma
temperature distribution. We present an overview of microflares
observed during the FOXSI-2 flight with concurrent brightening
observed in Extreme Ultraviolet (EUV) and soft X-rays, which indicates
emission beyond 10 MK. By combining data from FOXSI-2, XRT, and AIA, we
determined a well-constrained DEM for the microflares. The coordinated
FOXSI-2 observations produce one of the few definitive measurements
of the distribution and the amount of plasma above 5 MK in microflares.
---------------------------------------------------------
Title: First detection of non-thermal emission in a NuSTAR solar
microflare
Authors: Glesener, Lindsay; Krucker, Sam; Duncan, Jessie McBrayer;
Hannah, Iain; Grefenstette, Brian; Chen, Bin; Smith, David M.; White,
Stephen M.; Hudson, Hugh S.
2019AAS...23422503G Altcode:
We report the detection of emission from a non-thermal electron
distribution in a small solar microflare observed by the Nuclear
Spectroscopic Telescope Array (NuSTAR). On 2017 August 21, NuSTAR
observed a solar active region for approximately an hour before the
region was eclipsed by the Moon. The active region emitted several
small microflares of GOES class A and smaller. In this work, we present
spectroscopy demonstrating evidence of electron acceleration in one of
these microflares (GOES class A5.7) and we compare energetic aspects
of the accelerated distribution to commonly studied larger flares. The
flaring plasma observed by NuSTAR, with supporting observation by
the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI), is
well accounted for by a thick-target model of accelerated electrons
collisionally thermalizing within the loop, akin to the "coronal
thick target" behavior occasionally observed in larger flares. Future
observations of this kind will enable understanding of how flare
particle acceleration changes across energy scales, and will aid
the push toward the observational regime of nanoflares, which are a
possible source of significant coronal heating.
---------------------------------------------------------
Title: On the observation of a classical loop-prominence system
during the 2017 September 10 flare.
Authors: Martinez Oliveros, Juan Carlos; Krucker, Sam; Guevara Gomez,
Juan Camilo
2019AAS...23420411M Altcode:
We report observations of white-light ejecta in the low corona after
the 2017 September 10 flare, using data from the Helioseismic and
Magnetic Imager (HMI) of the Solar Dynamics Observatory. We report the
observation of a classical loop-prominence system, but are brighter
than expected and possibly seen here in the continuum rather than
line emission. We studied the spatial and temporal relation between
RHESSI X-ray and the white-light emissions. We also studied the HMI
spectroscopic data to determine the most probable emission mechanism
that can explain the observation of the loop-prominence system.
---------------------------------------------------------
Title: Hot Plasma in a Quiescent Solar Active Region as Measured by
RHESSI, XRT, and AIA
Authors: Ishikawa, Shin-nosuke; Krucker, Säm
2019ApJ...876..111I Altcode: 2019arXiv190311293I
This paper investigates a quiescent (nonflaring) active region observed
on 2010 July 13 in extreme ultraviolet (EUV), soft X-ray (SXR), and
hard X-rays to search for a hot component that is speculated to be
a key signature of coronal heating. We use a combination of Reuven
Ramaty High Energy Solar Spectroscopic Imager (RHESSI) imaging and
long-duration time integration (up to 40 minutes) to detect the active
regions in the 3-8 keV range during apparently nonflaring times. The
RHESSI imaging reveals a hot component that originates from the entire
active region, as speculated for a nanoflare scenario where the entire
active region is filled with a large number of unresolved small energy
releases. An isothermal fit to the RHESSI data gives temperatures
around ∼7 MK with an emission measure of several times 10<SUP>46</SUP>
cm<SUP>-3</SUP>. Adding EUV and SXR observations taken by AIA and the
X-ray Telescope, respectively, we derive a differential emission measure
(DEM) that shows a peak between 2 and 3 MK with a steeply decreasing
high-temperature tail, similar to what has been previously reported. The
derived DEM reveals that a wide range of temperatures contributes to
the RHESSI flux (e.g., 40% of the 4 keV emission being produced by
plasma below 5 MK, while emission at 7 keV is almost exclusively from
plasmas above 5 MK) indicating that the RHESSI spectrum should not
be fitted with an isothermal. The hot component has a rather small
emission measure (∼0.1% of the total EM is above 5 MK), and the
derived thermal energy content is of the order of 10% for a filling
factor of unity, or potentially below 1% for smaller filling factors.
---------------------------------------------------------
Title: Penetrating particle ANalyzer (PAN)
Authors: Wu, X.; Ambrosi, G.; Azzarello, P.; Bergmann, B.; Bertucci,
B.; Cadoux, F.; Campbell, M.; Duranti, M.; Ionica, M.; Kole, M.;
Krucker, S.; Maehlum, G.; Meier, D.; Paniccia, M.; Pinsky, L.;
Plainaki, C.; Pospisil, S.; Stein, T.; Thonet, P. A.; Tomassetti,
N.; Tykhonov, A.
2019AdSpR..63.2672W Altcode: 2019arXiv190104351W
PAN is a scientific instrument suitable for deep space and
interplanetary missions. It can precisely measure and monitor the flux,
composition, and direction of highly penetrating particles (> ∼
100 MeV/nucleon) in deep space, over at least one full solar cycle (11
years). The science program of PAN is multi- and cross-disciplinary,
covering cosmic ray physics, solar physics, space weather and space
travel. PAN will fill an observation gap of galactic cosmic rays in the
GeV region, and provide precise information of the spectrum, composition
and emission time of energetic particle originated from the Sun. The
precise measurement and monitoring of the energetic particles is also
a unique contribution to space weather studies. PAN will map the flux
and composition of penetrating particles, which cannot be shielded
effectively, precisely and continuously, providing valuable input for
the assessment of the related health risk, and for the development
of an adequate mitigation strategy. PAN has the potential to become a
standard on-board instrument for deep space human travel. <P />PAN is
based on the proven detection principle of a magnetic spectrometer,
but with novel layout and detection concept. It will adopt advanced
particle detection technologies and industrial processes optimized
for deep space application. The device will require limited mass
(20 kg) and power (20 W) budget. Dipole magnet sectors built from
high field permanent magnet Halbach arrays, instrumented in a modular
fashion with high resolution silicon strip detectors, allow to reach
an energy resolution better than 10% for nuclei from H to Fe at 1
GeV/n. The charge of the particle, from 1 (proton) to 26 (Iron), can
be determined by scintillating detectors and silicon strip detectors,
with readout ASICs of large dynamic range. Silicon pixel detectors
used in a low power setting will maintain the detection capabilities
for even the strongest solar events. A fast scintillator with silicon
photomultiplier (SiPM) readout will provide timing information to
determine the entering direction of the particle, as well as a high
rate particle counter. Low noise, low power and high density ASIC
will be developed to satisfy the stringent requirement of the position
resolution and the power consumption of the tracker.
---------------------------------------------------------
Title: NuSTAR's observations of tiny flares and big eruptions
Authors: Hannah, Iain; Grefenstette, Brian; Glesener, Lindsey; Krucker,
Sam; Smith, David; Hudson, Hugh; White, Stephen; Kuhar, Matej
2018csc..confE.118H Altcode:
NuSTAR is an astrophysics X-ray telescope, with direct imaging
spectroscopy providing a unique sensitivity for observing the Sun above
2.5keV. Targeting the faintest X-ray emission from the solar atmosphere
allows the study of the smallest flares, and their contribution to
heating the corona. However, it can also be used to observe weak
high-coronal sources that are associated with the energy release in
large, but occulted, eruptions. NuSTAR has observed the Sun over a
dozen times since Sep 2014, through to our latest observations in
2018: see http://ianan.github.io/nsovr/ for a quicklook overview of
NuSTAR's solar observations. We will present some of the latest solar
observations with NuSTAR and compare them to the emission seen at
lower energy wavelengths, particularly in EUV with SDO/AIA and also
the derived Fe18 emission.
---------------------------------------------------------
Title: Magnetic Reconnection Null Points as the Origin of
Semirelativistic Electron Beams in a Solar Jet
Authors: Chen, Bin; Yu, Sijie; Battaglia, Marina; Farid, Samaiyah;
Savcheva, Antonia; Reeves, Katharine K.; Krucker, Säm; Bastian,
T. S.; Guo, Fan; Tassev, Svetlin
2018ApJ...866...62C Altcode: 2018arXiv180805951C
Magnetic reconnection, the central engine that powers explosive
phenomena throughout the universe, is also perceived to be one
of the principal mechanisms for accelerating particles to high
energies. Although various signatures of magnetic reconnection
have been frequently reported, observational evidence that links
particle acceleration directly to the reconnection site has been rare,
especially for space plasma environments currently inaccessible to in
situ measurements. Here we utilize broadband radio dynamic imaging
spectroscopy available from the Karl G. Jansky Very Large Array to
observe decimetric type III radio bursts in a solar jet with high
angular (∼20″), spectral (∼1%), and temporal resolution (50
ms). These observations allow us to derive detailed trajectories of
semirelativistic (tens of keV) electron beams in the low solar corona
with unprecedentedly high angular precision (<0.″65). We found that
each group of electron beams, which corresponds to a cluster of type III
bursts with 1-2 s duration, diverges from an extremely compact region
(∼600 km<SUP>2</SUP>) in the low solar corona. The beam-diverging
sites are located behind the erupting jet spire and above the closed
arcades, coinciding with the presumed location of magnetic reconnection
in the jet eruption picture supported by extreme ultraviolet/X-ray
data and magnetic modeling. We interpret each beam-diverging site as a
reconnection null point where multitudes of magnetic flux tubes join
and reconnect. Our data suggest that the null points likely consist
of a high level of density inhomogeneities possibly down to 10 km
scales. These results, at least in the present case, strongly favor
a reconnection-driven electron-acceleration scenario.
---------------------------------------------------------
Title: Solar radio emission as a disturbance of aeronautical
radionavigation
Authors: Marqué, Christophe; Klein, Karl-Ludwig; Monstein, Christian;
Opgenoorth, Hermann; Pulkkinen, Antti; Buchert, Stephan; Krucker,
Säm; Van Hoof, Rudiger; Thulesen, Peter
2018JSWSC...8A..42M Altcode: 2018arXiv180806878M
On November 4th, 2015 secondary air traffic control radar was strongly
disturbed in Sweden and some other European countries. The disturbances
occurred when the radar antennas were pointing at the Sun. In this
paper, we show that the disturbances coincided with the time of peaks
of an exceptionally strong (∼10<SUP>5</SUP> Solar Flux Units) solar
radio burst in a relatively narrow frequency range around 1 GHz. This
indicates that this radio burst is the most probable space weather
candidate for explaining the radar disturbances. The dynamic radio
spectrum shows that the high flux densities are not due to synchrotron
emission of energetic electrons, but to coherent emission processes,
which produce a large variety of rapidly varying short bursts (such
as pulsations, fiber bursts, and zebra patterns). The radio burst
occurs outside the impulsive phase of the associated flare, about 30
min after the soft X-ray peak, and it is temporarily associated with
fast evolving activity occurring in strong solar magnetic fields. While
the relationship with strong magnetic fields and the coherent spectral
nature of the radio burst provide hints towards the physical processes
which generate such disturbances, we have so far no means to forecast
them. Well-calibrated monitoring instruments of whole Sun radio fluxes
covering the UHF band could at least provide a real-time identification
of the origin of such disturbances, which reports in the literature
show to also affect GPS signal reception.
---------------------------------------------------------
Title: Fermi-LAT Observations of the 2017 September 10 Solar Flare
Authors: Omodei, Nicola; Pesce-Rollins, Melissa; Longo, Francesco;
Allafort, Alice; Krucker, Säm
2018ApJ...865L...7O Altcode: 2018arXiv180307654O
The Fermi-Large Area Telescope detection of the X8.2 GOES class solar
flare of 2017 September 10 provides for the first time observations of a
long-duration high-energy gamma-ray flare associated with a ground-level
enhancement (GLE). The >100 MeV emission from this flare lasted for
more than 12 hr covering both the impulsive and extended phases. We
present the localization of the gamma-ray emission and find that it is
consistent with the active region from which the flare occurred over a
period lasting more than 6 hr. The temporal variation of the gamma-ray
flux and of the proton index inferred from the gamma-ray data seems to
suggest three phases in acceleration of the proton population. Based
on timing arguments we interpret the last phase to be tied to the
acceleration mechanism powering the powering the production of the
GLE particles.
---------------------------------------------------------
Title: Hard X-Ray Constraints on Small-scale Coronal Heating Events
Authors: Marsh, Andrew J.; Smith, David M.; Glesener, Lindsay;
Klimchuk, James A.; Bradshaw, Stephen J.; Vievering, Juliana; Hannah,
Iain G.; Christe, Steven; Ishikawa, Shin-nosuke; Krucker, Säm
2018ApJ...864....5M Altcode: 2018arXiv180802630M
Much evidence suggests that the solar corona is heated impulsively,
meaning that nanoflares may be ubiquitous in quiet and active regions
(ARs). Hard X-ray (HXR) observations with unprecedented sensitivity
>3 keV are now enabled by focusing instruments. We analyzed data
from the Focusing Optics X-ray Solar Imager (FOXSI) rocket and the
Nuclear Spectroscopic Telescope Array (NuSTAR) spacecraft to constrain
properties of AR nanoflares simulated by the EBTEL field-line-averaged
hydrodynamics code. We generated model X-ray spectra by computing
differential emission measures for homogeneous nanoflare sequences
with heating amplitudes H <SUB>0</SUB>, durations τ, delay times
between events t <SUB> N </SUB>, and filling factors f. The single
quiescent AR observed by FOXSI-2 on 2014 December 11 is well fit by
nanoflare sequences with heating amplitudes 0.02 erg cm<SUP>-3</SUP>
s<SUP>-1</SUP> <H <SUB>0</SUB> < 13 erg cm<SUP>-3</SUP>
s<SUP>-1</SUP> and a wide range of delay times and durations. We
exclude delays between events shorter than ∼900 s at the 90%
confidence level for this region. Three of five regions observed
by NuSTAR on 2014 November 1 are well fit by homogeneous nanoflare
models, while two regions with higher fluxes are not. Generally,
the NuSTAR count spectra are well fit by nanoflare sequences with
smaller heating amplitudes, shorter delays, and shorter durations than
the allowed FOXSI-2 models. These apparent discrepancies are likely
due to differences in spectral coverage between the two instruments
and intrinsic differences among the regions. Steady heating (t <SUB>
N </SUB> = τ) was ruled out with >99% confidence for all regions
observed by either instrument.
---------------------------------------------------------
Title: Microwave and Hard X-Ray Observations of the 2017 September
10 Solar Limb Flare
Authors: Gary, Dale E.; Chen, Bin; Dennis, Brian R.; Fleishman,
Gregory D.; Hurford, Gordon J.; Krucker, Säm; McTiernan, James M.;
Nita, Gelu M.; Shih, Albert Y.; White, Stephen M.; Yu, Sijie
2018ApJ...863...83G Altcode: 2018arXiv180702498G
We report the first science results from the newly completed Expanded
Owens Valley Solar Array (EOVSA), which obtained excellent microwave
(MW) imaging spectroscopy observations of SOL2017-09-10, a classic
partially occulted solar limb flare associated with an erupting flux
rope. This event is also well-covered by the Reuven Ramaty High Energy
Solar Spectroscopic Imager (RHESSI) in hard X-rays (HXRs). We present
an overview of this event focusing on MW and HXR data, both associated
with high-energy nonthermal electrons, and we discuss them within
the context of the flare geometry and evolution revealed by extreme
ultraviolet observations from the Atmospheric Imaging Assembly (AIA)
aboard the Solar Dynamics Observatory. The EOVSA and RHESSI data reveal
the evolving spatial and energy distribution of high-energy electrons
throughout the entire flaring region. The results suggest that the
MW and HXR sources largely arise from a common nonthermal electron
population, although the MW imaging spectroscopy provides information
over a much larger volume of the corona.
---------------------------------------------------------
Title: On the observation of a classical loop-prominence system
during the 2017 September 10 flare
Authors: Martinez Oliveros, Juan Carlos; Hudson, Hugh; Krucker, Säm;
Guevara Gomez, Juan Camilo
2018shin.confE.258M Altcode:
We report observations of white-light ejecta in the low corona after
the 2017 September 10 flare, using data from the Helioseismic and
Magnetic Imager (HMI) of the Solar Dynamics Observatory. We report the
observation of a classical loop-prominence system, but are brighter
than expected and possibly seen here in the continuum rather than
line emission. We studied the spatial and temporal relation between
RHESSI X-ray and the white-light emissions. We also studied the HMI
spectroscopic data to determine the most probable emission mechanism
that can explain the observation of the loop-prominence system.
---------------------------------------------------------
Title: Microwave Spectral Imaging of Bi-Directional Magnetic
Reconnection Outflow Region of the 2017 Sep 10 X8.2 Flare
Authors: Chen, Bin; Gary, Dale E.; Fleishman, Gregory D.; Krucker,
Sam; Nita, Gelu M.; Dennis, Brian R.; Yu, Sijie; Kuroda, Natsuha;
Reeves, Katharine K.; Polito, Vanessa; Shih, Albert
2018shin.confE.211C Altcode:
The newly commissioned Expanded Owens Valley Solar Array (EOVSA)
obtained microwave spectral imaging of the spectacular eruptive solar
flare on 2017 September 10 in 2.5-18 GHz. During the early impulsive
phase of the flare ( 15:54 UT), An elongated microwave source appears
to connect the top of the flare arcade to the bottom of the erupting
magnetic flux rope. Multi-frequency images reveal that the source
bifurcates into two parts: One is located at and above the hard X-ray
looptop source, and another located behind the flux rope. They appear to
follow closely with the bi-directional reconnection downflow and upflow
region as inferred from the SDO/AIA EUV images. The spatially resolved
spectra of this microwave source show characteristics of gyrosynchrotron
radiation, suggesting the presence of high-energy (100s of keV to MeV)
electrons throughout the bi-directional reconnection outflow region. We
derive physical parameters of the source region, and discuss their
implications in magnetic energy release and electron acceleration.
---------------------------------------------------------
Title: Identifying Typical Mg II Flare Spectra Using Machine Learning
Authors: Panos, Brandon; Kleint, Lucia; Huwyler, Cedric; Krucker,
Säm; Melchior, Martin; Ullmann, Denis; Voloshynovskiy, Sviatoslav
2018ApJ...861...62P Altcode: 2018arXiv180510494P
The Interface Region Imaging Spectrograph (IRIS) performs
solar observations over a large range of atmospheric heights,
including the chromosphere where the majority of flare energy is
dissipated. The strong Mg II h&k spectral lines are capable of
providing excellent atmospheric diagnostics, but have not been fully
utilized for flaring atmospheres. We aim to investigate whether the
physics of the chromosphere is identical for all flare observations by
analyzing if there are certain spectra that occur in all flares. To
achieve this, we automatically analyze hundreds of thousands of Mg
II h&k-line profiles from a set of 33 flares and use a machine
learning technique, which we call supervised hierarchical k-means, to
cluster all profile shapes. We identify a single peaked Mg II profile,
in contrast to the double-peaked quiet Sun profiles, appearing in
every flare. Additionally, we find extremely broad profiles with
characteristic blueshifted central reversals appearing at the front of
fast-moving flare ribbons. These profiles occur during the impulsive
phase of the flare, and we present results of their temporal and spatial
correlation with non-thermal hard X-ray signatures, suggesting that
flare-accelerated electrons play an important role in the formation
of these profiles. The ratio of the integrated Mg II h&k lines
can also serve as an opacity diagnostic, and we find higher opacities
during each flare maximum. Our study shows that machine learning is
a powerful tool for large scale statistical solar analyses.
---------------------------------------------------------
Title: Microwave Spectroscopic Imaging of the Magnetic Reconnection
Region in the 2017 September 10 Eruptive Solar Flare
Authors: Chen, Bin; Gary, Dale E.; Fleishman, Gregory D.; Krucker,
Sam; Nita, Gelu M.; Dennis, Brian R.; Yu, Sijie; Kuroda, Natsuha;
Reeves, Katharine; Polito, Vanessa; Shih, Albert Y.
2018tess.conf30603C Altcode:
The newly commissioned Expanded Owens Valley Solar Array (EOVSA)
obtained excellent high-cadence (1 s), microwave spectroscopic imaging
of the spectacular eruptive solar flare on 2017 September 10 in 2.5-18
GHz. During the early impulsive phase of the flare (~15:53-15:55 UT),
EOVSA images reveal an elongated microwave source that connects the top
of the cusp-shaped flare arcade to the bottom of the erupting magnetic
flux rope. The spatially resolved spectra of this microwave source show
characteristics of gyrosynchrotron radiation, suggesting the presence
of high-energy nonthermal electrons throughout the source region that
presumably encloses the magnetic reconnection site(s) and bi-directional
reconnection outflows. In addition, the lower and upper portions of
the source seem to have different spatial and spectral properties. We
derive physical parameters of the source region, and discuss their
implications in magnetic energy release and electron acceleration.
---------------------------------------------------------
Title: The Focusing Optics X-ray Solar Imager (FOXSI)
Authors: Christe, Steven; Shih, Albert Y.; Krucker, Sam; Glesener,
Lindsay; Saint-Hilaire, Pascal; Caspi, Amir; Allred, Joel C.; Chen,
Bin; Battaglia, Marina; Drake, James Frederick; Gary, Dale E.; Goetz,
Keith; Gburek, Szymon; Grefenstette, Brian; Hannah, Iain G.; Holman,
Gordon; Hudson, Hugh S.; Inglis, Andrew R.; Ireland, Jack; Ishikawa,
Shin-nosuke; Klimchuk, James A.; Kontar, Eduard; Kowalski, Adam F.;
Massone, Anna Maria; Piana, Michele; Ramsey, Brian; Schwartz, Richard;
Steslicki, Marek; Ryan, Daniel; Warmuth, Alexander; Veronig, Astrid;
Vilmer, Nicole; White, Stephen M.; Woods, Thomas N.
2018tess.conf40444C Altcode:
We present FOXSI (Focusing Optics X-ray Solar Imager), a Small Explorer
(SMEX) Heliophysics mission that is currently undergoing a Phase A
concept study. FOXSI will provide a revolutionary new perspective on
energy release and particle acceleration on the Sun. FOXSI's primary
instrument, the Direct Spectroscopic Imager (DSI), is a direct imaging
X-ray spectrometer with higher dynamic range and better than 10x the
sensitivity of previous instruments. Flown on a 3-axis-stabilized
spacecraft in low-Earth orbit, DSI uses high-angular-resolution
grazing-incidence focusing optics combined with state-of-the-art
pixelated solid-state detectors to provide direct imaging of solar hard
X-rays for the first time. DSI is composed of a pair of X-ray telescopes
with a 14-meter focal length enabled by a deployable boom. DSI has a
field of view of 9 arcminutes and an angular resolution of better than 8
arcsec FWHM; it will cover the energy range from 3 up to 50-70 keV with
a spectral resolution of better than 1 keV. DSI will measure each photon
individually and will be able to create useful images at a sub-second
temporal resolution. FOXSI will also measure soft x-ray emission down
to 0.8 keV with a 0.25 keV resolution with its secondary instrument,
the Spectrometer for Temperature and Composition (STC) provided by
the Polish Academy of Sciences. Making use of an attenuator-wheel and
high-rate-capable detectors, FOXSI will be able to observe the largest
flares without saturation while still maintaining the sensitivity to
detect X-ray emission from weak flares, escaping electrons, and hot
active regions. This presentation will cover the data products and
software that can be expected from FOXSI and how they could be used
by the community.
---------------------------------------------------------
Title: NuSTAR X-ray observations of tiny solar flares
Authors: Hannah, Iain G.; Krucker, Sam; Grefenstette, Brian; Glesener,
Lindsay; Kuhar, Matej; Miles Smith, David; Kleint, Lucia
2018tess.conf40801H Altcode:
NuSTAR is an astrophysics X-ray telescope, with direct imaging
spectroscopy providing a unique sensitivity for observing the Sun
above 2.5keV. This is ideal for capturing the response of the solar
atmosphere to the energy released in the smallest flares. NuSTAR has
observed the Sun several times since Sep 2014 and we present some of
the initial observations of tiny microflares within active regions,
events down to the GOES A0.1 equivalent level. These microflares
show thermal emission up to 10MK, and the possibility of non-thermal
emission powering this heating. In conjunction with observations at
lower energy wavelengths (Hinode/XRT, SDO/AIA and IRIS) we are able
to present a more complete picture of the solar atmosphere's thermal
response to these small flares. We also show tiny impulsive events
outside of active regions, that are orders of magnitude smaller,
and yet still emit a clear X-ray signature.
---------------------------------------------------------
Title: NGSPM Objectives Addressed by the FOXSI SMEX Mission
Authors: Shih, Albert Y.; Christe, Steven; Krucker, Sam; Glesener,
Lindsay; Saint-Hilaire, Pascal; Caspi, Amir
2018tess.conf41005S Altcode:
In addition to the baseline instrument complement recommended by the
NGSPM study, the study also identified a hard X-ray spectroscopic
imager as the next-highest-priority instrument. While there have been
significant advances in our understanding of impulsive energy release
at the Sun since the advent of RHESSI X-ray observations, there is
a clear need for new X-ray observations that can capture the full
range of emission in flares (e.g., faint coronal sources near bright
chromospheric sources), follow the intricate evolution of energy
release and changes in morphology, and search for the signatures of
impulsive energy release in even the quiescent Sun. The NGSPM study
acknowledges grazing-incidence focusing X-ray optics as the way to
achieve spectroscopic imaging with the combination of high dynamic
range and high sensitivity. <P />The FOXSI Small Explorer (SMEX)
mission, currently undergoing a Phase A concept study, addresses this
measurement need within the envelope of a NASA SMEX mission. FOXSI's
X-ray observations will provide quantitative information on (1) the
non-thermal populations of accelerated electrons and (2) the thermal
plasma distributions at the high temperatures inaccessible through
other wavelengths. FOXSI is proposed to launch in 2022, and thus there
will be synergy between its X-ray observations and the observations
made at other wavelengths by the mission(s) recommended by the NGSPM
study. Here we present examples with simulated observations to show
how FOXSI's capabilities address NGSPM science objectives.
---------------------------------------------------------
Title: The Micro Solar-Flare Apparatus (MiSolFA)
Authors: Christe, Steven; Casadei, Diego; Krucker, Sam; Lastufka, Erica
2018tess.conf40237C Altcode:
Advances in the study of coronal electron-accelerating regions
have so far been limited by a single view-point. This means that we
have limited information about the three dimensional structure of
sources. The micro-satellite MiSolFA (Micro Solar-Flare Apparatus)
is designed both as a stand-alone X-ray imaging spectrometer and a
complement to the Spectrometer/Telescope for Imaging X-rays (STIX)
mission to provide stereoscopic x-ray imaging. <P />MiSolFA will be the
most compact X-ray imaging spectrometer in space. Thanks to absorbing
grids produced using a novel approach and to new CdTe photon detectors
with small pixel size and excellent energy resolution, MiSolFA will
perform indirect imaging between 10 and 100 keV and with 10 arcsec
angular resolution, sufficient to separate most hard X-ray footpoint
sources from each other. The instrument is small enough to equip a
6-units cubesat such as the GSFC-developed Dellingr platform.
---------------------------------------------------------
Title: RHESSI Observations of the 2017 Sep 10 Solar Flare
Authors: Shih, Albert Y.; Krucker, Sam; Dennis, Brian R.
2018tess.conf31502S Altcode:
We present soft X-ray and hard X-ray observations of the 2017 September
10 X-class flare by the Reuven Ramaty High-Energy Solar Spectroscopic
Imager (RHESSI). RHESSI observed this flare from ~6 keV to ~300
keV with an angular resolution of 7" FWHM. We use RHESSI images to
determine the spatial distributions of hot plasma and non-thermal
electrons and their evolution in time, including their associations
with coronal loops, chromospheric footpoints, and the large-scale
current sheet. We analyze RHESSI spectra to obtain the temperature
of the hot plasma and the energy in non-thermal electrons over the
course of the flare. Finally, we compare the RHESSI observations with
observations at other wavelengths.
---------------------------------------------------------
Title: NuSTAR Detection of X-Ray Heating Events in the Quiet Sun
Authors: Kuhar, Matej; Krucker, Säm; Glesener, Lindsay; Hannah,
Iain G.; Grefenstette, Brian W.; Smith, David M.; Hudson, Hugh S.;
White, Stephen M.
2018ApJ...856L..32K Altcode: 2018arXiv180308365K
The explanation of the coronal heating problem potentially lies in
the existence of nanoflares, numerous small-scale heating events
occurring across the whole solar disk. In this Letter, we present
the first imaging spectroscopy X-ray observations of three quiet
Sun flares during the Nuclear Spectroscopic Telescope ARray (NuSTAR)
solar campaigns on 2016 July 26 and 2017 March 21, concurrent with
the Solar Dynamics Observatory/Atmospheric Imaging Assembly (SDO/AIA)
observations. Two of the three events showed time lags of a few minutes
between peak X-ray and extreme ultraviolet emissions. Isothermal fits
with rather low temperatures in the range 3.2-4.1 MK and emission
measures of (0.6-15) × 10<SUP>44</SUP> cm<SUP>-3</SUP> describe
their spectra well, resulting in thermal energies in the range (2-6)
× 10<SUP>26</SUP> erg. NuSTAR spectra did not show any signs of a
nonthermal or higher temperature component. However, as the estimated
upper limits of (hidden) nonthermal energy are comparable to the thermal
energy estimates, the lack of a nonthermal component in the observed
spectra is not a constraining result. The estimated Geostationary
Operational Environmental Satellite (GOES) classes from the fitted
values of temperature and emission measure fall between 1/1000 and
1/100 A class level, making them eight orders of magnitude fainter in
soft X-ray flux than the largest solar flares.
---------------------------------------------------------
Title: The Focusing Optics X-ray Solar Imager (FOXSI) SMEX Mission
Authors: Christe, S.; Shih, A. Y.; Krucker, S.; Glesener, L.;
Saint-Hilaire, P.; Caspi, A.; Allred, J. C.; Battaglia, M.; Chen, B.;
Drake, J. F.; Gary, D. E.; Goetz, K.; Gburek, S.; Grefenstette, B.;
Hannah, I. G.; Holman, G.; Hudson, H. S.; Inglis, A. R.; Ireland,
J.; Ishikawa, S. N.; Klimchuk, J. A.; Kontar, E.; Kowalski, A. F.;
Massone, A. M.; Piana, M.; Ramsey, B.; Schwartz, R.; Steslicki, M.;
Turin, P.; Ryan, D.; Warmuth, A.; Veronig, A.; Vilmer, N.; White,
S. M.; Woods, T. N.
2017AGUFMSH44A..07C Altcode:
We present FOXSI (Focusing Optics X-ray Solar Imager), a Small Explorer
(SMEX) Heliophysics mission that is currently undergoing a Phase A
concept study. FOXSI will provide a revolutionary new perspective
on energy release and particle acceleration on the Sun. FOXSI is
a direct imaging X-ray spectrometer with higher dynamic range and
better than 10x the sensitivity of previous instruments. Flown
on a 3-axis-stabilized spacecraft in low-Earth orbit, FOXSI uses
high-angular-resolution grazing-incidence focusing optics combined
with state-of-the-art pixelated solid-state detectors to provide direct
imaging of solar hard X-rays for the first time. FOXSI is composed of
a pair of x-ray telescopes with a 14-meter focal length enabled by a
deployable boom. Making use of a filter-wheel and high-rate-capable
solid-state detectors, FOXSI will be able to observe the largest flares
without saturation while still maintaining the sensitivity to detect
x-ray emission from weak flares, escaping electrons, and hot active
regions. This mission concept is made possible by past experience with
similar instruments on two FOXSI sounding rocket flights, in 2012 and
2014, and on the HEROES balloon flight in 2013. FOXSI's hard X-ray
imager has a field of view of 9 arcminutes and an angular resolution
of better than 8 arcsec; it will cover the energy range from 3 up to
50-70 keV with a spectral resolution of better than 1 keV; and it will
have sub-second temporal resolution.
---------------------------------------------------------
Title: Combined SDO/AIA, Hinode/XRT and FOXSI-2 microflare
observations - DEM analysis and energetics
Authors: Panchapakesan, S. A.; Glesener, L.; Vievering, J. T.; Ryan,
D.; Christe, S.; Inglis, A. R.; Buitrago-Casas, J. C.; Musset, S.;
Krucker, S.
2017AGUFMSH41A2745P Altcode:
The Focusing Optics X-ray Solar Imager (FOXSI) sounding rocket makes
directimaging and spectral observation of the Sun in hard X-rays
(HXRs) using highlysensitive focusing HXR optics. The second flight
of FOXSI was launchedsuccessfully on 11 December 2014 and observed
significant HXR emissions duringmicroflares. Some of these flares
showed heating up to severalmillion Kelvin and were visible in
the Extreme Ultraviolet (EUV) with the AtmosphericImaging Assembly
(SDO/AIA). Spectral observations from FOXSI suggest emission upto
10-12 MK. We utilize SDO/AIA EUV, Hinode/XRT soft X-ray, and FOXSI-2
highenergy X-ray observations to derive the differential emission
measure (DEM) ofthe microflares. The AIA and XRT observations provide
broad temperaturecoverage but are poorly constrained at the hotter
end. We therefore use FOXSI-2to better determine the high temperature
component, thus producing a moreconstrained DEM than is possible with
typically available observations. We usethis more highly constrained
DEM to investigate the energetics of the observedmicroflares.
---------------------------------------------------------
Title: Tracing Fast Electron Beams Emanating from the Magnetic
Reconnection Site in a Solar Jet
Authors: Chen, B.; Yu, S.; Battaglia, M.; Krucker, S.
2017AGUFMSH43C..06C Altcode:
Fast electron beams propagating in the solar corona can emit radio waves
commonly known as type III radio bursts. At decimetric wavelengths,
these bursts are emitted from the low corona where flare energy release
is thought to take place. As such, decimetric type III radio bursts
can serve as an excellent tool to directly trace fast electron beams
in the vicinity of the flare energy release site. Here we report
observations of decimetric type III bursts during a jet event using
the Jansky Very Large Array (VLA) in 1-2 GHz. Taking advantage of
VLA's highly sensitive spectral imaging capability with an ultra-high
cadence of 50 ms, we derive detailed trajectories of fast electron
beams (with a bulk speed of at least 0.3-0.5c, or several tens of keV)
and place them in the context of extreme ultraviolet and X-ray images
obtained by SDO/AIA and RHESSI. Our results show that the electron beams
originated in a region just below the jet and above the lower-lying
small-scale flare loops, presumably where the magnetic energy release
took place. We show that the electron beams appear in groups, each
with a duration of only a few seconds. Each group, consisting of beams
propagating along magnetic field lines at different angles, is seen
to emanate from a single site trailing the jet, interpreted as the
magnetic reconnection null point. Our results suggest, at least for
the present case, that the fast electron beams were energized directly
at the magnetic reconnection site which was highly inhomogeneous and
fragmentary possibly down to kilometer scales.
---------------------------------------------------------
Title: Joint NuSTAR and IRIS observation of a microflaring active
region
Authors: Hannah, I. G.; Kleint, L.; Krucker, S.; Glesener, L.;
Grefenstette, B.
2017AGUFMSH41A2743H Altcode:
We present observations of a weakly microflaring active region observed
in X-rays with NuSTAR, UV with IRIS and EUV with SDO/AIA. NuSTAR
was pointed at this unnamed active region near the East limb between
23:27UT and 23:37UT 26-July-2016, finding mostly quiescent emission
except for a small microflare about 23:35UT. The NuSTAR spectrum
for the pre-microflare time (23:27UT to 23:34UT) is well fitted by a
single thermal component of about 3MK and combined with SDO/AIA we
can determine the differential emission measure (DEM), finding it,
as expected, drops very sharply to higher temperatures. During the
subsequent microflare, the increase in NuSTAR counts matches a little
brightening loop observed with IRIS SJI 1400Å and SDO/AIA. Fortuitously
the IRIS slit crosses this microflaring loop and we find an increased
emission in Si IV 1394Å, Si IV 1403Å and O IV 1402Å but only
average line widths and velocities. The NuSTAR microflare spectrum
shows heating to higher temperatures and also allows us to investigate
the energetics of this event.
---------------------------------------------------------
Title: Electron Beams Escaping the Sun: Hard X-ray Diagnostics of
Jet-related Electron Acceleration
Authors: Glesener, L.; Musset, S.; Saint-Hilaire, P.; Fleishman,
G. D.; Krucker, S.; Christe, S.; Shih, A. Y.
2017AGUFMSH23D2704G Altcode:
Coronal jets, which arise via an interaction between closed and open
magnetic field, offer a convenient configuration for accelerated
electrons to escape the low corona. Jets occur in all regions of the
Sun, but those flare-related jets that occur in active regions are
associated with bremsstrahlung hard X-rays (HXRs) from accelerated
electrons. However, HXR measurement of the escaping beams themselves
is elusive as it requires extremely high sensitivity. Jets are
strongly correlated with Type III radio bursts in the corona and in
interplanetary space. In this poster we present RHESSI observations of
HXRs from flare-related jets, including multiwavelength analysis (with
extreme ultraviolet and radio emission) and modeling of the emitting
electron populations. We also present predicted observations of Type
III-emitting electron beams by the FOXSI Small Explorer, which is
currently undergoing a NASA Phase A concept study. FOXSI will measure
HXRs from jets and flares in the low corona, providing quantitative
diagnostics of accelerated electron beams at their origin. These same
electron beams will be measured at higher altitudes by instruments
aboard NASA's Parker Solar Probe and ESA's Solar Orbiter. With a
planned launch in the rising phase of Solar Cycle 25, FOXSI will be
ideally timed and optimized for collaborative study of electron beams
escaping the Sun.
---------------------------------------------------------
Title: Investigation of Energy Release in Microflares Observed by
the Second Sounding Rocket Flight of the Focusing Optics X-ray Solar
Imager (FOXSI-2)
Authors: Vievering, J. T.; Glesener, L.; Panchapakesan, S. A.; Ryan,
D.; Krucker, S.; Christe, S.; Buitrago-Casas, J. C.; Inglis, A. R.;
Musset, S.
2017AGUFMSH41A2744V Altcode:
Observations of the Sun in hard x-rays can provide insight into many
solar phenomena which are not currently well-understood, including
the mechanisms behind particle acceleration in flares. RHESSI is
the only solar-dedicated imager currently operating in the hard
x-ray regime. Though RHESSI has greatly added to our knowledge of
flare particle acceleration, the indirect imaging method of rotating
collimating optics is fundamentally limited in sensitivity and dynamic
range. By instead using a direct imaging technique, the structure and
evolution of even small flares and active regions can be investigated
in greater depth. FOXSI (Focusing Optics X-ray Solar Imager), a hard
x-ray instrument flown on two sounding rocket campaigns, seeks to
achieve these improved capabilities by using focusing optics for solar
observations in the 4-20 keV range. During the second of the FOXSI
flights, flown on December 11, 2014, two microflares were observed,
estimated as GOES class A0.5 and A2.5 (upper limits). Here we present
current imaging and spectral analyses of these microflares, exploring
the nature of energy release and comparing to observations from other
instruments. Additionally, we feature the first analysis of data from
the FOXSI-2 CdTe strip detectors, which provide improved efficiency
above 10 keV. Through this analysis, we investigate the capabilities
of FOXSI in enhancing our knowledge of smaller-scale solar events.
---------------------------------------------------------
Title: Anticipated Results from the FOXSI SMEX Mission
Authors: Shih, A. Y.; Christe, S.; Krucker, S.; Glesener, L.;
Saint-Hilaire, P.; Caspi, A.; Allred, J. C.; Battaglia, M.; Chen, B.;
Drake, J. F.; Gary, D. E.; Gburek, S.; Goetz, K.; Grefenstette, B.;
Gubarev, M.; Hannah, I. G.; Holman, G.; Hudson, H. S.; Inglis, A. R.;
Ireland, J.; Ishikawa, S. N.; Klimchuk, J. A.; Kontar, E.; Kowalski,
A. F.; Massone, A. M.; Piana, M.; Ramsey, B.; Ryan, D.; Schwartz,
R.; Steslicki, M.; Turin, P.; Veronig, A.; Vilmer, N.; Warmuth, A.;
White, S. M.; Woods, T. N.
2017AGUFMSH43C..03S Altcode:
While there have been significant advances in our understanding
of impulsive energy release at the Sun since the advent of RHESSI
observations, there is a clear need for new X-ray observations that
can capture the full range of emission in flares (e.g., faint coronal
sources near bright chromospheric sources), follow the intricate
evolution of energy release and changes in morphology, and search
for the signatures of impulsive energy release in even the quiescent
Sun. The FOXSI Small Explorer (SMEX) mission, currently undergoing a
Phase A concept study, combines state-of-the-art grazing-incidence
focusing optics with pixelated solid-state detectors to provide
direct imaging of hard X-rays for the first time on a solar
observatory. FOXSI's X-ray observations will provide quantitative
information on (1) the non-thermal populations of accelerated electrons
and (2) the thermal plasma distributions at the high temperatures
inaccessible through other wavelengths. FOXSI's major science questions
include: Where are electrons accelerated and on what time scales? Where
do escaping flare-accelerated electrons originate? What is the energy
input of accelerated electrons into the chromosphere and corona? How
much do flare-like processes heat the corona above active regions? Here
we present examples with simulated observations to show how FOXSI's
capabilities will address and resolve these and other questions.
---------------------------------------------------------
Title: High-Energy Aspects of Small-Scale Energy Release at the Sun
Authors: Glesener, L.; Vievering, J. T.; Wright, P. J.; Hannah,
I. G.; Panchapakesan, S. A.; Ryan, D.; Krucker, S.; Hudson, H. S.;
Grefenstette, B.; White, S. M.; Smith, D. M.; Marsh, A.; Kuhar, M.;
Christe, S.; Buitrago-Casas, J. C.; Musset, S.; Inglis, A. R.
2017AGUFMSM33E..04G Altcode:
Large, powerful solar flares have been investigated in detail for
decades, but it is only recently that high-energy aspects of small
flares could be measured. These small-scale energy releases offer
the opportunity to examine how particle acceleration characteristics
scale down, which is critical for constraining energy transfer theories
such as magnetic reconnection. Probing to minuscule flare sizes also
brings us closer to envisioning the characteristics of the small
"nanoflares" that may be responsible for heating the corona. A new
window on small-scale flaring activity is now opening with the use of
focusing hard X-ray instruments to observe the Sun. Hard X-rays are
emitted by flare-accelerated electrons and strongly heated plasma,
providing a relatively direct method of measuring energy release
and particle acceleration properties. This work will show the first
observations of sub-A class microflares using the FOXSI sounding
rocket and the NuSTAR astrophysics spacecraft, both of which directly
focus hard X-rays but have limited observing time on the Sun. These
instruments serve as precursors to a spacecraft version of FOXSI, which
will explore energy release across the entire range of flaring activity.
---------------------------------------------------------
Title: The size of coronal hard X-ray sources in solar flares:
How big are they?
Authors: Effenberger, F.; Krucker, S.; Rubio da Costa, F.
2017AGUFMSH41A2746E Altcode:
Coronal hard X-ray sources are considered to be one of the key
signatures of non-thermal particle acceleration and heating during
the energy release in solar flares. In some cases, X-ray observations
reveal multiple components spatially located near and above the loop
top and even further up in the corona. Here, we combine a detailed
RHESSI imaging analysis of near-limb solar flares with occulted
footpoints and a multi-wavelength study of the flare loop evolution in
SDO/AIA. We connect our findings to different current sheet formation
and magnetic break-out scenarios and relate it to particle acceleration
theory. We find that the upper and usually fainter emission regions can
be underestimated in their size due to the majority of flux originating
from the lower loops.
---------------------------------------------------------
Title: First NuSTAR Limits on Quiet Sun Hard X-Ray Transient Events
Authors: Marsh, Andrew J.; Smith, David M.; Glesener, Lindsay;
Hannah, Iain G.; Grefenstette, Brian W.; Caspi, Amir; Krucker, Säm;
Hudson, Hugh S.; Madsen, Kristin K.; White, Stephen M.; Kuhar, Matej;
Wright, Paul J.; Boggs, Steven E.; Christensen, Finn E.; Craig,
William W.; Hailey, Charles J.; Harrison, Fiona A.; Stern, Daniel;
Zhang, William W.
2017ApJ...849..131M Altcode: 2017arXiv171105385M
We present the first results of a search for transient hard X-ray (HXR)
emission in the quiet solar corona with the Nuclear Spectroscopic
Telescope Array (NuSTAR) satellite. While NuSTAR was designed as
an astrophysics mission, it can observe the Sun above 2 keV with
unprecedented sensitivity due to its pioneering use of focusing
optics. NuSTAR first observed quiet-Sun regions on 2014 November 1,
although out-of-view active regions contributed a notable amount
of background in the form of single-bounce (unfocused) X-rays. We
conducted a search for quiet-Sun transient brightenings on timescales
of 100 s and set upper limits on emission in two energy bands. We set
2.5-4 keV limits on brightenings with timescales of 100 s, expressed
as the temperature T and emission measure EM of a thermal plasma. We
also set 10-20 keV limits on brightenings with timescales of 30, 60,
and 100 s, expressed as model-independent photon fluxes. The limits in
both bands are well below previous HXR microflare detections, though
not low enough to detect events of equivalent T and EM as quiet-Sun
brightenings seen in soft X-ray observations. We expect future
observations during solar minimum to increase the NuSTAR sensitivity
by over two orders of magnitude due to higher instrument livetime and
reduced solar background.
---------------------------------------------------------
Title: Detection of nanoflare-heated plasma in the solar corona by
the FOXSI-2 sounding rocket
Authors: Ishikawa, Shin-nosuke; Glesener, Lindsay; Krucker, Säm;
Christe, Steven; Buitrago-Casas, Juan Camilo; Narukage, Noriyuki;
Vievering, Juliana
2017NatAs...1..771I Altcode:
The processes that heat the solar and stellar coronae to several million
kelvins, compared with the much cooler photosphere (5,800 K for the
Sun), are still not well known<SUP>1</SUP>. One proposed mechanism
is heating via a large number of small, unresolved, impulsive heating
events called nanoflares<SUP>2</SUP>. Each event would heat and cool
quickly, and the average effect would be a broad range of temperatures
including a small amount of extremely hot plasma. However, detecting
these faint, hot traces in the presence of brighter, cooler emission
is observationally challenging. Here we present hard X-ray data from
the second flight of the Focusing Optics X-ray Solar Imager (FOXSI-2),
which detected emission above 7 keV from an active region of the Sun
with no obvious individual X-ray flare emission. Through differential
emission measure computations, we ascribe this emission to plasma
heated above 10 MK, providing evidence for the existence of solar
nanoflares. The quantitative evaluation of the hot plasma strongly
constrains the coronal heating models.
---------------------------------------------------------
Title: On the Nature of Off-limb Flare Continuum Sources Detected
by SDO/HMI
Authors: Heinzel, P.; Kleint, L.; Kašparová, J.; Krucker, S.
2017ApJ...847...48H Altcode: 2017arXiv170906377H
The Helioseismic and Magnetic Imager on board the Solar Dynamics
Observatory has provided unique observations of off-limb flare
emission. White-light continuum enhancements were detected in the
“continuum” channel of the Fe 6173 Å line during the impulsive
phase of the observed flares. In this paper we aim to determine which
radiation mechanism is responsible for such enhancement being seen above
the limb, at chromospheric heights around or below 1000 km. Using a
simple analytical approach, we compare two candidate mechanisms, the
hydrogen recombination continuum (Paschen) and the Thomson continuum
due to scattering of disk radiation on flare electrons. Both mechanisms
depend on the electron density, which is typically enhanced during the
impulsive phase of a flare as the result of collisional ionization (both
thermal and also non-thermal due to electron beams). We conclude that
for electron densities higher than 10<SUP>12</SUP> cm<SUP>-3</SUP>,
the Paschen recombination continuum significantly dominates the
Thomson scattering continuum and there is some contribution from the
hydrogen free-free emission. This is further supported by detailed
radiation-hydrodynamical (RHD) simulations of the flare chromosphere
heated by the electron beams. We use the RHD code FLARIX to compute the
temporal evolution of the flare-heating in a semi-circular loop. The
synthesized continuum structure above the limb resembles the off-limb
flare structures detected by HMI, namely their height above the limb,
as well as the radiation intensity. These results are consistent with
recent findings related to hydrogen Balmer continuum enhancements,
which were clearly detected in disk flares by the IRIS near-ultraviolet
spectrometer.
---------------------------------------------------------
Title: Calibration of the hard x-ray detectors for the FOXSI solar
sounding rocket
Authors: Athiray, P. S.; Buitrago-Casas, Juan Camilo; Bergstedt,
Kendra; Vievering, Juliana; Musset, Sophie; Ishikawa, Shin-nosuke;
Glesener, Lindsay; Takahashi, Tadayuki; Watanabe, Shin; Courtade,
Sasha; Christe, Steven; Krucker, Säm.; Goetz, Keith; Monson, Steven
2017SPIE10397E..0AA Altcode:
The Focusing Optics X-ray Solar Imager (FOXSI) sounding rocket
experiment conducts direct imaging and spectral observation of
the Sun in hard X-rays, in the energy range 4 to 20 keV. These
high-sensitivity observations are used to study particle acceleration
and coronal heating. FOXSI is designed with seven grazing incidence
optics modules that focus X-rays onto seven focal plane detectors kept
at a 2m distance. FOXSI-1 was flown with seven Double-sided Si Strip
Detectors (DSSD), and two of them were replaced with CdTe detectors
for FOXSI-2. The upcoming FOXSI-3 flight will carry DSSD and CdTe
detectors with upgraded optics for enhanced sensitivity. The detectors
are calibrated using various radioactive sources. The detector's
spectral response matrix was constructed with diagonal elements
using a Gaussian approximation with a spread (sigma) that accounts
for the energy resolution of the detector. Spectroscopic studies of
past FOXSI flight data suggest that the inclusion of lower energy
X-rays could better constrain the spectral modeling to yield a more
precise temperature estimation of the hot plasma. This motivates us to
carry out an improved calibration to better understand the finer-order
effects on the spectral response, especially at lower energies. Here
we report our improved calibration of FOXSI detectors using experiments
and Monte-Carlo simulations.
---------------------------------------------------------
Title: Microflare Heating of an Active Region Observed with NuSTAR,
Hinode/XRT, and SDO/AIA
Authors: Wright, Paul James; Hannah, Iain; Grefenstette, Brian;
Glesener, Lindsay; Krucker, Sam; Hudson, Hugh S.; Smith, David M.;
Marsh, Andrew; White, Stephen M.; Kuhar, Matej
2017SPD....4810802W Altcode:
We present the first joint observation of a GOES equivalent A0.2
microflare that occurred on the 29 Apr 2015 with Hinode/XRT and
NuSTAR. During the three hours of combined observation we observe
distinctive loop heating in the soft X-rays from Hinode/XRT, and
the hottest channels from SDO/AIA. Crucially the impulsive phase of
this microflare was also observed by NuSTAR, a highly sensitive hard
X-ray (2.5-80 keV; Harrison et al. 2013) focussing optics imaging
spectrometer. The NuSTAR spectrum before and after the microflare
is well-fitted by a single thermal model of about 3.3 - 3.5 MK, but
at the impulsive phase shows additional material up to 10 MK. This
higher temperature emission is confirmed when we produce the DEM
using a combination of SDO/AIA, Hinode/XRT, and NuSTAR data. During
the impulsive phase of the microflare we determine the heating rate to
be about 3 x 10<SUP>25</SUP> erg s<SUP>-1</SUP>. Although non-thermal
emission is not detected we find upper-limits that are consistent with
the required heating rate.
---------------------------------------------------------
Title: Methods for reducing singly reflected rays on the Wolter-I
focusing mirrors of the FOXSI rocket experiment
Authors: Buitrago-Casas, Juan Camilo; Elsner, Ronald; Glesener,
Lindsay; Christe, Steven; Ramsey, Brian; Courtade, Sasha; Ishikawa,
Shin-nosuke; Narukage, Noriyuki; Turin, Paul; Vievering, Juliana;
Athiray, P. S.; Musset, Sophie; Krucker, Säm.
2017SPIE10399E..0JB Altcode:
In high energy solar astrophysics, imaging hard X-rays by direct
focusing offers higher dynamic range and greater sensitivity compared to
past techniques that used indirect imaging. The Focusing Optics X-ray
Solar Imager (FOXSI) is a sounding rocket payload that uses seven sets
of nested Wolter-I figured mirrors together with seven high-sensitivity
semiconductor detectors to observe the Sun in hard X-rays through direct
focusing. The FOXSI rocket has successfully flown twice and is funded
to fly a third time in summer 2018. The Wolter-I geometry consists
of two consecutive mirrors, one paraboloid and one hyperboloid, that
reflect photons at grazing angles. Correctly focused X-rays reflect
once per mirror segment. For extended sources, like the Sun, off-axis
photons at certain incident angles can reflect on only one mirror and
still reach the focal plane, generating a background pattern of singly
reflected rays (i.e., ghost rays) that can limit the sensitivity of the
observation to faint, focused sources. Understanding and mitigating
the impact of the singly reflected rays on the FOXSI optical modules
will maximize the instruments' sensitivity to background-limited
sources. We present an analysis of the FOXSI singly reflected rays
based on ray-tracing simulations and laboratory measurements, as well
as the effectiveness of different physical strategies to reduce them.
---------------------------------------------------------
Title: Methods for reducing ghost rays on the Wolter-I focusing
figures of the FOXSI rocket payload
Authors: Buitrago-Casas, Juan Camilo; Glesener, Lindsay; Christe,
Steven; Ramsey, Brian; Elsner, Ronald; Courtade, Sasha; Vievering,
Juliana; Subramania, Athiray; Krucker, Sam; Bale, Stuart
2017SPD....4830504B Altcode:
In high energy solar astrophysics, imaging hard X-rays by direct
focusing offers higher dynamic range and greater sensitivity compared
to past techniques that used indirect imaging. The Focusing Optics
X-ray Solar Imager (FOXSI) is a sounding rocket payload which uses
seven sets of nested Wolter-I figured mirrors that, together with
seven high-sensitive semiconductor detectors, observes the Sun in hard
X-rays by direct focusing. The FOXSI rocket has successfully flown
twice and is funded to fly a third time in summer 2018.The Wolter-I
geometry consists of two consecutive mirrors, one paraboloid, and one
hyperboloid, that reflect photons at grazing angles. Correctly focused
X-rays reflect twice, once per mirror segment. For extended sources,
like the Sun, off-axis photons at certain incident angles can reflect
on only one mirror and still reach the focal plane, generating a
pattern of single-bounce photons, or ‘ghost rays’ that can limit
the sensitivity of the observation of focused X-rays. Understanding
and cutting down the ghost rays on the FOXSI optics will maximize the
instrument’s sensitivity of the solar faintest sources for future
flights. We present an analysis of the FOXSI ghost rays based on
ray-tracing simulations, as well as the effectiveness of different
physical strategies to reduce them.
---------------------------------------------------------
Title: NuSTAR's X-ray observations of a microflaring active region
Authors: Hannah, Iain; Kleint, Lucia; Krucker, Sam; Wright, Paul James;
Glesener, Lindsay; Grefenstette, Brian
2017SPD....4820101H Altcode:
We present observations of a weakly microflaring active region observed
in X-rays with NuSTAR, UV with IRIS and EUV with SDO/AIA. NuSTAR
was pointed at this unnamed active region near the East limb between
23:27UT and 23:37UT 26-July-2016, finding mostly quiescent emission
except for a small microflare about 23:35UT. The NuSTAR spectrum for
the pre-microflare time (23:27UT to 23:34UT) is well fitted by a single
thermal of about 3MK and combined with SDO/AIA we can determine the
differential emission measure (DEM), finding it, as expected, drops
very sharply to higher temperatures. During the subsequent microflare,
the increase in NuSTAR counts matches a little brightening loop observed
with IRIS SJI 1400Å and SDO/AIA 94Å/Fe XVIII. Fortuitously the IRIS
slit was on this microflaring loop and we find that the IRIS spectrum
shows increased emission in Si IV 1394Å, O IV 1402Å and Si IV 1403Å
but only average line widths and velocities. The NuSTAR microflare
spectrum shows heating to higher temperatures and also allows us to
investigate the energetics of this event.
---------------------------------------------------------
Title: Results from NuSTAR: Dynamics and time evolution in a sub-A
class hard X-ray flare
Authors: Glesener, Lindsay; Krucker, Sam; Hannah, Iain; Hudson,
Hugh S.; Grefenstette, Brian; White, Stephen M.; Smith, David M.;
Marsh, Andrew
2017SPD....4810803G Altcode:
We report a NuSTAR observation of a solar microflare,
SOL2015-09-01T04. Although it was too faint to be observed by the
GOES X-ray Sensor, we estimate the flare to be an A0.2 class flare in
brightness. This flare, with only ∼5 counts s-1 detector-1 observed
by RHESSI, is fainter than any hard X-ray (HXR) flare in the existing
literature. The flare occurred during a solar pointing by the highly
sensitive NuSTAR astrophysical observatory, which used its direct
focusing optics to produce detailed HXR flare spectra and images. The
flare exhibits HXR properties commonly observed in larger flares,
including a fast rise and more gradual decay, earlier peaking time with
higher energy, similar spatial dimensions to the RHESSI microflares,
and a high-energy excess beyond an isothermal spectral component
during the impulsive phase. The flare is small in emission measure,
temperature, and energy, though not in physical size; observations
are consistent with its arising via the interaction of at least two
magnetic loops. We estimate the increase in thermal energy at the time
of the flare to be 1.8×1027 ergs. The observation suggests that flares
do indeed scale down to extremely small energies and retain what we
customarily think of as “flarelike” properties.
---------------------------------------------------------
Title: Nonequilibrium Processes in the Solar Corona, Transition
Region, Flares, and Solar Wind (Invited Review)
Authors: Dudík, Jaroslav; Dzifčáková, Elena; Meyer-Vernet, Nicole;
Del Zanna, Giulio; Young, Peter R.; Giunta, Alessandra; Sylwester,
Barbara; Sylwester, Janusz; Oka, Mitsuo; Mason, Helen E.; Vocks,
Christian; Matteini, Lorenzo; Krucker, Säm; Williams, David R.;
Mackovjak, Šimon
2017SoPh..292..100D Altcode: 2017arXiv170603396D
We review the presence and signatures of the non-equilibrium processes,
both non-Maxwellian distributions and non-equilibrium ionization, in
the solar transition region, corona, solar wind, and flares. Basic
properties of the non-Maxwellian distributions are described
together with their influence on the heat flux as well as on the
rates of individual collisional processes and the resulting optically
thin synthetic spectra. Constraints on the presence of high-energy
electrons from observations are reviewed, including positive detection
of non-Maxwellian distributions in the solar corona, transition
region, flares, and wind. Occurrence of non-equilibrium ionization
is reviewed as well, especially in connection to hydrodynamic and
generalized collisional-radiative modeling. Predicted spectroscopic
signatures of non-equilibrium ionization depending on the assumed
plasma conditions are summarized. Finally, we discuss the future
remote-sensing instrumentation that can be used for the detection of
these non-equilibrium phenomena in various spectral ranges.
---------------------------------------------------------
Title: NuSTAR Hard X-Ray Observation of a Sub-A Class Solar Flare
Authors: Glesener, Lindsay; Krucker, Säm; Hannah, Iain G.; Hudson,
Hugh; Grefenstette, Brian W.; White, Stephen M.; Smith, David M.;
Marsh, Andrew J.
2017ApJ...845..122G Altcode: 2017arXiv170704770G
We report a Nuclear Spectroscopic Telescope Array (NuSTAR) observation
of a solar microflare, SOL2015-09-01T04. Although it was too faint
to be observed by the GOES X-ray Sensor, we estimate the event to be
an A0.1 class flare in brightness. This microflare, with only ∼5
counts s<SUP>-1</SUP> detector<SUP>-1</SUP> observed by the Reuven
Ramaty High Energy Solar Spectroscopic Imager (RHESSI), is fainter
than any hard X-ray (HXR) flare in the existing literature. The
microflare occurred during a solar pointing by the highly sensitive
NuSTAR astrophysical observatory, which used its direct focusing optics
to produce detailed HXR microflare spectra and images. The microflare
exhibits HXR properties commonly observed in larger flares, including a
fast rise and more gradual decay, earlier peak time with higher energy,
spatial dimensions similar to the RHESSI microflares, and a high-energy
excess beyond an isothermal spectral component during the impulsive
phase. The microflare is small in emission measure, temperature,
and energy, though not in physical size; observations are consistent
with an origin via the interaction of at least two magnetic loops. We
estimate the increase in thermal energy at the time of the microflare
to be 2.4 × 10<SUP>27</SUP> erg. The observation suggests that flares
do indeed scale down to extremely small energies and retain what we
customarily think of as “flare-like” properties.
---------------------------------------------------------
Title: UV and X-ray Evolution of AR12230 as Observed with IRIS
and FOXSI-II
Authors: Ryan, Daniel; Christe, Steven; Glesener, Lindsay; Vievering,
Julie; Krucker, Sam; Ishikawa, Shin-Nosuke
2017SPD....4820401R Altcode:
We present a multi-spectral and spatio-temporal analysis of AR12230
using both UV and X-ray spectroscopic imaging obtained as part of
a coordinated observing campaign on 11 December 2014. The campaign
involved IRIS (Interface Region Imaging Spectrometer) -- which provides
both UV imaging and slit spectrograph observations of optically thick
chromospheric and transition region emission -- and FOXSI-II (Focusing
Optics X-ray Solar Imager) -- the second in a series of sounding rocket
flights which combines grazing incidence direct focusing optics to
produce solar X-ray spectroscopic imaging in the range 4-15keV. The
active region exhibits a prolonged compact brightening in the IRIS
1330 A and 1400 A slit-jaw channels near the center of the active
region throughout the duration of the observations. In the early phase
of the observations FOXSI-II shows an X-ray source approximately
20x20 arcsec centered at the same location. The X-ray spectra show
the presence of hot (~8 MK) thermal plasma and is suggestive of the
presence of non-thermal electrons.. Later, two additional transient,
spatially extended, simultaneous brightenings are observed, one of
which was captured by the IRIS slit spectrograph. We combine these
observations to explore the evolution and topology of the active
region. Hydrodynamic modeling of the chromosphere is used to place
a limit on the amount of non-thermal electrons required to produce
the observed UV emission. This result is then compared to the limit
inferred from the FOXSI-II X-ray spectra. Thus, we explore the role
of non-thermal electrons and hydrodynamics in the energization and
evolution of plasma in active regions.
---------------------------------------------------------
Title: DEM analysis of FOXSI-2 microflare using AIA observations
Authors: Athiray Panchapakesan, Subramania; Glesener, Lindsay;
Vievering, Juliana; Camilo Buitrago-Casas, Juan; Christe, Steven;
Inglis, Andrew; Krucker, Sam; Musset, Sophie
2017SPD....4810632A Altcode:
The second flight of Focusing Optics X-ray Solar Imager (FOXSI) sounding
rocket experiment was successfully completed on 11 December 2014. FOXSI
makes direct imaging and spectral observation of the Sun in hard X-rays
using grazing incidence optics modules which focus X-rays onto seven
focal plane detectors kept at a 2m distance, in the energy range 4 to
20 keV, to study particle acceleration and coronal heating. Significant
HXR emissions were observed by FOXSI during microflare events with A0.5
and A2.5 class, as classified by GOES, that occurred during FOXSI-2
flight.Spectral analysis of FOXSI data for these events indicate
presence of plasma at higher temperatures (>10MK). We attempt to
study the plasma content in the corona at different temperatures,
characterized by the differential emission measure (DEM), over the
FOXSI-2 observed flare regions using the Atmospheric Imaging Assembly
(SDO/AIA) data. We utilize AIA observations in different EUV filters
that are sensitive to ionized iron lines, to determine the DEM by using
a regularized inversion method. This poster will show the properties
of hot plasma as derived from FOXSI-2 HXR spectra with supporting DEM
analysis using AIA observations.
---------------------------------------------------------
Title: Hard X-Ray Constraints on Small-Scale Coronal Heating Events
Authors: Marsh, Andrew; Smith, David M.; Glesener, Lindsay; Klimchuk,
James A.; Bradshaw, Stephen; Hannah, Iain; Vievering, Juliana;
Ishikawa, Shin-Nosuke; Krucker, Sam; Christe, Steven
2017SPD....4810614M Altcode:
A large body of evidence suggests that the solar corona is heated
impulsively. Small-scale heating events known as nanoflares may be
ubiquitous in quiet and active regions of the Sun. Hard X-ray (HXR)
observations with unprecedented sensitivity >3 keV have recently been
enabled through the use of focusing optics. We analyze active region
spectra from the FOXSI-2 sounding rocket and the NuSTAR satellite to
constrain the physical properties of nanoflares simulated with the
EBTEL field-line-averaged hydrodynamics code. We model a wide range
of X-ray spectra by varying the nanoflare heating amplitude, duration,
delay time, and filling factor. Additional constraints on the nanoflare
parameter space are determined from energy constraints and EUV/SXR data.
---------------------------------------------------------
Title: Microflare Heating of a Solar Active Region Observed with
NuSTAR, Hinode/XRT, and SDO/AIA
Authors: Wright, Paul J.; Hannah, Iain G.; Grefenstette, Brian W.;
Glesener, Lindsay; Krucker, Säm; Hudson, Hugh S.; Smith, David M.;
Marsh, Andrew J.; White, Stephen M.; Kuhar, Matej
2017ApJ...844..132W Altcode: 2017arXiv170606108W
NuSTAR is a highly sensitive focusing hard X-ray (HXR) telescope and has
observed several small microflares in its initial solar pointings. In
this paper, we present the first joint observation of a microflare
with NuSTAR and Hinode/XRT on 2015 April 29 at ∼11:29 UT. This
microflare shows the heating of material to several million Kelvin,
observed in soft X-rays with Hinode/XRT, and was faintly visible in
the extreme ultraviolet with SDO/AIA. For three of the four NuSTAR
observations of this region (pre-flare, decay, and post-flare phases),
the spectrum is well fitted by a single thermal model of 3.2-3.5 MK,
but the spectrum during the impulsive phase shows additional emission
up to 10 MK, emission equivalent to the A0.1 GOES class. We recover
the differential emission measure (DEM) using SDO/AIA, Hinode/XRT,
and NuSTAR, giving unprecedented coverage in temperature. We find that
the pre-flare DEM peaks at ∼3 MK and falls off sharply by 5 MK;
but during the microflare’s impulsive phase, the emission above 3
MK is brighter and extends to 10 MK, giving a heating rate of about
2.5× {10}<SUP>25</SUP> erg s<SUP>-1</SUP>. As the NuSTAR spectrum is
purely thermal, we determined upper limits on the possible non-thermal
bremsstrahlung emission. We find that for the accelerated electrons to
be the source of heating, a power-law spectrum of δ ≥slant 7 with a
low-energy cutoff {E}<SUB>c</SUB>≲ 7 keV is required. In summary, this
first NuSTAR microflare strongly resembles much more powerful flares.
---------------------------------------------------------
Title: The Solar X-Ray Limb
Authors: Battaglia, Marina; Hudson, Hugh S.; Hurford, Gordon J.;
Krucker, Säm; Schwartz, Richard A.
2017ApJ...843..123B Altcode: 2017arXiv170511044B
We describe a new technique to measure the height of the X-ray limb
with observations from occulted X-ray flare sources as observed
by the RHESSI (the Reuven Ramaty High-Energy Spectroscopic Imager)
satellite. This method has model dependencies different from those
present in traditional observations at optical wavelengths, which depend
upon detailed modeling involving radiative transfer in a medium with
complicated geometry and flows. It thus provides an independent and
more rigorous measurement of the “true” solar radius, which means
that of the mass distribution. RHESSI’s measurement makes use of the
flare X-ray source’s spatial Fourier components (the visibilities),
which are sensitive to the presence of the sharp edge at the lower
boundary of the occulted source. We have found a suitable flare event
for analysis, SOL2011-10-20T03:25 (M1.7), and report a first result
from this novel technique here. Using a four-minute integration
over the 3-25 keV photon energy range, we find {R}<SUB>{{X</SUB>} -
{ray}}=960.11+/- 0.15+/- 0.29 arcsec, at 1 au, where the uncertainties
include statistical uncertainties from the method and a systematic
error. The standard VAL-C model predicts a value of 959.94 arcsec,
which is about 1σ below our value.
---------------------------------------------------------
Title: On the Origin of the Flare Emission in IRIS’ SJI 2832
Filter:Balmer Continuum or Spectral Lines?
Authors: Kleint, Lucia; Heinzel, Petr; Krucker, Säm
2017ApJ...837..160K Altcode: 2017arXiv170207167K
Continuum (“white-light,” WL) emission dominates the energetics of
flares. Filter-based observations, such as the IRIS SJI 2832 filter,
show WL-like brightenings during flares, but it is unclear whether
the emission arises from real continuum emission or enhanced spectral
lines, possibly turning into emission. The difficulty in filter-based
observations, contrary to spectral observations, is to determine which
processes contribute to the observed brightening during flares. Here
we determine the contribution of the Balmer continuum and the spectral
line emission to IRIS’ SJI 2832 emission by analyzing the appropriate
passband in simultaneous IRIS NUV spectra. We find that spectral line
emission can contribute up to 100% to the observed slitjaw images (SJI)
emission, that the relative contributions usually temporally vary,
and that the highest SJI enhancements that are observed are most likely
because of the Balmer continuum. We conclude that care should be taken
when calling SJI 2832 a continuum filter during flares, because the
influence of the lines on the emission can be significant.
---------------------------------------------------------
Title: Hard X-Ray Emission from Partially Occulted Solar Flares:
RHESSI Observations in Two Solar Cycles
Authors: Effenberger, Frederic; Rubio da Costa, Fatima; Oka, Mitsuo;
Saint-Hilaire, Pascal; Liu, Wei; Petrosian, Vahé; Glesener, Lindsay;
Krucker, Säm
2017ApJ...835..124E Altcode: 2016arXiv161202856E
Flares close to the solar limb, where the footpoints are occulted,
can reveal the spectrum and structure of the coronal looptop source
in X-rays. We aim at studying the properties of the corresponding
energetic electrons near their acceleration site, without footpoint
contamination. To this end, a statistical study of partially occulted
flares observed with Reuven Ramaty High-Energy Solar Spectroscopic
Imager is presented here, covering a large part of solar cycles
23 and 24. We perform detailed spectra, imaging, and light curve
analyses for 116 flares and include contextual observations from
SDO and STEREO when available, providing further insights into flare
emission that were previously not accessible. We find that most spectra
are fitted well with a thermal component plus a broken power-law,
non-thermal component. A thin-target kappa distribution model gives
satisfactory fits after the addition of a thermal component. X-ray
imaging reveals small spatial separation between the thermal and
non-thermal components, except for a few flares with a richer coronal
source structure. A comprehensive light curve analysis shows a very good
correlation between the derivative of the soft X-ray flux (from GOES)
and the hard X-rays for a substantial number of flares, indicative of
the Neupert effect. The results confirm that non-thermal particles
are accelerated in the corona and estimated timescales support the
validity of a thin-target scenario with similar magnitudes of thermal
and non-thermal energy fluxes.
---------------------------------------------------------
Title: Fermi-LAT Observations of High-energy Behind-the-limb Solar
Flares
Authors: Ackermann, M.; Allafort, A.; Baldini, L.; Barbiellini, G.;
Bastieri, D.; Bellazzini, R.; Bissaldi, E.; Bonino, R.; Bottacini, E.;
Bregeon, J.; Bruel, P.; Buehler, R.; Cameron, R. A.; Caragiulo, M.;
Caraveo, P. A.; Cavazzuti, E.; Cecchi, C.; Charles, E.; Ciprini, S.;
Costanza, F.; Cutini, S.; D'Ammando, F.; de Palma, F.; Desiante, R.;
Digel, S. W.; Di Lalla, N.; Di Mauro, M.; Di Venere, L.; Drell, P. S.;
Favuzzi, C.; Fukazawa, Y.; Fusco, P.; Gargano, F.; Giglietto, N.;
Giordano, F.; Giroletti, M.; Grenier, I. A.; Guillemot, L.; Guiriec,
S.; Jogler, T.; Jóhannesson, G.; Kashapova, L.; Krucker, S.; Kuss,
M.; La Mura, G.; Larsson, S.; Latronico, L.; Li, J.; Liu, W.; Longo,
F.; Loparco, F.; Lubrano, P.; Magill, J. D.; Maldera, S.; Manfreda,
A.; Mazziotta, M. N.; Mitthumsiri, W.; Mizuno, T.; Monzani, M. E.;
Morselli, A.; Moskalenko, I. V.; Negro, M.; Nuss, E.; Ohsugi, T.;
Omodei, N.; Orlando, E.; Pal'shin, V.; Paneque, D.; Perkins, J. S.;
Pesce-Rollins, M.; Petrosian, V.; Piron, F.; Principe, G.; Rainò,
S.; Rando, R.; Razzano, M.; Reimer, O.; Rubio da Costa, F.; Sgrò,
C.; Simone, D.; Siskind, E. J.; Spada, F.; Spandre, G.; Spinelli,
P.; Tajima, H.; Thayer, J. B.; Torres, D. F.; Troja, E.; Vianello, G.
2017ApJ...835..219A Altcode: 2017arXiv170200577A
We report on the Fermi-LAT detection of high-energy emission from the
behind-the-limb (BTL) solar flares that occurred on 2013 October 11,
and 2014 January 6 and September 1. The Fermi-LAT observations are
associated with flares from active regions originating behind both
the eastern and western limbs, as determined by STEREO. All three
flares are associated with very fast coronal mass ejections (CMEs)
and strong solar energetic particle events. We present updated
localizations of the >100 MeV photon emission, hard X-ray (HXR)
and EUV images, and broadband spectra from 10 keV to 10 GeV, as well
as microwave spectra. We also provide a comparison of the BTL flares
detected by Fermi-LAT with three on-disk flares and present a study
of some of the significant quantities of these flares as an attempt
to better understand the acceleration mechanisms at work during these
occulted flares. We interpret the HXR emission to be due to electron
bremsstrahlung from a coronal thin-target loop top with the accelerated
electron spectra steepening at semirelativistic energies. The >100
MeV gamma-rays are best described by a pion-decay model resulting
from the interaction of protons (and other ions) in a thick-target
photospheric source. The protons are believed to have been accelerated
(to energies >10 GeV) in the CME environment and precipitate down to
the photosphere from the downstream side of the CME shock and landed
on the front side of the Sun, away from the original flare site and
the HXR emission.
---------------------------------------------------------
Title: Fermi Large Area Telescope Observations of High-Energy
Gamma-ray Emission From Behind-the-limb Solar Flares
Authors: Omodei, Nicola; Pesce-Rollins, Melissa; Petrosian, Vahe;
Liu, Wei; Rubio da Costa, Fatima; Golenetskii, Sergei; Kashapova,
Larisa; Krucker, Sam; Palshin, Valentin; Fermi Large Area Telescope
Collaboration
2017APS..APR.Y3005O Altcode:
Fermi LAT >30 MeV observations of the active Sun have increased
the number of detected solar flares by almost a factor of 10 with
respect to previous space observations. Of particular interest are the
recent detections of three solar flares whose position behind the limb
was confirmed by the STEREO-B spacecraft. These observations sample
flares from active regions originating from behind both the eastern
and western limbs and include an event associated with the second
ground level enhancement event (GLE) of the 24th Solar Cycle. While
gamma-ray emission up to tens of MeV resulting from proton interactions
has been detected before from occulted solar flares, the significance
of these particular events lies in the fact that these are the first
detections of >100 MeV gamma-ray emission from footpoint-occulted
flares. These detections present an unique opportunity to diagnose
the mechanisms of high-energy emission and particle acceleration and
transport in solar flares. We will present the Fermi-LAT, RHESSI and
STEREO observations of these flares and discuss the various emission
scenarios for these sources.
---------------------------------------------------------
Title: Evidence of Significant Energy Input in the Late Phase of a
Solar Flare from NuSTAR X-Ray Observations
Authors: Kuhar, Matej; Krucker, Säm; Hannah, Iain G.; Glesener,
Lindsay; Saint-Hilaire, Pascal; Grefenstette, Brian W.; Hudson, Hugh
S.; White, Stephen M.; Smith, David M.; Marsh, Andrew J.; Wright, Paul
J.; Boggs, Steven E.; Christensen, Finn E.; Craig, William W.; Hailey,
Charles J.; Harrison, Fiona A.; Stern, Daniel; Zhang, William W.
2017ApJ...835....6K Altcode: 2017arXiv170107759K
We present observations of the occulted active region AR 12222
during the third Nuclear Spectroscopic Telescope ARray (NuSTAR) solar
campaign on 2014 December 11, with concurrent Solar Dynamics Observatory
(SDO)/AIA and FOXSI-2 sounding rocket observations. The active region
produced a medium-size solar flare 1 day before the observations, at
∼18 UT on 2014 December 10, with the post-flare loops still visible
at the time of NuSTAR observations. The time evolution of the source
emission in the SDO/AIA 335 Å channel reveals the characteristics
of an extreme-ultraviolet late-phase event, caused by the continuous
formation of new post-flare loops that arch higher and higher in
the solar corona. The spectral fitting of NuSTAR observations yields
an isothermal source, with temperature 3.8-4.6 MK, emission measure
(0.3-1.8) × 10<SUP>46</SUP> cm<SUP>-3</SUP>, and density estimated at
(2.5-6.0) × 10<SUP>8</SUP> cm<SUP>-3</SUP>. The observed AIA fluxes
are consistent with the derived NuSTAR temperature range, favoring
temperature values in the range of 4.0-4.3 MK. By examining the
post-flare loops’ cooling times and energy content, we estimate that
at least 12 sets of post-flare loops were formed and subsequently cooled
between the onset of the flare and NuSTAR observations, with their total
thermal energy content an order of magnitude larger than the energy
content at flare peak time. This indicates that the standard approach
of using only the flare peak time to derive the total thermal energy
content of a flare can lead to a large underestimation of its value.
---------------------------------------------------------
Title: NuSTAR's X-ray search for high energy emission from weakly
flaring active regions
Authors: Hannah, I. G.; Grefenstette, B.; Glesener, L.; Krucker, S.;
Hudson, H. S.; Smith, D. M.; White, S.; Marsh, A.; Wright, P. J.;
Kuhar, M.
2016AGUFMSH11D..07H Altcode:
The NuSTAR X-ray focusing optics telescope, provides highly sensitivity
imaging spectroscopy over 2-78 keV. Most of NuSTAR's time is spent on
targets outside of the solar system but some is devoted to the Sun;
solar observations began in late 2014 (Grefenstette et al. 2016
ApJ). Although not optimized for solar observations it is highly
capable of searching for the weak X-ray emission from high temperature
(> 5MK) or non-thermal components from the weakly or non-flaring
Sun. Such emission provides strong constraints on the nature of
energy release during these more quiescent times. NuSTAR has observed
quiescent/non-flaring active regions from its first observations late in
2014, finding sources between 3.1-4.4 MK. These data placed strict upper
limits on higher temperature emission (Hannah et al. 2016 ApJL). These
observations had limited spectral dynamic range due to short effective
exposures (duration and high deadtime), restricting our ability to
detect higher temperature or non-thermal emission. With weakening
solar activity since then we present further observations during 2015
and 2016 of microflares and non-flaring active regions with longer
exposures (in part due to the decreasing detector deadtime). We also
present robust multi-thermal emission measure distributions obtained
by combining our NuSTAR observations with EUV data from SDO/AIA and
softer X-rays from Hinode/XRT.
---------------------------------------------------------
Title: Hard X-Ray Observations of Coronal Sources: Implications for
Particle Acceleration
Authors: Effenberger, F.; Rubio da Costa, F.; Oka, M.; Saint-Hilaire,
P.; Liu, W.; Krucker, S.; Glesener, L.; Petrosian, V.
2016AGUFMSH51E2634E Altcode:
The properties of hard X-ray emission from solar flares can provide
insight into particle acceleration and transport processes. Commonly,
at higher energies the bright footpoint emission from the flare loop
prevents a detailed analysis of the weaker loop-top source due to
the limited dynamic range. Thus, flares close to the solar limb,
where the footpoints are occulted, are interesting events to study
because they can reveal the coronal loop-top emission and thus the
electron properties at their acceleration site. We present results of
a survey study of partially occulted flares observed with the Reuven
Ramaty High-Energy Solar Spectroscopic Imager (RHESSI). We found that
most of the flare spectra allow a fit to a thermal plus non-thermal
component, either with a broken power-law or a kappa function. The
spatial separation between the thermal and non-thermal component,
as derived from imaging, is usually small. The light curve analysis
shows for many flares a very good correlation between the derivative
of the soft X-ray flux and the hard X-rays. We discuss implications
for particle acceleration models that result from our study.
---------------------------------------------------------
Title: Analysis of Microflares from the Second Sounding Rocket Flight
of the Focusing Optics X-ray Solar Imager (FOXSI-2)
Authors: Vievering, J. T.; Glesener, L.; Krucker, S.; Christe, S.;
Buitrago-Casas, J. C.; Ishikawa, S. N.; Ramsey, B.; Takahashi, T.;
Watanabe, S.
2016AGUFMSH11D..05V Altcode:
Observations of the sun in hard x-rays can provide insight into many
solar phenomena which are not currently well-understood, including
the mechanisms behind particle acceleration in flares. Currently,
RHESSI is the only solar-dedicated spacecraft observing in the hard
x-ray regime. Though RHESSI has greatly added to our knowledge of flare
particle acceleration, the method of rotation modulation collimators
is limited in sensitivity and dynamic range. By instead using a
direct imaging technique, the structure and evolution of even small
flares and active regions can be investigated in greater depth. FOXSI
(Focusing Optics X-ray Solar Imager), a hard x-ray instrument flown
on two sounding rocket campaigns, seeks to achieve these improved
capabilities by using focusing optics for solar observations in the 4-20
keV range. During the second of the FOXSI flights, flown on December 11,
2014, two microflares were observed, estimated as GOES class A0.5 and
A2.5 (upper limits). Preliminary analysis of these two flares will be
presented, including imaging spectroscopy, light curves, and photon
spectra. Through this analysis, we investigate the capabilities of
FOXSI in enhancing our knowledge of smaller-scale solar events.
---------------------------------------------------------
Title: Focusing Solar Hard X-rays: Expected Results from a FOXSI
Spacecraft
Authors: Glesener, L.; Christe, S.; Shih, A. Y.; Dennis, B. R.;
Krucker, S.; Saint-Hilaire, P.; Hudson, H. S.; Ryan, D.; Inglis,
A. R.; Hannah, I. G.; Caspi, A.; Klimchuk, J. A.; Drake, J. F.;
Kontar, E.; Holman, G.; White, S. M.; Alaoui, M.; Battaglia, M.;
Vilmer, N.; Allred, J. C.; Longcope, D. W.; Gary, D. E.; Jeffrey,
N. L. S.; Musset, S.; Swisdak, M.
2016AGUFMSH13A2282G Altcode:
Over the course of two solar cycles, RHESSI has examined high-energy
processes in flares via high-resolution spectroscopy and imaging of
soft and hard X-rays (HXRs). The detected X-rays are the thermal
and nonthermal bremsstrahlung from heated coronal plasma and from
accelerated electrons, respectively, making them uniquely suited to
explore the highest-energy processes that occur in the corona. RHESSI
produces images using an indirect, Fourier-based method and has made
giant strides in our understanding of these processes, but it has also
uncovered intriguing new mysteries regarding energy release location,
acceleration mechanisms, and energy propagation in flares. Focusing
optics are now available for the HXR regime and stand poised to perform
another revolution in the field of high-energy solar physics. With
two successful sounding rocket flights completed, the Focusing Optics
X-ray Solar Imager (FOXSI) program has demonstrated the feasibility and
power of direct solar HXR imaging with its vastly superior sensitivity
and dynamic range. Placing this mature technology aboard a spacecraft
will offer a systematic way to explore high-energy aspects of the
solar corona and to address scientific questions left unanswered by
RHESSI. Here we present examples of such questions and show simulations
of expected results from a FOXSI spaceborne instrument to demonstrate
how these questions can be addressed with the focusing of hard X-rays.
---------------------------------------------------------
Title: VLA imaging spectroscopic observations of decimetric radio
bursts in the impulsive phase of a C7.2 solar flare
Authors: Yu, S.; Chen, B.; Gray, L.; Prijatelj, M.; Kouzmenkov, T.;
Krucker, S.
2016AGUFMSH51E2632Y Altcode:
Broadband radio dynamic imaging spectroscopic observations at decimetric
wavelengths provide powerful diagnostics for magnetic energy release
and particle acceleration processes in solar flares. On November 1st,
2014, we used the Karl G. Janskey Very Large Array (VLA) to observe a
C7.2 flare in 1-2 GHz with 512 spectral channels and a 50 millisecond
cadence. A variety of decimetric radio bursts were recorded during
the impulsive phase of the flare, including type III radio bursts,
spike bursts, and other bursts with unusual appearance in the dynamic
spectrum. We use simultaneous imaging and dynamic spectroscopy to
obtain the location and morphology of the radio sources as well as
their evolution in time and frequency, which are used to elucidate the
physical mechanisms of the radio bursts and trace energetic electrons
in the flaring region. The flare was also observed by SDO/AIA in EUV
and RHESSI in hard X-rays. We discuss implications of our results on
the energy release and particle acceleration processes of this flare.
---------------------------------------------------------
Title: RHESSI/SAS Observations of the Optical Solar Limb Over More
Than 15 Years
Authors: Fivian, M. D.; Hudson, H. S.; Krucker, S.
2016AGUFMSH42B..08F Altcode:
The Solar Aspect System (SAS) of the RHESSI satellite measures the
optical solar limb with a cadence typically set at 100 samples/s.RHESSI
has observed the Sun continuously since its launch in early 2002, and we
have acquired a unique data set ranging over more than a full 11-year
solar cycle and consisting of about 4x10^10 single data points.The
optics has a point spread of about 4.5 arcsec FWHM imaging the red
continuum onto three linear CCD sensors with a pixel resolution of 1.7
arcsec.However, careful study of systematics, masking of contaminated
data, and accumulation of data over appropriate time intervals has
led to measurementswith sub-milli arcsec accuracy.Analyzing data for
an initial period in 2004, these measurements have led to the most
accurate oblateness measurement to date, 8.01+-0.14 milli arcsec
(Fivian et al., 2008), a value consistent with models predicting
an oblateness from surface rotation.An excess oblateness term can
be attributed to magnetic elements possibly located in the enhanced
network.We also study photometric properties of our data. Previous
observations of latitude-dependent brightness variations at the limb
had suggested the presence of a polar temperature excess as large
as 1.5 K.The RHESSI observations, made with a rotating telescope in
space, have great advantages in the rejection of systematic errors
in the very precise photometry required for such an observation.Our
measurements of latitude-dependent brightness variations at the limb
lead to a quadrupolar term (a pole-to-equator temperature variation)
of the order of 0.1 K, an order of magnitude smaller than previously
reported.We present the analysis of these unique data, an overview of
some results and we report on our progress as we apply our developed
analysis method to the whole 15 years of data.
---------------------------------------------------------
Title: The Solar injection of ten electron/<SUP>3</SUP>He-rich
SEP events
Authors: Wang, L.; Krucker, S.; Mason, G. M.; Li, G.
2016AGUFMSH51E2631W Altcode:
We have derived the particle injections at the Sun for ten good
electron/3He-rich solar energetic particle (SEP) events, using a
1.2 AU particle path length (suggested by analysis of the velocity
dispersion). The inferred solar injections of high-energy ( 10 to 300
keV) electrons and of ∼MeV/nucleon ions (carbon and heavier) start
with a delay of 17 ± 3 min and 75 ± 14 min, respectively, after
the injection of low-energy ( 0.4 to 9 keV) electrons. The injection
duration (averaged over energy) ranges from 200 to 550 min for ions,
from 90 to 160 min for low-energy electrons, and from 10 to 30 min for
high-energy electrons. Most of the selected events have no reported
Hα flares or GOES SXR bursts, but all have type III radio bursts that
typically start after the onset of a low-energy electron injection. All
nine events with SOHO/LASCO coverage have a relatively fast (>570 km
s-1), mostly narrow (< 30°), west-limb coronal mass ejection (CME)
that launches near the start of the low-energy electron injection,
and reaches an average altitude of 1.0 and 4.7 RS, respectively,
at the start of the high-energy electron injection and of the ion
injection. The electron energy spectra show a continuous power law
extending across the transition from low to high energies, suggesting
that the low-energy electron injection may provide seed electrons
for the delayed high-energy electron acceleration. The delayed ion
injections and high ionization states may suggest an ion acceleration
along the lower altitude flanks, rather than at the nose of the CMEs.
---------------------------------------------------------
Title: The Focusing Optics X-ray Solar Imager (FOXSI) SMEX Mission
Authors: Christe, S.; Shih, A. Y.; Krucker, S.; Glesener, L.;
Saint-Hilaire, P.; Caspi, A.; Allred, J. C.; Battaglia, M.; Chen,
B.; Drake, J. F.; Gary, D. E.; Goetz, K.; Grefenstette, B.; Hannah,
I. G.; Holman, G.; Hudson, H. S.; Inglis, A. R.; Ireland, J.; Ishikawa,
S. N.; Klimchuk, J. A.; Kontar, E.; Kowalski, A. F.; Massone, A. M.;
Piana, M.; Ramsey, B.; Gubarev, M.; Schwartz, R. A.; Steslicki, M.;
Ryan, D.; Turin, P.; Warmuth, A.; White, S. M.; Veronig, A.; Vilmer,
N.; Dennis, B. R.
2016AGUFMSH13A2281C Altcode:
We present FOXSI (Focusing Optics X-ray Solar Imager), a recently
proposed Small Explorer (SMEX) mission that will provide a revolutionary
new perspective on energy release and particle acceleration on the
Sun. FOXSI is a direct imaging X-ray spectrometer with higher dynamic
range and better than 10x the sensitivity of previous instruments. Flown
on a 3-axis stabilized spacecraft in low-Earth orbit, FOXSI uses
high-angular-resolution grazing-incidence focusing optics combined
with state-of-the-art pixelated solid-state detectors to provide direct
imaging of solar hard X-rays for the first time. FOXSI is composed of
two individual x-ray telescopes with a 14-meter focal length enabled by
a deployable boom. Making use of a filter-wheel and high-rate-capable
solid-state detectors, FOXSI will be able to observe the largest flares
without saturation while still maintaining the sensitivity to detect
x-ray emission from weak flares, escaping electrons, and hot active
regions. This SMEX mission is made possible by past experience with
similar instruments on two sounding rocket flights, in 2012 and 2014,
and on the HEROES balloon flight in 2013. FOXSI will image the Sun
with a field of view of 9 arcminutes and an angular resolution of
better than 8 arcsec; it will cover the energy range from 3 to 100
keV with a spectral resolution of better than 1 keV; and it will have
sub-second temporal resolution.
---------------------------------------------------------
Title: Simulation of Quiet-Sun Hard X-rays Related to Solar Wind
Superhalo Electrons
Authors: Wang, W.; Wang, L.; Krucker, S.; Hannah, I. G.
2016AGUFMSH51E2635W Altcode:
Abstract. In this paper, we propose that the accelerated electrons in
the quiet-Sun could collide with the solar atmosphere to emit Hard
X-rays (HXRs) via non-thermal bremsstrahlung, while some of these
electrons would move upwards and escape into the interplanetary
medium, to form a superhalo electron population measured in the
solar wind. After considering the electron energy loss due to Coulomb
collisions and the ambipolar electrostatic potential, we find that
the sources of superhalo could only occur high in the corona (at a
heliocentric altitude ≥ 1.9 Rs), to remain a power-law shape of
electron spectrum as observed by STEREO at 1 AU near solar minimum
(Wang et al, 2012).The modeled quiet-Sun HXRs related to the superhalo
electrons fit well to a power-law spectrum, f(ɛ) ∝ ɛ-γ, with
an index γ ≈ 2.0-2.3 (3.3-3.7) at 10-100 keV, for the warm/cold
thick-target (thin-target) emissions produced by the downward-traveling
(upward-traveling) accelerated electrons. These simulated quiet-Sun
spectra are significantly harder than the observed spectra of most
solar HXR flares. Assuming that the quiet-Sun sources cover 5% of
the solar surface, the modeled thin-target HXRs are more than six
orders of magnitude weaker than the RHESSI upper limits of quiet-Sun
HXRs (Hannah et al., 2010). Using the thick-target model for the
downward-traveling electrons, the RHESSI upper limits restrict the
number of downward-traveling electrons to maximal ≈ 3 times the number
of escaping electrons. This ratio is fundamentally different from what
is observed during solar flares associated with escaping electrons where
the fraction of downward-traveling electrons dominates by a factor of
100 to 1000 over the escaping population. References: 1. Hannah et
al., APJ, 724, 487(2010) 2. Wang et al., APJ Letters,753,L23(2012)
3. Yang et al., RAA,Vol.15,No.3,348-362(2015) 4. Brown J.C., Solar
Physics,Vol.18,Issue 3,489,502(1971)
---------------------------------------------------------
Title: Quiet-sun and non-flaring active region measurements from
the FOXSI-2 sounding rocket
Authors: Buitrago-Casas, J. C.; Glesener, L.; Christe, S.; Ishikawa,
S. N.; Narukage, N.; Krucker, S.; Bale, S. D.
2016AGUFMSH13A2280B Altcode:
Solar hard X-ray (HXR) emissions are a cornerstone for understanding
particle acceleration and energy release in the corona. These
phenomena are present at different size scales and intensities, from
large eruptive events down to the smallest flares. The presence of
HXRs in small, unresolved flares would provide direct evidence of
small reconnection events, i.e. nano-flares, that are thought to
be be important for the unsolved coronal heating problem. Currently
operating solar-dedicated instruments that observe HXRs from the Sun
do not have the dynamic range, nor the sensitivity, crucial to observe
the faintest solar HXRs. The Focusing Optics X-ray Solar Imager (FOXSI)
sounding rocket payload is a novel experiment that develops and applies
direct focusing optics coupled with semiconductor detectors to observe
faint HXRs from the Sun. The FOXSI rocket has successfully completed
two flights, observing areas of the quiet-Sun, active regions and
micro-flares. We present recent data analysis to test the presence of
hot plasma in and outside of active regions observed during the two
flights, focusing on the differential emission measure distribution
of the non-flaring corona.
---------------------------------------------------------
Title: The FIELDS Instrument Suite for Solar Probe Plus. Measuring
the Coronal Plasma and Magnetic Field, Plasma Waves and Turbulence,
and Radio Signatures of Solar Transients
Authors: Bale, S. D.; Goetz, K.; Harvey, P. R.; Turin, P.; Bonnell,
J. W.; Dudok de Wit, T.; Ergun, R. E.; MacDowall, R. J.; Pulupa,
M.; Andre, M.; Bolton, M.; Bougeret, J. -L.; Bowen, T. A.; Burgess,
D.; Cattell, C. A.; Chandran, B. D. G.; Chaston, C. C.; Chen,
C. H. K.; Choi, M. K.; Connerney, J. E.; Cranmer, S.; Diaz-Aguado, M.;
Donakowski, W.; Drake, J. F.; Farrell, W. M.; Fergeau, P.; Fermin, J.;
Fischer, J.; Fox, N.; Glaser, D.; Goldstein, M.; Gordon, D.; Hanson,
E.; Harris, S. E.; Hayes, L. M.; Hinze, J. J.; Hollweg, J. V.; Horbury,
T. S.; Howard, R. A.; Hoxie, V.; Jannet, G.; Karlsson, M.; Kasper,
J. C.; Kellogg, P. J.; Kien, M.; Klimchuk, J. A.; Krasnoselskikh,
V. V.; Krucker, S.; Lynch, J. J.; Maksimovic, M.; Malaspina, D. M.;
Marker, S.; Martin, P.; Martinez-Oliveros, J.; McCauley, J.; McComas,
D. J.; McDonald, T.; Meyer-Vernet, N.; Moncuquet, M.; Monson, S. J.;
Mozer, F. S.; Murphy, S. D.; Odom, J.; Oliverson, R.; Olson, J.;
Parker, E. N.; Pankow, D.; Phan, T.; Quataert, E.; Quinn, T.; Ruplin,
S. W.; Salem, C.; Seitz, D.; Sheppard, D. A.; Siy, A.; Stevens, K.;
Summers, D.; Szabo, A.; Timofeeva, M.; Vaivads, A.; Velli, M.; Yehle,
A.; Werthimer, D.; Wygant, J. R.
2016SSRv..204...49B Altcode: 2016SSRv..tmp...16B
NASA's Solar Probe Plus (SPP) mission will make the first in situ
measurements of the solar corona and the birthplace of the solar
wind. The FIELDS instrument suite on SPP will make direct measurements
of electric and magnetic fields, the properties of in situ plasma waves,
electron density and temperature profiles, and interplanetary radio
emissions, amongst other things. Here, we describe the scientific
objectives targeted by the SPP/FIELDS instrument, the instrument
design itself, and the instrument concept of operations and planned
data products.
---------------------------------------------------------
Title: The FOXSI solar sounding rocket campaigns
Authors: Glesener, Lindsay; Krucker, Säm.; Christe, Steven; Ishikawa,
Shin-nosuke; Buitrago-Casas, Juan Camilo; Ramsey, Brian; Gubarev,
Mikhail; Takahashi, Tadayuki; Watanabe, Shin; Takeda, Shin'ichiro;
Courtade, Sasha; Turin, Paul; McBride, Stephen; Shourt, Van; Hoberman,
Jane; Foster, Natalie; Vievering, Juliana
2016SPIE.9905E..0EG Altcode:
The Focusing Optics X-ray Solar Imager (FOXSI) is, in its initial form,
a sounding rocket experiment designed to apply the technique of focusing
hard X-ray (HXR) optics to the study of fundamental questions about the
high-energy Sun. Solar HXRs arise via bremsstrahlung from energetic
electrons and hot plasma produced in solar flares and thus are one
of the most direct diagnostics of are-accelerated electrons and the
impulsive heating of the solar corona. Previous missions have always
been limited in sensitivity and dynamic range by the use of indirect
(Fourier) imaging due to the lack of availability of direct focusing
optics, but technological advances now make direct focusing accessible
in the HXR regime (as evidenced by the NuSTAR spacecraft and several
suborbital missions). The FOXSI rocket experiment develops and optimizes
HXR focusing telescopes for the unique scientific requirements of
the Sun. To date, FOXSI has completed two successful flights on 2012
November 02 and 2014 December 11 and is funded for a third flight. This
paper gives a brief overview of the experiment, which is sensitive
to solar HXRs in the 4-20 keV range, describes its first two flights,
and gives a preview of plans for FOXSI-3.
---------------------------------------------------------
Title: Fine-pitch CdTe detector for hard X-ray imaging and
spectroscopy of the Sun with the FOXSI rocket experiment
Authors: Ishikawa, Shin-nosuke; Katsuragawa, Miho; Watanabe, Shin;
Uchida, Yuusuke; Takeda, Shin'ichiro; Takahashi, Tadayuki; Saito,
Shinya; Glesener, Lindsay; Buitrago-Casas, Juan Camilo; Krucker,
Säm.; Christe, Steven
2016JGRA..121.6009I Altcode: 2016arXiv160603887I
We have developed a fine-pitch hard X-ray (HXR) detector using a cadmium
telluride (CdTe) semiconductor for imaging and spectroscopy for the
second launch of the Focusing Optics Solar X-ray Imager (FOXSI). FOXSI
is a rocket experiment to perform high sensitivity HXR observations
from 4 to 15 keV using the new technique of HXR focusing optics. The
focal plane detector requires <100μm position resolution (to take
advantage of the angular resolution of the optics) and ≈1 keV energy
resolution (full width at half maximum (FWHM)) for spectroscopy down
to 4 keV, with moderate cooling (>-30°C). Double-sided silicon
strip detectors were used for the first FOXSI flight in 2012 to meet
these criteria. To improve the detectors' efficiency (66% at 15 keV
for the silicon detectors) and position resolution of 75 μm for the
second launch, we fabricated double-sided CdTe strip detectors with a
position resolution of 60 μm and almost 100% efficiency for the FOXSI
energy range. The sensitive area is 7.67 mm × 7.67 mm, corresponding
to the field of view of 791” × 791”. An energy resolution of 1 keV
(FWHM) and low-energy threshold of ≈4 keV were achieved in laboratory
calibrations. The second launch of FOXSI was performed on 11 December
2014, and images from the Sun were successfully obtained with the CdTe
detector. Therefore, we successfully demonstrated the detector concept
and the usefulness of this technique for future HXR observations of
the Sun.
---------------------------------------------------------
Title: The injection of ten electron/3He-rich SEP events
Authors: Wang, Linghua; Krucker, Säm; Mason, Glenn M.; Li, Gang
2016shin.confE...1W Altcode:
We have derived the particle injections at the Sun for ten good
electron/3He-rich solar energetic particle (SEP) events, using a
1.2 AU particle path length (suggested by analysis of the velocity
dispersion). The inferred solar injections of high-energy ( 10 to 300
keV) electrons and of MeV/nucleon ions (carbon and heavier) start
with a delay of 17 ± 3 min and 75 ± 14 min, respectively, after
the injection of low-energy ( 0.4 to 9 keV) electrons. The injection
duration (averaged over energy) ranges from 200 to 550 min for ions,
from ∼90 to 160 min for low-energy electrons, and from 10 to 30 min
for high-energy electrons. Most of the selected events have no reported
Hα flares or GOES SXR bursts, but all have type III radio bursts that
typically start after the onset of a low-energy electron injection. All
nine events with SOHO/LASCO coverage have a relatively fast (>570
km/s), mostly narrow (< 30°), west-limb coronal mass ejection (CME)
that launches near the start of the low-energy electron injection,
and reaches an average altitude of ∼1.0 and 4.7 RS, respectively,
at the start of the high-energy electron injection and of the ion
injection. The electron energy spectra show a continuous power law
extending across the transition from low to high energies, suggesting
that the low-energy electron injection may provide seed electrons
for the delayed high-energy electron acceleration. The delayed ion
injections and high ionization states may suggest an ion acceleration
along the lower altitude flanks, rather than at the nose of the CMEs.
---------------------------------------------------------
Title: Flight production of Caliste-SO: the hard x-ray spectrometers
for solar orbiter/STIX instrument
Authors: Limousin, Olivier; Meuris, Aline; Gevin, Olivier; Blondel,
Claire; Donati, Modeste; Dumaye, Luc; Le Mer, Isabelle; Martignac,
Jérôme; Tourrette, Thierry; Vassal, Marie-Cécile; Blain, Dominique;
Boussadia, Mohamed; Fiant, Nicolas; Soufflet, Fabrice; Bednarzik,
Martin; Birrer, Guy; Stutz, Stefan; Wild, Christopher; Billot, Marc;
Fratter, Isabelle; Grimm, Oliver; Krucker, Säm.
2016SPIE.9905E..0FL Altcode:
Caliste-SO are CdTe hybrid detectors that will be used as spectrometer
units in the Spectrometer Telescope for Imaging X-rays (STIX)
on-board the Solar Orbiter space mission. Each unit is placed below
one collimator of this Fourier telescope to measure one visibility of
the image in the 4-150 keV energy range, with a spectral resolution of
1 keV FWHM at 6 keV. The paper presents the scientific requirements,
the design, the fabrication and the tests of the Caliste- SO devices
before mounting them onto printed circuits boards. Spectral response
was characterized on the 98 spacegrade units for various operating
parameters. The devices will equip the different instrument validation
models, including 32 units for the final instrument flight model to
be launched in 2018.
---------------------------------------------------------
Title: The First Focused Hard X-ray Images of the Sun with NuSTAR
Authors: Grefenstette, Brian W.; Glesener, Lindsay; Krucker, Säm;
Hudson, Hugh; Hannah, Iain G.; Smith, David M.; Vogel, Julia K.; White,
Stephen M.; Madsen, Kristin K.; Marsh, Andrew J.; Caspi, Amir; Chen,
Bin; Shih, Albert; Kuhar, Matej; Boggs, Steven E.; Christensen, Finn
E.; Craig, William W.; Forster, Karl; Hailey, Charles J.; Harrison,
Fiona A.; Miyasaka, Hiromasa; Stern, Daniel; Zhang, William W.
2016ApJ...826...20G Altcode: 2016arXiv160509738G
We present results from the the first campaign of dedicated solar
observations undertaken by the Nuclear Spectroscopic Telescope ARray
(NuSTAR) hard X-ray (HXR) telescope. Designed as an astrophysics
mission, NuSTAR nonetheless has the capability of directly imaging the
Sun at HXR energies (>3 keV) with an increase in sensitivity of at
least two magnitude compared to current non-focusing telescopes. In
this paper we describe the scientific areas where NuSTAR will make
major improvements on existing solar measurements. We report on the
techniques used to observe the Sun with NuSTAR, their limitations
and complications, and the procedures developed to optimize solar
data quality derived from our experience with the initial solar
observations. These first observations are briefly described, including
the measurement of the Fe K-shell lines in a decaying X-class flare,
HXR emission from high in the solar corona, and full-disk HXR images
of the Sun.
---------------------------------------------------------
Title: Implications Of The Mid-IR For ALMA Flare Observations
Authors: Hudson, Hugh; Krucker, Sam; Penn, Matt; Simoes, Paulo
2016csss.confE..49H Altcode:
Poster presented at the Cool Stars 19 meeting in Uppsala, Sweden,
June 2016
---------------------------------------------------------
Title: The Focusing Optics X-ray Solar Imager Small Explorer Concept
Mission
Authors: Christe, Steven; Shih, Albert Y.; Dennis, Brian R.; Glesener,
Lindsay; Krucker, Sam; Saint-Hilaire, Pascal; Gubarev, Mikhail;
Ramsey, Brian
2016SPD....47.0802C Altcode:
We present the FOXSI (Focusing Optics X-ray Solar Imager) small explorer
(SMEX) concept, a mission dedicated to studying particle acceleration
and energy release on the Sun. FOXSI is designed as a 3-axis stabilized
spacecraft in low-Earth orbit making use of state-of-the-art grazing
incidence focusing optics combined withpixelated solid-state detectors,
allowing for direct imaging of solar X-rays. The current design
being studied features multiple telescopes with a 14 meter focal
length enabled by a deployable boom.FOXSI will observe the Sun in the
3-100 keV energy range. The FOXSI imaging concept has already been
tested on two sounding rocket flights, in 2012 and 2014 and on the
HEROES balloon payload flight in 2013. FOXSI will image the Sun with
an angular resolution of 5”, a spectral resolution of 0.5 keV, and
sub-second temporal resolution. FOXSI is a direct imaging spectrometer
with high dynamic range and sensitivity and will provide a brand-new
perspective on energy release on the Sun. We describe the mission and
its science objectives.
---------------------------------------------------------
Title: The smallest hard X-ray flare?
Authors: Glesener, Lindsay; Krucker, Sam; Hannah, Iain; Smith, David
M.; Grefenstette, Brian; Marsh, Andrew; Hudson, Hugh S.; White,
Stephen M.; Chen, Bin
2016SPD....4740302G Altcode:
We report a NuSTAR observation of a small solar flare on 2015
September 1, estimated to be on the order of a GOES class A.05 flare
in brightness. This flare is fainter than any hard X-ray (HXR) flares
in the existing literature, and with a peak rate of only ∼5 counts
s<SUP>-1</SUP> detector<SUP>-1</SUP> observed by RHESSI, is effectively
the smallest that can just barely be detected by the current standard
(indirectly imaging) solar HXR instrumentation, though we expect
that smaller flares will continue to be discovered as instrumental
and observational techniques progress. The flare occurred during a
solar observation by the highly sensitive NuSTAR astrophysical HXR
spacecraft, which used its direct focusing optics to produce detailed
flare spectra and images. The flare exhibits properties commonly
observed in larger flares, including a fast rise and more gradual
decay, and similar spatial dimensions to the RHESSI microflares. We
will discuss the presence of non-thermal (flare-accelerated) electrons
during the impulsive phase. The flare is small in emission measure,
temperature, and energy, though not in physical dimensions. Its presence
is an indication that flares do indeed scale down to smaller energies
and retain what we customarily think of as “flarelike” properties.
---------------------------------------------------------
Title: The FOXSI sounding rocket: Latest analysis and results
Authors: Buitrago-Casas, Juan Camilo; Glesener, Lindsay; Christe,
Steven; Krucker, Sam; Ishikawa, Shin-Nosuke; Takahashi, Tadayuki;
Ramsey, Brian; Han, Raymond
2016SPD....4740702B Altcode: 2016SPD....4740702C
Hard X-ray (HXR) observations are a linchpin for studying
particle acceleration and hot thermal plasma emission in the solar
corona. Current and past indirectly imaging instruments lack the
sensitivity and dynamic range needed to observe faint HXR signatures,
especially in the presences of brighter sources. These limitations
are overcome by using HXR direct focusing optics coupled with
semiconductor detectors. The Focusing Optics X-ray Solar Imager (FOXSI)
sounding rocket experiment is a state of the art solar telescope that
develops and applies these capabilities.The FOXSI sounding rocket has
successfully flown twice, observing active regions, microflares, and
areas of the quiet-Sun. Thanks to its far superior imaging dynamic
range, FOXSI performs cleaner hard X-ray imaging spectroscopy than
previous instruments that use indirect imaging methods like RHESSI.We
present a description of the FOXSI rocket payload, paying attention
to the optics and semiconductor detectors calibrations, as well as
the upgrades made for the second flight. We also introduce some of
the latest FOXSI data analysis, including imaging spectroscopy of
microflares and active regions observed during the two flights, and
the differential emission measure distribution of the nonflaring corona.
---------------------------------------------------------
Title: Science Objectives of the FOXSI Small Explorer Mission Concept
Authors: Shih, Albert Y.; Christe, Steven; Alaoui, Meriem; Allred,
Joel C.; Antiochos, Spiro K.; Battaglia, Marina; Buitrago-Casas,
Juan Camilo; Caspi, Amir; Dennis, Brian R.; Drake, James; Fleishman,
Gregory D.; Gary, Dale E.; Glesener, Lindsay; Grefenstette, Brian;
Hannah, Iain; Holman, Gordon D.; Hudson, Hugh S.; Inglis, Andrew R.;
Ireland, Jack; Ishikawa, Shin-Nosuke; Jeffrey, Natasha; Klimchuk, James
A.; Kontar, Eduard; Krucker, Sam; Longcope, Dana; Musset, Sophie; Nita,
Gelu M.; Ramsey, Brian; Ryan, Daniel; Saint-Hilaire, Pascal; Schwartz,
Richard A.; Vilmer, Nicole; White, Stephen M.; Wilson-Hodge, Colleen
2016SPD....47.0814S Altcode:
Impulsive particle acceleration and plasma heating at the Sun, from the
largest solar eruptive events to the smallest flares, are related to
fundamental processes throughout the Universe. While there have been
significant advances in our understanding of impulsive energy release
since the advent of RHESSI observations, there is a clear need for
new X-ray observations that can capture the full range of emission
in flares (e.g., faint coronal sources near bright chromospheric
sources), follow the intricate evolution of energy release and changes
in morphology, and search for the signatures of impulsive energy
release in even the quiescent Sun. The FOXSI Small Explorer (SMEX)
mission concept combines state-of-the-art grazing-incidence focusing
optics with pixelated solid-state detectors to provide direct imaging
of hard X-rays for the first time on a solar observatory. We present
the science objectives of FOXSI and how its capabilities will address
and resolve open questions regarding impulsive energy release at the
Sun. These questions include: What are the time scales of the processes
that accelerate electrons? How do flare-accelerated electrons escape
into the heliosphere? What is the energy input of accelerated electrons
into the chromosphere, and how is super-heated coronal plasma produced?
---------------------------------------------------------
Title: Simulation of Quiet-Sun Hard X-Rays Related to Solar Wind
Superhalo Electrons
Authors: Wang, Wen; Wang, Linghua; Krucker, Säm; Hannah, Iain
2016SoPh..291.1357W Altcode: 2016arXiv160506339W; 2016SoPh..tmp...75W
In this paper, we propose that the accelerated electrons in the quiet
Sun could collide with the solar atmosphere to emit Hard X-rays (HXRs)
via non-thermal bremsstrahlung, while some of these electrons would
move upwards and escape into the interplanetary medium, to form
a superhalo electron population measured in the solar wind. After
considering the electron energy loss due to Coulomb collisions and
the ambipolar electrostatic potential, we find that the sources of
the superhalo could only occur high in the corona (at a heliocentric
altitude ≳1.9 R<SUB>⊙</SUB> (the mean radius of the Sun)), to
remain a power-law shape of electron spectrum as observed by Solar
Terrestrial Relations Observatory (STEREO) at 1 AU near solar minimum
(Wang et al. in Astrophys. J. Lett.753, L23, 2012). The modeled
quiet-Sun HXRs related to the superhalo electrons fit well to a
power-law spectrum, f ∼ε<SUP>−γ</SUP> in the photon energy
ε , with an index γ ≈2.0 -2.3 (3.3 - 3.7) at 10 - 100 keV, for
the warm/cold-thick-target (thin-target) emissions produced by the
downward-traveling (upward-traveling) accelerated electrons. These
simulated quiet-Sun spectra are significantly harder than the observed
spectra of most solar HXR flares. Assuming that the quiet-Sun sources
cover 5 % of the solar surface, the modeled thin-target HXRs are more
than six orders of magnitude weaker than the Reuven Ramaty High Energy
Solar Spectroscopic Imager (RHESSI) upper limit for quiet-Sun HXRs
(Hannah et al. in Astrophys. J.724, 487, 2010). Using the thick-target
model for the downward-traveling electrons, the RHESSI upper limit
restricts the number of downward-traveling electrons to at most ≈3
times the number of escaping electrons. This ratio is fundamentally
different from what is observed during solar flares associated with
escaping electrons where the fraction of downward-traveling electrons
dominates by a factor of 100 to 1000 over the escaping population.
---------------------------------------------------------
Title: RHESSI/SAS Observations of the Optical Solar Limb Over More
Than 14 Years
Authors: Fivian, Martin; Hudson, Hugh S.; Krucker, Sam
2016SPD....47.1204F Altcode:
The Solar Aspect System (SAS) of the RHESSI satellite measures the
optical solar limb with a cadence typically set at 100 samples/s.RHESSI
has observed the Sun continuously since its launch in early 2002, and we
have acquired a unique data set ranging over more than a full 11-year
solar cycle and consisting of about 4x10^10 single data points.The
optics has a point spread of about 4.5 arcsec FWHM imaging the red
continuum onto three linear CCD sensors with a pixel resolution of 1.7
arcsec.However, careful study of systematics, masking of contaminated
data, and accumulation of data over appropriate time intervals has
led to measurementswith sub-milli arcsec accuracy.Analyzing data for
an initial period in 2004, these measurements have led to the most
accurate oblateness measurement to date, 8.01+-0.14 milli arcsec
(Fivian et al., 2008), a value consistent with models predicting
an oblateness from surface rotation.An excess oblateness term can
be attributed to magnetic elements possibly located in the enhanced
network.We also study photometric properties of our data. Previous
observations of latitude-dependent brightness variations at the limb
had suggested the presence of a polar temperature excess as large
as 1.5 K.The RHESSI observations, made with a rotating telescope in
space, have great advantages in the rejection of systematic errors
in the very precise photometry required for such an observation.Our
measurements of latitude-dependent brightness variations at the limb
lead to a quadrupolar term (a pole-to-equator temperature variation)
of the order of 0.1 K, an order of magnitude smaller than previously
reported.We present the analysis of these unique data, an overview of
some results and we report on our progress as we apply our developed
analysis method to the whole 14 years of data.
---------------------------------------------------------
Title: White-light flares, Hard X-Rays, and Heights
Authors: Martinez Oliveros, Juan Carlos; Hudson, Hugh S.; Krucker, Sam
2016SPD....47.0617M Altcode:
The white-light continuum of a solar flare was the first manifestation
of a solar flare ever detected. Nevertheless, its mechanisms remain
unknown, even today. Improved observations confirm the identification
of white-light continuum emission and hard X-rays during the impulsive
phase of a solar flare, both in space and in time, to within the
observational limits. Two events observed near the limb, but not
occulted by it (SOL2011-02-24 and SOL2012-02-18), show that these
emissions appear to have physical heights lower than predicted by models
by hundreds of kms, referring height to the location of optical-depth
unity at disk center in the 500 nm continuum. We describe these results
and place them in the context of the three extreme-limb events (within
about 1<SUP>o</SUP>) reported by Krucker et al. (2015). The electrons
responsible for hard X-ray bremsstrahlung coincide with the most intense
flare energy release, but we do not presently understand the physics
of energy transport nor the nature of particle acceleration apparently
taking place at heights below the preflare temperature minimum.
---------------------------------------------------------
Title: The dynamics and magnetism of the X1 flare on 2014-03-29
Authors: Kleint, Lucia; Heinzel, Petr; Philip, Judge; Krucker, Sam
2016SPD....47.0613K Altcode:
The X1 flare on 2014-03-29 was observed with an unprecedented number
of instruments including chromospheric polarimetry and spectroscopy
from the UV to the IR. By combining data from these instruments, we can
answer several open questions: Where is the observed continuum emission
during flares formed and through which physical processes? How does
the magnetic field structure in the photosphere and in the chromosphere
change during a flare? We discuss the implications of our findings on
standard flare models.
---------------------------------------------------------
Title: Solar Observations with the Atacama Large
Millimeter/submillimeter Array (ALMA)
Authors: Kobelski, A.; Bastian, T. S.; Bárta, M.; Brajša, R.; Chen,
B.; De Pontieu, B.; Fleishman, G.; Gary, D.; Hales, A.; Hills, R.;
Hudson, H.; Hurford, G.; Loukitcheva, M.; Iwai, K.; Krucker, S.;
Shimojo, M.; Skokić, I.; Wedemeyer, S.; White, S.; Yan, Y.; ALMA
Solar Development Team
2016ASPC..504..327K Altcode:
The Atacama Large Millimeter/Submillimeter Array (ALMA) is a
joint North American, European, and East Asian project that opens
the mm-sub mm wavelength part of the electromagnetic spectrum for
general astrophysical exploration, providing high-resolution imaging
in frequency bands currently ranging from 84 GHz to 950 GHz (300
microns to 3 mm). It is located in the Atacama desert in northern
Chile at an elevation of 5000 m. Despite being a general purpose
instrument, provisions have been made to enable solar observations
with ALMA. Radiation emitted at ALMA wavelengths originates mostly
from the chromosphere, which plays an important role in the transport
of matter and energy, and the in heating the outer layers of the solar
atmosphere. Despite decades of research, the solar chromosphere remains
a significant challenge: both to observe, owing to the complicated
formation mechanisms of currently available diagnostics; and to
understand, as a result of the complex nature of the structure and
dynamics of the chromosphere. ALMA has the potential to change the
scene substantially as it serves as a nearly linear thermometer at
high spatial and temporal resolution, enabling us to study the complex
interaction of magnetic fields and shock waves and yet-to-be-discovered
dynamical processes. Moreover, ALMA will play an important role in
the study of energetic emissions associated with solar flares at
sub-THz frequencies.
---------------------------------------------------------
Title: Spectral and Imaging Observations of a White-light Solar
Flare in the Mid-infrared
Authors: Penn, Matt; Krucker, Säm; Hudson, Hugh; Jhabvala, Murzy;
Jennings, Don; Lunsford, Allen; Kaufmann, Pierre
2016ApJ...819L..30P Altcode: 2015arXiv151204449P
We report high-resolution observations at mid-infrared wavelengths of
a minor solar flare, SOL2014-09-24T17:50 (C7.0), using Quantum Well
Infrared Photodetector cameras at an auxiliary of the McMath-Pierce
telescope. The flare emissions, the first simultaneous observations
in two mid-infrared bands at 5.2 and 8.2 μ {{m}} with white-light and
hard X-ray coverage, revealed impulsive time variability with increases
on timescales of ∼4 s followed by exponential decay at ∼10 s in
two bright regions separated by about 13<SUP>\prime\prime</SUP> . The
brightest source is compact, unresolved spatially at the diffraction
limit (1\_\_AMP\_\_farcs;72 at 5.2 μ {{m}}). We identify the IR
sources as flare ribbons also seen in white-light emission at 6173 Å
observed by SDO/HMI, with twin hard X-ray sources observed by Reuven
Ramaty High Energy Solar Spectroscopic Imager, and with EUV sources
(e.g., 94 Å) observed by SDO/AIA. The two infrared points have
nearly the same flux density (f<SUB>ν</SUB>, W m<SUP>-2</SUP> Hz)
and extrapolate to a level of about an order of magnitude below that
observed in the visible band by HMI, but with a flux of more than two
orders of magnitude above the free-free continuum from the hot (∼15
MK) coronal flare loop observed in the X-ray range. The observations
suggest that the IR emission is optically thin; this constraint and
others suggest major contributions from a density less than about
4× {10}<SUP>13</SUP> cm<SUP>-3</SUP>. We tentatively interpret this
emission mechanism as predominantly free-free emission in a highly
ionized but cool and rather dense chromospheric region.
---------------------------------------------------------
Title: The First X-Ray Imaging Spectroscopy of Quiescent Solar Active
Regions with NuSTAR
Authors: Hannah, Iain G.; Grefenstette, Brian W.; Smith, David M.;
Glesener, Lindsay; Krucker, Säm; Hudson, Hugh S.; Madsen, Kristin
K.; Marsh, Andrew; White, Stephen M.; Caspi, Amir; Shih, Albert Y.;
Harrison, Fiona A.; Stern, Daniel; Boggs, Steven E.; Christensen,
Finn E.; Craig, William W.; Hailey, Charles J.; Zhang, William W.
2016ApJ...820L..14H Altcode: 2016arXiv160301069H
We present the first observations of quiescent active regions (ARs)
using the Nuclear Spectroscopic Telescope Array (NuSTAR), a focusing
hard X-ray telescope capable of studying faint solar emission from
high-temperature and non-thermal sources. We analyze the first directly
imaged and spectrally resolved X-rays above 2 keV from non-flaring ARs,
observed near the west limb on 2014 November 1. The NuSTAR X-ray images
match bright features seen in extreme ultraviolet and soft X-rays. The
NuSTAR imaging spectroscopy is consistent with isothermal emission of
temperatures 3.1-4.4 MK and emission measures 1-8 × 10<SUP>46</SUP>
cm<SUP>-3</SUP>. We do not observe emission above 5 MK, but our short
effective exposure times restrict the spectral dynamic range. With
few counts above 6 keV, we can place constraints on the presence of an
additional hotter component between 5 and 12 MK of ∼ {10}<SUP>46</SUP>
cm<SUP>-3</SUP> and ∼ {10}<SUP>43</SUP> cm<SUP>-3</SUP>, respectively,
at least an order of magnitude stricter than previous limits. With
longer duration observations and a weakening solar cycle (resulting
in an increased livetime), future NuSTAR observations will have
sensitivity to a wider range of temperatures as well as possible
non-thermal emission.
---------------------------------------------------------
Title: FOXSI-2: Upgrades of the Focusing Optics X-ray Solar Imager
for its Second Flight
Authors: Christe, Steven; Glesener, Lindsay; Buitrago-Casas, Camilo;
Ishikawa, Shin-Nosuke; Ramsey, Brian; Gubarev, Mikhail; Kilaru,
Kiranmayee; Kolodziejczak, Jeffery J.; Watanabe, Shin; Takahashi,
Tadayuki; Tajima, Hiroyasu; Turin, Paul; Shourt, Van; Foster, Natalie;
Krucker, Sam
2016JAI.....540005C Altcode:
The Focusing Optics X-ray Solar Imager (FOXSI) sounding rocket
payload flew for the second time on 2014 December 11. To enable
direct Hard X-Ray (HXR) imaging spectroscopy, FOXSI makes use of
grazing-incidence replicated focusing optics combined with fine-pitch
solid-state detectors. FOXSI’s first flight provided the first
HXR focused images of the Sun. For FOXSI’s second flight several
updates were made to the instrument including updating the optics and
detectors as well as adding a new Solar Aspect and Alignment System
(SAAS). This paper provides an overview of these updates as well as
a discussion of their measured performance.
---------------------------------------------------------
Title: Correlation of Hard X-Ray and White Light Emission in Solar
Flares
Authors: Kuhar, Matej; Krucker, Säm; Martínez Oliveros, Juan Carlos;
Battaglia, Marina; Kleint, Lucia; Casadei, Diego; Hudson, Hugh S.
2016ApJ...816....6K Altcode: 2015arXiv151107757K
A statistical study of the correlation between hard X-ray and white
light emission in solar flares is performed in order to search for a
link between flare-accelerated electrons and white light formation. We
analyze 43 flares spanning GOES classes M and X using observations
from the Reuven Ramaty High Energy Solar Spectroscopic Imager and
Helioseismic and Magnetic Imager. We calculate X-ray fluxes at 30
keV and white light fluxes at 6173 Å summed over the hard X-ray
flare ribbons with an integration time of 45 s around the peak hard-X
ray time. We find a good correlation between hard X-ray fluxes and
excess white light fluxes, with a highest correlation coefficient
of 0.68 for photons with energy of 30 keV. Assuming the thick target
model, a similar correlation is found between the deposited power by
flare-accelerated electrons and the white light fluxes. The correlation
coefficient is found to be largest for energy deposition by electrons
above ∼50 keV. At higher electron energies the correlation decreases
gradually while a rapid decrease is seen if the energy provided by
low-energy electrons is added. This suggests that flare-accelerated
electrons of energy ∼50 keV are the main source for white light
production.
---------------------------------------------------------
Title: Continuum Enhancements in the Ultraviolet, the Visible and
the Infrared during the X1 Flare on 2014 March 29
Authors: Kleint, Lucia; Heinzel, Petr; Judge, Phil; Krucker, Säm
2016ApJ...816...88K Altcode: 2015arXiv151104161K
Enhanced continuum brightness is observed in many flares (“white
light flares”), yet it is still unclear which processes contribute to
the emission. To understand the transport of energy needed to account
for this emission, we must first identify both the emission processes
and the emission source regions. Possibilities include heating in the
chromosphere causing optically thin or thick emission from free-bound
transitions of Hydrogen, and heating of the photosphere causing enhanced
H<SUP>-</SUP> continuum brightness. To investigate these possibilities,
we combine observations from Interface Region Imaging Spectrograph
(IRIS), SDO/Helioseismic and Magnetic Imager, and the ground-based
Facility Infrared Spectrometer instrument, covering wavelengths in
the far-UV, near-UV (NUV), visible, and infrared during the X1 flare
SOL20140329T17:48. Fits of blackbody spectra to infrared and visible
wavelengths are reasonable, yielding radiation temperatures ∼6000-6300
K. The NUV emission, formed in the upper photosphere under undisturbed
conditions, exceeds these simple fits during the flare, requiring
extra emission from the Balmer continuum in the chromosphere. Thus,
the continuum originates from enhanced radiation from photosphere
(visible-IR) and chromosphere (NUV). From the standard thick-target
flare model, we calculate the energy of the nonthermal electrons
observed by Reuven Ramaty High Energy Solar Spectroscope Imager (RHESSI)
and compare it to the energy radiated by the continuum emission. We
find that the energy contained in most electrons >40 keV, or
alternatively, of ∼10%-20% of electrons >20 keV is sufficient to
explain the extra continuum emission of ∼(4-8) × 10<SUP>10</SUP>
erg s<SUP>-1</SUP> cm<SUP>-2</SUP>. Also, from the timing of the RHESSI
HXR and the IRIS observations, we conclude that the NUV continuum is
emitted nearly instantaneously when HXR emission is observed with a
time difference of no more than 15 s.
---------------------------------------------------------
Title: The injection of ten electron/<SUP>3</SUP>He-rich SEP events
Authors: Wang, Linghua; Krucker, Säm; Mason, Glenn M.; Lin, Robert
P.; Li, Gang
2016A&A...585A.119W Altcode: 2016arXiv160507882W
We have derived the particle injections at the Sun for ten good
electron/<SUP>3</SUP>He-rich solar energetic particle (SEP) events,
using a 1.2 AU particle path length (suggested by analysis of the
velocity dispersion). The inferred solar injections of high-energy
(~10 to 300 keV) electrons and of ~MeV/nucleon ions (carbon and heavier)
start with a delay of 17 ± 3 min and 75 ± 14 min, respectively, after
the injection of low-energy (~0.4 to 9 keV) electrons. The injection
duration (averaged over energy) ranges from ~200 to 550 min for ions,
from ~90 to 160 min for low-energy electrons, and from ~10 to 30 min
for high-energy electrons. Most of the selected events have no reported
Hα flares or GOES SXR bursts, but all have type III radio bursts that
typically start after the onset of a low-energy electron injection. All
nine events with SOHO/LASCO coverage have a relatively fast (>570
km s<SUP>-1</SUP>), mostly narrow (≲30°), west-limb coronal
mass ejection (CME) that launches near the start of the low-energy
electron injection, and reaches an average altitude of ~1.0 and 4.7
R<SUB>S</SUB>, respectively, at the start of the high-energy electron
injection and of the ion injection. The electron energy spectra show a
continuous power law extending across the transition from low to high
energies, suggesting that the low-energy electron injection may provide
seed electrons for the delayed high-energy electron acceleration. The
delayed ion injections and high ionization states may suggest an ion
acceleration along the lower altitude flanks, rather than at the nose
of the CMEs.
---------------------------------------------------------
Title: EUV & X-ray observations of microflare heating of AR12333
Authors: Hannah, I. G.; Wright, P. J.; Grefenstette, B.; Glesener,
L.; Hudson, H. S.; Smith, D. M.; Krucker, S.; Marsh, A.; White, S. M.
2015AGUFMSH13B2442H Altcode:
We present a study of the heating in AR12333 due to small microflares
between 10:30 and 13:30UT on 29 April 2015. This region is well observed
in EUV by the Solar Dynamics Observatory/Atmospheric Imaging Assembly
(SDO/AIA) as well as Hinode's X-ray Telescope (XRT) operating in
a higher cadence mode, switching through the five thicker filters
(sensitive to the higher temperature range). The Hinode observations
were a coordinated campaign with the NuSTAR hard X-ray focusing
optics telescope (Harrison et al. 2013). NuSTAR was conducting a
full disk mosaic observation of the Sun and caught AR12333 several
times, providing imaging spectroscopy >2 keV. We investigate the
heating in the active region due to several small microflares (about
A1-Class). These were visible with the thicker XRT filters and only
clear in EUV once the FeXVIII component was extracted from SDO/AIA 94Å,
indicating heating primarily >3MK. Using the regularized inversion
method of Hannah & Kontar 2012, we recover the DEM from the SDO/AIA
and Hinode/XRT data and compare this to the thermal characteristics
derived from NuSTAR.
---------------------------------------------------------
Title: Hard X-ray Detectability of Small Impulsive Heating Events
in the Solar Corona
Authors: Glesener, L.; Klimchuk, J. A.; Bradshaw, S. J.; Marsh, A.;
Krucker, S.; Christe, S.
2015AGUFMSH13B2440G Altcode:
Impulsive heating events ("nanoflares") are a candidate to supply the
solar corona with its ~2 MK temperature. These transient events can
be studied using extreme ultraviolet and soft X-ray observations,
among others. However, the impulsive events may occur in tenuous
loops on small enough timescales that the heating is essentially not
observed due to ionization timescales, and only the cooling phase is
observed. Bremsstrahlung hard X-rays could serve as a more direct and
prompt indicator of transient heating events. A hard X-ray spacecraft
based on the direct-focusing technology pioneered by the Focusing
Optics X-ray Solar Imager (FOXSI) sounding rocket could search for these
direct signatures. In this work, we use the hydrodynamical EBTEL code to
simulate differential emission measures produced by individual heating
events and by ensembles of such events. We then directly predict hard
X-ray spectra and consider their observability by a future spaceborne
FOXSI, and also by the RHESSI and NuSTAR spacecraft.
---------------------------------------------------------
Title: NuSTAR X-ray observations of small flares and non-flaring
active regions
Authors: Hannah, I. G.; Grefenstette, B.; Smith, D. M.; Marsh, A.;
Glesener, L.; Krucker, S.; Hudson, H. S.; White, S.; Madsen, K.;
Caspi, A.; Vogel, J.; Shih, A.
2015AGUFMSH31D..03H Altcode:
We present imaging spectroscopy of the Sun with the NuSTAR hard X-ray
(HXR) telescope, an astrophysics mission that uses focusing optics to
directly image X-rays between ~2-80 keV. Although not optimized for
solar observations, NuSTAR's high sensitivity can probe previously
inaccessible X-ray emission from the Sun - crucial for searching for
high temperature and non-thermal emission from "non-flaring" active
regions. We present analysis of the first NuSTAR solar observations,
that began in late 2014 and continued through 2015. These include
using its imaging spectroscopy capabilities to derive the thermal
characteristics of several "non-flaring" active regions, providing
limits to the high temperature emission. We also show NuSTAR
observations of several small microflares that were also observed
by Hinode/XRT (in multiple thicker filters sensitive to higher
temperatures) and RHESSI. This combination of three separate X-ray
telescopes provides a broad observational characterization of active
region heating by these very small microflares.
---------------------------------------------------------
Title: Particle acceleration by a solar flare termination shock
Authors: Chen, Bin; Bastian, Timothy S.; Shen, Chengcai; Gary, Dale
E.; Krucker, Säm; Glesener, Lindsay
2015Sci...350.1238C Altcode: 2015arXiv151202237C
Solar flares—the most powerful explosions in the solar system—are
also efficient particle accelerators, capable of energizing a large
number of charged particles to relativistic speeds. A termination
shock is often invoked in the standard model of solar flares as a
possible driver for particle acceleration, yet its existence and
role have remained controversial. We present observations of a solar
flare termination shock and trace its morphology and dynamics using
high-cadence radio imaging spectroscopy. We show that a disruption of
the shock coincides with an abrupt reduction of the energetic electron
population. The observed properties of the shock are well reproduced
by simulations. These results strongly suggest that a termination
shock is responsible, at least in part, for accelerating energetic
electrons in solar flares.
---------------------------------------------------------
Title: Observations and Simulations of a Termination Shock in an
Eruptive Solar Flare as a Possible Particle Accelerator
Authors: Chen, B.; Bastian, T.; Shen, C.; Gary, D. E.; Krucker, S.;
Glesener, L.
2015AGUFMSH11F..05C Altcode:
A termination shock has been often invoked in the standard model for
eruptive solar flares as a possible driver for particle acceleration. It
is hypothesized as a standing shock wave generated by super-magnetosonic
reconnection outflows impinging upon dense, newly-reconnected magnetic
loops during the flare energy release process. However, such shock
wave has largely remained a theoretical concept inferred from model
predictions due to the lack of observational evidence. Here we present
observations of a termination shock in a solar flare and trace its
morphology and dynamics using high-cadence radio imaging spectroscopy
enabled by the upgraded Karl G. Jansky Very Large Array. The observed
properties of the shock, including its location, morphology, and
dynamics, are well-reproduced by magnetohydrodynamics simulations
in a standard Kopp-Pneuman-type reconnection geometry for two-ribbon
flares. We further show that a disruption of the shock coincides with
an abrupt reduction of the energetic electron population. These results
strongly suggest that a termination shock is responsible, at least in
part, for accelerating energetic electrons in solar flares.
---------------------------------------------------------
Title: FOXSI-2 Observations and Coronal Heating
Authors: Christe, S.; Glesener, L.; Krucker, S.; Ramsey, B.; Ishikawa,
S. N.; Buitrago Casas, J. C.; Takahashi, T.; Foster, N.
2015AGUFMSH31D..07C Altcode:
Energy release and particle acceleration on the Sun is a frequent
occurrence associated with a number of different solar phenomenon
including but not limited to solar flares, coronal mass ejections and
nanoflares. The exact mechanism through which particles are accelerated
and energy is released is still not well understood. This issue is
related to the unsolved coronal heating problem, the mystery of the
heating mechanism for the million degree solar corona. One prevalent
theory posits the existence of a multitude of small flares, dubbed
nanoflares. Recent observations of active region AR11890 by IRIS
(Testa et al. 2014) are consistent with numerical simulations of
heating by impulsive beams of nonthermal electrons, suggesting that
nanoflares may be similar to large flares in that they accelerate
particles. Furthermore, observations by the EUNIS sounding rocket
(Brosius et al. 2014) of faint Fe XIX (592.2 Angstrom) emission in an
active region is indicative of plasma at temperatures of at least 8.9
MK providing further evidence of nanoflare heating. One of the best
ways to gain insight into accelerated particles on the Sun and the
presence of hot plasma is by observing the Sun in hard X-rays (HXR). We
present on observations taken during the second successful flight of the
Focusing Optics X-ray Solar Imager (FOXSI-2). FOXSI flew on December 11,
2014 with upgraded optics as well as new CdTe strip detectors. FOXSI-2
observed thermal emission (4-15 keV) from at least three active regions
(AR#12234, AR#12233, AR#12235) and observed regions of the Sun without
active regions. We present on using FOXSI observations to test the
presence of hot temperatures in and outside of active regions.
---------------------------------------------------------
Title: Capabilities of a FOXSI Small Explorer
Authors: Inglis, A. R.; Christe, S.; Glesener, L.; Krucker, S.; Dennis,
B. R.; Shih, A.; Wilson-Hodge, C.; Gubarev, M.; Hudson, H. S.; Kontar,
E.; Buitrago Casas, J. C.; Drake, J. F.; Caspi, A.; Holman, G.; Allred,
J. C.; Ryan, D.; Alaoui, M.; White, S. M.; Saint-Hilaire, P.; Klimchuk,
J. A.; Hannah, I. G.; Antiochos, S. K.; Grefenstette, B.; Ramsey,
B.; Jeffrey, N. L. S.; Reep, J. W.; Schwartz, R. A.; Ireland, J.
2015AGUFMSH43B2456I Altcode:
We present the FOXSI (Focusing Optics X-ray Solar Imager) small explorer
(SMEX) concept, a mission dedicated to studying particle acceleration
and energy release on the Sun. FOXSI is designed as a 3-axis stabilized
spacecraft in low-Earth orbit making use of state-of-the-art grazing
incidence focusing optics, allowing for direct imaging of solar
X-rays. The current design being studied features three telescope
modules deployed in a low-inclination low-earth orbit (LEO). With a 15
meter focal length enabled by a deployable boom, FOXSI will observe
the Sun in the 3-50 keV energe range. The FOXSI imaging concept has
already been tested on two sounding rocket flights, in 2012 and 2014
and on the HEROES balloon payload flight in 2013. FOXSI will image
the Sun with an angular resolution of 5”, a spectral resolution of
0.5 keV, and sub-second temporal resolution using CdTe detectors. In
this presentation we investigate the science objectives and targets
which can be accessed from this mission. Because of the defining
characteristic of FOXSI is true imaging spectroscopy with high dynamic
range and sensitivity, a brand-new perspective on energy release on the
Sun is possible. Some of the science targets discussed here include;
flare particle acceleration processes, electron beams, return currents,
sources of solar energetic particles (SEPs), as well as understanding
X-ray emission from active region structures and the quiescent corona.
---------------------------------------------------------
Title: Second flight of the Focusing Optics X-ray Solar Imager
sounding rocket [FOXSI-2]
Authors: Buitrago-Casas, J. C.; Krucker, S.; Christe, S.; Glesener,
L.; Ishikawa, S. N.; Ramsey, B.; Foster, N. D.
2015AGUFMSH43B2454B Altcode:
The Focusing Optics X-ray Solar Imager (FOXSI) is a sounding rocket
experiment that has flown twice to test a direct focusing method for
measuring solar hard X-rays (HXRs). These HXRs are associated with
particle acceleration mechanisms at work in powering solar flares and
aid us in investigating the role of nanoflares in heating the solar
corona. FOXSI-1 successfully flew for the first time on November 2,
2012. After some upgrades including the addition of extra mirrors
to two optics modules and the inclusion of new fine-pitch CdTe strip
detectors, in addition to the Si detectors from FOXSI-1, the FOXSI-2
payload flew successfully again on December 11, 2014. During the
second flight four targets on the Sun were observed, including at
least three active regions, two microflares, and ~1 minute of quiet
Sun observation. This work is focused in giving an overview of the
FOXSI rocket program and a detailed description of the upgrades for the
second flight. In addition, we show images and spectra investigating
the presence of no thermal emission for each of the flaring targets
that we observed during the second flight.
---------------------------------------------------------
Title: Hard X-ray Detector Calibrations for the FOXSI Sounding Rocket
Authors: Lopez, A.; Glesener, L.; Buitrago Casas, J. C.; Han, R.;
Ishikawa, S. N.; Christe, S.; Krucker, S.
2015AGUFMSH43B2455L Altcode:
In the study of high-energy solar flares, detailed X-ray images and
spectra of the Sun are required. The Focusing Optics X-ray Solar Imager
(FOXSI) sounding rocket experiment is used to test direct-focusing
X-ray telescopes and Double-sided Silicon Strip Detectors (DSSD)
for solar flare study and to further understand coronal heating. The
measurement of active region differential emission measures, flare
temperatures, and possible quiet-Sun emission requires a precisely
calibrated spectral response. This poster describes recent updates
in the calibration of FOXSI's DSSDs based on new calibration tests
that were performed after the second flight. The gain for each strip
was recalculated using additional radioactive sources. Additionally,
the varying strip sensitivity across the detectors was investigated and
based on these measurements, the flight images were flatfielded. These
improvements lead to more precise X-ray data for future FOXSI flights
and show promise for these new technologies in imaging the Sun.
---------------------------------------------------------
Title: SDO/HMI -- RHESSI White-Light Flare Catalog: Matsushita
Analysis
Authors: Martinez Oliveros, J. C.; Hudson, H. S.; Krucker, S.
2015AGUFMSH52A..05M Altcode:
In recent years several observation of white-light flare features in
the low corona using data from the Helioseismic and Magnetic Imager
onboard the Solar Dynamics Observatory have been reported. We present
the first results of a white-light flare catalog based on SDO/HMI
6173A Intensity observations and the RHESSI flare catalog. We selected
flares during 2011 and 2013 with GOES classification above M1.0 that
were fully or partially observed by RHESSI as reported in the RHESSI
flare catalog. We found that at least one third of the flares present
white-light enhancement in the 6173A line and at least one fifth of the
events show above the limb white-light sources. We will also discuss the
results of a comparative analysis between the Hard X-ray and white-light
positions. This analysis show the statistical average-height variation
using the analysis technique described by Matsushita et al. 1992.
---------------------------------------------------------
Title: Characterizing Temperatures of FOXSI-2 Microflares Using
RHESSI and AIA Observations
Authors: Han, R.; Glesener, L.; Buitrago Casas, J. C.; Lopez, A.;
Badman, S.; Krucker, S.
2015AGUFMSH43B2453H Altcode:
The second flight of the Focusing Optics X-ray Solar Imager sounding
rocket payload (FOXSI-2) was successfully completed on December 11,
2014. FOXSI's direct imaging technology allows it to measure hard
X-ray (HXR) emissions from the Sun with superior dynamic range
and sensitivity relative to indirect HXR observations. During the
December FOXSI-2 flight, several microflares were observed. We wish
to characterize the temperature distributions of these microflares
using supporting measurements in order to validate the FOXSI spectral
response. The temperature distribution of solar plasma is best
described by the differential emission measure (DEM), a physical
quantity that characterizes the amount of material present in the
corona in each temperature range. To determine the DEM, we employ
multi-wavelength extreme ultraviolet observations by the Atmospheric
Imaging Assembly (SDO/AIA) using a regularization method. We also
perform isothermal fitting of thermal X-ray spectra from the Reuven
Ramaty Solar Spectroscopic Imager (RHESSI). This poster will show the
temperature distribution for each of the December 11 microflares and
compare these results with those obtained from FOXSI-2.
---------------------------------------------------------
Title: The NuSTAR Sensitivity to Quiet-Sun Transient Events
Authors: Marsh, A.; Smith, D. M.; Glesener, L.; Hannah, I. G.; Krucker,
S.; Hudson, H. S.; Grefenstette, B.; Madsen, K.; Caspi, A.
2015AGUFMSH13B2441M Altcode:
We present the NuSTAR sensitivity to quiet Sun (QS) transient events,
which have been seen in wavelengths from UV to soft X-rays. Although
not optimized for solar observations, NuSTAR can observe X-ray
emission from the Sun with unprecedented sensitivity in the hard X-ray
range; this is crucial for detecting individual events in the quiet
corona. While NuSTAR has not yet detected any such events, we use QS
data from the 01-November-2014 observations (at a GOES-level ~B4) to
determine what types of events we could have detected. In particular,
we place lower limits on the detectable flare emission measure for
isothermal temperatures between 2 - 10 MK. While our sensitivity to
date has been limited by noise due to active regions outside the field
of view, these limits are still >2 orders of magnitude below the
RHESSI detection limits at corresponding temperatures. We expect to
increase our sensitivity by at least an order of magnitude with future
observations at lower solar flux levels and with fewer active regions
on the disk.
---------------------------------------------------------
Title: The Energy Spectrum of Solar Energetic Electrons
Authors: Wang, L.; Yang, L.; Krucker, S.; Wimmer-Schweingruber, R. F.;
Bale, S. D.
2015AGUFMSH21A2385W Altcode:
Here we present a statistical survey of the energy spectrum of solar
energetic electron events (SEEs) observed by the WIND 3DP instrument
from 1995 though 2014. For SEEs with the minimum energy below 10 keV and
the maximum energy above 100 keV, ~85% (~2%) have a double-power-law
energy spectrum with a steepening (hardening) above the break energy,
while ~13% have a single-power-law energy spectrum at all energies. The
average spectral index is ~2.4 below the energy break and is ~4.0 above
the energy break. For SEEs detected only at energies <10 keV (>20
keV), they generally show a single-power-law spectrum with the average
index of ~3.0 (~3.3). The spectrum of SEEs detected only below 10 keV
appears to get harder with increasing solar activity, but the spectrum
of SEEs with higher-energy electrons shows no clear correlation with
solar activity. We will also investigate whether the observed energy
spectrum of SEEs at 1 AU mainly reflects the electron acceleration at
the Sun or the electron transport in the interplanetary medium.
---------------------------------------------------------
Title: A Statistical Correlation of Sunquakes Based on Their Seismic
and White-Light Emission
Authors: Buitrago-Casas, J. C.; Martínez Oliveros, J. C.; Lindsey,
C.; Calvo-Mozo, B.; Krucker, S.; Glesener, L.; Zharkov, S.
2015SoPh..290.3151B Altcode: 2015arXiv150207798B; 2015SoPh..tmp..169B
Several mechanisms have been proposed to explain the transient seismic
emission, i.e. "sunquakes," from some solar flares. Some theories
associate high-energy electrons and/or white-light emission with
sunquakes. High-energy charged particles and their subsequent heating
of the photosphere and/or chromosphere could induce acoustic waves in
the solar interior. We carried out a correlative study of solar flares
with emission in hard X-rays, enhanced continuum emission at 6173 Å,
and transient seismic emission. We selected those flares observed by
the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI)
with a considerable flux above 50 keV between 1 January 2010 and
26 June 2014. We then used data from the Helioseismic and Magnetic
Imager onboard the Solar Dynamic Observatory to search for excess
visible-continuum emission and new sunquakes not previously reported. We
found a total of 18 sunquakes out of 75 flares investigated. All of the
sunquakes were associated with an enhancement of the visible continuum
during the flare. Finally, we calculated a coefficient of correlation
for a set of dichotomic variables related to these observations. We
found a strong correlation between two of the standard helioseismic
detection techniques, and between sunquakes and visible-continuum
enhancements. We discuss the phenomenological connectivity between these
physical quantities and the observational difficulties of detecting
seismic signals and excess continuum radiation.
---------------------------------------------------------
Title: How Important Are Electron Beams in Driving Chromospheric
Evaporation in the 2014 March 29 Flare?
Authors: Battaglia, Marina; Kleint, Lucia; Krucker, Säm; Graham, David
2015ApJ...813..113B Altcode: 2015arXiv150909186B
We present high spatial resolution observations of chromospheric
evaporation in the flare SOL2014-03-29T17:48. Interface Region Imaging
Spectrograph observations of the Fe xxi λ 1354.1 line indicate
evaporating plasma at a temperature of 10 MK along the flare ribbon
during the flare peak and several minutes into the decay phase with
upflow velocities between 30 and 200 km s<SUP>-1</SUP>. Hard X-ray (HXR)
footpoints were observed by the Ramaty High Energy Solar Spectroscopic
Imager for two minutes during the peak of the flare. Their locations
coincided with the locations of the upflows in parts of the southern
flare ribbon but the HXR footpoint source preceded the observation
of upflows in Fe xxi by 30-75 s. However, in other parts of the
southern ribbon and in the northern ribbon, the observed upflows were
not coincident with an HXR source in time or space, most prominently
during the decay phase. In this case evaporation is likely caused by
energy input via a conductive flux that is established between the hot
(25 MK) coronal source, which is present during the whole observed
time-interval, and the chromosphere. The presented observations suggest
that conduction may drive evaporation not only during the decay phase
but also during the flare peak. Electron beam heating may only play
a role in driving evaporation during the initial phases of the flare.
---------------------------------------------------------
Title: Hard X-Ray Imaging of Individual Spectral Components in
Solar Flares
Authors: Caspi, Amir; Shih, Albert Y.; McTiernan, James M.; Krucker,
Säm
2015ApJ...811L...1C Altcode: 2015arXiv150806003C
We present a new analytical technique, combining Reuven Ramaty High
Energy Solar Spectroscopic Imager (RHESSI) high-resolution imaging
and spectroscopic observations, to visualize solar flare emission as
a function of spectral component (e.g., isothermal temperature) rather
than energy. This computationally inexpensive technique is applicable
to all spatially invariant spectral forms and is useful for visualizing
spectroscopically determined individual sources and placing them in
context, e.g., comparing multiple isothermal sources with nonthermal
emission locations. For example, while extreme ultraviolet images can
usually be closely identified with narrow temperature ranges, due to the
emission being primarily from spectral lines of specific ion species,
X-ray images are dominated by continuum emission and therefore have a
broad temperature response, making it difficult to identify sources of
specific temperatures regardless of the energy band of the image. We
combine RHESSI calibrated X-ray visibilities with spatially integrated
spectral models including multiple isothermal components to effectively
isolate the individual thermal sources from the combined emission
and image them separately. We apply this technique to the 2002 July
23 X4.8 event studied in prior works, and image for the first time
the super-hot and cooler thermal sources independently. The super-hot
source is farther from the footpoints and more elongated throughout
the impulsive phase, consistent with an in situ heating mechanism for
the super-hot plasma.
---------------------------------------------------------
Title: Magnetic field line lengths inside interplanetary magnetic
flux ropes
Authors: Hu, Qiang; Qiu, Jiong; Krucker, Sam
2015JGRA..120.5266H Altcode: 2015arXiv150205284H
We report on the detailed and systematic study of field line twist
and length distributions within magnetic flux ropes embedded in
interplanetary coronal mass ejections (ICMEs). The Grad-Shafranov
reconstruction method is utilized together with a constant-twist
nonlinear force-free (Gold-Hoyle) flux rope model to reveal the
close relation between the field line twist and length in cylindrical
flux ropes, based on in situ Wind spacecraft measurements. We show
that the field line twist distributions within interplanetary flux
ropes are inconsistent with the Lundquist model. In particular,
we utilize the unique measurements of magnetic field line lengths
within selected ICME events as provided by Kahler et al. (<link
href="#jgra51898-bib-0016"/>) based on energetic electron burst
observations at 1 AU and the associated type III radio emissions
detected by the Wind spacecraft. These direct measurements are compared
with our model calculations to help assess the flux rope interpretation
of the embedded magnetic structures. By using the different flux rope
models, we show that the in situ direct measurements of field line
lengths are consistent with a flux rope structure with spiral field
lines of constant and low twist, largely different from that of the
Lundquist model, especially for relatively large-scale flux ropes.
---------------------------------------------------------
Title: The Fast Filament Eruption Leading to the X-flare on 2014
March 29
Authors: Kleint, Lucia; Battaglia, Marina; Reardon, Kevin; Sainz Dalda,
Alberto; Young, Peter R.; Krucker, Säm
2015ApJ...806....9K Altcode: 2015arXiv150400515K
We investigate the sequence of events leading to the solar X1 flare
SOL2014-03-29T17:48. Because of the unprecedented joint observations of
an X-flare with the ground-based Dunn Solar Telescope and the spacecraft
IRIS, Hinode, RHESSI, STEREO, and the Solar Dynamics Observatory, we can
sample many solar layers from the photosphere to the corona. A filament
eruption was observed above a region of previous flux emergence, which
possibly led to a change in magnetic field configuration, causing
the X-flare. This was concluded from the timing and location of the
hard X-ray emission, which started to increase slightly less than a
minute after the filament accelerated. The filament showed Doppler
velocities of ∼2-5 km s<SUP>-1</SUP> at chromospheric temperatures
for at least one hour before the flare occurred, mostly blueshifts,
but also redshifts near its footpoints. Fifteen minutes before the
flare, its chromospheric Doppler shifts increased to ∼6-10 km
s<SUP>-1</SUP> and plasma heating could be observed before it lifted
off with at least 600 km s<SUP>-1</SUP> as seen in IRIS data. Compared
to previous studies, this acceleration (∼3-5 km s<SUP>-2</SUP>) is
very fast, while the velocities are in the common range for coronal
mass ejections. An interesting feature was a low-lying twisted second
filament near the erupting filament, which did not seem to participate
in the eruption. After the flare ribbons started on each of the second
filament’s sides, it seems to have untangled and vanished during the
flare. These observations are some of the highest resolution data of
an X-class flare to date and reveal some small-scale features yet to
be explained.
---------------------------------------------------------
Title: FOXSI: Properties of optics and detectors for hard-X rays
Authors: Buitrago-Casas, Juan Camilo; Glesener, Lindsay; Christe,
Steven; Krucker, Sam; Ishikawa, Shin-nosuke; Foster, Natalie
2015TESS....140305B Altcode: 2015TESS....140305C
The Focusing Optics X-ray Solar Imager (FOXSI) is a state-of-the-art
direct focusing X-ray telescope designed to observe the Sun. This
experiment completed its second flight onboard a sounding rocket last
December 11, 2014 from the White Sands Missile Range in New Mexico. The
optics use a set of iridium-coated nickel/cobalt mirrors made using
a replication technique based on an electroformed perfect polished
surface. Since this technique creates full shells that no need to
be co-aligned with other segments, an angular resolution of up to ~5
arcsec is gotten. The FOXSI focal plane consists of seven double-sided
strip detectors. Five Silicon and 2 CdTe detectors were used during
the second flight.We present on various properties of Wolter-I optics
that are applicable to solar HXR observation, including ray-tracing
simulations of the single-bounce (“ghost ray”) patterns from sources
outside the field of view and angular resolution for different source
angles and effective area measurements of the FOXSI optics. We also
present the detectors calibration results, paying attention to energy
resolution (~0.5 keV), energy thresholds (~4-15 keV for Silicon and
~4-20 keV for CdTe detectors), and spatial coherence of these values
over the entire detector.
---------------------------------------------------------
Title: The Focusing Optics X-ray Solar Imager: Second Flight and
Recent Results
Authors: Christe, Steven; Krucker, Sam; Glesener, Lindsay; Ramsey,
Brian; Ishikawa, Shin-nosuke; Buitrago Casas, Juan Camilo; Foster,
Natalie; Takahashi, Tadayuki
2015TESS....121301C Altcode:
Energy release and particle acceleration on the Sun is a frequent
occurrence associated with a number of different solar phenomenon
including but not limited to solar flares and coronal mass
ejections. The exact mechanism through which particle are accelerated
is still not well understood. One of the best ways to gain insight into
accelerated particles on the Sun is by observing the Sun in hard X-rays
(HXR) which provide one of the most direct diagnostics of energetic
electrons. Past and current HXR observations lack the sensitivity and
dynamic range necessary to observe the faint signature of accelerated
electrons where they are accelerated in the solar corona. However
these limitations can be overcome through the use of HXR focusing
optics coupled with solid-state pixelated detectors. We present on the
second successful launch of the Focusing Optics X-ray Solar Imager,
a sounding rocket payload which flew on December 11, 2014. In this
flight, the FOXSI optics were upgraded for better sensitivity and
new CdTe strip detectors were included to provide increased detection
efficiency. During this flight, FOXSI observed thermal emission from
at least three active regions (AR#12234, AR#12233, AR#12235). Another
observation target for FOXSI was the quiet Sun. In this presentation
we summarize the flight as well as the latest observations and analysis.
---------------------------------------------------------
Title: High-sensitivity search for transient solar X-ray emission
with NuSTAR
Authors: Marsh, Andrew; Hannah, Iain; Glesener, Lindsay; Smith, David
M.; Grefenstette, Brian; Madsen, Kristin; Krucker, Sam; Hudson, Hugh;
White, Stephen; Caspi, Amir; Christe, Steven; Shih, Albert; Mewaldt,
Richard; Pivovaroff, Michael; Vogel, Julia
2015TESS....121302M Altcode:
We present the first results of a search for transient X-ray emission in
quiet solar regions with the NuSTAR astrophysics satellite. Transient
brightenings of 10<SUP>24</SUP>-10<SUP>27 </SUP>ergs, or "nanoflares,"
have been observed as thermal emission in EUV and soft X-rays, but
never in hard X-rays (HXRs) due to lack of sensitivity. Frequent
nanoflares could account for a significant fraction of the energy
release needed to heat the corona to >1 MK. NuSTAR directly images
X-rays from ~2-80 keV, with much higher sensitivity than dedicated
solar HXR instruments. More importantly it can point at the Sun without
suffering damage, a rare capability for an astrophysics instrument. We
have developed an algorithm to search the NuSTAR data in space and
time for transient events, while taking into account instrumental
and systematic effects. Preliminary analysis yields a sensitivity to
events ~0.001 times as bright as an “typical” RHESSI microflare
(Hannah et al. 2008), for linear scaling and event duration of 10
seconds. Future observations at full-Sun flux levels below GOES ~B5
will increase our sensitivity by an order of magnitude or more.
---------------------------------------------------------
Title: An Investigation of Flare Footpoint DEMs using AIA Diffraction
Patterns
Authors: Raftery, Claire; Bain, Hazel; Schwartz, Richard; Torre,
Gabriele; Krucker, Sam
2015TESS....130211R Altcode:
The heating of flare footpoints by accelerated electrons is a
well-established component of the standard flare model. However,
limitations of current instruments make it challenging to obtain high
cadence, high resolution observations of the brightest footpoint
regions, predominantly due to low cadence, or pixel saturation.In
moderate and large flares observed by the Solar Dynamics Observatory’s
Atmospheric Imaging Assembly, CCD pixels in the footpoint regions are
frequently saturated despite the automatic exposure control. Using the
method of Schwartz et al. (2014), we reconstruct saturated footpoint
kernels in the brightest flaring regions and investigate the evolving
footpoint differential emission measure at the full 12 second AIA
cadence. This is compared to the changing electron fluxes observed with
the Reuven Ramaty Solar Spectroscopic Imager (RHESSI) to investigate
the relationship between the non-thermal electron energy flux and the
footpoint thermal response.(Schwartz, R. A., Torre, G., & Piana,
M. (2014), Astrophysical Journal Letters, 793, LL23 )
---------------------------------------------------------
Title: The Miniature X-ray Spectrograph (MiXS)
Authors: Martinez Oliveros, Juan Carlos; Glesener, Lindsay; Saint
Hilaire, Pascal; Sundkvist, David; Hurford, Gordon; Bain, Hazel;
Bale, Stuart D.; Krucker, Sam
2015TESS....140301M Altcode:
The Miniature X-ray Spectrograph (MiXS) is an innovative, small,
and fully functional solar X-ray observatory concept designed to fit
within a 6U CubeSat platform. MiXS will provide the community with X-ray
spectroscopy up to 100 keV of solar flares at a small fraction of the
cost of a conventional mission. It includes layered Si/CdTe detectors,
providing routine observations of both soft and hard X-ray emission
with low background. If selected for funding, MiXS will provide hard
X-ray (HXR) spectroscopy throughout the declining phase of this solar
cycle allowing continuous solar observations while new generation
HXR instrumentation put in orbit. MiXS is the first stage of a much
ambitious cube design the Miniature Xray Imager (MiXI), which can
provide to the community X-ray imaging up to 40 - 50 keV. In the next
solar cycle, coordinated observations between Solar Orbiter’s STIX
instrument and future MiXS or MiXI iterations will enable solar flare
observation from two vantage points, while new observatories will
be commissioned. This will provide new insight into the directivity
of flare HXR emission and will allow detailed study of both coronal
and footpoint sources within the same flare. These results may have
profound implications for theories of flare acceleration processes. We
describe here the MiXS concept and its usefulness to the solar and
heliophysics communities.
---------------------------------------------------------
Title: Hard X-ray imaging spectroscopy of hot coronal sources and
active regions with NuSTAR
Authors: Hannah, Iain; Marsh, Andrew; Glesener, Lindsay; Smith,
David; Grefenstette, Brian; Madsen, Kristin; Krucker, Sam; Hudson,
Hugh; White, Stephen; Shih, Albert Y.
2015TESS....120402H Altcode:
We present imaging spectroscopy of the Sun with the NuSTAR hard X-ray
(HXR) telescope, searching for high temperature and non-thermal emission
in the “non-flaring” Sun. Launched in 2012, NASA's astrophysics
mission NuSTAR uses focusing optics to directly image X-rays between
~2-80 keV. In the band below ~50 keV the field of view is 12'x12'
and the instrument has an energy resolution of ~0.4 keV. Although not
optimized for solar observations, NuSTAR’s high sensitivity can probe
previously inaccessible X-ray emission from the Sun. NuSTAR observed the
Sun three times during late 2014 and we present these first directly
imaged hard X-rays from non-flaring active regions. Using NuSTAR’s
imaging spectroscopy capabilities we are able to derive the active
region’s multi-thermal characteristics. We will also discuss a hot
(>3MK) source that appears to linger high in the corona and could
be associated with the occulted active region AR12192.
---------------------------------------------------------
Title: Magnetic Field-line Twist and Length Distributions inside
Interplanetary Magnetic Flux Ropes
Authors: Hu, Qiang; Qiu, Jiong; Krucker, Sam
2015TESS....121204H Altcode:
We report on the detailed and systematic study of field-line
twist and length distributions within magnetic flux ropes embedded
in Interplanetary Coronal Mass Ejections (ICMEs). The Grad-Shafranov
reconstruction method is utilized together with a constant-twist
nonlinear force-free (Gold-Hoyle) flux rope model and the commonly
known Lundquist (linear force-free) model to reveal the close relation
between the field-line twist and length in cylindrical flux ropes,
based on in-situ spacecraft magnetic field and plasma measurements. In
particular, we utilize energetic electron burst observations at 1 AU
together with associated type III radio emissions detected by the Wind
spacecraft to provide unique measurements of magnetic field-line lengths
within selected ICME events. These direct measurements are compared with
flux-rope model calculations to help assess the fidelity of different
models and to provide diagnostics of internal structures. We show that
our initial analysis of field-line twist indicates clear deviation
from the Lundquist model, but better consistency with the Gold-Hoyle
model. By using the different flux-rope models, we conclude that the
in-situ direct measurements of field-line lengths are consistent with a
flux-rope structure with spiral field lines of constant and low twist,
largely different from that of the Lundquist model, especially for
relatively large-scale flux ropes. We will also discuss the implications
of our analysis of flux-rope structures on the origination and evolution
processes in their corresponding solar source regions.
---------------------------------------------------------
Title: Sub-Milli Arcsecond Resolution Observations of the Optical
Solar Limb with RHESSI/SAS
Authors: Fivian, Martin D.; Hudson, Hugh; Krucker, Sam
2015TESS....120325F Altcode:
The Solar Aspect System (SAS) of the RHESSI satellite measures the
optical solar limb with a cadence typically set at 100 samples/s. RHESSI
has observed the Sun continuously since its launch in early 2002, and we
have acquired a unique data set ranging over more than a full 11-year
solar cycle and consisting of about 4x10^10 single data points. The
optics has a point spread of about 4.5 arcsec FWHM imaging the red
continuum onto three linear CCD sensors with a pixel resolution of 1.7
arcsec. However, careful study of systematics, masking of contaminated
data, and accumulation of data over appropriate time intervals has led
to measurements with sub-milli arcsec accuracy. Analyzing data for
an initial period in 2004, these measurements have led to the most
accurate oblateness measurement to date, 8.01+-0.14 milli arcsec
(Fivian et al., 2008), a value consistent with models predicting
an oblateness from surface rotation. An excess oblateness term can
be attributed to magnetic elements possibly located in the enhanced
network. We also study photometric properties of our data. Previous
observations of latitude-dependent brightness variations at the limb
had suggested the presence of a polar temperature excess as large as
1.5 K. The RHESSI observations, made with a rotating telescope in
space, have great advantages in the rejection of systematic errors
in the very precise photometry required for such an observation. Our
measurements of latitude-dependent brightness variations at the limb
lead to a quadrupolar term (a pole-to-equator temperature variation)
of the order of 0.1 K, an order of magnitude smaller than previously
reported. We present the analysis of these unique data, an overview of
some results and we report on our progress as we apply our developed
analysis method to the whole 13 years of data.
---------------------------------------------------------
Title: Hard X-ray imaging spectroscopy of FOXSI microflares
Authors: Glesener, Lindsay; Krucker, Sam; Christe, Steven;
Buitrago-Casas, Juan Camilo; Ishikawa, Shin-nosuke; Foster, Natalie
2015TESS....140601G Altcode:
The ability to investigate particle acceleration and hot thermal
plasma in solar flares relies on hard X-ray imaging spectroscopy using
bremsstrahlung emission from high-energy electrons. Direct focusing
of hard X-rays (HXRs) offers the ability to perform cleaner imaging
spectroscopy of this emission than has previously been possible. Using
direct focusing, spectra for different sources within the same field
of view can be obtained easily since each detector segment (pixel
or strip) measures the energy of each photon interacting within that
segment. The Focusing Optics X-ray Solar Imager (FOXSI) sounding rocket
payload has successfully completed two flights, observing microflares
each time. Flare images demonstrate an instrument imaging dynamic range
far superior to the indirect methods of previous instruments like the
RHESSI spacecraft.In this work, we present imaging spectroscopy of
microflares observed by FOXSI in its two flights. Imaging spectroscopy
performed on raw FOXSI images reveals the temperature structure of
flaring loops, while more advanced techniques such as deconvolution
of the point spread function produce even more detailed images.
---------------------------------------------------------
Title: Observation and Modeling of a Termination Shock in a Solar
Eruption as a Possible Particle Accelerator
Authors: Gary, Dale E.; Chen, Bin; Bastian, Timothy S.; Shen, Chengcai;
Krucker, Sam
2015TESS....130402G Altcode:
Solar eruptions and their associated solar flares are the most energetic
particle accelerators in our solar system. Yet the acceleration
mechanism remains uncertain. A possible candidate often invoked in the
standard picture of solar eruptions is a termination shock, produced
by fast reconnection outflows impinging upon dense, closed loops in
a helmet-type geometry. However, the importance of termination shocks
in solar particle acceleration remains controversial, mainly because
there has been no direct detection of such shocks. Here we report direct
imaging of the location and evolution of a termination shock during the
rise phase of a solar eruption. The shock appears at radio wavelengths
as a narrow surface sandwiched between multitudes of downward-moving
plasma blobs and the underlying, newly-reconnected flaring loops,
and evolves coherently with a loop-top hard X-ray source in the shock
downstream region. The shock produces many short-lived, point-like
radio sources, each interpreted as emission from a turbulence cell
interacting with fast (nonthermal) electrons. These point-like radio
sources clearly outline the termination shock front and their positions
change in reaction to the arrival of the fast plasma blobs, which are
well-reproduced by our numerical simulations based on a resistive
magnetohydrodynamics reconnection model in a standard two-ribbon
flare geometry. We further show that a temporary disruption of the
shock coincides with a reduction of radio and hard X-ray emission
associated with the energetic electron population. Our observations
strongly favor a scenario in which the termination shock is responsible
for accelerating electrons to high energies.
---------------------------------------------------------
Title: STEREO- Wind Radio Positioning of an Unusually Slow Drifting
Event
Authors: Martínez-Oliveros, J. C.; Raftery, C.; Bain, H.; Liu, Y.;
Pulupa, M.; Saint-Hilaire, P.; Higgins, P.; Krupar, V.; Krucker,
Säm; Bale, S. D.
2015SoPh..290..891M Altcode: 2014arXiv1410.3352M
On 13 March 2010 an unusually long-duration event was observed by
radio spectrographs onboard the STEREO-B and Wind spacecraft. The
event started at about 13:00 UT and ended at approximately 06:00 UT on
14 March. The event presents itself as slow drifting, quasi-continuous
emission in a very narrow frequency interval, with an apparent frequency
drift from about 625 kHz to approximately 425 kHz. Using the Leblanc,
Dulk, and Bougeret (Solar Phys.183, 165, 1998) interplanetary density
model, we determined that the drift velocities of the radio source
are ≈ 33 km s<SUP>−1</SUP> and ≈ 52 km s<SUP>−1</SUP> for 0.2
and 0.5 times the densities of Leblanc model, respectively, with a
normalization density of 7.2 cm<SUP>−3</SUP> at 1 AU and assuming
harmonic emission. A joint analysis of the radio direction-finding
data, coronograph white-light observations and modeling revealed that
the radio sources appear to be located in interaction regions with
relatively high density and slow solar wind speed.
---------------------------------------------------------
Title: Co-Spatial White Light and Hard X-Ray Flare Footpoints Seen
Above the Solar Limb
Authors: Krucker, Säm; Saint-Hilaire, Pascal; Hudson, Hugh S.;
Haberreiter, Margit; Martinez-Oliveros, Juan Carlos; Fivian, Martin
D.; Hurford, Gordon; Kleint, Lucia; Battaglia, Marina; Kuhar, Matej;
Arnold, Nicolas G.
2015ApJ...802...19K Altcode:
We report analysis of three solar flares that occur within 1° of limb
passage, with the goal to investigate the source height of chromospheric
footpoints in white light (WL) and hard X-rays (HXR). We find the
WL and HXR (≥30 keV) centroids to be largely co-spatial and from
similar heights for all events, with altitudes around 800 km above
the photosphere or 300-450 km above the limb height. Because of the
extreme limb location of the events we study, emissions from such
low altitudes are influenced by the opacity of the atmosphere and
projection effects. STEREO images reveal that for SOL2012-11-20T12:36
the projection effects are smallest, giving upper limits of the absolute
source height above the nominal photosphere for both wavelengths of
∼1000 km. To be compatible with the standard thick target model,
these rather low altitudes require very low ambient densities within
the flare footpoints, in particular if the HXR-producing electrons
are only weakly beamed. That the WL and HXR emissions are co-spatial
suggests that the observed WL emission mechanism is directly linked
to the energy deposition by flare accelerated electrons. If the WL
emission is from low-temperature (≤slant {{10}<SUP>4</SUP>} K) plasma
as currently thought, the energy deposition by HXR-producing electrons
above ∼30 keV seems only to heat chromospheric plasma to such low
temperatures. This implies that the energy in flare-accelerated
electrons above ∼30 keV is not responsible for chromospheric
evaporation of hot (\gt {{10}<SUP>6</SUP>} K) plasma, but that their
energy is lost through radiation in the optical range.
---------------------------------------------------------
Title: Sign Singularity and Flares in Solar Active Region NOAA 11158
Authors: Sorriso-Valvo, L.; De Vita, G.; Kazachenko, M. D.; Krucker,
S.; Primavera, L.; Servidio, S.; Vecchio, A.; Welsch, B. T.; Fisher,
G. H.; Lepreti, F.; Carbone, V.
2015ApJ...801...36S Altcode: 2015arXiv150104279S
Solar Active Region NOAA 11158 has hosted a number of strong flares,
including one X2.2 event. The complexity of current density and
current helicity are studied through cancellation analysis of their
sign-singular measure, which features power-law scaling. Spectral
analysis is also performed, revealing the presence of two separate
scaling ranges with different spectral index. The time evolution of
parameters is discussed. Sudden changes of the cancellation exponents
at the time of large flares and the presence of correlation with
Extreme-Ultra-Violet and X-ray flux suggest that eruption of large
flares can be linked to the small-scale properties of the current
structures.
---------------------------------------------------------
Title: Electron Energy Partition in the Above-the-looptop Solar Hard
X-Ray Sources
Authors: Oka, Mitsuo; Krucker, Säm; Hudson, Hugh S.; Saint-Hilaire,
Pascal
2015ApJ...799..129O Altcode:
Solar flares produce non-thermal electrons with energies up to tens
of MeVs. To understand the origin of energetic electrons, coronal
hard X-ray (HXR) sources, in particular above-the-looptop sources,
have been studied extensively. However, it still remains unclear how
energies are partitioned between thermal and non-thermal electrons
within the above-the-looptop source. Here we show that the kappa
distribution, when compared to conventional spectral models, can
better characterize the above-the-looptop HXRs (gsim15 keV) observed
in four different cases. The widely used conventional model (i.e., the
combined thermal plus power-law distribution) can also fit the data,
but it returns unreasonable parameter values due to a non-physical sharp
lower-energy cutoff E<SUB>c</SUB>. In two cases, extreme-ultraviolet
data were available from SDO/AIA and the kappa distribution was still
consistent with the analysis of differential emission measure. Based
on the kappa distribution model, we found that the 2012 July 19 flare
showed the largest non-thermal fraction of electron energies about
50%, suggesting equipartition of energies. Considering the results of
particle-in-cell simulations, as well as density estimates of the four
cases studied, we propose a scenario in which electron acceleration is
achieved primarily by collisionless magnetic reconnection, but the
electron energy partition in the above-the-looptop source depends
on the source density. In low-density above-the-looptop regions
(few times 10<SUP>9</SUP> cm<SUP>-3</SUP>), the enhanced non-thermal
tail can remain and a prominent HXR source is created, whereas in
higher-densities (>10<SUP>10</SUP> cm<SUP>-3</SUP>), the non-thermal
tail is suppressed or thermalized by Coulomb collisions.
---------------------------------------------------------
Title: Hmi and Rhessi Measurements of the Radial Location of Solar
Flare Footpoints to Subarcsecond Accuracy
Authors: Krucker, S.; Saint-Hilaire, P.; Hudson, H. S.; Haberreiter,
M.; Kleint, L.; Hurford, G. J.; Fivian, M. D.; Battaglia, M.; Martinez
Oliveros, J. C.
2014AGUFMSH31C..05K Altcode:
We report analysis of three solar flares that occur within one degree
of limb passage, with the goal to investigate the source height of
chromospheric footpoints in white light (WL) and hard X-rays (HXR). The
optical observations are from the Helioseismic and Magnetic Imager
(HMI) around 617.3 nm, providing high precision observations with an
absolute positional accuracy in the radial direction below 0.1 arcsec
(~70 km), as referred to the adjacent limb. The Reuven Ramaty Higher
Energy Solar Spectroscopic Imager (RHESSI) gives HXR source centroids to
a similar accuracy depending on counting statistics. The observed height
of the emissions at either wavelength is influenced by the opacity of
the atmosphere at that wavelength and the height must correspond to a
radial distance from Sun center that is greater than the solar limb at
that wavelength (~350 km for WL and ~450 km for HXR). We find the WL
and HXR (~30 keV) centroids to be largely co-spatial and from similar
heights for all events, with altitudes around 800 km above the height of
the photosphere. The observed altitudes are limited by the uncertainty
of the precise heliographic locations near the limb and the resulting
projection effects. STEREO images reveal that for SOL2012-11-20T12:36
the projection effects are smallest, giving upper limits of the absolute
source height of 979+-70 km for the WL emission and 926+-51 km for HXR
source. Hence, the peak of the WL and HXR must be below 1000 km. To
be compatible with the standard thick target model, these rather low
altitudes require low ambient densities within the flare footpoints, in
particular if the HXR-producing electrons are only weakly beamed. That
the WL and HXR emissions are co-spatial suggests that the observed WL
emission mechanism is directly linked to the energy deposition by flare
accelerated electrons with energies above ~30 keV. If the WL emission
is from low-temperature (~10 000 K) plasma as currently thought, the
energy deposition by HXR-producing electrons above ~30 keV seems only
to heat chromospheric plasma to such low temperatures. This implies
that the energy in flare-accelerated electrons above ~30 keV is lost
through radiation in the optical range rather than heating chromospheric
plasma to coronal (> MK) temperatures.
---------------------------------------------------------
Title: Electron Energy Partition in the Above-the-looptop Solar Hard
X-ray Sources
Authors: Oka, M.; Krucker, S.; Hudson, H. S.; Saint-Hilaire, P.
2014AGUFMSH23A4147O Altcode:
Solar flares produce non-thermal electrons with energies up to tens of
MeVs. To understand the origin of energetic electrons, coronal hard
X-ray sources in particular 'above-the-looptop' sources have been
studied extensively. However, it still remains unclear how energies
are partitioned between thermal and non-thermal electrons within the
above-the-looptop source. Here we show that the kappa distribution,
when compared to conventional spectral models, can better characterize
the above-the- looptop hard X-rays (>~15 keV) observed in four
different cases. The combined thermal plus power-law distribution can
also fit the data, but it returns unreasonable parameter values due to
the artifact of its sharp, lower-energy cutoff Ec. In two cases with
extreme-ultraviolet data from SDO/AIA, the analysis of differential
emission measure (DEM) did not rule out the kappa distribution
model. Based on the kappa distribution model, we found that the 2012
July 19 flare showed the largest non-thermal fraction of electron
energies about 50%, suggesting equipartition. Considering results of
particle-in-cell (PIC) simulations as well as density estimates of the
four cases studied, we propose a scenario in which electron energization
is achieved primarily by collisionless magnetic reconnection but the
non-thermal tail can be suppressed or thermalized by Coulomb collisions.
---------------------------------------------------------
Title: NuSTAR's First Solar Observations: Search for Transient
Brightenings / Nanoflares
Authors: Marsh, A.; Hannah, I. G.; Glesener, L.; Smith, D. M.;
Grefenstette, B.; Krucker, S.; Hudson, H. S.; Hurford, G. J.; White,
S.; Caspi, A.; Christe, S.; Shih, A.; Mewaldt, R. A.; Pivovaroff,
M.; Vogel, J.
2014AGUFMSH13C4129M Altcode:
We present a timing analysis of the Sun with the NuSTAR hard X-ray
(HXR) telescope, searching for transient brightenings / nanoflares in
the quiet Sun and active regions. A substantial amount of flare energy
goes into accelerating electrons. HXR observations are a crucial tool
for understanding this non-thermal emission and the energy release
in flares. RHESSI is able to study this emission over many orders of
magnitude (active region flares from X-class to A-class microflares),
but it cannot detect the emission from smaller events. Such "nanoflares"
have been postulated as a possible source of coronal heating and their
existence and relationship to larger flares is still uncertain. In
order to detect these events in HXRs, instruments more sensitive
than RHESSI are required. Launched in 2012, the astrophysics mission
NuSTAR uses focusing optics to directly image X-rays between ~2-80
keV. Although not optimized for solar observations, NuSTAR's highly
sensitive imaging spectroscopy will be used to search for the faintest
X-ray emission from the Sun. These solar observations will begin in
September 2014; here we present the first results of our search for
transient brightenings that could relate to nanoflares.
---------------------------------------------------------
Title: The Focusing Optics X-ray Solar Imager: Second Flight and
Recent Results
Authors: Christe, S.; Krucker, S.; Glesener, L.; Ishikawa, S. N.;
Ramsey, B.; Buitrago Casas, J. C.; Foster, N.
2014AGUFMSH53B4227C Altcode:
Solar flares accelerate particles up to high energies through various
acceleration mechanisms which are not currently understood. Hard X-rays
are the most direct diagnostic of flare-accelerated electrons. However
past and current hard x-ray observation lack the sensitivity and dynamic
range necessary to observe the faint signature of accelerated electrons
in the acceleration region, the solar corona. These limitations can be
easily overcome through the use of HXR focusing optics coupled with
solid state pixelated detectors. We present on recent updates on the
FOXSI sounding rocket program. During its first flight FOXSI observed
imaged a microflare with simultaneous observations by RHESSI. We
present recent imaging analysis of the FOXSI observations and detailed
comparison with RHESSI. New detector calibration results are also
presented and, time-permitting, preliminary results from the second
launch of FOXSI scheduled for December 2014.
---------------------------------------------------------
Title: Constraining hot plasma in a non-flaring solar active region
with FOXSI hard X-ray observations
Authors: Ishikawa, Shin-nosuke; Glesener, Lindsay; Christe, Steven;
Ishibashi, Kazunori; Brooks, David H.; Williams, David R.; Shimojo,
Masumi; Sako, Nobuharu; Krucker, Säm
2014PASJ...66S..15I Altcode: 2015arXiv150905288I; 2014PASJ..tmp..102I
We present new constraints on the high-temperature emission measure
of a non-flaring solar active region using observations from the
recently flown Focusing Optics X-ray Solar Imager (FOXSI) sounding
rocket payload. FOXSI has performed the first focused hard X-ray
(HXR) observation of the Sun in its first successful flight on 2012
November 2. Focusing optics, combined with small strip detectors,
enable high-sensitivity observations with respect to previous
indirect imagers. This capability, along with the sensitivity of
the HXR regime to high-temperature emission, offers the potential
to better characterize high-temperature plasma in the corona as
predicted by nanoflare heating models. We present a joint analysis of
the differential emission measure (DEM) of active region 11602 using
coordinated observations by FOXSI, Hinode/XRT, and Hinode/EIS. The
Hinode-derived DEM predicts significant emission measure between
1 MK and 3 MK, with a peak in the DEM predicted at 2.0-2.5 MK. The
combined XRT and EIS DEM also shows emission from a smaller population
of plasma above 8 MK. This is contradicted by FOXSI observations that
significantly constrain emission above 8 MK. This suggests that the
Hinode DEM analysis has larger uncertainties at higher temperatures and
that > 8 MK plasma above an emission measure of 3 × 10<SUP>44</SUP>
cm<SUP>-3</SUP> is excluded in this active region.
---------------------------------------------------------
Title: Statistical Study of Coronal Hard X-ray Source Heights
and Fluxes
Authors: Glesener, L.; Oka, M.; Krucker, S.; Hudson, H. S.
2014AGUFMSH11D..03G Altcode:
Hard X-ray observation of partly occulted flares has proven useful
for studying flare-accelerated electrons in the solar corona. These
nonthermal electrons emit bremsstrahlung hard X-rays (HXRs), but
are difficult to observe for on-disk flares because the much brighter
chromospheric footpoints tend to dominate HXR images. Previous research
using the RHESSI spacecraft has performed individual and statistical
study of HXR sources in partly occulted flares to investigate,
for example, spectral characteristics and the relationship between
nonthermal coronal sources and thermal loops. Source heights are not
usually measured in these cases because of the difficulty in determining
heliographic locations of flares beyond the limb. Occasionally,
multi-spacecraft observation will identify a source location and thus
calculate an absolute HXR source height. Microflare source heights have
also been studied statistically by fitting distribution functions to the
observed projected locations. But so far, a statistical study of coronal
HXR sources in which source heights are determined individually for each
flare has not been performed. In this work, we study flares jointly
observed by RHESSI and STEREO/EUVI over a 2+ year time range. From
RHESSI data, we obtain coronal HXR source positions and fluxes. The
alternate viewing angle offered by STEREO provides flare locations,
enabling RHESSI source positions to be translated into absolute heights
above the photosphere. We will present the distribution of coronal HXR
source heights and will discuss their possible corresponding features
in the standard flare model, i.e. thermal loops, nonthermal looptop
sources, above-the-looptop sources, and ejecta.
---------------------------------------------------------
Title: MiXI: The Miniature X-ray Imager
Authors: Martinez Oliveros, J. C.; Glesener, L.; Sundkvist, D. J.;
Saint-Hilaire, P.; Bain, H. M.; Fivian, M. D.; Hurford, G. J.; Sample,
J. G.; Bale, S. D.; Krucker, S.
2014AGUFMSH53B4222M Altcode:
The Miniature X-ray Imager (MiXI) is an innovative, small, and fully
functional solar X-ray observatory concept designed to fit within a 6U
CubeSat platform. MiXI will provide the community with X-ray imaging
in the energy range from ~6 to 40-50 keV and spectroscopy up to 100
keV of solar flares at a small fraction of the cost of a conventional
mission. It includes rotation modulation collimators and layered Si/CdTe
detectors, providing routine observations of both soft and hard X-ray
emission with low background. Coordinated observations between MiXI
and the STIX instrument onboard Solar Orbiter will enable solar flare
observation from two vantage points, providing new insights into the
directivity of flare HXR emission and will allow detailed study of both
coronal and footpoint sources within the same flare. These results may
have profound implications for theories of flare acceleration processes.
---------------------------------------------------------
Title: Harnessing AIA Diffraction Patterns to Determine Flare
Footpoint Temperatures
Authors: Bain, H. M.; Schwartz, R. A.; Torre, G.; Krucker, S.; Raftery,
C. L.
2014AGUFMSH13B4105B Altcode:
In the "Standard Flare Model" energy from accelerated electrons
is deposited at the footpoints of newly reconnected flare loops,
heating the surrounding plasma. Understanding the relation between
the multi-thermal nature of the footpoints and the energy flux
from accelerated electrons is therefore fundamental to flare
physics. Extreme ultraviolet (EUV) images of bright flare kernels,
obtained from the Atmospheric Imaging Assembly (AIA) onboard the Solar
Dynamics Observatory, are often saturated despite the implementation
of automatic exposure control. These kernels produce diffraction
patterns often seen in AIA images during the most energetic flares. We
implement an automated image reconstruction procedure, which utilizes
diffraction pattern artifacts, to de-saturate AIA images and reconstruct
the flare brightness in saturated pixels. Applying this technique to
recover the footpoint brightness in each of the AIA EUV passbands, we
investigate the footpoint temperature distribution. Using observations
from the Ramaty High Energy Solar Spectroscopic Imager (RHESSI), we
will characterize the footpoint accelerated electron distribution of
the flare. By combining these techniques, we investigate the relation
between the nonthermal electron energy flux and the temperature response
of the flare footpoints.
---------------------------------------------------------
Title: Coronal Signatures of Magnetic Energy Dissipation in Solar
Eruptions
Authors: Krucker, S.
2014AGUFMSM43C..01K Altcode:
This talk will highlight recent observational results of magnetic
energy dissipation in solar flares and coronal mass ejection with a
focus on hard X-ray observations provided by the Reuven Ramaty High
Energy Solar Spectroscopic Imager (RHESSI).
---------------------------------------------------------
Title: High-resolution Observations of the X-flare on 2014-03-29
Authors: Kleint, L.; Battaglia, M.; Krucker, S.; Reardon, K.; Sainz
Dalda, A.
2014AGUFMSH31C..06K Altcode:
We investigate the sequence of events leading to the X1 flare
SOL2014-03-29T17:48. Because of the unprecedented joint observations of
an X-flare with the ground-based Dunn Solar Telescope and the spacecraft
IRIS, Hinode, RHESSI, STEREO, and SDO, we can sample many solar layers
from the photosphere to the corona. We find that a filament eruption,
which was possibly caused by a thermal instability, was the cause of
this X-flare. The filament was rising in the chromosphere for at least
one hour before the flare occurred with a velocity of ∼2--5 km/s. 15
minutes before the flare, its chromospheric rise velocity increased to
∼6--10 km/s, before it lifted off with at least 600 km/s, as seen by
IRIS in the transition region. Doppler velocities from H-alpha images
reveal intriguing small scale flows along the filament and enable us to
derive its probable shape. An unusual feature was a low-lying twisted
flux rope near the filament, which did not participate in the filament
eruption. After the flare ribbons started on each of its sides, the
flux rope seems to have untangled and vanished during the flare. We
present a comprehensive overview of the flare, including polarimetric
and spectroscopic data at subarcsecond resolution.
---------------------------------------------------------
Title: Does There Exist a Relationship Between Acoustic and
White-Light Emission in Hard-X ray Solar Flares?
Authors: Buitrago-Casas, J. C.; Martinez Oliveros, J. C.; Glesener,
L.; Krucker, S.; Calvo-Mozo, B.
2014AGUFMSH11D..02B Altcode:
Several mechanisms have been proposed to explain the observed seismicity
during some solar flares. One theory associates high-energy electrons
and white-light emission with sunquakes. This relationship is based
on the back-warming model, where high-energy electrons and their
subsequent heating of the photosphere induce acoustic waves in the
solar interior. We carried out a correlative study of solar flares
with emission in hard-X rays (HXRs) above 50 keV, enhanced white light
emission at 6573Å, and acoustic sources. We selected those flares
observed by RHESSI (Reuven Ramaty High Energy Solar Spectroscopic
Imager) with a considerable flux in the 50-100 and 100-300 keV bands
between January 1, 2010 and June 26, 2014. Additionally, we restricted
the sample to flares close to disk center where it is observationally
easiest to detect a sunquake. We then used data from the Helioseismic
and Magnetic Imager onboard the Solar Dynamic Observatory (SDO/HMI) to
search for white-light emission and helioseismic signatures. Finally,
we calculated a coefficient of correlation for this set of dichotomic
observables. We discuss the phenomenological connectivity between
these physical quantities and the observational difficulties of
detecting seismic signals and white-light radiation with terrestrial
and space-borne observations.
---------------------------------------------------------
Title: NuSTAR's first solar observations: Search for a high energy
X-ray component to the "non-flaring" Sun
Authors: Marsh, A.; Hannah, I. G.; Glesener, L.; Smith, D. M.;
Grefenstette, B.; Krucker, S.; Hudson, H. S.; Hurford, G. J.; White,
S.; Caspi, A.; Christe, S.; Shih, A.; Mewaldt, R. A.; Pivovaroff,
M.; Vogel, J.
2014AGUFMSH12A..04M Altcode:
We present spectroscopy of the Sun with the NuSTAR hard X-ray (HXR)
telescope, searching for high temperature and non-thermal emission
in the "non-flaring" Sun. A substantial amount of flare energy goes
into accelerating electrons. HXR observations are a crucial tool
for understanding this non-thermal emission and the energy release
in flares. RHESSI is able to study this emission over many orders of
magnitude (active region flares from X-class to A-class microflares),
but it cannot detect the emission from smaller events. Such "nanoflares"
have been postulated as a possible source of coronal heating and their
existence and relationship to larger flares is still uncertain. In
order to detect these events in HXRs, instruments more sensitive
than RHESSI are required. Launched in 2012, the astrophysics mission
NuSTAR uses focusing optics to directly image X-rays between ~2-80
keV. Although not optimized for solar observations, NuSTAR's highly
sensitive imaging spectroscopy will be used to search for the faintest
X-ray emission from the Sun. These solar observations will begin in
September 2014. Here we present the first results of our search for
transient brightenings in active and quiet Sun regions with NuSTAR.
---------------------------------------------------------
Title: Magnetic Field-line Length and Twist Distributions within
Interplanetary Flux Fopes from Wind Spacecraft Measurements
Authors: Hu, Q.; Qiu, J.; Krucker, S.; Wang, L.; Wang, B.; Chen, Y.;
Moestl, C.
2014AGUFMSH31A4104H Altcode:
We report on the detailed and systematic study of field-line twist
and length distributions within magnetic flux ropes embedded in
Interplanetary Coronal Mass Ejections (ICMEs). In particular we
will utilize energetic electron burst observations at 1 AU together
with associated type III radio emissions detected by the Wind
spacecraft to provide unique measurements of magnetic field-line
lengths within selected ICME events. These direct measurements will
be compared with flux-rope model calculations to help assess the
fidelity of different models and to provide diagnostics of internal
structures. The Grad-Shafranov reconstruction method will be utilized
together with a constant-twist nonlinear force-free (Gold-Hoyle) flux
rope model and the commonly known Lundquist (linear force-free) model
to reveal the close relation between the field-line twist and length
in cylindrical flux ropes, based on in-situ Wind spacecraft magnetic
field and plasma measurements. We show that our initial analysis of
field-line twist indicates clear deviation from the Lundquist model, but
better consistency with the Gold-Hoyle model. We will also discuss the
implications of our analysis of flux-rope structures on the origination
and evolution processes in their corresponding solar source regions.
---------------------------------------------------------
Title: First Images from the Focusing Optics X-Ray Solar Imager
Authors: Krucker, Säm; Christe, Steven; Glesener, Lindsay; Ishikawa,
Shin-nosuke; Ramsey, Brian; Takahashi, Tadayuki; Watanabe, Shin; Saito,
Shinya; Gubarev, Mikhail; Kilaru, Kiranmayee; Tajima, Hiroyasu; Tanaka,
Takaaki; Turin, Paul; McBride, Stephen; Glaser, David; Fermin, Jose;
White, Stephen; Lin, Robert
2014ApJ...793L..32K Altcode:
The Focusing Optics X-ray Solar Imager (FOXSI) sounding rocket payload
flew for the first time on 2012 November 2, producing the first focused
images of the Sun above 5 keV. To enable hard X-ray (HXR) imaging
spectroscopy via direct focusing, FOXSI makes use of grazing-incidence
replicated optics combined with fine-pitch solid-state detectors. On
its first flight, FOXSI observed several targets that included active
regions, the quiet Sun, and a GOES-class B2.7 microflare. This Letter
provides an introduction to the FOXSI instrument and presents its first
solar image. These data demonstrate the superiority in sensitivity and
dynamic range that is achievable with a direct HXR imager with respect
to previous, indirect imaging methods, and illustrate the technological
readiness for a spaceborne mission to observe HXRs from solar flares
via direct focusing optics.
---------------------------------------------------------
Title: Caliste-SO: the x-ray spectrometer unit of the STIX instrument
onboard the Solar Orbiter space mission
Authors: Meuris, Aline; Limousin, Olivier; Gevin, Olivier; Vassal,
Marie-Cécile; Soufflet, Fabrice; Fiant, Nicolas; Bednarzik, Martin;
Wild, Christopher; Stutz, Stefan; Birrer, Guy; Blondel, Claire;
Le Mer, Isabelle; Huynh, Duc-Dat; Donati, Modeste; Grimm, Oliver;
Commichau, Volker; Hurford, Gordon; Krucker, Säm.; Gonzalez,
François; Billot, Marc
2014SPIE.9154E..0YM Altcode:
Caliste-SO is a hybrid detector integrating in a volume of 12 × 14 ×
18 mm<SUP>3</SUP> a 1 mm-thick CdTe pixel detector, a frontend IDeF-X
HD ASIC and passive parts to perform high resolution spectroscopy
in the 4-200 keV energy range with high count rate capability
(10<SUP>4</SUP>-10<SUP>5</SUP> photons/s/cm<SUP>2</SUP>). The detector
hybridization concept was designed by CEA and 3DPlus to realize CdTe
cameras for space astronomy missions with various pixel patterns. For
the STIX instrument onboard the Solar Orbiter mission, the imaging
system is made by 32 collimators that sample the visibilities of the
spatial Fourier transform and doesn't require fine pitch pixels. The
Al-Schottky CdTe detectors produced by Acrorad are then patterned and
tested by the Paul Scherrer Institute to produce 12 pixels surrounded
by a guard ring within 1 cm<SUP>2</SUP>. Electrical and spectroscopic
performance tests of the Caliste-SO samples are performed in France at
key manufacturing steps, before sending the samples to the principal
investigator to mount them in the Detector Electronics Module of STIX in
front of each collimator. Four samples were produced in 2013 to be part
of the STIX engineering model. Best pixels show an energy resolution
of 0.7 keV FWHM at 6 keV (1 keV resolution requirement for STIX) and
a low-level detection threshold below 3 keV (4 keV requirement for
STIX). The paper describes the design and the production of Caliste-SO
and focuses on main performance tests performed so far to characterize
the spectrometer unit.
---------------------------------------------------------
Title: Observing the Sun in hard X-rays using grazing incidence
optics: the FOXSI and HEROES projects
Authors: Christe, Steven; Glesener, Lindsay; Krucker, Sam; Shih,
Albert Y.; Gaskin, Jessica; Wilson, Colleen
2014AAS...22412362C Altcode:
Solar flares accelerate particles up to high energies through various
acceleration mechanisms which are not currently understood. Hard X-rays
are the most direct diagnostic of flare-accelerated electrons. However
past and current hard x-ray observation lack the sensitivity and dynamic
range necessary to observe the faint signature of accelerated electrons
in the acceleration region, the solar corona. These limitations can be
easily overcome through the use of HXR focusing optics coupled with
solid state pixelated detectors. We present results from the recent
flights of two sub-orbital payloads that have applied grazing incidence
HXR optics to solar observations. FOXSI, short for Focusing Optics X-Ray
Solar Imager, was launched on a sounding rocket in November 2012 from
White Sanda and observed a solar flare. HEROES, short for High Energy
Replicated Optics to Explore the Sun, observed the sun for 7 hours from
a high altitude balloon on September 21, 2013. We present recent results
as well as the capabilities of a possible future satellite mission
---------------------------------------------------------
Title: RHESSI and EIS observations of an above-the-looptop
reconnection region
Authors: Glesener, Lindsay; Hara, Hirohisa; Krucker, Sam
2014AAS...22410406G Altcode:
A variety of solar flare observations suggest particle acceleration in
the corona, at or above the flare looptop. Hard X-ray (HXR) studies,
for example, occasionally reveal accelerated electrons above flare
looptops, in some cases suggesting the location of the acceleration
region. However, since coronal HXR sources are faint and the structure
of the flare as seen in extreme ultraviolet (EUV) images is complicated,
it is difficult to say where these sources lie with respect to,
for example, the reconnection region.HXR and EUV observations can
provide complementary information for investigating this topic. EUV
imaging spectroscopy reveals bulk flows and locations of line-broadened
(potentially turbulent) sources. Such observations can, for example,
identify outflows from the reconnection region. HXR imaging places
the flare-accelerated electrons in the context of the overall flare
geometry, allowing comparison of the locations of accelerated electrons,
the reconnection region, and the flare loop.In this work, data from
RHESSI and Hinode/EIS are used to investigate above-the-looptop sources
in the 2013 May 15 X-class flare. Above-the-looptop EIS flows and
loop-top line-broadening are compared with RHESSI HXR sources in the
preimpulsive phase of the flare.
---------------------------------------------------------
Title: Current and future solar observation using focusing hard
X-ray imagers
Authors: Glesener, Lindsay; Caspi, Amir; Christe, Steven; Hannah, Iain;
Hudson, Hugh S.; Hurford, Gordon J.; Grefenstette, Brian; Krucker,
Sam; Marsh, Andrew; Mewaldt, Richard A.; Pivovaroff, Michael; Shih,
Albert Y.; Smith, David M.; Vogel, Julia; White, Stephen M.
2014AAS...22412364G Altcode:
The efficient processes that accelerate particles in solar flares
are not currently understood. Hard X-rays (HXRs) are one of the
best diagnostics of flare-accelerated electrons, and therefore of
acceleration processes. Past and current solar HXR observers rely on
indirect Fourier imaging and thus lack the necessary sensitivity and
imaging dynamic range to make detailed studies of faint HXR sources in
the solar corona (where particle acceleration is thought to occur). A
future generation of solar HXR observers will instead likely rely on
direct HXR focusing, which can provide far superior sensitivity and
imaging dynamic range.The first wave of focused solar HXR studies
is already underway, including sounding rocket and high-altitude
balloon payloads, and, in the near future, solar observation by the
NuSTAR astrophysics observatory. This poster will (1) summarize the
capabilities of current solar HXR instruments, comparing the science
that can be done from each platform, and (2) discuss the scientific
power of a future, dedicated, spaceborne observatory optimized to
observe HXRs from the Sun.
---------------------------------------------------------
Title: Energetic analysis of the white light emission associated to
seismically active flares in solar cycle 24
Authors: Buitrago-Casas, Juan Camilo; Martinez Oliveros, Juan Carlos;
Glesener, Lindsay; Krucker, Sam
2014AAS...22412338B Altcode:
Solar flares are explosive phenomena, thought to be driven by magnetic
free energy accumulated in the solar corona. Some flares release seismic
transients, "sunquakes", into the Sun's interior. Different mechanisms
are being considered to explain how sunquakes are generated. We are
conducting an analysis of white-light emission associated with those
seismically active solar flares that have been reported by different
authors within the current solar cycle. Seismic diagnostics are based
upon standard time-distance techniques, including seismic holography,
applied to Dopplergrams obtained by SDO/HMI and GONG. The relation
between white-light emissions and seismic activity may provide
important information on impulsive chromospheric heating during
flares, a prospective contributor to seismic transient emission, at
least in some instances. We develop a method to get an estimation of
Energy associated whit white-light emission and compare those results
whit values of energy needed to generate a sunquake according with
holographic helioseismology techniques.
---------------------------------------------------------
Title: Chromospheric and Coronal HMI Flare Sources
Authors: Martinez Oliveros, Juan Carlos; Saint-Hilaire, Pascal;
Couvidat, Sebastien; Hudson, Hugh S.; Krucker, Sam
2014AAS...22412331M Altcode:
We present observations of white-light features in the low corona, for
three flares SOL20110308T1935, SOL20110308T0230 and SOL2013-05-13T16:01,
using data from the Helioseismic and Magnetic Imager (HMI) of the
Solar Dynamics Observatory. At least two distinct kinds of sources
appear (chromospheric and coronal) in the early and later phases of
flare development, in addition to the white-light footpoint sources
commonly observed in the lower atmosphere. The gradual emissions have
a clear identification with the classical loop-prominence system,
with emission contributions from electron scattering and from the
free-free continuum (as seen in soft X-rays). These sources may also
contain other continuum and/or line emissions and lead clearly to
coronal rain in some cases observed
---------------------------------------------------------
Title: Helioseismic and Magnetic Imager Observations of Linear
Polarization from a Loop Prominence System
Authors: Saint-Hilaire, Pascal; Schou, Jesper; Martinez Oliveros, Juan
Carlos; Hudson, Hugh S.; Krucker, Sam; Bain, Hazel; Couvidat, Sebastien
2014AAS...22412311S Altcode:
White-light observations by the Solar Dynamics Observatory's
Helioseismic and Magnetic Imager of a loop-prominence system occurring
in the aftermath of an X-class flare on 2013 May 13 near the eastern
solar limb show a linearly polarized component, reaching up to 20%
at an altitude of 33 Mm, about the maximal amount expected if the
emission were due solely to Thomson scattering of photospheric light
by the coronal material. The mass associated with the polarized
component was 8.2x10^14 g. At 15 Mm altitude, the brightest part of
the loop was 3(+/-0.5)% linearly polarized, only about 20% of that
expected from pure Thomson scattering, indicating the presence of an
additional unpolarized component at wavelengths near Fe I (617.33 nm),
probably thermal emission. We estimated the free electron density of
the white-light loop system to possibly be as high as 1.8x10^12 cm^-3.
---------------------------------------------------------
Title: Observations of Linear Polarization in a Solar Coronal Loop
Prominence System Observed near 6173 Å
Authors: Saint-Hilaire, Pascal; Schou, Jesper; Martínez Oliveros,
Juan-Carlos; Hudson, Hugh S.; Krucker, Säm; Bain, Hazel; Couvidat,
Sébastien
2014ApJ...786L..19S Altcode: 2014arXiv1402.7016S
White-light observations by the Solar Dynamics Observatory's
Helioseismic and Magnetic Imager of a loop-prominence system occurring
in the aftermath of an X-class flare on 2013 May 13 near the eastern
solar limb show a linearly polarized component, reaching up to ~20%
at an altitude of ~33 Mm, about the maximum amount expected if the
emission were due solely to Thomson scattering of photospheric light by
the coronal material. The mass associated with the polarized component
was 8.2 × 10<SUP>14</SUP> g. At 15 Mm altitude, the brightest part
of the loop was 3(±0.5)% linearly polarized, only about 20% of that
expected from pure Thomson scattering, indicating the presence of an
additional unpolarized component at wavelengths near Fe I (617.33
nm). We estimate the free electron density of the white-light loop
system to possibly be as high as 1.8 × 10<SUP>12</SUP> cm<SUP>-3</SUP>.
---------------------------------------------------------
Title: Radio Imaging of a Type IVM Radio Burst on the 14th of
August 2010
Authors: Bain, H. M.; Krucker, S.; Saint-Hilaire, P.; Raftery, C. L.
2014ApJ...782...43B Altcode:
Propagating coronal mass ejections (CMEs) are often accompanied by burst
signatures in radio spectrogram data. We present Nançay Radioheliograph
observations of a moving source of broadband radio emission, commonly
referred to as a type IV radio burst (type IVM), which occurred in
association with a CME on the 14th of August 2010. The event was
well observed at extreme ultraviolet (EUV) wavelengths by SDO/AIA and
PROBA2/SWAP, and by the STEREO SECCHI and SOHO LASCO white light (WL)
coronagraphs. The EUV and WL observations show the type IVM source to
be cospatial with the CME core. The observed spectra is well fitted by a
power law with a negative slope, which is consistent with optically thin
gyrosynchrotron emission. The spectrum shows no turn over at the lowest
Nançay frequencies. By comparing simulated gyrosynchrotron spectra
with Nançay Radioheliograph observations, and performing a rigorous
parameter search we are able to constrain several key parameters of the
underlying plasma. Simulated spectra found to fit the data suggest a
nonthermal electron distribution with a low energy cutoff of several
tens to 100 keV, with a nonthermal electron density in the range
10<SUP>0</SUP>-10<SUP>2</SUP> cm<SUP>-3</SUP>, in a magnetic field of
a few Gauss. The nonthermal energy content of the source is found to
contain 0.001%-0.1% of the sources thermal energy content. Furthermore,
the energy loss timescale for this distribution equates to several
hours, suggesting that the electrons could be accelerated during the
CME initiation or early propagation phase and become trapped in the
magnetic structure of the CME core without the need to be replenished.
---------------------------------------------------------
Title: Statistical Properties of Super-hot Solar Flares
Authors: Caspi, Amir; Krucker, Säm; Lin, R. P.
2014ApJ...781...43C Altcode: 2013arXiv1312.0371C
We use Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI)
high-resolution imaging and spectroscopy observations from ~6 to 100 keV
to determine the statistical relationships between measured parameters
(temperature, emission measure, etc.) of hot, thermal plasma in 37
intense (GOES M- and X-class) solar flares. The RHESSI data, most
sensitive to the hottest flare plasmas, reveal a strong correlation
between the maximum achieved temperature and the flare GOES class, such
that "super-hot" temperatures >30 MK are achieved almost exclusively
by X-class events; the observed correlation differs significantly
from that of GOES-derived temperatures, and from previous studies. A
nearly ubiquitous association with high emission measures, electron
densities, and instantaneous thermal energies suggests that super-hot
plasmas are physically distinct from cooler, ~10-20 MK GOES plasmas,
and that they require substantially greater energy input during the
flare. High thermal energy densities suggest that super-hot flares
require strong coronal magnetic fields, exceeding ~100 G, and that
both the plasma β and volume filling factor f cannot be much less
than unity in the super-hot region.
---------------------------------------------------------
Title: Chromospheric and Coronal Observations of Solar Flares with
the Helioseismic and Magnetic Imager
Authors: Martínez Oliveros, Juan-Carlos; Krucker, Säm; Hudson, Hugh
S.; Saint-Hilaire, Pascal; Bain, Hazel; Lindsey, Charles; Bogart,
Rick; Couvidat, Sebastien; Scherrer, Phil; Schou, Jesper
2014ApJ...780L..28M Altcode: 2013arXiv1311.7412M
We report observations of white-light ejecta in the low corona, for
two X-class flares on 2013 May 13, using data from the Helioseismic
and Magnetic Imager (HMI) of the Solar Dynamics Observatory. At least
two distinct kinds of sources appeared (chromospheric and coronal),
in the early and later phases of flare development, in addition to
the white-light footpoint sources commonly observed in the lower
atmosphere. The gradual emissions have a clear identification
with the classical loop-prominence system, but are brighter than
expected and possibly seen here in the continuum rather than line
emission. We find the HMI flux exceeds the radio/X-ray interpolation
of the bremsstrahlung produced in the flare soft X-ray sources by at
least one order of magnitude. This implies the participation of cooler
sources that can produce free-bound continua and possibly line emission
detectable by HMI. One of the early sources dynamically resembles
"coronal rain", appearing at a maximum apparent height and moving
toward the photosphere at an apparent constant projected speed of 134
± 8 km s<SUP>-1</SUP>. Not much literature exists on the detection of
optical continuum sources above the limb of the Sun by non-coronagraphic
instruments and these observations have potential implications for our
basic understanding of flare development, since visible observations
can in principle provide high spatial and temporal resolution.
---------------------------------------------------------
Title: Particle Densities within the Acceleration Region of a
Solar Flare
Authors: Krucker, Säm; Battaglia, Marina
2014ApJ...780..107K Altcode:
The limb flare SOL2012-07-19T05:58 (M7.7) provides the best example of
a non-thermal above-the-loop-top hard X-ray source with simultaneous
observations by the Reuven Ramaty High Energy Solar Spectroscopic
Imager (RHESSI) and the Atmospheric Imaging Assembly on board the Solar
Dynamic Observatory. By combining the two sets of observations, we
present the first direct measurement of the thermal proton density and
non-thermal electron density within the above-the-loop-top source where
particle acceleration occurs. We find that both densities are of the
same order of magnitude of a few times 10<SUP>9</SUP> cm<SUP>-3</SUP>,
about 30 times lower than the density in the underlying thermal flare
loops. The equal densities indicate that the entire electron population
within the above-the-loop-top source is energized. While the derived
densities depend on the unknown source depth and filling factor,
the ratio of these two densities does not. Within the uncertainties,
the ratio is one for a low energy cutoff of the non-thermal electron
spectrum between 10 and 15 keV. RHESSI observations only constrain
the cutoff energy to below ~15 keV, leaving the spectral shape of
the electrons within the above-the-loop-top source at lower energies
unknown. Nevertheless, these robust results strongly corroborate earlier
findings that the above-the-loop-top source is the acceleration region
where a bulk energization process acts on all electrons.
---------------------------------------------------------
Title: Observation of Heating by Flare-accelerated Electrons in a
Solar Coronal Mass Ejection
Authors: Glesener, Lindsay; Krucker, Säm; Bain, Hazel M.; Lin,
Robert P.
2013ApJ...779L..29G Altcode:
We report a Reuven Ramaty High Energy Solar Spectroscopic Imager
(RHESSI) observation of flare-accelerated electrons in the core of
a coronal mass ejection (CME) and examine their role in heating the
CME. Previous CME observations have revealed remarkably high thermal
energies that can far surpass the CME's kinetic energy. A joint
observation by RHESSI and the Atmospheric Imaging Assembly of a partly
occulted flare on 2010 November 3 allows us to test the hypothesis that
this excess energy is collisionally deposited by flare-accelerated
electrons. Extreme ultraviolet (EUV) images show an ejection forming
the CME core and sheath, with isothermal multifilter analysis revealing
temperatures of ~11 MK in the core. RHESSI images reveal a large (~100
× 50 arcsec<SUP>2</SUP>) hard X-ray (HXR) source matching the location,
shape, and evolution of the EUV plasma, indicating that the emerging
CME is filled with flare-accelerated electrons. The time derivative of
the EUV emission matches the HXR light curve (similar to the Neupert
effect observed in soft and HXR time profiles), directly linking the CME
temperature increase with the nonthermal electron energy loss, while
HXR spectroscopy demonstrates that the nonthermal electrons contain
enough energy to heat the CME. This is the most direct observation
to date of flare-accelerated electrons heating a CME, emphasizing the
close relationship of the two in solar eruptive events.
---------------------------------------------------------
Title: Plasma Heating in a Post Eruption Current Sheet: A Case Study
Based on Ultraviolet, Soft, and Hard X-Ray Data
Authors: Susino, R.; Bemporad, A.; Krucker, Säm
2013ApJ...777...93S Altcode: 2013arXiv1310.2853S
Off-limb observations of the solar corona after coronal mass ejections
(CMEs) often show strong, compact, and persistent UV sources behind the
eruption. They are primarily observed by the SOHO/UVCS instrument in the
"hot" [Fe XVIII] λ974 line and are usually interpreted as a signature
of plasma heating due to magnetic reconnection in the post-CME current
sheet (CS). Nevertheless, the physical process itself and the altitude
of the main energy release are currently not fully understood. In
this work, we study the evolution of plasma heating after the CME of
2004 July 28 by comparing UV spectra acquired by UVCS with soft and
hard X-ray (SXR, HXR) images of the post-flare loops taken by GOES/SXI
and RHESSI. The X-ray data show a long-lasting extended source that is
rising upward, toward the high-temperature source detected by UVCS. UVCS
data show the presence of significant non-thermal broadening in the
CS (a signature of turbulent motions) and a strong density gradient
across the CS region. The thermal energy released in the HXR source
is on the order of ~10<SUP>32</SUP> erg, a factor ~2-5 larger than
the energy required to explain the high-temperature plasma sampled
by UVCS. Nevertheless, the very different time evolutions of SXR and
HXR sources compared with the UV emission suggest that reconnection
occurring above the post-eruption arcades is not directly responsible
for the high-temperature plasma sampled higher up by UVCS. We conclude
that an additional plasma heating mechanism (such as turbulent
reconnection) in the CS is likely required.
---------------------------------------------------------
Title: Solar Eruptive Events (SEE) 2020 Mission Concept
Authors: Lin, R. P.; Caspi, A.; Krucker, S.; Hudson, H.; Hurford,
G.; Bandler, S.; Christe, S.; Davila, J.; Dennis, B.; Holman, G.;
Milligan, R.; Shih, A. Y.; Kahler, S.; Kontar, E.; Wiedenbeck, M.;
Cirtain, J.; Doschek, G.; Share, G. H.; Vourlidas, A.; Raymond, J.;
Smith, D. M.; McConnell, M.; Emslie, G.
2013arXiv1311.5243L Altcode:
Major solar eruptive events (SEEs), consisting of both a large flare and
a near simultaneous large fast coronal mass ejection (CME), are the most
powerful explosions and also the most powerful and energetic particle
accelerators in the solar system, producing solar energetic particles
(SEPs) up to tens of GeV for ions and hundreds of MeV for electrons. The
intense fluxes of escaping SEPs are a major hazard for humans in space
and for spacecraft. Furthermore, the solar plasma ejected at high speed
in the fast CME completely restructures the interplanetary medium
(IPM) - major SEEs therefore produce the most extreme space weather
in geospace, the interplanetary medium, and at other planets. Thus,
understanding the flare/CME energy release process(es) and the related
particle acceleration processes are major goals in Heliophysics. To
make the next major breakthroughs, we propose a new mission concept,
SEE 2020, a single spacecraft with a complement of advanced new
instruments that focus directly on the coronal energy release and
particle acceleration sites, and provide the detailed diagnostics of
the magnetic fields, plasmas, mass motions, and energetic particles
required to understand the fundamental physical processes involved.
---------------------------------------------------------
Title: Hard X-Ray Emission from the February 15, 2011 Solar Flare
Authors: Marsh, Andrew; Smith, D. M.; Krucker, S.; Glesener, L.
2013SPD....44...57M Altcode:
The February 15, 2011 solar flare was the first X-class flare of
Solar Cycle 24, and as such has received much attention in the
literature. This flare has many interesting features, including the
presence of a sun-quake, a large coronal mass ejection, and a distinct
“pre-impulsive” phase during which the thermal emission rises
appreciably before any significant rise in non-thermal flux. It is
presently unknown why certain flares exhibit pre-impulsive behavior,
or what heating or particle acceleration mechanisms are responsible for
these events. In addition, there has been limited analysis of the hard
x-ray emission from this particular flare. We present x-ray images,
spectra, and analysis of RHESSI data from this event with particular
focus on the pre-impulsive phase. Additional SDO AIA and HMI images will
be used to determine the context of the RHESSI observations. Looking
at the early stages of this and similar flares presents excellent
opportunities to constrain acceleration and heating modes and to learn
more about what physical processes underlie flare and CME initiation.
---------------------------------------------------------
Title: Using X-ray absorption to measure the height of the solar
atmosphere
Authors: Hudson, Hugh S.; Battaglia, M.; Hurford, G. J.; Krucker,
S.; Schwartz, R. A.
2013SPD....44..117H Altcode:
The X-ray image of a partially-occulted solar flare, one occurring
just behind the limb of the Sun, can have a sharply defined X-ray edge
resulting from attenuation in the atmosphere of the quiet Sun in the
foreground. Our analysis makes use of RHESSI's direct measurement of
image Fourier visibilities, and we estimate that the ultimate precision
of the limb height will on the order of the photospheric scale height in
the region of dominant absorption. This occurs at an altitude depending
on the X-ray photon energies used for the measurement, but generally in
the upper photosphere and chromosphere. We give a preliminary report on
analysis of one suitable event, the flare SOL2002-04-04T15:32 (M6.1),
where we find a clean signature of this attenuation up to the RHESSI
hard X-ray range 12-25 keV. At this energy Compton scattering begins
to dominate the attenuation, greatly reducing the model-dependence of
the result; at lower photon energies photoelectric absorption becomes
more important. These data determine the physical altitude of the
mean atmospheric density, with minimal model dependence, and therefore
provide an independent calibration of the atmospheric height scale.
---------------------------------------------------------
Title: White-light and Hard X-ray source heights of the
SOL2011-01-28T00:24 solar flare
Authors: Martinez Oliveros, Juan Carlos; Glesener, L.; Hudson, H. S.;
Krucker, S.; Hurford, G. J.
2013SPD....44...86M Altcode:
White-light continuum and hard X-ray emission in flares have strong
correlations in time, but at present we do not have a clear idea about
their height structures. Recently, several studies of the relative
positions of the white-light and hard X-ray sources have been made using
observations of flares near the solar limb. However, these results are
still inconclusive due to the small number of flares observed. On 28
January 2011 a white-light flare (SOL2011-01-28T07:35) was observed
on the western limb, observed simultaneously by the Helioseismic
Magnetic Imager (HMI) on the Solar Dynamics Observatory (SDO), the
Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and the
Solar TErrestrial RElations Observatory (STEREO). This observation
provides the heights of these emissions directly, limited only by
the limb references for the two spacecraft, with almost no projection
uncertainty. We report the results of this analysis and discuss our
findings in terms of present models of particle acceleration and energy
transport in the impulsive phase.Abstract (2,250 Maximum Characters):
White-light continuum and hard X-ray emission in flares have strong
correlations in time, but at present we do not have a clear idea about
their height structures. Recently, several studies of the relative
positions of the white-light and hard X-ray sources have been made using
observations of flares near the solar limb. However, these results are
still inconclusive due to the small number of flares observed. On 28
January 2011 a white-light flare (SOL2011-01-28T07:35) was observed
on the western limb, observed simultaneously by the Helioseismic
Magnetic Imager (HMI) on the Solar Dynamics Observatory (SDO), the
Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and the
Solar TErrestrial RElations Observatory (STEREO). This observation
provides the heights of these emissions directly, limited only by
the limb references for the two spacecraft, with almost no projection
uncertainty. We report the results of this analysis and discuss our
findings in terms of present models of particle acceleration and energy
transport in the impulsive phase.
---------------------------------------------------------
Title: MiXI: The Miniature X-ray Imager
Authors: Martinez Oliveros, Juan Carlos; Glesener, L.; Hurford, G. J.;
Sundkvist, D.; Krucker, S.; Bale, S.
2013SPD....44..144M Altcode:
The Miniature X-ray Imager (MiXI) is an ambitious, innovative, small,
and fully functional solar X-ray observatory concept designed to fit
within a 6U CubeSat platform. MiXI will provide the community with X-ray
imaging and spectroscopy of solar flares, but at a small fraction of the
cost of a conventional mission. MiXI will observe from 3 to 50 keV. It
includes rotation modulation collimators and layered Si/CdTe detectors,
providing routine observations of both soft and hard X-ray emission
with low background. If selected for funding, MiXI could launch in
2017 to coincide with the launch of Solar Orbiter. In the next solar
cycle, coordinated observations between the STIX instrument onboard
Solar Orbiter and a future version of MiXI will enable solar flare
observation from two vantage points. This will provide new insight into
the directivity of flare HXR emission and will allow detailed study of
both coronal and footpoint sources in the same flare. These results
may have profound implications for theories of flare acceleration
processes. We describe here the MiXI concept and its usefulness to
the solar and heliophysics communities.
---------------------------------------------------------
Title: The Focusing Optics X-ray Solar Imager (FOXSI) sounding rocket,
first flight
Authors: Christe, Steven; Glesener, L.; Ishikawa, S.; Ramsey, B.;
Takahashi, T.; Watanabe, S.; Saito, S.; Lin, R. P.; Krucker, S.
2013SPD....44...88C Altcode:
Understanding electron acceleration in solar flares requires
X-ray studies with greater sensitivity and dynamic range than are
available with current solar hard X-ray observers (i.e. the RHESSI
spacecraft). RHESSI employs an indirect Fourier imaging method that is
intrinsically limited in dynamic range and therefore can rarely image
faint coronal flare sources in the presence of bright footpoints. With
greater sensitivity and dynamic range, electron acceleration sites in
the corona could be studied in great detail. Both these capabilities
can be advanced by the use of direct focusing optics. The recently
flown Focusing Optics X-ray Solar Imager (FOXSI) sounding rocket
payload demonstrates the feasibility and usefulness of hard X-ray
focusing optics for observations of solar hard X-rays. FOXSI features
grazing-incidence replicated nickel optics made by the NASA Marshall
Space Flight Center and fine-pitch silicon strip detectors developed by
the Astro-H team at JAXA/ISAS. FOXSI flew successfully on November 2,
2012, producing images and spectra of a microflare and performing a
search for nonthermal emission (4-15 keV) from nanoflares in the quiet
Sun. Nanoflares are a candidate for providing the required energy
to heat the solar corona to its high temperature of a few million
degrees. A future satellite version of FOXSI, featuring similar optics
and detectors, could make detailed observations of hard X-rays from
flare-accelerated electrons, identifying and characterizing particle
acceleration sites and mapping out paths of energetic electrons as they
leave these sites and propagate throughout the solar corona.Abstract
(2,250 Maximum Characters): Understanding electron acceleration in solar
flares requires X-ray studies with greater sensitivity and dynamic range
than are available with current solar hard X-ray observers (i.e. the
RHESSI spacecraft). RHESSI employs an indirect Fourier imaging method
that is intrinsically limited in dynamic range and therefore can
rarely image faint coronal flare sources in the presence of bright
footpoints. With greater sensitivity and dynamic range, electron
acceleration sites in the corona could be studied in great detail. Both
these capabilities can be advanced by the use of direct focusing
optics. The recently flown Focusing Optics X-ray Solar Imager (FOXSI)
sounding rocket payload demonstrates the feasibility and usefulness of
hard X-ray focusing optics for observations of solar hard X-rays. FOXSI
features grazing-incidence replicated nickel optics made by the NASA
Marshall Space Flight Center and fine-pitch silicon strip detectors
developed by the Astro-H team at JAXA/ISAS. FOXSI flew successfully
on November 2, 2012, producing images and spectra of a microflare and
performing a search for nonthermal emission (4-15 keV) from nanoflares
in the quiet Sun. Nanoflares are a candidate for providing the required
energy to heat the solar corona to its high temperature of a few million
degrees. A future satellite version of FOXSI, featuring similar optics
and detectors, could make detailed observations of hard X-rays from
flare-accelerated electrons, identifying and characterizing particle
acceleration sites and mapping out paths of energetic electrons as
they leave these sites and propagate throughout the solar corona.
---------------------------------------------------------
Title: The Spectrometer Telescope for Imaging X-rays (STIX)
Authors: Krucker, Sam; STIX Team
2013SPD....44..140K Altcode:
The Spectrometer Telescope for Imaging X-rays (STIX) is one of 10
instruments on board Solar Orbiter, a confirmed M-class mission of the
European Space Agency (ESA) within the Cosmic Vision program scheduled
to be launched in 2017. STIX applies a Fourier-imaging technique using
a set of tungsten grids (at pitches from 0.038 to 1 mm) in front of
32 pixelized CdTe detectors to provide imaging spectroscopy of solar
thermal and non-thermal hard X-ray emissions from 4 to 150 keV. The
status of the instrument that will be presented at the Critical Design
Review (CDR) later this year will be discussed in this poster.
---------------------------------------------------------
Title: Densities within the Acceleration Region of a Solar Flare
Authors: Krucker, Sam; Battaglia, M.
2013SPD....4440204K Altcode:
The limb flare of 2012 July 19 around 5UT provides the best example of
a non-thermal above-the-loop-top hard X-ray source with simultaneous
observations by the Reuven Ramaty High Energy Solar Spectroscopic Imager
(RHESSI) and the Atmospheric Imaging Assembly (AIA) onboard the Solar
Dynamic Observatory (SDO). Combining the two sets of observations we
present the first direct measurement of the thermal proton density
and non-thermal electron density within the above-the-loop-top source
of a solar flare. We find that both densities are of the same order of
magnitude of a few times 1e9 cm-3, about 30 times lower than the density
in the underlying thermal flare loops. The equal densities indicate
that the entire electron distribution within the above-the-loop-top
source is energized. While the derived densities depend on the unknown
source depth and filling factor, the ratio of these two densities does
not. Within the uncertainties, the ratio is one for a low energy cut-off
of the non-thermal electron spectrum between 10 and 16 keV, consistent
with equal densities. RHESSI observations only constrain the cut-off
energy to below 20 keV, leaving the spectral shape of the electrons
within the above-the-loop-top source below 20 keV unknown. Nevertheless,
these robust results strongly corroborate earlier findings that the
above-the-loop-top source is the acceleration region where a bulk
energization process acts on all electrons.
---------------------------------------------------------
Title: Radio imaging spectroscopy of synchrotron emission associated
with a CME on the 14th of August 2010
Authors: Bain, Hazel; Krucker, S.; Saint-Hilaire, P.; Raftery, C.
2013SPD....4440203B Altcode:
We present Nancay Radioheliograph observations of a moving type IV
solar radio burst which occurred in association with a CME on the 14th
of August 2010. The event was well observed at extreme ultraviolet
wavelengths by the Atmospheric Imaging Assembly onboard the Solar
Dynamics Observatory, the SWAP instrument onboard Proba2 and by the
LASCO white light coronograph. The burst emission was found to be
cospatial with the core of the CME. Using radio imaging spectroscopy
we are able to characterize the underlying electron distribution and
plasma parameters within the source. Fitted spectra reveal a clear
power law component consistent with optically thin synchrotron emission
from accelerated electrons trapped in the erupting flux rope. As
is often observed in type IV bursts, polarization measurements show
the source to be moderately polarized during the peak of the burst,
before steadily increasing to around 70% as the brightness temperature
of the burst decays.
---------------------------------------------------------
Title: Observations of a CME heated by flare-accelerated electrons
Authors: Glesener, Lindsay; Krucker, S.; Bain, H.; Lin, R. P.
2013SPD....44...35G Altcode:
We present an updated analysis of a RHESSI observation of
flare-accelerated electrons in the core of a coronal mass ejection
(CME) and examine their role in heating the CME. Previous CME
observations have found remarkably high temperatures in the core. A
joint observation by RHESSI and AIA of a partly occulted flare on 2010
November 3 allows us to test the hypothesis that this excess energy
is collisionally deposited by flare-accelerated electrons. Extreme
ultraviolet (EUV) images from AIA show an ejected plasmoid forming the
CME core and sheath, with isothermal multifilter analysis revealing
temperatures of &gt 11 MK in the core. RHESSI images produced
via the Two-Step CLEAN algorithm reveal a large 100 x 50 arcsec2),
diffuse hard X-ray source matching the location, shape, and evolution
of the AIA plasma, indicating that the emerging CME core is filled
with energetic electrons. Spectroscopy demonstrates that the nonthermal
electrons contain enough energy to heat the CME. The time integral of
the EUV emission matches the hard X-ray light curve (similar to the
“Neupert effect” observed in soft and hard X-ray time profiles),
directly linking the CME temperature increase with the nonthermal
electron energy loss. This is the most direct observation to date
of energetic flare electrons heating a CME, emphasizing the close
relationship of the two in solar eruptive events.
---------------------------------------------------------
Title: Results from the first flight of the Focusing Optics X-ray
Solar Imager (FOXSI) sounding rocket
Authors: Glesener, Lindsay; Christe, S.; Ishikawa, S.; Ramsey, B.;
Takahashi, T.; Watanabe, S.; Saito, S.; Lin, R. P.; Krucker, S.;
FOXSI Team
2013SPD....4420004G Altcode:
Understanding electron acceleration in solar flares requires
X-ray studies with greater sensitivity and dynamic range than are
available with current solar hard X-ray observers (i.e. the RHESSI
spacecraft). RHESSI employs an indirect Fourier imaging method that is
intrinsically limited in dynamic range and therefore can rarely image
faint coronal flare sources in the presence of bright footpoints. With
greater sensitivity and dynamic range, electron acceleration sites in
the corona could be studied in great detail. Both these capabilities
can be advanced by the use of direct focusing optics. The recently
flown Focusing Optics X-ray Solar Imager (FOXSI) sounding rocket
payload demonstrates the feasibility and usefulness of hard X-ray
focusing optics for solar study. FOXSI features grazing-incidence
replicated nickel optics from the NASA Marshall Space Flight Center
and fine-pitch silicon strip detectors developed by the Astro-H team at
JAXA/ISAS. FOXSI flew successfully on November 2, 2012, producing images
and spectra of a microflare and performing a search for nonthermal
emission (4-15 keV) from nanoflares in the quiet Sun. Nanoflares are a
candidate for providing the required energy to heat the solar corona
to its high temperature of a few million degrees. A future satellite
version of FOXSI, featuring similar optics and detectors, could make
detailed observations of hard X-rays from flare-accelerated electrons,
identifying and characterizing particle acceleration sites and mapping
out paths of energetic electrons as they leave these sites and propagate
throughout the solar corona.
---------------------------------------------------------
Title: The Focusing Optics X-ray Solar Imager (FOXSI): Instrument
and First Flight
Authors: Glesener, Lindsay; Christe, S.; Ishikawa, S.; Ramsey, B.;
Takahashi, T.; Saito, S.; Lin, R. P.; Krucker, S.; FOXSI Team
2013HEAD...1312314G Altcode:
Understanding electron acceleration in solar flares requires hard
X-ray studies with greater sensitivity and dynamic range than are
available with current solar hard X-ray observers (i.e. the RHESSI
spacecraft). Both these capabilities can be advanced by the use of
direct focusing optics instead of the indirect Fourier methods of
current and previous generations. The Focusing Optics X-ray Solar
Imager (FOXSI) sounding rocket payload demonstrates the feasibility and
usefulness of hard X-ray focusing optics for solar observation. FOXSI
flew for the first time on 2012 November 2, producing images and
spectra of a microflare and performing a search for nonthermal
X-rays from the quiet Sun. Such measurements are important for
characterizing the impact of small "nanoflares" on the solar coronal
heating problem. A spaceborne solar observer featuring similar optics
could make detailed observations of hard X-rays from flare-accelerated
electrons, identifying and characterizing particle acceleration sites
and mapping out paths of energetic electrons as they leave these sites
and propagate throughout the solar corona. Solar observations from
NuSTAR are also expected to be an important step in this direction.
---------------------------------------------------------
Title: Suzaku/WAM and RHESSI Observations of Non-thermal Electrons
in Solar Microflares
Authors: Ishikawa, Shin-nosuke; Krucker, Säm; Ohno, Masanori; Lin,
Robert P.
2013ApJ...765..143I Altcode:
We report on hard X-ray spectroscopy of solar microflares observed by
the Wide-band All-sky Monitor (WAM), on board the Suzaku satellite, and
by RHESSI. WAM transient data provide wide energy band (50 keV-5 MeV)
spectra over a large field of view (~2π sr) with a time resolution
of 1 s. WAM is attractive as a hard X-ray solar flare monitor due to
its large effective area (~800 cm<SUP>2</SUP> at 100 keV, ~13 times
larger than that of RHESSI). In particular, this makes it possible to
search for high energy emission in microflares that is well below the
RHESSI background. The WAM solar flare list contains six GOES B-class
microflares that were simultaneously observed by RHESSI between the
launch of Suzaku in 2005 July and 2010 March. At 100 keV, the detected
WAM fluxes are more than ~20 times below the typical RHESSI instrumental
background count rates. The RHESSI and WAM non-thermal spectra are in
good agreement with a single power law with photon spectral indices
between 3.3 and 4.5. In a second step, we also searched the RHESSI
microflare list for events that should be detectable by WAM, assuming
that the non-thermal power-law emission seen by RHESSI extends to
>50 keV. From the 12 detectable events between 2005 July and 2007
February, 11 were indeed seen by WAM. This shows that microflares,
similar to regular flares, can accelerate electrons to energies up to
at least 100 keV.
---------------------------------------------------------
Title: Solar flares at submillimeter wavelengths
Authors: Krucker, Säm; Giménez de Castro, C. G.; Hudson, H. S.;
Trottet, G.; Bastian, T. S.; Hales, A. S.; Kašparová, J.; Klein,
K. -L.; Kretzschmar, M.; Lüthi, T.; Mackinnon, A.; Pohjolainen, S.;
White, S. M.
2013A&ARv..21...58K Altcode:
We discuss the implications of the first systematic observations of
solar flares at submillimeter wavelengths, defined here as observing
wavelengths shorter than 3 mm (frequencies higher than 0.1 THz). The
events observed thus far show that this wave band requires a new
understanding of high-energy processes in solar flares. Several events,
including observations from two different observatories, show during
the impulsive phase of the flare a spectral component with a positive
(increasing) slope at the highest observable frequencies (up to 405
GHz). To emphasize the increasing spectra and the possibility that
these events could be even more prominent in the THz range, we term
this spectral feature a "THz component". Here we review the data and
methods, and critically assess the observational evidence for such
distinct component(s). This evidence is convincing. We also review the
several proposed explanations for these feature(s), which have been
reported in three distinct flare phases. These data contain important
clues to flare development and particle acceleration as a whole, but
many of the theoretical issues remain open. We generally have lacked
systematic observations in the millimeter-wave to far-infrared range
that are needed to complete our picture of these events, and encourage
observations with new facilities.
---------------------------------------------------------
Title: Kappa Distribution Model for Hard X-Ray Coronal Sources of
Solar Flares
Authors: Oka, M.; Ishikawa, S.; Saint-Hilaire, P.; Krucker, S.; Lin,
R. P.
2013ApJ...764....6O Altcode: 2012arXiv1212.2579O
Solar flares produce hard X-ray emission, the photon spectrum of
which is often represented by a combination of thermal and power-law
distributions. However, the estimates of the number and total energy
of non-thermal electrons are sensitive to the determination of the
power-law cutoff energy. Here, we revisit an "above-the-loop" coronal
source observed by RHESSI on 2007 December 31 and show that a kappa
distribution model can also be used to fit its spectrum. Because
the kappa distribution has a Maxwellian-like core in addition to a
high-energy power-law tail, the emission measure and temperature of
the instantaneous electrons can be derived without assuming the cutoff
energy. Moreover, the non-thermal fractions of electron number/energy
densities can be uniquely estimated because they are functions of only
the power-law index. With the kappa distribution model, we estimated
that the total electron density of the coronal source region was ~2.4
× 10<SUP>10</SUP> cm<SUP>-3</SUP>. We also estimated without assuming
the source volume that a moderate fraction (~20%) of electrons in the
source region was non-thermal and carried ~52% of the total electron
energy. The temperature was 28 MK, and the power-law index δ of the
electron density distribution was -4.3. These results are compared
to the conventional power-law models with and without a thermal core
component.
---------------------------------------------------------
Title: Opportunities for Solar Science with NuSTAR
Authors: Glesener, Lindsay; Boggs, S. E.; Christensen, F.; Craig,
W. W.; Hailey, C. J.; Grefenstette, B.; Harrison, F.; Hudson, H. S.;
Hurford, G. J.; Krucker, S.; Marsh, A.; Mewaldt, R. A.; Pivovaroff,
M.; Smith, D. M.; Stern, D.; Vogel, J.; White, S. M.; Zhang, W.;
NuSTAR Team
2013AAS...22124423G Altcode:
While NuSTAR was designed to observe faint cosmic sources in hard
X-rays (HXR), its unprecedented sensitivity can also be used to address
several outstanding questions in high energy solar physics. Medium- and
large-sized solar flares have been well -studied in HXR by the Reuven
Ramaty High Energy Solar Spectroscopic Imager (RHESSI), launched in
2002. These flares are always found in active regions and usually
emit nonthermal HXR from accelerated electrons, along with thermal
bremsstrahlung as those electrons lose their energy and heat the
ambient plasma. To date, no HXR flares outside active regions have been
observed, though thermal brightenings in soft X-rays and EUV suggest
that small "nanoflares" may occur frequently across the entire solar
disk, even at quiet times when no active regions are present. Even a
few minutes of NuSTAR solar observations will allow a search for HXR
from quiet-Sun nanoflares with better sensitivity than any previous
study. These observations will have important implications for the
role of flares in supplying the corona with its surprisingly hot
temperature (1--2 MK, as compared with the photospheric temperature
of 5800 K). NuSTAR will also make the first observations of escaping
flare electrons associated with Type III radio emission, can image
faint coronal sources in partially occulted flares that are below
RHESSI's sensitivity, and, combined with RHESSI data, could study the
faint, earliest phase of flares, where direct signatures of particle
acceleration are most likely to be observed.
---------------------------------------------------------
Title: Observational Aspects of Particle Acceleration in Large
Solar Flares
Authors: Raymond, John C.; Krucker, Säm; Lin, Robert P.; Petrosian,
Vahé
2013pacp.book..197R Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Focusing Optics X-ray Solar Imager
Authors: Glesener, L.; Christe, S.; Krucker, S.; Ishikawa, S.; Ramsey,
B.; Takahashi, T.; Saito, S.; Lin, R. P.
2012AGUFMSH13A2237G Altcode:
Measurements of the nonthermal energies and occurrence frequencies
of nanoflares are important for understanding the overall flare
contribution to coronal heating. Nanoflares have been observed to be
ubiquitous in the quiet Sun in extreme ultraviolet and soft X-ray
wavelengths, but so far remain undetected at nonthermal hard X-ray
(HXR) energies, likely due to the insufficient sensitivity of current
instruments. The Focusing Optics X-ray Solar Imager (FOXSI) sounding
rocket payload has been designed for high sensitivity in the 5-15 keV
range by combining grazing-incidence HXR optics with fine-resolution
silicon strip detectors. FOXSI will make the first measurement of
nonthermal HXR from accelerated electrons in nanoflares, and will also
measure hot (5-10 MK) components of active region temperatures. FOXSI
is scheduled for a first flight in October 2012, and the first results
of this flight will be presented.
---------------------------------------------------------
Title: Radio imaging of synchrotron emission associated with a CME
on the 14th of August 2010
Authors: Bain, H. M.; Krucker, S.; Raftery, C. L.; Saint-Hilaire, P.
2012AGUFMSH51A2215B Altcode:
Radio observations can be used to identify sources of electron
acceleration within flares and CMEs. In a small number of events,
radio imaging has revealed the presence of synchrotron emission from
nonthermal electrons in the expanding loops of the CME (Bastian et
al. (2001), Maia et al. (2007) and Démoulin et al. (2012)). Events
in which the synchrotron emission is sufficiently bright to be
identified in the presence of plasma emission from radio bursts,
which are prevalent at meter wavelengths, are infrequent. Using radio
images from the Nançay Radioheliograph (NRH) we present observations
of synchrotron emission associated with a CME which occurred on the
14th of August 2010. Using context observations from the Atmospheric
Imaging Assembly (AIA) onboard the Solar Dynamics Observatory,
the SWAP instrument onboard Proba2, the LASCO coronograph onboard
SOHO and the Reuven Ramaty High Energy Solar Spectroscopic Imager
(RHESSI), we follow the propagation of the CME out to 2-3 solar radii
and characterize the associated electron distribution. We find that
the synchrotron emission is cospatial with the CME core.
---------------------------------------------------------
Title: STEREO-Wind Direction Finding of a Stream Interaction Region
Authors: Martinez Oliveros, J. C.; Raftery, C. L.; Liu, Y.; Krupar,
V.; Higgins, P. A.; Bale, S. D.; Krucker, S.
2012AGUFMSH43A2137M Altcode:
On 13 March 2010 a long duration event was seen by spectrographs onboard
the STEREO-B and Wind spacecraft. The event started at about 13:00 UT
and ended at approximately 05:00 UT on 14 March. The event presents
itself as a slow drifting quasi-continuous emission in a very narrow
frequency band, ranging from about 425 MHz to approximately 625 MHz. We
study the propagation behavior of the radio event, which appears to
be a stream interaction region (SIR). Using radio direction-finding
techniques, we locate the SIR in the interplanetary space and then
correlate these results with visible and in-situ data from instruments
onboard STEREO and Wind.
---------------------------------------------------------
Title: The Spectrometer Telescope for Imaging X-rays STIX on Solar
Orbiter
Authors: Csillaghy, A.; Battaglia, M.; Krucker, S.; Hurford, G. J.
2012AGUFMSA11A2119C Altcode:
The Spectrometer Telescope for Imaging X-rays (STIX) will provide
imaging spectroscopy of solar thermal and non-thermal X-ray emissions
from ~4 to 150 keV. STIX will play an important role in answering two of
Solar Orbiter's main science questions: (1) How and where are energetic
particles accelerated at the Sun, and how are they transported into
interplanetary space? X-ray images and spectra will provide information
on the location, spectrum and intensity of flare accelerated electrons
near the Sun. (2) What is the magnetic connection from Solar Orbiter
back to the Sun? STIX will play a key role in linking remote sensing
and in-situ observations on Solar Orbiter. Radio signatures of flare
accelerated electrons will be observed by the Radio and Plasma
wave instrument (RPW), while the SupraThermal Electron sensor
(STE) of the Energetic Particle Detector suite (EPD) will detect
electrons in-situ. Thus, the magnetic structure, field line length
and connectivity can be tracked. STIX is based on a Fourier-transform
imaging technique similar to that used successfully by the Hard X-ray
Telescope (HXT) on the Japanese Yohkoh mission, and related to that
used for the Reuven Ramaty High Energy Solar Spectroscopic Imager
mission. STIX has a higher sensitivity than RHESSI, with comparable
image quality and spectral and spatial resolution. It will be able
to observe thermal and non-thermal emission from nanoflares up to the
largest X- class events. STIX consists of three main parts: 1. An X-ray
window, 2. An imager with 32 subcollimators, and 3. A spectrometer
with 32 Cadmium Telluride (CdTe) X-ray detectors The transmission
through the grid pairs to the detectors is a very sensitive function
of the direction of incidence of the X-ray flux. The relative count
rates of the detectors behind the different sets of grids encode the
spatial information that can be subsequently decoded on the ground to
reconstruct images of the source region at different X-ray energies.
---------------------------------------------------------
Title: A Space Weather Mission to the Earth's 5th Lagrangian Point
(L5)
Authors: Howard, R. A.; Vourlidas, A.; Ko, Y.; Biesecker, D. A.;
Krucker, S.; Murphy, N.; Bogdan, T. J.; St Cyr, O. C.; Davila, J. M.;
Doschek, G. A.; Gopalswamy, N.; Korendyke, C. M.; Laming, J. M.;
Liewer, P. C.; Lin, R. P.; Plunkett, S. P.; Socker, D. G.; Tomczyk,
S.; Webb, D. F.
2012AGUFMSA13D..07H Altcode:
The highly successful STEREO mission, launched by NASA in 2006,
consisted of two spacecraft in heliocentric orbit, one leading and
one trailing the Earth and each separating from Earth at the rate
of about 22.5 degrees per year. Thus the two spacecraft have been
probing different probe/Sun/Earth angles. The utility of having remote
sensing and in-situ instrumentation away from the Sun-Earth line was
well demonstrated by STEREO. Here we propose the concept of a mission
at the 5th Lagrangian "point" in the Earth/Sun system, located behind
Earth about 60 degrees to the East of the Sun-Earth line. Such a mission
would enable many aspects affecting space weather to be well determined
and thus improving the prediction of the conditions of the solar wind
as it impinges on geospace. For example, Coronal Mass Ejections can
tracked for a significant distance toward Earth, new active regions
can be observed before they become visible to the Earth observer, the
solar wind can be measured before it rotates to Earth. The advantages
of such a mission will be discussed in this presentation.
---------------------------------------------------------
Title: A Survey of Pre-Impulsive Hard X-Ray Emission from RHESSI
Flares
Authors: Marsh, A.; Smith, D. M.; Krucker, S.
2012AGUFMSH43B2158M Altcode:
We present a survey of pre-impulsive flare emission in a set of
non-occulted flares observed with the Reuven Ramaty High Energy Solar
Spectroscopic Imager (RHESSI). This emission can take the form of a
gradual rise, and sometimes a distinct, brief plateau (e.g. the November
4, 2003 X28 flare). Pre-impulsive emission gives an opportunity to look
for coronal acceleration sites without contamination from footpoint
or thermal loop emission, even without occultation. There is also the
opportunity to directly compare the pre-impulsive emission to where
the footpoint and loop emission later appears, something that is not
possible for occulted flares. We are preparing a systematic sample of
images and spectra of all non-occulted RHESSI flares above GOES-class X1
that have good coverage of the pre-impulsive and impulsive data. While
the hard x-ray images later in the flare will serve as a starting
point to discuss magnetic loop and reconnection geometries, we will
ultimately include EUV, magnetogram, and other data to clarify the
geometry of the initial reconnection. As with all spectra from hard
x-ray coronal sources, the energy spectra from these pre-impulsive
measurements may allow a relatively direct measurement of the electron
spectrum produced by the reconnection process.
---------------------------------------------------------
Title: Spectrometer Telescope for Imaging X-rays (STIX)
Authors: Benz, A. O.; Gallagher, P.; Veronig, A.; Grimm, O.; Sylwester,
J.; Orleanski, P.; Arnold, N.; Bednarzik, M.; Farnik, F.; Hurford,
G.; Krucker, S.; Limousin, O.; Mann, G.; Vilmer, N.
2012IAUSS...6E.509B Altcode:
The Solar Orbiter Mission has been confirmed within ESA's M-class
Cosmic Vision plan. Launch date is January 2017 into an orbit that
reaches nearly one quarter AU in the perihelion. STIX is one of
the 10 instruments selected for close cooperation. STIX applies
a Fourier-imaging technique using shading tungsten grids. A total
of 32 pixelized CdTe detectors will permit high resolution imaging
spectroscopy. The design has passed ESA's Preliminary Design Review
and will be finalized by the end of 2012. The instrument specification
will be presented and its scientific potential discussed.
---------------------------------------------------------
Title: Observational Aspects of Particle Acceleration in Large
Solar Flares
Authors: Raymond, John C.; Krucker, Säm; Lin, Robert P.; Petrosian,
Vahé
2012SSRv..173..197R Altcode: 2012SSRv..tmp...47R
Solar flares efficiently accelerate electrons to several tens of MeV
and ions to 10 GeV. The acceleration is usually thought to be associated
with magnetic reconnection occurring high in the corona, though a shock
produced by the Coronal Mass Ejection (CME) associated with a flare can
also accelerate particles. Diagnostic information comes from emission at
the acceleration site, direct observations of Solar Energetic Particles
(SEPs), and emission at radio wavelengths by escaping particles,
but mostly from emission from the chromosphere produced when the
energetic particles bombard the footpoints magnetically connected to
the acceleration region. This paper provides a review of observations
that bear upon the acceleration mechanism.
---------------------------------------------------------
Title: A Statistical Study of Solar Electron Events over One Solar
Cycle
Authors: Wang, Linghua; Lin, R. P.; Krucker, Säm; Mason, Glenn M.
2012ApJ...759...69W Altcode:
We survey the statistical properties of 1191 solar electron events
observed by the WIND 3DP instrument from <1 keV to gsim300 keV
for a solar cycle (1995 through 2005). After taking into account
times of high background, the corrected occurrence frequency of solar
electron events versus peak flux exhibits a power-law distribution
over three orders of magnitude with exponents between -1.0 and -1.6
for different years, comparable to the frequency distribution of
solar proton events, microflares, and coronal mass ejections (CMEs),
but significantly flatter than that of soft X-ray (SXR) flares. At
40 keV (2.8 keV), the integrated occurrence rate above ~0.29 (~330)
cm<SUP>-2</SUP> s<SUP>-1</SUP> sr<SUP>-1</SUP> keV<SUP>-1</SUP>
near 1 AU is ~1000 year<SUP>-1</SUP> (~600 year<SUP>-1</SUP>) at
solar maximum and ~35 year<SUP>-1</SUP> (~25 year<SUP>-1</SUP>) at
solar minimum, about an order of magnitude larger than the observed
occurrence rate. We find these events typically extend over ~45°
in longitude, implying the occurrence rate over the whole Sun is
~10<SUP>4</SUP> year<SUP>-1</SUP> near solar maximum. The observed
solar electron events have a 98.75% association with type III radio
bursts, suggesting all type III bursts may be associated with a solar
electron event. They have a close (~76%) association with the presence
of low-energy (~0.02-2 MeV nucleon<SUP>-1</SUP>), <SUP>3</SUP>He-rich
(<SUP>3</SUP>He/<SUP>4</SUP>He >= 0.01) ion emissions measured
by the ACE ULEIS instrument. For these electron events, only ~35%
are associated with a reported GOES SXR flare, but ~60% appear to be
associated with a CME, with ~50% of these CMEs being narrow. These
electrons are often detected down to below 1 keV, indicating a source
high in the corona.
---------------------------------------------------------
Title: Asymmetry of Hard X-Ray Emissions at Conjugate Footpoints in
Solar Flares
Authors: Yang, Ya-Hui; Cheng, C. Z.; Krucker, Säm; Hsieh, Min-Shiu;
Chen, Nai-Hwa
2012ApJ...756...42Y Altcode:
The chromospheric double hard X-ray (HXR) sources generally appear
at the conjugate footpoints of flaring loops with asymmetric flux
distributions. The behavior of such HXR footpoint asymmetry should be
affected by several effects simultaneously and cannot be attributed
to a single effect easily. In this study, we attempt to address the
properties of photospheric magnetic fields in the areas coinciding
with asymmetric HXR footpoints based on RHESSI observations during
2002-2009. A total of 172 time intervals in 22 flares closed to
the solar disk center with available pre-flare MDI magnetograms are
investigated. The strong HXR footpoint is found to preferentially
(75%) locate at the region with weak magnetic field strength, which
is qualitatively consistent with the asymmetric magnetic mirror
scenario. The HXR footpoint fluxes become more asymmetric when the
footpoints move to the areas with more asymmetric field strength. A
feature of asymmetry reversal between different energy ranges is
observed in some flares, although no significant energy dependence
of footpoint asymmetry is found in our statistical results. We also
investigated the possible causes of time-dependent HXR footpoint
asymmetry by examining the 2004 November 4 M5.4 flare and the 2004
November 6 M3.6 flare. By comparing the estimated asymmetry quantities
with the HXR light curves, the asymmetry reversal in the late period
of the M5.4 flare is mainly attributed to the difference of coronal
energy release or acceleration processes in different periods, while
it is associated with the location changes of HXR footpoints moving
to different magnetic field regions in the M3.6 flare.
---------------------------------------------------------
Title: The spectrometer telescope for imaging x-rays on board the
Solar Orbiter mission
Authors: Benz, A. O.; Krucker, S.; Hurford, G. J.; Arnold, N. G.;
Orleanski, P.; Gröbelbauer, H. -P.; Klober, S.; Iseli, L.; Wiehl,
H. J.; Csillaghy, A.; Etesi, L.; Hochmuth, N.; Battaglia, M.;
Bednarzik, M.; Resanovic, R.; Grimm, O.; Viertel, G.; Commichau, V.;
Meuris, A.; Limousin, O.; Brun, S.; Vilmer, N.; Skup, K. R.; Graczyk,
R.; Stolarski, M.; Michalska, M.; Nowosielski, W.; Cichocki, A.;
Mosdorf, M.; Seweryn, K.; Przepiórka, A.; Sylwester, J.; Kowalinski,
M.; Mrozek, T.; Podgorski, P.; Mann, G.; Aurass, H.; Popow, E.;
Onel, H.; Dionies, F.; Bauer, S.; Rendtel, J.; Warmuth, A.; Woche,
M.; Plüschke, D.; Bittner, W.; Paschke, J.; Wolker, D.; Van Beek,
H. F.; Farnik, F.; Kasparova, J.; Veronig, A. M.; Kienreich, I. W.;
Gallagher, P. T.; Bloomfield, D. S.; Piana, M.; Massone, A. M.;
Dennis, B. R.; Schwarz, R. A.; Lin, R. P.
2012SPIE.8443E..3LB Altcode:
The Spectrometer Telescope for Imaging X-rays (STIX) is one of 10
instruments on board Solar Orbiter, a confirmed Mclass mission of the
European Space Agency (ESA) within the Cosmic Vision program scheduled
to be launched in 2017. STIX applies a Fourier-imaging technique
using a set of tungsten grids (at pitches from 0.038 to 1 mm) in
front of 32 pixelized CdTe detectors to provide imaging spectroscopy
of solar thermal and non-thermal hard X-ray emissions from 4 to 150
keV. The status of the instrument reviewed in this paper is based on
the design that passed the Preliminary Design Review (PDR) in early
2012. Particular emphasis is given to the first light of the detector
system called Caliste-SO.
---------------------------------------------------------
Title: Hard X-Ray Observations of a Jet and Accelerated Electrons
in the Corona
Authors: Glesener, Lindsay; Krucker, Säm; Lin, R. P.
2012ApJ...754....9G Altcode:
We report the first hard X-ray observation of a solar jet on the limb
with flare footpoints occulted, so that faint emission from accelerated
electrons in the corona can be studied in detail. In this event on
2003 August 21, RHESSI observed a double coronal hard X-ray source in
the pre-impulsive phase at both thermal and nonthermal energies. In
the impulsive phase, the first of two hard X-ray bursts consists of a
single thermal/nonthermal source coinciding with the lower of the two
earlier sources, and the second burst shows an additional nonthermal,
elongated source, spatially and temporally coincident with the coronal
jet. Analysis of the jet hard X-ray source shows that collisional
losses by accelerated electrons can deposit enough energy to generate
the jet. The hard X-ray time profile above 20 keV matches that of the
accompanying Type III and broadband gyrosynchrotron radio emission,
indicating both accelerated electrons escaping outward along the jet
path and electrons trapped in the flare loop. The double coronal hard
X-ray source, the open field lines indicated by Type III bursts, and
the presence of a small post-flare loop are consistent with significant
electron acceleration in an interchange reconnection geometry.
---------------------------------------------------------
Title: Determination of Electromagnetic Source Direction as an
Eigenvalue Problem
Authors: Martínez-Oliveros, Juan C.; Lindsey, Charles; Bale, Stuart
D.; Krucker, Säm
2012SoPh..279..153M Altcode: 2012arXiv1205.2393M; 2012SoPh..tmp...98M
Low-frequency solar and interplanetary radio bursts are generated at
frequencies below the ionospheric plasma cutoff and must therefore
be measured in space, with deployable antenna systems. The problem
of measuring both the general direction and polarization of an
electromagnetic source is commonly solved by iterative fitting
methods such as linear regression that deal simultaneously with both
directional and polarization parameters. We have developed a scheme
that separates the problem of deriving the source direction from that of
determining the polarization, avoiding iteration in a multi-dimensional
manifold. The crux of the method is to first determine the source
direction independently of concerns as to its polarization. Once
the source direction is known, its direct characterization in terms
of Stokes vectors, in a single iteration if desired, is relatively
simple. This study applies the source-direction determination to radio
signatures of flares received by STEREO. We studied two previously
analyzed radio type III bursts and found that the results of the
eigenvalue decomposition technique are consistent with those obtained
previously by Reiner et al. (Solar Phys.259, 255, 2009). For the type
III burst observed on 7 December 2007, the difference in travel times
from the derived source location to STEREO A and B is the same as the
difference in the onset times of the burst profiles measured by the
two spacecraft. This is consistent with emission originating from a
single, relatively compact source. For the second event of 29 January
2008, the relative timing does not agree, suggesting emission from
two sources separated by 0.1 AU, or perhaps from an elongated region
encompassing the apparent source locations.
---------------------------------------------------------
Title: The Height of a White-light Flare and Its Hard X-Ray Sources
Authors: Martínez Oliveros, Juan-Carlos; Hudson, Hugh S.; Hurford,
Gordon J.; Krucker, Säm; Lin, R. P.; Lindsey, Charles; Couvidat,
Sebastien; Schou, Jesper; Thompson, W. T.
2012ApJ...753L..26M Altcode: 2012arXiv1206.0497M
We describe observations of a white-light (WL) flare
(SOL2011-02-24T07:35:00, M3.5) close to the limb of the Sun, from which
we obtain estimates of the heights of the optical continuum sources and
those of the associated hard X-ray (HXR) sources. For this purpose, we
use HXR images from the Reuven Ramaty High Energy Spectroscopic Imager
and optical images at 6173 Å from the Solar Dynamics Observatory. We
find that the centroids of the impulsive-phase emissions in WL and HXRs
(30-80 keV) match closely in central distance (angular displacement
from Sun center), within uncertainties of order 0farcs2. This directly
implies a common source height for these radiations, strengthening the
connection between visible flare continuum formation and the accelerated
electrons. We also estimate the absolute heights of these emissions
as vertical distances from Sun center. Such a direct estimation has
not been done previously, to our knowledge. Using a simultaneous 195
Å image from the Solar-Terrestrial RElations Observatory spacecraft
to identify the heliographic coordinates of the flare footpoints,
we determine mean heights above the photosphere (as normally defined;
τ = 1 at 5000 Å) of 305 ± 170 km and 195 ± 70 km, respectively, for
the centroids of the HXR and WL footpoint sources of the flare. These
heights are unexpectedly low in the atmosphere, and are consistent
with the expected locations of τ = 1 for the 6173 Å and the ~40 keV
photons observed, respectively.
---------------------------------------------------------
Title: Investigating the Dependency of Footpoint Temperature on Hard
X-ray Energy using AIA Dispersion Spectra and RHESSI Imaging
Authors: Raftery, Claire; Bain, H. M.; Krucker, S.
2012AAS...22030906R Altcode:
The nature of energy deposition in solar flares remains largely
a mystery. There have been many studies, analyzing the energy and
temperature distributions of solar flares throughout their lifetime,
though few that directly characterize the temperature distribution
of flaring footpoint plasma as a function of footpoint energy. Here
we will use the SDO/AIA dispersion spectra technique (Raftery et
al. 2011) to identify the differential emission measure distribution of
individual flaring ribbons as a function of time. We will interpret the
distributions with respect to the energies observed in those footpoints
using RHESSI imaging and spectroscopy techniques. In doing so, we will
be directly observing the changes in the temperature response of the
footpoint plasma to electron beams of varying energies.
---------------------------------------------------------
Title: Radio Imaging of Shock-accelerated Electrons Associated with
an Erupting Plasmoid on 2010 November 3
Authors: Bain, H. M.; Krucker, Säm; Glesener, L.; Lin, R. P.
2012ApJ...750...44B Altcode:
We present observations of a metric type II solar radio burst that
occurred on the 3rd of November 2010 in association with an erupting
plasmoid. The eruption was well observed by the Atmospheric Imaging
Assembly (AIA) on board the Solar Dynamics Observatory and the Reuven
Ramaty High Energy Solar Spectroscopic Imager, while the burst occurred
in the frequency range of the Nançay Radioheliograph (NRH). Such
events, where the type II emission occurs in the NRH frequency range,
allowing us to image the burst, are infrequent. Combining these
data sets, we find that the type II is located ahead of the hot
(~11 MK) core of the plasmoid, which is surrounded by a well-defined
envelope of cool (few MK) plasma. Using two methods, we determine the
propagation velocity of the shock: (1) fitting the type II emission
observed in PHOENIX and HUMAIN radio spectrogram data; (2) direct
imaging of the type II source location using NRH observations. We use
LASCO C2 polarized brightness images to normalize our coronal density
model. However, we find that information from imaging is required in
order to fine-tune this normalization. We determine a shock propagation
velocity between 1900 km s<SUP>-1</SUP> and 2000 km s<SUP>-1</SUP>. This
is faster than the plasmoid observed at extreme-ultraviolet wavelengths
by AIA (v = 670-1440 km s<SUP>-1</SUP>, where the cooler plasma
propagates faster than the hot core). The positioning of the type II,
ahead of the plasmoid, suggests that the electrons are accelerated in
a piston-driven shock.
---------------------------------------------------------
Title: The Focusing Optics X-ray Solar Imager (FOXSI)
Authors: Christe, Steven; Krucker, S.; Glesener, L.; Ishikawa, S.;
Ramsey, B.; Takahashi, T.; Lin, R.
2012AAS...22041003C Altcode:
Hard x-ray (HXR) observations are a powerful diagnostic tool
providing quantitative measurements of nonthermal energetic (>10
keV) electrons. Energetic electrons traveling in a plasma radiate
HXR emission through the well-known process of bremsstrahlung. Solar
eruptive events are the most powerful particle accelerators in the solar
system, accelerating electrons up to hundreds of MeV. It is thought that
the energy release process and particle acceleration occur somewhere
in the corona. Since bremsstrahlung emission depends on the density of
the ambient medium, solar HXR emission is strongest when electron beams
enter the chromosphere where they lose their energy quickly through
collisions. Energetic electrons moving in the relatively tenuous corona
suffer few collisions, losing little energy and producing only faint
HXR emission. Present-day instruments do not have the sensitivity to see
the faint HXR emission from electrons traveling in the corona, nor the
dynamic range to see such faint emission in the presence of bright HXR
footpoint emission. Existing observations therefore show us only where
energetic electrons are stopped but not where they are accelerated,
nor along what path they escape from the acceleration site. Thus, to
make the next breakthrough in understanding the energy release in solar
eruptive events requires HXR imaging with much higher sensitivity and
dynamic range. HXR focusing optics combined with position sensitive
solid state detectors can provide both. We discuss the current state
of technological development in this area and the science it would
make possible.
---------------------------------------------------------
Title: On The Energetics Of Seismic Excitation Mechanisms
Authors: Martinez Oliveros, Juan Carlos; Bain, H.; Krucker, S.; Donea,
A.; Hudson, H.; Lin, R. P.; Lindsey, C.
2012AAS...22020503M Altcode:
Some solar flares emit strong acoustic transients into the solar
interior during their impulsive phases (Kosovichev and Zharkova,
1998). These transients penetrate thousands of kilometers beneath the
active region photosphere and refract back to the surface, where they
produce a characteristic helioseismic signature tens of thousands
of kilometers from their origin over the succeeding hour. Several
mechanisms of seismic excitation have been proposed, ranging from
hydrodynamic shocks to Lorentz force perturbations. However, regardless
of the mechanism of generation, it is clear that not all flares induce
an acoustic response in the interior of the Sun. A concrete hypothesis
or theory about the nature of this is still a topic of ongoing
investigations. For some particular flares, we present a comparative
study between the energy deposited by the proposed mechanisms of seismic
excitation and the acoustic energy deduced using holographic techniques.
---------------------------------------------------------
Title: Solar Hard X-ray Observations with NuSTAR
Authors: Marsh, Andrew; Smith, D. M.; Krucker, S.; Hudson, H. S.;
Hurford, G. J.; White, S. M.; Mewaldt, R. A.; Harrison, F. A.;
Grefenstette, B. W.; Stern, D.
2012AAS...22052112M Altcode:
High-sensitivity imaging of coronal hard X-rays allows detection
of freshly accelerated nonthermal electrons at the acceleration
site. A few such observations have been made with Yohkoh and RHESSI,
but a leap in sensitivity could help pin down the time, place, and
manner of reconnection. Around the time of this meeting, the Nuclear
Spectroscopic Telescope ARray (NuSTAR), a NASA Small Explorer for high
energy astrophysics that uses grazing-incidence optics to focus X-rays
up to 80 keV, will be launched. Three weeks will be dedicated to solar
observing during the baseline two-year mission. NuSTAR will be 200
times more sensitive than RHESSI in the hard X-ray band. This will allow
the following new observations, among others: 1) Extrapolation of the
micro/nanoflare distribution by two orders of magnitude down in flux;
2) Search for hard X-rays from network nanoflares (soft X-ray bright
points) and evaluation of their role in coronal heating; 3) Discovery
of hard X-ray bremsstrahlung from the electron beams driving type III
radio bursts, and measurement of their electron spectrum; 4) Hard X-ray
studies of polar soft X-ray jets and impulsive solar energetic particle
events at the edge of coronal holes; 5) Study of coronal bremsstrahlung
from particles accelerated by coronal mass ejections as they are
first launched; 6) Study of particles at the coronal reconnection
site when flare footpoints and loops are occulted; 7) Search for weak
high-temperature coronal plasmas in active regions that are not flaring;
and 8) Search for hypothetical axion particles created in the solar
core via the hard X-ray signal from their conversion to X-rays in the
coronal magnetic field. NuSTAR will also serve as a pathfinder for a
future dedicated space mission with enhanced capabilities, such as a
satellite version of the FOXSI sounding rocket.
---------------------------------------------------------
Title: Radio Imaging Of Shock-accelerated Electrons Associated With
An Erupting Plasmoid On The 3rd Of November 2010
Authors: Bain, Hazel; Krucker, S.; Glesener, L.; Lin, R. P.
2012AAS...22050804B Altcode:
We present observations of a metric type II solar radio burst that
occurred in association with an erupting plasmoid on the 3rd of November
2010. The event was well observed by the Nancay Radioheliograph (NRH)
and the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics
Observatory (SDO). Events in which the type II emission is present in
the NRH frequency range of a few hundred MHz are infrequent. From NRH
images, we found the type II source location to be situated ahead of the
hot ( 11 MK) core of the plasmoid, which is surrounded by an envelope
of cooler (few MK) plasma. Fitting the type II emission observed in
radio spectrogram data, we were able to determine that the shock which
produced the type II emission, was propagating with a velocity of 1900 -
2000 km/s. Using a combination of direct imaging from NRH and polarized
brightness images from LASCO C2, we were able to normalize the coronal
density model used for the fit. The shock velocity was found to be
significantly greater than the velocity of the hot core and cooler
envelope (670 - 1440 km/s) seen at extreme ultraviolet wavelengths
with AIA. The location of the burst emission ahead of the core and
the relative velocities of the shock and the plasmoid are indicative
of a piston-driven shock. <P />This work was supported in part by the
RHESSI project, NASA contract NAS598033. LG was partly supported by
NASA GSRP grant NNX09AM40H. RPL was partly supported by the WCU grant
(No. R31-10016) funded by the Korean Ministry of Education, Science
and Technology.
---------------------------------------------------------
Title: Instrument data processing unit for spectrometer/telescope
for imaging x-rays (STIX)
Authors: Skup, Konrad R.; Cichocki, A.; Graczyk, R.; Michalska,
M.; Mosdorf, M.; Nowosielski, W.; Orleański, P.; Przepiórka, A.;
Seweryn, K.; Stolarski, M.; Winkler, M.; Sylwester, J.; Kowalinski,
M.; Mrozek, T.; Podgorski, P.; Benz, A. O.; Krucker, S.; Hurford,
G. J.; Arnold, N. G.; Önel, H.; Meuris, A.; Limousin, O.; Grimm, O.
2012SPIE.8454E..0KS Altcode:
The Spectrometer/Telescope for Imaging X-rays (STIX) is one of 10
instruments on board Solar Orbiter, an M-class mission of the European
Space Agency (ESA) scheduled to be launch in 2017. STIX applies a
Fourier-imaging technique using a set of tungsten grids in front of
32 pixelized CdTe detectors to provide imaging spectroscopy of solar
thermal and non-thermal hard X-ray emissions from 4 to 150 keV. These
detectors are source of data collected and analyzed in real-time by
Instrument Data Processing Unit (IDPU). Besides the data processing
the IDPU controls and manages other STIX's subsystems: ASICs and ADCs
associated with detectors, Aspect System, Attenuator, PSU and HK. The
instrument reviewed in this paper is based on the design that passed the
Instrument Preliminary Design Review (IPDR) in early 2012 and Software
Preliminary Design Review (SW PDR) in middle of 2012. Particular
emphasis is given to the IDPU and low level software called Basic SW
(BSW).
---------------------------------------------------------
Title: Direct Measurement Of The Height Of A White-light Flare
Authors: Hudson, Hugh S.; Martinez-Oliveros, J.; Krucker, S.; Hurford,
G.; Thompson, W.; Schou, J.; Couvidat, S.; Lindsey, C.
2012AAS...22020441H Altcode:
We have used RHESSI and HMI observations to observe hard X-ray and
white-light continuum sources of the limb flare SOL2011-02-24, and
find the source centroids to coincide within errors of about 0.2 arc s,
with the conclusion that the emissions form at the same height in the
atmosphere. This greatly strengthens the known association between
non-thermal electrons and white-light continuum formation. We also
use STEREO observations to find the heliographic coordinates of the
flare. This determines the projected height of the photosphere directly
below the flare emissions. With this information, the RHESSI metrology
determines the absolute height of the sources to be remarkably low
in the solar atmosphere: the two footpoints have comparable heights,
which we estimate at about 290 +- 138 km above the photosphere. This
location lies significantly below the visible-light limb height,
estimated at 500 km by Brown & Christensen-Dalsgaard (1998), and
the height of optical depth unity to Thomson scattering, estimated
at a higher altitude. The results are not consistent with any current
models of these processes.
---------------------------------------------------------
Title: Solar Particle Acceleration Radiation and Kinetics (SPARK). A
mission to understand the nature of particle acceleration
Authors: Matthews, Sarah A.; Williams, David R.; Klein, Karl-Ludwig;
Kontar, Eduard P.; Smith, David M.; Lagg, Andreas; Krucker, Sam;
Hurford, Gordon J.; Vilmer, Nicole; MacKinnon, Alexander L.; Zharkova,
Valentina V.; Fletcher, Lyndsay; Hannah, Iain G.; Browning, Philippa
K.; Innes, Davina E.; Trottet, Gerard; Foullon, Clare; Nakariakov,
Valery M.; Green, Lucie M.; Lamoureux, Herve; Forsyth, Colin; Walton,
David M.; Mathioudakis, Mihalis; Gandorfer, Achim; Martinez-Pillet,
Valentin; Limousin, Olivier; Verwichte, Erwin; Dalla, Silvia; Mann,
Gottfried; Aurass, Henri; Neukirch, Thomas
2012ExA....33..237M Altcode: 2011ExA...tmp..124M
Energetic particles are critical components of plasma populations
found throughout the universe. In many cases particles are accelerated
to relativistic energies and represent a substantial fraction of
the total energy of the system, thus requiring extremely efficient
acceleration processes. The production of accelerated particles
also appears coupled to magnetic field evolution in astrophysical
plasmas through the turbulent magnetic fields produced by diffusive
shock acceleration. Particle acceleration is thus a key component
in helping to understand the origin and evolution of magnetic
structures in, e.g. galaxies. The proximity of the Sun and the range
of high-resolution diagnostics available within the solar atmosphere
offers unique opportunities to study the processes involved in particle
acceleration through the use of a combination of remote sensing
observations of the radiative signatures of accelerated particles, and
of their plasma and magnetic environment. The SPARK concept targets the
broad range of energy, spatial and temporal scales over which particle
acceleration occurs in the solar atmosphere, in order to determine how
and where energetic particles are accelerated. SPARK combines highly
complementary imaging and spectroscopic observations of radiation from
energetic electrons, protons and ions set in their plasma and magnetic
context. The payload comprises focusing-optics X-ray imaging covering
the range from 1 to 60 keV; indirect HXR imaging and spectroscopy
from 5 to 200 keV, γ-ray spectroscopic imaging with high-resolution
LaBr<SUB>3</SUB> scintillators, and photometry and source localisation
at far-infrared wavelengths. The plasma environment of the regions
of acceleration and interaction will be probed using soft X-ray
imaging of the corona and vector magnetography of the photosphere
and chromosphere. SPARK is designed for solar research. However,
in addition it will be able to provide exciting new insights into the
origin of particle acceleration in other regimes, including terrestrial
gamma-ray flashes (TGF), the origin of γ-ray bursts, and the possible
existence of axions.
---------------------------------------------------------
Title: The 2010 August 1 Type II Burst: A CME-CME Interaction and
its Radio and White-light Manifestations
Authors: Martínez Oliveros, Juan Carlos; Raftery, Claire L.; Bain,
Hazel M.; Liu, Ying; Krupar, Vratislav; Bale, Stuart; Krucker, Säm
2012ApJ...748...66M Altcode: 2012arXiv1202.2375M
We present observational results of a type II burst associated with
a CME-CME interaction observed in the radio and white-light (WL)
wavelength range. We applied radio direction-finding techniques to
observations from the STEREO and Wind spacecraft, the results of which
were interpreted using WL coronagraphic measurements for context. The
results of the multiple radio direction-finding techniques applied
were found to be consistent both with each other and with those derived
from the WL observations of coronal mass ejections (CMEs). The results
suggest that the type II burst radio emission is causally related to
the CMEs interaction.
---------------------------------------------------------
Title: Electron Acceleration Associated with Solar Jets
Authors: Krucker, Säm; Kontar, E. P.; Christe, S.; Glesener, L.;
Lin, R. P.
2011ApJ...742...82K Altcode:
This paper investigates the solar source region of supra-thermal
(few keV up to the MeV range) electron beams observed near Earth
by combining in situ measurements of the three-dimensional Plasma
and Energetic Particles experiment on the WIND spacecraft with
remote-sensing hard X-ray observations by the Reuven Ramaty High
Energy Solar Spectroscopic Imager. The in situ observations are used
to identify events, and the hard X-ray observations are then searched
for signatures of supra-thermal electrons radiating bremsstrahlung
emission in the solar atmosphere. Only prompt events detected above
50 keV with a close temporal correlation between the flare hard
X-ray emission and the electrons seen near Earth are selected,
limiting the number of events to 16. We show that for 7 of these
16 events, hard X-ray imaging shows three chromospheric sources:
two at the footpoints of the post-flare loop and one related to an
apparently open field line. The remaining events show two footpoints
(seven events, four of which show elongated sources possibly hiding
a third source) or are spatially unresolved (two events). Out of the
16 events, 6 have a solar source region within the field of view of
the Transition Region and Corona Explorer (TRACE). All events with
TRACE data show EUV jets that have the same onset as the hard X-ray
emission (within the cadence of tens of seconds). After the hard X-ray
burst ends, the jets decay. These results suggest that escaping prompt
supra-thermal electron events observed near Earth are accelerated in
flares associated with reconnection between open and closed magnetic
field lines, the so-called interchange reconnection scenario.
---------------------------------------------------------
Title: Statistical Properties of Super-Hot Solar Flares
Authors: Caspi, A.; Krucker, S.; Lin, R. P.
2011AGUFMSH13B1942C Altcode:
Observations by the Reuven Ramaty High Energy Solar Spectroscopic
Imager (RHESSI) have shown that "super-hot" (T > 30 MK) plasma
temperatures appear to be commonly achieved by intense, GOES M-
and X-class flares. Recent studies of individual X-class flares
(Caspi & Lin 2010, ApJ 725, L161; Longcope et al. 2010, SolPhys
267, 107) showed that the super-hot thermal component is spectrally
and spatially distinct from the ubiquitous ~10-20 MK plasma observed
by GOES, suggesting that the two populations are heated by different
physical mechanisms. However, how and why some flares achieve super-hot
temperatures while others do not, and on what physical parameters this
depends, remains unknown; consequently, the origins of super-hot plasmas
remain poorly understood. We present results from a survey of 37 M-
and X-class flares observed by RHESSI and GOES, which show that the
maximum plasma temperature measured by RHESSI is strongly correlated
with GOES class, such that super-hot temperatures are achieved almost
exclusively by X-class flares. This correlation is significantly
steeper than that observed for the peak GOES temperature. The maximum
(instantaneous) thermal energy of the RHESSI-observed plasma, which
occurs after the peak RHESSI temperature, is also correlated with GOES
class, and super-hot flares are observed to be strongly associated
with high number densities and (instantaneous) thermal energy densities
compared to cooler flares, suggesting that strong magnetic fields are
a requirement for the formation of super-hot plasma. We discuss these
results and their implications for flare plasma heating, temperature
distribution evolution, and energy transport.
---------------------------------------------------------
Title: Imaging Spectroscopy Using AIA Diffraction Patterns in
Conjunction with RHESSI and EVE Observations
Authors: Raftery, Claire L.; Krucker, Säm; Lin, Robert P.
2011ApJ...743L..27R Altcode:
Extreme-ultraviolet (EUV) spectroscopy is a very powerful tool that
can be used for probing the dynamic response of the solar corona
and chromosphere during solar flares. Here we present a unique
application of observations from the Atmospheric Imaging Assembly
(AIA) on board the Solar Dynamics Observatory using the artifacts
of diffraction and dispersion. Using these techniques we can achieve
imaging spectroscopy at the resolution of AIA (0farcs6 plate scale)
and at the revolutionary cadence of the instrument (nominally 12 s)
for the brightest (saturated) pixels during solar flares. Analyzing
the dispersion and diffraction effects that are observed as a result
of the support grids used for the instrument's front filters, we can
achieve up to 0.5 Å spectral resolution across the EUV, optically
thin passbands. Here we describe the technique used and present the
first result of its application—the emission measure distribution
for a single pixel at the top of a flaring loop. We analyze the AIA
dispersion spectrum in conjunction with Extreme Ultraviolet Variability
Experiment observations and spectroscopic and imaging results from
the Reuven Ramaty High Energy Solar Spectroscopic Imager.
---------------------------------------------------------
Title: Suzaku/WAM and RHESSI observation of non-thermal electrons
in solar microflares
Authors: Ishikawa, S.; Krucker, S.; Ohno, M.; Hudson, H. S.; Christe,
S.; Lin, R. P.
2011AGUFMSH41A1908I Altcode:
We report on hard X-ray spectroscopy of solar microflares observed by
the Wide-band All-sky Monitor (WAM), onboard the Suzaku satellite,
and by the Reuven Ramaty High Energy Solar Spectroscopic Imager
(RHESSI). WAM transient data provide wide energy band (50 keV - 5 MeV)
spectrum over a large field of view (~2π str) with a time resolution
of 1 s. While WAM was mainly designed for gamma-ray bursts and other
bursts of hard X-ray and gamma-ray from transient non-solar objects,
it is also attractive as a hard X-ray solar flare monitor thanks to
its large effective area (~800 cm<SUP>2</SUP> at 100 keV, ~13 times
larger than that of RHESSI). Hard X-ray (>50 keV) emissions from
17 GOES B-class flares were detected by WAM by Febrary 2010, and
7 of them were also observed by RHESSI. The GOES classes of these
events range from B1.3 to B9.5, and the RHESSI non-thermal spectra
are well-fit by power-laws with photon spectral indices between 3 and
5. The durations of both the WAM and RHESSI non-thermal emissions are
~1 minute, and the detected WAM fluxes are more than ~20 times smaller
than RHESSI backgrounds at energies above ~100 keV. The WAM spectra show
the high-energy extension of the non-thermal power-law distribution
seen by RHESSI, showing that microflares, similar to regular flares,
accelerate electrons to energies above 50 keV. We discuss high-energy
(>50 keV) particle acceleration in solar microflares and its relation
to large flares.
---------------------------------------------------------
Title: High Energy Solar Physics Data in Europe (HESPE): a European
project for the exploitation of hard X-ray data in solar flare physics
Authors: Piana, M.; Csillaghy, A.; Kontar, E. P.; Fletcher, L.;
Veronig, A. M.; Vilmer, N.; Hurford, G. J.; Dennis, B. R.; Schwartz,
R. A.; Massone, A.; Krucker, S.; Benvenuto, F.; Etesi, L. I.; Guo,
J.; Hochmuth, N.; Reid, H.
2011AGUFMSH33B2068P Altcode:
It has been recognized since the early days of the space program that
high-energy observations play a crucial role in understanding the basic
mechanisms of solar eruptions. Unfortunately, the peculiar nature of
this radiation makes it so difficult to extract useful information
from it that non-conventional observational techniques together with
complex data analysis procedures must be adopted. HESPE is a European
project funded within the seventh Framework Program, with the aim of
realizing computational methods for solar high-energy data analysis and
technological tools for the intelligent exploitation of science-ready
products. Such products and methods are put at disposal of the solar,
heliospheric and space weather communities, who will exploit them in
order to build flare prediction models and to integrate the information
extracted from hard X-rays and gamma rays data, with the one extracted
from other wavelengths data.
---------------------------------------------------------
Title: Electron Acceleration in Solar Flares and in Impulsive Solar
Energetic Particle (SEP) Events
Authors: Lin, R. P.; Wang, L.; Krucker, S.; Glesener, L.
2011AGUFMSH44A..01L Altcode:
We investigate the relationship between electron acceleration in
solar flares [detected via the electron bremsstrahlung hard x-ray
(HXR) emission], and in impulsive solar energetic particle (SEP)
events where the escaping electrons are detected in situ near 1
AU. These are most common types of particle acceleration by the Sun,
with of order ~104 events occurring per year over the whole Sun
during solar maximum. Both types predominantly accelerate electrons
(large electron/proton ratios) to ~1-10s of keV energies, typically
with double power-law energy spectra with a downward break at a few
tens of keV. Impulsive electron events are associated one-to-one
with low frequency type III radio bursts, but only ~25% have reported
chromospheric flares or detectable HXR emission. Wind 3DP observations
show that the electrons often exhibit nearly scatter-free propagation
in the interplanetary medium. Relative to the start of the type III
burst at the Sun, the injection of the electrons above ~20 keV is
found to be often (~80%) delayed by ~10-30 minutes. Where HXR emission
is detected, RHESSI imaging often shows coronal sources that suggests
magnetic reconnection between open field lines and an underlying loop
(interchange reconnection), resulting in a jet. Comparison of the
electron energy spectrum derived from the RHESSI hard X-ray measurements
with the impulsive event electron spectrum observed by Wind for prompt
(not delayed) events, suggests a model where electrons accelerated
by this interchange magnetic reconnection on open field lines escape,
while electrons on closed field lines are further accelerated as those
fields collapse, to produce the spectrum and much larger (by a factor
of >100) total number of HXR emitting electrons. Impulsive electron
events also appear to be associated with fast (>~580 km/s) narrow
CMEs as well as jets, suggesting that in delayed events the escaping
>~20 keV electrons (and ions) might be accelerated by the jet/fast
narrow CME as it travels outward through the corona.
---------------------------------------------------------
Title: Hard X-ray and Extreme Ultraviolet Correlations in Solar Flares
Authors: Bain, H. M.; Raftery, C. L.; Krucker, S.; Allred, J. C.
2011AGUFMSH13B1936B Altcode:
It is well established that there is a connection between hard X-ray and
extreme ultraviolet (EUV) flare emission. In the thick target model,
hard X-ray emission is produced by a beam of nonthermal electrons,
which deposit their energy in the chromosphere as a result of Coulomb
collisions with the denser plasma. At EUV wavelengths we observe
the response of the chromosphere and corona to this deposition of
energy. Using X-ray observations from the Ramaty High Energy Solar
Spectroscopic Imager (RHESSI) and EUV observations from the Atmospheric
Imaging Assembly (AIA) and the Extreme Ultraviolet Variability
Experiment (EVE) instruments onboard the Solar Dynamics Observatory
(SDO) we investigate the spectral, spatial and temporal correlations
between these emission mechanisms. With these new observations from SDO
we can achieve this with unprecedented spectral coverage and cadence.
---------------------------------------------------------
Title: Imaging spectroscopy with AIA: using AIA diffraction patterns
to probe flare DEM curves in conjunction with RHESSI and EVE spectra
Authors: Raftery, C. L.; Krucker, S.
2011AGUFMSH13B1945R Altcode:
Here we present an innovative use of AIA data, utilizing diffraction and
dispersion artifacts to preform imaging spectroscopy with AIA. Using
this technique, we probe the time and spatial dependence of emission
measure distributions in a compact flare. This is achieved through
comparison of AIA dispersion spectra to EVE and CHIANTI spectra. In this
way, we can no only probe the original spectral components of the flare
emission but we can achieve this with the cadence and spatial resolution
of AIA. To further extend the dataset, RHESSI spectral results are
included and, in a break from tradition, RHESSI images are used to
place AIA spectra in context. Unlike traditional spectroscopic methods,
imaging spectroscopy with AIA allows us to capture the highly dynamic
nature of flares with no compromise for spatial resolution or cadence.
---------------------------------------------------------
Title: The Focusing Optics X-ray Solar Imager (FOXSI)
Authors: Krucker, S.
2011AGUFMSH13B1950K Altcode:
The Focusing Optics X-ray Solar Imager (FOXSI) is a NASA Low Cost
Access to Space sounding rocket payload that will launch in early
2012. A larger sensitivity and dynamic range than currently available
are needed in order to image faint X-rays from electron beams in the
tenuous corona, particularly those near the coronal acceleration region
and those that escape into interplanetary space. FOXSI combines nested,
grazing-incidence replicated optics with double-sided silicon strip
detectors to achieve a dynamic range of >100 and a sensitivity
100 times that of RHESSI. Advances in the fabrication and assembly
of the optics at the NASA Marshall Space Flight Center provide a
spatial resolution of 8 arcseconds (FWHM), while the silicon detectors,
developed by the Astro-H team at ISAS/JAXA, offer an energy resolution
of 0.4 keV. FOXSI's first flight will conduct a search for nonthermal
electrons in the quiet Sun, possibly related to nanoflares. FOXSI
will serve as a pathfinder for future space-based solar hard X-ray
spectroscopic imagers, which will be able to image nonthermal electrons
in flare acceleration sites and provide quantitative measurements
such as energy spectra, densities, and energy content in accelerated
electrons.
---------------------------------------------------------
Title: Observations of Langmuir Waves Associated with Type III
Radio Bursts : Wind Observations and Further Improvements with Inner
Heliospheric Missions
Authors: Maksimovic, M.; Vidojevic, S.; Arnaud, Z.; Krucker, S.
2011AGUFMSH44B..08M Altcode:
Interplanetary electron beams, produced by solar flares or CMEs, are
unstable in the solar wind and generate Langmuir waves at the local
plasma frequency Fp. These waves are then converted into the so-called
Type III radio bursts which are freely propagating electromagnetic
emissions at Fp or its harmonic. We present a statistical analysis
of both in-situ Langmuir Waves, associated energetic electrons and
Type III bursts recorded simultaneously by the WIND spacecraft. We
discuss the relevance of these observations with respect to the latest
developments of Type III generation theories. Finally we describe the
possible improvements that the forthcoming inner heliospheric mission
can bring to this topic.
---------------------------------------------------------
Title: Energetics And Heating In A Solar Plasma Ejection Observed
By RHESSI And AIA
Authors: Glesener, L.; Krucker, S.; Bain, H. M.; Lin, R. P.
2011AGUFMSH44A..04G Altcode:
For the past nine years, hard X-ray observations by the Reuven
Ramaty High Energy Solar Spectroscopic Imager (RHESSI) have provided
remarkable insight into the locations and spectra of energetic flare
particles. With the advent of high-cadence, multiwavelength observations
by the Atmospheric Imaging Assembly (AIA) aboard the Solar Dynamics
Observatory, it is now more possible to study the dynamic structures
among which these energetic particles move. On November 3, 2010, a
C4.9 solar flare occurred just behind the eastern limb of the Sun,
accompanied by a coronal mass ejection. Because the bright flare
footpoints were occulted by the solar disk (by about 6 degrees), faint
coronal X-ray sources can be studied in detail. Extreme ultraviolet
(EUV) images from AIA show a mass of plasma ejected from the solar
surface. Isothermal analysis using multiple EUV filters shows that
this erupting plasma reaches a high temperature of ~11 MK. Meanwhile,
RHESSI X-ray images reveal a large, diffuse hard X-ray source matching
the location, shape, and evolution of the ejecting plasma, suggesting
the presence of nonthermal electrons that may be magnetically trapped in
the region. Here, RHESSI spectroscopy and AIA temperature analysis are
combined in order to examine the relationship between the populations
of thermal and nonthermal electrons in the ejected plasma. Electron
spectra, locations, and temporal evolution are examined in order to
test the hypothesis that nonthermal electrons collisionally heat the
erupting plasma to this high temperature.
---------------------------------------------------------
Title: A diffusive description of the focused transport of solar
energetic particles. Intensity- and anisotropy-time profiles as
a powerful diagnostic tool for interplanetary particle transport
conditions
Authors: Artmann, S.; Schlickeiser, R.; Agueda, N.; Krucker, S.; Lin,
R. P.
2011A&A...535A..92A Altcode:
The transport of solar energetic charged particles along the
interplanetary magnetic field in the ecliptic plane of the sun can be
described roughly by a one-dimensional diffusion equation. Large-scale
spatial variations of the guide magnetic field can be taken into account
by adding an additional term to the diffusion equation that includes
the effect of adiabatic focusing. We solve this equation analytically
by assuming a point-like particle injection in time and space and a
spatial power-law dependence for the focusing length and the spatial
diffusion coefficient. We infer the intensity- and anisotropy-time
profiles of solar energetic particles from this solution. Through these
the influence of different assumptions for the diffusion parameters can
be seen in a mathematically closed form. The comparison of calculated
and measured intensity- and anisotropy-time profiles, which are a
powerful diagnostic tool for interplanetary particle transport, gives
information about the large-scale spatial dependence of the focusing
length and the diffusion coefficient. For an exceptionally large solar
energetic particle event, which did occur on 2001 April 15, we fit the
27 - 512 keV electron intensities and anisotropies observed by the Wind
spacecraft using the theoretically derived profiles. We find a linear
spatial dependence of the mean free path along the guiding magnetic
field. We also find the mean free path to be energy independent, which
supports the theory of "velocity-dependent diffusion". This means that
the intensity profiles for the discussed energies exhibit the same shape
if they are plotted against the traveled distance and not against the
time. In this case the profiles differ only in their maximum values and
we can determine the energy spectra of the solar flare electrons out of
the scaling factor we need to fit the data. The derived spectra exhibits
a power-law dependence ∝ E_kin<SUP>-3</SUP> in an energy range from ~
50 keV to ~ 500 keV. <P />Appendices are available in electronic form
at <A href="http://www.aanda.org">http://www.aanda.org</A>
---------------------------------------------------------
Title: High-resolution Imaging of Solar Flare Ribbons and Its
Implication on the Thick-target Beam Model
Authors: Krucker, Säm; Hudson, H. S.; Jeffrey, N. L. S.; Battaglia,
M.; Kontar, E. P.; Benz, A. O.; Csillaghy, A.; Lin, R. P.
2011ApJ...739...96K Altcode:
We report on high-resolution optical and hard X-ray observations of
solar flare ribbons seen during the GOES X6.5 class white-light flare
of 2006 December 6. The data consist of imaging observations at 430 nm
(the Fraunhofer G band) taken by the Hinode Solar Optical Telescope
with the hard X-rays observed by the Reuven Ramaty High Energy Solar
Spectroscopic Imager. The two sets of data show closely similar ribbon
structures, strongly suggesting that the flare emissions in white light
and in hard X-rays have physically linked emission mechanisms. While
the source structure along the ribbons is resolved at both wavelengths
(length ~ 30”), only the G-band observations resolve the width of the
ribbon, with values between ~0farcs5 and ~1farcs8. The unresolved
hard X-ray observations reveal an even narrower ribbon in hard
X-rays (the main footpoint has a width perpendicular to the ribbon
of <1farcs1 compared to the G-band width of ~1farcs8) suggesting
that the hard X-ray emission comes from the sharp leading edge of
the G-band ribbon. Applying the thick-target beam model, the derived
energy deposition rate is >5 × 10<SUP>12</SUP> erg s<SUP>-1</SUP>
cm<SUP>-2</SUP> provided by an electron flux of 1 × 10<SUP>20</SUP>
electrons s<SUP>-1</SUP> cm<SUP>-2</SUP> above 18 keV. This requires
that the beam density of electrons above 18 keV be at least 1 ×
10<SUP>10</SUP> cm<SUP>-3</SUP>. Even if field lines converge toward
the chromospheric footpoints, the required beam in the corona has too
high a density to be described as a dilute tail population on top of
a Maxwellian core. We discuss this issue and others associated with
this extreme event, which poses serious questions to the standard
thick target beam interpretation of solar flares.
---------------------------------------------------------
Title: An Observational Overview of Solar Flares
Authors: Fletcher, L.; Dennis, B. R.; Hudson, H. S.; Krucker, S.;
Phillips, K.; Veronig, A.; Battaglia, M.; Bone, L.; Caspi, A.; Chen,
Q.; Gallagher, P.; Grigis, P. T.; Ji, H.; Liu, W.; Milligan, R. O.;
Temmer, M.
2011SSRv..159...19F Altcode: 2011SSRv..tmp..261F; 2011arXiv1109.5932F
We present an overview of solar flares and associated phenomena,
drawing upon a wide range of observational data primarily from the
RHESSI era. Following an introductory discussion and overview of
the status of observational capabilities, the article is split into
topical sections which deal with different areas of flare phenomena
(footpoints and ribbons, coronal sources, relationship to coronal mass
ejections) and their interconnections. We also discuss flare soft X-ray
spectroscopy and the energetics of the process. The emphasis is to
describe the observations from multiple points of view, while bearing
in mind the models that link them to each other and to theory. The
present theoretical and observational understanding of solar flares is
far from complete, so we conclude with a brief discussion of models,
and a list of missing but important observations.
---------------------------------------------------------
Title: Microflares and the Statistics of X-ray Flares
Authors: Hannah, I. G.; Hudson, H. S.; Battaglia, M.; Christe, S.;
Kašparová, J.; Krucker, S.; Kundu, M. R.; Veronig, A.
2011SSRv..159..263H Altcode: 2011SSRv..tmp..262H; 2011SSRv..tmp...87H; 2011arXiv1108.6203H;
2011SSRv..tmp..243H; 2011SSRv..tmp..163H
This review surveys the statistics of solar X-ray flares, emphasising
the new views that RHESSI has given us of the weaker events (the
microflares). The new data reveal that these microflares strongly
resemble more energetic events in most respects; they occur solely
within active regions and exhibit high-temperature/nonthermal emissions
in approximately the same proportion as major events. We discuss the
distributions of flare parameters (e.g., peak flux) and how these
parameters correlate, for instance via the Neupert effect. We also
highlight the systematic biases involved in intercomparing data
representing many decades of event magnitude. The intermittency of
the flare/microflare occurrence, both in space and in time, argues
that these discrete events do not explain general coronal heating,
either in active regions or in the quiet Sun.
---------------------------------------------------------
Title: The Focusing Optics X-ray Solar Imager (FOXSI)
Authors: Krucker, Säm; Christe, Steven; Glesener, Lindsay; Ishikawa,
Shin-nosuke; McBride, Stephen; Glaser, David; Turin, Paul; Lin,
R. P.; Gubarev, Mikhail; Ramsey, Brian; Saito, Shinya; Tanaka,
Yasuyuki; Takahashi, Tadayuki; Watanabe, Shin; Tanaka, Takaaki;
Tajima, Hiroyasu; Masuda, Satoshi
2011SPIE.8147E..05K Altcode: 2011SPIE.8147E...4K
The Focusing Optics x-ray Solar Imager (FOXSI) is a sounding rocket
payload funded under the NASA Low Cost Access to Space program to
test hard x-ray (HXR) focusing optics and position-sensitive solid
state detectors for solar observations. Today's leading solar HXR
instrument, the Reuven Ramaty High Energy Solar Spectroscopic Imager
(RHESSI) provides excellent spatial (2 arcseconds) and spectral (1
keV) resolution. Yet, due to its use of an indirect imaging system,
the derived images have a low dynamic range (typically <10) and
sensitivity. These limitations make it difficult to study faint x-ray
sources in the solar corona which are crucial for understanding the
particle acceleration processes which occur there. Grazing-incidence
x-ray focusing optics combined with position-sensitive solid state
detectors can overcome both of these limitations enabling the next
breakthrough in understanding impulsive energy release on the Sun. The
FOXSI project is led by the Space Sciences Laboratory at the University
of California, Berkeley. The NASA Marshall Space Flight Center is
responsible for the grazingincidence optics, while the Astro-H team at
JAXA/ISAS has provided double-sided silicon strip detectors. FOXSI is
a pathfinder for the next generation of solar hard x-ray spectroscopic
imagers. Such observatories will be able to image the non-thermal
electrons within the solar flare acceleration region, trace their paths
through the corona, and provide essential quantitative measurements such
as energy spectra, density, and energy content in accelerated electrons.
---------------------------------------------------------
Title: Solar Hard X-ray Observations with NuSTAR
Authors: Smith, David M.; Krucker, S.; Hurford, G.; Hudson, H.; White,
S.; Mewaldt, R.; Grefenstette, B.; Harrison, F.; NuSTAR Science Team
2011HEAD...12.4309S Altcode:
High-sensitivity imaging of solar hard X-rays allows detection of
freshly accelerated nonthermal electrons at the acceleration site. A
few such observations have been made with Yohkoh and RHESSI, but a
leap in sensitivity could help pin down the time, place, and manner
of reconnection. The Nuclear Spectroscopic Telescope Array (NuSTAR)
Small Explorer will be capable of solar pointing, and three weeks
will be dedicated to solar observing during the baseline two-year
mission. NuSTAR will be 200 times more sensitive than RHESSI in the
hard X-ray band. This will allow the following new solar observations,
among others: <P />1) Extrapolation of the micro/nanoflare distribution
by two orders of magnitude down in flux <P />2) Search for hard X-rays
from network nanoflares (soft X-ray bright points) and evaluation
of their role in coronal heating <P />3) Discovery of hard X-ray
bremsstrahlung from the electron beams driving type III radio bursts,
and measurement of their electron spectrum <P />4) Hard X-ray studies
of polar soft X-ray jets and impulsive solar energetic particle events
at the edge of coronal holes, and comparison of these events with
observations of 3He and other particles in interplanetary space <P />5)
Study of coronal bremsstrahlung from particles accelerated by coronal
mass ejections as they are first launched <P />6) Study of particles
at the coronal reconnection site when flare footpoints are occulted;
and <P />7) Search for hypothetical axion particles created in the
solar core via the hard X-ray signal from their conversion to X-rays
in the coronal magnetic field.
---------------------------------------------------------
Title: On the Relation of Above-the-loop and Footpoint Hard X-Ray
Sources in Solar Flares
Authors: Ishikawa, S.; Krucker, Säm; Takahashi, T.; Lin, R. P.
2011ApJ...737...48I Altcode:
We report on the most prominent example of an above-the-loop hard X-ray
source in the extensive solar flare database of RHESSI. The limb flare
of 2003 October 22 around 20 UT resembles the famous Masuda flare,
except that only one of the footpoint sources is visible with the
other one occulted. However, even for this very prominent event,
the above-the-loop source is only visible during one of the four
hard X-ray peaks, highlighting the rare occurrence of above-the-loop
sources that are equally bright as footpoint sources. The relative
timing between the above-the-loop and footpoint sources shows that the
coronal source peaks about 10 s before the footpoint source and decays
during the time the footpoint source is most prominent. Furthermore,
the derived number of non-thermal electrons within the above-the-loop
source is large enough to provide the needed number of precipitating
electrons to account for the footpoint emission over the duration
of the hard X-ray peak. Hence, these observations support the simple
scenario where bulk energization is accelerating all electrons within
the above-the-loop source and precipitating electrons are emptying
out of the above-the-loop source to produce the footpoint emissions.
---------------------------------------------------------
Title: On the Near-Earth Observation of Protons and Electrons from
the Decay of Low-energy Solar Flare Neutrons
Authors: Agueda, Neus; Krucker, Säm; Lin, Robert P.; Wang, Linghua
2011ApJ...737...53A Altcode:
We investigate the near-Earth observation of interplanetary protons
and electrons that result from the in-flight beta decay of low-energy
(1-10 MeV) solar neutrons. We use in situ measurements throughout
solar cycle 23 of 1-11 MeV protons and 50-400 keV electrons by the 3DP
experiment on board the Wind spacecraft. We select a sample of isolated
large (X-class) eastern hemisphere flares occurring during quiescent
interplanetary conditions with the goal of discriminating neutron-decay
particles from primary solar energetic particles. Unfortunately,
all major flares of solar cycle 23 have to be excluded, with the
largest flare in our sample being a X3.6 flare. For these relatively
small event sizes, no in situ events due to the decay of solar flare
neutrons are observed by Wind. From the one event with simultaneous
γ-ray observations, we estimate the expected signal of neutron-decay
protons in the Wind/3DP detectors. We use theoretical calculations
of the spectrum of escaping neutrons at the Sun combined with an
interplanetary propagation model to predict the neutron-decay proton
spectrum expected near the Earth. We find that the expected spectrum
is indeed well below the background intensities. However, using the
estimates derived from the largest solar event of cycle 23 (2003 October
28) and assuming the flare would have occurred isolated in the eastern
hemisphere, a clear signal above 5 MeV is expected to be seen.
---------------------------------------------------------
Title: Fine-Pitch Semiconductor Detector for the FOXSI Mission
Authors: Ishikawa, S.; Saito, S.; Tajima, H.; Tanaka, T.; Watanabe,
S.; Odaka, H.; Fukuyama, T.; Kokubun, M.; Takahashi, T.; Terada, Y.;
Krucker, S.; Christe, S.; McBride, S.; Glesener, L.
2011ITNS...58.2039I Altcode: 2015arXiv150905717I
The Focusing Optics X-ray Solar Imager (FOXSI) is a NASA sounding rocket
mission which will study particle acceleration and coronal heating on
the Sun through high sensitivity observations in the hard X-ray energy
band (5-15 keV). Combining high-resolution focusing X-ray optics and
fine-pitch imaging sensors, FOXSI will achieve superior sensitivity;
two orders of magnitude better than that of the RHESSI satellite. As the
focal plane detector, a Double-sided Si Strip Detector (DSSD) with a
front-end ASIC (Application Specific Integrated Circuit) will fulfill
the scientific requirements of spatial and energy resolution, low
energy threshold and time resolution. We have designed and fabricated a
DSSD with a thickness of 500 μm and a dimension of 9.6 mm × 9.6 mm,
containing 128 strips with a pitch of 75 μm, which corresponds to
8 arcsec at the focal length of 2 m. We also developed a low-noise
ASIC specified to FOXSI. The detector was successfully operated in
the laboratory at a temperature of -20°C and with an applied bias
voltage of 300 V. Extremely good energy resolutions of 430 eV for the
p-side and 1.6 keV for the n-side at a 14 keV line were achieved for
the detector. We also demonstrated fine-pitch imaging successfully
by obtaining a shadow image. Hence the implementation of scientific
requirements was confirmed.
---------------------------------------------------------
Title: Estimates of Densities and Filling Factors from a Cooling
Time Analysis of Solar Microflares Observed with RHESSI
Authors: Baylor, R. N.; Cassak, P. A.; Christe, S.; Hannah, I. G.;
Krucker, Säm; Mullan, D. J.; Shay, M. A.; Hudson, H. S.; Lin, R. P.
2011ApJ...736...75B Altcode: 2011arXiv1107.3997B
We use more than 4500 microflares from the RHESSI microflare data set to
estimate electron densities and volumetric filling factors of microflare
loops using a cooling time analysis. We show that if the filling factor
is assumed to be unity, the calculated conductive cooling times are
much shorter than the observed flare decay times, which in turn are
much shorter than the calculated radiative cooling times. This is likely
unphysical, but the contradiction can be resolved by assuming that the
radiative and conductive cooling times are comparable, which is valid
when the flare loop temperature is a maximum and when external heating
can be ignored. We find that resultant radiative and conductive cooling
times are comparable to observed decay times, which has been used as an
assumption in some previous studies. The inferred electron densities
have a mean value of 10<SUP>11.6</SUP> cm<SUP>-3</SUP> and filling
factors have a mean of 10<SUP>-3.7</SUP>. The filling factors are lower
and densities are higher than previous estimates for large flares,
but are similar to those found for two microflares by Moore et al.
---------------------------------------------------------
Title: Temperature and Density Estimates of Extreme-ultraviolet
Flare Ribbons Derived from TRACE Diffraction Patterns
Authors: Krucker, Säm; Raftery, Claire L.; Hudson, Hugh S.
2011ApJ...734...34K Altcode:
We report on Transition Region And Coronal Explorer 171 Å observations
of the GOES X20 class flare on 2001 April 2 that shows EUV flare ribbons
with intense diffraction patterns. Between the 11th to 14th order, the
diffraction patterns of the compact flare ribbon are dispersed into two
sources. The two sources are identified as emission from the Fe IX line
at 171.1 Å and the combined emission from Fe X lines at 174.5, 175.3,
and 177.2 Å. The prominent emission of the Fe IX line indicates that
the EUV-emitting ribbon has a strong temperature component near the
lower end of the 171 Å temperature response (~0.6-1.5 MK). Fitting
the observation with an isothermal model, the derived temperature is
around 0.65 MK. However, the low sensitivity of the 171 Å filter to
high-temperature plasma does not provide estimates of the emission
measure for temperatures above ~1.5 MK. Using the derived temperature
of 0.65 MK, the observed 171 Å flux gives a density of the EUV ribbon
of 3 × 10<SUP>11</SUP> cm<SUP>-3</SUP>. This density is much lower
than the density of the hard X-ray producing region (~10<SUP>13</SUP>
to 10<SUP>14</SUP> cm<SUP>-3</SUP>) suggesting that the EUV sources,
though closely related spatially, lie at higher altitudes.
---------------------------------------------------------
Title: The RHESSI Microflare Height Distribution
Authors: Christe, S.; Krucker, S.; Saint-Hilaire, P.
2011SoPh..270..493C Altcode: 2011arXiv1104.2624C; 2011SoPh..tmp..107C
We present the first in-depth statistical survey of flare source
heights observed by RHESSI. Flares were found using a flare-finding
algorithm designed to search the 6 - 10 keV count-rate when RHESSI's
full sensitivity was available in order to find the smallest events
(Christe et al. in Astrophys. J.677, 1385, 2008). Between March 2002
and March 2007, a total of 25 006 events were found. Source locations
were determined in the 4 - 10 keV, 10 - 15 keV, and 15 - 30 keV energy
ranges for each event. In order to extract the height distribution
from the observed projected source positions, a forward-fit model was
developed with an assumed source height distribution where height is
measured from the photosphere. We find that the best flare height
distribution is given by g(h)∝exp (−h/λ) where λ=6.1±0.3
Mm is the scale height. A power-law height distribution with a
negative power-law index, γ=3.1±0.1 is also consistent with the
data. Interpreted as thermal loop-top sources, these heights are
compared to loops generated by a potential-field model (PFSS). The
measured flare heights distribution are found to be much steeper than
the potential-field loop height distribution, which may be a signature
of the flare energization process.
---------------------------------------------------------
Title: The Focusing Optics X-ray Solar Imager
Authors: Glesener, Lindsay; Krucker, S.; Christe, S.; Ramsey, B.;
Ishikawa, S.; Takahashi, T.; Saito, S.
2011SPD....42.1511G Altcode: 2011BAAS..43S.1511G
The Focusing Optics X-ray Solar Imager (FOXSI) is a NASA Low Cost
Access to Space sounding rocket payload that will launch in late
2011. A larger sensitivity and dynamic range than currently available
are needed in order to image faint X-rays from electron beams in
the tenuous corona, particularly those near any coronal acceleration
region and those that escape into interplanetary space. FOXSI combines
fast-replication, nested, grazing-incidence optics with double-sided
silicon strip detectors to achieve a dynamic range of >100 and
a sensitivity 100 times that of RHESSI. Advances in the fabrication
and assembly of the optics at the NASA Marshall Space Flight Center
provide a spatial resolution of 8 arcseconds, while the silicon
detectors, developed by the Astro-H team at ISAS/JAXA, offer an energy
resolution of 0.5 keV. FOXSI's first flight will be used to conduct a
search for X-ray emission from nonthermal electron beams in quiet Sun
nanoflares. In addition, FOXSI will serve as a pathfinder for future
space-based solar hard X-ray spectroscopic imagers, which will be
able to image nonthermal electrons in flare acceleration sites and
provide quantitative measurements such as energy spectra, densities,
and energy content in accelerated electrons.
---------------------------------------------------------
Title: Solar Hard X-ray Observations with NuSTAR
Authors: Smith, David M.; Krucker, S.; Hudson, H. S.; Hurford, G. J.;
White, S. M.; Mewaldt, R. A.; Stern, D.; Grefenstette, B. W.; Harrison,
F. A.
2011SPD....42.1501S Altcode: 2011BAAS..43S.1501S
High-sensitivity imaging of coronal hard X-rays allows detection of
freshly accelerated nonthermal electrons at the acceleration site. A
few such observations have been made with Yohkoh and RHESSI, but a
leap in sensitivity could help pin down the time, place, and manner
of reconnection. <P />In 2012, the Nuclear Spectroscopic Telescope
Array (NuSTAR), a NASA Small Explorer for high energy astrophysics
that uses grazing-incidence optics to focus X-rays up to 80 keV, will
be launched. NuSTAR is capable of solar pointing, and three weeks
will be dedicated to solar observing during the baseline two-year
mission. NuSTAR will be 200 times more sensitive than RHESSI in the
hard X-ray band. This will allow the following new observations, among
others: <P />1) Extrapolation of the micro/nanoflare distribution by two
orders of magnitude down in flux <P />2) Search for hard X-rays from
network nanoflares (soft X-ray bright points) and evaluation of their
role in coronal heating <P />3) Discovery of hard X-ray bremsstrahlung
from the electron beams driving type III radio bursts, and measurement
of their electron spectrum <P />4) Hard X-ray studies of polar soft
X-ray jets and impulsive solar energetic particle events at the edge
of coronal holes, and comparison of these events with observations
of 3He and other particles in interplanetary space <P />5) Study of
coronal bremsstrahlung from particles accelerated by coronal mass
ejections as they are first launched <P />6) Study of particles at
the coronal reconnection site when flare footpoints are occulted; and
<P />7) Search for hypothetical axion particles created in the solar
core via the hard X-ray signal from their conversion to X-rays in the
coronal magnetic field. <P />NuSTAR will also serve as a pathfinder
for a future dedicated space mission with enhanced capabilities,
such as a satellite version of the FOXSI sounding rocket.
---------------------------------------------------------
Title: Estimation of the Reconnection Electric Field in the 2003
October 29 X10 Flare
Authors: Yang, Ya-Hui; Cheng, C. Z.; Krucker, Säm; Hsieh, Min-Shiu
2011ApJ...732...15Y Altcode:
The electric field in the reconnecting current sheet of the 2003
October 29 X10 flare is estimated to be a few kV m<SUP>-1</SUP> in
this study, based on the rate of change in the photospheric magnetic
flux in the newly brightened areas of Transition Region and Coronal
Explorer (TRACE) UV ribbons. For comparison, the motion speed of
Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) hard
X-ray (HXR) footpoints and the photospheric magnetic field strength
are also used for the electric field calculation. This X10 flare
event is selected due to its distinct two-phase HXR kernel motion,
two arcade systems with different magnetic shear, and the high cadence
and complete coverage of the TRACE 1600 Å Michelson Doppler Imager
(MDI) magnetogram and RHESSI HXR observations. We pay particular
attention to the electric field characteristics in different flare
phases, as well as the temporal correlation with the HXR emission
and its power-law spectral index and the photospheric magnetic field
strength. We found that in the early impulsive phase, the reconnection
electric field peaks just before the HXR emission peaks and the energy
spectrum hardens. The result is consistent with the scenario that more
particles are accelerated to higher energies by larger reconnection
electric fields and then precipitate into the lower chromosphere to
produce stronger HXR emissions. Such a particle acceleration mechanism
plays its most significant role in the impulsive phase of this flare. In
addition, our results provide evidence that the highly sheared magnetic
field lines are mapped to the magnetic reconnection diffusion region
to produce a large reconnection electric field.
---------------------------------------------------------
Title: The Height of White-light Flare Continuum Formation
Authors: Martinez Oliveros, J.; Hudson, Hugh; Krucker, S.; Schou,
J.; Couvidat, S.
2011SPD....42.2211M Altcode: 2011BAAS..43S.2211M
White-light continuum and hard X-ray emission in flares have strong
correlations in time and in horizontal position, but at present we do
not have a clear idea about their height structures. On 24 February
2011 a white-light flare (SOL2011-02-24T07:35) was observed on the
east limb, simultaneously by the Helioseismic Magnetic Imager (HMI)
on the Solar Dynamics Observatory (SDO), and by the Reuven Ramaty
High Energy Solar Spectroscopic Imager (RHESSI). This observation
gives us the opportunity to determine the heights of these emissions
directly, limited only by the limb references for the two spacecraft,
with almost no projection undertainty. HMI obtained clear images in
the pseudo-continuum around 6173A, and RHESSI obtained hard X-ray
images. For both data sets, the precision of centroid determination
is of order 0.1 arc s. We believe that the position of the white-light
limb, as a local reference, can also be understood at a corresponding
level of accuracy for the two data sets. We report the results of
this analysis and discuss our findings in terms of present models of
particle acceleration and energy transport in the impulsive phase.
---------------------------------------------------------
Title: Estimation of Reconnection Electric Field in the 2003 October
29 X10 Flare
Authors: Cheng, C. Z.; Yang, Ya-Hui; Krucker, S.; Hsieh, M.
2011SPD....42.2235C Altcode: 2011BAAS..43S.2235C
The electric field in the reconnecting current sheet is estimated
from the change rate of photospheric magnetic flux in the newly
brightened areas of TRACE UV ribbons. The X10 flare on 2003 October
29 is selected due to its distinct two-phase HXR footpoint motion,
two arcade systems with different magnetic shear, and the high-cadence
and complete coverage of TRACE 1600 Å, MDI magnetogram, and RHESSI
HXR observations. Besides the strengths of reconnection electric
field in different flare phases, we particularly pay attention to the
temporal correlation between the reconnection electric field and the
corresponding characteristics at the conjugate HXR footpoints (such as
the HXR emissions, HXR power-law spectral indexes, and the photospheric
magnetic field strengths). We found that in the early impulsive phase,
the reconnection electric field peaks just before the HXR emission
peaks and the energy spectrum hardens. The result could be consistent
with the scenario that more particles are accelerated to higher
energies by larger reconnection electric field and then precipitate
into lower chromosphere to produce stronger HXR emissions. Moreover,
such particle acceleration mechanism plays most significant role
in the impulsive phase of this X10 flare. In addition, our results
provide the evidence that the highly-sheared magnetic field lines are
mapped to the magnetic reconnection diffusion region to produce large
reconnection electric field.
---------------------------------------------------------
Title: Synchrotron Emission in Expanding CME Loops
Authors: Bain, Hazel; Raftery, C.; Krucker, S.
2011SPD....42.2307B Altcode: 2011BAAS..43S.2307B
Hard X-ray and radio observations can be used to identify source of
electron acceleration within the flare-CME system. Observations at meter
wavelengths using the Nancay Radioheliograph are ideal for locating
sources of synchrotron and plasma emission. Bastian et al. (2001)
and Maia et al. (2007) imaged synchrotron emission present in the
expanding loops of two independent CME's. It is unclear how these
electrons were accelerated. Previous suggestions include acceleration
at the reconnection site/current sheet between the flare and CME or in
the foreshock generated as the ejected material propagates outward. We
present observations of an event which occurred on the 14th of August
which is a new candidate for studying these events.
---------------------------------------------------------
Title: Solar Eruptive Events (SEE) Mission for the Next Solar Maximum
Authors: Lin, Robert P.; Krucker, S.; Caspi, A.; Hurford, G.; Dennis,
B.; Holman, G.; Christe, S.; Shih, A. Y.; Bandler, S.; Davila, J.;
Milligan, R.; Kahler, S.; Weidenbeck, M.; Doschek, G.; Vourlidas,
A.; Share, G.; Raymond, J.; McConnell, M.; Emslie, G.
2011SPD....42.2204L Altcode: 2011BAAS..43S.2204L
Major solar eruptive events consisting of both a large flare and a
near simultaneous large fast coronal mass ejection (CME), are the most
powerful explosions and also the most powerful and energetic particle
accelerators in the solar system, producing solar energetic particles
(SEPs) up to tens of GeV for ions and 10s-100s of MeV for electrons. The
intense fluxes of escaping SEPs are a major hazard for humans in space
and for spacecraft. Furthermore, the solar plasma ejected at high speed
in the fast CME completely restructures the interplanetary medium,
producing the most extreme space weather in geospace, at other planets,
and in the heliosphere. Thus, the understanding of the flare/CME
energy release process and of the related particle acceleration
processes in SEEs is a major goal in Heliophysics. Here we present
a concept for a Solar Eruptive Events (SEE) mission, consisting of a
comprehensive set of advanced new instruments on the single spacecraft
in low Earth orbit, that focus directly on the coronal energy release
and particle acceleration in flares and CMEs. SEE will provide new
focussing hard X-ray imaging spectroscopy of energetic electrons in the
flare acceleration region, new energetic neutral atom (ENA) imaging
spectroscopy of SEPs being accelerated by the CME at altitudes above
2 solar radii, gamma-ray imaging spectroscopy of flare-accelerated
energetic ions, plus detailed EUV/UV/Soft X-ray diagnostics of the
plasmas density, temperature, and mass motions in the energy release and
particle acceleration regions. Together with ground-based measurements
of coronal magnetic fields from ATST, FASR, and COSMO, SEE will enable
major breakthroughs in our understanding of the fundamental physical
processes involved in major solar eruptive events.
---------------------------------------------------------
Title: Metric Type II Emission in Association with an Erupting
Plasmoid
Authors: Bain, Hazel; Glesener, L.; Krucker, S.
2011SPD....42.2319B Altcode: 2011BAAS..43S.2319B
We present observations of a metric type II radio burst that occurred
on the 3rd of November 2010. The burst, which occurred in the frequency
range of the Nancay Radioheliograph (NRH), was observed in conjunction
with an erupting plasmoid. Events occurring in this frequency range are
infrequent and as such makes this an interesting event to study. Using
NRH observations together with images from the Atmospheric Imaging
Assembly onboard the Solar Dynamics Observatory (SDO/AIA) we investigate
the generation mechanism of this coronal type II burst.
---------------------------------------------------------
Title: On the Relation of Above-the-loop-top and Footpoint Hard
X-ray Sources in Solar Flares
Authors: Ishikawa, Shin-Nosuke; Krucker, S.; Takahashi, T.; Lin, R. P.
2011SPD....42.1104I Altcode: 2011BAAS..43S.1104I
We report on the most prominent example of an above-the-loop-top hard
X-ray source in the extensive solar flare data base of the Reuven
Ramaty High Energy Solar Spectroscopic Imager (RHESSI). The limb flare
of 2003 October 22 around 20UT shows a coronal source in the 40-100
keV range that is significantly above the thermal flare loop top,
similar as seen in the famous Masuda flare. The above-the-loop-top
source is only visible during one of the four hard X-ray peaks,
highlighting the rare occurrence of above-the-loop-top sources that
are equally bright as footpoint sources. The chromospheric emissions
for the peak with above-the-loop-top source shows a single footpoint,
with the second footpoint occulted. The relative timing between the
above-the-loop-top and footpoint source shows that the coronal source
peaks about 10 s before the footpoint source and decays during the
time the footpoint source is most prominent. <P />Thanks to the good
energy resolution of RHESSI, this is the first time to obtain accurate
spectra of both above-the-loop-top and footpoint sources. Photon indices
of the above-the-loop-top and the footpoint sources are 4.8±0.4 and
3.7±0.5, respectively. These difference of indices 1.1±0.6 suggests
that these emission are from the same components of electrons through
thin- and thick-target bremsstrahlung. <P />The large intensity of
the above-the-loop-top source and the absence of thermal emission
in the above-the-loop-top region suggests that the electrons within
above-the-loop-top source are purely non-thermal. Based on this
assumption, we estimate the number of the accelerated electrons and
time scale. We discuss on the relation of the above-the-loop-top and
footpoint sources and a possible model to explain this event.
---------------------------------------------------------
Title: High-Resolution Imaging of Solar Flare Ribbons and its
Implication on the Thick-Target Beam Model
Authors: Krucker, Sam
2011SPD....42.2208K Altcode: 2011BAAS..43S.2208K
We report on high-resolution optical and hard X-ray observations of
solar flare ribbons seen during the GOES X6.5 class white-light flare
of 2006 December 6. The data consist of imaging observations at 430 nm
(the Fraunhofer G-band) taken by the Hinode Solar Optical Telescope
(SOT) with the hard X-rays observed by the Reuven Ramaty High Energy
Solar Spectroscopic Imager (RHESSI). The two sets of data show closely
similar ribbon structures, strongly suggesting that the flare emissions
in white light and in hard X-rays have physically linked emission
mechanisms. While the source structure along the ribbons is resolved at
both wavelengths (length 30”), only the G-band observations resolve
the width of the ribbon, with values between 0.5" and 1.8”. The
unresolved hard X-ray observations reveal an even narrower ribbon
in hard X-rays (the main footpoint has a width perpendicular to the
ribbon of <1.1” compared to the G-band width of 1.8” suggesting
that the hard X-ray emission comes from the sharp leading edge of
the G-band ribbon. Applying the thick-target beam model, the derived
energy deposition rate is >5x10<SUP>12</SUP> erg s<SUP>-1</SUP>
cm<SUP>-2</SUP> provided by electrons above 18 keV. This requires that
the beam density of electrons above 18 keV be at least 1x10<SUP>10</SUP>
cm<SUP>-3</SUP>. Even if field lines converge towards the chromospheric
footpoints, the required beam in the corona has a too high density
to be described as a dilute tail population on top of a Maxwellian
core. We discuss this issue and others associated with this extreme
event, which poses serious questions to the standard thick target beam
interpretation of solar flares.
---------------------------------------------------------
Title: Probing Flare Temperatures Using AIA Dispersion Patterns In
Conjunction With EVE And RHESSI Spectra
Authors: Raftery, Claire; Krucker, S.
2011SPD....42.2106R Altcode: 2011BAAS..43S.2106R
The thermal distribution of plasma in solar flares has been studied
extensively and yet, remains evasive. Using the revolutionary spatial
resolution and cadence of SDO/AIA, we analyze diffraction and dispersion
effects out to more than 40 orders. Comparing the dispersion patterns
to spectral results from SDO/EVE and RHESSI, and synthetic spectra
from CHIANTI, we identify the distribution of plasma temperatures in
a compact flaring loop with 12 second cadence. Unlike traditional
spectroscopy, the high cadence of the AIA observations allow us to
capture the highly dynamic nature of these eruptions with no compromise
for spatial resolution or cadence.
---------------------------------------------------------
Title: Energetics And Heating In A Solar Plasma Ejection Observed
By RHESSI And AIA
Authors: Glesener, Lindsay; Krucker, S.; Bain, H. M.; Lin, R. P.
2011SPD....42.0903G Altcode: 2011BAAS..43S.0903G
For the past nine years, hard X-ray observations by the Reuven
Ramaty High Energy Solar Spectroscopic Imager (RHESSI) have provided
remarkable insight into the locations and spectra of energetic
flare particles. With the advent of high-cadence, multiwavelength
observations by the Atmospheric Imaging Assembly (AIA) aboard the
Solar Dynamics Observatory, it is now possible to study the dynamic
structures among which these energetic particles move. On November 3,
2010, a C4.9 solar flare emerged from behind the eastern limb of the
Sun, accompanied by a coronal mass ejection. Because the bright flare
footpoints were occulted by the solar disk (by about 12 degrees), the
coronal X-ray sources can be studied in detail without contamination by
the footpoints. Extreme ultraviolet (EUV) images from AIA show a mass
of plasma ejected from the solar surface. Isothermal analysis using
multiple EUV filters shows that this erupting plasma reaches a high
temperature of 11 MK. Meanwhile, RHESSI X-ray images reveal a large,
diffuse hard X-ray source that matches the location and shape of the
ejecting plasma, suggesting the presence of nonthermal electrons
magnetically trapped in the region. Using RHESSI spectroscopy and
AIA temperature analysis, we will test the hypothesis that nonthermal
electrons in the erupting plasma are responsible for heating the plasma
to this high temperature via collisional heating.
---------------------------------------------------------
Title: Imaging Spectroscopy of a White-Light Solar Flare
Authors: Martínez Oliveros, J. C.; Couvidat, S.; Schou, J.; Krucker,
S.; Lindsey, C.; Hudson, H. S.; Scherrer, P.
2011SoPh..269..269M Altcode: 2011SoPh..tmp....7M; 2010arXiv1012.0344M
We report observations of a white-light solar flare
(SOL2010-06-12T00:57, M2.0) observed by the Helioseismic Magnetic
Imager (HMI) on the Solar Dynamics Observatory (SDO) and the Reuven
Ramaty High Energy Solar Spectroscopic Imager (RHESSI). The HMI data
give us the first space-based high-resolution imaging spectroscopy
of a white-light flare, including continuum, Doppler, and magnetic
signatures for the photospheric Fe I line at 6173.34 Å and its
neighboring continuum. In the impulsive phase of the flare, a bright
white-light kernel appears in each of the two magnetic footpoints. When
the flare occurred, the spectral coverage of the HMI filtergrams (six
equidistant samples spanning ±172 mÅ around nominal line center)
encompassed the line core and the blue continuum sufficiently far from
the core to eliminate significant Doppler crosstalk in the latter, which
is otherwise a possibility for the extreme conditions in a white-light
flare. RHESSI obtained complete hard X-ray and γ-ray spectra (this
was the first γ-ray flare of Cycle 24). The Fe I line appears to be
shifted to the blue during the flare but does not go into emission; the
contrast is nearly constant across the line profile. We did not detect
a seismic wave from this event. The HMI data suggest stepwise changes
of the line-of-sight magnetic field in the white-light footpoints.
---------------------------------------------------------
Title: RHESSI Imaging Survey of γ-ray Bremsstrahlung Emission in
Solar Flares
Authors: Ishikawa, S.; Krucker, Säm; Takahashi, T.; Lin, R. P.
2011ApJ...728...22I Altcode:
We present a high-energy (>150 keV) imaging survey of all solar
γ-ray flares observed by the Reuven Ramaty High Energy Solar
Spectroscopic Imager (RHESSI) to study bremsstrahlung emission from
relativistic electrons. Using RHESSI rear segment data, images in the
energy range from 150 to 450 keV integrated over the total duration
of the impulsive phase of the flare are derived. Out of the 29 γ-ray
peaks in 26 RHESSI flares, we successfully obtained images for 21
γ-ray peaks in 20 flares. The remaining eight peaks have >150
keV fluences of less than a few hundred photons per cm<SUP>2</SUP>
and counting statistics are too poor for detailed imaging. The flux
ratio of the footpoint sources is found to be similar at 50 keV and
above 150 keV, indicating that relativistic electrons are present
in both footpoints of the flare loop. No correlation between the
footpoint separation and the fluence ratio of the 2.2 MeV line and
the >300 keV photons is found. This indicates that the relative
efficiency of proton to electron acceleration does not depend on
loop length, as could have been expected from stochastic acceleration
models. As previously reported, the three flares with the best counting
statistics show not only footpoint emission, but also a coronal γ-ray
bremsstrahlung source. For events with lower counting statistics, no
coronal source could be identified. However, instrumental limitation
could easily hide a coronal source for events with lower statistics,
suggesting that coronal γ-ray bremsstrahlung sources are nevertheless
a general feature of γ-ray flares.
---------------------------------------------------------
Title: Pitch-angle Distributions and Temporal Variations of 0.3-300
keV Solar Impulsive Electron Events
Authors: Wang, L.; Lin, R. P.; Krucker, Säm
2011ApJ...727..121W Altcode:
We investigate the propagation of ~0.3-300 keV electrons in five solar
impulsive electron events, observed by the WIND three-dimensional Plasma
and Energetic Particle instrument, that have rapid-rise and rapid-decay
temporal profiles. In two events, the temporal profiles above 25
keV show a second peak of inward-traveling electrons tens of minutes
after the first peak, followed by a third peak due to outward-traveling
electrons minutes later—likely due to reflection/scattering first at
~0.7-1.7 AU past the Earth, and then in the inner heliosphere inside
1 AU. In the five events, below a transition energy E <SUB>0</SUB>
(~10-40 keV), the pitch-angle distributions are highly anisotropic with
a pitch-angle width at half-maximum (PAHM) of <15° (unresolved)
through the time of the peak; the ratio Λ of the peak flux of scattered
(22fdg5-90° relative to the outward direction) to field-aligned
scatter-free (0°-22fdg5) electrons is lsim0.1. Above E <SUB>0</SUB>,
the PAHM at the flux peak increases with energy up to 85° at 300
keV, and Λ also increases with energy up to ~0.8 at 300 keV. Thus,
low-energy electrons propagated essentially scatter-free through
the interplanetary medium, while high-energy electrons experienced
pitch-angle scattering, with scattering strength increasing with
energy. The transition energy E <SUB>0</SUB> between the two populations
is always such that the electron gyroradius (ρ<SUB> e </SUB>) is
approximately equal to the local thermal proton gyroradius (ρ<SUB>
Tp </SUB>), suggesting that the higher energy electrons were scattered
by resonance with turbulent fluctuations at scale gsimρ<SUB> Tp </SUB>
in the solar wind.
---------------------------------------------------------
Title: Solar energetic electron probes of magnetic cloud field
line lengths
Authors: Kahler, S. W.; Krucker, S.; Szabo, A.
2011JGRA..116.1104K Altcode:
Magnetic clouds (MCs) are large interplanetary coronal mass ejections
of enhanced and low-variance fields with rotations indicative of
magnetic flux ropes originally connected to the Sun. The MC flux
rope models require field lines with larger pitch angles and longer
lengths with increasing distance from the MC axis. While the models
can provide good fits to the in situ solar wind observations, there
have not been definitive observational tests of the global magnetic
field geometry, particularly for the field line lengths. However,
impulsive solar energetic (E > 10 keV) electron events occasionally
occur within an MC, and the electron onsets can be used to infer Le,
the magnetic field line lengths traveled by the electrons from the Sun
to the points in the MC where the electron onsets occur. We selected
8 MCs in and near which 30 solar electron events were observed by the
3DP instrument on the Wind spacecraft. We compared the corresponding
Le values with calculated model field line lengths to test two MC
models. Some limitations on the technique are imposed by variations
of the models and uncertainly about MC boundary locations. We found
generally poor correlations between the computed electron path lengths
and the model field line lengths. Only one value of Le inside an MC,
that of 18 October 1995, exceeded 3.2 AU, indicating an absence of
the long path lengths expected in the highly wound outer regions of
MC models. We briefly consider the implications for MC models.
---------------------------------------------------------
Title: AIA and RHESSI Observations of Solar Coronal Jets
Authors: Krucker, S.; Lin, R. P.; Csillaghy, A.
2010AGUFMSH23A1826K Altcode:
The broad temperature coverage and high time resolution of AIA
observations provides an excellent diagnostic tool to study solar
flares. We combine AIA EUV observations with RHESSI hard X-ray
observations to study the geometry, time evolution, and energetics of
solar coronal jets. We report on a jet event of 2010 July 20 2UT (GOES
A5) that shows the standard jet geometry of interchange reconnection
(e.g. Moore et al. 2010). The hard X-ray observations reveal a very
compact (4"x4") thermal flare loop. Counts in the non-thermal range,
however, are too low to make an image of the HXR footpoints. The jet
itself is only seen at EUV wavelengths with a width of 15" and a length
of 60". The EUV time evolution shows two components, one that closely
follows the HXR time profile and a second one that lasts longer that
represents the jet evolution. RHESSI temperature diagnostics of hot
flare plasma nicely complements the three AIA filters (193A, 131A, and
94A) with temperature responses to hot plasma (>6 MK). The fit to the
HXR spectrum reveals a peak temperature of 9.1 MK. For this temperature
range, the filter ratio of 131A and 94A is unique and steeply increasing
with temperature. The 131/94 ratio gives a peak temperatures around 9.9
MK with the same temperature trend as seen with RHESSI. This indicates
that the presently available filter response curves for 131A and 94A
can be used as temperature diagnostics of hot (6-18 MK) flare plasmas.
---------------------------------------------------------
Title: Electron Acceleration by Multi-Island Coalescence
Authors: Oka, M.; Phan, T.; Krucker, S.; Fujimoto, M.; Shinohara, I.
2010AGUFMSH33A1812O Altcode:
Energetic electrons of up to tens of MeV are created during explosive
phenomena in the solar corona. While many theoretical models consider
magnetic reconnection as a possible way of generating energetic
electrons, the precise roles of magnetic reconnection during
acceleration and heating of electrons still remain unclear. Here
we show from 2D particle-in-cell simulations that coalescence of
magnetic islands that naturally form as a consequence of tearing mode
instability and associated magnetic reconnection leads to efficient
energization of electrons. The key process is the secondary magnetic
reconnection at the merging points, or the `anti-reconnection', which
is, in a sense, driven by the converging outflows from the initial
magnetic reconnection regions. By following the trajectories of the
most energetic electrons, we found a variety of different acceleration
mechanisms but the energization at the anti-reconnection is found to
be the most important process. We discuss possible applications to
the energetic electrons observed in the solar flares. We anticipate
our results to be a starting point for more sophisticated models of
particle acceleration during the explosive energy release phenomena.
---------------------------------------------------------
Title: itch angle distributions and temporal variations of 0.3-300
keV solar impulsive electron events
Authors: Wang, L.; Lin, R. P.; Krucker, S.
2010AGUFMSH33A1828W Altcode:
We investigate pitch angle distributions (PADs) and temporal
variations for five solar impulsive electron events with a rapid
rise and rapid decay, observed by the WIND 3D Plasma & Energetic
Particle instrument in the energy range from ∼0.3 to 300 keV. Below
a transition energy E<SUB>0</SUB> ( ∼10-30 keV), the PADs are
highly anisotropic with a pitch-angle width at half maximum (PAHM)
of <15<SUP>o</SUP> (unresolved) through the peak; the ratio of
the peak flux of scattered (22.5-90<SUP>o</SUP>) to field-aligned
scatter-free (0-22.5<SUP>o</SUP>, relative to the outward direction)
electrons is <0.1. Above E<SUB>0</SUB>, the PADs become broader
and the PAHM at peak increases with energy up to 85<SUP>o</SUP> at 300
keV; the peak-flux ratio of scattered to scatter-free electrons also
increases with energy up to ∼0.8 at 300 keV. Thus, in these events,
electrons with energies below E<SUB>0</SUB> propagated essentially
scatter-free through the interplanetary medium, while electrons at
higher energies experienced pitch-angle scattering, with scattering
strength increasing with energy. In the five events studied, the
transition energy E<SUB>0</SUB> between the two populations occurs
where the electron gyroradius (ρ <SUB>e</SUB>) is equal to the local
thermal proton gyroradius (ρ <SUB>Tp</SUB>), suggesting that the
higher energy electrons were scattered by resonance with turbulent
fluctuations at scale ≳ ρ <SUB>Tp</SUB> in the solar wind. In two
events, the temporal profiles above 25 keV also show a second peak
of inward-traveling electrons tens of minutes after the first peak,
possibly due to reflection/scattering at ∼0.5-1 AU past the Earth,
and a third peak due to outward-traveling electrons minutes later,
likely caused by reflection/scattering of those inward-traveling
populations between the Sun and Earth.
---------------------------------------------------------
Title: AIA observations of a flare/CME system in conjunction with
X-ray and radio data
Authors: Bain, H. M.; Krucker, S.
2010AGUFMSH11A1600B Altcode:
During solar flares and coronal mass ejections, free magnetic energy,
previously stored within the magnetic field, is released and used
to accelerate particles and heat the surrounding plasma. Using
multiwavelength observations from AIA onboard SDO, combined with
X-ray observations from RHESSI and radio observations from the Nancay
Radioheliograph we investigate sources of particle acceleration and
heating within the flare-CME system of an event which occurred on
the 14 August 2010. This was the first proton event in more than
three years. In addition, radio spectrogram data show Type III
radio bursts associated with the impulsive/launch phase of the CME,
indicating the location of energetic electrons as they escape into
the interplanetary medium. A Type II radio burst is also observed as
electrons are accelerated in a CME driven shock as the ejected material
propagates away from the Sun.
---------------------------------------------------------
Title: A "black light flare" observed by HMI?
Authors: Martinez Oliveros, J.; Hudson, H. S.; Krucker, S.
2010AGUFMSH23A1827M Altcode:
We report the observation of a negative precursor to the white-light
flare SOL2010-06-12T00:57 (M2.0), as observed via SDO/HMI in narrow
bands near 6173.3 A. RHESSI and Fermi hard X-ray and gamma-ray
observations for this flare are also available and reveal an unusually
hard spectrum in the hard X-ray range. The "black light flare" locations
are in both of the two white-light flare emission regionsand precede
them by about one HMI time step of 45 s. The timing thus resembles
that predicted by Henoux et al. (1990) based on the interplay between
ionization (leading to opacity) and heating (leading to emission). We
discuss this remarkable event in its full observational scope.
---------------------------------------------------------
Title: SDO and RHESSI Observations of Microflares
Authors: Christe, S.; Krucker, S.; Hannah, I. G.
2010AGUFMSH23A1830C Altcode:
We present observations of 10 microflares (B and A GOES Class) with
simultaneous observations in both hard X-ray (HXR) and ultraviolet
(UV/EUV) observations by RHESSI and SDO/AIA, respectively. The
microflares were chosen by visually searching for time-isolated events
in the available concurrent RHESSI/SDO dataset. Identifying the flare
region using RHESSI imaging observations, we compare HXR (thermal and
nonthermal) light curves and images with those observed by AIA in every
available UV/EUV channels. Many events show complex morphologies with
multiple flaring loops observed in AIA images. For the simplest event
with a single flare loop, good correlation is found between the hot
thermal (~10 MK) HXR emission and emission from 131 and 94 Angstrom
and the filter-ratio derived temperatures agree with the HXR-derived
temperatures during the decay phase. During the impulsive phase,
footpoints emission was observed at all wavelengths with filter-ratios
suggest the presence of multiple temperatures.
---------------------------------------------------------
Title: Multi-temperature Observations of Solar Microflares using
RHESSI and SDO/AIA
Authors: McTiernan, J. M.; Krucker, S.
2010AGUFMSH23A1831M Altcode:
In this work, we estimate the Differential Emission Measure for solar
microflares using data from the AIA instrument on SDO and RHESSI
data. Seven of the AIA wavelength bands (94, 131, 171, 193, 304,
and 335 Angstroms), which detect emission from solar plasma with
temperatures ranging from less than 20,000 K to as high as 20 MK,
are useful for this calculation. The RHESSI instrument observes solar
flare plasma in the temperature range above 8 MK. Preliminary results
for a microflare observed on 11 July 2010 show loop footpoint sources
with broad emission measure distributions, with both low (<100,000
K) and high (>10 MK) plasma. There is also a loop visible in the
AIA 131 and 94 Angstrom wavelength bands, which are sensitive to high
temperatures. RHESSI images in the energy range between 4 and 10 keV
also show the loop, which has a relatively narrow (in temperature)
emission measure distribution, concentrated above 10 MK. Implications
for flare heating scenarios which may result in this sort of emission
measure distribution will be discussed.
---------------------------------------------------------
Title: Thermal Imaging of Multi-Temperature Flare Plasma with RHESSI
Visibilities
Authors: Caspi, A.; Krucker, S.; Hurford, G. J.; McTiernan, J. M.
2010AGUFMSH53B..08C Altcode:
Solar flares contain thermal plasma at multiple
temperatures. Observations with the Reuven Ramaty High Energy Solar
Spectroscopic Imager (RHESSI) have shown that the most intense (GOES
X-class) flares commonly achieve maximum temperatures of ~30-50
MK. Recent analysis reveals that in such flares, the "super-hot"
(>30 MK) plasma exists simultaneously with, but separately from,
the ~10-20 MK plasma observed in nearly all flares; the X-ray spectra
are well-fit by two distinct isothermal components. RHESSI images at
multiple energies can show differing morphologies and/or centroid
positions, as each thermal component contributes a different but
varying amount to the flux in each energy band, making it difficult to
separately characterize the spatial properties of the multiple emitting
plasmas. We present a novel technique to visualize the individual
thermal plasmas by combining RHESSI X-ray visibilities using the
precise but spatially-unresolved spectra and model fits, isolating
the contributions from each thermal component to effectively image
as a function of temperature rather than energy. This technique is
computationally inexpensive and generalizable to an arbitrary number
of thermal components, e.g. a binned differential emission measure
model of the temperature distribution. We apply this technique to the
2002 July 23 X4.8 event to show that the super-hot and cool plasmas
are spatially distinct and to examine their evolution throughout the
flare. We then discuss the implications for plasma heating in large
solar flares. <P />FIGURE CAPTION: Image contours of the two individual
thermal components in the 2002 Jul 23 X4.8 event, derived using our
technique; the non-thermal contours are shown for reference. (Figure
not shown in ADS abstract.)
---------------------------------------------------------
Title: Longitudinal Spread of Protons from the Decay of Solar Flare
Neutrons
Authors: Agueda, N.; Krucker, S.; Lin, R. P.
2010AGUFMSH33B1834A Altcode:
Solar energetic particle events which arise from the in-flight decay
of energetic neutrons emitted from the Sun were observed in three
cases during solar cycle 21. These events were associated with X-class
gamma-ray solar flares occurring on the eastern hemisphere of the Sun
during quiescent interplanetary conditions. The beginning of solar
cycle 24 presents the opportunity to observe this kind of events from
spacecraft separated in longitude and distance from the Sun. We present
a Monte Carlo simulation of the propagation of solar flare neutrons
and its resulting decay protons in the heliosphere. We find that the
longitudinal spread of solar neutron decay protons depends on the
angular distribution of solar neutrons at the Sun. We conclude that
the observation of neutron decay proton events by spacecraft separated
in longitude could be used to infer the degree of neutron isotropy at
the Sun and constrain theoretical models of high-energy processes in
solar flares.
---------------------------------------------------------
Title: Probing flare temperatures using AIA dispersion effects and
RHESSI imaging/spectroscopy
Authors: Raftery, C. L.; Krucker, S.
2010AGUFMSH23A1828R Altcode:
Diffraction patterns that occur as a result of very intense, often
saturated emission can provide many useful tools not ordinarily
available. Specifically, analyzing dispersion features of high order
diffraction patterns can provide useful insight into the thermal
distributions of plasma within a structure. Here, we analyze the
diffraction patterns of multiple SDO/AIA wavelength bands to more
than 40 orders. Combining these results with imaging and spectroscopy
from RHESSI enabled us to probe temperature and emission measure
distributions within saturated flare sources.
---------------------------------------------------------
Title: Study of Flare Energetics Using X-ray, Radio, and EUV
Observations
Authors: Glesener, L.; Krucker, S.; Bain, H. M.; Lin, R. P.
2010AGUFMSH23A1829G Altcode:
In studying solar flare energetics, the following questions are usually
considered: What is the number of accelerated electrons? What are
their energies? Where are they located and where do they ultimately
end up? Multiwavelength studies are useful for answering these
questions. Information on the instantaneous number of electrons and
their energies can be inferred from the nonthermal bremsstrahlung
spectrum in the hard X-ray (HXR) regime. HXR images can also identify
locations of energetic electrons. Thermal bremsstrahlung emission in
soft X-rays provides an estimate of the energy that goes into heating
ambient electrons, as well as temperature and density diagnostics. UV
and EUV observations provide more detailed temperature estimates,
show the dynamics of plasma structures, and help to locate emitting
volumes. Radio emissions offer information on the energy lost to
escaping electrons (which emit Type III bursts) and the energies of
gyrosynchrotron-emitting electrons in flare loops. Here, we combine
X-ray data from the Reuven Ramaty High Energy Solar Spectrometer
(RHESSI), EUV data from the Atmospheric Imaging Assembly (AIA) aboard
the Solar Dynamics Observatory, and radio data from various solar radio
spectrographs. Several case studies of flare electron energetics will be
presented. The time profiles of HXR and radio emission will be compared,
along with the electron energies derived from each. EUV and UV data
will be used to image flare loops and investigate temperature evolution.
---------------------------------------------------------
Title: Statistically Derived Flaring Chromospheric-Coronal Density
Structure from Non-thermal X-ray Observations of the Sun
Authors: Saint-Hilaire, P.; Krucker, S.; Lin, R. P.
2010ApJ...721.1933S Altcode: 2011arXiv1111.4251S
For the first time, we have used RHESSI's spatial and energy resolution
to determine the combined chromospheric and coronal density profile
of the flaring solar atmosphere in a statistical manner, using a data
set of 838 flares observable in hard X-rays above 25 keV. Assuming
the thick-target beam model, our "average flaring atmosphere" was
found to have density scale heights of 131 ± 16 km at low altitudes
(chromosphere, up to ≈1-1.5 Mm above photosphere), and of 5-6 Mm
at high altitudes (corona, above ≈2-3 Mm). Assuming a unit step
change in ionization level, modeling yields a height of 1.3 ±
0.2 Mm for the transition between fully neutral and fully ionized
atmosphere. Furthermore, centroids of emission above 50 keV, produced
by electrons of similar or higher energies, are located mostly in a
small region ~0.5 Mm in vertical extent, where neutral densities are
beyond 3 × 10<SUP>13</SUP> cm<SUP>-3</SUP>.
---------------------------------------------------------
Title: Development of double-sided silicon strip detectors for solar
hard x-ray observation
Authors: Saito, Shinya; Ishikawa, Shin-Nosuke; Watanabe, Shin; Odaka,
Hirokazu; Sugimoto, Soichiro; Fukuyama, Taro; Kokubun, Motohide;
Takahashi, Tadayuki; Terada, Yukikatsu; Tajima, Hiroyasu; Tanaka,
Takaaki; Krucker, Säm; Christe, Steven; McBride, Steve; Glesener,
Lindsay
2010SPIE.7732E..0QS Altcode: 2010SPIE.7732E..19S
The Focusing Optics X-ray Solar Imager (FOXSI) is a rocket experiment
scheduled for January 2011 launch. FOXSI observes 5 - 15 keV hard
X-ray emission from quiet-region solar flares in order to study
the acceleration process of electrons and the mechanism of coronal
heating. For observing faint hard X-ray emission, FOXSI uses focusing
optics for the first time in solar hard X-ray observation, and attains
100 times higher sensitivity than RHESSI, which is the present solar
hard X-ray observing satellite. Now our group is working on developments
of both Double-sided Silicon Strip Detector (DSSD) and read-out analog
ASIC "VATA451" used for FOXSI. Our DSSD has a very fine strip pitch
of 75 μm, which has sufficient position resolution for FOXSI mirrors
with angular resolution (FWHM) of 12 arcseconds. DSSD also has high
spectral resolution and efficiency in the FOXSI's energy range of 5 -
15 keV, when it is read out by our 64-channel analog ASIC. In advance
of the FOXSI launch, we have established and tested a setup of 75 μm
pitch DSSD bonded with "VATA451" ASICs. We successfully read out from
almost all the channels of the detector, and proved ability to make
a shadow image of tungsten plate. We also confirmed that our DSSD
has energy resolution (FWHM) of 0.5 keV, lower threshold of 5 keV,
and position resolution less than 63 μm. These performance satisfy
FOXSI's requirements.
---------------------------------------------------------
Title: The Focusing Optics Solar X-ray Imager (FOXSI)
Authors: Christe, Steven; Glesener, L.; Krucker, S.; Ramsey, B.;
Ishikawa, S.; Takahashi, T.; Tajima, H.
2010AAS...21641309C Altcode:
The Focusing Optics x-ray Solar Imager (FOXSI) is a sounding rocket
payload funded under the NASA Low Cost Access to Space program to
test hard x-ray focusing optics and position-sensitive solid state
detectors for solar observations. Today's leading solar hard x-ray
instrument, the Reuven Ramaty High Energy Solar Spectroscopic Imager
(RHESSI) provides excellent spatial (2 arcseconds) and spectral (1 keV)
resolution. Yet, due to its use of indirect imaging, the derived images
have a low dynamic range (<30) and sensitivity. These limitations
make it difficult to study faint x-ray sources in the solar corona
which are crucial for understanding the solar flare acceleration
process. Grazing-incidence x-ray focusing optics combined with
position-sensitive solid state detectors can overcome both of these
limitations enabling the next breakthrough in understanding particle
acceleration in solar flares. The FOXSI project is led by the Space
Science Laboratory at the University of California. The NASA Marshall
Space Flight Center, with experience from the HERO balloon project, is
responsible for the grazing-incidence optics, while the Astro H team
(JAXA/ISAS) will provide double-sided silicon strip detectors. FOXSI
will be a pathfinder for the next generation of solar hard x-ray
spectroscopic imagers. Such observatories will be able to image the
non-thermal electrons within the solar flare acceleration region, trace
their paths through the corona, and provide essential quantitative
measurements such as energy spectra, density, and energy content in
accelerated electrons.
---------------------------------------------------------
Title: Electron Acceleration by Multi-Island Coalescence
Authors: Oka, M.; Phan, T. -D.; Krucker, S.; Fujimoto, M.; Shinohara,
I.
2010ApJ...714..915O Altcode: 2010arXiv1004.1154O
Energetic electrons of up to tens of MeV are created during
explosive phenomena in the solar corona. While many theoretical
models consider magnetic reconnection as a possible way of generating
energetic electrons, the precise roles of magnetic reconnection during
acceleration and heating of electrons still remain unclear. Here, we
show from two-dimensional particle-in-cell simulations that coalescence
of magnetic islands that naturally form as a consequence of tearing mode
instability and associated magnetic reconnection leads to efficient
energization of electrons. The key process is the secondary magnetic
reconnection at the merging points, or the "anti-reconnection," which
is, in a sense, driven by the converging outflows from the initial
magnetic reconnection regions. By following the trajectories of the
most energetic electrons, we found a variety of different acceleration
mechanisms but the energization at the anti-reconnection is found to
be the most important process. We discuss possible applications to
the energetic electrons observed in the solar flares. We anticipate
our results to be a starting point for more sophisticated models of
particle acceleration during the explosive energy release phenomena.
---------------------------------------------------------
Title: Measurements of the Coronal Acceleration Region of a Solar
Flare
Authors: Krucker, Sam; Hudson, H. S.; White, S. M.; Masuda, S.; Lin,
R. P.
2010AAS...21630603K Altcode:
The most discussed coronal hard X-ray source has been the
above-the-loop-top source observed in the Masuda flare. We present new
RHESSI hard X-ray observations of a similar event with simultaneous
microwave observations by NoRH. These observations clearly establish
the non-thermal nature of the above-the-loop-top source and triggered
a new interpretation. To account for the extremely bright hard X-ray
source in a rather low ambient density plasma, all electrons in the
above-the-loop-top source seem to be accelerated, suggesting that the
above-the-loop-top source is itself the electron acceleration region.
---------------------------------------------------------
Title: Solar Flare Electrons at the Sun and in the Interplanetary
Space
Authors: Krucker, Säm.
2010EGUGA..12.5954K Altcode:
How solar flares accelerate electrons up to relativistic energies is
one of the main unsolved problems in Heliophysics. Remote sensing
at radio and hard X-ray wavelengths provide crucial diagnostics of
flare-accelerated electrons in the solar corona. In-situ observations
in interplanetary space give direct observations of flare-accelerated
electrons escaping the Sun. In this review, recent results combining
these observations are discussed, with a special emphases on future
observations with Solar Orbiter and Solar Probe +.
---------------------------------------------------------
Title: Statistics Of Solar Flares Hard X-ray Spectra Using RHESSI.
Authors: Alaoui, Meriem; Krucker, S.; Saint-Hilaire, P.; Lin, R. P.
2010AAS...21640437A Altcode:
Non-thermal X-ray emission provides a key diagnostic on particle
acceleration and transport during solar flares. Thermal and non-thermal
emissions can be derived from the spatially integrated photon flux
spectrum obtained by the Ramaty High Energy Solar Spectroscopic Imager
(RHESSI). The thermal radiation is usually represented by a maxwellian
and the non-thermal emission is represented by a single or a double
(broken) power-law. We present spectral characteristics of 65 flares
observed by RHESSI and having emission to at least 150keV above the
background level. The spectra are integrated over 8s at the hard x-ray
peak time. Our major findings are as follows: (1)The spectra almost
always break downwards i.e. the spectral indices above the breakenergy
are greater than indices below the break energy (47/65). (2) The break
energy is typically around 60 keV (3) There is no dependence of the
break energy upon the spectral index. (4) A small dependence of the
photon flux upon the break energy.
---------------------------------------------------------
Title: G-band and Hard X-ray Emissions of the 2006 December 14 Flare
Observed by Hinode/SOT and Rhessi
Authors: Watanabe, Kyoko; Krucker, Säm; Hudson, Hugh; Shimizu,
Toshifumi; Masuda, Satoshi; Ichimoto, Kiyoshi
2010ApJ...715..651W Altcode: 2010arXiv1004.4259W
We report on G-band emission observed by the Solar Optical Telescope on
board the Hinode satellite in association with the X1.5-class flare on
2006 December 14. The G-band enhancements originate from the footpoints
of flaring coronal magnetic loops, coinciding with nonthermal hard
X-ray bremsstrahlung sources observed by the Reuven Ramaty High Energy
Solar Spectroscopic Imager. At the available 2 minute cadence, the
G-band and hard X-ray intensities are furthermore well correlated in
time. Assuming that the G-band enhancements are continuum emission from
a blackbody, we derived the total radiative losses of the white-light
flare (white-light power). If the G-band enhancements additionally have
a contribution from lines, the derived values are overestimates. We
compare the white-light power with the power in hard X-ray producing
electrons using the thick-target assumption. Independent of the cutoff
energy of the accelerated electron spectrum, the white-light power and
the power of accelerated electrons are roughly proportional. Using
the observed upper limit of ~30 keV for the cutoff energy, the hard
X-ray producing electrons provide at least a factor of 2 more power
than needed to produce the white-light emission. For electrons above
40 keV, the powers roughly match for all four of the time intervals
available during the impulsive phase. Hence, the flare-accelerated
electrons contain enough energy to produce the white-light flare
emissions. The observed correlation in time, space, and power strongly
suggests that electron acceleration and white-light production in solar
flares are closely related. However, the results also call attention
to the inconsistency in apparent source heights of the hard X-ray
(chromosphere) and white-light (upper photosphere) sources.
---------------------------------------------------------
Title: RHESSI and TRACE Observations of Emerging Flux Reconnection
in a Solar Jet on August 21, 2003
Authors: Glesener, Lindsay; Krucker, S.; Lin, R. P.
2010AAS...21632006G Altcode: 2010BAAS...41R.911G
Solar jets, or transient, collimated brightenings, were first observed
in soft x-rays by Yohkoh in the 1990's. These observations provided
confirmation for the emerging flux model predicted by Heyvaerts and
Shibata to describe reconnection in the corona. This model also predicts
hard x-ray (HXR) emission during jet production, and recent RHESSI and
HINODE observations appear to support this prediction. Unfortunately,
projection effects make it difficult to determine whether HXR sources
on the solar disk correspond to flare footpoints or to the jet
itself. Observation of jets near or at the solar limb can mitigate
projection effects and allow the jet geometry to be resolved. <P
/>Here, we present RHESSI observations of a partially occulted HXR
flare that coincided with an EUV jet observed by TRACE on August 21,
2003. Comparisons between TRACE and RHESSI images and time profiles are
used to illustrate several aspects of the emerging flux reconnection
model, including the locations of outflow jets, post-reconnection
flare loops, and HXR emission from the jet base. Radio data is used to
provide corroborating evidence for the sources and timing of particle
acceleration.
---------------------------------------------------------
Title: Search for Heliospheric Electrons and Protons from the Decay
of Solar Flare Neutrons over Solar Cycle 23
Authors: Agueda, Neus; Krucker, S.; Lin, R. P.; Wang, L.; Larson, D. E.
2010AAS...21640435A Altcode:
The observation of solar neutrons has long been recognized as a probe
to learn the properties of ions accelerated in solar flares. To date,
few energetic particle events produced by decaying solar flare neutrons
have been reported, and the information they have provided has covered
the 20-200 MeV energy range of the spectrum. <P />We present preliminary
results of a comprehensive search of in-situ measurements of 1-10 MeV
protons and 0.05-1 MeV electrons related to the decay of solar flare
neutrons. We select a sample of 39 isolated large eastern hemisphere
flares through solar cycle 23, and characterize the interplanetary
magnetic field conditions after each flare. We analyze the electron
and proton in-situ intensities observed by Wind following each solar
flare and discuss the challenges that poses the detection of the
decay products of low energy solar flare neutrons. <P />This work was
supported by NASA under research grants NNX08AE34G and NAS5-98033.
---------------------------------------------------------
Title: Observations of a Sun-grazing Comet's Interaction with the
Solar Corona
Authors: Raftery, Claire; Martinez Oliveros, J. C.; Krucker, S.
2010AAS...21640723R Altcode:
A number of the "Kreutz” family of Sun-grazing comets were observed to
fly in close proximity to the Sun on 12 March 2010. Here we present
observations of one such comet during its initial approach and
"impact” with the Sun. This study makes use of observations from a
range of spacecraft with multiple points of view including both STEREO
spacecraft and multiple instruments on board SOHO.
---------------------------------------------------------
Title: Observing Solar Hard X-rays from Heliospheric Orbits
Authors: Hurford, Gordon J.; Benz, A.; Dennis, B.; Krucker, S.;
Limousin, O.; Lin, R.; Vilmer, N.
2010AAS...21640416H Altcode: 2010BAAS...41Q.902H
The coming decade provides two opportunities to acquire a different
observational perspective on solar hard x-ray emission. Both ESA's Solar
Orbiter and NASA's Solar Probe Plus missions will be in heliocentric
orbits with perihelia of 0.28 au and 0.05 au respectively. This
poster indicates the unique scientific advantages of hard x-ray
imaging/spectroscopy observations from such platforms. <P />These
advantages stem from three factors: First, in combination with other
payload elements, the hard x-rays provide the ability to observationally
link accelerated electrons at the Sun to radio observations of
the propagating electrons and to direct observations of in situ
electrons. Second, the substantial gain in sensitivity afforded by
close-in vantage points enables exploration of the origin of non-flare
associated SEP events to be studied and the character of quiescent
active-region heating and electron acceleration to be evaluated. Third,
the different observational perspectives provided by the heliocentric
orbits compared to low-Earth orbits enable improved separation of
coronal and footpoint sources as well as measurements of the isotropy
of the x-ray emission. <P />Despite the limited payload resources
(mass, power, telemetry) afforded by such missions, scientifically
effective hard x-ray imaging spectroscopy from 5 keV to 150 keV is
still feasible. The Spectrometer/Telescope for Imaging X-rays (STIX),
accepted as part of the Solar Orbiter payload, combines high spectral
resolution ( 1 keV FWHM at 10 keV) with spatial resolution as good
as 1500 km, and can efficiently encode the data for several hundred
optimized images per hour within a modest telemetry allocation and
4 kg / 4 watt budget. The X-ray Imaging Spectrometer (XIS) proposed
for Solar Probe Plus, views the Sun through its thermal shield. It
also features high spectral resolution from 6 to 150 keV and spatial
resolution of 1500 km at perihelion. The poster describes the imaging
principles and current configurations of both instruments.
---------------------------------------------------------
Title: Magnetic Connectivity of the CME Event on December 31, 2007
Authors: Martinez Oliveros, Juan Carlos; Raftery, C.; Agueda, N.;
Krucker, S.; Bale, S.; Lindsey, C.
2010AAS...21640624M Altcode:
Dynamical changes in the solar magnetic field are responsible for
coronal mass ejections (CMEs), one of the most violent phenomena
observed in the Sun. During a CME, mass is lifted away from the Sun and
accelerated into the interplanetary medium, sometimes interacting with
the Earth or other planets. Some of these CMEs produce radio emission,
including type II bursts, which are generated by a strong upstream
interplanetary (IP) shocks. The emission is strongest in regions
where the direction of the IP shock is quasi-perpendicular to the
interplanetary magnetic field. Due to small-scale shock structure, this
occurs at various sites along the shock front. We study the different
emissions associated with the CME observed on 2007 December 31 using
data from the RHESSI, Proba-2/SWAP, and STEREO A/B spacecraft. We track
and probe the CME and associated shock signatures from its origin in
the lower corona through the interplanetary medium.
---------------------------------------------------------
Title: Measurements of the Coronal Acceleration Region of a Solar
Flare
Authors: Krucker, Säm; Hudson, H. S.; Glesener, L.; White, S. M.;
Masuda, S.; Wuelser, J. -P.; Lin, R. P.
2010ApJ...714.1108K Altcode:
The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and
the Nobeyama Radioheliograph (NoRH) are used to investigate coronal
hard X-ray and microwave emissions in the partially disk-occulted solar
flare of 2007 December 31. The STEREO mission provides EUV images of the
flare site at different viewing angles, establishing a two-ribbon flare
geometry and occultation heights of the RHESSI and NoRH observations
of ~16 Mm and ~25 Mm, respectively. Despite the occultation, intense
hard X-ray emission up to ~80 keV occurs during the impulsive phase
from a coronal source that is also seen in microwaves. The hard X-ray
and microwave source during the impulsive phase is located ~6 Mm above
thermal flare loops seen later at the soft X-ray peak time, similar in
location to the above-the-loop-top source in the Masuda flare. A single
non-thermal electron population with a power-law distribution (with
spectral index of ~3.7 from ~16 keV up to the MeV range) radiating
in both bremsstrahlung and gyrosynchrotron emission can explain
the observed hard X-ray and microwave spectrum, respectively. This
clearly establishes the non-thermal nature of the above-the-loop-top
source. The large hard X-ray intensity requires a very large number
(>5 × 10<SUP>35</SUP> above 16 keV for the derived upper limit
of the ambient density of ~8 × 10<SUP>9</SUP> cm<SUP>-3</SUP>)
of suprathermal electrons to be present in this above-the-loop-top
source. This is of the same order of magnitude as the number of ambient
thermal electrons. We show that collisional losses of these accelerated
electrons would heat all ambient electrons to superhot temperatures
(tens of keV) within seconds. Hence, the standard scenario, with hard
X-rays produced by a beam comprising the tail of a dominant thermal core
plasma, does not work. Instead, all electrons in the above-the-loop-top
source seem to be accelerated, suggesting that the above-the-loop-top
source is itself the electron acceleration region.
---------------------------------------------------------
Title: Combined STEREO/RHESSI Study of Coronal Mass Ejection
Acceleration and Particle Acceleration in Solar Flares
Authors: Temmer, M.; Veronig, A. M.; Kontar, E. P.; Krucker, S.;
Vršnak, B.
2010ApJ...712.1410T Altcode: 2010arXiv1002.3080T
Using the potential of two unprecedented missions, Solar Terrestrial
Relations Observatory (STEREO) and Reuven Ramaty High-Energy Solar
Spectroscopic Imager (RHESSI), we study three well-observed fast coronal
mass ejections (CMEs) that occurred close to the limb together with
their associated high-energy flare emissions in terms of RHESSI hard
X-ray (HXR) spectra and flux evolution. From STEREO/EUVI and STEREO/COR1
data, the full CME kinematics of the impulsive acceleration phase up to
~4 R <SUB>sun</SUB> is measured with a high time cadence of <=2.5
minutes. For deriving CME velocity and acceleration, we apply and
test a new algorithm based on regularization methods. The CME maximum
acceleration is achieved at heights h <= 0.4 R <SUB>sun</SUB>,
and the peak velocity at h <= 2.1 R <SUB>sun</SUB> (in one case,
as small as 0.5 R <SUB>sun</SUB>). We find that the CME acceleration
profile and the flare energy release as evidenced in the RHESSI HXR flux
evolve in a synchronized manner. These results support the "standard"
flare/CME model which is characterized by a feedback relationship
between the large-scale CME acceleration process and the energy release
in the associated flare.
---------------------------------------------------------
Title: Time-resolved hard X-Ray hardness variation of solar flares
observed by Suzaku Wide-band All-sky monitor
Authors: Endo, Akira; Morigami, Kouichi; Tashiro, Makoto; Terada,
Yukikatsu; Yamaoka, Kazutaka; Sonoda, Eri; Minoshima, Takashi; Krucker,
Sam; HXD-WAM Team
2010ecsa.conf..194E Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Focusing Optics X-Ray Solar Imager
Authors: Glesener, L.; Krucker, S.; Christe, S.
2010LPICo1534...30G Altcode:
No abstract at ADS
---------------------------------------------------------
Title: On The Brightness and Waiting-Time Distributions of a Type
III Radio Storm Observed By Stereo/Waves
Authors: Eastwood, J. P.; Wheatland, M. S.; Hudson, H. S.; Krucker,
S.; Bale, S. D.; Maksimovic, M.; Goetz, K.; Bougeret, J. -L.
2010ApJ...708L..95E Altcode: 2009arXiv0911.4131E
Type III solar radio storms, observed at frequencies below ~16 MHz
by space-borne radio experiments, correspond to the quasi-continuous,
bursty emission of electron beams onto open field lines above active
regions. The mechanisms by which a storm can persist in some cases
for more than a solar rotation whilst exhibiting considerable radio
activity are poorly understood. To address this issue, the statistical
properties of a type III storm observed by the STEREO/WAVES radio
experiment are presented, examining both the brightness distribution
and (for the first time) the waiting-time distribution (WTD). Single
power-law behavior is observed in the number distribution as a function
of brightness; the power-law index is ~2.1 and is largely independent of
frequency. The WTD is found to be consistent with a piecewise-constant
Poisson process. This indicates that during the storm individual type
III bursts occur independently and suggests that the storm dynamics are
consistent with avalanche-type behavior in the underlying active region.
---------------------------------------------------------
Title: The white-light continuum in the impulsive phase of a solar
flare.
Authors: Hudson, H. S.; Fletcher, L.; Krucker, S.
2010MmSAI..81..637H Altcode: 2010arXiv1001.1005H
We discuss the IR/visible/VUV continuum emission of the impulsive
phase of a solar flare, using TRACE UV and EUV images to characterize
the spectral energy distribution. This continuum has been poorly
observed but energetically dominates the radiant energy output
. Recent bolometric observations of solar flares furthermore point to
the impulsive phase as the source of a major fraction of the radiant
energy. This component appears to exhibit a Balmer jump and thus must
originate in an optically thin region above the quiet photosphere,
with an elevated temperature and strong ionization.
---------------------------------------------------------
Title: Evidence for extended acceleration of solar flare ions from 1-8
MeV solar neutrons detected with the MESSENGER Neutron Spectrometer
Authors: Feldman, William C.; Lawrence, David J.; Goldsten, John O.;
Gold, Robert E.; Baker, Daniel N.; Haggerty, Dennis K.; Ho, George C.;
Krucker, SäM.; Lin, Robert P.; Mewaldt, Richard A.; Murphy, Ronald
J.; Nittler, Larry R.; Rhodes, Edgar A.; Slavin, James A.; Solomon,
Sean C.; Starr, Richard D.; Vilas, Faith; Vourlidas, Angelos
2010JGRA..115.1102F Altcode: 2010JGRA..11501102F
Neutrons produced on the Sun during the M2 flare on 31 December 2007
were observed at 0.48 AU by the MESSENGER Neutron Spectrometer. These
observations are the first detection of solar neutrons inside 1 AU. This
flare contained multiple acceleration episodes as seen in type III
radio bursts. After these bursts ended, both the energetic particle and
neutron fluxes decayed smoothly to background with an e-folding decay
time of 2.84 h, spanning a 9 h time period. This time is considerably
longer than the mean lifetime of a neutron, which indicates that
either the observed neutrons were generated in the spacecraft by solar
energetic particle protons, or they originated on the Sun. If most of
the neutrons came from the Sun, as our simulations of neutron production
on the spacecraft show, they must have been continuously produced. A
likely explanation of their long duration is that energetic ions were
accelerated over an extended time period onto closed magnetic arcades
above the corona and then slowly pitch angle-scattered by coronal
turbulence into their chromospheric loss cones. Because of their
relatively low energy loss in the Neutron Spectrometer (0.5-7.5 MeV),
most of these neutrons beta decay to energetic protons and electrons
close to the Sun, thereby forming an extended seed population available
for further acceleration by subsequent shocks driven by coronal mass
ejections in interplanetary space.
---------------------------------------------------------
Title: The McClymont Jerk: A driver of solar seismicity
Authors: Martinez Oliveros, J. C.; Hudson, H. S.; Krucker, S.
2009AGUFMSH13A1508M Altcode:
Our Sun is a vibrating and amazing celestial object, full of
little-understood phenomena. One such phenomenon is the so-called
Sunquake, originally observed by Kosovichev and Zharkova (1998). Studies
by Donea and Lindsey (2005) and Besliu et al. (2008) now show that
the sunquake is not a rare phenomenon. They are powerful events,
hardly visible on the solar surface, and result directly from energy
release. They can be detected using acoustic techniques as high
frequencies oscillations in the sun. The first models proposed to
explain sunquakes involved pressure pulses, perhaps associated with
heating revealed by white-light flare emission. Hudson, Fisher and Welsh
(2008) proposed a mechanism to generate seismic waves based on the
dynamical behavior of the solar magnetic field during flares. In this
poster we study the variations of the magnetic field as an alternative
mechanism for the generation of seismic waves.
---------------------------------------------------------
Title: A Statistical Study of Spectral Hardening in Solar Flares
and Related Solar Energetic Particle Events
Authors: Grayson, James A.; Krucker, Säm; Lin, R. P.
2009ApJ...707.1588G Altcode:
Using hard X-ray observations from the Reuven Ramaty High Energy
Solar Spectroscopic Imager (RHESSI), we investigate the reliability of
spectral hardening during solar flares as an indicator of related solar
energetic particle (SEP) events at Earth. All RHESSI data are analyzed,
from 2002 February through the end of Solar Cycle 23, thereby expanding
upon recent work on a smaller sample of flares. Previous investigations
have found very high success when associating soft-hard-harder (SHH)
spectral behavior with energetic proton events, and confirmation of this
link would suggest a correlation between electron acceleration in solar
flares and SEPs seen in interplanetary space. In agreement with these
past findings, we find that of 37 magnetically well-connected flares
(W30-W90), 12 of 18 flares with SHH behavior produced SEP events and
none of 19 flares without SHH behavior produced SEPs. This demonstrates
a statistically significant dependence of SHH and SEP observations, a
link that is unexplained in the standard scenario of SEP acceleration
at the shock front of coronal mass ejections and encourages further
investigation of the mechanisms which could be responsible.
---------------------------------------------------------
Title: STATISTICAL STUDY of HARD X-RAY SPECTRAL CHARACTERISTICS OF
SOLAR FLARES
Authors: Alaoui, M.; Krucker, S.; Saint-Hilaire, P.; Lin, R. P.
2009AGUFMSH23A1519A Altcode:
We investigate the spectral characteristics of 75 solar flares at
the hard X-ray peak time observed by RHESSI (Ramaty High Energy Solar
Spectroscopic Imager) in the energy range 12-150keV. At energies above
40keV, the Hard X-ray emission is mostly produced by bremsstrahlung
of suprathermal electrons as they interact with the ambient plasma
in the chromosphere. The observed photon spectra therefore provide
diagnostics of electron acceleration processes in Solar flares. We
will present statistical results of spectral fitting using two models:
a broken power law plus a thermal component which is a direct fit of
the photon spectrum and a thick target model plus a thermal component
which is a fit of the photon spectra with assumptions on the electrons
emitting bremsstrahlung in the thick target approximation.
---------------------------------------------------------
Title: RHESSI and TRACE Observations of Emerging Flux Reconnection
in a Solar Jet on August 21, 2003
Authors: Glesener, L.; Krucker, S.; Lin, R. P.
2009AGUFMSH21C..02G Altcode:
Solar jets, or transient, collimated brightenings, were first observed
in soft x-rays by Yohkoh in the 1990’s. These observations led to
the establishment of the Shibata-Heyvaerts emerging flux model for
reconnection associated with jets. This model also predicts hard x-ray
(HXR) emission during jet production, and recent RHESSI and HINODE
observations appear to support this prediction. Unfortunately,
projection effects make it difficult to determine whether HXR
sources on the solar disk correspond to flare footpoints or the jet
itself. Observation of jets near or at the solar limb can lessen this
type of source confusion. Here, we present RHESSI observations of a
partially occulted HXR flare that coincides with an EUV jet observed
by TRACE on August 21, 2003. A double coronal HXR source is observed
shortly before the emission of the jet, indicating the location of
reconnection outflows. Thick- and thin-target models are used to
estimate the electron energies and densities in the HXR sources,
and allow for comparison with the emerging flux model.
---------------------------------------------------------
Title: High Resolution Imaging of Solar Flare Footpoints in White
Light and Hard X-rays
Authors: Krucker, S.; Hudson, H. S.; Lin, R. P.
2009AGUFMSH21C..01K Altcode:
We test the standard thick target beam scenario of solar flares
using high resolution G-band (430 nm) observations (~0.2 arcsec)
taken by HINODE/SOT and hard X-ray observations (2.3 arcsec) from
the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI)
of the December 6, 2006 GOES X9 flare. At both wavelengths, several
co-spatial footpoint sources are seen on the flare ribbons that show
similar relative intensities. This excellent correlation suggests that
the suprathermal electrons producing the hard X-ray emission are also
the source of energy for the white light emission, excluding energetic
protons as a possible source. If both emissions indeed come from the
same location, the higher resolution G-band observations suggest that
the individual hard X-ray sources are unresolved. Using the footpoint
area from the G-band images, the energy deposition rate by the hard
X-ray producing electron beam in cold thick target approximation become
enormous with values of 2x10 12 erg/s/cm2 for 25 keV (9x1012 erg/s/cm2
for 10 keV). This corresponds to a giant electron beam density within
the hard X-ray source of 0.3x1010 cm-3 above 25 keV (5x1010 cm-3 above
10 keV). These estimates pose serious questions for the thick-target
beam interpretation. We will discuss alternative scenarios, including
the idea of a purely non-thermal electron distribution as the source
of the hard X-ray emission.
---------------------------------------------------------
Title: A Statistical Study of Spectral Hardening in Solar Flares
and Related Solar Energetic Particle Events
Authors: Grayson, J.; Krucker, S.; Lin, R. P.
2009AGUFMSH33A1480G Altcode:
Using hard X-ray observations from the Reuven Ramaty High Energy
Solar Spectroscopic Imager (RHESSI), we investigate the reliability of
spectral hardening during solar flares as an indicator of related solar
energetic particle (SEP) events at Earth. All RHESSI data are analyzed,
from February 2002 through the end of Solar Cycle 23, thereby expanding
upon recent work on a smaller sample of flares. Previous investigations
have found very high success when associating soft-hard-harder (SHH)
spectral behavior with energetic proton events, and confirmation of this
link would suggest a correlation between electron acceleration in solar
flares and SEPs seen in interplanetary space. In agreement with these
past findings, we find that of 37 magnetically well-connected flares
(W30-W90), 12 of 18 flares with SHH behavior produced SEP events and
none of 19 flares without SHH behavior produced SEPs. This demonstrates
a statistically significant dependence of SHH and SEP observations, a
link that is unexplained in the standard scenario of SEP acceleration
at the shock front of coronal mass ejections, and encourages further
investigation of the mechanisms which could be responsible.
---------------------------------------------------------
Title: Solar flare electrons at the Sun and near the Earth. Insights
from interplanetary transport simulations
Authors: Agueda, N.; Krucker, S.; Lin, R. P.; Vainio, R. O.; Lario,
D. D.; Sanahuja, B.
2009AGUFMSH33A1478A Altcode:
We study 15 prompt 3He-rich electron events observed near 1 AU by
Wind/3DP above 50 keV. The events are associated with magnetically
well-connected flares (between W10 and W85) with rather short soft
X-ray duration (~10 min) and HXR sources seen by RHESSI on the visible
solar disk. We compare the Wind/3DP observations with the results of
interplanetary transport simulations, to infer the properties of the
propagation and constrain the injection of solar energetic electrons
in the interplanetary medium. We discuss the correlation between
the inferred total number of injected electrons with the number of
electrons required to produce the observed X-ray flux.
---------------------------------------------------------
Title: Energetic Electron Probes of Magnetic Cloud Topology
Authors: Kahler, S. W.; Krucker, S.; Szabo, A.
2009AGUFMSH41A1643K Altcode:
Magnetic clouds (MCs) are large interplanetary coronal mass ejections
(ICMEs) of enhanced fields with rotations indicative of magnetic flux
ropes originally connected to the Sun. The MC models require field
lines with larger pitch angles and longer lengths with increasing
distance from the MC axis. While the models can provide good fits to the
observations, the global magnetic field geometry has not been tested,
particularly for the field-line lengths. Impulsive solar energetic (E
> 30 keV) electron events occurring within MCs can be used to infer
the propagation distance and magnetic field-line length from the Sun at
the point in the MC where the electron onset occurs. We surveyed the
18 MCs listed on the Wind/MFI MC list in or near which solar electron
events were observed by the SSL/Berkeley 3DP instrument on the Wind
spacecraft. By plotting the electron onset times as functions of their
speeds for each event and taking their solar injections to coincide
with observed type III radio bursts in the Wind/WAVES detector, the
travel distances of the electrons along the field lines of the MCs
were computed. Those electron travel distances were compared with the
field-line lengths computed from two different MC model fits to test
the models.
---------------------------------------------------------
Title: The Focusing Optics Solar X-ray Imager (FOXSI)
Authors: Christe, S.; Glesener, L.; Krucker, S.; Ramsey, B.; Ishikawa,
S.; Takahashi, T.
2009AGUFMSH33B1496C Altcode:
The Focusing Optics x-ray Solar Imager is a sounding rocket payload
funded under the NASA Low Cost Access to Space program to test hard
x-ray focusing optics and position-sensitive solid state detectors for
solar observations. Today's leading solar hard x-ray instrument, the
Reuven Ramaty High Energy Solar Spectroscopic Imager provides excellent
spatial (2 arcseconds) and spectral (1~keV) resolution. Yet, due to
its use of indirect imaging, the derived images have a low dynamic
range (<30) and sensitivity. These limitations make it difficult
to study faint x-ray sources in the solar corona which are crucial for
understanding the solar flare acceleration process. Grazing-incidence
x-ray focusing optics combined with position-sensitive solid state
detectors can overcome both of these limitations enabling the
next breakthrough in understanding particle acceleration in solar
flares. The foxsi project is led by the Space Science Laboratory at
the University of California. The NASA Marshall Space Flight Center,
with experience from the HERO balloon project, is responsible for
the grazing-incidence optics, while the Astro H team (JAXA/ISAS) will
provide double-sided silicon strip detectors. FOXSI will be a pathfinder
for the next generation of solar hard x-ray spectroscopic imagers. Such
observatories will be able to image the non-thermal electrons within the
solar flare acceleration region, trace their paths through the corona,
and provide essential quantitative measurements such as energy spectra,
density, and energy content in accelerated electrons.
---------------------------------------------------------
Title: Submillimeter and X-ray observations of an X class flare
Authors: Giménez de Castro, C. G.; Trottet, G.; Silva-Valio, A.;
Krucker, S.; Costa, J. E. R.; Kaufmann, P.; Correia, E.; Levato, H.
2009A&A...507..433G Altcode: 2009arXiv0908.2339G
The GOES X1.5 class flare that occurred on August 30, 2002 at
1327:30 UT is one of the few events detected so far at submillimeter
wavelengths. We present a detailed analysis of this flare combining
radio observations from 1.5 to 212 GHz (an upper limit of the flux
is also provided at 405 GHz) and X-ray. Although the observations of
radio emission up to 212 GHz indicates that relativistic electrons
with energies of a few MeV were accelerated, no significant hard X-ray
emission was detected by RHESSI above ~250 keV. Images at 12-20 and
50-100 keV reveal a very compact, but resolved, source of about ~10
arcsec×10 arcsec. EUV TRACE images show a multi-kernel structure
suggesting a complex (multipolar) magnetic topology. During the
peak time the radio spectrum shows an extended flatness from ~7 to
35 GHz. Modeling the optically thin part of the radio spectrum as
gyrosynchrotron emission we obtained the electron spectrum (spectral
index δ, instantaneous number of emitting electrons). It is shown that
in order to keep the expected X-ray emission from the same emitting
electrons below the RHESSI background at 250 keV, a magnetic field
above 500 G is necessary. On the other hand, the electron spectrum
deduced from radio observations ≥50 GHz is harder than that
deduced from ~70-250 keV X-ray data, meaning that there must exist
a breaking energy around a few hundred keV. During the decay of the
impulsive phase, a hardening of the X-ray spectrum is observed which
is interpreted as a hardening of the electron distribution spectrum
produced by the diffusion due to Coulomb collisions of the trapped
electrons in a medium with an electron density of n<SUB>e</SUB> ~
3-5 × 10<SUP>10</SUP> cm<SUP>-3</SUP>.
---------------------------------------------------------
Title: Physics of ion acceleration in the solar flare on 2005
September 7 determines γ-ray and neutron production
Authors: Watanabe, K.; Lin, R. P.; Krucker, S.; Murphy, R. J.; Share,
G. H.; Harris, M. J.; Gros, M.; Muraki, Y.; Sako, T.; Matsubara, Y.;
Sakai, T.; Shibata, S.; Valdés-Galicia, J. F.; González, L. X.;
Hurtado, A.; Musalem, O.; Miranda, P.; Martinic, N.; Ticona, R.;
Velarde, A.; Kakimoto, F.; Tsunesada, Y.; Tokuno, H.; Ogio, S.
2009AdSpR..44..789W Altcode:
Relativistic neutrons were observed by the neutron monitors at
Mt. Chacaltaya and Mexico City and by the solar neutron telescopes
at Chacaltaya and Mt. Sierra Negra in association with an X17.0
flare on 2005 September 7. The neutron signal continued for more
than 20 min with high statistical significance. Intense emissions of
γ-rays were also registered by INTEGRAL, and during the decay phase
by RHESSI. We analyzed these data using the solar-flare magnetic-loop
transport and interaction model of Hua et al. [Hua, X.-M., Kozlovsky,
B., Lingenfelter, R.E. et al. Angular and energy-dependent neutron
emission from solar flare magnetic loops, Astrophys. J. Suppl. Ser. 140,
563-579, 2002], and found that the model could successfully fit the data
with intermediate values of loop magnetic convergence and pitch-angle
scattering parameters. These results indicate that solar neutrons were
produced at the same time as the γ-ray line emission and that ions
were continuously accelerated at the emission site.
---------------------------------------------------------
Title: The Focusing Optics X-ray Solar Imager (FOXSI)
Authors: Krucker, Sam; Christe, Steven; Glesener, Lindsay; McBride,
Steve; Turin, Paul; Glaser, David; Saint-Hilaire, Pascal; Delory,
Gregory; Lin, R. P.; Gubarev, Mikhail; Ramsey, Brian; Terada,
Yukikatsu; Ishikawa, Shin-Nosuke; Kokubun, Motohide; Saito, Shinya;
Takahashi, Tadayuki; Watanabe, Shin; Nakazawa, Kazuhiro; Tajima,
Hiroyasu; Masuda, Satoshi; Minoshima, Takashi; Shomojo, Masumi
2009SPIE.7437E..05K Altcode: 2009SPIE.7437E...4K
The Focusing Optics x-ray Solar Imager (FOXSI) is a sounding rocket
payload funded under the NASA Low Cost Access to Space program to
test hard x-ray focusing optics and position-sensitive solid state
detectors for solar observations. Today's leading solar hard x-ray
instrument, the Reuven Ramaty High Energy Solar Spectroscopic Imager
(RHESSI) provides excellent spatial (2 arcseconds) and spectral (1 keV)
resolution. Yet, due to its use of indirect imaging, the derived images
have a low dynamic range (<30) and sensitivity. These limitations
make it difficult to study faint x-ray sources in the solar corona
which are crucial for understanding the solar flare acceleration
process. Grazing-incidence x-ray focusing optics combined with
position-sensitive solid state detectors can overcome both of these
limitations enabling the next breakthrough in understanding particle
acceleration in solar flares. The FOXSI project is led by the Space
Science Laboratory at the University of California. The NASA Marshall
Space Flight Center, with experience from the HERO balloon project, is
responsible for the grazing-incidence optics, while the Astro H team
(JAXA/ISAS) will provide double-sided silicon strip detectors. FOXSI
will be a pathfinder for the next generation of solar hard x-ray
spectroscopic imagers. Such observatories will be able to image the
non-thermal electrons within the solar flare acceleration region, trace
their paths through the corona, and provide essential quantitative
measurements such as energy spectra, density, and energy content in
accelerated electrons.
---------------------------------------------------------
Title: Acceleration of Relativistic Protons During the 20 January
2005 Flare and CME
Authors: Masson, S.; Klein, K. -L.; Bütikofer, R.; Flückiger, E.;
Kurt, V.; Yushkov, B.; Krucker, S.
2009SoPh..257..305M Altcode: 2009arXiv0905.1816M
The origin of relativistic solar protons during large flare/CME
events has not been uniquely identified so far. We perform a detailed
comparative analysis of the time profiles of relativistic protons
detected by the worldwide network of neutron monitors at Earth with
electromagnetic signatures of particle acceleration in the solar corona
during the large particle event of 20 January 2005. The intensity -
time profile of the relativistic protons derived from the neutron
monitor data indicates two successive peaks. We show that microwave,
hard X-ray, and γ-ray emissions display several episodes of particle
acceleration within the impulsive flare phase. The first relativistic
protons detected at Earth are accelerated together with relativistic
electrons and with protons that produce pion-decay γ rays during the
second episode. The second peak in the relativistic proton profile at
Earth is accompanied by new signatures of particle acceleration in the
corona within ≈1R<SUB>⊙</SUB> above the photosphere, revealed by
hard X-ray and microwave emissions of low intensity and by the renewed
radio emission of electron beams and of a coronal shock wave. We
discuss the observations in terms of different scenarios of particle
acceleration in the corona.
---------------------------------------------------------
Title: X-ray Emission from the Base of a Current Sheet in the Wake
of a Coronal Mass Ejection
Authors: Saint-Hilaire, P.; Krucker, S.; Lin, R. P.
2009ApJ...699..245S Altcode: 2011arXiv1111.4248S
Following a coronal mass ejection (CME) which started on 2002
November 26, RHESSI observed for 12 hr an X-ray source above the
solar limb, at altitudes between 0.1 and 0.3 R<SUB>S</SUB> above the
photosphere. The Geostationary Operational Environmental Satellite
baseline was remarkably high throughout this event. The X-ray source's
temperature peaked around 10-11 MK, and its emission measure increased
throughout this time interval. Higher up, at 0.7 R<SUB>S</SUB> , hot
(initially >8 MK) plasma has been observed by Ultraviolet Coronograph
Spectrometer on Solar and Heliospheric Observatory for 2.3 days. This
hot plasma was interpreted as the signature of a current sheet (CS)
trailing the CME. The thermal energy content of the X-ray source is more
than an order of magnitude larger than in the CS. Hence, it could be
the source of the hot plasma in the CS, although CS heating by magnetic
reconnection within it cannot be discounted. To better characterize the
X-ray spectrum, we have used novel techniques (back-projection-based
and visibility-based) for long-integration (several hours) imaging
spectroscopy. There is no observed nonthermal hard X-ray bremsstrahlung
emission, leading to the conclusion that there is either very little
particle acceleration occurring in the vicinity of this postflare
X-ray source, or that either the photon spectral index would have
had to be uncharacteristically (in flare parlance) high (γ gsim 8)
and/or the low-energy cutoff very low (E<SUB>c</SUB> lsim 6 keV).
---------------------------------------------------------
Title: Energy Deposition and Hard X-Ray Source Motions in the 2002
July 23 γ-Ray Flare
Authors: Fivian, M. D.; Krucker, S.; Lin, R. P.
2009ApJ...698L...6F Altcode:
Reconnection models of solar flares predict a systematic motion of hard
X-ray (HXR) footpoints as magnetic energy is released and electrons are
accelerated. While the correlation of the HXR flux with the apparent
motion of the footpoints has previously been investigated, we derive
and investigate for the first time the correlation between cumulative
deposited energy at the footpoints and their separation. Providing
excellent statistics, data from the Reuven Ramaty High Energy Solar
Spectroscopic Imager of the 2002 July 23 flare are re-analyzed. The data
show an excellent correlation for most of the time intervals. However,
despite the good correlation, for some time ranges the derived amount
of released magnetic energy is far too small to account for the energy
in HXR-producing electrons.
---------------------------------------------------------
Title: Hard X-ray Emission From Partially Occulted Solar Flares
Authors: Glesener, Lindsay; Krucker, S.; Lin, R. P.
2009SPD....40.1914G Altcode:
The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) is
optimized for fine energy and spatial resolution observations of hard
x-ray (HXR) emission from solar flares. In many RHESSI flares, most of
this emission originates from the bright flare loop footpoints. Fainter
sources located higher in the corona are often not visible due to
RHESSI's limited dynamic range. However, flares which are partially
occulted (i.e. have footpoints obscured by the solar disk) allow
for detailed observations of faint coronal HXR sources. <P />We have
determined occultation heights for 55 RHESSI flares from 2002-2004
and compared the occultation heights with nonthermal flux. Compared
with statistical results of on-disk flares, it is estimated that the
coronal emission is 10-100 times fainter than the expected footpoint
emission, confirming that these coronal sources would not have been
visible in the presence of footpoints. <P />Occulted flares can be
used to investigate the role of flare-accelerated electrons in heating
ambient coronal plasma. With this aim, we compare RHESSI images of
nonthermal HXR from energetic electrons with TRACE images made using
the 195-angstrom filter, which has a sensitivity maximum at 1.5 MK. The
locations and time profiles of the nonthermal and thermal sources are
compared to investigate whether the heating seen by TRACE is due to
the same electrons that emit the HXR seen by RHESSI.
---------------------------------------------------------
Title: The X-ray Detectability of Electron Beams Escaping from the Sun
Authors: Saint-Hilaire, Pascal; Krucker, Säm; Christe, Steven; Lin,
Robert P.
2009ApJ...696..941S Altcode: 2011arXiv1111.4250S
We study the detectability and characterization of electron beams
as they leave their acceleration site in the low corona toward
interplanetary space through their nonthermal X-ray bremsstrahlung
emission. We demonstrate that the largest interplanetary electron
beams (gsim10<SUP>35</SUP> electrons above 10 keV) can be detected
in X-rays with current and future instrumentation, such as RHESSI or
the X-Ray Telescope (XRT) onboard Hinode. We make a list of optimal
observing conditions and beam characteristics. Amongst others, good
imaging (as opposed to mere localization or detection in spatially
integrated data) is required for proper characterization, putting
the requirement on the number of escaping electrons (above 10 keV)
to gsim3 × 10<SUP>36</SUP> for RHESSI, gsim3 × 10<SUP>35</SUP> for
Hinode/XRT, and gsim10<SUP>33</SUP> electrons for the FOXSI sounding
rocket scheduled to fly in 2011. Moreover, we have found that simple
modeling hints at the possibility that coronal soft X-ray jets could
be the result of local heating by propagating electron beams.
---------------------------------------------------------
Title: Radio Emission from RHESSI Microflares observed by the Nobeyama
Radio Heliograph
Authors: Christe, Steven; White, S.; Krucker, S.
2009SPD....40.1923C Altcode:
We present a statistical survey of RHESSI microflares observed in
hard X-rays (HXR) with simultaneous observations by the Nobeyama
Radio Heliograph at 17 GHz and 34 GHz. These microflares were found
using a new flare-finding algorithm designed to search the 6-12 keV
count-rate when RHESSI's full sensitivity was available, in order to
find the smallest events. Between March 2002 and March 2007, a total
of 25000 events are identified, of which 8500 are also observed by
Nobeyama at 17 and 34 GHz. We compare HXR and radio fluxes and analyze
the relationship statistically. When the events are detected at both
wavelengths, the relationship between the lightcurves falls into
several classes. Radio and HXR images of a small selection of events
are used to investigate the relative locations of the two emissions,
and we discuss the physical conditions that affect the relationship
between the HXR and radio emission.
---------------------------------------------------------
Title: The Focusing Optics X-ray Solar Imager (FOXSI)
Authors: Christe, Steven; Glesener, L.; Krucker, S.; Ramsey, B.;
Takahashi, T.; Lin, R.
2009SPD....40.3303C Altcode:
The Focusing Optics X-ray Solar Imager (FOXSI) is a NASA Low Cost
Access to Space sounding rocket payload scheduled for launch late
2010. FOXSI will provide imaging spectroscopy with high sensitivity (
50 times RHESSI) and high dynamic range ( 100) in hard X-rays (HXR)
up to 15 keV. For the first time, it will be possible to search
for nonthermal emission of thermal network flares occurring in the
quiet corona in order to determine whether they are similar to active
region flares. Additionally, FOXSI will extend the active-region flare
distribution to events two orders of magnitude smaller than previously
observed and determine their contribution to coronal heating. FOXSI is
able to achieve this unprecendeted advance in solar HXR observations
through the combination of nested HXR optics developped by the Marshall
Space Flight Center and novel silicon strip detectors provided by ISAS
Japan. The FOXSI mission will provide HXR spectroscopic imaging with an
angular resolution of 12" (FWHM) and 1 keV energy resolution. FOXSI will
be a pathfinder for the future generation of solar HXR spectroscopic
imagers.
---------------------------------------------------------
Title: High Resolution Imaging of Solar Flare Footpoints in White
Light and Hard X-rays
Authors: Krucker, Sam; Hudson, H. S.; Lin, R. P.
2009SPD....40.1919K Altcode:
High resolution G-band (430 nm) observations ( 0.2 arcsec) taken by
HINODE/SOT and hard X-ray observations (2.3 arcsec) from the Reuven
Ramaty High Energy Solar Spectroscopic Imager (RHESSI) are used to study
footpoint sources of the December 6, 2006 GOES X9 class solar flare. At
both wavelengths, several co-spatial footpoint sources are seen on the
flare ribbons that show similar relative intensities. This excellent
correlation suggests that the energetic electrons producing the hard
X-ray emission are also the source of energy for the white light
emission, excluding energetic protons as a possible source. If both
emissions indeed come from the same location, the higher resolution
G-band observation suggest that the individual hard X-ray sources
are unresolved. Using the area from the G-band images, the cold
thick target model gives the enormous energy deposition rate of 2e12
erg/s/cm2 for 25 keV (9e12 erg/s/cm2 for 10 keV) and huge densities of
beam electrons within the hard X-ray source of 0.3e10 cm-3 above 25 keV
(5d10 cm-3 above 10 keV). These estimates pose serious questions for
the thick-target interpretation. We will discuss alternative scenarios,
including the idea of a purely non-thermal electron distribution as
the source of the hard X-ray emission.
---------------------------------------------------------
Title: White Light Emission from RHESSI flares
Authors: Christe, Steven; Watanabe, K.; Krucker, S.
2009SPD....40.1922C Altcode:
In this study we investigate the relationship between RHESSI flares
and white light emission as observed by Hinode/SOT at 3968.6 angstroms
(Ca II H line), 4306.4 angstroms (G band), and 4505.1 angstroms(blue
continuum) images. The emission mechanism for white light emission
in flares is not yet understood though it is believed that emission
at these wavelengths originates from the chromosphere, photosphere,
and deep photosphere respectively. Using a combination of the official
RHESSI flare list and a microflare-finding algorithm, we investigate all
RHESSI flares with simultaneous observations by RHESSI and Hinode/SOT
(a total of 200 events). Single event studies suggest that microflares
are frequently observed in Ca II H line. Photospheric emission is
searched for using the location of the Ca II H line chromospheric
emission. We discuss the physical conditions necessary that may affect
the relationship between HXR and white light emission.
---------------------------------------------------------
Title: Statistical Study of Solar Flare Hard X-ray Spectra
Authors: Alaoui, Meriem; Krucker, S.; Saint-Hilaire, P.; Lin, R. P.
2009SPD....40.1901A Altcode:
We present a statistical study of solar flare hard X-ray spectra above
40 keV observed by the Reuven Ramaty High Energy Solar Spectroscopic
Imager (RHESSI). At these energies, the hard X-ray emission is mostly
produced by bremsstrahlung of suptrathermal electrons as the interact
with the ambient plasma in the chromosphere. The observed photon spectra
therefore provide diagnostics of electron acceleration processes
in solar flare. We will present statistical results of broken power
law fits of almost 100 flares with emission seen above 100 keV. The
derived results will be compared to statistical results of electron
spectra observed in interplanetary space.
---------------------------------------------------------
Title: A Statistical Study of Spectral Hardening in Solar Flares
and Related Solar Energetic Proton Events
Authors: Grayson, James; Krucker, S.; Lin, R. P.
2009SPD....40.1902G Altcode:
We investigate the reliability of hard x-ray spectral hardening during
solar flares as an indicator of related energetic proton events at
Earth, using high-resolution observations from the Reuven Ramaty High
Energy Solar Spectroscopic Imager (RHESSI). Previous work by Kiplinger
(1995) obtained as high as 96% success when associating hardening
behavior with energetic proton events, suggesting a correlation
between electron acceleration in solar flares and solar energetic
protons seen in interplanetary space. Here we further this study,
and more recent work with smaller sets of RHESSI observations, by
investigating all observed large flares during RHESSI's lifetime. Our
data set includes over 400 M- and X-class flares with at least partial
coverage by RHESSI, approximately half of which allow us to determine
spectral behavior accurately.
---------------------------------------------------------
Title: Multiwavelength Observation of Electron Acceleration in the
2006 December 13 Flare
Authors: Minoshima, T.; Imada, S.; Morimoto, T.; Kawate, T.;
Koshiishi, H.; Kubo, M.; Inoue, S.; Isobe, H.; Masuda, S.; Krucker,
S.; Yokoyama, T.
2009ApJ...697..843M Altcode: 2009arXiv0903.1144M
We present a multiwavelength observation of a solar flare occurring on
2006 December 13 with Hinode, RHESSI, and the Nobeyama Radio Observatory
to study the electron acceleration site and mechanism. The Solar Optical
Telescope (SOT) on board Hinode observed elongated flare ribbons, and
RHESSI observed double-footpoint hard X-ray (HXR) sources appearing
in part of the ribbons. A photospheric vector magnetogram obtained
from SOT reveals that the HXR sources are located at the region where
horizontal magnetic fields change direction. The region is interpreted
as the footpoint of magnetic separatrix. Microwave images taken
with the Nobeyama Radioheliograph show a loop structure connecting
the HXR sources. The brighter parts of the microwave intensity are
located between the top and footpoints of the loop. We consider these
observations as evidence of electron acceleration near the magnetic
separatrix and injection parallel to the field line.
---------------------------------------------------------
Title: Direct Observations of the Coronal Acceleration Region of a
Solar Flare
Authors: Krucker, Sam; Hudson, H. S.; White, S. M.; Lin, R. P.
2009SPD....40.3601K Altcode:
Solar flares essentially convert the intrinsic energy of coronal
magnetic field into the kinetic energy of accelerated particles. Hard
X-ray emission from flare-accelerated electrons produced by the
bremsstrahlung mechanism provides the most direct diagnostics of
electron acceleration. The most discussed coronal hard X-ray source has
been the above-the-loop-top source observed in the Masuda flare. The
poor spectral resolution of these observations, however, made an
interpretation ambiguous, and the exact location of the acceleration
remained elusive. We present high spatial and spectral resolution
RHESSI hard X-ray observations of an above-the-loop-top source with
simultaneous microwave observations from NoRH. These observations
provide a unambiguous interpretation of above-the-loop-top sources:
The above-the-loop-top source itself is the acceleration region, where
all electrons within an extended volume (1e27 cm3) are accelerated. The
distribution of the accelerated electrons is definitely non-thermal,
with a power law distribution extending from 10 keV up to the
relativistic range (few MeV). The plasma beta in the acceleration region
changes from the pre-flare value of 0.01 to 1, indicating that roughly
half of the magnetic energy has been transformed into kinetic energy.
---------------------------------------------------------
Title: The RHESSI Microflare Height Distribution
Authors: Christe, S.; Krucker, Sam; Hudson, H.; Lin, R.
2009SPD....40.1903C Altcode:
We present the first in-depth statistical survey of flare source
heights observed by RHESSI between March 2002 and March 2007, a total
of 25,705 events. These flares were found using a new flare-finding
algorithm designed to search the 6-12 keV count-rate when RHESSI's
full sensitivity was available in order to find the smallest
events. Thermal (4-10 keV) and nonthermal (15-25 keV) images were
made for all microflares and source centroid locations were found for
each event. In order to extract the height information from source
positions, a Monte-Carlo model was developed with an assumed source
height distribution where height is measured from the photosphere. We
find that the best source height model is given by an exponential
distribution with a scale height of 2.1 (0.3) Mm and a minimum height
of 3.1 (0.3) Mm. Comparing with previously published loop length
measurements, we find that the average loop tilt is 44 degrees as
measured from the vertical.
---------------------------------------------------------
Title: Imaging Spectroscopy on Preflare Coronal Nonthermal Sources
Associated with the 2002 July 23 Flare
Authors: Asai, Ayumi; Nakajima, Hiroshi; Shimojo, Masumi; Yokoyama,
Takaaki; Masuda, Satoshi; Krucker, Säm
2009ApJ...695.1623A Altcode: 2009arXiv0901.3591A
We present a detailed examination on the coronal nonthermal emissions
during the preflare phase of the X4.8 flare that occurred on 2002 July
23. The microwave (17 GHz and 34 GHz) data obtained with Nobeyama
Radioheliograph, at Nobeyama Solar Radio Observatory and the hard
X-ray (HXR) data taken with RHESSI obviously showed nonthermal sources
that are located above the flare loops during the preflare phase. We
performed imaging spectroscopic analyses on the nonthermal emission
sources both in microwaves and in HXRs, and confirmed that electrons
are accelerated from several tens of keV to more than 1 MeV even in this
phase. If we assume the thin-target model for the HXR emission source,
the derived electron spectral indices (~4.7) is the same value as that
from microwaves (~4.7) within the observational uncertainties, which
implies that the distribution of the accelerated electrons follows a
single power law. The number density of the microwave-emitting electrons
is, however, larger than that of the HXR-emitting electrons, unless
we assume low-ambient plasma density of about 1.0 × 10<SUP>9</SUP>
cm<SUP>-3</SUP> for the HXR-emitting region. If we adopt the
thick-target model for the HXR emission source, on the other hand,
the electron spectral index (~6.7) is much different, while the gap
of the number density of the accelerated electrons is somewhat reduced.
---------------------------------------------------------
Title: Diagnostics of interplanetary and flaring plasmas in impulsive
solar energetic particle events
Authors: Kartavykh, Y. Y.; Dröge, W.; Klecker, B.; Mason, G. M.;
Möbius, E.; Popecki, M.; Krucker, S.
2009BRASP..73..291K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Sub-terahertz, Microwaves and High Energy Emissions During
the 6 December 2006 Flare, at 18:40 UT
Authors: Kaufmann, Pierre; Trottet, Gérard; Giménez de Castro,
C. Guillermo; Raulin, Jean-Pierre; Krucker, Säm; Shih, Albert Y.;
Levato, Hugo
2009SoPh..255..131K Altcode: 2008arXiv0811.3488K
The presence of a solar burst spectral component with flux density
increasing with frequency in the sub-terahertz range, spectrally
separated from the well-known microwave spectral component, bring
new possibilities to explore the flaring physical processes, both
observational and theoretical. The solar event of 6 December 2006,
starting at about 18:30 UT, exhibited a particularly well-defined
double spectral structure, with the sub-THz spectral component
detected at 212 and 405 GHz by the Solar Submilimeter Telescope
(SST) and microwaves (1 - 18 GHz) observed by the Owens Valley Solar
Array (OVSA). Emissions obtained by instruments onboard satellites
are discussed with emphasis to ultra-violet (UV) obtained by the
Transition Region And Coronal Explorer (TRACE), soft X-rays from
the Geostationary Operational Environmental Satellites (GOES) and X-
and γ-rays from the Ramaty High Energy Solar Spectroscopic Imager
(RHESSI). The sub-THz impulsive component had its closer temporal
counterparts only in the higher energy X- and γ-rays ranges. The
spatial positions of the centers of emission at 212 GHz for the first
flux enhancement were clearly displaced by more than one arc-minute from
positions at the following phases. The observed sub-THz fluxes and burst
source plasma parameters were difficult to be reconciled with a purely
thermal emission component. We discuss possible mechanisms to explain
the double spectral components at microwaves and in the THz ranges.
---------------------------------------------------------
Title: A Statistical Study of Hard X-Ray Footpoint Motions in Large
Solar Flares
Authors: Yang, Ya-Hui; Cheng, C. Z.; Krucker, Säm; Lin, R. P.; Ip,
W. H.
2009ApJ...693..132Y Altcode:
A statistical analysis of the temporal evolution of hard X-ray
(HXR) footpoint motions in 27 M- and X-class solar flares observed
by the Reuven Ramaty High Energy Solar Spectroscopic Imager is
presented. Extreme UV images from TRACE and SOHO/EIT, Hα images, and
magnetograms from SOHO/MDI are used to put the HXR footpoint motions
in context of flare ribbons and the magnetic neutral line. Footpoint
motions are often found to be complex making a statistical analysis
difficult. In an attempt to simplify the analysis, each event
was searched for motions predominantly parallel and predominantly
perpendicular to the neutral line or flare ribbons. Four kinds of
complex motions are described and their relationships to the possible
magnetic reconnection processes are discussed. In the soft X-ray
(SXR) rise phase, motions along the neutral line or flare ribbons
are most common (20 out of 27) and only two events show perpendicular
motions (for the remaining five events a simple classification was not
possible). However, at later times around the SXR peak, perpendicular
motion is more frequently observed (~40%) than motions along the neutral
line or ribbons ~27%). The preference of HXR kernels appearing at the
footpoints of highly-sheared magnetic loops at the start of the SXR
rise phase is consistent with the magnetic reconnection theory that
the reconnection occurring at sheared magnetic arcade field lines
produces most HXR energy release in the impulsive phase of large flares.
---------------------------------------------------------
Title: The Focusing Optics X-ray Solar Imager
Authors: Glesener, Lindsay; Krucker, S.; Christe, S.; Turin, P.;
McBride, S.
2009AAS...21347508G Altcode: 2009BAAS...41..436G
The Focusing Optics X-ray Solar Imager (FOXSI) is a NASA Low Cost
Access to Space sounding rocket payload scheduled to fly in late 2010
to observe hard X-ray emission (HXR) from the quiet Sun. Particle
acceleration in small "nanoflares" in the quiet Sun is thought to play
an important role in the heating of the corona to millions of degrees
Kelvin. FOXSI HXR observations of these flares will provide first
estimates of the non-thermal energy content in small flares from the
quiet Sun. <P />Imaging nanoflares requires high energy sensitivity
and a large dynamic range. To date, the most sensitive HXR images are
made using a rotating modulating collimator aboard the Reuven Ramaty
High Energy Spectroscopic Imager satellite (RHESSI). However, the
rotating modulation technique is intrinsically limited in sensitivity
and dynamic range. <P />The focusing optics of FOXSI will achieve a
sensitivity 100 times better than that of RHESSI at energies around
10 keV. FOXSI uses nested-shell, grazing-angle optics and silicon
strip detectors to achieve an angular resolution of 12" (FWHM) and 1
keV energy resolution. FOXSI will observe the quiet Sun in the 4 to
15 keV range for 5 minutes. The focusing optics technique developed
by FOXSI will prove useful to future solar HXR observing missions,
especially those interested in imaging faint HXR emission from particle
acceleration regions in the corona.
---------------------------------------------------------
Title: The Solar Flare: A Strongly Turbulent Particle Accelerator
Authors: Vlahos, L.; Krucker, S.; Cargill, P.
2009LNP...778..157V Altcode:
The topics of explosive magnetic energy release on a large scale
(a solar flare) and particle acceleration during such an event are
rarely discussed together in the same article. Many discussions of
magnetohydrodynamic (MHD) mod- eling of solar flares and/or CMEs
have appeared (see [143] and references therein) and usually address
large-scale destabilization of the coronal mag- netic field. Particle
acceleration in solar flares has also been discussed exten- sively
[74, 164, 116, 166, 87, 168, 95, 122, 35] with the main emphasis being
on the actual mechanisms for acceleration (e.g., shocks, turbulence,
DC electric fields) rather than the global magnetic context in which
the acceleration takes place.
---------------------------------------------------------
Title: Spectra of Solar Impulsive Electron Events Observed Near Earth
Authors: Krucker, Säm; Oakley, P. H.; Lin, R. P.
2009ApJ...691..806K Altcode:
A statistical survey of the spectral shapes of 62 solar impulsive
electron events detected in the ~ 1 to 300 keV range near Earth
by the three-dimensional Plasma and Energetic Particles experiment
on the WIND spacecraft is presented. The electron peak flux spectra
generally show a broken power-law dependence with a steepening above ~
60 keV. The break in the spectrum is pronounced with averaged power-law
indices below and above the break of δ<SUB>low</SUB> = 1.9 ± 0.3
and δ<SUB>high</SUB> = 3.6 ± 0.7, respectively, and an averaged
ratio δ<SUB>low</SUB>/δ<SUB>high</SUB> of 0.54 with a standard
deviation of 0.09. Two correlations are found: (1) δ<SUB>low</SUB> and
δ<SUB>high</SUB> are linearly correlated (correlation coefficient of
0.61), (2) The peak fluxes around the break energy and δ<SUB>low</SUB>
are anticorrelated (coefficient 0.74), with larger events having flatter
spectra below the break. Whether the observed spectral breaks are direct
signatures of the solar acceleration processes or whether they are due
to transport effects from the Sun to Earth is currently not understood.
---------------------------------------------------------
Title: Particle Acceleration and Transport on the Sun
Authors: Bastian, T. S.; Emslie, G.; Fleishman, G.; Gary, D. E.;
Holman, G.; Hudson, H.; Hurford, G.; Krucker, S.; Lee, J.; Miller,
J.; White, S.
2009astro2010S..13B Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Observational Study of Particle Acceleration in the 2006
December 13 Flare
Authors: Minoshima, T.; Morimoto, T.; Kawate, T.; Imada, S.;
Koshiishi, H.; Masuda, S.; Kubo, M.; Inoue, S.; Isobe, H.; Krucker,
S.; Yokoyama, T.
2008AGUFMSH41B1619M Altcode:
We study the particle acceleration in a flare on 2006 December 13,
by using the Hinode, RHESSI, Nobeyama Radio Polarimeters (NoRP) and
Nobeyama Radioheliograph (NoRH) observations. For technical reasons,
both RHESSI and NoRH have a problem in imaging in this flare. Since we
have succeeded in solving the problem, it is now possible to discuss
the particle acceleration mechanism from an image analysis. This flare
shows very long-lasting (1 hour) non-thermal emissions, consisting of
many spikes. We focus on the second major spike at 02:29 UT, because
the RHESSI image is available only in this period. The RHESSI 35-100 keV
HXR image shows double sources located at the footpoints of the western
soft X-ray (SXR) loop seen by the Hinode/XRT. The non-linear force-free
(NLFF) modeling based on a magnetogram data by Inoue et al. shows the
NLFF to potential magnetic transition of the loop, which would induce
the electric field and then accelerate particles. Overlaying the HXR
image on the photospheric three-dimensional magnetic field map taken
by the Hinode Spectro-Polarimeter, we find that the HXR sources are
located at the region where the horizontal magnetic fields invert. The
NoRH 34 GHz microwave images show the loop structure connecting the
HXR sources. The microwave peaks do not located at the top of the
loop but between the loop top and the footpoints. The NoRP microwave
spectrum shows the soft-hard-soft pattern in the period, same as the
HXR spectrum (Ning 2008). From these observational results we suggest
that the electrons were accelerated parallel to the magnetic field
line near the magnetic separatrix.
---------------------------------------------------------
Title: The Microflare Height Distribution
Authors: Christe, S. D.; Krucker, S.
2008AGUFMSH13A1510C Altcode:
We present an in-depth statistical survey of flare heights of all
X-ray microflares as observed by RHESSI between March 2002 and
March 2007, a total of >25,000 events, an order of magnitude
larger then previous studies. The microflares were found using a new
flare-finding algorithm designed to search the 6-- 12~keV count-rate
when RHESSI's full sensitivity was available in order to find the
smallest events. The flare centroid position are found at the peak time
and as a function of energy (3-6, 6-12, 12-25 keV). Flares are found
to occur only in active regions, not in the "quiet" Sun. Flare heights
are found using two independent methods including that of Matsushita
(1992). The distribution of flares at the limb are fitted with a Monte
Carlo simulations and the flare height deduced. We find that the 6-12
keV flare heights are consistent with a flare height of ~3 arcseconds
above the photosphere with small separation between the thermal and
nonthermal sources. The Matsushita method confirm these values.
---------------------------------------------------------
Title: A RHESSI search for chromospheric evaporation in super-hot
flares
Authors: Caspi, A.; Krucker, S.; Lin, R. P.; McTiernan, J. M.
2008AGUFMSH51C..04C Altcode:
Chromospheric evaporation (CE) - thought to occur when
downward-accelerated coronal electrons impact the denser chromosphere,
heating the ambient material which then rises to fill the flaring loop -
has often been suggested as the primary source of hot thermal looptop
plasma. Evidence for CE is given by crystal spectrometer observations
of blueshifted spectral lines from footpoints. RHESSI observations show
that peak flare temperatures generally occur soon after the hard X-ray
(HXR; >20~keV) peak, when the emission measure is still only ~20% of
its later peak value. Imaging places the thermal plasma at the looptop
even for large (GOES X-class) flares that exceed super-hot (Te >
30~MK) temperatures. In such flares, non-thermal energy deposition can
exceed ~1029~erg/s, which for typical electron energies, chromospheric
densities, and footpoint areas should yield CE temperatures of ~10-20~MK
with emission measures of ~1050~cm-3. Such bright thermal emission
has never been observed from footpoints, suggesting that while CE
may contribute the bulk loop material, heating occurs primarily in
the corona. We present an analysis of selected RHESSI M- and X-class
flares, employing imaging spectroscopy to determine the temperature and
emission measure (or limits thereof) of spatially-separated footpoint
and looptop sources. We compare the time evolution of the thermal
footpoint signatures to HXR lightcurves and to the looptop thermal
component. We calculate the energy contained in non-thermal electrons,
the thermal energies at the looptop and footpoints, and discuss the
implications for heating by non-thermal electrons and the contribution
of CE to the thermal flare plasma.
---------------------------------------------------------
Title: Beam speeds and Langmuir wave growth in interplanetary type
III bursts
Authors: Bale, S. D.; Krucker, S.
2008AGUFMSH24A..08B Altcode:
We use measurements from the Wind spacecraft to study Langmuir wave
growth in the source region of four separate in situ interplanetary
type III bursts. Velocity distributions of energetic electron are used
to compute the advection beam speed and growth rate (in the cold beam
approximation) and these beam speeds and growth rates are compared
with Langmuir wave activity and detailed features of the Langmuir wave
spectra and waveforms. Knowledge of the electron beam speed lets us
compute the resonant wavenumber and the expected spacecraft frequency
of both daughter Langmuir waves and ion acoustic waves required for the
parametric decay process. We find no strong evidence for decay products
nor carrier splitting that is consistent with parametric decay. On
the other hand, many of the Langmuir waveforms show 'polarization'
signatures that may be consistent with linear mode conversion or
trapped eigenmode interpretations of type III wave processes.
---------------------------------------------------------
Title: Hard X-ray Emission From Partially Occulted Solar Flares
Authors: Glesener, L.; Krucker, S.; Lin, R. P.
2008AGUFMSH13B1540G Altcode:
The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI)
is optimized for fine energy and spatial resolution of hard X-ray
(HXR) emission from solar flares. Most RHESSI flares exhibit strong
HXR emission from bright flare loop footpoints, drowning out faint
emission from higher in the corona. Partial occultation of flares by
the solar disk obscures these bright footpoints, allowing for detailed
observations of faint coronal HXR sources. It is therefore necessary to
identify and characterize flares with high occultation heights. Here
we present a technique for determining flare occultation heights
by extrapolating the paths of active flare regions across the solar
disk. A statistical study of 55 flares compares occultation heights
to nonthermal X-ray emission. Compared with statistical results of
on-disk flares, it is estimated that this coronal emission is at least
20 times fainter than the expected footpoint emission, confirming
that partial occultation is crucial for the study of faint coronal
sources. HXR images made by RHESSI are also compared with images at
longer wavelengths from the TRACE, EIT, and STEREO instruments.
---------------------------------------------------------
Title: Evidence for the Magnetic Trapping of Solar-Flare Ions
from 1-8-MeV Solar Neutrons Detected with the MESSENGER Neutron
Spectrometer
Authors: Feldman, W. C.; Lawrence, D. J.; Goldsten, J. O.; Gold, R. E.;
Baker, D. N.; Haggerty, D. K.; Krucker, S.; Lin, R. P.; Murphy, R. J.;
Nittler, L. R.; Slavin, J. A.; Solomon, S. C.; Starr, R. D.; Vilas,
F.; Vourlidas, A.
2008AGUFM.U12A..02F Altcode:
Neutrons produced on the Sun during the M2 flare on 31 December 2007
were observed throughout an approximately 10-hour period at 0.48 AU
by the MESSENGER Neutron Spectrometer. This flare contained multiple
acceleration episodes as seen in Type III radio bursts and fine
structure in the energetic-particle environment at MESSENGER. After
these bursts ended, both the energetic-particle and neutron fluxes
decayed smoothly to background with an e-folding decay time of 2.84
hours. The fact that this time is considerably longer than the mean
life of a neutron (886 s) indicates that neutrons at the Sun must have
been continuously produced. A likely explanation is that a considerable
fraction of the energetic ions accelerated during the flare remained
trapped on closed magnetic arcades high in the corona and were slowly
pitch-angle scattered by coronal turbulence into their chromospheric
loss cones. Subsequent interactions with chromospheric ions generated
neutrons that scatter to form a population of upward-going albedo
neutrons.
---------------------------------------------------------
Title: Magnetic flux cancellation associated with a recurring solar
jet observed with Hinode, RHESSI, and STEREO/EUVI
Authors: Chifor, C.; Isobe, H.; Mason, H. E.; Hannah, I. G.; Young,
P. R.; Del Zanna, G.; Krucker, S.; Ichimoto, K.; Katsukawa, Y.;
Yokoyama, T.
2008A&A...491..279C Altcode:
Aims: We study the physical properties of a recurring solar active
region jet observed in X-rays and extreme-ultraviolet (EUV). <BR
/>Methods: Multi-wavelength data from all three instruments on
board Hinode were analysed. X-ray imaging and spectroscopy of the
microflaring emission associated with the jets was performed with the
Reuven Ramaty High Energy Spectroscopic Imager (RHESSI). Associated
EUV jets were observed with the Sun Earth Connection Coronal and
Heliospheric Investigation (SECCHI)/Extreme Ultraviolet Imager
(EUVI) on board STEREO. <BR />Results: We found a correlation between
recurring magnetic flux cancellation close to a pore, the X-ray jet
emission, and associated Ca II H ribbon brightenings. We estimated the
lower limit for the decrease in magnetic energy associated with the
X-ray jet emission at 3 × 10<SUP>29</SUP> erg. The recurring plasma
ejection was observed simultaneously at EUV and X-ray temperatures,
associated with type III radio bursts and microflaring activity at the
jet footpoint. <BR />Conclusions: The recurring jet (EUV and X-ray)
emissions can be attributed to chromospheric evaporation flows due to
recurring coronal magnetic reconnection. In this process, the estimated
minimum loss in the magnetic energy is sufficient to account for the
total energy required to launch the jet. <P />Movie of Fig. 3 is only
available in electronic form via http://www.aanda.org
---------------------------------------------------------
Title: Hard X-ray emission from the solar corona
Authors: Krucker, S.; Battaglia, M.; Cargill, P. J.; Fletcher, L.;
Hudson, H. S.; MacKinnon, A. L.; Masuda, S.; Sui, L.; Tomczak, M.;
Veronig, A. L.; Vlahos, L.; White, S. M.
2008A&ARv..16..155K Altcode: 2008A&ARv.tmp....8K
This review surveys hard X-ray emissions of non-thermal electrons in the
solar corona. These electrons originate in flares and flare-related
processes. Hard X-ray emission is the most direct diagnostic of
electron presence in the corona, and such observations provide
quantitative determinations of the total energy in the non-thermal
electrons. The most intense flare emissions are generally observed
from the chromosphere at footpoints of magnetic loops. Over the years,
however, many observations of hard X-ray and even γ-ray emission
directly from the corona have also been reported. These coronal sources
are of particular interest as they occur closest to where the electron
acceleration is thought to occur. Prior to the actual direct imaging
observations, disk occultation was usually required to study coronal
sources, resulting in limited physical information. Now RHESSI has
given us a systematic view of coronal sources that combines high
spatial and spectral resolution with broad energy coverage and high
sensitivity. Despite the low density and hence low bremsstrahlung
efficiency of the corona, we now detect coronal hard X-ray emissions
from sources in all phases of solar flares. Because the physical
conditions in such sources may differ substantially from those of
the usual “footpoint” emission regions, we take the opportunity
to revisit the physics of hard X-radiation and relevant theories of
particle acceleration.
---------------------------------------------------------
Title: STEREO and RHESSI Observations of Electron Acceleration in
a Partially Disk-Occulted Solar Flare
Authors: Krucker, S.; Wuelser, J. -P.; Vourlidas, A.; Davila, J.;
Thompson, W. T.; White, S.; Lin, R. P.
2008ESPM...12.2.84K Altcode:
RHESSI hard X-ray observations of partially-disk occulted solar flares
provide crucial information on faint coronal hard X-ray sources in the
absence of generally much brighter emissions from footpoints of flare
loops. Coronal hard X-ray sources can differ fundamentally from the
classical footpoint sources of the flare impulsive phase and provide
unique information about the supra-thermal electrons closest to the
site in the corona where their acceleration is believed to occur. The
different view-angles provided by the STEREO spacecraft allow us to put
the partially occulted hard X-ray sources observed by RHESSI in context
with the EUV flare ribbons and the EUV emission from CME observed by
STEREO/EUVI. <P />In this presentation we report on the GOES C8 flare
observed on December 31, 2007 peaking around 01:11UT. From Earth-view
(RHESSI), the flare occurs about 12 degrees behind the eastern limb
giving an occultation height of 16 Mm. From STEREO B, the flare
ribbons are seen on the disk (about 10 degrees from the limb), while
the flare is highly occulted (130 Mm) for STEREO A observations so
that emissions related to the associated CME are seen. Despite the
occultation, RHESSI observes strong non-thermal emissions up to 100
keV that entirely originate from the corona. Initially, the coronal
hard X-ray emission is seen from above the EUV flare ribbons similar
to what is reported in the Masuda flare. Later on, emissions from a
radially extended (approximately 20 Mm) source is seen. The radial
extension is in the same direction as the current sheet of the outward
moving CME suggesting that the HXR emission might be a direct signature
of electrons accelerated in the reconnection process.
---------------------------------------------------------
Title: Microflares with RHESSI and Hinode/XRT
Authors: Hannah, I. G.; Krucker, S.; Christe, S.; Hudson, H. S.; Lin,
R. P.
2008ASPC..397..169H Altcode:
In this article we discuss the opportunities for analyzing microflares
with RHESSI and Hinode/XRT. We present analysis of one microflare,
using the RHESSI to obtain the thermal and non-thermal spectral
parameters and compare the RHESSI images of the thermal (4-8 keV) and
non-thermal (12-50 keV) emission with the Hinode/XRT images. The RHESSI
non-thermal emission in this event matches spatial and temporally the
initial brightest emission from XRT.
---------------------------------------------------------
Title: Open magnetic flux tubes in the corona and the transport of
solar energetic particles
Authors: Klein, K. -L.; Krucker, S.; Lointier, G.; Kerdraon, A.
2008A&A...486..589K Altcode:
We investigate how magnetic fields guide energetic particles through
the corona into interplanetary space and eventually to a spacecraft
near the Earth. A set of seven simple particle events is studied,
where energetic electrons (30-500 keV; Wind spacecraft) or protons
(5-55 MeV; SoHO) were released together with low-energy electron beams
producing metric-to-kilometric type III emission. Imaging of the coronal
(metre-wave) part of this emission with the Nançay Radioheliograph is
used to identify the open flux tubes that guide these electrons - and by
inference all particles detected at 1 AU. Open coronal field lines are
also computed using potential magnetic field extrapolations, constrained
by a source surface and by SoHO/MDI measurements in the photosphere
(code by Schrijver and DeRosa). We find that in all events the type III
radio sources lie in open flux tubes in the potential magnetic field
extrapolations. The open flux tubes are rooted in small parts of the
parent active region, covering a heliocentric angle of a few degrees in
the photosphere. But they expand rapidly above the neighbouring closed
magnetic structures and cover several tens of degrees in longitude on
the source surface. Some of these open field lines are found to connect
the parent active region to the footpoint of the nominal Parker spiral
on the source surface, within the uncertainty of about ±10° inherent
to the evaluation of its connection longitude. This is so even when
the parent active region is as far as 50° away. In two cases where
the coronal flux tubes point to high heliolatitudes, the detection of
Langmuir waves at the Wind spacecraft in the ecliptic plane suggests
that the interplanetary field lines curve down to the ecliptic before
reaching 1 AU. We conclude that non-radial open flux tubes in the corona
can transport particles over several tens of degrees in longitude even
in simple impulsive particle events. In all events we studied, potential
magnetic field models give an adequate description of these structures.
---------------------------------------------------------
Title: A Statistical Survey of Hard X-ray Spectral Characteristics
of Solar Flares with Two Footpoints
Authors: Saint-Hilaire, P.; Krucker, S.; Lin, R. P.
2008SoPh..250...53S Altcode: 2008SoPh..tmp...93S; 2011arXiv1111.4247S
Using RHESSI data, we have analyzed 172 hard X-ray (HXR) peaks during 53
solar flares that exhibited a double-footpoint structure. Fitting both
footpoints with power laws, we find that spectral index differences Δγ
range mostly between 0 to 0.6, and only rarely go beyond. Asymmetries
between footpoints were not observed to be significantly dependent
on their mean heliographic position, their relative position with
respect to each other, nor their orientation with respect to the solar
equator. By assuming a symmetric acceleration process, it is also clear
that differences in footpoint spectral indices and footpoint flux
ratios can seldom be attributed to a difference in column densities
between the two legs of a coronal loop. Our results corroborate better
the magnetic mirror trap scenario. Moreover, footpoint asymmetries are
more marked during times of peak HXR flux than when averaging over the
whole HXR burst, suggesting that the magnetic configuration evolves
during individual HXR bursts. We also observed a linear correlation
between the peak 50-keV flux and the peak GOES 1 - 8 Å channel flux
and that HXR burst duration seem correlated with loop length.
---------------------------------------------------------
Title: Coronal Hard X-Ray Emission Associated with Radio Type
III Bursts
Authors: Krucker, Säm; Saint-Hilaire, P.; Christe, S.; White, S. M.;
Chavier, A. D.; Bale, S. D.; Lin, R. P.
2008ApJ...681..644K Altcode:
We report on a purely coronal hard X-ray source detected in a partially
disk-occulted solar flare by the Reuven Ramaty High Energy Solar
Spectroscopic Imager (RHESSI) that is associated with radio type
III bursts and a suprathermal electron event detected near 1 AU by
the WIND 3-D Plasma and Energetic Particle (3DP) instrument. Several
observational characteristics suggest that the coronal hard X-ray source
is thin target bremsstrahlung emission from the escaping electrons that
produce the radio type III bursts. The hard X-ray emission correlates
in time with the radio type III bursts and originates from a radially
elongated source in the corona with a length (~65 Mm) similar to typical
coronal density scale heights. Furthermore, the difference between the
hard X-ray photon spectral index (γ = 4.1 +/- 0.4) and the electron
spectral index of the in situ observed event (δ<SUB>in situ</SUB> = 2.9
+/- 0.3) is around 1, consistent with the thin target interpretation. A
further test for the thin target scenario is to compare the number of
electrons needed to produce the observed hard X-ray emission with the
number of in situ observed electrons. However, the number of escaping
electrons derived from the single-spacecraft WIND measurement is in the
best case an order of magnitude estimate and could easily underestimate
the actual number of escaping electrons. Using the WIND observations,
the estimated number of escaping electrons is about an order of
magnitude too low. Thus, the thin target interpretation only holds
if the WIND measurements are significantly underestimating the actual
number of escaping electrons. Future multispacecraft observations with
STEREO, Solar Orbiter, and Sentinels will resolve this uncertainty.
---------------------------------------------------------
Title: Hard X-Rays Associated with Type III Radio Bursts
Authors: Christe, S.; Krucker, S.; Lin, R. P.
2008ApJ...680L.149C Altcode:
During a period of 12 minutes on 2002 July 19 14:23-14:35 UT, the
WAVES instrument on WIND observed six interplanetary type III radio
bursts, one approximately every 2 minutes, and each was accompanied by
a weak hard X-ray (HXR) burst (12-15 keV) observed by RHESSI. The radio
bursts are observed up to 150 MHz with some up to 600 MHz. Simultaneous
observations by TRACE show jetlike eruptions emanating from the region
of HXR emission. The observed HXRs are inconsistent with emission from
the escaping type III-producing nonthermal electrons. We suggest that
the type III acceleration process may be associated with an explosive
release of lsim5 × 10<SUP>26</SUP> ergs in the form of a "superhot"
(26 MK) thermal plasma in the corona, an energy comparable to that
associated with the type III-producing electrons.
---------------------------------------------------------
Title: Coronal γ-Ray Bremsstrahlung from Solar Flare-accelerated
Electrons
Authors: Krucker, Säm; Hurford, G. J.; MacKinnon, A. L.; Shih, A. Y.;
Lin, R. P.
2008ApJ...678L..63K Altcode:
The Reuven Ramaty High Energy Spectroscopic Imager (RHESSI) provides
for the first time imaging spectroscopy of solar flares up to the
γ-ray range. The three RHESSI flares with best counting statistics are
analyzed in the 200-800 keV range revealing γ-ray emission produced by
electron bremsstrahlung from footpoints of flare loops, but also from
the corona. Footpoint emission dominates during the γ-ray peak, but as
the γ-ray emission decreases the coronal source becomes more and more
prominent. Furthermore, the coronal source shows a much harder spectrum
(with power-law indices γ between 1.5 and 2) than the footpoints (with
γ between 3 and 4). These observations suggest that flare-accelerated
high-energy (~MeV) electrons stay long enough in the corona to lose
their energy by collisions producing γ-ray emission, while lower
energetic electrons precipitate more rapidly to the footpoints.
---------------------------------------------------------
Title: STEREO and RHESSI Observations of a Partially Disk-Occulted
Solar Flare
Authors: Krucker, S.; Wuelser, J.; Lin, R. R.
2008AGUSMSP44A..02K Altcode:
RHESSI hard X-ray observations of partially-disk occulted solar flares
provide crucial information on faint coronal hard X-ray sources in the
absence of generally much brighter emissions from footpoints of flare
loops. Coronal hard X-ray sources can differ fundamentally from the
classical footpoint sources of the flare impulsive phase and provide
unique information about the supra-thermal electrons closest to the
site in the corona where their acceleration is believed to occur. The
different view-angles provided by the STEREO spacecraft allow us to put
the partially occulted hard X-ray sources observed by RHESSI in context
with the EUV flare ribbons and the EUV emission from CMEs observed by
STEREO/EUVI. In this talk we report on the GOES C8 flare observed on
December 31, 2007 peaking around 01:11UT. From Earth-view (RHESSI),
the flare occurs about 12 degrees behind the eastern limb giving an
occultation height of 16 Mm. From STEREO B, the flare ribbons are seen
on the disk (about 10 degrees from the limb), while the flare is highly
occulted (130 Mm) for STEREO A observations so that emissions related to
the associated CME are seen. Despite the occultation, RHESSI observes
strong non-thermal emissions up to 100 keV that entirely originate
from the corona. Initially, the coronal hard X-ray emission is seen
from above the EUV flare ribbons, while later on, emissions from a
radially extended (~20 Mm) source located about 20 Mm to the south
of the flare ribbon are seen. Preliminary results suggest that the
elongated hard X-ray source is possibly related to the CME.
---------------------------------------------------------
Title: Radio Submillimeter and γ-Ray Observations of the 2003
October 28 Solar Flare
Authors: Trottet, G.; Krucker, Säm; Lüthi, T.; Magun, A.
2008ApJ...678..509T Altcode:
Radio observations at 210 GHz taken by the Bernese Multibeam Radiometer
for KOSMA (BEMRAK) are combined with hard X-ray and γ-ray observations
from the SONG instrument on board CORONA-F and the Reuven Ramaty High
Energy Solar Spectroscopic Imager (RHESSI) to investigate high-energy
particle acceleration during the energetic solar flare of 2003 October
28. Two distinct components at submillimeter wavelengths are found. The
first is a gradual, long-lasting (>30 minutes) component with
large apparent source sizes (~60”). Its spectrum below ~200 GHz is
consistent with synchrotron emission from flare-accelerated electrons
producing hard X-ray and γ-ray bremsstrahlung assuming a magnetic
field strength of >=200 G in the radio source and a confinement
time of the radio-emitting electrons in the source of less than 30
s. The other component is impulsive and starts simultaneously with
high-energy (>200 MeV nucleon<SUP>-1</SUP>) proton acceleration
and the production of pions. The derived radio source size is compact
(<=10”), and the emission is cospatial with the location of
precipitating flare-accelerated >30 MeV protons as seen in γ-ray
imaging. The close correlation in time and space of radio emission
with the production of pions suggests that synchrotron emission
of positrons produced in charged-pion decay might be responsible
for the observed compact radio source. However, order-of-magnitude
approximations rather suggest that the derived numbers of positrons from
charged-pion decay are probably too small to account for the observed
radio emission. Synchrotron emission from energetic electrons therefore
appears as the most likely emission mechanism for the compact radio
source seen in the impulsive phase, although it does not account for
its close correlation, in time and space, with pion production.
---------------------------------------------------------
Title: RHESSI Hard X-ray Microflare Statistics
Authors: Christe, S. D.; Hannah, I.; Krucker, S.; Hurford, G.;
McTiernan, J.; Lin, R. P.
2008AGUSMSP51C..09C Altcode:
We present the first in-depth statistical survey of all X-ray
microflares observed by RHESSI between March 2002 and March 2007,
a total of 25,705 events. These microflares were found using a new
flare-finding algorithm designed to search the 6--12~keV count rate
when RHESSIs full sensitivity was available in order to find the
smallest events. These microflares are small flares, from low GOES
C Class to below A Class (background subtracted) and associated with
active regions. Each microflare is automatically analyzed at the peak
time of the 6--12~keV emission. Spectral parameters are determined
both by considering ratios between energy channels and by forward
fitting a thermal plus non-thermal model. Flare images are created
using back-projection and centroids and by forward fitting the complex
visibilities. The combination of imaging and spectral parameters allow
for the first time analysis of the thermal and non-thermal energy at
the peak time of these microflares.
---------------------------------------------------------
Title: Electron beam velocities derived from Wind Type III radio
bursts.
Authors: Saint-Hilaire, P.; Krucker, S.; Bale, S.
2008AGUSMSP51C..13S Altcode:
Using a novel approach based on the Radon Transform to identify radio
Type III bursts observed by the Waves instrument on board the Wind
spacecraft, we find and characterize all interplanetary Type III bursts
observed by Wind since its launch. We will be interested in those whose
exciter also struck the spacecraft, allowing us to make a statistical
study of exciter velocities spanning more that a solar cycle, for the
first time.
---------------------------------------------------------
Title: Can chromospheric evaporation explain superhot flare plasmas?
Authors: Caspi, A.; Krucker, S.; Lin, R. P.
2008AGUSMSP44A..03C Altcode:
Most flares show strong soft X-ray (SXR; <20 keV) emission from
a hot looptop source and hard X-ray (HXR; >20 keV) emission from
footpoint sources. The latter is generally interpreted as non-thermal
bremsstrahlung from downward-accelerated coronal electrons impacting the
denser chromosphere, where they deposit most of their kinetic energy
as heat. Heated chromospheric material then rises and fills the flare
loop - a process known as chromospheric evaporation (CE). Evidence
for CE is given by crystal spectrometer observations of blueshifted
spectral line emission from footpoints, suggesting upward motion of
hot evaporated plasma. CE has often been thought to be the primary
source of thermal looptop plasma. In large (GOES X-class) flares,
non-thermal energy deposition can be as much as ~1e29~erg/s. For
typical non-thermal electron energies, chromospheric densities, and
footpoint areas, this input power should yield ~5-20~MK CE material at
the footpoints, but with an emission measure calculated at ~1e50~cm-
3. Such bright thermal emission should be clearly visible in RHESSI
imaging spectra of large flares, but no such emission has yet been
observed. This suggests that while CE may indeed be the primary
source of thermal loop plasma, heating by non-thermal electrons may
be less efficient than once believed. A heating efficiency of only
~10-50% would result in a CE emission measure of only ~1e48~cm-3,
which would be more consistent with observed footpoint fluxes. We
present an analysis of selected RHESSI flares, ranging in GOES class
from ~M1 to ~X17. We employ imaging spectroscopy to obtain spectra of
spatially-separated footpoints and looptop sources to determine the
strength of the thermal components in each. The time evolution of the
thermal footpoint signatures is compared to that of the HXR emission as
well as to the looptop thermal emission and temperature. We calculate
the energy contained in non-thermal electrons, the thermal energies at
the footpoints and the looptop, and discuss the implications for the
efficiency of heating by non-thermal electrons and the contribution
of chromospheric evaporation to the flare thermal plasma.
---------------------------------------------------------
Title: S/WAVES: The Radio and Plasma Wave Investigation on the
STEREO Mission
Authors: Bougeret, J. L.; Goetz, K.; Kaiser, M. L.; Bale,
S. D.; Kellogg, P. J.; Maksimovic, M.; Monge, N.; Monson, S. J.;
Astier, P. L.; Davy, S.; Dekkali, M.; Hinze, J. J.; Manning, R. E.;
Aguilar-Rodriguez, E.; Bonnin, X.; Briand, C.; Cairns, I. H.; Cattell,
C. A.; Cecconi, B.; Eastwood, J.; Ergun, R. E.; Fainberg, J.; Hoang,
S.; Huttunen, K. E. J.; Krucker, S.; Lecacheux, A.; MacDowall, R. J.;
Macher, W.; Mangeney, A.; Meetre, C. A.; Moussas, X.; Nguyen, Q. N.;
Oswald, T. H.; Pulupa, M.; Reiner, M. J.; Robinson, P. A.; Rucker,
H.; Salem, C.; Santolik, O.; Silvis, J. M.; Ullrich, R.; Zarka, P.;
Zouganelis, I.
2008SSRv..136..487B Altcode: 2008SSRv..tmp....9B
This paper introduces and describes the radio and plasma wave
investigation on the STEREO Mission: STEREO/WAVES or S/WAVES. The
S/WAVES instrument includes a suite of state-of-the-art experiments
that provide comprehensive measurements of the three components of the
fluctuating electric field from a fraction of a hertz up to 16 MHz, plus
a single frequency channel near 30 MHz. The instrument has a direction
finding or goniopolarimetry capability to perform 3D localization
and tracking of radio emissions associated with streams of energetic
electrons and shock waves associated with Coronal Mass Ejections
(CMEs). The scientific objectives include: (i) remote observation and
measurement of radio waves excited by energetic particles throughout
the 3D heliosphere that are associated with the CMEs and with solar
flare phenomena, and (ii) in-situ measurement of the properties of
CMEs and interplanetary shocks, such as their electron density and
temperature and the associated plasma waves near 1 Astronomical Unit
(AU). Two companion papers provide details on specific aspects of the
S/WAVES instrument, namely the electric antenna system (Bale et al.,
Space Sci. Rev., 2007) and the direction finding technique (Cecconi
et al., Space Sci. Rev., 2007).
---------------------------------------------------------
Title: An intriguing solar microflare observed with RHESSI, Hinode,
and TRACE
Authors: Hannah, I. G.; Krucker, S.; Hudson, H. S.; Christe, S.; Lin,
R. P.
2008A&A...481L..45H Altcode: 2007arXiv0712.0369H
Aims:We investigate particle acceleration and heating in a solar
microflare. <BR />Methods: In a microflare with non-thermal emission
to remarkably high energies (>50 keV), we investigate the hard
X-rays with RHESSI imaging and spectroscopy and the resulting thermal
emission seen in soft X-rays with Hinode/XRT and in EUV with TRACE. <BR
/>Results: The non-thermal footpoints observed with RHESSI spatially and
temporally match bright footpoint emission in soft X-rays and EUV. There
is the possibility that the non-thermal spectrum extends down to 4
keV. The hard X-ray burst clearly does not follow the expected Neupert
effect, with the time integrated hard X-rays not matching the soft
X-ray time profile. So, although this is a simple microflare with good
X-ray observation coverage it does not fit the standard flare model.
---------------------------------------------------------
Title: RHESSI Microflare Statistics. II. X-Ray Imaging, Spectroscopy,
and Energy Distributions
Authors: Hannah, I. G.; Christe, S.; Krucker, S.; Hurford, G. J.;
Hudson, H. S.; Lin, R. P.
2008ApJ...677..704H Altcode: 2007arXiv0712.2544H
We present the first statistical analysis of the thermal and nonthermal
X-ray emission of all 25,705 microflares (RHESSI) observed between
2002 March and 2007 March. These events were found by searching
the 6-12 keV energy range (see Paper I) and are small active region
flares, from low (GOES) C class to below A class. Each microflare is
automatically analyzed at the peak time of the 6-12 keV emission:
the thermal source size is found by forward-fitting the complex
visibilities for 4-8 keV, and the spectral parameters (temperature,
emission measure, power-law index) are found by forward-fitting a
thermal plus nonthermal model. The resulting wealth of information we
determine about the events allows a range of the thermal and nonthermal
properties to be investigated. In particular, we find that there is
no correlation between the thermal loop size and the flare magnitude,
indicating that microflares are not necessarily spatially small. We
present the first thermal energy distribution of RHESSI flares and
compare it to previous thermal energy distributions of transient
events. We also present the first nonthermal power distribution of
RHESSI flares and find that a few microflares have unexpectedly large
nonthermal powers up to 10<SUP>28</SUP> erg s<SUP>-1</SUP>. The total
microflare nonthermal energy, however, is still small compared to
that of large flares as it occurs for shorter durations. These large
energies and difficulties in analyzing the steep nonthermal spectra
suggest that a sharp broken power law and thick-target bremsstrahlung
model may not be appropriate for microflares.
---------------------------------------------------------
Title: RHESSI Microflare Statistics. I. Flare-Finding and Frequency
Distributions
Authors: Christe, S.; Hannah, I. G.; Krucker, S.; McTiernan, J.; Lin,
R. P.
2008ApJ...677.1385C Altcode:
We present the first in-depth statistical survey of all X-ray
microflares observed by RHESSI between 2002 March and 2007 March,
a total of 25,705 events, an order of magnitude larger then previous
studies. These microflares were found using a new flare-finding
algorithm designed to search the 6-12 keV count rate when RHESSI's full
sensitivity was available in order to find the smallest events. The
peak and total count rate are automatically obtained along with
count spectra at the peak and the microflare centroid position. Our
microflare magnitudes are below GOES C class, on average GOES A class
(background subtracted). They are found to occur only in active regions,
not in the "quiet" Sun, and are similar to large flares. The monthly
average microflaring rate is found to vary with the solar cycle
and ranges from 90 to 5 flares a day during active and quiet times,
respectively. Most flares are found to be impulsive (74%), with rise
times shorter than decay times. The mean flare duration is ~6 minutes
with a 1 minute minimum set by the flare-finding algorithm. The
frequency distributions of the peak count rate in the energy bands,
3-6, 6-12, and 12-25 keV, can be represented by power-law distributions
with a negative power-law index of 1.50 +/- 0.03, 1.51 +/- 0.03, and
1.58 +/- 0.02, respectively. We find that these power-law indices are
constant as a function of time. The X-ray photon spectra for individual
events can be approximated with a power-law spectrum [dJ/d(hν) ~
(hν)<SUP>-γ</SUP>]. Using the ratio of photon fluxes between 10-15
and 15-20 keV, we find 4 < γ < 12, with an average of 7.4. Based
on these values, the nonthermal power is calculated. The microflare
occurrence frequency varies with the rate of energy release consistent
with a power law with an exponent of -1.7 +/- 0.1. We estimate the
total energy flux deposited in active regions by microflare-associated
accelerated electrons (>10 keV) over the five years of observations
to be, on average, below 10<SUP>26</SUP> erg s<SUP>-1</SUP>.
---------------------------------------------------------
Title: Coronal Jet Observed by Hinode as the Source of
a<SUP>3</SUP>He-rich Solar Energetic Particle Event
Authors: Nitta, Nariaki V.; Mason, Glenn M.; Wiedenbeck, Mark E.;
Cohen, Christina M. S.; Krucker, Säm; Hannah, Iain G.; Shimojo,
Masumi; Shibata, Kazunari
2008ApJ...675L.125N Altcode:
We study the solar source of the <SUP>3</SUP>He-rich solar
energetic particle (SEP) event observed on 2006 November 18. The
SEP event showed a clear velocity dispersion at energies below 1 MeV
nucleon<SUP>-1</SUP>, indicating its solar origin. We associate the SEP
event with a coronal jet in an active region at heliographic longitude
of W50°, as observed in soft X-rays. This jet was the only noticeable
activity in full-disk X-ray images around the estimated release time of
the ions. It was temporally correlated with a series of type III radio
bursts detected in metric and longer wavelength ranges and was followed
by a nonrelativistic electron event. The jet may be explained in terms
of the model of an expanding loop reconnecting with a large-scale
magnetic field, which is open to interplanetary space for the particles
to be observed at 1 AU. The open field lines appear to be anchored at
the boundary between the umbra and penumbra of the leading sunspot,
where a brightening is observed in both soft and hard X-rays during
the jet activity. Other flares in the same region possibly associated
with <SUP>3</SUP>He-rich SEP events were not accompanied by a jet,
indicative of different origins of this type of SEP event.
---------------------------------------------------------
Title: Hard X-Ray Spectral Evolution and Production of Solar Energetic
Particle Events during the January 2005 X-Class Flares
Authors: Saldanha, R.; Krucker, Säm; Lin, R. P.
2008ApJ...673.1169S Altcode:
High-resolution hard X-ray observations provided by the Reuven Ramaty
High Energy Solar Spectroscopic Imager (RHESSI) are used to study the
spectral evolution of ~50-200 keV nonthermal electron bremsstrahlung
emissions of five X-class flares observed during the January 2005 solar
storm events. Four of these flares show progressive spectral hardening
during at least some hard X-ray peaks, while only one event shows
the otherwise more commonly observed soft-hard-soft behavior. Imaging
observations reveal that ~50-100 keV nonthermal electron bremsstrahlung
emissions originate from footpoints of flare loops at all times,
including during times of progressive spectral hardening, indicating
that the spectral hardening component is produced by precipitating
electrons, and not by electrons trapped in the corona. The four flares
with progressive spectral hardening are all related to solar energetic
particle (SEP) events, while the only X-class flare with soft-hard-soft
behavior is not. This finding is consistent with earlier studies,
suggesting that electron acceleration and transport in flares is somehow
linked to the production of SEPs escaping into interplanetary space.
---------------------------------------------------------
Title: Hard X-Ray Emissions from Partially Occulted Solar Flares
Authors: Krucker, Säm; Lin, R. P.
2008ApJ...673.1181K Altcode:
Observations of solar flares partially occulted by the solar limb
provide diagnostics of coronal hard X-ray (HXR) emissions in the
absence of generally much brighter emissions from footpoints of
flare loops. In this paper, a statistical survey of 55 partially
occulted flares observed by the Reuven Ramaty High-Energy Solar
Spectroscopic Imager (RHESSI) is presented, revealing the existence
of two different components of coronal HXR emissions. Below ~15 keV
thermal HXR emission with a gradual time profile is generally dominant,
while at higher energies an additional component is seen in 50 out of
55 events. This additional component shows faster time variations in
the order of tens of seconds and is most prominent during the rise
of the thermal emission. A comparison of the centroid positions of
these two emissions shows that they are most often cospatial within
~2000 km, although for a few events clear separations are observed as
well. The spectra of the high-energy component show a rather steep
(soft) power law with indices mostly between ~4 and ~6. Thin target
emission in the corona from flare-accelerated electrons is discussed
as a possible origin of the fast time variation component.
---------------------------------------------------------
Title: Ion acceleration and neutral emission mechanisms for 2005
September 7 flare
Authors: Watanabe, K.; Murphy, R. J.; Share, G. H.; Hurford, G. J.;
Krucker, S.; Lin, R. P.; Harris, M. J.; Gros, M.; Sako, T.; Muraki,
Y.; Matsubara, Y.; Shibata, S.; Sakai, T.; Valdés-Galicia, J. F.;
González, L. X.; Hurtado, A.; Musalem, O.; Velarde, A.; Ticona, R.;
Martinic, N.; Miranda, P. P.; Kakimoto, F.; Tsunesada, Y.; Tokuno,
H.; Ogio, S.
2008ICRC....1...45W Altcode: 2008ICRC...30a..45W
In association with an X17.0 flare on 2005 September 7, strong neutral
emissions were detected both in space and on the ground. In space,
intense emissions of gamma-rays were registered by INTEGRAL and
by RHESSI during the decay phase. Gamma-ray lines at 0.511, 2.2,
4.4, and 6.1 MeV were observed and there was evidence for pion-decay
radiation. On the ground, relativistic neutrons were observed by the
neutron monitors at Mt. Chacaltaya and Mexico City and by the solar
neutron telescopes at Chacaltaya and Mt. Sierra Negra. The neutron
signal continued for more than 20 minutes with high statistical
significance. The long decay of the signals suggests that ions were
continuously accelerated or trapped in the emission site. We also find
that gamma-rays were emitted over a corresponding extended period. Only
when we in cooperate the high-energy gamma-ray emission time history
can we explain the long-lasting neutron emission. We also use the
Hua et al. (2002) solar-flare magnetic loop transport and interaction
model to find the best model to explain the data.
---------------------------------------------------------
Title: Physics of ion acceleration in the solar flare on 2005
September 7 determines gamma-ray and neutron production
Authors: Watanabe, Kyoko; Krucker, Sam; Lin, Robert; Murphy, Ronald;
Share, Gerald; Harris, Michael; Gros, Maurice
2008cosp...37.3429W Altcode: 2008cosp.meet.3429W
Relativistic neutrons were observed by the neutron monitors at
Mt. Chacaltaya and Mexico City and by the solar neutron telescopes at
Chacaltaya and Mt. Sierra Negra in association with an X17.0 flare
on 2005 September 7. The neutron signal continued for more than 20
minutes with high statistical significance. Intense emissions of
gamma-rays were also registered by INTEGRAL, and by RHESSI during the
decay phase. We analyzed these data using the solar-flare magnetic-loop
transport and interaction model of Hua et al. (2002), and found that
the model could successfully fit the data with intermediate values of
loop magnetic convergence and pitch angle scattering parameters. These
results indicate that solar neutrons were produced at the same time as
the gamma-ray line emission and that ions were continuously accelerated
at the emission site.
---------------------------------------------------------
Title: Evidence of a Two-Temperature Source Region in the
<SUP>3</SUP>He-Rich Solar Energetic Particle Event of 2000 May 1
Authors: Kartavykh, Y. Y.; Dröge, W.; Klecker, B.; Mason, G. M.;
Möbius, E.; Popecki, M.; Krucker, S.
2007ApJ...671..947K Altcode:
Using instruments on the ACE and Wind spacecraft, we investigate the
temporal evolution, spectra, and ionization states of solar energetic
particle (SEP) Fe in the impulsive event of 2000 May 1. Proton and
electron intensities and anisotropies were used to help constrain
the characteristics of the interplanetary propagation, taking into
account focusing, pitch-angle scattering, adiabatic deceleration,
and convection. We find that the event was nearly scatter-free, with
an interplanetary scattering mean free path larger than 1 AU. The Fe
spectrum spectral form is consistent with stochastic acceleration, but
the observed increase of the ionization state of Fe between 200-600
keV nucleon<SUP>-1</SUP> is larger than can be explained using a
single temperature source even after including the effect of adiabatic
deceleration in the solar wind. A two-temperature source region is
required to fit the observed range of Fe charge states, with the bulk
(>80%) of the particles coming from a T~10<SUP>6</SUP> K region,
and the remainder from a region with T~1.6×10<SUP>7</SUP> K.
---------------------------------------------------------
Title: X-ray Microflares with Hinode and RHESSI.
Authors: Hannah, I. G.; Christe, S.; Krucker, S.; Hudson, H.; Lin,
R. P.; Deluca, E.
2007AGUFMSH52C..07H Altcode:
We present analysis of microflares (small active region associated
flares below GOES C class) using RHESSI and Hinode/XRT. RHESSI has
observed well over 1,000 microflares since Hinode launched late in 2006
and of these over 150 have good Hinode/XRT coverage. We use RHESSI to
obtain the temperature, emission measure and non-thermal power-law
parameters from spectral fitting. We compare RHESSI and Hinode/XRT
images to locate the thermal and non-thermal emissions. Taking advantage
of the sensitive high-resolution capability of XRT for the softer
X-rays, we investigate the resulting heating and evaporation from the
accelerated electrons observed via the non-thermal emission by RHESSI.
---------------------------------------------------------
Title: RHESSI and Hinode X-Ray Observations of a Partially Occulted
Solar Flare
Authors: Krucker, Säm; Hannah, I. G.; Lin, R. P.
2007ApJ...671L.193K Altcode:
This Letter presents X-ray observations of a partially limb-occulted
solar flare taken by the Reuven Ramaty High-Energy Solar Spectroscopic
Imager and the X-ray telescope on board Hinode. Thermal emission
originates from a simple loop at the western limb that rises slowly
(~7 km s<SUP>-1</SUP>) until the flare peak time. Above 18 keV, faint
nonthermal emission with a hard/flat spectrum (γ~4) and fast time
variations (of the order of tens of seconds) is seen that comes from
a loop slightly above (~2000 km) the thermal loop, if compared at the
same time. However, the nonthermal loop agrees well in altitude with
the thermal flare loop seen later, at the soft X-ray peak time. This
is consistent with simple flare models where nonthermal electrons in
a flare loop produce thin-target hard X-ray emission in the corona as
they travel to the loop footpoints. There they lose all their energy
and heat chromospheric plasma that fills the loop earlier seen in
nonthermal hard X-rays. This suggests that electron acceleration in
solar flares occurs in the corona.
---------------------------------------------------------
Title: HINODE and RHESSI Observations of Partially Occulted Solar
Flares
Authors: Krucker, S.; Hannah, I. G.; Lin, R. P.
2007AGUFMSH52C..04K Altcode:
We present X-ray imaging and spectral observations of a series of
partially occulted solar flares observed by HINODE/XRT and RHESSI
on November 21, 2006. Despite that the footpoints of the flare loops
are occulted, non-thermal hard X-ray emissions from flare-accelerated
electrons are observed in all of these flares. Although the non-thermal
emissions are faint, their spectra are rather flat/hard with power
law indices between ~4 and ~5. All events look similar with a simple
thermal loop seen at the limb that rises slowly (~10 km/s) during the
impulsive phase. The non-thermal emission can only be imaged when
averaged in time over the impulsive phase, but it originate from a
similarly sized loop. Compared at the same time, the non-thermal
loop is about 1500 km higher up than the thermal loop. However,
the location of the non-thermal loop seen in the impulsive phase is
about at the same altitude as the thermal loop seen at the soft X-ray
peak time. These observations are consistent with the standard flare
model where flare-accelerated electrons produce faint non-thermal hard
X-ray emissions in flare loops (thin target emission) and then lose
their energy in the footpoints of these loops and heat chromospheric
plasma. Evaporation of heated chromospheric plasma then fills up
the flare loops earlier seen in non-thermal hard X-rays and makes it
visible in thermal X-ray emission.
---------------------------------------------------------
Title: RHESSI Hard X-ray Microflare Statistics
Authors: Christe, S.; Hannah, I.; Krucker, S.; Hurford, G.; McTiernan,
J.; Lin, R.
2007AGUFMSH22A0849C Altcode:
We present the first in-depth statistical survey of all X-ray
microflares observed by RHESSI between March 2002 and March 2007,
a total of 25,705 events. These microflares were found using a new
flare-finding algorithm designed to search the 6--12~keV count rate
when RHESSIs full sensitivity was available in order to find the
smallest events. These microflares are small flares, from low GOES
C Class to below A Class (background subtracted) and associated with
active regions. Each microflare is automatically analyzed at the peak
time of the 6--12~keV emission. Spectral parameters are determined
both by considering ratios between energy channels and by forward
fitting a thermal plus non-thermal model. Flare images are created
using back-projection and centroids and by forward fitting the complex
visibilities. The combination of imaging and spectral parameters allow
for the first time analysis of the thermal and non-thermal energy at
the peak time of these microflares.
---------------------------------------------------------
Title: Statistics of Hot Plasmas in M/X Flares Using RHESSI Fe &
Fe/Ni Line and Continuum Observations
Authors: Caspi, A.; Krucker, S.; Lin, R. P.
2007AGUFMSH41A0309C Altcode:
Observations by the Reuven Ramaty High Energy Solar Spectroscopic
Imager (RHESSI) have shown that "super-hot" (T > ~30~MK) thermal
plasmas are a somewhat common feature of (large) flares of GOES class
M or X. However, the origins and evolution of such super-hot plasmas
are still poorly understood. RHESSI observes solar photons >3 keV
with a spectral resolution of ~1~keV FWHM, and is especially sensitive
to flare plasmas hotter than ~10~MK. RHESSI's rich data set allows an
unprecedented level of analysis of thermal flare plasma through imaging
and spectroscopic observations of the thermal bremsstrahlung continuum
and the Fe and Fe/Ni line complexes at ~6.7 and ~8~keV. Accurately
characterizing the thermal plasma provides information about flare
heating and cooling mechanisms and improves our understanding of flare
energy transport and release. We present early results of a statistical
survey of hot thermal plasmas in ~300 RHESSI-observed M/X-class flares
(2002-2005), utilizing an analytical method to compare the thermal
continuum and line emission (Caspi & Lin 2007). We present flare
energetics, size, and temperature distributions and discuss the
implications for the origins of super-hot thermal plasmas.
---------------------------------------------------------
Title: Ion acceleration and neutral emission mechanisms for 2005
September 7 flare
Authors: Watanabe, K.; Lin, R. P.; Krucker, S.; Murphy, R. J.; Share,
G. H.; Harris, M. J.; Gros, M.; Sako, T.; Muraki, Y.
2007AGUFMSH52A0908W Altcode:
In association with an X17.0 flare on 2005 September 7, strong neutral
emissions were detected both in space and on the ground. In space,
intense emissions of gamma-rays were registered by INTEGRAL and
by RHESSI during the decay phase. Gamma-ray lines at 0.511, 2.2,
4.4, and 6.1 MeV were observed and there was evidence for pion-decay
radiation. On the ground, relativistic neutrons were observed by the
neutron monitors at Mt. Chacaltaya and Mexico City and by the solar
neutron telescopes at Chacaltaya and Mt. Sierra Negra. The neutron
signal continued for more than 20 minutes with high statistical
significance. The long decay of the signals suggests that ions were
continuously accelerated or trapped in the emission site. We also find
that gamma-rays were emitted over a corresponding extended period. Only
when we in cooperate the high-energy gamma-ray emission time history
can we explain the long-lasting neutron emission. We also use the Hua
et al. (2002) solar- flare magnetic loop transport and interaction
model to find the best model to explain the data.
---------------------------------------------------------
Title: Comparing flare temperature distributions from RHESSI and
from Hinode
Authors: Saint-Hilaire, P.; Krucker, S.; Lin, R. P.
2007AGUFMSH53A1057S Altcode:
Using flares that were observed by both RHESSI and Hinode (XRT and
EIS), as well as available data from STEREO, TRACE and SOHO's EIT, we
compare the spatial and temporal temperature distributions of flaring
plasma in the X-ray and EUV regimes.
---------------------------------------------------------
Title: What is There Before a Flare?
Authors: Hudson, H. S.; Hannah, I. G.; Krucker, S.; Watanabe, K.
2007AGUFMSH53A1055H Altcode:
The physical parameters in a coronal volume prior to the occurrence
of a flare are generally unknown, but may play an important role
in identifying the processes involved in flaring or eruption. Now
we have observations from Hinode at very high resolution that can
provide the best possible values for preflare temperature and density,
for example. We make use of Hinode XRT observations of the preflare
magnetic structure with the same footpoint locations as a flaring
loop, as identified in RHESSI images. We additionally introduce a
method based on conductive equilibrium (RTV scaling) to reduce the
uncertainty on estimates of physical parameters due to lack of knowledge
of the detailed geometry. Preliminary results are consistent with the
finding at lower resolution from Yohkoh: in a majority of cases, the
preflare conditions are not observable in soft X-rays. We discuss the
upper limits that result, which point to low temperatures, densities,
and plasma beta values, but high Alfven speeds. We hope also to be
able to extend this conclusion with EIS observations.
---------------------------------------------------------
Title: Solar Flare Hard X-Ray Emission from the High Corona
Authors: Krucker, Säm; White, S. M.; Lin, R. P.
2007ApJ...669L..49K Altcode:
One of the largest solar hard X-ray (HXR) flares and solar energetic
particle (SEP) events recorded by the Mars Odyssey mission while
orbiting Mars occurred on 2002 October 27 and is related to a very
fast (~2300 km s<SUP>-1</SUP>) coronal mass ejection (CME). From the
Earth, the flare site is 40.4° +/- 3.5° behind the solar limb and
only emissions from the high corona at least 1.5 × 10<SUP>5</SUP>
km radially above the main flare site can be seen. Nevertheless,
the Earth-orbiting Reuven Ramaty High Energy Solar Spectroscopic
Imager (RHESSI) observed HXR emission up to 60 keV with a relatively
flat, nonthermal spectrum (γ between 3 and 3.5) that has an onset
simultaneous with the main HXR emission observed above 60 keV by
the Gamma-Ray Spectrometer (GRS) orbiting Mars. While GRS records
several smaller enhancements after the main peak, the high coronal
source observed by RHESSI shows a long exponential decay (τ = 135
+/- 5 s) with progressive spectral hardening. The emissions from the
high corona originate from an extended source (~1.5 × 10<SUP>5</SUP>
km in diameter) that expands (390 +/- 70 km s<SUP>-1</SUP>) and moves
upwards (750 +/- 80 km s<SUP>-1</SUP>) in the same direction as the
CME. These observations reveal the existence of energetic electrons in
the high corona in closed magnetic structures related to the CME that
are accelerated at the same time as the main energy release in the
flare. Although the number of energetic electrons in the high corona
is only a small fraction of the total accelerated electrons, about 10%
of all electrons in the high coronal source are nonthermal (>10 keV).
---------------------------------------------------------
Title: Solar Flare Electron Spectra at the Sun and near the Earth
Authors: Krucker, Säm; Kontar, E. P.; Christe, S.; Lin, R. P.
2007ApJ...663L.109K Altcode:
We compare hard X-ray (HXR) photon spectra observed by the RHESSI with
the spectra of the electrons in the associated solar impulsive particle
events observed near 1 AU by the WIND 3D Plasma and Energetic Particle
(3DP) instrument. For prompt events, where the inferred injection
time at the Sun coincides with the HXR burst, the HXR photon power-law
spectral index γ and the in situ observed electron spectral index δ
measured above 50 keV show a good linear fit, δ=γ+0.1(+/-0.1), with
correlation coefficient of 0.83, while for delayed events (inferred
injection >10 minutes after the HXR burst) only a weak correlation
with a coefficient of 0.43 is seen. The observed relationship for
prompt events is inconsistent, however, with both the thin target case,
where the escaping electrons come from the X-ray-producing electron
population, and the thick target case where some of the accelerated
source population escapes to 1 AU and the rest produce the HXRs while
losing all their energy to collisions. Furthermore, the derived total
number of escaping electrons correlates with the number of electrons
required to produce observed X-ray flux but is only about ~0.2% of
the number of HXR-producing electrons.
---------------------------------------------------------
Title: First Results of the S/WAVES Experiment on the STEREO Mission.
Authors: Bonnin, X.; Maksimovic, M.; Bougeret, J. -L.; Goetz, K.;
Bale, S. D.; Kaiser, M. L.; Reiner, M. J.; Cecconi, B.; Briand, C.;
Krucker, S.; S/Waves Team
2007sf2a.conf..582B Altcode:
We present the first results of the STEREO/Waves (S/Waves) investigation
on the STEREO Mission. The S/Waves instrument includes a suite of
state-of-the-art sub-instruments that provide comprehensive measurements
of the three components of the electric field from a fraction of a
Hertz up to 16 MHz, plus a single frequency channel near 30 MHz. The
instrument has a direction finding or goniopolarimetry capability,
used to perform 3-D localization and tracking of streams of energetic
electrons and of shock waves associated with Coronal Mass Ejections
(CMEs). The scientific objectives include (i) remote observation and
measurement of energetic phenomena throughout the 3-D heliosphere
that are associated with the CMEs and with solar flare phenomena,
and (ii) in-situ measurement of the properties of CMEs, such as their
electron density and temperature and the associated plasma waves near
1 Astronomical Unit.
---------------------------------------------------------
Title: RHESSI Microflare Statistics: X-ray Imaging And Spectral
Analysis
Authors: Hannah, Iain; Christe, S.; Krucker, S.; Hurford, G.; Lin, R.
2007AAS...210.9315H Altcode: 2007BAAS...39R.213H
We present x-ray imaging and spectral analysis of all microflares the
Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) observed
between March 2002 and March 2007, a total of 24,799 events. These
microflares are small flares, below C Class, and were found by searching
6-12 keV during periods when the full sensitivity RHESSI’s detectors
were available. Each microflare is automatically analysed: the thermal
source size is found by forward fitting the complex visibilities for
4-8 keV and the spectral parameters (temperature, emission measure,
power-law index) are found by forward fitting a thermal plus non-thermal
spectral model. This wealth of information allows us to investigate
many aspects of these microflares and in particular the thermal and
non-thermal energy distributions of these events.
---------------------------------------------------------
Title: Statistical Properties of Hot Thermal Plasmas in M/X Flares
Using RHESSI Fe & Fe/Ni Line and Continuum Observations
Authors: Caspi, Amir; Krucker, S.; Lin, R. P.
2007AAS...210.9318C Altcode: 2007BAAS...39..214C
Observations by the Reuven Ramaty High Energy Solar Spectroscopic Imager
(RHESSI) have shown that "super-hot" (T > ∼30 MK) thermal plasmas
are a common feature of (large) flares of GOES class M or X. However,
the origins and evolution of such super-hot plasmas are still poorly
understood. RHESSI observes solar photons >3 keV with a spectral
resolution of ∼1 keV FWHM, and is especially sensitive to emission
from flare plasmas hotter than ∼10 MK. RHESSI's rich data set allows
an unprecedented level of analysis of thermal flare plasma through
imaging and spectroscopic observations of the thermal bremsstrahlung
X-ray continuum above ∼3 keV and the Fe and Fe/Ni line complexes
at ∼6.7 and ∼8 keV. Accurately characterizing the thermal plasma
provides information about flare heating and cooling mechanisms and
improves our understanding of flare energy transport and release. We
present first results of a statistical survey of hot thermal plasmas
in RHESSI-observed M/X-class flares, utilizing a newly-developed
analytical method to compare the thermal continuum and Fe & Fe/Ni
line emission (Caspi & Lin 2007). We present flare energetics and
temperature distributions and discuss the implications for the origins
of super-hot thermal plasmas.
---------------------------------------------------------
Title: Hard X-ray Spectral Characteristics Of Solar Flares With Two
Footpoints: a survey
Authors: Saint-Hilaire, Pascal; Krucker, S.; Lin, R. P.
2007AAS...210.9317S Altcode: 2007BAAS...39..214S
Using RHESSI data, we have analyzed the spectral characteristics of
some 172 hard X-ray peaks during 53 solar flares which exhibited
a double-footpoint structure. Spectral index differences between
footpoints do not always exist, even in large flares. It is also
clear that differences in footpoint spectral indices cannot always be
attributed to a difference in column densities between the two legs of
a coronal loop. Moreover, footpoint asymmetries seem to be more marked
during times of peak HXR flux. <P />This work has been supported by
SNSF grant PBEZ2-108928, and NASA HGI grant NNX07AH74G.
---------------------------------------------------------
Title: Properties Of A Type III Storm Associated With Active Region
10923 Observed By The STEREO/WAVES Radio Receiver
Authors: Eastwood, Jonathan; Bale, S. D.; Krucker, S.; Lin, R. P.;
Bougeret, J. L.; Maksimovic, M.; Goetz, K.; Kaiser, M. L.; SWAVES Team
2007AAS...210.2808E Altcode: 2007BAAS...39..136E
We analyze a Type III storm associated with Active Region 10923 observed
by the Stereo spacecraft between November 11 - 17 2006. This active
region developed on the central disk and provides an unprecedented
opportunity to study how a Type III storm develops at kilometric
wavelengths. Bursts are first observed at low frequency (high coronal
altitude). We investigate the power law behavior in the brightness
distribution of individual type III bursts. Time series of brightness
at specific frequencies also appear to follow power law behavior
over a restricted frequency range, indicating specific constraints
on the structure of the source region. We discuss the implications
these results have for the magnetic topology of the corona above
active regions.
---------------------------------------------------------
Title: RHESSI Microflare Statistics: Flare Finding and Frequency
Distribution
Authors: Christe, Steven; Hannah, I.; Krucker, S.; McTiernan, J.;
Lin, R. P.
2007AAS...210.9310C Altcode: 2007BAAS...39..212C
The Reuven Ramaty High Energy Spectroscopic Solar Imager (RHESSI) is
uniquely suited to observe solar microflares due to its sensitivity in
the 3-15 keV energy range (up to 100 times better than previous solar
instruments). As such, it provides new information on these low level
transients. In this study, we present, for the first time, initial
results from a new flare-finding algorithm which searches the 6 to 12
keV energy band in order to find the smallest flares. Over the lifetime
of RHESSI (5 years), we have found a total of 24 799 microflares. We
describe the flare-finding algorithm and present microflare statistics
including the frequency distribution. This work was supported by NASA
contract NAS5-98033.
---------------------------------------------------------
Title: STEREO measurements of shocks and in situ interplanetary
type III radio bursts: early results from the S/WAVES and IMPACT
experiments
Authors: Bale, S. D.; Krucker, S.; Bougeret, J.; Goetz, K.; Kaiser,
M. L.; Kellogg, P. J.; Larson, D. E.; Lin, R. P.; Luhmann, J. G.;
Maksimovic, M.; Russell, C. T.
2007AGUSMSH33A..05B Altcode:
We present early measurements by the STEREO/WAVES and IMPACT instruments
of bow shocks, interplanetary shocks, and in situ interplanetary type
III bursts. S/WAVES measures spectral density and 'burst' electric
waveforms on three orthogonal antennas. IMPACT burst mode samples
the thermal and suprathermal electrons and magnetic field at high
cadence for several 10 minute intervals each day. The S/WAVES and
IMPACT bursts systems have triggered together on several occasions at
shocks and foreshock electron beams. Here we present results of these
high time resolution bursts. Early in the mission, several in situ IP
type III bursts were observed at 1 AU by STEREO and Wind. The similar
instrumentation on the three spacecraft (with separations greater than
100 Re) allows us to limit the spatial extent of the type III radio
emission region.
---------------------------------------------------------
Title: Coronal Gamma-ray Sources in Solar Flares
Authors: Krucker, Sam; Hurford, G. J.; Shih, A. Y.; Lin, R. P.
2007AAS...210.9314K Altcode: 2007BAAS...39..213K
The Reuven Ramaty High Energy Spectroscopic Imager (RHESSI) provides
for the first time imaging spectroscopy of solar flares up to the
gamma-ray range. The three RHESSI flares with best counting statistics
are analyzed in the 200-800 keV range revealing gamma-ray emission
produced by electron bremsstrahlung from footpoints of flare loops, but
also from the corona.Footpoint emission dominates during the gamma-ray
peak, but as the gamma-ray emission decreases the coronal source becomes
more and more prominent. Furthermore, the coronal source shows a much
harder spectrum (with power law indices between 1.5 and 2) than the
footpoints (with indices between 3 and 4). These observations suggest
that flare-accelerated high energy electrons ( MeV) are staying long
enough in the corona and lose their energy by collisions producing
gamma-ray emission, while lower energetic electrons precipitate to
the footpoints without losing significant energy in the corona.
---------------------------------------------------------
Title: Radio and White-Light Coronal Signatures Associated with the
RHESSI Hard X-Ray Event of 2002 July 23
Authors: Reiner, M. J.; Krucker, S.; Gary, D. E.; Dougherty, B. L.;
Kaiser, M. L.; Bougeret, J. -L.
2007ApJ...657.1107R Altcode:
Simultaneous radio, white-light, and hard X-ray (HXR) observations
for the 2002 July 23 γ-ray flare event are used to establish the
relationship of a complex type III-like burst to the corresponding
coronal mass ejection (CME) and the coronal electron acceleration
signatures observed in the decimeter/microwave (dm-cm) emissions
and X-rays. We find that the onset of the type III-like emissions
for this event is coincident with the impulsive RHESSI HXR event,
the dm-cm radio emissions and with the linearly extrapolated liftoff
time of the CME. The overall intensity-time characteristics of the
complex type III-like burst resembles that of both the dm-cm flux and
the HXR light curve that correspond to an electron acceleration event
deep in the corona. Furthermore, the complex radiation characteristics
of the type III-like emissions are found to be directly related to
the CME kinematics, which is directly related to the frequency drift
of the associated low-frequency (kilometric) type II emissions. The
frequency-drift characteristics of the high-frequency (metric) type
II emissions observed for this event, on the other hand, are not
clearly related to the kilometric type II emissions and therefore to
the observed CME height-time characteristics, indicating that these
emissions may correspond to an independent coronal shock wave.
---------------------------------------------------------
Title: Review of Selected RHESSI Solar Results
Authors: Dennis, Brian R.; Hudson, Hugh S.; Krucker, Säm
2007LNP...725...33D Altcode:
We review selected science results from RHESSI solar observations made
since launch on 5 February 2002. A brief summary of the instrumentation
is given followed by a sampling of the major science results obtained
from the soft X-ray, hard X-ray, and gamma-ray energy domains. The
thermal continuum measurements and detection of Fe-line features are
discussed as they relate to parameters of the thermal flare plasma for
several events, including microflares. Observations of X-ray looptop,
and rising above-the-loop sources are discussed as they relate to
standard models of eruptive events and the existence of a current
sheet between the two. Hard X-ray spectra and images of footpoints and
coronal sources are presented, showing how they can be used to separate
thermal and nonthermal sources and determine the magnetic reconnection
rate. Gamma-ray line images and spectra are presented as they relate
to determining the location, spectra, and angular distribution of
the accelerated ions and the temperature of the chromospheric target
material. Finally, we discuss the overall energy budget for two of
the larger events seen with RHESSI.
---------------------------------------------------------
Title: Solar Flares With two Hard X-ray Footpoints: a Statistical
Survey
Authors: Saint-Hilaire, P.; Krucker, S.; Lin, R. P.
2006AGUFMSH23A0331S Altcode:
Using RHESSI data and its unprecedented spectral and spatial coverage,
we have done a statistical survey of the spectral and spatial
characteristics of flares displaying two footpoints in hard X-rays
(HXR). We have manually selected 53 of RHESSI's best observed flares,
where both HXR footpoints could be imaged above 50 keV. Numerous
correlations could thus be investigated, providing additional
constraints to coronal particle acceleration models. Assuming both
footpoints are fed by the same electron injections, we will in
particular address the question as to whether differences in HXR
footpoint spectral indices and flux ratios can be attributed to a
difference in column density between the acceleration site and both
chromospheric footpoints.
---------------------------------------------------------
Title: A Study of Pitch Angle Distributions for Six Solar Impulsive
Electron Events
Authors: Wang, L.; Lin, R. P.; Li, G.; Krucker, S.
2006AGUFMSH43B1530W Altcode:
The pitch angle distributions (PADs) of solar impulsive electron events
measured at 1 AU provide important information on the solar release
of the electrons and their interplanetary transport. Using WIND/3DP
electron data, we investigate the energy and time dependence of the
PADs for six impulsive electron events with short time durations, low
pre-event background, good count statistics and the electron energy
range from ~0.5 keV to ~200 keV. The PADs of the pre-event background
are anisotropic below ~2 keV due to halo electrons continuously
streaming out from the Sun at these energies and isotropic above. In the
preliminary study of two events (1998 Aug 29 and 2002 Oct 20), we remove
such pre-event background. We find that the PAD width at half maximum
is roughly constant from ~0.5 keV to 10-13 keV. It remains below ~30
degrees and changes slightly through the rapid-rise, rapid-decay phase,
and it increases up to ~50-60 degrees at the slow-decay phase. We also
find that the PAD width above ~13-20 keV increases with both energy
and time. At the peak phase the width increases from ~40-50 degrees
at 30 keV to ~60-70 degrees at 200 keV. At 200 keV it increases from
~50-60 degrees at the rapid-rise phase to ~80 degrees at the slow-decay
phase. These properties suggest that, for these solar impulsive electron
events, the low-energy electrons propagate nearly scatter-free in the
interplanetary medium, while the high energy electrons experience
some energy-dependent scattering. Furthermore, at a fixed energy,
after electrons are scattered to different pitch angles, they travel
to 1 AU at different speeds parallel to the magnetic field line. We
analyze the pitch-angle dispersion and estimate the distance between
the scattering site and 1 AU. We find that for high-energy electrons
the average is ~0.03±0.06 AU.
---------------------------------------------------------
Title: Interplanetary Type II Intensity Emission as a Function of
the Heliospheric Position and Coronal Mass Ejection Speed
Authors: Aguilar-Rodriguez, E.; Bale, S. D.; Krucker, S.
2006AGUFMSH51A1464A Altcode:
Interplanetary Type II radio burst radiation results from the excitation
of plasma waves in the ambient medium by a coronal mass ejection
(CME) driven shock, propagating outward from the Sun. However, not
all disturbances are able to produce Type II emission. Factors such
as the speed of the disturbance and the properties of the medium
through which the disturbance propagates are fundamental to produce
Type II emission. Shock formation requires that the velocity of the
disturbance excedes the local fast magnetosonic speed. Some studies
have evaluated the radial behaviour of the fast magnetosonic speed as
well as the Alfven speed above active regions. By combining both white
light (SOHO/LASCO) and radio (Wind/WAVES) observations, we present
preliminary results on the Type II intensity emission as a function
of heliospheric position and CME speed. This might reveal the average
Alfven speed profile in the inner heliosphere which, in turn, would
tell us about the location of the solar wind acceleration. This work
is supported by the University of California Institute for Mexico and
the United States (UC MEXUS).
---------------------------------------------------------
Title: RHESSI Microflare Statistics: Energy Budget
Authors: Hannah, I. G.; Christe, S. D.; Krucker, S.; Lin, R. P.
2006AGUFMSH23A0348H Altcode:
We present analysis of all RHESSI microflares, covering GOES class
A through C , encompassing over 4 years of observations. Well over
10,000 microflares were found using our own flare finding algorithm,
that scans the 6-12 keV energy range, finding considerably more than the
standard RHESSI flare list. Each microflare is automatically analysed:
the source size is found by forward fitting the complex visibilities
and the spectral parameters (temperature, emission measure, power-law
index) are found by forward fitting a thermal plus non-thermal
spectral model. The combination of these allows us to compute the
thermal and non-thermal energy in each of the microflares. We present
the thermal and non-thermal energy distributions of these microflares
and how the ratio of these two energy components changes for different
characteristics of the microflares. This work is funded by NASA grant
NAS5-98033.
---------------------------------------------------------
Title: A Radio Burst and Its Associated CME on March 17, 2002
Authors: Yan, Y.; Pick, M.; Wang, M.; Krucker, S.; Vourlidas, A.
2006SoPh..239..277Y Altcode: 2006SoPh..tmp...78Y
In this study, we present a detailed analysis, based on multiwavelength
observations and magnetic field extrapolation, of a radio and X-ray
event observed on March 17, 2002. This event was accompanied by a
Coronal Mass Ejection (CME) observed by the Large-Angle Spectrometric
Coronagraph (LASCO) aboard SOHO. During the main event, the Reuven
Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) mission observed
a hard X-ray emission correlated in time with the development of a type
III burst group. The CME development, the hard X-ray emission, and the
type III burst group appear to be closely associated. The multifrequency
Nançay Radioheliograph (NRH) shows that the type III bursts are
produced at a distance from the active region that progressively
increases with time. Their emitting sources are distributed along
the western edge of the CME. We conclude the type III electron beams
propagate in the interface region between the ascending CME and the
neighboring open field lines. Due to the development of the CME,
this region becomes progressively highly compressed. By measuring,
at each frequency, the shift versus time of the type III positions, we
estimate that the electron density in this compression region increased
roughly by a factor of 10 over a few minutes. Another signature of
this compression region is a narrow white light feature interpreted
as a coronal shock driven by the CME lateral expansion.
---------------------------------------------------------
Title: High Energy Solar Physics from Lunar-Based Observatories
Authors: Emslie, G.; Dennis, B. R.; Holman, G. D.; Hudson, H. S.;
Krucker, S.; Lin, R. P.; Murphy, R.; Ryan, J. M.; Share, G. H.
2006AGUFMSM52A..05E Altcode:
High-energy and optical solar observations from large telescopes will
greatly improve our understanding of the physical processes responsible
for particle acceleration in the Sun and in other astrophysical
sources. In addition, such observations will allow us to identify
the conditions preceding solar eruptive events that are potentially
hazardous to astronauts and equipment on the Moon and on interplanetary
flights, and to power transmission and communications on the Earth. The
moon provides an exceptionally large and stable platform on which to
position instruments that can be used to observe the Sun (and other
astrophysical sources) at photon energies from < 1 keV to >
100 MeV. Uninterrupted measurements over extended periods of time
(1/2 lunar day or 14 days, which is also the duration of the East-West
passage of an active region on the Sun) would be possible, and the
gradual rotation rate of the Moon also allows horizon occultation
measurements (at a drift rate ~ 0.5 arc seconds/second) to be made.
---------------------------------------------------------
Title: Pre-Impulsive Hard X-Ray Emission from Coronal Sources in
X-Class Flares
Authors: Caspi, A.; Krucker, S.; Lin, R. P.
2006AGUFMSH43B1528C Altcode:
The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI)
has observed significant non- thermal emission up to ~70~keV from
coronal sources prior to the impulsive phase in some flares, which
has interesting implications for particle trapping and flare energy
release. Images and spectra during the pre- impulsive phase of the
GOES-class X4.8 event on 23~July~2002 suggests that the coronal
hard X-ray (HXR) emission is non-thermal, as analysis of the Fe
(~6.7~keV) and Fe/Ni (~8~keV) line complexes constrains the thermal
component to have little or no thermal continuum emission above
~15~keV. The over-the-limb X3.1 event on 24~Aug~2002 displays similar
spectral characteristics, and occultation of the footpoints places
the non-thermal HXR emission unambiguously in the corona. RHESSI
images of this event show an extended source below ~35~keV and a
compact looptop source above ~35~keV, possibly indicating trapping of
energetic electrons. Spatially-integrated spectra are well-fit by a
broken power-law and the spectral indices above and below the break
differ by ~2, suggesting an observation of the transition between
thin- and thick-target bremsstrahlung. The collisional lifetime of
non-thermal electrons is <10~sec at the inferred densities of
~1011~cm-3 and the rising HXR flux thus suggests continuous particle
injection. We apply the technique of Johns &Lin (1992) to invert the
pre-impulsive phase photon spectra and recover the original electron
energy spectra as a function of time, both for spatially-integrated
spectra and source-isolated spectra using imaging spectroscopy. We then
compare the inverted spectra with a forward-modeling analysis of the
photon spectra using the iron line analysis technique of Caspi &Lin
(2006) to constrain the thermal model. We compare the results for both
24~Aug~2002 and 23~Jul~2002, and discuss the implications for electron
acceleration and flare plasma heating in the corona during this period.
---------------------------------------------------------
Title: Occulted Hard X-ray Flare Observations with Sentinels
Authors: Krucker, S.; Hannah, I.; Hudson, H. S.; Hurford, G.; Lin,
R. P.
2006AGUFMSH53C..07K Altcode:
Multi-spacecraft hard X-ray observations provided by the Sentinel
mission will give different view angles of solar flares, including the
possibility of partially occulted observations. Occultation of the main
flare emission by the solar limb allows us to study fainter coronal
emission that otherwise would be hidden by the limited dynamic range
of the observations. Furthermore, it will allow to measure directivity
and reconstruct the 3 dimensional geometry of X-ray sources. We use
statistical results from RHESSI observations to derive the probability
of occulted flare observations with Sentinels. A statistical study of
19 giant flares with fast CMES (v>1500 km/s) occurring behind the
solar limb is presented. We find that all events occurring 45 degrees
or less behing the limb (corresponding to an occultation height h <
0.4~Rsun) show X-ray enhancements, while the three events occurring
more than 50 degrees (h>1~Rsun) behind the limb do not. This result
indicates that on the average, two Sentinels will see the total flare,
while one spacecraft will see the flare partially occulted. The most
prominent event in our RHESSI survey will be discussed in detail. It
has an occultation high of 0.3 solar radius, but still shows emission
up to 80 keV. The observed hard X-ray source is extended, with a size
of about 200", and moves upwards with a velocity of ~1000 km/s.
---------------------------------------------------------
Title: Observations of a soft X-ray rising loop associated with a
type II burst and a coronal mass ejection in the 03 November 2003
X-ray flare
Authors: Dauphin, C.; Vilmer, N.; Krucker, S.
2006A&A...455..339D Altcode:
Context: .We report observations of a type II burst - the signature
of a shock wave - starting at the unusual high frequency of 650 MHz
during the 03 November 2003 flare. This flare is associated with
the propagation of a soft X-ray coronal loop and with a coronal mass
ejection (CME).<BR /> Aims: .We study in this paper the origin of the
shock wave in the low corona and present a kinematics analysis of the
soft X-ray coronal loop and of the CME observed a few tens of minutes
later.<BR /> Methods: .We study in this paper the spatial and temporal
relation between the soft X-ray rising loop observed by the GOES soft
X-ray Imager (GOES/SXI), the type II sources observed by the Nançay
Radio Heliograph (NRH) and the CME observed by LASCO (Large Angle
and Spectroscopic Coronograph).<BR /> Results: .This analysis shows
that the type II burst observed during this flare is driven by the
X-ray rising loop. Furthermore, the kinematics analysis of the X-ray
coronal loop and CME shows that the two structures are related.<BR />
Conclusions: .The direct comparison of the type II sources with the
GOES/SXI observations clearly shows that the type II burst is ignited
by the shock wave created ahead of the rising X-ray loop. Finally, we
propose to interpret these different observations in the framework of
an ascending magnetic structure with a reconnecting process operating
in the tail of this ascending structure.
---------------------------------------------------------
Title: Statistical Study of Hard X-ray Footpoint Motions in Solar
Flares
Authors: Yang, Ya-Hui; Krucker, S.; Lin, R. P.; Ip, W. H.
2006SPD....37.1312Y Altcode: 2006BAAS...38..242Y
Hard X-ray (HXR) source locations in solar flares are thought to
be the sites where flare-produced energetic electrons precipitate
along reconnected magnetic loops and then lose their energy via
collisions with ambient plasma in the lower atmosphere. According to a
standard reconnection model, HXR footpoints should separate due to the
successive reconnections. However, several previous studies showed that
the footpoints move along ribbons or move approach to each other. To
understand the characteristics of footpoint motions more systematically,
a statistical survey of HXR footpoint motions in M-class and X-class
flares is done using RHESSI observations. Such investigation provides
a good way to test flare models.
---------------------------------------------------------
Title: Gamma-Ray Imaging of the 2003 October/November Solar Flares
Authors: Hurford, G. J.; Krucker, S.; Lin, R. P.; Schwartz, R. A.;
Share, G. H.; Smith, D. M.
2006ApJ...644L..93H Altcode:
We present RHESSI imaging of three flares (2003 October 28 and 29
and November 2) in the 2.223 MeV neutron-capture gamma-ray line with
angular resolution as high as 35". Comparisons of imaged and spatially
integrated fluences show that in all cases most, if not all, of the
emission was confined to compact sources with size scales of tens
of arcseconds or smaller that are located within the flare active
region. Thus, the gamma-ray-producing ions appear to be accelerated
by the flare process and not by a widespread shock driven by a fast
coronal mass ejection. The 28 October event yielded the first such
image to show double-footpoint gamma-ray line sources. These footpoint
sources straddled the flaring loop arcade but were displaced from the
corresponding 0.2-0.3 MeV electron-bremsstrahlung emission footpoints by
14" and 17" +/- 5". As with the previously studied 2002 July 23 event,
this implies spatial differences in acceleration and/or propagation
between the flare-accelerated ions and electrons.
---------------------------------------------------------
Title: A Statistical Study of Solar Energetic Electron Events over
one Solar Cycle
Authors: Wang, Linghua; Lin, R. P.; Krucker, S.; Mason, G. M.
2006SPD....37.2903W Altcode: 2006BAAS...38Q.256W
Impulsive solar electron events are the most common impulsive solar
particle acceleration phenomenon, often observed at 1-100 keV. Most of
these events are associated with small SXR bursts and type III radio
bursts, while unaccompanied by HXR bursts. We survey the WIND/3DP
electron data ( 1-400 keV) from 1995 through 2005 and identify 1100
solar energetic electron events over one solar cycle. The number of
electron events shows a time variation similar to sunspots and SXR
flares. Taking into account the effect of instrumental background,
we find a quite power-law relationship between the event number N
and the event peak flux J: dN/dJ=A×J^(-γ). The integration over the
electron flux estimates that >= 10^4 events/year occur the whole Sun
near solar maximum. Meanwhile, we survey ion data ( 0.02-10 MeV/nuc)
observed by the ACE/ULEIS from 1998 to 2005. We find a good correlation:
80 % of impulsive electron events are accompanied by MeV/nuc, 3He-rich
(3He/4He >= 1%) ion emissions; >93% of 3He-rich ion events
with a clear velocity dispersion appear to have an impulsive electron
event. Finally, We present the correlation between the peak flux of
impulsive electrons and the count of 3He and 4He ions.
---------------------------------------------------------
Title: Spectral Evolution of Coronal Hard X-ray Sources during
Solar Flares
Authors: Krucker, Sam; Lin, R. P.
2006SPD....37.2706K Altcode: 2006BAAS...38..254K
Hard X-ray (HXR) emissions during solar flares are most prominent
at chromospheric footpoints of flare loops which reveal where
flare-accelerated electrons lose their energy by collision. The lower
density in the corona makes it much more difficult to detect coronal
HXR emissions, but coronal HXR sources directly reveal insights into
the acceleration region (e.g. Masuda et al. 1994). Observations with
Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) give for
the first time detailed spatial and spectral observations in the HXR
range. Initial results of a few events reveal at least two different
spectral behavior, possibly indicating two different acceleration
mechanisms: (1) Coronal HXR sources with a 'soft-hard-soft' behavior
(Battaglia & Benz 2006), and (2) sources that show spectral
hardening in time, i.e. a 'soft-hard-harder' behavior (Krucker et
al. 2005). After a short review of recent RHESSI observations, we will
present statistical results on the spectral evolution of coronal HXR
sources of 50 partly occulted limb flares seen by RHESSI.
---------------------------------------------------------
Title: Gamma-ray And Hard X-ray Imaging Of The 20 January 2005
Solar Flare
Authors: Hurford, Gordon J.; Krucker, S.; Lin, R. P.; Schwartz, R. A.;
Share, G. H.; Smith, D. M.
2006SPD....37.2804H Altcode: 2006BAAS...38..255H
Gamma-ray imaging provides a direct indication of the spatial properties
of accelerated ions near the Sun. We present the results of RHESSI
gamma-ray imaging of the 20 January 2005 solar flare. While maps
in the electron-bremsstrahlung dominated >250-500 keV band show
two comparable sources coinciding with footpoints seen with TRACE,
corresponding 35 arcsecond resolution images in the neutron-capture
line at 2.223 MeV show the ion-generated source to be preferentially
associated with just one of these two footpoints. This result is
supported by imaging with 183 arcseconds resolution in four gamma-ray
continuum bands between 600 keV and 8 MeV. The relative locations
of their centroids show relative displacements that correlate
with the relative contribution of electrons and ions at different
energies. Comparison of the intensity of the imaged neutron-capture
line image with the spatially integrated line intensity obtained
spectroscopically is consistent with all of the neutron-capture line
flux originating in a compact source (diameter <20 arcseconds). While
shock-acceleration high in the corona might be expected to produce a
large, diffuse source, the observed compact source indicates that the
gamma-ray producing ions were flare-accelerated. These flare-integrated
results are presented in the context of a hard X-ray movie showing the
movement and time-evolution of the lectron-associated footpoints. This
work has been supported by NASA under NAS5-98033.
---------------------------------------------------------
Title: Measuring the Temperature of Hot Flare Plasma Using RHESSI
Fe and Fe/Ni Line Observations
Authors: Caspi, Amir; Krucker, S.; Lin, R. P.
2006SPD....37.2704C Altcode: 2006BAAS...38..253C
Solar flares of GOES class M or X can produce "super-hot" (temperatures
of 30 MK or higher) thermal plasmas. Continuum emission from such
flare plasmas can dominate the X-ray spectrum up to 20-50 keV,
making it difficult to separate from the non-thermal bremsstrahlung
from accelerated, tens of keV electrons. Observations suggest that
flare-accelerated electrons contribute significantly to heating of
the thermal plasma (manifesting as the empirical "Neupert effect"),
although recent studies also suggest that other heating mechanisms may
be equally significant. Accurately characterizing the thermal plasma
provides information about the heating and cooling mechanisms involved,
and also improves our understanding of the non-thermal emission. The
Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) observes
solar photons with energies >3 keV, with a spectral resolution
of 1 keV FWHM, and is especially sensitive to flare plasmas above 10
MK. RHESSI observes both the thermal continuum and the Fe and Fe/Ni
line complexes at 6.7 and 8 keV. The fluxes and equivalent widths of
the line complexes are strongly temperature-dependent and are useful as
probes of the thermal plasma. We present an analytical method using the
ratio of the line fluxes (Fe to Fe/Ni) as a temperature diagnostic to
compare with and constrain the continuum temperature measurements. We
discuss results of this analysis on two X-class flares (23/Jul/2002 and
02/Nov/2003) and compare our method to the equivalent width method of
Phillips et al. (2005). We also present an application of this analysis
by examining the "pre-impulsive" phase of two flares (23/Jul/2002 and
24/Aug/2002) which both appear to show strong non-thermal emission
from a coronal source.
---------------------------------------------------------
Title: Hard X-rays Associated With Solar Type III Radio Bursts
Authors: Christe, Steven; Krucker, S.; Lin, R. P.
2006SPD....37.2904C Altcode: 2006BAAS...38..256C
During a period of 12 minutes on 19 July 2002 14:23-14:35 UT, the WAVES
instrument on the Wind spacecraft observed interplanetary type III
radio bursts approximately every 2 minutes, and each was accompanied
by a hard X-ray burst (12-15 keV) observed by RHESSI. We investigate
the emission mechanism for the observed hard X-rays for each event. We
find that the four smallest X-ray events are inconsistent with emission
from a thermal plasma or a nonthermal beam suggesting that the observed
emission is related to the acceleration mechanism of type IIIs.
---------------------------------------------------------
Title: A study of the solar injection for nine scatter-free impulsive
electron events
Authors: Wang, L.; Lin, R. P.; Krucker, S.
2006AGUSMSH41A..03W Altcode:
Using WIND/3DP electron data from 1996 through 2005, we investigate
the injection at the Sun of nine scatter- free impulsive electron
events with good count statistics, the electron energy range from
~1keV to above 60 keV and the simple association with the solar and
interplanetary type III radio burst observed by WIND/WAVES. Taking into
account the effects of interplanetary propagation and instrumental
response, we find that the observed time profiles of electron fluxes
at all energies fit quite well to triangular injections at the Sun
with equal rise and fall times. We find double injections: One for
<~ 1 to 6-9 keV electrons begins 4.6± 2.6 min before the start of
type III burst and lasts for an averaged duration of 90 min, while a
second for electrons above 9-13 keV starts 6.9±0.7 min after the start
of the type III burst and lasts for an average of 24 min. Electrons
of low-energy injection are likely to be the source of the type III
burst. The majority of events might have a B or C class GOES SXR flare
related to the start of low-energy electron injection, three might have
a B or C class SXR flare related to the start of high-energy injection,
and five might have a west- limb CME with the velocity <~600km/s.
---------------------------------------------------------
Title: Imaging of a radio type II burst relative to the CME shock
Authors: Krucker, S.; Dauphin, C.; Vilmer, N. R.
2006AGUSMSH32A..03K Altcode:
Radio type II emission is thought to be produced by energetic electrons
that are accelerated at shocks in the solar corona and in interplanetary
space. Simultaneous imaging observations of both, the radio emission
and the shock, are rare, especially in the lower corona. Here, we
present imaging observations in radio waves and soft X-rays of a type
II burst that occurred on November 3, 2003 during a GOES X3 flare. The
radio type II burst starts at an unusally high frequency (600 MHz) and
can therefore be imaged with the Nancay radio heliograph. Simultaneous
soft X-ray observations provided by GOES SXI show a faint, fast moving
(~800 km/s) loop-like emission in the lower corona (0.1-0.3 solar
radius above the photosphere) that spatially and temporally correlates
with the Coronal Mass Ejection (CME) later seen in white-light with
SOHO/LASCO. Therefore, the observed SXR front is most likely emitted
when the CME shock is in the lower corona. The radio type II emission
is observed to occur infront of the SXR emission and is only seen from
a single location, but not all along the shock front.
---------------------------------------------------------
Title: Solar HXR Emission and Energetic Electron events seen at 1~AU:
A temporal and spectral comparison
Authors: Krucker, S.; Christe, S. D.; Kontar, E. P.; Lin, R. P.
2006AGUSMSH41A..04K Altcode:
Onset time analysis of impulsive electron events at 1 AU suggest that
there are two classes of events: (1) events with a solar release time
in close temporal agreement with solar hard X-ray (HXR) flares, and (2)
events with a delayed solar release time relative to the HXR emission
that seem to be accelerated later possibly at coronal shocks. Simple
onset time analysis, however, are criticized for not taking propagation
effect properly into account producing falsely delayed release
times. Here we present a further test of the existence of two classes
of events by comparing the in-situ observed electron spectra with the
solar HXR photon spectra remotely observed by RHESSI. For events with
a good temporal agreement, a clear correlation between the spectra has
been reported indicating a common acceleration mechanism. Delayed events
have not yet been analyzed. However, if indeed a second acceleration
mechanism independent of the HXR flare exists that release energetic
electron into interplanetary space, no correlation between the in-situ
observed electron spectra and the HXR photon spectra is expected to
be found.
---------------------------------------------------------
Title: Evidence for double injections in scatter-free solar impulsive
electron events
Authors: Wang, L.; Lin, R. P.; Krucker, S.; Gosling, J. T.
2006GeoRL..33.3106W Altcode: 2006GeoRL..3303106W
We investigate the injection at the Sun for three scatter-free impulsive
electron events (7 August 1999, 28 June and 22 August 2000) observed
from ~0.4 to 300 keV by the WIND 3DP instrument. Taking into account
the interplanetary scatter-free propagation and instrumental effects,
we find that the observed time profiles of electron fluxes at all
energies fit well to triangular injections at the Sun with equal rise
and fall times. We find two distinct injections: that of ~0.4 to 6-9
keV electrons begins 9.1 +/- 4.7 min before the start of the type
III radio burst and lasts for 50-300 min, while that of ~13 to 300
keV electrons starts 7.6 +/- 1.3 min after the start of the type III
burst and lasts for a factor of 5-10 times shorter. Electrons of the
low-energy injection are likely to be the source of the type III radio
bursts, and the delayed high-energy injection occurs when the associated
CME passes altitudes of ~1-6 R<SUB>$\odot$</SUB>. The observed electron
energy spectra fit to double power-law with a downward break at ~40-50
keV, but exhibit a smooth power-law across the transition (~6-13 keV)
between the two injections, suggesting the low-energy injection may
provide the seed electrons for the high-energy injection.
---------------------------------------------------------
Title: Investigations of spatially resolved 1.28 GHz radio activity
associated with solar X-Ray micro flares
Authors: Sawant, H. S.; Krucker, S.; Madsen, F. R. H.; Kane, S.;
Karlicky, M.; Ananthakrishnan, S.; Subramanian, P.
2006cosp...36.3365S Altcode: 2006cosp.meet.3365S
On 20 and 22 November 2005 Solar observations at 1280 MHz were carried
out from 04 00 to 11 30UT at the Giant Metrewave Radio Telescope --
GMRT - with time resolution of 512 ms and spatial resolution of sim
5 arc sec During GMRT observation time on both days micro flares have
been observed by RHESSI satellites in the range of 3 -- 10 keV Detailed
investigation of the X-ray emission shows the presence of both thermal
and non-thermal components Imaging analysis of one of the micro flares
in x ray shows the RHESSI data can produce image with resolution of 7
Here we report an investigation of simultaneous 1 28 GHz radio activity
and 3-10 keV X-ray observations of solar X-ray micro flares as observed
on 20 November during the interval 08 00-08 30 UT
---------------------------------------------------------
Title: Radio and magnetic field constraints on SEP propagation from
the Sun to Earth
Authors: Klein, K. -L.; Krucker, S.; Hoang, S.; Lointier, G.
2006cosp...36.1787K Altcode: 2006cosp.meet.1787K
Energetic particles accelerated in the solar corona can be detected
in space provided an appropriate magnetic connection exists The
interplanetary magnetic field is usually described by an Archimedian
spiral Parker spiral starting at a source surface sim 2 5 R odot
from the Sun s centre While statistically speaking impulsive solar
energetic particle events SEP events are associated with flares at
longitudes close to the root of the Parker spiral in the western solar
hemisphere there is some scatter sometimes deviations of more than 10
heliocentric degrees We present a sample of impulsive electron events
where we identify coronal and interplanetary paths of the particles
through their type III radio emission We identify flux tubes into which
electrons are injected at the Sun by imaging their radio emission with
the Nan c c ay Radioheliograph below 0 5 R footnotesize odot above
the photosphere The arrival of the electrons at Earth is traced by
kilometric radio emission and Langmuir waves detected by the WAVES
radio receiver aboard the it Wind mission and by the 3DP particle
detector We compare the propagation paths of the electrons with the
nominal Parker spiral and with the magnetic field below the source
surface inferred from the potential field and source surface model
developed by Schrijver DeRosa 2003 whose results are available within
the SolarSoft package First results suggest that the type III emission
is a reliable tracer of the magnetic field topology below the source
surface even in cases when the parent flare
---------------------------------------------------------
Title: The magnetic field configuration for 17-3-2002 radio bursts
and associated CME
Authors: Yan, Y.; Pick, M.; Wang, M.; Krucker, S.
2006cosp...36.3130Y Altcode: 2006cosp.meet.3130Y
The coronal magnetic field configuration is important for understanding
the energy storage and release processes that account for flares
and or CMEs Using a model which is based on the work for potential
magnetic field problems that only applies the condition at infinity
with the boundary condition on the solar surface specified we have
reconstructed the global coronal mangetic field structure for 17
March 2002 radio bursts and CME event It is found that the March
17 2002 event occurred in a complex multipolarity configuration of
the magnetic field due to the localization of AR 9871 inside an old
remnant region and to the local inclusion of small interacting loops
around the leading negative polarity of this AR and open field lines
from negative polarities The small interacting loops can be inferred
from RHESSI SXR HXR observations The origin and evolution of the radio
sources during the CME flare processes on 17 March 2002 in AR 9871 can
be explained under such reconstructed magnetic field configurations
The successive type III bursts are along the extrapolated open field
lines and the continuum sources are confined in closed loops The west
leg of the CME is approximately in the direction of the open fields
suggesting the progressive interactions with these open field lines The
close coupling among the CME the hard X-ray emission and the group of
type III bursts and more particularly the close temporal relationship
between the hard X-ray and type III emissions may be due to the dynamic
processes between bipolar loops and the surrounding
---------------------------------------------------------
Title: Properties of acceleration sites in active regions as derived
from heavy ion charge states
Authors: Kartavykh, Y.; Dröge, W.; Klecker, B.; Möbius, E.; Popecki,
M.; Mason, G.; Krucker, S.
2006cosp...36.1248K Altcode: 2006cosp.meet.1248K
Charge states of heavy ions in solar energetic particle SEP events are
determined by both the plasma conditions in the acceleration region
and propagation effects The steep increase of the ionic charge of
heavy ions as observed in all 3He- and Fe-rich SEP events suggests that
stripping in a dense environment in the low corona is important in all
these events The observed charge states and energy spectra of iron ions
are used to infer the plasma conditions in the acceleration region by
modelling the observations with a combined acceleration and propagation
model that includes charge stripping acceleration coulomb losses
and recombination in the corona and interplanetary propagation The
interplanetary propagation includes anisotropic pitch-angle scattering
on magnetic irregularities as well as magnetic focusing convection and
adiabatic deceleration in the expanding solar wind To accurately derive
the value of the scattering mean free path of particles the intensity
profiles and anisotropy data from ACE and Wind spacecraft were used
The comparison of the deduced parameters of the acceleration region
with coronal density profiles shows that the acceleration of these
ions takes place in closed magnetic structures in the low corona
---------------------------------------------------------
Title: A study of the solar injection for nine scatter-free impulsive
electron events
Authors: Wang, L.; Lin, R. P.; Krucker, S.
2006cosp...36.1406W Altcode: 2006cosp.meet.1406W
Using WIND 3DP electron data from 1996 through 2005 we investigate
the injection at the Sun of nine scatter-free impulsive electron
events with good count statistics the electron energy range from sim
1keV to above 60 keV and the simple association with the solar and
interplanetary type III radio burst observed by WIND WAVES Taking into
account the effects of interplanetary propagation and instrumental
response we find that the observed time profiles of electron fluxes at
all energies fit quite well to triangular injections at the Sun with
equal rise and fall times We find double injections One for lesssim
1 to 6-9 keV electrons begins 4 6 pm 2 6 min before the start of type
III burst and lasts for an averaged duration of 90 min while a second
for electrons above 9-13 keV starts 6 9 pm 0 7 min after the start
of the type III burst and lasts for an average of 24 min Electrons
of low-energy injection are likely to be the source of the type III
burst The majority of events might have a B or C class GOES SXR flare
related to the start of low-energy electron injection three might have
a B or C class SXR flare related to the start of high-energy injection
and five might have a west-limb CME with the velocity lesssim 600km s
---------------------------------------------------------
Title: A statistical study of solar energetic electron events over
one solar cycle
Authors: Wang, L.; Lin, R. P.; Krucker, S.; Mason, G. M.
2006cosp...36.1404W Altcode: 2006cosp.meet.1404W
Using WIND 3DP electron data from 1995 through 2005 we study the number
distribution of solar energetic electron events over one solar cycle We
find a quite power-law relationship between the event number N and the
event peak flux J dN dJ A cdot J - gamma Taking into account the effect
of instrumental background we estimate the yearly number of electron
events from the integration using the minimal peak flux observed at
3 and 40 keV We find that both the observed and estimated numbers of
electron events show a time variation similar to the sunspot numbers
We also present the statistical study of electron events accompanied
by low-energy sim MeV nucleon and highly 3 He-rich ion emissions and
estimate the effects of electron scattering in the interplanetary
medium from in situ observations at 1 AU
---------------------------------------------------------
Title: Solar flare imaging in X-rays and g-rays
Authors: Krucker, S.
2006ilws.conf...49K Altcode:
Recent results of solar flare imaging at X-ray and γ-ray energies
are briefly summarized using observations from the Reuven Ramaty High
Energy Solar Spectroscopic Imager (RHESSI) Small Explorer mission.
---------------------------------------------------------
Title: Stereoscopic observations of Coronal Hard X-ray flare emissions
with Ulysses and RHESSI
Authors: Krucker, S.; Trottet, G.; McTiernan, J. M.; Lin, R. P.
2005AGUFMSH13A0289K Altcode:
One of the major findings of the Yohkoh mission is the existence of
coronal hard X-ray (HXR) sources above hot flare loops. The emission
mechanism that produces these coronal sources is not understood. Coronal
HXR sources are rather rarely observed, possible because of limited
dynamic range of our observations that makes it difficult to see faint
coronal sources next to bright footpoint emissions. For flares occurring
just behind the solar limb, the footpoint emission can be occulted
and coronal sources can therefore be observed independently. In this
work, we were using Ulysses and RHESSI hard X-ray observations that
often provides largely different flare view angles to study coronal HXR
sources in partly occulted flares. Out of 112 flares seen by Ulysses, we
find 5 flares that are partly occulted (5 to 15 degrees) from the RHESSI
point of view, but are fully seen (i.e. on disk) by Ulysses. The flare
with the best statistics (April 4, 2002, 16UT) reveals the existence of
a coronal HXR source that is about 12±2 times fainter than the total
HXR emission seen by Ulysses. The coronal source seen with RHESSI shows
a relatively steep, progressively hardening power law spectra clearly
favoring a non-thermal interpretation. The power law index decreases
from ~4.5 at the beginning of the main peak to around 3.0 at the end
with a value of ~3.5±0.1 at the peak. Reliable Ulysses spectra can
only be derived during the peak of the event showing a power spectra
slightly stepper around 3.0±0.2. The relatively small difference
in the power law indices clearly excludes a simple explanation of
thick target emission in footpoints and thin target emission in the
corona. The observed spectral hardening can be partly explained by
trapping of electrons in the corona.
---------------------------------------------------------
Title: Gamma-ray Imaging of the 2005 January 20 Solar Flare
Authors: Hurford, G. J.; Krucker, S.; Lin, R. P.; Schwartz, R. A.;
Smith, D. M.
2005AGUFMSH23A0313H Altcode:
Solar gamma-ray emission provides the only available observational
perspective on flare-accelerated ions near the Sun. Inelastic collisions
between accelerated ions and the ambient solar atmosphere produces
prompt line emission, a Doppler-broadened gamma-ray continuum as well
as narrow lines due to positron annihilation (511 keV) and neutron
capture (2.2 MeV). In addition to this ion-related emission, there
is a significant electron-bremsstrahlung contribution from electrons
accelerated to energies up to 10 MeV and beyond. The 2002 January 20
event provided one of the hardest ion and electron spectra observed
to date. In this paper, we report on the results of gamma-ray imaging
from ~100 keV through 7 MeV, including the gamma-ray continuum and
neutron-capture line. Emphasis will be on the spatial distribution of
the emission as a function of energy.
---------------------------------------------------------
Title: Late-phase hard X-ray emission from flares
Authors: Hudson, H. S.; Krucker, S.
2005AGUFMSH13A0290H Altcode:
In a few major flare events observed by RHESSI, we see hard X-ray
signatures long after (tens of minutes to more than one hour) the
impulsive-phase onset. A prototype for such phenomena was the flare
of March 31, 1969 (Frost and Dennis, 1971). Recent RHESSI examples
include January 19, 2005, a GOES X1.5 event with gradual variations, and
September 7, 2005 an X17 event with more impulsive variability. RHESSI
images show footpoint emissions in both cases; the spectra are hard and
become harder with time as in the Frost-Dennis event. The existence
of impulsive variability and footpoint emission allows us to discuss
trapping and injection. We discuss the morphology of these events,
including other examples such as April 21, 2002 (X1.5).
---------------------------------------------------------
Title: HXR spectral evolution and SEP production in the January 2005
X-class flares
Authors: Saldanha, R.; Krucker, S.; Lin, R. P.
2005AGUFMSH23A0312S Altcode:
The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI)
was used to study non-thermal hard X-ray spectra of four solar X-class
flares of January 2005 (January 15th, 17th, 19th and 20th). Excluding
the January 19th event, the other three flares were found to be
associated with Solar Energetic Particles (SEPs). The three SEP related
flares show progressive spectral hardening over individual flux peaks
(a soft-hard-harder spectral evolution) with spectral power-law indices
ranging from 4.0 to 2.0, while the January 19th flare displayed
a more typical soft-hard-soft spectral evolution in the main peak
(i.e. spectral power law indices are anti-correlated with the HXR
flux). These findings are consistent with earlier results of Kiplinger
(1995).
---------------------------------------------------------
Title: Hard X-ray Emission From Coronal Electron Beams Associated
With Type III Radio Bursts
Authors: Saint-Hilaire, P.; Krucker, S.; Lin, R. P.
2005AGUFMSH31A..03S Altcode:
For the first time, a concerted effort to find X-ray emission from Type
III-generating electron beams propagating in the solar corona has been
undertaken. Spatially-integrated and imaging spectroscopy of partial
thick-target hard X-ray bremsstrahlung emission from electron beams
propagating outwards in the solar corona have been numerically modeled,
and folded through the RHESSI (and GOES) instrumental response. The
extensive RHESSI simulation software has been used to evaluate the
possibility of detection of such X-rays. Under the assumption that
Type III radio bursts are produced by the same kind of electron beams
that generate the usually-observed flare hard X-rays (in our case,
we are interested in beams going outwards rather than precipitating
in the chromosphere), it is clear that they should be observable in
X-rays with RHESSI, at least when the flare is occulted (footpoints
behind the solar limb). I will present and discuss observations made
with RHESSI and radio observatories.
---------------------------------------------------------
Title: RHESSI X-ray and Gamma-ray observations of the January 20,
2005 event
Authors: Krucker, S.; Hurford, G. J.; Lin, R. P.
2005AGUFMSH21A..01K Altcode:
Spacecraft and ground-based solar observations of the January 20, 2005
flare are summarized with a focus on X-ray and Gamma-ray observations
from the Reuven Ramaty High Energy Solar Spectroscopic Imager
(RHESSI). X-ray and Gamma-ray observations are excellent diagnostics
of accelerated particles: Accelerated electrons colliding with the
ambient solar atmosphere produce bursts of bremsstrahlung hard X-rays;
and collisions of accelerated ions with the atmosphere result in a
complex spectrum of narrow and broad gamma-ray lines. In the January 20
flare, HXR emission is observed from footpoints of the flare loop. The
statistics in the 2.2 MeV line emission is rather poor and a detailed
spatial comparison with HXR images is inconclusive. The spectral
evolution in HXRs during the main peak shows hardening during the event
(so-called 'soft-hard-harder' behavior) as often observed from flares
related to solar energetic particle events (Kiplinger 1995). The flare
accelerated proton spectrum derived from the Gamma-ray observations is
flat with a power law index of 2.3±0.3 and is surprisingly similar to
the insitu observed proton spectrum near 1~AU (2.1±0.1). This suggest
that the insitu observed protons are possibly flare associated and not
necessarily shock accelerated. We will discuss this possibility also
considering the relative timing between flare emissions, the coronal
mass ejection, and the onset of solar energetic particles observed at
1 AU.
---------------------------------------------------------
Title: Marnetic and Energy Characteristics in Two-Ribbon X-Class
Flares
Authors: Yang, Y.; Ip, W.; Krucker, S.; Lin, R.
2005AGUFMSH13A0271Y Altcode:
Several two-ribbon X-class flares which occurred near the solar disk
center are investigated in this study. Some specific sub-regions within
an active region are selected to estimate the change in magnetic
flux from pre-flare to post-flare states. The high-cadence (1-min
averaged) MDI magnetogram provides comprehensive photospheric magnetic
measurement during these flares. The extreme ultraviolet observations
in the wavelengths of 171, 195, and 1600 nm obtained by TRACE are used
to search for the correlation of magnetic changes in the corona and
transition region. We also used the RHESSI data to identify the hard
X-ray source regions and compare them with the magnetic observations. We
found that the magnetic fluxes within or in the vicinity of ribbons
can change permanently after a flare eruption. Transient magnetic
changes are also found in sub-regions. The characteristics of these
X-class flares will be discussed in detail in this presentation.
---------------------------------------------------------
Title: Solar Impulsive Electron Events and RHESSI Hard X-ray Emission
Authors: Christe, S.; Krucker, S.; Wang, L.; Lin, R. P.
2005AGUFMSH31A..04C Altcode:
Solar impulsive electron events (SIEEs) are often related to radio
type III bursts and are thought to be accelerated in the solar
corona. It is therefore expected that they should produce X-rays
through bremmsstrahlung whose flux is dependent on the height of the
accelerator. Previous studies have shown that low energy SEIIs appear to
be the source of type III radio bursts. The Reuven Ramaty High Energy
Solar Spectroscopic Imager (RHESSI) provides unique sensitivity in the
3-15 keV energy range, with an effective area ~100 times larger than
similar past instruments. RHESSI is therefore uniquely suited to study
small events. The 3-D Plasma and Energetic Particle Instrument (3DP) on
the WIND spacecraft measures in-situ energetic particles with energies
from ~0.4 to 300 keV. WIND has observed a total of 547 SIEEs since
the launch of RHESSI. A total of 303 of these events have simultaneous
RHESSI observations and 444 events show an associated type III radio
burst. We concentrate on low energy events as observed by 3DP (SEIIs
energy < 20 keV) with an associated radio type III and simulatenous
RHESSI observations. We find a total of 68 such events. We compare
the X-ray producing and SEII particle populations and the expected to
observed X-ray flux based on the acceleration height derived from the
starting frequency of the metric type III observations. This work was
supported by GSRP Grant NNM-05ZA12H and NASA contract NAS5-98033.
---------------------------------------------------------
Title: Pre-Impulsive Hard X-Ray Emission from Coronal Sources in
X-Class Flares
Authors: Caspi, A.; Krucker, S.; Lin, R. P.
2005AGUFMSH13A0285C Altcode:
The GOES-class X4.8 event on 23 July 2002 exhibits significant hard
X-ray emission prior to the impulsive phase of the flare. Images from
the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI)
show that most of the emission appears to originate from a coronal
source. Analysis of the Fe (~6.7 keV) and Fe/Ni (~8 keV) line complexes
seen in RHESSI spectra suggests that there is little or no thermal
continuum emission above ~15 keV, thus implying that non-thermal
electron bremsstrahlung dominates the spectrum even to low energies. The
X3.1 event on 24 Aug 2002 displays similar characteristics, and
its position at the west limb occults the footpoints and shows
that the bulk of the HXR emission unambiguously originates in the
corona. Preliminary analysis shows that the pre-impulsive spectra
are well-fit by a broken power-law characteristic of non-thermal
emission. The spectral indices above and below the break energy differ
by ~2, suggesting that we may be observing the transition between thin-
and thick-target bremsstrahlung. The break energy increases with time,
which suggests an average electron pitch angle of ~45°, assuming an
increasing column density derived from the thermal emission measure. We
analyze and compare the RHESSI X-ray spectra for the pre-impulsive
phases of both 24 Aug 2002 and 23 July 2002, using the Fe and Fe/Ni
line complexes to constrain the thermal parameters. We derive the
time-varying characteristics of the electron populations and discuss
the implications for acceleration and heating of electrons in the
corona during this period.
---------------------------------------------------------
Title: A Study of the Solar Injection for Eleven Impulsive
Electron/3He-Rich Sep Events
Authors: Wang, L.; Lin, R. P.; Krucker, S.; Mason, G. M.
2005ESASP.592..457W Altcode: 2005soho...16E..81W; 2005ESASP.592E..81W
No abstract at ADS
---------------------------------------------------------
Title: The Relationship between Synchrotron and Bremsstrahlung
Emission of Nonthermal Electrons during a Solar Flare. A detailed
study during the August 30, 2002 X1.5 event
Authors: Giménez de Castro, C. G.; Silva, A. V. R.; Trottet, G.;
Krucker, S.; Costa, J. E. R.; Kaufmann, P.; Correia, E.; Lüthi, T.;
Magun, A.; Levato, H.
2005AIPC..784..566G Altcode:
Synchrotron emission from nonthermal electrons has a strong dependence
on the magnetic field of the medium. On the contrary, Bremsstrahlung
emission does not depend on the magnetic field. The simultaneous
observations of both forms of radiation may give us clues about local
magnetic field configuration. In this report we use the optically thin
part of the radio spectrum during the microwave maximum of the flare
occurred on August 30, 2002, at 1328 UT to determine different mean
magnetic field intensities and nonthermal electron density distributions
compatible with the observed data. Assuming that the same electrons
emit by coulomb interactions, the obtained distributions are used to
compute the photon spectrum of the X-Ray emission by Bremsstrahlung
and the spectra are compared with observations obtained by instruments
on board the RHESSI satellite. We discuss the effects of the trapping
on the Bremsstrahlung emitted radiation, giving constraints on both
magnetic field intensity and trapping time.
---------------------------------------------------------
Title: Solar energetic electrons related to the 28 October 2003 flare
Authors: Klassen, A.; Krucker, S.; Kunow, H.; Müller-Mellin, R.;
Wimmer-Schweingruber, R.; Mann, G.; Posner, A.
2005JGRA..110.9S04K Altcode: 2005JGRA..11009S04K
We investigate the solar origin of near-relativistic electrons
and protons during the X17.2/4B flare as observed by the
Comprehensive Suprathermal and Energetic Particle Analyser (COSTEP)
and Three-Dimensional Plasma (3DP) analyzer experiments on board
the SOHO and Wind spacecraft. These observations are combined with
ground- and space-based spectral radio data obtained by the Potsdam
spectrograph and the Wind/Waves instrument. Additionally, we use
measurements of relativistic protons (ground-level event (GLE))
by neutron monitors (Kiel and Moscow). Timing and electron energy
spectrum analysis suggest that there are three separate stages of
electron injection into interplanetary space: (1) An injection of
radio type III-producing electrons is observed first; (2) an impulsive
injection with an almost symmetric time profile with a short duration
(∼18 min) is released ∼11 min later, followed by (3) a gradual,
long (>1 hour) lasting injection, with an onset ∼25 min after
the first type III burst. While the first escaping type III-producing
electrons are more likely related to the reconnection processes during
the impulsive flare phase, the association of the two delayed electron
injections with solar events is not well understood.
---------------------------------------------------------
Title: Radio and X-Ray Signatures of Magnetic Reconnection behind
an Ejected Flux Rope
Authors: Pick, M.; Démoulin, P.; Krucker, S.; Malandraki, O.; Maia, D.
2005ApJ...625.1019P Altcode:
We present a detailed study of a complex solar event observed on
2002 June 2. Joint imaging EUV, X-ray, and multiwavelength radio
observations allow us to trace the development of the magnetic structure
involved in this solar event up to a radial distance of the order of 2
R<SUB>solar</SUB>. The event involves type II, III, and IV bursts. The
type IV burst is formed by two sources: a fast-moving one (M) and a
“quasi-stationary” one (S). The time coincidence in the flux peaks
of these radio sources and the underlying hard X-ray sources implies
a causal link. In the first part of our paper we provide a summary
of the observations without reference to any coronal mass ejection
(CME) model. The experimental results impose strong constraints on
the physical processes. In the second part of our paper, we find that
a model with an erupting twisted flux rope, with the formation of a
current sheet behind, best relates the different observations in a
coherent physical evolution (even if there is no direct evidence of
the twisted flux rope). Our results show that multiwavelength radio
imaging represents a powerful tool to trace the dynamical evolution of
the reconnecting current sheet behind ejected flux ropes (in between
sources M and S) and over an altitude range not accessible by X-ray
observations.
---------------------------------------------------------
Title: Relationship of Solar Flare Accelerated Particles to Solar
Energetic Particles (SEPs) Observed in the Interplanetary Medium
Authors: Lin, R. P.; Shih, A. Y.; Krucker, S.; Smith, D. M.; Murphy,
R.; Share, G.; Mewaldt, R.; Cohen, C. M.; Looper, M. D.; Mason, G. M.;
Haggerty, D. K.
2005AGUSMSH33A..02L Altcode:
Observations of hard X-ray/gamma-ray continuum and gamma-ray lines
produced by energetic electrons and ions, respectively, colliding
with the solar atmosphere, have shown that large solar flares
can accelerate ions up to many GeV and electrons up to hundreds of
MeV. Solar energetic particles (SEPs) are observed by spacecraft near
1 AU and by ground-based instrumentation to extend up to similar
energies in large SEP events, but it is believed that a different
acceleration process associated with fast Coronal mass Ejections (CMEs)
is responsible. The Ramaty High Energy Solar Spectroscopic Imager
(RHESSI) mission provides high-resolution spectroscopy and imaging
of flare hard X-rays and gamma-rays. Such observations can provide
information on the location, energy spectra, and composition of the
flare accelerated energetic particles at the Sun. We compare the
RHESSI observations of the 2003 October 28, 2003 November 2, and 2005
January 20 flares with both energetic electron and ion observations
near 1 AU from ACE, SAMPEX, Wind,and GOES spacecraft, and discuss the
implications for the particle acceleration and escape processes
---------------------------------------------------------
Title: Microflare Statistics and Frequency Distribution
Authors: Christe, S. D.; Hannah, I.; Rauscher, E.; Krucker, S.;
McTiernan, J.; Lin, R. P.
2005AGUSMSP42A..04C Altcode:
RHESSI is uniquely suited to observe solar microflares due to its unique
sensitivity in the 3-15 keV energy range (up to ~100 times better than
previous solar instruments). As such, it provides new information on
these low level transients. Initial results (Krucker et al 2002, Benz
& Grigis 2002) suggest that microflares are different from larger
flares. They are more often associated with steep nonthermal spectra
(power law index -5 to -7). In this study, we present microflare
statistics from times of low activity. A list of microflares was
created by applying the standard RHESSI flare-finding algorithm to
the lower 6-12 keV energy range (~10,000 events). Imaging was used
in order to obtain positions of solar events and reject non-solar
events. These solar events were then each spectrally analyzed. We
present microflare statistics, including active region productivity,
and the microflare frequency distribution. This work was supported by
NASA contract NAS5-98033.
---------------------------------------------------------
Title: RHESSI Observations of Hard X-ray Footpoint Motions in
Solar Flares
Authors: Fivian, M. D.; Krucker, S.; Lin, R. P.
2005AGUSMSH31A..02F Altcode:
Solar HXR bremsstrahlung from energetic electrons accelerated in
the impulsive phase of a flare is observed to be primarily from the
footpoints of magnetic loops. Standard magnetic reconnection models
predict increasing separation of the footpoints during the flare as
longer and larger loops are produced. If the reconnection process
results in accelerated electrons, the HXR footpoints should show
this motion.The motion is only apparent; it is due to the HXR emission
shifting to footpoints of neighboring newly reconnected field lines. The
speed of footpoint separation reflects the rate of magnetic reconnection
and should be roughly proportional to the total energy deposition
in the footpoints. We studied footpoint motions in several large
flares with the Reuven Ramaty High Energy Solar Spectroscopic Imager
(RHESSI) and compared the motion with the energy deposition rate in the
footpoints. A correlation between the motion and the energy deposition
rate is found during some periods in these flares, but not always.
---------------------------------------------------------
Title: Coronal X-ray Emission in Long-Duration Occulted Flares
Authors: Balciunaite, P.; Christe, S. D.; Krucker, S.; Lin, R. P.
2005AGUSMSP52A..06B Altcode:
To investigate coronal X-ray (≥ 6 keV) emission using RHESSI, we
select long-duration (≥ 3 h) flares with completely or partially
occulted footpoints. Flare footpoint sources dominate and thus
obstruct the observation of coronal emission; short durations do not
allow enough time for observation of various changes. We examine the
location, morphology, size, height, and spectral characteristics in 20
long-duration flares seen with RHESSI whose footpoints are occulted
by the solar limb to at most 30°. In 4 cases we see hard X-ray (≥
20 keV) coronal sources; in the remaining cases hard X-ray emission
comes from partially occulted footpoints only or the coronal emission
is thermal.
---------------------------------------------------------
Title: Comparison between impulsive 3He-rich events and energetic
electron events
Authors: Wang, L.; Lin, R. P.; Krucker, S.; Mason, G. M.
2005AGUSMSH33A..01W Altcode:
Impulsive solar energetic particle (SEP) events with large enrichments
of 3He are associated with ~2-100 keV impulsive electrons. Electron
observations with the energy range of ~3 eV - 500 keV by the WIND 3-D
Plasma and Energetic Particle experiment (3DP) and ion measurements
with the energy range of ~ 0.02 - 10 MeV/nucleon by the ACE Ultra-Low
Energy Isotopic Spectrometer (ULEIS) provide the first possibility of
an accurate timing comparison of between impulsive 3He-rich events
and energetic electron events. We select eleven solar impulsive
events with enhanced 3He/4He ratios (~0.1 - 1.5) and a clear velocity
dispersion of both ion and electron events over a wide energy range. We
remove the contaminations of higher energy electrons in Solid State
Telescopes (SST) on WIND, determine the interplanetary path length
from peak times of WIND electron data observed in situ, and obtain
the electron injection profiles at the Sun from triangular fits to in
situ observations. The onsets and peaks of the injection of 3He-rich
ion events at the Sun are derived from those of ACE ion data observed
in situ by taking into account the travel time along the path length
comparable to electron events. The comparison study shows a systematic
delay of the injection of 3He-rich ions events with respect to the
injection of electron events. Nine of ten events have a fast (>
570 km/s) west CME observed by SOHO/LASCO with the onset of electron
injection close to the origin of the CME, and with the onset of ion
injection corresponding to a median height ~ 5 Rs of CME.
---------------------------------------------------------
Title: Solar Hard X-ray Emissions in the Decay Phase of Gradual
X-class Flares
Authors: Krucker, S.; Lin, R. P.
2005AGUSMSH31A..01K Altcode:
Solar hard X-ray emissions are most often observed during the impulsive
phase of solar flares and are produced by non-thermal electrons that
lose their energy in the chromosphere by collisions. During large
gradual flares, however, HXR emissions can last well into the decay
phase showing progressively hardening X-ray spectra (e.g. Cliver
et al. 1986). Here we present X-ray observations of such events
obtained by the Reuven Ramaty High Energy Solar Spectroscopic Imager
(RHESSI). RHESSI provides for the first time high resolution (1 keV)
spectral observations with simultaneous imaging down to 2". In about
half of all X-class flares seen by RHESSI, enhanced HXR emission
lasting well into decay phase (up to two hours after the flare onset)
is observed. These emissions are generally ~10 times fainter and
show much less time variations than HXR bursts occurring during
the impulsive phase. The X-ray spectra are found to be hard and can
generally be fitted with broken power law functions. First results
from one of the most prominent events (2005 January 19, 8UT) show
clear spectral hardening. Over 15 minutes, the spectral power law
index hardens below the break (from 2.6 to 2.1) as well as above it
(from 3.3 to 2.6). Furthermore, the break energy increases from 40
to 60 keV. Imaging reveals that the emission come from footpoints of
post flare loops suggesting that it is produced in a thick target. No
HXR emission is seen to originate from the post flare loops. We
are presently investigating the importance of trapping of energetic
electrons in these events.
---------------------------------------------------------
Title: An exceptionally bright flare from SGR 1806-20 and the origins
of short-duration γ-ray bursts
Authors: Hurley, K.; Boggs, S. E.; Smith, D. M.; Duncan, R. C.; Lin,
R.; Zoglauer, A.; Krucker, S.; Hurford, G.; Hudson, H.; Wigger, C.;
Hajdas, W.; Thompson, C.; Mitrofanov, I.; Sanin, A.; Boynton, W.;
Fellows, C.; von Kienlin, A.; Lichti, G.; Rau, A.; Cline, T.
2005Natur.434.1098H Altcode: 2005astro.ph..2329H
Soft-γ-ray repeaters (SGRs) are galactic X-ray stars that emit
numerous short-duration (about 0.1s) bursts of hard X-rays during
sporadic active periods. They are thought to be magnetars: strongly
magnetized neutron stars with emissions powered by the dissipation
of magnetic energy. Here we report the detection of a long (380s)
giant flare from SGR 1806-20, which was much more luminous than any
previous transient event observed in our Galaxy. (In the first 0.2s,
the flare released as much energy as the Sun radiates in a quarter
of a million years.) Its power can be explained by a catastrophic
instability involving global crust failure and magnetic reconnection
on a magnetar, with possible large-scale untwisting of magnetic field
lines outside the star. From a great distance this event would appear
to be a short-duration, hard-spectrum cosmic γ-ray burst. At least
a significant fraction of the mysterious short-duration γ-ray bursts
may therefore come from extragalactic magnetars.
---------------------------------------------------------
Title: Coronal phenomena at the release of solar energetic electron
events
Authors: Klein, K. -L.; Krucker, S.; Trottet, G.; Hoang, S.
2005A&A...431.1047K Altcode:
We investigate dynamical processes in the solar corona at the release
of electrons (~30-500 keV) detected by the Wind/3DP experiment, with
the aim to clarify the relationship between coronal acceleration
and the escape of electrons to interplanetary space. Energetic
electrons and plasma in the corona are traced using radio, EUV and
X-ray observations. 40 events were identified where the release
time of the electrons could be determined within an uncertainty of a
few minutes and occurred during the observing hours of the Nançay
Radioheliograph. All were accompanied by decametric-to-kilometric
type III bursts (Wind/WAVES), and 30 by metric radio emission in
the western hemisphere. The main findings from these 30 events are:
(i) Electrons detected at Wind are released at the time of distinct
episodes of electron acceleration in the corona signalled by radio
emission. The release may occur at the start of the radio event or
up to an hour later. (ii) The most conspicuous examples of delayed
electron release occur in events associated with complex, long lasting
(>10 min, up to seveal hours) radio emission. Radio observations
suggest that in these cases the earlier accelerated electrons remain
confined in the corona or are injected into flux tubes which are not
connected to the spacecraft. (iii) Type II bursts revealing shock
waves in the corona accompany about a third of the events. But the
shock waves occur in general together with type IV radio signatures
due to long lasting acceleration not related to the shock. With a few
exceptions these type IV emissions have a clearer timing relationship
with the electron release to space than the type II bursts. We conclude
that the combination of time-extended acceleration at heights ≲0.5
R_⊙ above the photosphere with the injection of electrons into a
variety of closed and open magnetic field structures explains the
broad variety of timing shown by the radio observations and the in
situ measurements. <P />Appendix A is only available in electronic
form at http://www.edpsciences.org
---------------------------------------------------------
Title: New insingts into solar physics from RHESSI
Authors: Krucker, S.; Lin, R. P.
2005ESASP.560..101K Altcode: 2005csss...13..101K
No abstract at ADS
---------------------------------------------------------
Title: Hard X-ray footpoint motions in solar flares: Comparing
magnetic reconnection models with observations
Authors: Krucker, Säm; Fivian, M. D.; Lin, R. P.
2005AdSpR..35.1707K Altcode:
Hard X-ray observations from the Reuven Ramaty High Energy Solar
Spectroscopic Imager (RHESSI) of the October 29, 2003 GOES X10
two-ribbon flare are used together with magnetic field observations
from the Michelson Doppler Imager (MDI) onboard SoHO to compare
footpoint motions with predictions from magnetic reconnection
models. The temporal variations of the velocity v of the hard X-ray
footpoint motions and the photospheric magnetic field strength B in
footpoints are investigated. The underlying photospheric magnetic field
strength is generally higher ( B ∼ 700-1200 G) in the slower moving
( v ∼ 20-50 km s <SUP>-1</SUP>) western footpoint than in the faster
( v ∼ 20-100 km s <SUP>-1</SUP>) moving eastern source (∼100-600
G). Furthermore, a rough temporal correlation between the HXR flux
and the product vB<SUP>2</SUP> is observed.
---------------------------------------------------------
Title: Modulations of broad-band radio continua and X-ray emissions
in the large X-ray flare on 03 November 2003
Authors: Dauphin, C.; Vilmer, N.; Lüthi, T.; Trottet, G.; Krucker,
S.; Magun, A.
2005AdSpR..35.1805D Altcode:
The GOES X3.9 flare on 03 November 2003 at ∼09:45 UT was observed
from metric to millimetric wavelengths by the Nançay Radioheliograph
(NRH), the Radio Solar Telescope Network (RSTN) and by radio instruments
operated by the Institute of Applied Physics (University of Bern). This
flare was simultaneously observed and imaged up to several 100 keV
by the RHESSI experiment. The time profile of the X-ray emission
above 100 keV and of the radio emissions shows two main parts,
impulsive emission lasting about 3 min and long duration emission
(partially observed by RHESSI) separated in time by 4 min. We shall
focus here on the modulations of the broad-band radio continua and
of the X-ray emissions observed in the second part of the flare. The
observations suggest that gyrosynchrotron emission is the prevailing
emission mechanism even at decimetric wavelengths for the broad-band
radio emission. Following this interpretation, we deduce the density
and the magnetic field of the decimetric sources and briefly comment
on possible interpretations of the modulations.
---------------------------------------------------------
Title: CME Interaction and the Intensity of Solar Energetic Particle
Events
Authors: Gopalswamy, N.; Yashiro, S.; Krucker, S.; Howard, R. A.
2005IAUS..226..367G Altcode:
Large Solar Energetic Particles (SEPs) are closely associated with
coronal mass ejections (CMEs). The significant correlation observed
between SEP intensity and CME speed has been considered as the evidence
for such a close connection. The recent finding that SEP events with
preceding wide CMEs are likely to have higher intensities compared to
those without was attributed to the interaction of the CME-driven shocks
with the preceding CMEs or with their aftermath. It is also possible
that the intensity of SEPs may also be affected by the properties
of the solar source region. In this study, we found that the active
region area has no relation with the SEP intensity and CME speed,
thus supporting the importance of CME interaction. However, there is a
significant correlation between flare size and the active region area,
which probably reflects the spatial scale of the flare phenomenon as
compared to that of the CME-driven shock.
---------------------------------------------------------
Title: Rhessi Microflare Statistics
Authors: Hannah, I. G.; Christe, S.; Krucker, S.; Hudson, H. S.;
Fletcher, L.; Hendry, M. A.
2004ESASP.575..259H Altcode: 2004soho...15..259H
No abstract at ADS
---------------------------------------------------------
Title: Intensity variation of large solar energetic particle events
associated with coronal mass ejections
Authors: Gopalswamy, N.; Yashiro, S.; Krucker, S.; Stenborg, G.;
Howard, R. A.
2004JGRA..10912105G Altcode:
We studied the coronal mass ejections (CMEs) and flares associated
with large solar energetic particle (SEP) events of solar cycle 23
(1996-2002) in order to determine what property of the solar eruptions
might order the SEP intensity. The SEP events were divided into three
groups: (1) events in which the primary CME was preceded by one or more
wide CMEs from the same solar source, (2) events with no such preceding
CMEs, and (3) events in which the primary CME might have interacted
with a streamer or with a nearby halo CME. The SEP intensities are
distinct for groups 1 and 2 although the CME properties were nearly
identical. Group 3 was similar to group 1. The primary findings of this
study are as follows: (1) Higher SEP intensity results whenever a CME
is preceded by another wide CME from the same source region. (2) The
average flare size was also larger for high-intensity SEP events. (3)
The intensity of SEP events with preceding CMEs showed a tighter
correlation with CME speed. The extent of scatter in the CME speed
versus SEP intensity plots was reduced when various subgroups were
considered separately. (4) The intensities of energetic electrons were
better correlated with flare size than with CME speed. (5) The SEP
intensity showed poor correlation with the flare size, except for group
3 events. Since only a third of the events did not have preceding CMEs,
we conclude that the majority of SEP producing CMEs propagate through
the near-Sun interplanetary medium severely disturbed and distorted by
the preceding CMEs. Furthermore, the preceding CMEs are faster and wider
on the average, so they may provide seed particles for CME-driven shocks
that follow. Therefore we conclude that the differing intensities of
SEP events in the two groups may not have resulted due to the inherent
properties of the CMEs. The presence of preceding CMEs seems to be the
discriminating characteristic of the high- and low-intensity SEP events.
---------------------------------------------------------
Title: RHESSI Observations of Hard X-ray Footpoint Motions in
Solar Flares
Authors: Fivian, M. D.; Krucker, S.; Lin, R. P.
2004AGUFMSH13A1132F Altcode:
Solar HXR bremsstrahlung from energetic electrons accelerated in
the impulsive phase of a flare is observed to be primarily from
the footpoints of magnetic loops. Standard magnetic reconnection
models predict increasing separation of the footpoints during the
flare as longer and larger loops are produced. If the reconnection
process results in accelerated electrons, the HXR footpoints should
show this motion. The motion is only apparent; it is due to the HXR
emission shifting to footpoints of neighboring newly reconnected
field lines. The speed of footpoint separation reflects the rate
of <P />magnetic reconnection and should be roughly proportional to
the total energy deposition in the footpoints. We studied footpoint
motions in several large flares with the Reuven Ramaty High Energy
Solar Spectroscopic Imager (RHESSI) and compared the motion with the
energy deposition rate in the footpoints. A correlation between the
motion and the energy deposition rate is found during some periods in
these flares, but not always.
---------------------------------------------------------
Title: Coronal Hard X-ray Emission in Occulted Flares
Authors: Balciunaite, P.; Krucker, S.; Lin, R. P.
2004AGUFMSH13A1131B Altcode:
We use RHESSI to investigate coronal sources in six behind-the-limb
flares with occulted or partially occulted footpoints. The major
findings are as follows: (1) Coronal hard X-ray (> 20 keV) emission
is observed above thermal soft X-ray (< 12 keV) and EUV (195 Å)
emission (which confirms Masuda's above-the-loop-top source); (2)
Coronal HXR sources show impulsive (on a scale of tens of seconds)
variation in their HXR time profiles; (3) The data can be fitted with
a power law component with an index of 5 to 7 or with several thermal
components with very high temperatures up to 100 MK. Presently, using
the derived physical parameters, we test various heating models to
determine whether the coronal HXR emission is thermal or non-thermal.
---------------------------------------------------------
Title: Nonthermal X-ray Microflares
Authors: Christe, S.; Rauscher, E.; Krucker, S.; Lin, R. P.
2004AGUFMSH24A..03C Altcode:
The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI)
provides unique sensitivity in the 3-15 keV energy range, with an
effective area ∼100 times larger than similar past instruments. Along
with its high spectral resolution (1 keV) RHESSI is uniquely suited to
study small events. Microflares have been observed by Benz & Grigis
(2002) and Krucker et al. (2002) to have anomalously steep spectra (
spectral index between -5 and -8) extending down to ∼ 7 keV. Thermal
emission is found to dominate below ∼ 7 keV. In many other respects,
microflares show properties similar to larger flares. We present
single event studies of different types of x-ray microflares. RHESSI
observations during quiet times (04-May 10-14; GOES level low B class)
reveal a set 5 microflares (>=A Class). These microflares show
power law spectra (spectral index of ∼4-8) with little or no thermal
emission in the 3- ∼7 keV energy range above the nonthermal part of
the spectrum. Other microflares in the same GOES class range, however,
have been found which show extremely hard spectra with emission up to
50 keV (power law index ∼2). At lower energies, emission is dominated
by a hot thermal component (20 MK). This work was supported by NASA
contract NAS5-98033.
---------------------------------------------------------
Title: Overview of Early Rhessi Results
Authors: Krucker, S.; Hudson, H. S.
2004ESASP.575..247K Altcode: 2004soho...15..247K
No abstract at ADS
---------------------------------------------------------
Title: Solar X-rays and Energetic Electrons Escaping from the Sun
Authors: Krucker, S.; Kontar, E. P.; Lin, R. P.
2004AGUFMSH13A1129K Altcode:
The Sun frequently accelerates electrons in solar flares and type III
radio bursts. Some of the accelerated electrons lose their energy
by collisions in the denser, lower solar atmosphere producing hard
X-ray (HXR) emissions and heat the corona, while others escape into
interplanetary space. Whether the HXR producing and the escaping
electrons are accelerated by the same mechanism is not known. Combining
RHESSI X-ray observations with in-situ observations of energetic
electrons from the WIND spacecraft allows for the first time a detailed
temporal, spatial, and spectral study. Statistical results of 16 events
with a close temporal agreement between the HXR and the in-situ detected
electrons (taking the time of flight of the escaping electrons into
account) show a correlation between the HXR photon spectral index and
the electron spectral index observed in-situ thus indicating a common
acceleration mechanism. Furthermore, the solar X-ray source structure of
these events look similar showing hot loops with HXR footpoints plus an
additional HXR source separated from the loop by typically ~15". This
source structure can be explained by a simple magnetic reconnection
model with newly emerging flux tubes that reconnect with previously open
field lines, so-called interchange reconnection. Events with a delayed
timing between the HXRs and the solar release of escaping electrons
(Krucker et al. 1999, Haggerty & Roelof 2002) are presently
investigated. If these delayed events are indeed accelerated later in
the event by shocks, no correlation between the HXR photon spectrum
and the in-situ observed electron spectrum is expect to be found.
---------------------------------------------------------
Title: Spectra of Solar Energetic Electrons in Flares and near Earth
Authors: Kontar, E. P.; Krucker, S.; Lin, R. P.
2004AGUFMSH13A1130K Altcode:
Successful operation of Reuven Ramaty High Energy Solar Spectroscopic
Imager (RHESSI) allows us to observe hard X-ray spectrum of many solar
flares with unprecendent energy resolution. X-ray spectra provide us
with vital information about the spectral properties of highly energetic
electrons at the Sun. The mean electron spectrum of the solar flare
can be recovered through a newly developed constraint regularization
analysis of the bremsstrahlung photon spectra that energetic electrons
produce. We emphasize the use of non-square inversion techniques
combined with the correction of the observed spectrum for the effect
of Compton photon back-scatter to achieve the most meaningful solution
to the problem. For 16 solar flares the electron spectrum of temporally
related solar energetic electron events has been measured using WIND/3DP
allowing us simultaneous analysis of electron flux spectral properties
at the Sun and near the Earth. Electron spectra from solar flares show
strong correlations with the spectrum of solar energetic electrons,
though vary with energy. Weaker correlation at lower energies can be
viewed as a propogation and/or escape effect. Results suggest that
the commonly used model of a collisional transport (thick-target)
for flare electrons plus free streaming for interplanetary particles
cannot explain the observed spectra.
---------------------------------------------------------
Title: The hard X-ray spectral structure of flare ribbons
Authors: Hudson, H.; Fletcher, L.; Krucker, S.; Pollock, J.
2004AGUFMSH24A..02H Altcode:
We examine the spatial distribution of hard X-ray spectral parameters
in flares exhibiting the classic two-ribbon structure using RHESSI
observations. The flares studied include July~15 and July~17, 2002,
and October~29, 2003. We confirm the existence of a tendency for
the localization of the hard X-ray sources into dominant bright
“footpoint” regions which do not show ribbon structure as extensive
as that seen in Hα or UV~images. As a part of the study we characterize
the ribbons photometrically in the EUV as observed by TRACE, confirming
earlier results that find complicated relationships between EUV and
hard X-rays. We seek an empirical explanation for the restricted
hard X-ray footpoints in terms of a spatial analog of the well-known
“soft-hard-soft” morphology: the regions of weaker hard X-ray emission
correspond to steeper X-ray energy spectra and hence to softer electron
precipitation spectra. This relationship may be as predicted by the
1D radiation hydrodynamics models of flaring loops.
---------------------------------------------------------
Title: Comparison between impulsive energetic electron events and
<SUP>3</SUP>He-rich events
Authors: Wang, L.; Mason, G.; Lin, R. P.; Krucker, S.
2004AGUFMSH31A1153W Altcode:
We present a comparison between electron data by WIND/3DP and
<SUP>3</SUP>He data by ACE/ULEIS. Our study shows that impulsive
energetic electron events occur more often than impulsive
<SUP>3</SUP>He-rich events. For a set of 41 <SUP>3</SUP>He-rich
events from 1997 November to 2003 April, WIND/3DP has good
measurements during 29 events. An injection analysis shows
that among the 29 <SUP>3</SUP>He-rich events, 13 events with the
<SUP>3</SUP>He/<SUP>4</SUP>He ratio ( ∼0.002-0.34) have an electron
event with an injection time difference < 1 hour, 6 events with the
<SUP>3</SUP>He/<SUP>4</SUP>He ratio ( ∼0.081-6.2) have an electron
event with an injection time difference of 1-2 hours, and the other 8
events with the <SUP>3</SUP>He/<SUP>4</SUP>He ratio ( ∼0.0054-0.25)
have an electron event with a injection difference > 2 hour or have
no electron events. Further analysis including type III bursts and
energy spectra may provide important information about the association
between <SUP>3</SUP>He-rich events and electron events.
---------------------------------------------------------
Title: Comparing Solar Hard X-ray Emissions and Impulsive Electron
Events seen at 1AU
Authors: Krucker, S.; Lin, R. P.; Kontar, E. P.
2004AGUSMSH22A..02K Altcode:
The Sun frequently accelerates electrons in solar flares and type III
radio bursts. Some of the accelerated electrons lose their energy by
collisions in the denser, lower solar atmosphere producing hard X-ray
(HXR) emissions, while others escape into interplanetary space. Whether
the HXR producing and the escaping electrons are accelerated by the
same mechanism is not known. We present a combined study of RHESSI
X-ray observations and WIND/3dp in situ electron observations taken
near 1~AU. Electron events with a solar release time in close temporal
agreement with the HXR peak time are selected. For these events,
the electron spectrum measured at 1~AU is compared with the electron
spectrum derived from the HXR observations. We find the derived and the
observed electron spectrum do not agree with a simple model of electron
acceleration high in the corona with downward moving electrons producing
HXRs in the lower, denser corona (thick target model) and upwards moving
electrons escaping into interplanetary space without energy changes;
the observed electron spectrum at 1 AU would predict a much harder
HXR spectrum than what is observed. More complicated models including
the effects of how particle escape from the acceleration cite are
need. That a high coronal acceleration can be excluded makes it hard
to explain how the low energy electrons, down to a few hundred eV,
can escape to ~1~AU. This suggests that two different mechanisms may
be accelerating electrons and that the HXR emission is not related to
the electrons seen at 1~AU despite the close temporal correlation.
---------------------------------------------------------
Title: Above the Loop-top Non-thermal Hard X-ray Source in a Partially
Occulted Flare
Authors: Balciunaite, P.; Krucker, S.; Lin, R. P.
2004AAS...204.5407B Altcode: 2004BAAS...36..758B
We present RHESSI hard X-ray imaging, spectroscopy, and source motion
analysis of a long-duration GOES M 4.5 class flare (18 Nov 2003
0923-1101 UT), whose footpoints were occulted by the east limb of the
Sun. During the rise phase of the flare a non-thermal hard X-ray source
is seen above 20 keV with a spectral power-law index around 6. The
source is initially detected 50 arcseconds above the limb and seen to
rise with a velocity of 12 km/s. Its time profile varies on a scale
of tens of seconds. At lower energies, a thermal hard X-ray source
with a temperature of up to 17 MK is detected 10 arcseconds below the
non-thermal source, and seen to be rising with a lower velocity of 7
km/s. Its time variation is slow and gradual, similar to that of the
GOES time profile. Both sources are seen above GOES SXI soft X-ray and
SOHO EIT EUV loops. We also discuss the energetics of the non-thermal
above-the-loop-top source.
---------------------------------------------------------
Title: Characterizing thermal and non-thermal electron populations
in solar flares using RHESSI
Authors: Caspi, A.; Krucker, S.; Lin, R. P.
2004AAS...204.8704C Altcode: 2004BAAS...36..819C
Solar flare plasmas contain both non-thermal and thermal electron
populations, with the latter often reaching temperatures above
∼10 MK. These high-temperature plasmas emit a characteristic line
and continuum X-ray spectrum, including emission in the Fe/Ni line
complexes at ∼6.7 and ∼8 keV. The fluxes and equivalent widths
of these line complexes are strongly temperature-dependent, and thus
they are a useful probe of thermal flare plasmas. The Reuven Ramaty
High Energy Solar Spectroscopic Imager (RHESSI) observes photons with
energies above 3 keV, with a spectral resolution of ∼1 keV FWHM, and
is especially sensitive to flare plasmas above ∼10 MK. In many flares,
RHESSI detects emission in both of the Fe/Ni line complexes, as well
as in the thermal continuum. Recent results using RHESSI for full-Sun
spectroscopy show that the relationship between the 6.7- to 8-keV line
flux ratio and the continuum temperature does not agree with theoretical
values, and further, varies from flare to flare. RHESSI provides imaging
spectroscopy, and hence the individual spectra of spatially-separated
sources can be obtained and and the source morphology can be studied at
different energies. We perform such an analysis on a variety of flares,
including the X4.8 event on 23 July 2002 and the X1.5 event on 21 April
2002, to determine the time-varying characteristics of the multiple
thermal and non-thermal electron populations. We estimate and compare
the energy contained in these electron populations, and discuss the
implications for heating and energy transport in solar flares.
---------------------------------------------------------
Title: Scatter-free Impulsive Electron Event from the October 28,
2003 X17 Flare
Authors: Krucker, S.; Lin, R. P.
2004AAS...204.0206K Altcode: 2004BAAS...36..668K
The October 28, 11 UT X-17 flare produced an impulsive electron event
detected near the Earth by the WIND 3D Plasma & Energetic Particles
instrument from ∼200 keV down to ∼1 keV, even though the flare was
located around E08 and therefore unlikely to be magnetically connected
to Earth. This impulsive event exhibited a fast rise and decay (
∼10 min each), and was followed by a second, much slower rising
event that lasted for many hours. The peak times of the impulsive
event show clear velocity dispersion consistent with a path length of
1.27±0.1 AU, indicating that the electron propagation is essentially
scatter-free. Onset time analysis reveal that the impulsive event
electrons left the Sun at 11:14±3 UT, just after the end of a group
of type III bursts seen at 14 MHz (11:03-11:11 UT) and when the type
II burst reaches 14 MHz. Radio pulsations at 0.6-1.3 GHz were seen
between 11:14:30 and 11:22:00 UT. The velocity dispersion seen in the
peak times suggests that the peak of the injection occurred around
11:25±3 UT roughly consistent with the end of the decimeter radio
emission. This impulsive electron event also was observed near Mars at
energies from ∼1 to 20 keV by the Electron Reflectometer instrument
on Mars Global Surveyor spacecraft.
---------------------------------------------------------
Title: Solar Origin of Interplanetary Impulsive Electron Events
Authors: Pick, M.; Maia, D. J.; Malandraki, O.; Krucker, S.;
Demoulin, P.
2004AGUSMSH22A..03P Altcode:
Many solar impulsive electron events have been traditionally associated
with type III radio emissions. Several recent studies however showed
that, in the majority of the events, the solar release of electrons at
high energies can present delays of up-to-half an hour with respect of
the onset of type III bursts. We have revisited the origin of a large
number of events using multiwave-length observations. For each event,
we investigated the coronal restructuring using EUV, white-light,
radio imaging and spectral observations in a wide frequency range that
allows us to follow the evolution of the corona from a few tenths above
the solar limb up to a few solar radii. Radio observations revealed
direct energetic electron signatures, close in time with the electron
release. The release time for the delayed events always coincides with
the onset or major changes in the complex of radio emissions. This
close association indicates that the coronal processes involved in the
radio emissions are at the origin of the electron acceleration. We
illustrate our results by presenting, more particularly, one recent
event for which the observations were also coupled with imaging
spectroscopy measurements obtained by the RHESSI mission (from 3 keV to
17 MeV). RHESSI observed a hard X-ray emission, which lasted for more
than fifteen minutes. This emission was closely associated in time and
space with the radio emission imaged by the Nançay Radioheliograph. The
results suggest that, for this event, both electrons detected in the
corona and those injected in the interplanetary medium are due to a
similar process involving coronal magnetic field interactions. Their
respective sites of acceleration/injection are however distinct in
space and time. The energetic electrons detected in the interplanetary
medium are not released during the X-ray burst.
---------------------------------------------------------
Title: RHESSI Gamma-ray Imaging of the Flares of October/November 2003
Authors: Hurford, G. J.; Krucker, S.; Lin, R. P.; Schwartz, R. A.;
Share, G. H.; Smith, D. M.
2004AAS...204.0202H Altcode: 2004BAAS...36..667H
Imaging of gamma-ray line emission from solar flares provides
direct information on the spatial characteristics of accelerated
ions near the Sun. To date this has been possible for only one event
(July 23, 2002) whose neutron-capture line image at 2.2 MeV implied a
significant spatial difference between high energy ions and electrons
in acceleration and/or transport. <P />Observations of the X17, X10
and X8 events on October 28, October 29 and November 2, 2003 by the
Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) provided
a ten-fold increase in the detected nuclear-line photons available for
imaging. This paper presents the results of imaging of these flares
in the 2.2 MeV neutron capture line, including source size, location
and motion estimates and comparisons with simultaneous gamma-ray and
x-ray emission in the electron-bremsstrahlung continuum. <P />This
work is supported by NASA contract NAS5-98033.
---------------------------------------------------------
Title: The Microflare Frequency Distribution observed by RHESSI
Authors: Christe, S.; Krucker, S.; Lin, R. P.; Hannah, I.
2004AAS...204.8703C Altcode: 2004BAAS...36R.818C
The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI)
provides uniquely high sensitivity in the 3-15 keV energy range with
an effective area from 14 to 130 times larger then previous solar
instruments. We present a microflare frequency distribution derived
from RHESSI observations. Times were chosen such that activity is very
low (GOES B Class background) and includes a 24 hour period when no
active regions were present on the Sun. Microflares were found through
searching for 5σ increases in count rate, summing over all detectors,
between two adjacent time bins in the 3-10 keV energy band. Count
rates were binned from 4 seconds to 3 minutes, increasing by 4 seconds
increments, and the search repeated in order to detect flares of various
time scales. Each microflare was individually checked in order to reject
non-solar events. Few microflares are seen to occur below the time scale
of minutes. The observed occurence rate is one microflare every ∼ 8
minutes above a threshold flux of ∼ 0.4 cm<SUP>-2</SUP> s<SUP>-1</SUP>
keV<SUP>-1</SUP> over the 5-10 keV energy range. Microflares were also
found when no active regions were present on the Solar disk. This work
was supported by NASA contract NAS5-98033.
---------------------------------------------------------
Title: The hard X-ray spectral structure of flare ribbons
Authors: Fletcher, L.; Hudson, H. S.; Krucker, S.; Pollock, J. A.
2004AAS...204.5403F Altcode: 2004BAAS...36..758F
We examine the spatial distribution of hard X-ray spectral parameters
in flares exhibiting the classic two-ribbon structure using RHESSI
observations. The flares studied include July 15 and July 17, 2002,
and October 29, 2003. We confirm the existence of a tendency for
the localization of the hard X-ray sources into dominant bright
“footpoint” regions which do not show ribbon structure as extensive
as that seen in Hα or UV images. As a part of the study we characterize
the ribbons photometrically in the EUV as observed by TRACE, confirming
earlier results that find complicated relationships between EUV and
hard X-rays. We seek an empirical explanation for the restricted
hard X-ray footpoints in terms of a spatial analog of the well-known
“soft-hard-soft” morphology: the regions of weaker hard X-ray emission
correspond to steeper X-ray energy spectra and hence to softer electron
precipitation spectra. This relationship may be as predicted by the
1D radiation hydrodynamics models of flaring loops.
---------------------------------------------------------
Title: High-frequency slowly drifting structures and X-ray sources
observed by RHESSI
Authors: Karlický, M.; Fárník, F.; Krucker, S.
2004A&A...419..365K Altcode:
Three solar flares (April 4, 2002, May 17, 2002, and August 30, 2002)
with the 0.4-2.0 GHz slowly drifting structures were selected and
analyzed together with RHESSI X-ray observations. Two events (April 4,
2002 and May 17, 2002) were observed above and one event (August 30,
2002) close to the solar limb. While in April 4, 2002 and August 30,
2002 the radio drifting structures with relatively high frequency
drifts (-32- -25 MHz s<SUP>-1</SUP>) were recorded at times of the
start of a motion of the X-ray flare source, in May 17, 2002 event
a splitting of the X-ray source into two sources was observed before
observation of the 0.8-1.8 GHz radio structure drifting with very slow
frequency drift (-0.4 MHz s<SUP>-1</SUP>). The X-ray source of the May
17, 2002 was much softer (<40 keV) than those in April 4, 2002 and
August 30, 2002 (>100 keV). Velocities of the X-ray sources in the
image plane were estimated as 12 km s<SUP>-1</SUP> for April 4, 2002
and 10 km s<SUP>-1</SUP> for August 30, 2002. Analyzing GOES data and
X-ray RHESSI spectra of the May 17, 2002 flare the plasma thermal and
non-thermal electron densities in the X-ray sources were determined. For
two cases (April 4, 2002 and May 17, 2002) it was found that the plasma
density in the coronal X-ray source is higher than maximum one derived
from the radio drifting structure. The cross-correlation of the radio
drifting structure and hard X-ray flux for the August 30, 2002 event
reveals that the hard X-ray emission is delayed 0.5-0.7 s after the
radio and it is partly correlated with an enhanced background of the
drifting structure. All these results are discussed and interpreted
considering the flare model with the plasmoid ejection.
---------------------------------------------------------
Title: RHESSI Observations of Hard X-ray Microflares with
Interplanetary Type III Radio Bursts
Authors: Rauscher, E.; Christe, S.; Krucker, S.; Lin, R. P.
2004AAS...204.5408R Altcode: 2004BAAS...36..758R
RHESSI's increased sensitivity compared to previous instruments (100
times at 10 keV) and its ability to create high resolution images
makes it ideal to study microflares. During the period of 2003 May 11
through 2003 May 13, RHESSI observed a series of microflares, eight
of which occurred with temporally coincident interplanetary type III
radio bursts, as observed by WIND (1-14 MHz). Statistically we would
have expected one coincident event. These flares range from GOES
class A1 to B1 above an average background of B1. Imaging shows that
many of these flares originate from the same active region. For three
of the flares TRACE 1600Å movies show similar geometric structures,
with two bright spots spatially coincident with X-ray emission (3-15
keV) and jets leaving the flare site. A similar set of events was
analyzed by Christe et al (SPD 2003). We present an investigation
of the structure of these flares in order to determine why they are
associated with type III bursts.
---------------------------------------------------------
Title: Title Requested
Authors: Krucker, S.; RHESSI Team
2004AAS...204.4704K Altcode: 2004BAAS...36R.737K
The high solar activity during the last weeks of October and
beginning of November 2003 with 11 X-class flares provides a great
opportunity to study particle acceleration in flares. Recent results
of multi-spacecraft and ground-based observations are summarized with
a focus on X-ray observations from the Reuven Ramaty High Energy Solar
Spectroscopic Imager (RHESSI).
---------------------------------------------------------
Title: Triggering of the Two X-class Flares of 28 and 29 October 2003
Authors: Choudhary, D. P.; Moore, R. L.; Falconer, D.; Pojoga, S.;
Tian-Sen, H.; Krucker, S.; Uddin, W.
2004AAS...204.0225C Altcode: 2004BAAS...36..982C
From H-alpha movies from Aryabhatta Research Institute of Observational
Sciences and from Prairie View Solar Observatory, hard X-ray movies
from RHESSI, line-of-sight magnetogram movies from SOHO/MDI, and
vector magnetograms from Marshal Space Flight Center, we examine the
magnetic structure and evolution of the large delta-sunspot active
region NOAA 10486 in relation to the onset and development of the
two X-class flares that occurred in this active region on 28 and 29
October 2003. We find evidence that each of these flares was triggered
by strongly sheared magnetic field via “tether-cutting" reconnection
with adjacent/overlying strongly sheared field. In the first flare, the
initial brightening in H-alpha (1) was partly rooted in emerging sheared
magnetic field along the edge of the large positive-polarity flux
domain of the delta sunspot, and (2) consisted of four flare kernels,
two in negative magnetic flux and two in positive magnetic flux. In
the second flare, the brightening started in the core of a Z-shaped
sigmoidal sheared magnetic field and the inner two of four H-alpha
kernels were visible in 30-50 Kev hard x-ray image from RHESSI. Each
flare spread from the initial quadrupolar brightening to develop into
a much larger two-ribbon flare straddling a much more extensive swath
of strongly sheared field along the edge of the large positive-flux
domain of the delta sunspot, the first flare on the leading side and
the second flare on the trailing side of this domain. Thus, localized
internal reconnection triggered the explosion of these extensive sheared
magnetic fields. <P />This research was supported by NASA's Office
of Space Science through the Solar and Heliospheric Physics SR&T
Program, and was done during Dr. Choudhary's tenure at MSFC/NSSTC as
an NRC Senior Resident Research Associate.
---------------------------------------------------------
Title: Scatter-free impulsive electron event from the October 28,
2003 X17 flare
Authors: Krucker, S.; Lin, R. P.; Mitchell, D.
2004AGUSMSH51A..10K Altcode:
The October 28, 11 UT X-17 flare produced an impulsive electron
event detected near the Earth by the WIND 3D Plasma & Energetic
Particles instrument from ~200 keV down to ~1~keV, even though the
flare was located around E08 and therefore unlikely to be magnetically
connected to Earth. This impulsive event exhibited a fast rise and
decay (~10 min each), and was followed by a second, much slower rising
event that lasted for many hours. The peak times of the impulsive
event show clear velocity dispersion consistent with a path length of
1.27±0.1 AU, indicating that the electron propagation is essentially
scatter-free. Onset time analysis reveal that the impulsive event
electrons left the Sun at 11:14±3 UT, just after the end of a group
of type III bursts seen at 14~MHz (11:03-11:11~UT) and when the type
II burst reaches 14~MHz. Radio pulsations at 0.6-1.3~GHz were seen
between 11:14:30 and 11:22:00~UT. The velocity dispersion seen in the
peak times suggests that the peak of the injection occurred around
11:25±3~UT roughly consistent with the end of the decimeter radio
emission. This impulsive electron event also was observed near Mars
at energies from ~1 to 20~keV by the Electron Reflectometer instrument
on Mars Global Surveyor spacecraft.
---------------------------------------------------------
Title: Magnetic Field, Hα, and RHESSI Observations of the 2002 July
23 Gamma-Ray Flare
Authors: Yurchyshyn, Vasyl; Wang, Haimin; Abramenko, Valentyna;
Spirock, Thomas J.; Krucker, Säm
2004ApJ...605..546Y Altcode:
In this paper we examine two aspects of the 2002 July 23 gamma-ray flare
by using multiwavelength observations. First, the data suggest that the
interaction of the erupted field with an overlying large-scale coronal
field can explain the offset between the gamma-ray and the hard X-ray
sources observed in this event. Second, we pay attention to rapid and
permanent changes in the photospheric magnetic field associated with
the flare. MDI and BBSO magnetograms show that the following magnetic
flux had rapidly decreased by 1×10<SUP>20</SUP> Mx immediately after
the flare, while the leading polarity was gradually increasing for
several hours after the flare. Our study also suggests that the changes
were most probably associated with the emergence of new flux and the
reorientation of the magnetic field lines. We interpret the magnetograph
and spectral data for this event in terms of the tether-cutting model.
---------------------------------------------------------
Title: Studies of Microflares in RHESSI Hard X-Ray, Big Bear Solar
Observatory Hα, and Michelson Doppler Imager Magnetograms
Authors: Liu, Chang; Qiu, Jiong; Gary, Dale E.; Krucker, Säm;
Wang, Haimin
2004ApJ...604..442L Altcode:
In this paper, we present a study of the morphology of 12 microflares
jointly observed by RHESSI in the energy range from 3 to 15 keV
and by Big Bear Solar Observatory (BBSO) at the Hα line. They
are A2-B3 events in GOES classification. From their time profiles,
we find that all of these microflares are seen in soft X-ray, hard
X-ray, and Hα wavelengths, and their temporal evolution resembles
that of large flares. Co-aligned hard X-ray, Hα, and magnetic field
observations show that the events all occurred in active regions and
were located near magnetic neutral lines. In almost all of the events,
the hard X-ray sources are elongated structures connecting two Hα
bright kernels in opposite magnetic fields. These results suggest
that, similar to large flares, the X-ray sources of the microflares
represent emission from small magnetic loops and that the Hα bright
kernels indicate emission at footpoints of these flare loops in the
lower atmosphere. Among the 12 microflares, we include five events that
are clearly associated with type III radio bursts as observed by the
radio spectrometer on board Wind. Spectral fitting results indicate
the nonthermal origin of the X-ray emission at over ~10 keV during
the impulsive phase of all the events, and the photon spectra of the
microflares associated with type III bursts are generally harder than
those without type III bursts. TRACE observations at EUV wavelengths are
available for five events in our list, and in two of these, coincident
EUV jets are clearly identified to be spatially associated with the
microflares. Such findings suggest that some microflares are produced
by magnetic reconnection, which results in closed compact loops and
open field lines. Electrons accelerated during the flare escape along
the open field lines to interplanetary space.
---------------------------------------------------------
Title: Nobeyama Radioheliograph and RHESSI Observations of the X1.5
Flare of 2002 April 21
Authors: Kundu, M. R.; Garaimov, V. I.; White, S. M.; Krucker, S.
2004ApJ...600.1052K Altcode:
We present an overview of the radio observations of the X1.5 flare of
2002 April 21 and complementary data from other wavelengths. This flare
was fairly well observed by the Ramaty High Energy Spectroscopic Imager
(RHESSI) spacecraft and fully observed by the Nobeyama Radioheliograph
(NoRH) at 17 and 34 GHz. This long-duration event lasted more than 2
hr and featured a beautiful arcade of rising loops on the limb visible
at X-ray, EUV, and radio wavelengths. The main flare was preceded by
a small event 90 minutes earlier showing a long EUV loop connecting
well-separated radio and hard X-ray sources. The main flare itself
starts with a compact radio and hard X-ray source at the eastern end of
the region that develops into emission close to the solar surface (and
well inside the solar limb) over a large region to the northwest. As
the flare proceeds, a large set of loops is seen to rise well above the
solar limb. Distinct regions of radio emission with very different time
behavior can be identified in the radio images, and, in particular,
a peculiar nonthermal source seen in radio and hard X-rays low in the
corona at the base of the arcade is seen to turn on 30 minutes after
the start of the impulsive phase. At about the same time, an extremely
intense burst of coherent radio emission is seen from 500 to 2000 MHz;
we speculate that this lower-frequency burst is produced by electrons
that are accelerated in the nonthermal source at the base of the arcade
and injected into the loop system where they radiate plasma emission
in the 10<SUP>10</SUP> cm<SUP>-3</SUP> density plasma at the top of
the arcade of loops. This event is striking as a demonstration of
the many ways in which a flare can produce radio emission, and the
combined data at different wavelengths reveal a diversity of energy
release and nonthermal acceleration sites.
---------------------------------------------------------
Title: Characterizing thermal and non-thermal electron populations
in solar flares using RHESSI
Authors: Caspi, A.; Krucker, S.; Lin, R. P.
2004cosp...35.3582C Altcode: 2004cosp.meet.3582C
Solar flare plasmas contain both non-thermal and thermal electron
populations, with the latter often reaching temperatures above
∼10 MK. These high-temperature plasmas emit a characteristic line
and continuum X-ray spectrum, including emission in the Fe/Ni line
complexes at ∼6.7 and ∼8 keV. The fluxes and equivalent widths
of these line complexes are strongly temperature-dependent, and thus
they are a useful probe of thermal flare plasmas. The Reuven Ramaty
High Energy Solar Spectroscopic Imager (RHESSI) observes photons with
energies above 3 keV, with a spectral resolution of ∼1 keV FWHM, and
is especially sensitive to flare plasmas above ∼10 MK. In many flares,
RHESSI detects emission in both of the Fe/Ni line complexes, as well
as in the thermal continuum. Recent results using RHESSI for full-Sun
spectroscopy show that the relationship between the 6.7- to 8-keV line
flux ratio and the continuum temperature does not agree with theoretical
values, and further, varies from flare to flare. RHESSI provides imaging
spectroscopy, and hence the individual spectra of spatially-separated
sources can be obtained and and the source morphology can be studied at
different energies. We perform such an analysis on a variety of flares,
including the X4.8 event on 23 July 2002 and the X1.5 event on 21 April
2002, to determine the time-varying characteristics of the multiple
thermal and non-thermal electron populations. We estimate and compare
the energy contained in these electron populations, and discuss the
implications for heating and energy transport in solar flares.
---------------------------------------------------------
Title: RHESSI Hard X-ray Observations during the October/November
2003 Events
Authors: Krucker, S.; Fivian, M. D.; Lin, R. P.
2004cosp...35.1789K Altcode: 2004cosp.meet.1789K
The high solar activity during October-November 2003 with 11 X-class
flares provides a great opportunity to study particle acceleration
in flares. The Reuven Ramaty High Energy Solar Spectroscopic Imager
(RHESSI) coverage was best for the October 29, ∼20UT (GOES X-11)
event, missing only the later decay phase. This event shows a two ribbon
structure with non-thermal hard X-ray emission from the ribbons and
thermal emission from the loops connecting the ribbons. The ribbons
move apart with averaged speeds of ∼35 km s<SUP>-1</SUP> and ∼15
km s<SUP>-1</SUP>, respectively. The faster moving ribbon travels
through a region with lower magnetic field strength (∼400-800 G)
whereas the slower moving ribbons goes through a stronger field region
(∼1000-1700 G). This is roughly consistent with the simplest models
of magnetic reconnection theory, predicting B v ≈ const. We are
analyzing the relation between the footpoint motions, the magnetic
field strength and the total released energy derived from the HXR
observations. If magnetic reconnection is indeed responsible for the
observed energy release, the temporal evolution of the footpoint speeds
and the magnetic field strength at the footpoints should be correlated
with the energy deposited in the HXR footpoints.
---------------------------------------------------------
Title: X-ray and radio observations of energetic electrons produced
in the 3 November 2003 solar flare at ~09:5000 UT
Authors: Dauphin, C.; Vilmer, N.; Lüthi, T.; Magun, A.; Krucker,
S.; Schwartz, R.; Trottet, G.
2004cosp...35.2506D Altcode: 2004cosp.meet.2506D
Hard X-ray and radio observations provide complementary observations
of energetic electrons produced in solar flares. The GOES X4 flare on
03 November 2003 at ∼ 09:50 UT was observed and imaged up to several
100 keV by the RHESSI experiment. It was simultaneously observed at
metric/decimetric wavelengths by the Nançay Radioheliograph (NRH)
and at centimetric/millimetric wavelengths by radio instruments
operated by the Institute of Applied Physics (University of Bern). We
present in this contribution an analysis of these radio and X-ray
data. The time profiles of the X-ray emission above 50 keV and of
the centimetric/millimetric emissions show two main parts (impulsive
emission lasting about three minutes) and a long duration emission
(partially observed by RHESSI) separated in time by four minutes. At
metric/decimetric wavelengths a type II burst with an unusually high
frequency is observed between the impulsive emissions and the long
duration radio continuum. Combined analysis of RHESSI sources at
energies above a few hundred keV and of metric/decimetric sources
observed by the NRH shows the extension in space of both X-ray and
radio sources traced by energetic electrons between the impulsive
part of the event and the late energetic X-ray phase associated with
the strong radio continuum. Spectral analysis of the high energy X-ray
continuum and of the centimetric/millimetric will be performed to infer
the characteristics of energetic electrons in both parts of the events
and to further investigate in this event the relationship between
centimetric-millimetric emitting electrons and HXR/GR bremsstrahlung
emitting ones.
---------------------------------------------------------
Title: On the Photometric Accuracy of RHESSI Imaging and Spectrosocopy
Authors: Aschwanden, Markus J.; Metcalf, Thomas R.; Krucker, Säm;
Sato, Jun; Conway, Andrew J.; Hurford, G. J.; Schmahl, Edward J.
2004SoPh..219..149A Altcode: 2003astro.ph..9499A
We compare the photometric accuracy of spectra and images in flares
observed with the Ramaty High-Energy Solar Spectroscopic Imager
(RHESSI) spacecraft. We test the accuracy of the photometry by
comparing the photon fluxes obtained in different energy ranges from the
spectral-fitting software SPEX with those fluxes contained in the images
reconstructed with the Clean, MEM, MEM-Vis, Pixon, and Forward-fit
algorithms. We quantify also the background fluxes, the fidelity of
source geometries, and spatial spectra reconstructed with the five image
reconstruction algorithms. We investigate the effects of grid selection,
pixel size, field of view, and time intervals on the quality of image
reconstruction. The detailed parameters and statistics are provided
in an accompanying CD-ROM and web page. We find that Forward-fit,
Pixon, and Clean have a robust convergence behavior and a photometric
accuracy in the order of a few percent, while MEM does not converge
optimally for large degrees of freedom (for large field of view and/or
small pixel sizes), and MEM-Vis suffers in the case of time-variable
sources. This comparative study documents the current status of the
RHESSI spectral and imaging software, one year after launch.
---------------------------------------------------------
Title: The Microflare Frequency Distribution observed by RHESSI
Authors: Christe, S.; Krucker, S.; Lin, R. P.
2004cosp...35.3164C Altcode: 2004cosp.meet.3164C
The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI)
provides uniquely high sensitivity in the 3-15 keV energy range with
an effective area from 14 to 130 times larger then previous solar
instruments. We present a microflare frequency distribution derived
from RHESSI observations. Times were chosen such that activity is very
low (GOES B Class background) and includes a 24 hour period when no
active regions were present on the Sun. Microflares were found through
searching for 5σ increases in count rate, summing over all detectors,
between two adjacent time bins in the 3-10 keV energy band. Count
rates were binned from 4 seconds to 3 minutes, increasing by 4 seconds
increments, and the search repeated in order to detect flares of various
time scales. Each microflare was individually checked in order to reject
non-solar events. Few microflares are seen to occur below the time scale
of minutes. The observed occurence rate is one microflare every ∼ 8
minutes above a threshold flux of ∼ 0.4 cm<SUP>-2</SUP> s<SUP>-1</SUP>
keV<SUP>-1</SUP> over the 5-10 keV energy range. Microflares were also
found when no active regions were present on the Solar disk. This work
was supported by NASA contract NAS5-98033.
---------------------------------------------------------
Title: Gamma-ray imaging of the X-class flares of October/November
2003
Authors: Hurford, G.; Krucker, S.; Lin, R.; Schwartz, R.; Share, G.;
Smith, D.
2004cosp...35.2402H Altcode: 2004cosp.meet.2402H
Imaging of gamma-ray line emission from solar flares provides
direct information on the spatial characteristics of accelerated
ions near the Sun. To date this has been possible for only one event
(July 23, 2002) whose neutron-capture line image at 2.2 MeV implied a
significant spatial difference between high-energy ions and electrons
in acceleration and/or transport. Observations of the X17, X10 and X8
events on October 28, October 29 and November 2, 2003 by the Reuven
Ramaty High Energy Solar Spectroscopic Imager (RHESSI) provided a
ten-fold increase in the detected nuclear-line photons available for
imaging. This paper presents the results of imaging of these flares
in the 2.2 MeV neutron capture line, including source size, location
and motion estimates and comparisons with simultaneous gamma-ray and
X-ray emission in the electron-bremsstrahlung continuum. This work is
supported by NASA contract NAS5-98033.
---------------------------------------------------------
Title: Variations in Solar X-ray Flux and its Relevance to the
Coronal Heating Problem
Authors: Benz, A. O.; Grigis, P. C.; Krucker, S.
2004cosp...35..343B Altcode: 2004cosp.meet..343B
The thermal X-ray and EUV emissions of the solar corona are known
to fluctuate in time and space. The fluctuations seem to be of a
similar nature as the flares and thus are termed microflares or
nanoflares. Several forms of such small events have been observed
in the solar corona by SoHO, TRACE, Yohkoh, and RHESSI. The reported
nanoevents in the quiet regions are about 3 orders of magnitude smaller
than microevents reported in active regions and are radio-poor. Magnetic
energy dissipation by small flares is one of the scenarios for coronal
heating. These micro-events obviously increase the energy in the corona
and are signatures of coronal heating, the question is whether they
dominate coronal heating and are the cause of the existence of the
corona. The main uncertainty in determining the role of flare heating
is the flare energetics, in particular the forms of energy into which
the magnetic energy is dissipated. A large fraction becomes observable
in electrons having energies of some tens of keV. Another less known
fraction is thought to be associated with the reconnection jets and
MHD waves. The heating process(es) must be able to account for the
coronae of the quiet sun including coronal holes, active regions,
as well as more active stars showing coronal emissions at levels of
more than 3 orders of magnitude higher than the Sun.
---------------------------------------------------------
Title: Impulsive Electron Events seen at 1 AU and their Solar Origin:
a Review of Recent Results
Authors: Krucker, S.
2004cosp...35.1773K Altcode: 2004cosp.meet.1773K
Thanks to new spacecraft missions like WIND and ACE, new insights
into the solar origin of impulsive electron events seen at 1 AU are
revealed. Onset time studies show that there are likely at least two
different mechanisms that release electrons into interplanetary space:
1) electron events that are related to radio type III bursts 2) and
a later released, second population at higher energies (∼100 keV)
that is possibly related to a shock, or might be related to coronal
process seen in radio occurring after the shock already left the
corona. I will review the most recent results including a discuss of
the controversial points and I will outline how we can use the upcoming
STEREO mission to test the existence of these two classes of events.
---------------------------------------------------------
Title: Comparision of HXR Footpoint Motions of solar Flares with
Reconnection Model
Authors: Fivian, M. D.; Krucker, S.; Lin, R. P.
2004cosp...35.3248F Altcode: 2004cosp.meet.3248F
In a reconnection model of solar flares, the motion of the footpoints
and the magnetic field along the footpoints suppose to be correlated
with the deposited energy at the footpoints produced by the non-thermal
electrons. RHESSI hard X-ray imaging is used to measure the location
of the footpoints and RHESSI imaging spectroscopy is used to determine
the deposited energy in the footpoints. While a correlation can be
found for some events (e.g. July 23, 2002) the footpoint motion seems
to be completely uncorrelated in others (e.g. linear motion with 15
km/sec with clearly modulated HXR flux in the Mar-18, 2003 event). We
present a statistical analysis of these correlations as function of
various parameters.
---------------------------------------------------------
Title: The X-ray source region of <SUP>3</SUP>He-rich solar energetic
particle events
Authors: Krucker, S.; Lin, R. P.; Kontar, E. P.; Mason, G. M.;
Wiedenbeck, M. E.
2003AGUFMSH11D1130K Altcode:
The Reuven Ramaty High Energy Spectroscopic Imager RHESSI allows
for the first time to study simultaneously the spatial and spectral
characteristics of solar hard X-ray emission in detail. In this paper,
RHESSI X-ray imaging spectroscopy is used to investigate the source
regions of <SUP>3</SUP>He-rich solar energetic particles observed
at 1 AU. A series of very large <SUP>3</SUP>He-rich events that
occurred between August 19-21, 2002 are investigated. The timing
of the simultaneously observed electron events suggests that these
particle events are related to a series of GOES M class flares. All
events originated from the same active region AR0069 and show a
similar behavior: Next to the main flaring loops, TRACE observations
additionally show an EUV jet that appears to escape from the Sun with
a speed of up to 500 km s<SUP>-1</SUP>. X-ray emission is seen from
the main flaring loops as well; however, the most prominent source in
>30 keV hard X-rays appears to be displaced from the flaring loops,
at the footpoint of the field lines along which the jet is moving
outward. We investigate the possibility that the HXR footpoint is
produced by the downward moving part of the same electron population
that escapes to 1 AU, by comparing the derived electron spectrum from
the HXR footpoint source with the in situ observed electron spectrum.
---------------------------------------------------------
Title: RHESSI Microflares Statistics
Authors: Rauscher, E.; Christe, S.; Hannah, I.; Krucker, S.; Lin, R. P.
2003AGUFMSH21B0167R Altcode:
The Reuven Ramaty High Energy Solar Spectroscopic Imager
(RHESSI) provides unique sensitivity in the 3-15 keV energy
range with an effective area ∼100 times larger then similar past
instruments. Microflares have been observed with little emission above
15 keV due to extremely steep spectra (spectral index of ~8 with a low
energy cutoff near 8 keV). Since the energy in non-thermal electrons is
very sensitive to the value of the power law and the low energy cutoff,
observations by RHESSI will give a better estimate of the total energy
input into the corona. We present the first statistical analysis of
RHESSI microflares during times with activity below GOES C Class and
without solar type III radio storms. Currently, we have analyzed June
2002 and May 2003. Microflares are found through searching for peaks
3σ above background in the 6-12 keV data range. The fluxes in the 3-6
keV 12-25 keV, and 50-100 keV bands are also recorded. Statistics on the
following flare characteristics are presented; max counts, total counts,
duration, GOES level. We have also analyzed the equivalent WIND data set
in search of correlated solar type III radio bursts. Initial results
show that only a small number of RHESSI microflares are associated
with interstellar type III radio bursts. This work was supported by
NASA contract NAS5-98033.
---------------------------------------------------------
Title: RHESSI Observations of High-Temperature Plasmas in Solar Flares
Authors: Caspi, A.; Krucker, S.; Lin, R. P.
2003AGUFMSH22A0170C Altcode:
Solar flare plasmas are multi-thermal, and in particular may contain
high-temperature components above ∼20 MK. While other solar
instruments (e.g. TRACE, GOES SXI) are sensitive to low-temperature
plasmas below ∼20 MK, the Reuven Ramaty High Energy Solar
Spectroscopic Imager (RHESSI) observes photons with energies above
3 keV, and thus is especially sensitive to high-temperature plasmas
above ∼10 MK. High-temperature plasma emission includes the Fe/Ni line
complexes at ∼6.7 and ∼8 keV, and both the equivalent widths and the
fluxes of these line complexes are strongly temperature-dependent. In
many flares, RHESSI detects emission in both of these line complexes,
as well as emission in the thermal continuum, with spectral resolution
of ∼1 keV FWHM. Through imaging and spectroscopy with RHESSI, and
by comparison with other solar instruments, we can thus constrain the
temperatures and emission measures over the entire range of thermal
plasmas. We present a spectroscopic analysis of a variety of flares,
including the X1.5 event on 21 April 2002, to determine the time-varying
characteristics of the thermal electron populations. We estimate
the energy contained in thermal electrons and compare with the energy
contained in the time-varying non-thermal electron population. Finally,
we discuss the implications for heating and energy transport in flares
with high-temperature components.
---------------------------------------------------------
Title: First Detection of Hard X-ray Emission From Solar Type III
Radio Bursts
Authors: Christe, S.; Krucker, S.; Lin, R. P.; Arzner, K.; Benz, A. O.
2003AGUFMSH11D1131C Altcode:
We present the first detection of non-flare related hard X-ray
emission from type III radio bursts as observed by the Reuven Ramaty
High Energy Solar Spectroscopic Imager (RHESSI). During a period of
15 minutes on 19 July 2002 14:23-14:35 UT, the WAVES instrument on
the Wind spacecraft observed interplanetary type III radio bursts
approximately every 2 minutes and each was accompanied by a 12-15 keV
X-ray brightening observed by RHESSI. The radio and X-rays fluxes were
found to be strongly correlated. No flares were reported in the SEC
solar event reports during this time and only the strongest brightening
is associated with a detectable enhancement in the GOES levels (A3
above a B8 background). Phoenix-2, a ground-based radio spectrometer,
observed each interplanetary type III to extend down to 300 MHz
(0.1 R<SUB>sun</SUB>) The strongest type III was also accompagnied
by a cluster of decimetric radio emission in the frequency range 1
to 2 GHz. A close correlation is found between X-ray fluxes and the
decimetric fluxes. X-ray spectra show non-thermal emission (9-30 keV)
with an electron spectral power-law index of ∼4, from the footpoint
of a TRACE loop observed in FeXII (195 Å). Subsequently, jets are
seen to originate from the RHESSI footpoint emission travelling with
apparent speeds of ∼ 100 km s<SUP>-1</SUP>. The observed RHESSI hard
X-ray fluxes require ∼10<SUP>33</SUP> electrons above 10 keV. This
work was supported by NASA contract NAS5-98033.
---------------------------------------------------------
Title: Temporal Variability of Gamma-Ray Lines from the X-Class
Solar Flare of 2002 July 23
Authors: Shih, A. Y.; Smith, D. M.; Lin, R. P.; Hurford, G. J.;
Krucker, S.; Schwartz, R. A.; Share, G. H.; Murphy, R. J.
2003AGUFMSH11D1132S Altcode:
On 2002 July 23, the Reuven Ramaty High Energy Solar Spectroscopic
Imager (RHESSI) detected a GOES X4.8 solar flare, and analysis of the
data has produced the first spectrally resolved measurement of nuclear
de-excitation gamma-ray lines in a solar flare. These narrow lines
result from accelerated ions colliding with the ambient medium (which
includes Fe, Mg, Ne, Si, C, and O nuclei), and thus their relative
fluxes probe the composition of the solar atmosphere. We investigate
the temporal variability of the flux of these lines by analyzing
the observed spectrum for two separate time intervals of the flare
(00:27:20-00:32:56 UT and 00:32:56-00:43:20 UT). The relative fluxes in
the gamma-ray lines change dramatically between these two intervals,
suggesting that the circumstances of ion acceleration change as the
flare progresses. We consider the possibility of variations in the
nuclear abundances, as well as other possible causes such as a varying
alpha/proton ratio or a varying energy spectrum for the accelerated
ions. Finally, we compare these variations with the source locations
as determined by RHESSI's gamma-ray imaging. The work at the University
of California, Berkeley, was supported by NASA contract NAS 5-98033.
---------------------------------------------------------
Title: A study of interplanetary propagation of solar impulsive
~1-100keV electron events
Authors: Wang, L.; Lin, R. P.; Krucker, S.; Larson, D. E.
2003AGUFMSH21B0153W Altcode:
We study solar impulsive ~1-100keV electron events that exhibit
clear velocity dispersion, as observed by the 3D Plasma and Energetic
Particle (3DP) instrument on the WIND spacecraft. At the onset of those
impulsive electron events, the outward travelling electrons are observed
first, then the backward travelling electrons are detected, mostly
in high energy range (>20keV) and sometimes in low energy range
(<20keV). The backward population must be produced by scattering
and/or magnetic mirroring in the interplanetary medium beyond 1 AU. For
one impulsive event observed on December 5,1997, we determine the delay
between the onset of the outward travelling electrons and the onset of
the backward travelling electrons in different pitch angle ranges. We
find that the delays for 27, 40 and 66 keV electrons, respectively, are
10, 5.8 and 7.3 minutes at ~10° pitch angle; 1.7,1.8 and 3 minutes at
~30° pitch angle; 10.5, 6 and 7.2 minutes at ~55° pitch angle; 6.2,
3.2 and 4.2 minutes at ~80° pitch angle.Those time differences are not
inversly proportional to parallel velocity along the magnetic field B,
so the simple magnetic mirror is unlikely to be an explanation. We
investigate pitch angle scattering by magnetic field fluctuations as
a possible mechanism.
---------------------------------------------------------
Title: Non-thermal Coronal Hard X-ray Emission Observed During a
Partially Occulted Flare
Authors: Balciunaite, P.; Krucker, S.; Caspi, A.; Lin, R. P.
2003AGUFMSH22A0172B Altcode:
We will present an analysis of RHESSI hard X-ray imaging and
spectroscopy of a GOES M1.5 class flare which occurred on 1 June 2003
0210-0700 UT in NOAA active region 0375 and was located 9 degrees behind
the east limb of the Sun. Flare footpoints were thus occulted from
view by ~9000 km, allowing for a relatively unobstructed detection of
emission from the associated coronal sources. Throughout the duration
of the flare, a thermal hard X-ray source can be seen rising with
a velocity of ~7 km/s above GOES SXI soft X-ray and EIT ultraviolet
(304A) loops. During the impulsive phase of the flare (0237-0310 UT), a
non-thermal source is additionally seen above 15 keV with a relatively
soft power law spectrum with an index around -5. This non-thermal
source is displaced from the thermal source by up to 10,000 km and
shows greater variations on a time scale of tens of seconds. We will
present total energy estimates and a detailed analysis of the source
motions during the flare. We will also discuss implications for models
that postulate above-the-loop-top sources.
---------------------------------------------------------
Title: Hard X-Ray Source Motions in the 2002 July 23 Gamma-Ray Flare
Authors: Krucker, Säm; Hurford, G. J.; Lin, R. P.
2003ApJ...595L.103K Altcode:
The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) is
used to study the hard X-ray (HXR) source motions of the 2002 July 23
γ-ray flare. Above 30 keV, at least three HXR sources are observed
during the impulsive phase that can be identified with footpoints of
coronal magnetic loops that form an arcade. On the northern ribbon
of this arcade, a source is seen that moves systematically along the
ribbon for more than 10 minutes. On the other ribbon, at least two
sources are seen that do not seem to move systematically for more
than half a minute, with different sources dominating at different
times. The northern source motions are fast during times of strong HXR
flux but almost absent during periods with low HXR emission. This is
consistent with magnetic reconnection if a higher rate of reconnection
of field lines (resulting in a higher footpoint speed) produces more
energetic electrons per unit time and therefore more HXR emission. The
absence of footpoint motion in one ribbon is inconsistent with simple
reconnection models but can be explained if the magnetic configuration
there is more complex.
---------------------------------------------------------
Title: Radio and Hard X-Ray Images of High-Energy Electrons in an
X-Class Solar Flare
Authors: White, S. M.; Krucker, S.; Shibasaki, K.; Yokoyama, T.;
Shimojo, M.; Kundu, M. R.
2003ApJ...595L.111W Altcode:
We present the first comparison between radio images of high-energy
electrons accelerated by a solar flare and images of hard X-rays
produced by the same electrons at photon energies above 100 keV. The
images indicate that the high-energy X-rays originate at the footpoints
of the loops dominating the radio emission. The radio and hard X-ray
light curves match each other well and are quantitatively consistent
with an origin in a single population of nonthermal electrons with
a power-law index of around 4.5-5. The high-frequency radio spectral
index suggests a flatter energy spectrum, but this is ruled out by the
X-ray spectrum up to 8 MeV. The preflare radio images show a large
hot long-lived loop not visible at other wavelengths. Flare radio
brightness temperatures exceed 10<SUP>9</SUP> K, and the peak in the
radio spectrum is as high as 35 GHz: both these two features and the
hard X-ray data require very high densities of nonthermal electrons,
possibly as high as 10<SUP>10</SUP> cm<SUP>-3</SUP> above 20 keV at
the peak of the flare.
---------------------------------------------------------
Title: RHESSI Hard X-Ray Imaging Spectroscopy of the Large Gamma-Ray
Flare of 2002 July 23
Authors: Emslie, A. Gordon; Kontar, Eduard P.; Krucker, Säm; Lin,
Robert P.
2003ApJ...595L.107E Altcode:
We present Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI)
hard X-ray images in different energy bands for the large X-class flare
of 2002 July 23; these images are used to construct spatially resolved
hard X-ray spectra for each of four prominent features: a bright,
soft source high in the corona, two localized, hard footpoints in
opposite polarity magnetic regions that show highly correlated flux
and spectral variations in time, and a third footpoint bounded by
the other three sources. The power-law spectral indices of the two
correlated footpoints differ by ~0.3-0.4, which may be the result of
differing column densities from the electron source.
---------------------------------------------------------
Title: First Gamma-Ray Images of a Solar Flare
Authors: Hurford, G. J.; Schwartz, R. A.; Krucker, S.; Lin, R. P.;
Smith, D. M.; Vilmer, N.
2003ApJ...595L..77H Altcode:
Imaging of gamma-ray lines, produced by nuclear collisions of energetic
ions with the solar atmosphere, provides the only direct indication of
the spatial properties of accelerated ions near the Sun. We present the
first gamma-ray images of a solar flare, obtained with the Reuven Ramaty
High Energy Solar Spectroscopic Imager (RHESSI) for the X4.8 flare of
2002 July 23. Two rotating modulation collimators (with 35" and 183"
resolution) were used to obtain images for the same time interval in
four energy bands: the narrow deuterium line at 2.223 MeV formed by the
thermalization and capture of neutrons produced in the collisions; the
3.25-6.5 MeV band that includes the prompt de-excitation lines of C and
O; and the 0.3-0.5 and 0.7-1.4 MeV bands that are dominated by electron
bremsstrahlung. The centroid of the 2.223 MeV image was found to be
displaced by 20<SUP>”</SUP>+/-6<SUP>”</SUP> from that of the 0.3-0.5
MeV image, implying a difference in acceleration and/or propagation
between the accelerated electron and ion populations near the Sun.
---------------------------------------------------------
Title: RHESSI Observations of Particle Acceleration and Energy
Release in an Intense Solar Gamma-Ray Line Flare
Authors: Lin, R. P.; Krucker, S.; Hurford, G. J.; Smith, D. M.; Hudson,
H. S.; Holman, G. D.; Schwartz, R. A.; Dennis, B. R.; Share, G. H.;
Murphy, R. J.; Emslie, A. G.; Johns-Krull, C.; Vilmer, N.
2003ApJ...595L..69L Altcode:
We summarize Reuven Ramaty High Energy Solar Spectroscopic Imager
(RHESSI) hard X-ray (HXR) and γ-ray imaging and spectroscopy
observations of the intense (X4.8) γ-ray line flare of 2002 July 23. In
the initial rise, a new type of coronal HXR source dominates that has
a steep double-power-law X-ray spectrum and no evidence of thermal
emission above 10 keV, indicating substantial electron acceleration
to tens of keV early in the flare. In the subsequent impulsive phase,
three footpoint sources with much flatter double-power-law HXR spectra
appear, together with a coronal superhot (T~40 MK) thermal source. The
north footpoint and the coronal source both move systematically to the
north-northeast at speeds up to ~50 km s<SUP>-1</SUP>. This footpoint's
HXR flux varies approximately with its speed, consistent with magnetic
reconnection models, provided the rate of electron acceleration
varies with the reconnection rate. The other footpoints show similar
temporal variations but do not move systematically, contrary to simple
reconnection models. The γ-ray line and continuum emissions show that
ions and electrons are accelerated to tens of MeV during the impulsive
phase. The prompt de-excitation γ-ray lines of Fe, Mg, Si, Ne, C,
and O-resolved here for the first time-show mass-dependent redshifts
of 0.1%-0.8%, implying a downward motion of accelerated protons and
α-particles along magnetic field lines that are tilted toward the
Earth by ~40°. For the first time, the positron annihilation line is
resolved, and the detailed high-resolution measurements are obtained
for the neutron-capture line. The first ever solar γ-ray line and
continuum imaging shows that the source locations for the relativistic
electron bremsstrahlung overlap the 50-100 keV HXR sources, implying
that electrons of all energies are accelerated in the same region. The
centroid of the ion-produced 2.223 MeV neutron-capture line emission,
however, is located ~20<SUP>”</SUP>+/-6<SUP>”</SUP> away, implying
that the acceleration and/or propagation of the ions must differ from
that of the electrons. Assuming that Coulomb collisions dominate the
energetic electron and ion energy losses (thick target), we estimate
that a minimum of ~2×10<SUP>31</SUP> ergs is released in accelerated
>~20 keV electrons during the rise phase, with ~10<SUP>31</SUP>
ergs in ions above 2.5 MeV nucleon<SUP>-1</SUP> and about the same
in electrons above 30 keV released in the impulsive phase. Much more
energy could be in accelerated particles if their spectra extend to
lower energies.
---------------------------------------------------------
Title: Properties of high heliolatitude solar energetic particle
events and constraints on models of acceleration and propagation
Authors: Dalla, S.; Balogh, A.; Krucker, S.; Posner, A.;
Müller-Mellin, R.; Anglin, J. D.; Hofer, M. Y.; Marsden, R. G.;
Sanderson, T. R.; Tranquille, C.; Heber, B.; Zhang, M.; McKibben, R. B.
2003GeoRL..30.8035D Altcode: 2003GeoRL..30sULY9D
We analyse 9 large solar energetic particle (SEP) events detected
by the Ulysses spacecraft at high heliolatitudes during the
recent solar maximum polar passes. Properties of time intensity
profiles from the Ulysses/COSPIN instrument are compared with those
measured by SOHO/COSTEP and Wind/3DP near Earth. We find that onset
times and times to maximum at high latitude are delayed compared to
in-ecliptic values. We show that the parameter which best orders these
characteristics of time profiles is the difference in latitude between
the associated flare and the spacecraft. We find that the presence of a
shock is not necessary for the establishing of near equal intensities
at Ulysses and in the ecliptic during the decay phase. The model of
SEP acceleration by coronal mass ejection driven shocks does not appear
to account for our observations, which would more easily be explained
by particle diffusion across the interplanetary magnetic field.
---------------------------------------------------------
Title: Characterization of SEP events at high heliographic latitudes
Authors: dalla, S.; Balogh, A.; Krucker, S.; Posner, A.;
Müller-Mellin, R.; Anglin, J. D.; Hofer, M. Y.; Marsden, R. G.;
Sanderson, T. R.; Heber, B.; Zhang, M.; McKibben, R. B.
2003AIPC..679..656D Altcode:
Between February 2000 and May 2002, the Ulysses spacecraft made the
first ever measurements of solar energetic particles (SEPs) at high
heliographic latitudes. Nine large gradual SEP events were detected
at latitudes greater than 45°, their signatures being clearest at
high particle energies, i.e. protons >30 MeV and electrons >0.1
MeV. In this paper we measure the onset times of Ulysses high latitude
events in several energy channels, and plot them versus inverse particle
speed. We repeat the procedure for near Earth observations by Wind and
SOHO. Velocity dispersion is observed in all the events near Earth and
in most of them at Ulysses. The plots of onset times versus inverse
speed allow to derive an experimental path length and time of release
from the solar atmosphere. We find that the derived path lengths at
Ulysses are longer than the length of a Parker spiral magnetic field
line connecting it to the Sun, by a factor between 1.2-2.7. The time
of particle release from the Sun is typically between 100 and 200 mins
later than the relase time derived from in-ecliptic measurements. Unlike
near Earth observations, Ulysses measurements are therefore not
compatible with scatter-free propagation from the Sun to the spacecraft.
---------------------------------------------------------
Title: A multiwavelength analysis of the august 30, 2002 flare
Authors: Gimenez de Castro, C. G.; Silva, A. V. R.; Trottet, G.;
Costa, J. E. R.; Kaufman, P.; Correia, E.; Krucker, S.; Martinez,
G.; Rovira, M.; Lüthi, T.; Magun, A.; Levato, H.
2003BASBr..23...23G Altcode:
We present preliminary results of a multiwavelength analysis of the
flare classified as X1.5 which occurred on Aug 30 2002 at about 1328
UT in NOAA region 0095. The event was observed by the Ramaty High
Energy Solar Spectrometer Imager (RHESSI) up to 200-300 keV and by
the new Solar Submillimeter Telescope (SST) at 212 GHz. Radio image
observations at long decimetric-metric waves obtained with the Nançay
Radioheliograph are also being included. Our analysis utilizes radio
flux from different telescopes to cover an extended range from 160
MHz up to 212 GHz. HxR images show two sources above 30-40 keV, 5
and 3 arc sec in diameter separated by about 7 arc sec, and a third
source at low energies. Both the total photon spectra and individual
source spectra, were fitted by a double power-law. We compare HxR with
Ha images obtained with the HASTA telescope. During peak time the
microwave spectrum is flat between 10 to 35 GHz. We discuss a flare
model with a homogeneous source which includes both gyrosynchrotron
and free-free emission/absorption to explain the observed flatness.
---------------------------------------------------------
Title: RHESSI Discovery of a Coronal Non-Thermal Hard X-Ray Source
in the 23 July 2002 Gamma-Ray Line Flare
Authors: Lin, R. P.; Krucker, S.; Holman, G. D.; Sui, L.; Hurford,
G. J.; Schwartz, R. A.
2003ICRC....6.3207L Altcode: 2003ICRC...28.3207L
The Reuven Ramaty High Energy Solar spectroscopic Imager (RHESSI)
observations of the gamma-ray line flare of 2002 July 23, show that
in the ∼8 minute rise prior to the impulsive phase, the hard X-ray
emission comes from a coronal source which has no counterpart in
the simultaneous TRACE EUV images or in the Hα images. The spectrum
above ∼10 keV fits to a double-powerlaw shape with break energies
at ∼20-40 keV and exponents of ∼5 below and ∼7 above, with
no obvious thermal emission above ∼10 keV. This coronal nonthermal
source implies that substantial energy release and electron acceleration
occurs before the impulsive phase.
---------------------------------------------------------
Title: First Gamma-Ray Images of a Solar Flare
Authors: Hurford, G. J.; Schwartz, R. A.; Krucker, S.; Lin, R. P.;
Smith, D. M.; Vilmer, N.
2003ICRC....6.3203H Altcode: 2003ICRC...28.3203H; 2003ICRC....6.3203L
We present the first gamma-ray images of a solar flare, obtained with
the Reuven Ramaty High Energy Solar spectroscopic Imager (RHESSI) for
the X4.8 flare of 2002 July 23. Two rotating modulation collimators
(35 & 180 resolution) provided images of the narrow deuterium line
at 2.223 MeV formed by thermalization and capture of neutrons produced
in energetic ion collisions, the 3.25-6.5 MeV band that includes the
prompt de-excitation lines of C and O, and the 0.30.5 and 0.7-1.4 MeV
bands that are dominated by electron-bremsstrahlung. The centroid of
the 2.223 MeV image was found to be displaced by ∼20(±6) arcsec from
that of the 0.3-0.5 MeV band, implying a difference in acceleration
and/or propagation between the accelerated electron and ion populations
near the Sun.
---------------------------------------------------------
Title: Onsets and Release Times in Solar Particle Events
Authors: Tylka, A. J.; Cohen, C. M. S.; Dietrich, W. F.; Krucker, S.;
McGuire, R. E.; Mewaldt, R. A.; Ng, C. K.; Reames, D. V.; Share, G. H.
2003ICRC....6.3305T Altcode: 2003ICRC...28.3305T
The time at which solar energetic particles (SEPs) are first released
into interplanetary space, and its relation to CMEs and various
photon emissions, are important clues to the site and nature of the
SEP acceleration mechanism [15,7,8,10,11]. We examine velocity disp
ersion among onsets in electrons and ions from Wind, ACE, and IMP8,
as well as available neutron monitors, to determine the solar release
time. We present results for two large impulsive events (1 May 2000
and 14 April 2001) and three western ground level events (GLEs;
6 November 1997, 6 May 1998, and 15 April 2001). In the impulsive
events, the particle release coincides with hard x-ray emission. But
the large GLEs show delayed release with respect to γ -ray emission,
consistent with acceleration by the CME-driven shock.
---------------------------------------------------------
Title: Delay in solar energetic particle onsets at high heliographic
latitudes
Authors: dalla, S.; Balogh, A.; Krucker, S.; Posner, A.;
Müller-Mellin, R.; Anglin, J. D.; Hofer, M. Y.; Marsden, R. G.;
Sanderson, T. R.; Heber, B.; Zhang, M.; McKibben, R. B.
2003AnGeo..21.1367D Altcode:
Ulysses observations have shown that solar energetic particles (SEPs)
can easily reach high heliographic latitudes. To obtain information on
the release and propagation of SEPs prior to their arrival at Ulysses,
we analyse the onsets of nine large high-latitude particle events. We
measure the onset times in several energy channels, and plot them versus
inverse particle speed. This allows us to derive an experimental path
length and time of release from the solar atmosphere. We repeat the
procedure for near-Earth observations by Wind and SOHO. We find that
the derived path lengths at Ulysses are 1.06 to 2.45 times the length
of a Parker spiral magnetic field line connecting the spacecraft
to the Sun. The time of particle release from the Sun is between
100 and 350 min later than the release time derived from in-ecliptic
measurements. We find no evidence of correlation between the delay in
release and the inverse of the speed of the CME associated with the
event, or the inverse of the speed of the corresponding interplanetary
shock. The main parameter determining the magnitude of the delay
appears to be the difference in latitude between the flare and the
footpoint of the spacecraft.
---------------------------------------------------------
Title: RHESSI Observations of Two Occulted Solar Flares
Authors: Balciunaite, P.; Krucker, S.; Christe, S.; Lin, R. P.
2003SPD....34.1810B Altcode: 2003BAAS...35..841B
We present RHESSI (Reuven Ramaty Solar Spectroscopic Imager)
observations of two limb-occulted solar flares (GOES C2 and C5 class),
which occurred on 13 May 2002 10:50-22:20 UT and 14 October 2002 14:00
UT-15 October 2002 16:00 UT, respectively. The observations of such
flares allow the investigation of coronal hard X-ray sources without
the interference of the generally brighter footpoint sources. The
aforementioned events are inferred to originate 34 and 44 degrees behind
the east limb by tracking the most probable originating active regions
(NOAA 9957 and 0162, respectively). The 8-15 keV source centroid
position as a function of time initially shows a constant position
just above the solar limb for approximately 2 hours. Then the source
appears to rise above the limb, in a manner that could not be due
solely to solar rotation. Presently we are investigating spectra for
both events to derive the temperature and emission measures and search
for a possible non-thermal contribution.
---------------------------------------------------------
Title: RHESSI Hard X-Ray Imaging Spectroscopy of the July 23, 2002
Solar Flare
Authors: Emslie, A. G.; Kontar, E. P.; Krucker, S.; Lin, R. P.
2003SPD....34.2208E Altcode: 2003BAAS...35..851E
We present hard X-ray images in different energy bands, as obtained
by the unique combination of rotating modulation collimators and
high-resolution germanium spectrometers on the RHESSI satellite,
for the large X-class flare of July 23, 2002. These data are then
used to construct spatially-resolved hard X-ray spectra for each
of four prominent features evident in these images. These four main
features are a bright, low-energy (soft spectrum) source high in the
corona, two localized high-energy (hard spectrum) footpoints and a
bright feature bounded by the other three, which could either be an
additional footpoint or a source near the top of a magnetic structure
connecting the other two. The temporal evolution of the spectrum of
each feature is described, with allowances for the dynamic range of
the RHESSI instrument, which obscures weak sources at a given photon
energy when one or more much brighter sources at this photon energy
are also present. A comparison with the temporal evolution of the
spatially-integrated hard X-ray spectrum is also provided. <P />This
work was supported by NASA's Office of Space Science and by a PPARC
Award.
---------------------------------------------------------
Title: RHESSI Hard X-ray observations of the March 18, 2003 X-class
Flare
Authors: Fivian, M. D.; Krucker, S.; Lin, R. P.
2003SPD....34.1814F Altcode: 2003BAAS...35..842F
Hard X-ray observations of the GOES X1.5 flare on March 18, 2003, taken
by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI)
are presented. This long duration event (GOES duration slightly longer
than 1 hour) occurred at S15W46. Enhanced hard X-ray emission (>30
keV) is seen for more than 25 minutes with the strongest HXR bursts
detected almost 10 minutes after the peak emission seen in GOES soft
X-ray. Signatures of escaping energetic electrons, such as radio type
III bursts seen with WIND/WAVES in the interplanetary space and a large
electron event detected by WIND/3DP at 1 AU, are only observed with the
later HXR burst but not during the rise phase of the SXR. In order to
understand this, the HXR sources and their motions are investigated
and compared to TRACE (171 /AA) EUV images and MDI magnetograms. <P
/>This work is funded by NASA grant NAS5-98033-05/03.
---------------------------------------------------------
Title: RHESSI Observations of Solar Hard X-ray Flares with and
without Interplanetary Type III Radio Bursts
Authors: Rauscher, E.; Krucker, S.; Lin, R. P.
2003SPD....34.1813R Altcode: 2003BAAS...35..841R
The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI)
allows more detailed imaging of hard X-ray flares than has been
available before. The hard X-ray sources seen in solar flares are
believed to be the result of accelerated electrons streaming down
the legs of magnetic loops and colliding with the denser plasma at
the footpoints, releasing hard X-rays through bremsstrahlung. Type
III interplanetary radio bursts are evidence of accelerated electrons
leaving the Sun. Previous studies have indicated a correlation between
hard X-ray flares and type III interplanetary radio bursts for some
events. We are now studying the structure of hard X-ray flares using
RHESSI and TRACE (EUV) movies to investigate possible differences
between flares that have corresponding radio bursts (as detected by
WIND/WAVES) and ones that do not. We have collected over 600 events from
the first year of RHESSI. From that group, we have chosen a half dozen
events with good coverage from both instruments for more detailed study.
---------------------------------------------------------
Title: Type III Radio Bursts and Microflares
Authors: Christe, S.; Krucker, S.; Arzner, K.; Lin, R. P.
2003SPD....34.1501C Altcode: 2003BAAS...35..830C
We present recent observations of microflares observed simultaneously
in EUV (TRACE), radio (Nancay, Phoenix-2), and X-rays (RHESSI). During
a period of 15 min on 19 July 2002 14:23-14:35 UT, RHESSI observed
microflares approximately every 2 minutes. Each microflare was
accompagnied by a radio Type III burst. The largest flare (14:29:25
UT) was also accompagnied by a cluster of decimetric radio spikes in
the frequency range 1 to 2 GHz. In addition, FeXII (195 Å) images
provided by TRACE show two jets-like emissions originating from a
complex double arche structure. The centroid of the jets were found to
travel at apparent speeds of ∼ 100 km s<SUP>-1</SUP>, consistent with
observations by Shimojo et al. (1996). X-ray images show non-thermal
emission (9-30 keV) from the footpoints of the TRACE arches. Strong
correlation in flux amplitude is found between emissions in the radio
( ∼1340 MHz) and non-thermal X-ray (9-30 keV integrated). The
event is interpreted as an anemone-jet in the model by Shibata et
al. (1994). This research is supported by NASA contract NAS 5-98033.
---------------------------------------------------------
Title: Statistical Properties of Solar Impulsive Electron Events
Authors: Oakley, P.; Krucker, S.; Lin, R. P.
2003SPD....34.1607O Altcode: 2003BAAS...35..833O
This is a statistical study with the WIND 3DP instrument of 509 solar
impulsive electron events from 1994 to 2001. We analyze and compare
the peak flux spectra, the presence of Langmuir waves, the frequency
distribution of events and the time delay between the injection of
electrons and the emission of the related radio type III bursts. Our
most interesting finding is that electron events occurring during
a radio type II burst show significantly harder spectra, indicating
that shocks with type II bursts might be more efficient at accelerating
higher energy particles. <P />We also observed that the number of events
seen only at low energies increases after the peak of the solar cycle
has passed, while the number of events seen only at higher energies
peaks with the solar cycle. Solar wind density is not observed to play
any role in determining at what energies a solar event is visible.
---------------------------------------------------------
Title: First Gamma-Ray Images of a Solar Flare: 2002 July 23
Authors: Hurford, G. J.; Schwartz, R. A.; Krucker, S.; Lin, R. P.;
Smith, D. M.; Vilmer, N.
2003SPD....34.1409H Altcode: 2003BAAS...35..830H
Imaging of gamma-ray lines, produced by nuclear collisions of energetic
ions with the solar atmosphere, provides the only direct indication
of the spatial properties of accelerated ions near the Sun. The Reuven
Ramaty High Energy Solar Spectroscopic Imager (RHESSI) was used to make
the first gamma-ray line images of a solar flare. The X4.8 flare of
2002 July 23, was imaged in the 2218-2228 band containing the narrow
neutron-capture deuteriom line at 2223 keV, and in the 3250-6500 keV
band which includes the prompt de-excitation lines of C and O. These are
compared with images at 300-500 and 700-1400 keV, which are dominated
by electron-bremsstrahlung. The centroids of 35-arcsecond resolution
images made at these two lower-energy bands were displaced by 20
arcseconds from the 2223 keV image of the neutron capture line. Such
an offset indicates a difference in acceleration and/or propagation
between the accelerated electron and ion populations near the Sun. <P
/>This work was supported by NASA grant NAS5-98033-05/03.
---------------------------------------------------------
Title: Temporal Variability of Gamma-Ray Lines from the X-Class
Solar Flare of 2002 July 23
Authors: Shih, A. Y.; Smith, D. M.; Lin, R. P.; Krucker, S.; Schwartz,
R. A.; Share, G. H.; Murphy, R. J.
2003SPD....34.1801S Altcode: 2003BAAS...35..839S
On 2002 July 23, the Reuven Ramaty High Energy Solar Spectroscopic
Imager (RHESSI) detected a GOES X4.8 solar flare, and analysis of
the data has produced the first spectrally resolved measurement of
nuclear de-excitation lines in a solar flare. We investigate the
temporal variability of the flux of these lines by analyzing the
observed spectrum for distinct time intervals of the flare. After the
first major burst of the flare, the gamma-ray lines produced by iron,
carbon, and oxygen show statistically significant changes in their
fluxes relative to the neon line, suggesting that the circumstances of
ion acceleration change as the flare progresses. We consider possible
causes such as a varying alpha/proton ratio or a varying energy spectrum
for the ions. Finally, we correlate these variations with other observed
variations of the flare. <P />The work at the University of California,
Berkeley, was supported by NASA contract NAS 5-98033.
---------------------------------------------------------
Title: Energy estimates for solar flares using RHESSI and GOES SXI
Authors: Caspi, A.; Krucker, S.; Lin, R. P.
2003SPD....34.2204C Altcode: 2003BAAS...35..850C
We examine energy release and transport in a variety of midsize-to-large
flares (GOES X-ray class C5 and above) using the Ramaty High Energy
Solar Spectroscopic Imager (RHESSI) and the GOES Solar X-ray Imager
(SXI). Previous observations suggest that high-energy electrons
accelerated during the flare lose their energy primarily through
Coulomb collisions with the solar atmosphere, heating it to produce
the soft X-ray emitting plasma. This results in the so-called
Neupert effect, where the soft X-ray time profile corresponds to the
time-integrated hard X-ray profile from non-thermal bremsstrahlung of
the electrons. RHESSI can, for the first time, obtain both images and
spectra over the full range of 3 keV to 17 MeV with excellent resolution
spatially, temporally, and energetically. <P />We calculate the thermal
energy vs. time for each flare by using temperature and emission measure
values derived from GOES data, using SXI images to estimate source
volumes. Using RHESSI imaging and spectroscopic data, we calculate the
energies in non-thermal energetic electrons and in thermal plasmas
at temperatures above the peak response of SXI/GOES. We compare the
RHESSI- and SXI-obtained energies and discuss the implications for
energy release and transport in flares by energetic electrons.
---------------------------------------------------------
Title: Hard X-ray Sources and Motions in the July 23, 2002 Gamma-Ray
Flare
Authors: Krucker, Sam; Hurford, G. H.; Lin, R. P.
2003SPD....34.1406K Altcode: 2003BAAS...35..829K
The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI)
is used to study the Hard X-ray (HXR) emission of the July 23, 2002
gamma-ray flare. In the inital rise (00:21-00:26 UT), a probably
non-thermal coronal HXR source dominates with weaker emission from
foopoints in two ribbons visible in H-alpha and EUV. During the main
impulsive phase (00:27-00:42 UT), HXR footpoint emission dominates
above 30 keV, whereas at lower energies a gradual coronal HXR source is
seen. The footpoint emission originates from several sources close to
a magnetic neutral line. The time profiles of two footpoints apparently
in opposite polarity regions are similar, suggesting magnetic connection
between them. The relative separation of these two footpoints increases
in time. On the positive polarity side, a source moves roughly parallel
to the neutral line with a speed of up to ∼100 km s<SUP>-1</SUP>;
on the negative polarity side with slightly weaker magnetic fluxes,
several sources are seen that do not seem to move systematically, with
different sources dominating at different times. The coronal source
shows an elongated structure, possibly containing more than one source,
that moves with a similar speed in almost the same direction as the
moving footpoint. The speed of both the footpoint and the coronal
source motions are roughly correlated with the total HXR flux of the
footpoints. The source motions are fast during times of strong HXR
flux, but almost absent during periods with low HXR emission. These
observations are consistent with magnetic reconnection models in
which a higher rate of reconnection of field lines produces more
energetic electrons per unit time and therefore more HXR emission,
and also results in a higher apparent speed of the HXR sources at the
footpoints of newly reconnected loops.
---------------------------------------------------------
Title: Studies of Microflares with RHESSI, Hard X-ray in BBSO Hα
and MDI Magnetograms
Authors: Liu, C.; Qiu, J.; Gary, D.; Krucker, S.; Wang, H.
2003SPD....34.2203L Altcode: 2003BAAS...35..850L
Microflares are among the most interesting subjects because its
accumulated energy may be a substantial part to heat the upper solar
atmosphere. The unique high sensitivity in the energy range from ∼3
to 15 keV and excellent spectral and spatial resolution provided by the
Ramaty High Energy Solar Spectroscopic Imager mission (RHESSI) allow
for the first time the detailed study of the locations and the spectra
property of solar microflares. We studied a number of microflares in the
energy range from 3 to 15 keV which occurred in several active regions,
in July 2002, and we found: (1) from the co-alignment results between
RHESSI and MDI, we confirm that almost all the microflares are located
in magnetic neutral lines; (2) from the co-alignment results between
RHESSI and Hα images, we find that the hard X-ray source tends to
connect two Hα footpoints and the RHESSI peaks always occur ∼1
minute earlier than the observable Hα brightening, which indicates
the possible loop-top source before the Hα footpoint brightening;
(3) We also notice that some microflares are associated with type III
radio bursts as observed by the radio spectrometer on-board WIND.
---------------------------------------------------------
Title: Rapid Changes in the Longitudinal Magnetic Field Associated
with the July 23, 2002 γ -ray Flare
Authors: Yurchyshyn, V. B.; Wang, H.; Abramenko, V. I.; Spirock,
T. J.; Krucker, S.
2003SPD....34.1508Y Altcode: 2003BAAS...35Q.832Y
In this paper we analyze and discuss rapid changes of the magnetic field
associated with the July 23, 2002 γ -ray flare. MDI magnetic flux
profiles and BBSO vector magnetograms showed that immediately after
the flare the leading polarity had increased by 2*E<SUP>20</SUP>Mx,
while the following polarity decreased only by 1*E<SUP>20</SUP>Mx. The
observed changes were permanent and not caused by variations in seeing
or changes in the line profile, which we used to measure the magnetic
field. In this active region we distinguish two separate locations,
which show the most dramatic changes in the magnetic field. The
first location showed an increase in the magnetic field strength
and a new penumbra area and it was associated with emergence of new
magnetic flux. At the second location the position of the neutral
line had changed and it coincided with the footpoints of a rapidly
growing post-flare loop system. Linear force-free field simulations
showed that the re-orientation of the magnetic field during the flare
was capable of producing the observed changes in the total magnetic
flux. We also discuss a possible magnetic configuration responsible
for the flare. This work was supported in part by NSF ATM-0086999 and
ATM-0205157 and under NASA NAG5-10910 NAG5-12782 grants.
---------------------------------------------------------
Title: Why was there no Solar Energetic Particle Event Associated
with the Gamma-ray-line Flare of 2002 July 23?
Authors: Gopalswamy, N.; Dennis, B. R.; Kaiser, M. L.; Krucker, S.;
Lin, R. P.; Vourlidas, A.
2003SPD....34.2202G Altcode: 2003BAAS...35..850G
We investigated the coronal and interplanetary (IP) events associated
with two X-class flares on 2002 July 20 and 23. Both flares were
associated with ultra-fast (>2000 km s<SUP>-1</SUP>) coronal mass
ejections (CMEs) and IP shocks. We use white-light, EUV, hard X-ray and
radio observations to trace the origin of the CMEs to active region
0039 located close to the east limb. The July 20 flare was partly
occulted by the east limb, yet it resulted in a major solar energetic
particle event with intensity ∼ 20 pfu in the >10 MeV channel
(1 pfu = 1 particle per (cm<SUP>2</SUP> s sr MeV)). The July 23 event
was the first gamma-ray-line flare detected by RHESSI, but it did
not show any enhancement in SEPs above the elevated background from
the July 20 event. We identified two distinguishing factors between
the July 2 and July 23 CMEs: (1) The July 20 CME had a higher kinetic
energy, and (2) The July 20 CME was interacting with another fast CME
(1350 km s<SUP>-1</SUP>) that preceded by less than an hour from the
same region; there were also two other CMEs on July 19 from the same
region. Thus the coronal and IP environment of the July 20 event was
highly disturbed due to preceding CMEs (as compared to the July 23
event). We suggest that the different coronal/IP environments may be
responsible for the lack of SEP event associated with the July 23 event.
---------------------------------------------------------
Title: Nobeyama Radio Heliograph and RHESSI Observations of the X1.5
Flare of April 21, 2002
Authors: Kundu, M. R.; Garaimov, V. I.; White, S. M.; Krucker, S.
2003SPD....34.1812K Altcode: 2003BAAS...35..841K
We present radio observations of the X1.5 flare of April 21, 2002, and
complementary data from other wavelengths. This flare was fairly well
observed by the spacecraft RHESSI and fully observed by the Nobeyama
Radio Heliograph at 17 and 34 GHz. This long-duration event lasted
more than 2 hours and features a beautful arcade of rising loops on
the limb visible at X-ray, EUV and radio wavelengths. The main flare
was preceded by a small event 90 minutes earlier showing a long EUV
loop connecting well-separated radio and hard X-ray sources. The main
flare itself starts with a compact radio and hard X-ray source at the
eastern end of the region that is followed by emission close to the
solar surface (well inside the solar limb) over a large region to the
northwest. As the flare proceeds a large set of loops is seen to rise
well above the solar limb. Distinct regions of radio emission with very
different time behaviour can be identified in the radio images, and in
particular a peculiar nonthermal 17 GHz source low in the corona at the
base of the arcade is seen to turn on 30 minutes after the start of the
impulsive phase. At about the same time an extremely intense burst of
coherent radio emission is seen from 500 to 2000 MHz: we speculate that
this lower-frequency burst is located at the top of the arcade of loops.
---------------------------------------------------------
Title: On the Origin of Solar Energetic Particle Events
Authors: Krucker, S.
2003LNP...612..179K Altcode: 2003ecpa.conf..179K
The 3-D Plasma and Energetic Particles (3DP) instruments on the WIND
spacecraft are providing new insights on the origin of solar energetic
particles. In this paper, recent results on the solar origin of 1-300
keV electrons and 0.1-6 MeV protons are reviewed. The main findings are
that one class of electron events escaping into interplanetary space
are related to the impulsive phase of the flare (radio type III burst
related events), but there is also a second class of events released
after the impulsive phase of the flare that is possibly related to
coronal shocks. In two thirds of all proton events, low energy protons
(<6 MeV) are released roughly an hour later than the electrons
and are most likely accelerated at the shock fronts of coronal mass
ejections. The remaining third of proton events show a puzzling velocity
dispersion that could be explained by a non-simultaneous release of
protons at different energy.
---------------------------------------------------------
Title: Hard X-ray and metric/decimetric spatially resolved
observations of the 10 April 2002 solar flare
Authors: Vilmer, N.; Krucker, S.; Trottet, G.; Lin, R. P.
2003AdSpR..32.2509V Altcode:
The GOES M8.2 flare on 10 April 2002 at ∼ 1230 UT was observed at
X-ray wavelengths by RHESSI and atmetric/decimetric wavelengths by the
Nançay Radioheliograph (NRH). We discuss the temporal evolution of
X-ray sources together with the evolution of the radio emission sites
observed at different coronal heights by the NRH. While the first strong
HXR peak at energies above 50 keV arises from energy release in compact
magnetic structures (with spatial scales of a few 10 <SUP>4</SUP> km)
and is not associated with strong radio emission, the second one leads
to energy release in magnetic structures with scales larger than 10
<SUP>5</SUP> km and is associated with intense decimetric/metric and
dekametric emissions. We discuss these observations in the context of
the acceleration sites of energetic electrons interacting at the Sun
and of escaping ones.
---------------------------------------------------------
Title: WIND/3DP and Nancay radioheliograph observations of solar
energetic electron events
Authors: Klein, K. -L.; Krucker, S.; Trottet, G.
2003AdSpR..32.2521K Altcode:
Initial results of a combined study of electron events using the 3DP
experiment on the WIND spacecraftand the Nançay Radioheliograph
(NRH) are presented. A total of 57 electron events whose solar
release time could be inferred from WIND/3DP observations occurred
during NRH observing times. In 40 of them a distinct signature was
detected in maps at decimetric and metric wavelengths (dm-m-λ) taken
by the NRH. These events are equally distributed among two categories:
(1) Electron release together with dm-m-λ bursts of a few minutes
duration: these events are also accompanied by decametric-hectometric
type III bursts seen by WAVES/WIND. They correspond to the well-known
impulsive electron events. (2) Electron release during long duration
(several tens of minutes) dm-m-λ emission: the electrons are most often
released more than ten minutes after the start of the radio event. In
the majority of cases the dm-m-λ radio source changes position, size,
and/or intensity near the time of electron release.
---------------------------------------------------------
Title: Hard X-ray Microflares down to 3~keV
Authors: Christe, S.; Krucker, S.; Lin, R.; Hurford, G.; Schwartz,
R. A.
2002AGUFMSH52A0484C Altcode:
The excellent sensitivity, spectral and spatial resolution, and
energy coverage down to 3~keV provided by the Reuven Ramaty Solar
Spectroscopic Imager mission (RHESSI) allows for the first time the
detailed study of the locations and the spectra of solar microflares
down to 3~keV. During a one hour quiet interval (GOES soft X-ray level
around B6) on May~2, 1:40-2:40UT, at least 7 microflares occurred with
the largest peaking at A6 GOES level. The microflares are found to come
from 4 different active regions including one behind the west limb. At
7" resolution, some events show elongated sources, while others are
unresolved point sources. In the impulsive phase of the microflares,
the spectra can generally be fitted best with a thermal model plus
power-law above ~6-7~keV. The decay phase sometimes can be fit with
a thermal only, but in some events, power-law emission is detected
late in the event indicating particle acceleration after the thermal
peak of the event. The power-law fits extend to below 7~keV with
exponents between -5 and -8, and imply a total non-thermal electron
energy content between 10<SUP>26</SUP>-10<SUP>27</SUP> ergs. Since the
total energy in non-thermal electrons is very sensitive to the value
of the power-law and the energy cutoff, these observations will give us
better estimates of the total energy input into the corona. Presently,
microflare observations on June~26 02:20-06:10UT are being analyzed
and are expected to corroborate these results.
---------------------------------------------------------
Title: RHESSI Hard X-ray imaging spectroscopy of the 2002 July 23
Solar Gamma-ray Flare
Authors: Krucker, S.; Hurford, G. J.; Schwartz, R. A.; Lin, R. P.;
Smith, D. M.
2002AGUFMSH52A0481K Altcode:
The GOES X4.8 flare of 23 July 2002 is the first gamma-ray flare seen
by the recently launched Reuven Ramaty High Energy Solar Spectroscopic
Imager (RHESSI). In this paper, first results of X-ray (>3~keV)
imaging spectroscopy of this flare are discussed and related to images
seen in EUV (TRACE, EIT), Hα (BBSO), and radio waves (Nobeyama). In
particular, solar flare bremsstrahlung emission above 300 keV is imaged
for the first time. During the first minutes of the impulsive phase of
the flare, three main hard X-ray (>30~keV) sources are seen. Two
of the sources show similar spectra with a break around 60~keV and
exponents around -1.7 below the break and -3.0 above. The spectrum of
the third source (that is observed in-between the two other sources,
but slightly displaced to the side) is best fitted with a single power
law with an exponent of around -3.0. Later in the impulsive phase, only
the two sources with similar spectra are seen. The spatial separation of
these sources is linearly increasing with a velocity of ~36 km/s over
more than 5 minutes. This separation is comparable to the footpoint
separation reported by Sakao et al. (1994). More detailed analysis on
the temporal evolution of the spectra and a comparison with imaging
observations at other wavelengths will be presented.
---------------------------------------------------------
Title: Statistical Study of Solar Impulsive Electron Events
Authors: Oakley, P.; Krucker, S.; Lin, R. P.
2002AGUFMSH61A0426O Altcode:
This study with the WIND 3DP instrument of 380 solar impulsive electron
events shows many different temporal delays with respect to the
correlated radio type III bursts. The majority of events have a delayed
electron injection time with respect to the radio type III bursts of
about 14 minutes. This is similar to other published findings, but
with more events and better statistics. To try and determine why some
events showed on-time arrivals of electrons while most were delayed
we analyzed their peak flux spectra and the frequency distribution
of events. The frequency distribution for all events is a power law
with a slope of 1.35+/-.05 for energies 30 keV to 300 keV. The peak
flux spectra for individual events usually shows a double power law
dependence with a steepening at 50 keV. The two slopes have average
indices of -3.5 and -2, and are slightly correlated. However there
was no difference in these distributions between on-time events and
delayed events. It is also found that there is a strong tendency for
events with in-situ detected Langmuir waves at 1 AU to be delayed less
(12 minutes on average) than those without visible waves (17 minutes
on average). An interesting finding is that electron events occurring
during a radio type II burst show significantly harder spectra,
indicating that shocks with type II bursts might be more efficient at
accelerating higher energy particles.
---------------------------------------------------------
Title: RHESSI Observations of Solar Hard X-ray Flares and the Relative
Timing of Interplanetary Type III Radio Bursts
Authors: Rauscher, E.; Krucker, S.; Lin, R. P.
2002AGUFMSH52A0482R Altcode:
The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) allows
more detailed imaging of solar hard X-ray flares than has been available
before. The hard X-ray sources seen in solar flares are believed to
be the result of accelerated electrons streaming down the legs of a
magnetic loop and colliding with the denser plasma at the footpoints,
releasing hard X-rays through bremstrahlung. Interplanetary type III
radio bursts are evidence of accelerated electrons leaving the Sun. We
have begun a study of the temporal correlation of hard X-ray flares and
interplanetary type III radio bursts to investigate the acceleration
processes behind these events. The RHESSI data from February 12 through
July 23 have been temporally compared with WIND/WAVES data to find
correlation between hard X-ray events and interplanetary type III radio
bursts. Relatively simple events with and without type III bursts have
been chosen for spatial investigation. This involves making RHESSI hard
X-ray (>20~keV) images and overlaying these onto corresponding EUV
images from EIT and TRACE and images from MDI.
---------------------------------------------------------
Title: RHESSI Observation of an Occulted Hard X-ray Flare
Authors: Balciunaite, P.; Christe, S.; Krucker, S.; Lin, R. P.
2002AGUFMSH52A0480B Altcode:
We present an observation of an occulted GOES C2 class flare
which occurred on 13 May 2002 10:50-22:20 UT taken by the recently
launched Reuven Ramaty Solar Spectroscopic Imager mission (RHESSI) in
open-shutter mode. The flare is inferred to originate 34 degrees behind
the east limb by tracking the most probable originating active region
(NOAA Active Region 9957). This relatively large occultation could
explain why no impulsive flare phase was observed. The hard X-ray
(8-15 keV) source centroid position as a function of time initially
shows a constant position just above the solar limb for approximately
2 hours. Then the source appears to rise above the limb, in a manner
that could not be due solely to solar rotation. RHESSI CLEANed images
at 7" resolution demonstrate coronal source motion as a function of
time. Presently we are investigating spectra with resolution down to 1
keV and energy coverage down to 3 keV and the derived temperature and
emission measure. A limb flare observed on 21 April 2002 00:30-04:00
UT is presented as a comparison.
---------------------------------------------------------
Title: Correlation of RHESSI and TRACE Observations of the Rise
Phase of the 21 April 2002 X1.5 Flare
Authors: Caspi, A.; Krucker, S.; Lin, R. P.
2002AGUFMSH52A0465C Altcode:
The X1.5 flare of 21 April 2002 was observed simultaneously by RHESSI
and TRACE, and thus provides a great opportunity to correlate RHESSI
findings with independent observations from another instrument. We
begin by analyzing the thermal and non-thermal parts of the flare
spectrum observed by RHESSI. We then use the temperature and volume
emission measure obtained from fitting to predict the TRACE instrument
response. We correlate these predictions with the observed TRACE flux
at the flare site. Gallagher et al. (2002) state that TRACE showed
EUV (195 Å) brightening some 4 minutes after RHESSI, but preliminary
results indicate a small initial brightening in the 195 Å passband
around 00:40 UT, at the same time as RHESSI, with the primary flux
increase occurring around 00:44 UT. This roughly matches predictions
of TRACE response based on RHESSI spectral data.
---------------------------------------------------------
Title: RHESSI Gamma-Ray Line Spectroscopy of the X-class Flare of
July 23, 2002
Authors: Smith, D. M.; Schwartz, R. A.; Lin, R. P.; Share, G. H.;
Murphy, R. J.; Hurford, G.; Krucker, S.; Fivian, M.; Hyatt, L.;
Dennis, B. R.; Vilmer, N.
2002AGUFMSH52A0461S Altcode:
The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) is
the first spacecraft with high-spectral-resolution germanium gamma-ray
detectors to be dedicated to solar flare studies. We will report on
RHESSI's first observation of a major gamma-ray line flare, the X4.8
limb flare of July 23, 2002. RHESSI observed nuclear de-excitation lines
of Mg, Si, Ne, O, and C from this flare, plus the neutron-capture line
at 2.223 MeV. There is evidence for redshifts and Doppler broadening
of the de-excitation lines, as well as temporal variations of the line
spectrum. We will discuss the implications of these observations for
the spectrum, time variation, angular distribution, and composition of
the accelerated ions and for the composition of the interaction region.
---------------------------------------------------------
Title: Gamma-ray Imaging of the 2002 July 23 Solar Flare
Authors: Hurford, G. J.; Schwartz, R. A.; Krucker, S.; Lin, R. P.;
Smith, D. M.
2002AGUFMSH52A0462H Altcode:
Gamma-ray spectroscopy of nuclear line emission has provided the
primary observational tool for the study of accelerated nuclei near
the Sun. The Reuven Ramaty High Energy Solar Spectroscopic Imager
(RHESSI), launched in February 2002, is an imaging-spectrometer
which not only does high-resolution spectroscopy from 3 keV to 17
MeV, but also images gamma-rays with angular resolution as high as 35
arcseconds. This enables the emission sites of the 2.2 MeV capture line
(from thermalized neutrons), of the Doppler-broadened, prompt gamma-ray
continuum and of the hard x-ray continuum (from accelerated electrons)
to be located within the flaring active region. Comparison of these
locations can provide a new perspective on acceleration and transport
processes. The X4.8 limb flare of 23 July 2002 provided the first
opportunity to observe solar gamma-ray lines with RHESSI. This paper
presents the first solar gamma-ray images of the 2.2 MeV line and of
the 4 to 7 MeV gamma-ray range which is dominated by excitation lines
of C and O. The locations of these gamma-ray sources will be compared
to those of the hard x-ray sources.
---------------------------------------------------------
Title: Hard X-ray Microflares down to 3 keV
Authors: Krucker, Säm; Christe, Steven; Lin, R. P.; Hurford, Gordon
J.; Schwartz, Richard A.
2002SoPh..210..445K Altcode:
The excellent sensitivity, spectral and spatial resolution, and energy
coverage down to 3 keV provided by the Reuven Ramaty High-Energy Solar
Spectroscopic Imager mission (RHESSI) allows for the first time the
detailed study of the locations and the spectra of solar microflares
down to 3 keV. During a one-hour quiet interval (GOES soft X-ray level
around B6) on 2 May, 1:40-2:40 UT, at least 7 microflares occurred
with the largest peaking at A6 GOES level. The microflares are found
to come from 4 different active regions including one behind the
west limb. At 7” resolution, some events show elongated sources,
while others are unresolved point sources. In the impulsive phase of
the microflares, the spectra can generally be fitted better with a
thermal model plus power law above ∼ 6-7 keV than with a thermal
only. The decay phase sometimes can be fitted with a thermal only,
but in some events, power-law emission is detected late in the
event indicating particle acceleration after the thermal peak of the
event. The behind-the-limb microflare shows thermal emissions only,
suggesting that the non-thermal power law emission originates lower, in
footpoints that are occulted. The power-law fits extend to below 7 keV
with exponents between −5 and −8, and imply a total non-thermal
electron energy content between 10<SUP>26</SUP>-10<SUP>27</SUP>
erg. Except for the fact that the power-law indices are steeper than
what is generally found in regular flares, the investigated microflares
show characteristics similar to large flares. Since the total energy
in non-thermal electrons is very sensitive to the value of the power
law and the energy cutoff, these observations will give us better
estimates of the total energy input into the corona. (Note that color
versions of figures are on the accompanying CD-ROM.)
---------------------------------------------------------
Title: Hard x-ray and Metric/Decimetric Radio Observations of the
20 February 2002 Solar Flare
Authors: Vilmer, N.; Krucker, S.; Lin, R. P.; The Rhessi Team
2002SoPh..210..261V Altcode:
The GOES C7.5 flare on 20 February 2002 at 11:07 UT is one of the
first solar flares observed by RHESSI at X-ray wavelengths. It was
simultaneously observed at metric/decimetric wavelengths by the Nançay
radioheliograph (NRH) which provided images of the flare between 450
and 150 MHz. We present a first comparison of the hard X-ray images
observed with RHESSI and of the radio emission sites observed by the
NRH. This first analysis shows that: (1) there is a close occurrence
between the production of the HXR-radiating most energetic electrons and
the injection of radio-emitting non-thermal electrons at all heights
in the corona, (2) modifications with time in the pattern of the HXR
sources above 25 keV and of the decimetric radio sources at 410 MHz are
observed occurring on similar time periods, (3) in the late phase of
the most energetic HXR peak, a weak radio source is observed at high
frequencies, overlying the EUV magnetic loops seen in the vicinity
of the X-ray flaring sites above 12 keV. These preliminary results
illustrate the potential of combining RHESSI and NRH images for the
study of electron acceleration and transport in flares.
---------------------------------------------------------
Title: The RHESSI Imaging Concept
Authors: Hurford, G. J.; Schmahl, E. J.; Schwartz, R. A.; Conway,
A. J.; Aschwanden, M. J.; Csillaghy, A.; Dennis, B. R.; Johns-Krull,
C.; Krucker, S.; Lin, R. P.; McTiernan, J.; Metcalf, T. R.; Sato,
J.; Smith, D. M.
2002SoPh..210...61H Altcode:
The Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI)
observes solar hard X-rays and gamma-rays from 3 keV to 17 MeV
with spatial resolution as high as 2.3 arc sec. Instead of focusing
optics, imaging is based on nine rotating modulation collimators that
time-modulate the incident flux as the spacecraft rotates. Starting from
the arrival time of individual photons, ground-based software then uses
the modulated signals to reconstruct images of the source. The purpose
of this paper is to convey both an intuitive feel and the mathematical
basis for this imaging process. Following a review of the relevant
hardware, the imaging principles and the basic back-projection method
are described, along with their relation to Fourier transforms. Several
specific algorithms (Clean, MEM, Pixons and Forward-Fitting) applicable
to RHESSI imaging are briefly described. The characteristic strengths
and weaknesses of this type of imaging are summarized.
---------------------------------------------------------
Title: Relative Timing and Spectra of Solar Flare Hard X-ray Sources
Authors: Krucker, Säm; Lin, R. P.
2002SoPh..210..229K Altcode:
Hard X-ray lightcurves, spectrograms, images, and spectra of three
medium-sized flares observed by the Reuven Ramaty High-Energy Solar
Spectroscopic Imager (RHESSI) are presented. Imaging spectroscopy
of the 20 February 2002, 11:06 UT flare at 10” spatial resolution,
comparable to the best previous hard X-ray imaging from Yohkoh,
shows two footpoints with an ∼ 8 s delay of peak emission between
footpoints. Subsequent imaging at le4” shows three sources consistent
with two separate loops and simultaneous brightening in connected
footpoints. Imaging for the simple two footpoint flare of 2 June 2002
also shows simultaneous footpoint brightening. The more complex 17
March 2002 flare shows at least four different sources during the
main peak of the event, and it is difficult to clearly demonstrate
simultaneous brightening of connected footpoints. Non-thermal power
laws are observed down to ∼ 12-13 keV without flattening in all these
events, indicating the energy content in energetic electrons may be
significantly greater than previously estimated from assumed 25 keV
low energy cutoff. Simultaneously brightening footpoints show similar
spectra, at least in the three flares investigated. Double-power-law
spectra with a relatively sharp break are often observed.
---------------------------------------------------------
Title: Modeling Images and Spectra of a Solar Flare Observed by
RHESSI on 20 February 2002
Authors: Sui, Linhui; Holman, Gordon D.; Dennis, Brian R.; Krucker,
Säm; Schwartz, Richard A.; Tolbert, Kim
2002SoPh..210..245S Altcode:
We have analyzed a C7.5 limb flare observed by RHESSI on 20 February
2002. The RHESSI images appear to show two footpoints and a loop-top
source. Our goal was to determine if the data are consistent with
a simple steady-state model in which high-energy electrons are
continuously injected at the top of a semicircular flare loop. A
comparison of the RHESSI images with simulated images from the model
has made it possible for us to identify spurious sources and fluxes in
the RHESSI images. We find that the RHESSI results are in many aspects
consistent with the model if a thermal source is included between the
loop footpoints, but there is a problem with the spectral index of the
loop-top source. The thermal source between the footpoints is likely
to be a low-lying loop interacting with the northern footpoint of a
higher loop containing the loop-top source.
---------------------------------------------------------
Title: Rhessi and Trace Observations of the 21 April 2002 x1.5 Flare
Authors: Gallagher, Peter T.; Dennis, Brian R.; Krucker, Säm;
Schwartz, Richard A.; Tolbert, A. Kimberley
2002SoPh..210..341G Altcode:
Observations of the X1.5 flare on 21 April 2002 are reviewed using the
Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) and the
Transition Region and Coronal Explorer (TRACE). The major findings are
as follows: (1) The 3-25 keV X-rays started < 4 min before the EUV
(195 Å) emission suggesting that the initial energy release heated
plasma directly to ≳20 MK, well above the 1.6 MK needed to produce the
Fe xii (195 Å) line. (2) Using coaligned 12-25 keV RHESSI and TRACE
images, further evidence is found for the existence of hot (15-20 MK)
plasma in the 195 Å passband. This hot, diffuse emission is attributed
to the presence of the Fe xxiv (192 Å) line within the TRACE 195 Å
passband. (3) The 12-25 keV source centroid moves away from the limb
with an apparent velocity of ∼ 9.9 km s<SUP>−1</SUP>, slowing to ∼
1.7 km s<SUP>−1</SUP> after 3 hours, its final altitude being ∼ 140
Mm after ∼ 12 hours. This suggests that the energy release site moves
to higher altitudes in agreement with classical flare models. (4) The
50-100 keV emission correlates well with EUV flare ribbons, suggesting
thick-target interactions at the footpoints of the magnetic arcade. The
50-100 keV time profile matches the time derivative of the GOES light
curve (Neupert effect), which suggests that the same electrons that
produced the thick-target hard X-ray emission also heat the plasma seen
in soft X-rays. (5) X-ray footpoint emission has an E<SUP>−3</SUP>
spectrum down to ∼ 10 keV suggesting a lower electron cutoff energy
than previously thought. (6) The hard X-ray (25-200 keV) peaks have
FWHM durations of ∼ 1 min suggesting a more gradual energy release
process than expected. (7) The TRACE images reveal a bright symmetric
front propagating away from the main flare site at speeds of ≥ 120
km s<SUP>−1</SUP>. This may be associated with the fast CME observed
several minutes later by LASCO. (8) Dark sinuous lanes are observed in
the TRACE images that extend almost radially from the post-flare loop
system. This `fan of spines' becomes visible well into the decay phase
of the flare and shows evidence for both lateral and downward motions.
---------------------------------------------------------
Title: The Reuven Ramaty High-Energy Solar Spectroscopic Imager
(RHESSI)
Authors: Lin, R. P.; Dennis, B. R.; Hurford, G. J.; Smith, D. M.;
Zehnder, A.; Harvey, P. R.; Curtis, D. W.; Pankow, D.; Turin, P.;
Bester, M.; Csillaghy, A.; Lewis, M.; Madden, N.; van Beek, H. F.;
Appleby, M.; Raudorf, T.; McTiernan, J.; Ramaty, R.; Schmahl, E.;
Schwartz, R.; Krucker, S.; Abiad, R.; Quinn, T.; Berg, P.; Hashii,
M.; Sterling, R.; Jackson, R.; Pratt, R.; Campbell, R. D.; Malone,
D.; Landis, D.; Barrington-Leigh, C. P.; Slassi-Sennou, S.; Cork, C.;
Clark, D.; Amato, D.; Orwig, L.; Boyle, R.; Banks, I. S.; Shirey,
K.; Tolbert, A. K.; Zarro, D.; Snow, F.; Thomsen, K.; Henneck,
R.; Mchedlishvili, A.; Ming, P.; Fivian, M.; Jordan, John; Wanner,
Richard; Crubb, Jerry; Preble, J.; Matranga, M.; Benz, A.; Hudson,
H.; Canfield, R. C.; Holman, G. D.; Crannell, C.; Kosugi, T.; Emslie,
A. G.; Vilmer, N.; Brown, J. C.; Johns-Krull, C.; Aschwanden, M.;
Metcalf, T.; Conway, A.
2002SoPh..210....3L Altcode:
RHESSI is the sixth in the NASA line of Small Explorer (SMEX)
missions and the first managed in the Principal Investigator mode,
where the PI is responsible for all aspects of the mission except
the launch vehicle. RHESSI is designed to investigate particle
acceleration and energy release in solar flares, through imaging and
spectroscopy of hard X-ray/gamma-ray continua emitted by energetic
electrons, and of gamma-ray lines produced by energetic ions. The
single instrument consists of an imager, made up of nine bi-grid
rotating modulation collimators (RMCs), in front of a spectrometer
with nine cryogenically-cooled germanium detectors (GeDs), one behind
each RMC. It provides the first high-resolution hard X-ray imaging
spectroscopy, the first high-resolution gamma-ray line spectroscopy,
and the first imaging above 100 keV including the first imaging of
gamma-ray lines. The spatial resolution is as fine as ∼ 2.3 arc sec
with a full-Sun (≳ 1°) field of view, and the spectral resolution
is ∼ 1-10 keV FWHM over the energy range from soft X-rays (3 keV)
to gamma-rays (17 MeV). An automated shutter system allows a wide
dynamic range (>10<SUP>7</SUP>) of flare intensities to be handled
without instrument saturation. Data for every photon is stored in a
solid-state memory and telemetered to the ground, thus allowing for
versatile data analysis keyed to specific science objectives. The
spin-stabilized (∼ 15 rpm) spacecraft is Sun-pointing to within ∼
0.2° and operates autonomously. RHESSI was launched on 5 February
2002, into a nearly circular, 38° inclination, 600-km altitude orbit
and began observations a week later. The mission is operated from
Berkeley using a dedicated 11-m antenna for telemetry reception and
command uplinks. All data and analysis software are made freely and
immediately available to the scientific community.
---------------------------------------------------------
Title: Imaging Spectroscopy of the February 20, 2002 flare: RHESSI
Observations
Authors: Krucker, S.; Lin, R. P.; Caspi, A.; Hudson, H.; Schwartz,
R. S.; Johns-Krull, C. M.; RHESSI Team
2002AAS...200.7607K Altcode: 2002BAAS...34..776K
First RHESSI imaging spectroscopy results of X-ray bursts from solar
flares are presented. In a first step, different spatial features
such as footpoints, loops in-between footpoints, etc. are identified
in the X-ray images. By reconstructing images at different energies
(with a spectral resolution down to 1 keV) the X-ray photon spectrum
for each of the spatial features can be extracted independently of each
other. Here, images at high cadence (4 seconds) are analyzed allowing
to follow the temporal evolution of the spectra of different spatial
features. The results from imaging spectroscopy of individual spatial
features are compared with the total spectrum and the differences are
discussed. For the event of February 20, 11:05 UT, in-situ observations
at 1 AU of 1-300 keV electron taken by the 3DP instrument on the WIND
spacecraft show an impulsive electron event of solar origin. The
electron onset times at different energies reveal that the solar
release time of these escaping electrons coincides with the HXR burst
seen with RHESSI. Therefore, a direct comparison of the different X-ray
photon spectra and the electron spectrum observed at 1 AU is possible.
---------------------------------------------------------
Title: RHESSI Observations of Continuous Solar X-Ray Emission from
3 to 10 keV
Authors: Hurford, G. J.; Krucker, Sam; RHESSI Team
2002AAS...200.7604H Altcode: 2002BAAS...34..776H
The RHESSI spacecraft, launched 5 February 2002, provides imaging
spectroscopy of solar x-ray emission from 3 keV to 15 MeV. In addition
to its high spatial and spectral resolution (2.3 arcsec and ~ 1
keV respectively), RHESSI also has exceptional sensitivity at low
energies. This sensitivity has enabled it to observe continuous x-ray
emission from the non-flaring Sun at energies from 3 to ~ 10 keV. This
paper presents preliminary observations of the spatial and spectral
characteristics of this emission.
---------------------------------------------------------
Title: Modeling Images and Spectra of Solar Flares Observed by RHESSI
Authors: Sui, L.; Holman, G. D.; Krucker, S.; Dennis, B. R.
2002AAS...200.6908S Altcode: 2002BAAS...34R.758S
The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) was
successfully launched on February 5, 2002. RHESSI is now obtaining
high resolution spectra and images of solar flares in x-rays and
gamma-rays. In order to interpret these images and spectra, we have
been developing flare models incorporating thermal and nonthermal
bremsstrahlung emission. In our models electrons with a power-law
energy distribution are continuously injected into a flare loop, with
Coulomb scattering and magnetic mirroring included. We use the EFLUXPROG
Fokker-Planck code ( http://hesperia.gsfc.nasa.gov/hessi/modelware.htm )
to obtain the steady-state electron distribution throughout the loop. We
then compute the spectral and spatial distribution of the bremsstrahlung
hard x-ray flux. We have compared these model results with results
obtained by RHESSI during the impulsive phase of several moderate
size flare. We will present some of these early RHESSI observations
and the comparison with our flare models. This work is supported in
part by the NASA Sun-Earth Connection Program and the RHESSI Project.
---------------------------------------------------------
Title: X-ray and radio observations of the 20 February 2002 solar
flare
Authors: Vilmer, N. R.; Krucker, S.; Lin, R. P.; Schwartz, R. A.;
Klassen, A.; RHESSI Team
2002AAS...200.7606V Altcode: 2002BAAS...34..776V
The GOES C7.5 flare on 20 February 2002 at 11:06 UT was simultaneously
observed at X-ray wavelengths by RHESSI and at metric/decimetric
wavelengths by the Nançay radioheliograph (NRH). We shall present a
first comparison of the time evolution of X-ray images in different
energy bands observed with RHESSI and of the time evolution of images
of the radio emission sites at 5 frequencies in the 150-450 MHz
range observed by the NRH. The identification of the different radio
spectral features is provided by the Tremsdorf radiospectrograph. We
shall also investigate whether the evolution in space and time of the
X-ray sources are related to any spectral evolution.
---------------------------------------------------------
Title: Soft-hard-soft spectral evolution observed at HESSI resolution
Authors: Hudson, H. S.; Dennis, B. R.; Krucker, S.; Lin, R. P.; Sato,
J.; Schwartz, R. A.; Smith, D. M.
2002AAS...200.6905H Altcode: 2002BAAS...34..758H
Hard X-ray bursts from solar flares show a strong correlation between
spectral index and flux level in the 20-100 keV hard X-ray range. Prior
to HESSI this pattern could mainly be studied only with scintillation
counters, at relatively low resolution. HESSI has observed more than
ten M-class flares, and we will report how this morphology appears
at ~1-keV resolution for these events. Specifically, we address the
question of whether the time evolution of the break energy in a double
power-law fit can play a role in defining the soft-hard-soft morphology.
---------------------------------------------------------
Title: X-ray Signatures related to Energetic Electrons Escaping from
the Sun: First HESSI results
Authors: Krucker, S.; Lin, R. P.; Klein, K.
2002AGUSMSH21D..01K Altcode:
Solar energetic electrons are observed to escape from the Sun into
interplanetary space. However, the release machanism(s) and the involved
magnetic field geometry are not well understood. In particular,
it is not well understood why some solar events release energetic
electrons into interplanetary and others are not. Here, radio emissions
(type III bursts) and insitu observations of escaping electron beams
(impulsive electron events) are compared with X-ray signatures produced
by electrons moving towards the Sun. The Nancay Radio Heliograph and
the WAVES experiment on WIND provide the radio observations, WIND/3DP
the insitu 1-300~keV electron observations. The X-ray observations are
taken by the recently launched High Energy Solar Spectroscopic Imager
(HESSI). First results of this study are presented.
---------------------------------------------------------
Title: Solar energetic electron events and coronal shocks
Authors: Klassen, A.; Bothmer, V.; Mann, G.; Reiner, M. J.; Krucker,
S.; Vourlidas, A.; Kunow, H.
2002A&A...385.1078K Altcode:
Mildly relativistic electrons appear during the solar energetic particle
events. A detailed investigation on the origin of such electrons is
presented for four particlular solar events. The mildly relativistic
electrons have been detected at energies of 0.25-0.7 MeV by COSTEP/SOHO
and below 0.392 MeV by Wind 3-DP experiments. Coronal shocks associated
with these electron events are identified from the metric-to-decametric
solar type II radio bursts. All selected events were associated with
solar activity at western longitudes, so that the magnetic footpoints
connecting the spacecraft with the Sun were close to the flare/shock/CME
site. The associated type II bursts were accompanied by so-called shock
accelerated (SA) type III bursts appearing to be emerging from the type
II emission site. We found: (1) that all of the 0.25-0.7 MeV electron
events were released during or after, but never simultaneously with the
onset of type II bursts and CMEs. The time delay between the type II
burst onset and the release of the mildly relativistic electrons is in
the range of 11.5-45 min; (2) that the mildly relativistic electrons
were released rather at the end of SA type III bursts or somewhat
later; (3) that the mildly relativistic electrons were released when
the associated type II burst and the CME reached a certain height
(h ~ 1-4 R_s) above the photosphere. For the four events studied,
it is concluded that mildly relativistic electrons at 0.25-0.7 MeV
energies measured in the interplanetary medium from solar energetic
particle events are accelerated by coronal shock waves, commonly in
association with white-light CMEs.
---------------------------------------------------------
Title: Energy Distribution of Microevents in the Quiet Solar Corona
Authors: Benz, Arnold O.; Krucker, Säm
2002ApJ...568..413B Altcode: 2001astro.ph..9027B
Recent imaging observations of EUV line emissions have shown evidence
for frequent flarelike events in a majority of the pixels in quiet
regions of the solar corona. The changes in coronal emission
measure indicate impulsive heating of new material to coronal
temperatures. These heating or evaporation events are candidate
signatures of “nanoflares” or “microflares” proposed to interpret
the high temperature as well as the very existence of the corona. The
energy distribution of these microevents reported in the literature
differ widely, and so do the estimates of their total energy input into
the corona. Here we analyze the assumptions of the different methods,
compare them by using the same data set, and discuss their results. We
also estimate the different forms of energy input and output, keeping
in mind that the observed brightenings are most likely secondary
phenomena. A rough estimate of the energy input observed by EIT on the
SOHO satellite is of the order of 10% of the total radiative output in
the same region. It is considerably smaller for the two reported TRACE
observations. The discrepancy can be explained by flare selection and
different thresholds for flare detection. There is agreement on the
slope and the absolute value of the distribution if the same methods
are used and a numerical error is corrected. The extrapolation of the
power law to unobserved energies that are many orders of magnitude
smaller remains questionable. Nevertheless, these microevents and
unresolved smaller events are currently the best source of information
on the heating process of the corona.
---------------------------------------------------------
Title: Energetic Particle Observations during the 2000 July 14
Solar Event
Authors: Bieber, John W.; Dröge, Wolfgang; Evenson, Paul A.;
Pyle, Roger; Ruffolo, David; Pinsook, Udomsilp; Tooprakai, Paisan;
Rujiwarodom, Manit; Khumlumlert, Thiranee; Krucker, Säm
2002ApJ...567..622B Altcode:
Data from nine high-latitude neutron monitors are used to deduce
the intensity-time and anisotropy-time profiles and pitch-angle
distributions of energetic protons near Earth during the major solar
event on 2000 July 14 (also known as the Bastille Day event). In
addition, particle and magnetic field measurements from Wind,
the Advanced Composition Explorer, and the Solar and Heliospheric
Observatory (SOHO) are used in the analysis. The observations are
fitted with good agreement between two independent numerical models of
interplanetary transport. The rapid decrease of anisotropy from a high
initial value cannot be explained by a simple model of interplanetary
transport. Hence, we invoke a barrier or magnetic bottleneck consistent
with an observed magnetic disturbance from an earlier coronal mass
ejection that was located ~0.3 AU beyond Earth's orbit at the time
of the Bastille Day event. This work includes the first treatment of
focused transport through a magnetic bottleneck. We conclude that the
bottleneck reflected a major fraction (~85%) of the relativistic solar
protons back toward Earth.
---------------------------------------------------------
Title: RHESSI and TRACE Observations of an X-class Flare
Authors: Hudson, H.; Dennis, B.; Gallagher, P.; Krucker, S.; Reeves,
K.; Warren, H.
2002cosp...34E3101H Altcode: 2002cosp.meetE3101H
RHESSI and TRACE both obtained excellent observations of an X1.5 flare
on April 21, 2002. In this paper we provide an overview of the flare
and discuss the high- energy imaging and spectra in detail. The TRACE
images in the 195A passband (Fe XII and FeXXIV) reveal this flare to
have a spiky arcade with post-flare flow field in the "supra-arcade
downflow" pattern discovered by Yohkoh. Below the spikes, but above
the FeXII loops, TRACE observes a region with complex motions and fine
structure. We confirm with RHESSI that this region has an elevated
temperature and discuss the transition between thermal and non-thermal
sources. RHESSI also observes footpoint emission distributed along
the flare ribbons.
---------------------------------------------------------
Title: WIND/3DP and Nançay Radioheliograph observations of solar
energetic electron events
Authors: Klein, K.; Krucker, S.; Trottet, G.
2002cosp...34E2239K Altcode: 2002cosp.meetE2239K
Initial results of a combined study of electron events with the
3DP experiment on the WIND satellite and the Nançay Radioheliograph
(NRH) are presented. A total of 60 electron events whose solar release
time could be inferred from WIND/3DP observations occurred during NRH
observing times. In 25 of them a distinct radio signature was detected
in dm-m- maps of the NRH. These events are about equally distributed
among two categories: (1) Electron release together with dm-m-
bursts of a few minutes duration: these events are also accompanied
by decametric-to-kilometric type III bursts seen by WAVES/WIND. They
correspond to the well-known impulsive electron events. (2) Electron
release during long duration (several tens of minutes) dmm- emission:
the electrons are most often released more than ten minutes after the
start of the radio event, frequently without simultaneous decametric
type III bursts. In the majority of cases the dm-m- radio source
changes position, size, and/or intensity near the time of electron
release. The findings suggest that a spectrum of solar causes can be
responsible for energetic electron events at 1 AU. We show examples
of the different categories of events and discuss them with respect
to different scenarios of acceleration and transport in the corona.
---------------------------------------------------------
Title: Hard X-ray and metric/decimetric radio observations of the
10 April 2002 solar flare
Authors: Vilmer, N.; Trottet, G.; Krucker, S.; Lin, R.
2002cosp...34E2370V Altcode: 2002cosp.meetE2370V
The GOES M8.2 flare on 10 April 2002 at1230 UT was observed at Xray-
wavelengths by RHESSI and at metric/decimetric wavelengths by the
Nançay Radioheliograph (NRH). We shall present some comparisons of
the time evolutions of the X-ray images in different energy bands
observed by RHESSI and of the time evolution of the radio emission
sites at 5 frequencies in the 150-450 MHz range observed by the
NRH. We shall investigate whether the evolution in space and time of
X-ray and radio sources are related to any spectral evolution of the
energetic particles. We shall also investigate the magnetic structures
at different spatial scales in which accelerated electrons propagate
from the flare region as well as the existence of coronal acceleration
regions remote from the flare site.
---------------------------------------------------------
Title: Nanoflare heating in the quiet corona: evidence and problems
Authors: Krucker, S.
2002AdSpR..30..493K Altcode:
The content of coronal material in the quiet Sun is not constant,
as soft X-ray (SXR) and high-temperature EUV line observations
have shown. The observed temporal variations in SXR and EUV flux
are interpreted as heating events showing many analogies to flares
occurring in active regions. Compared to a large flare in an active
region, the thermal energy input of a heating event in the quiet corona
is 6 or more orders of magnitude smaller. Heating events in the quiet
corona are therefore often called microflares or nanoflares. The total
heating rate of nanoflares in the quiet corona has been investigated in
several works by many different authors: Do nanoflares provide enough
energy to heat the quiet corona? The results of the different studies
are controversial. In this review, recent observational studies on
nanoflare heating in the quiet corona and their different results are
compared and discussed.
---------------------------------------------------------
Title: Micro-events in the Quiet Solar Corona
Authors: Benz, A.; Krucker, S.
2002cosp...34E1929B Altcode: 2002cosp.meetE1929B
The content of coronal material in the quiet Sun is not constant as
soft X-ray and high-temperature EUV line observations have shown. New
material, probably heated and evaporated from the chromosphere is
occasionally injected even in the faintest parts above the magnetic
network cell interiors. We discuss the characteristics of the largest
of these events, based on simultaneous transition region observations
(in EUV and radio) and the observed analogies to flares. Assuming that
the smaller events follow the same pattern, we estimate the total
mass input and compare it to the requirements observed in the solar
wind. A rough estimate of the energy input observed by EIT on the SoHO
satellite is of the order of 10% of the total radiative output in the
same region. The simulation indicates that the extrapolation to smaller
events is problematic and that smaller events may play an even more
decisive role than previously assumed. The hypothesis of nanoflare
heating is consistent with these observations if the lower corona is
not just heated, but continuously replenished by chromospheric material
heated to coronal temperatures. These micro-events are currently the
best source of information on the heating process of the corona.
---------------------------------------------------------
Title: Understanding of Particle Acceleration from in situ Electron
Measurements
Authors: Krucker, S.
2002cosp...34E2888K Altcode: 2002cosp.meetE2888K
Solar particle acceleration can be studied by investigating
particles escaping from the Sun into interplanetary space. Next to
the quasi-steady solar wind streaming away from the Sun, there are
also large, sudden increases in particle flux observed. These events
detected from GeV down to keV energies are called solar energetic
particle (SEP) events. In this talk, new results obtained by the 3-D
Plasma and Energetic Particle Instruments on the WIND spacecraft are
reviewed. The discussion on the identification of solar events possibly
involved in the acceleration of SEP is emphasized. Additonally, in
situ electron observations are compared to RHESSI X-ray obseravations.
---------------------------------------------------------
Title: The Frequency Distribution and Other Statistical Properties
of Solar Impulsive Electron Events
Authors: Oakley, P. H.; Krucker, S.; Lin, R. P.
2001AGUFMSH42A0770O Altcode:
A statistical survey of 391 impulsive electron events (1-300~keV)
observed at 1 AU by the 3-D Plasma and Energetic Particles experiment
on the WIND spacecraft is presented. The data was taken during the
first 7 years of operations of the WIND spacecraft covering the last
solar minimum and maximum. Two main results are presented: (a) The event
frequency distribution as a function of peak flux is found to be a power
law with an index around -1.4 over 3 orders of magnitude. Assuming the
peak flux is proportional to the total energy released in an event,
this result suggests that the few big events are energetically most
important. (b) In most events, the peak flux spectra in the energy range
of 1-300 keV can be fitted with a double power law with a turnover
energy around 50~keV. The averaged slope at lower energies is around
-1.8 and it steepens at higher energies to an average value of -3.3
---------------------------------------------------------
Title: On the Velocity Dispersion in Solar Impulsive Electron Events
Authors: Krucker, S.; Lin, R. P.; Larson, D. E.; Bale, S. D.
2001AGUFMSH31C..08K Altcode:
A statistical survey of 26 solar impulsive electron events in the 1 to
300 keV range is presented, as observed by the 3D Plasma and Energetic
Particle experiment on the Wind spacecraft. This study was triggered
by results of ACE/EPAM observations (Roelof et al.) reporting the
absence of velocity dispersion in solar impulsive electrons events
in the energy range of 40-300~keV. Here we find that the majority
of events show clear velocity dispersion. Events with high peak flux
had to be first corrected for spurious counts of electrons depositing
only a fraction of their energy before they are scattered out of the
detector. Without this correction the onset times wrongly appeared
to be almost without velocity dispersion. For the large events, the
reduced distribution functions show positive slopes at high energies
(around 100 keV). The calculated linear growth rates are large, but
no in-situ waves are observed by the WIND/WAVES instrument.
---------------------------------------------------------
Title: Solar Origin of a Series of Well-collimated Electron Events
Authors: Maia, Dalmiro; Pick, Monique; Hawkins, S. Edward, III;
Krucker, Sam
2001ApJ...560.1058M Altcode:
We investigate the solar origin and propagation of a series of
well-collimated energetic electron events on 1997 November 28, measured
in situ by the Electron, Proton, and Alpha Monitor (EPAM) experiment
on the Advanced Composition Explorer (ACE) spacecraft. EPAM measures
electrons in the energy range from 40 to 300 keV over a wide range of
look directions and with better than 1 minute time resolution. During
the events in our study, the particles are strongly collimated along
the magnetic field. As such, these near-relativistic (beta=0.4-0.7)
particles tend to be scatter free; their observed arrival at ACE
provides a good estimate of the release time back to the Sun. EPAM
results are extended to 20 keV using energetic electron data from
the Three-Dimensional Plasma Analyzer (3DP) experiment on the Wind
spacecraft. We combine these observations with fast imaging of
the solar corona in the meter wave domain, provided by the Nançay
radioheliograph, and dynamic spectral information from the WAVES
experiment on the Wind spacecraft. Together, this complement of
observations of solar energetic particle events provides insight
into the onset times and sites of particle acceleration in the low
corona. The imaging observations at metric radio frequencies show three
series of similar events off the west limb of the Sun that extend
into interplanetary type III bursts. The third event corresponds
only to a strong in situ particle event. During this time period,
the coronagraphic observations from the Large Angle and Spectrometric
Coronagraph experiment on the Solar and Heliospheric Observatory
reveal a dimming and some morphological changes in the vicinity of
the injection site, suggesting the opening of the magnetic field. We
find that for the strongest event, the onset times for the energetic
particles measured at the spacecraft reveal a dispersion in the
inferred release times back at the Sun versus energy. The onset times
measured for electrons with energies above 60 keV are not compatible
with the release time back at the Sun inferred from particles of lower
energies. This dispersion can be successfully explained by the radio
data: the radio observations reveal two successive energetic electron
injections separated by a few minutes. The observations thus show that
there is a later release of the particles detected in situ at higher
energies and that the origin of the phenomenon is in the low corona.
---------------------------------------------------------
Title: The Source Regions of Impulsive Solar Electron Events
Authors: Benz, Arnold O.; Lin, Robert P.; Sheiner, Olga A.; Krucker,
Säm; Fainberg, Joe
2001SoPh..203..131B Altcode:
Low-energy (2-19 keV) impulsive electron events observed in
interplanetary space have been traced back to the Sun, using
their interplanetary type III radiation and metric/decimetric
radio-spectrograms. For the first time we are able to study the
highest frequencies and thus the radio signatures closest to the source
region. All the selected impulsive solar electron events have been found
to be associated with an interplanetary type III burst. This allows
to time the particle events at the 2 MHz plasma level and identify
the associated coronal radio emissions. Except for 5 out of 27 cases,
the electron events were found to be associated with a coronal type
III burst in the metric wavelength range. The start frequency yields a
lower limit to the density in the acceleration region. We also search
for narrow-band spikes at the start of the type III bursts. In about
half of the observed cases we find metric spikes or enhancements of
type I bursts associated with the start of the electron event. If
interpreted as the plasma emission of the acceleration process, the
observed average frequency of spikes suggests a source density of the
order of 3×10<SUP>8</SUP> cm<SUP>−3</SUP> consistent with the energy
cut-off observed.
---------------------------------------------------------
Title: High energy particle acceleration by solar flares and fast
coronal mass ejections
Authors: Lin, R. P.; Hurford, G.; Krucker, S.; Bale, S.
2001ESASP.493..275L Altcode: 2001sefs.work..275L
No abstract at ADS
---------------------------------------------------------
Title: Solar Impulsive Electron Events with Unusual Velocity
Dispersions
Authors: Krucker, S.; Bale, S. D.; Lin, R. P.
2001ICRC....8.3235K Altcode: 2001ICRC...27.3235K
A statistical survey of 26 solar impulsive electron events in the
∼1 to 300 keV range is presented, as observed by the 3D Plasma and
Energetic Particle experiment on the Wind spacecraft. This study was
triggered by results of ACE/EPAM observations (Roelof et al.) reporting
the absence of velocity dispersion in solar energetic electrons events
in the energy range of 40-300 keV. Earlier studies also reported the
absence of dispersion in some events, whereas Roelof et al. claim to
see no velocity dispersion in most of the events. The presented survey
in this paper shows that besides normal dispersive events (14 out of
26), there are indeed events with unusual velocity dispersion (12 out
of 26) showing nearly simultaneous onsets at 1 AU above 100 keV. A
strong correlation between the peak flux and the absence of velocity
dispersion is found: Events with high peak flux (≥0.2 cm-2 s-1 ster-1
eV-1 at 27 keV) show unusual velocity dispersion. This explains that
earlier WIND/3DP surveys taken during solar minimum where events were
smaller did not show events with unusual velocity dispersion.
---------------------------------------------------------
Title: Are Heating Events in the Quiet Solar Corona Small Flares?
Authors: Krucker, S.; Benz, A. O.
2001AGUSM..SP52B03K Altcode:
Temporary enhancements of the coronal emission measure in a quiet
region have been shown to constitute a significant energy input. Here
some relatively large events (Yohkoh, EIT Observations) are discussed
and tested for characteristics known from full-sized impulsive flares
in active regions. The differences to active region flares seem to be
mainly quantitative, and the analyzed heating events may in principle
be considered as microflares or large nanoflares, thus small versions
of regular flares. In this presentation we focus on the following
questions: (1) Are there related events seen in transient region lines
(CDS/SUMER observations)? (2) What are the relative contributions
of the thermal, potential, and expansion energy to the total energy
released in these events. (3) Are the reported flare temperatures in
quiet regions of 1-2 MK possibly higher (3-5 MK)?
---------------------------------------------------------
Title: Solar Impulsive Electron Events with Unusual Velocity
Dispersions
Authors: Krucker, S.; Bale, S. D.; Lin, R. P.
2001AGUSM..SH62B10K Altcode:
Solar impulsive electron events observed at 1 AU occasionally do
not show the expected velocity dispersion normally produced by the
difference in time of flight of electrons at different energies. These
unusual velocity dispersions were brought to attention by Roelof
et al. investigating ACE EPAM data (40-300 keV). They explain the
absence of velocity dispersion by wave-particle interaction slowing
down the electrons. Here we present WIND/3DP observations of those
events investigating the following questions: 1) Are the onset times
simultaneously at all energies? In particularly, what are the onset
times at low energies (1-40 keV)? 2) Are the spectra flat at the
beginning of the events as predicted by wave-particle interaction? 3)
Is there an enhanced level of insitu waves at the time when 40-300 keV
electrons arrive at 1 AU as a result of the wave-particle interaction?
---------------------------------------------------------
Title: Heating the Quiet Corona by Nanoflares: Evidence and Problems
Authors: Benz, A. O.; Krucker, S.
2001IAUS..203..471B Altcode: 2000astro.ph.12106B
The content of coronal material in the quiet Sun is not constant as
soft X-ray and high-temperature EUV line observations have shown. New
material, probably heated and evaporated from the chromosphere is
occasionaly injected even in the faintest parts above the magnetic
network cell interiors. We discuss the characteristics of the largest
of these events, based on simultaneous transition region observations
(in EUV and radio) and the observed analogies to flares. Assuming
that the smaller events follow the same pattern, we estimate the
total energy input. Various recent analyses are compared and briefly
discussed. Finally we present the results of a simulation, extrapolating
the observed range of microflares to smaller energies. The simultation
indicates that the extrapolation to smaller events is problematic and
that smaller events may play an even more decisive role than previously
assumed. The hypothesis of nanoflare heating is consistent with these
observations if the lower corona is not just heated, but continuously
replenished by chromospheric material heated to coronal temperatures.
---------------------------------------------------------
Title: Mechanisms for Coronal Mass Supply by Evaporative Micro-Events
Authors: Brown, J. C.; Krucker, S.; Güdel, M.; Benz, A. O.
2001IAUS..203..498B Altcode:
There is extensive evidence from SoHO and other data that
“micro-events” play an important role in sustaining at least
some components of the solar corona. These are often termed coronal
micro-“heating events” though a major part of their role is feeding
coronal loops through chromospheric evaporation. We consider what can
be learnt from these data concerning the energy release and transport
mechanisms driving the evaporation, including thermal conduction and
fast particles, and what model constraints are available from other
data (such as hard X-rays and radio events). We conclude, from one
large event and the statistics of many small ones, that conductive
evaporation alone does not fit observations and that fast particles or
some other nonthermal driver must be involved. As well as the problem
of single loop events, we consider the global implications for supply
of the corona and wind.
---------------------------------------------------------
Title: Relativistic solar protons on Bastille Day 2000
Authors: Bieber, J. W.; Droege, W.; Evenson, P.; Pyle, R.; Ruffolo,
D.; Pinsook, U.; Tooprakai, P.; Rujiwarodom, M.; Khumlumlert, T.;
Krucker, S.
2001ICRC...27Q....B Altcode:
Relativistic protons emitted by the Sun on 2000 July 14 (Bastille Day)
caused the count rate of high-latitude neutron monitors to increase
by 25-40% relative to the preexisting background of galactic cosmic
rays. We use a 9monitor network to show that an initial large anisotropy
decreased very rapidly to a small value. We model the event with
numerical codes based upon the Boltzmann equation, and conclude that
solar particle transport was significantly influenced by a reflecting
boundary or magnetic bottleneck from an earlier CME that was located
∼ 0.3 AU beyond Earth at the time of the Bastille Day event. We also
model Wind low-energy electrons to determine the parallel mean free
path over a rigidity range spanning 4 orders of magnitude.
---------------------------------------------------------
Title: Two Classes of Solar Proton Events Derived from Onset Time
Analysis
Authors: Krucker, Säm; Lin, R. P.
2000ApJ...542L..61K Altcode:
We analyze onset times for 26 solar energetic (from 30 keV to 6 MeV)
proton events that exhibit clear velocity dispersion, as observed by
the three-dimensional Plasma and Energetic Particles experiment on the
Wind spacecraft. Assuming that the particles are injected simultaneously
at all energies and travel the same path length, we find two classes of
proton events: (1) for the 18 class 1 events, the derived path lengths
are between 1.1 and 1.3 AU, indicating that the first arriving protons
travel essentially scatter-free and (2) the eight class 2 events show
longer path lengths around 2 AU. For all proton events, the observed
temporally related electron events all have electron path lengths
around 1.1-1.3 AU. Relative to the electron injection time at the Sun,
the protons of the first class are injected ~0.5-2 hr later. Assuming
these particles are accelerated by the associated coronal mass ejection
(CME) shock, the protons at all energies between 0.03 and 6 MeV appear
to be accelerated (or released) simultaneously high in the corona,
roughly ~1-10 R<SUB>solar</SUB> above the electrons. The pitch-angle
distributions are observed to be similar for both classes of events,
making it unlikely that propagation effects are responsible for the
longer path length of class 2 events. The late proton onset times at 1
AU of class 2 are therefore more likely explained by a successively
later solar release (or escape) of protons at successive lower
energies. Assuming again acceleration at the CME shock, the release
(or escape) of the protons of class 2 events appears to depend on
energy and occur at a higher altitude for lower energies, with the most
energetic protons possibly released simultaneously with the electrons.
---------------------------------------------------------
Title: On the solar release of Energetic Particles detected at 1 AU
Authors: Krucker, Säm; Lin, Robert P.
2000AIPC..528...87K Altcode: 2000atep.conf...87K
The 3-D Plasma and Energetic Particles experiment on the WIND
spacecraft was designed to provide high sensitivity measurements of
both suprathermal ions and electrons down to solar wind energies. A
statistical survey of 26 solar proton events has been investigated. For
all these proton events, a temporally related electron event is
observed. The presented results focus on the properties of protons
released near the Sun which show a velocity dispersion when detected
at 1 AU. The particle flux onset times observed at 1 AU in the energy
range between 30 keV and 6 MeV suggest that there are two classes
of proton events: (1) For one class (70% of the events), the first
arriving protons are traveling almost scatterfree as indicated by the
derived path lengths between 1.1 and 1.3 AU, (2) whereas the events
of the second class show significantly larger path lengths of around
2 AU. Relative to the electron release time at the Sun, the almost
scatterfree traveling protons of the first class of events are release
delayed by 0.5 to 2 hours. For the events of the second class, protons
and electrons seemed to be released simultaneously within the accuracy
of 20 minutes. .
---------------------------------------------------------
Title: Mechanisms for dynamic coronal mass supply via evaporative
solar “micro-events”
Authors: Brown, J. C.; Krucker, S.; Güdel, M.; Benz, A. O.
2000A&A...359.1185B Altcode:
The idea that the corona is at least in part supplied by chromospheric
evaporation in loop “micro-events" is quantified in terms of the
power requirements of evaporation mechanisms, using recent analyses of
data on such events in high temperature EUV lines from the SoHO EIT
instrument. Estimates are derived for the pre-event and event values
of loop density and temperature and it is shown, using the conductive
scaling law, that the event emission measure enhancements are too large
to be accounted for solely by enhanced conductive flux from coronal
heating. That is, observations demand that supply of coronal mass by
evaporation events need a mechanism which enhances upper chromospheric
heating and not just conductively driven evaporation. Thus coronal mass
supply in transients is inextricably linked to direct chromospheric
heating processes. Using parametric models of a chromospheric heating
function and of the pre-event chromosphere, an estimate is made of the
extra power required to yield the emission measure enhancement of a
large event evaporatively. The dependence of the result on just how the
EUV solar images are interpreted is emphasised and observational tests
are discussed for the case of heating by fast particles. Implications
of the results in terms of the global supply of the hot corona and
wind mass loss are briefly mentioned.
---------------------------------------------------------
Title: Coronal Replenishment and Heating
Authors: Benz, A. O.; Krucker, S.; Mitra Kraev, U.
2000SPD....31.0217B Altcode: 2000BAAS...32..814B
The content of coronal material in the quiet Sun is not constant as
soft X-ray and high-temperature EUV line observations have shown. New
material, probably heated and evaporated from the chromosphere is
occasionaly injected even in the faintest parts above the magnetic
network cell interiors. We discuss the characteristics of the largest
of these events, based on simultaneous transition region observations
(in EUV and radio) and the observed analogies to flares. Assuming
that the smaller events follow the same pattern, we estimate the
total mass input and compare it to the requirements observed in
the solar wind. Various recent analyses are compared and briefly
discussed. Finally we present the results of a simulation, extrapolating
the observed range of microflares to smaller energies. The simultation
indicates that the extrapolation to smaller events is problematic and
that smaller events may play an even more decisive role than previously
assumed. The hypothesis of nanoflare heating is consistent with these
observations if the lower corona is not just heated, but continuously
replenished by chromospheric material heated to coronal temperatures.
---------------------------------------------------------
Title: Two Classes of Solar Proton Events derived from Onset Time
Analysis
Authors: Krucker, S.; Lin, R. P.
2000SPD....31.0287K Altcode: 2000BAAS...32R.826K
The 3-D Plasma and Energetic Particles experiment on the WIND
spacecraft was designed to provide high sensitivity measurements of
both suprathermal ions and electrons down to solar wind energies. A
statistical survey of 26 solar proton events has been investigated. For
all these proton events, a temporally related electron event is
observed. The presented results focus on the properties of protons
released near the Sun which show a velocity dispersion when detected
at 1 AU. The particle flux onset times observed at 1 AU in the energy
range between 30keV and 6 MeV suggest that there are two classes
of proton events: (1) For one class (70% of the events), the first
arriving protons are traveling almost scatterfree as indicated by
the derived path lengths between 1.1 and 1.3 AU, (2) whereas the
events of the second class show significantly larger path lengths
around 2 AU. Relative to the electron release time at the Sun, the
almost scatterfree traveling protons of the first class of events are
release delayed by 0.5 to 2 hours. For the events of the second class,
protons and electrons seemed to be released simultaneously within the
accuracy of 20 minutes. Events of the first class are most likely shock
accelerated and the late release times suggest an acceleration region
at several solar radii away from the Sun. The puzzeling events of the
second class could as well be explained by pitch angle scattering or
by a succesive later release time at lower energies as by an actual
longer path length.
---------------------------------------------------------
Title: Are Heating Events in the Quiet Solar Corona Small
Flares? Multiwavelength Observations of Individual Events
Authors: Krucker, Säm; Benz, Arnold O.
2000SoPh..191..341K Altcode: 1999astro.ph.12501K
Temporary enhancements of the coronal emission measure in a quiet region
have been shown to constitute a significant energy input. Here some
relatively large events are studied for simultaneous brightenings in
transition region lines and in radio emission. Associated emissions
are discussed and tested for characteristics known from full-sized
impulsive flares in active regions. Heating events and flares are found
to have many properties in common, including (i) associated polarized
radio emission, which usually precedes the emission measure peak
(Neupert effect) and sometimes has a non-thermal spectrum, and (ii)
associated and often preceding peaks in O v and He i emission. On
the other hand, heating events also differ from impulsive flares:
(i) In half of the cases, their radio emission at centimeter waves
shows a spectrum consistent with thermal radiation, (ii) the ratio
of the gyro-synchrotron emission to the estimated thermal soft X-ray
emission is smaller than in flares, and (iii) the associated emission
in the O v transition region line shows red shifts and blue shifts,
indicating upflows in the rise phase and downflows in the decay
phase, respectively. Nevertheless, the differences seem to be mainly
quantitative, and the analyzed heating events with thermal energies
around 10<SUP>26</SUP> erg may in principle be considered as microflares
or large nanoflares, thus small versions of regular flares.
---------------------------------------------------------
Title: WIND Observations of Energetic Solar Proton Events Down to
keV Energies: Onset Time Analysis
Authors: Krucker, S.; Lin, R. P.
2000ASPC..206..187K Altcode: 2000hesp.conf..187K
No abstract at ADS
---------------------------------------------------------
Title: Heating Events Observed in the Quiet Corona
Authors: Benz, A. O.; Krucker, S.
1999ESASP.448..547B Altcode: 1999ESPM....9..547B; 1999mfsp.conf..547B
No abstract at ADS
---------------------------------------------------------
Title: Heating Events in the Quiet Solar Corona
Authors: Krucker, S.; Benz, A. O.
1999spro.proc...25K Altcode:
Sensitive observations of the quiet Sun provided by (1) the SXT on
board the Yohkoh satellite, (2) the EIT on board the SoHO spacecraft in
high-temperature iron line emission, and (3) the Very Large Array (VLA)
in the centimeter radio range are investigated in view of the coronal
heating problem. The observed enhancements in coronal emission measure
are interpreted as heating events (microflares) bringing chromospheric
material to coronal temperatures, whereas the radio observations
show the existence of non-thermal emission related to some of these
heating events. Assuming an effective height of 5000~km, the thermal
energy inputs by such microflares have been found in the range from
8× 10<SUP>24</SUP> erg to 1.6× 10<SUP>26</SUP> erg, and the total
energy input amounts to about 16% of the average radiated power of
the coronal plasma in the quiet corona. The frequency distribution of
microflares is an approximate power-law of the form f(E) = f<SUB>0</SUB>
E<SUP>-δ</SUP> with a power-law index δ between 2.3 and 2.6. As the
low-energy cutoff is due to sensitivity limitations and the power-law
index is steeper than 2, these observations demonstrate the possibility
that microflares dominate the energy input into the quiet corona.
---------------------------------------------------------
Title: On the Origin of Impulsive Electron Events Observed at 1 AU
Authors: Krucker, Säm; Larson, Davin E.; Lin, Robert P.; Thompson,
Barbara J.
1999ApJ...519..864K Altcode:
A statistical survey of 12 impulsive electron events detected
at energies down below 1 keV and 58 events detected above 25
keV observed at 1 AU by the 3-D Plasma and Energetic Particles
experiment on the Wind spacecraft is presented. Timing analysis of the
velocity dispersion reveals two different kinds of electron events:
(1) events released from the Sun at the onset of a radio type III
burst, which suggest that these electrons are part of the population
producing the type III radio emission; and (2) events in which the
electrons are released up to half an hour later than the onset of
the type III burst. These electrons therefore may be produced by a
different acceleration mechanism than the population producing the
radio emission. Both types of behavior can be observed during the
same impulsive electron event at different energies, but most events
show the same timing at all energies. At lower energies (<25 keV),
type III-related impulsive electron events are more often observed
(nine of 12 events), whereas at higher energies (>25 keV), events
not related to type III bursts are more numerous (41 of 58). However,
events of both classes are observed below 1 keV. Impulsive electron
events not related to type III radio bursts are observed to be proton
rich, with an order-of-magnitude lower electron-to-proton ratio than
events related to type III bursts. <P />For roughly 3/4 of the events
not related to type III bursts, large-scale coronal transient waves,
also called EIT waves or coronal Moreton waves, are observed by the
Extreme Ultraviolet Imaging Telescope (EIT) on board SOHO. Temporal
and spatial correlations together with hydromagnetic simulations show
that at least some impulsive electron events are more likely related
to the propagating Moreton wave than to the flare phenomenon itself.
---------------------------------------------------------
Title: The source region of an interplanetary type II radio burst
Authors: Bale, S. D.; Reiner, M. J.; Bougeret, J. -L.; Kaiser, M. L.;
Krucker, S.; Larson, D. E.; Lin, R. P.
1999GeoRL..26.1573B Altcode:
We present the first observation of the source region of an
interplanetary type II radio burst, using instruments on the Wind
spacecraft. Type II radio emission tracks the motion of a CME-driven
interplanetary (IP) shock which encounters the spacecraft. Upstream of
the IP shock backstreaming electrons are observed, first antiparallel
to the interplanetary magnetic field (IMF), and then later parallel as
well. Langmuir waves are observed concomitant with the shock-accelerated
electrons. The electron energy spectrum and Langmuir wave amplitudes
are very similar to those observed in the terrestrial electron
foreshock. From the connection times to the shock, we infer the
existence and characteristic size of large scale structure on the shock
front. The type II radio emission seems to be generated in a small
bay upstream of the shock, and this may account for some splitting
structure observed in the frequency spectrum of many type II bursts.
---------------------------------------------------------
Title: Quantitative results on heating events in the quiet corona
Authors: Benz, Arnold O.; Krucker, Säm
1999AIPC..471...67B Altcode: 1999sowi.conf...67B
The emission measure of the quiet corona, defined by the plasma
hotter than one million degrees, is fluctuating in nearly every 1900
km×1900 km pixel observed by EIT on SoHO. In the average, the larger
the emission measure in a pixel, the more it fluctuates. Increases
in emission measure constitute a major energy input into the corona,
suggesting that the lower corona is not just heated, but continuously
replenished by chromospheric material heated to coronal temperatures.
---------------------------------------------------------
Title: Heating events in the quiet solar corona: multiwavelength
correlations
Authors: Benz, Arnold O.; Krucker, Säm
1999A&A...341..286B Altcode:
Coronal, transition region and chromospheric lines and centimeter
radio emission of the quiet Sun have been simultaneously observed
by SoHO and the VLA. The corona above the magnetic network has a
higher pressure and is more variable than above the interior of
supergranular cells. The Fourier transform in time is found to have
steeper spectra in the corona and upper chromosphere than in the
transition region. The temporal sequence of brightenings has been
determined by cross-correlations of identical picture elements in
different emissions. The method allows to study statistically the
faintest fluctuations in the corona and relate them to the layers
below. The cross-correlations yield that (i) the first emissions to
peak in time are O V and He I originating in the transition region
and the upper chromosphere, respectively. (ii) The coronal line of Fe
XII lags by about 5 minutes and Fe IX/X peaks a further half a minute
later in the average, latest of all emissions. The interpretation of
these lags follows readily from analogy with regular flares in active
regions, where O V and He I correlate closely with hard X-rays emitted
by beam electrons impinging on the chromosphere. The coronal iron lines
are then emitted by the evaporating plasma expanding into the corona
and cooling by conducting part of the energy to increase the emission
in Fe IX. (iii) The radio emission peaks before the coronal emission
measure, similar to the Neupert effect in flares, but shows considerable
variation relative to O V. It is proposed that there are two emission
processes at work radiating both thermal emission and non-thermal
gyrosynchrotron emission at various fluxes. These statistical results
show that the coronal heating events follow the properties of regular
solar flares and thus may be interpreted as microflares or nanoflares.
---------------------------------------------------------
Title: Heating Events in the Quiet Solar Corona
Authors: Benz, Arnold O.; Krucker, Säm
1998SoPh..182..349B Altcode:
Sensitive observations of the quiet Sun observed by EIT on the SOHO
satellite in high-temperature iron-line emission originating in the
corona are presented. The thermal radiation of the quiet corona is
found to fluctutate significantly, even on the shortest time scale of
2 min and in the faintest pixels. The power spectrum of the emission
measure time variations is approximately a power law with an exponent
of 1.79±0.08 for the brightest pixels and 1.69±0.08 for the average
and the faintest pixels. The more prominent enhancements are identified
with previously reported X-ray network flares (Krucker et al., 1997)
above the magnetic network of the quiet chromosphere. In coronal
EUV iron lines they are amenable to detailed analysis suggesting
that the brightenings are caused by additional plasma injected from
below and heated to slightly higher temperature than the preexisting
corona. Statistical investigations are consistent with the hypothesis
that the weaker emission measure enhancements originate from the
same parent population. The power input derived from the impulsive
brightenings is linearly proportional to the radiative loss in the
observed part of the corona. The absolute amount of impulsive input
is model-dependent. It cannot be excluded that it can satisfy the
total requirement for heating. These observations give strong evidence
that a significant fraction of the heating in quiet coronal regions
is impulsive.
---------------------------------------------------------
Title: Energy Distribution of Heating Processes in the Quiet Solar
Corona
Authors: Krucker, Säm; Benz, Arnold O.
1998ApJ...501L.213K Altcode:
We have determined the variations in the emission measure of the solar
corona using EUV Imaging Telescope/Solar and Heliospheric Observatory
observations of iron lines in a quiet region of the Sun. The
emission measure is found to vary significantly in at least 85%
of all the pixels within 42 minutes. The variations are interpreted
as heating events that bring chromospheric material above the one
million degree threshold of the observed lines and that cool the
coronal plasma below that limit. A method to assess heating events
has been developed. The thermal energy input by such microflares is
calculated from the observed increases in emission measure and the
derived temperature. Heating events have been found in the range from
8×10<SUP>24</SUP> to 1.6×10<SUP>26</SUP> ergs. The energy input
by >=3 σ events of the emission measure increase the amounts to
about 16% of the average radiated power of the coronal plasma in
the quiet corona. The frequency distribution of microflares is an
approximate power law of the form f(E)=f<SUB>0</SUB>E<SUP>-δ</SUP>,
with a power-law index δ between 2.3 and 2.6. Since the low-energy
cutoff is due to sensitivity limitations and the power-law index
is steeper than 2, these observations demonstrate the possibility
that microflares dominate the energy input into the quiet corona. The
observed power law would have to continue to about 3×10<SUP>23</SUP>
ergs in order to match the observed minimum heating requirement.
---------------------------------------------------------
Title: Radio Wave and Soft X-ray Diagnostics of Heating Events in
the Quiet Solar Corona
Authors: Krucker, S.; Benz, A. O.
1998cee..workE...7K Altcode:
Sensitive observations of the quiet Sun observed by (1) SXT on the
Yohkoh satellite, (2) EIT on the SoHO satellite in high-temperature iron
line emission, (3) CDS on the SoHO satellite in the He I and O V line
emission, and (4) the Very Large Array (VLA) in the centimeter radio
range are investigate in the view of the coronal heating problem. The
observed enhancements are interpreted as heating events (mircoflares)
bringing chromospheric material to coronal temperatures. The thermal
energy inputs by such microflares have been found in the range from
8 times 10<SUP>24</SUP>erg to 1.6 times 10<SUP>26</SUP>erg, and the
total energy input amounts to about 16% of the average radiated power
of the coronal plasma in the quiet corona. The frequency distribution
of microflares is an approximate power-law of the form f(E) = f_0
E^{-delta} with a power-law index delta between 2.3 and 2.6. As the
low-energy cutoff is due to sensitivity limitations and the power-law
index is steeper than 2, these observations demonstrate the possibility
that microflares dominate the energy input into the quiet corona.
---------------------------------------------------------
Title: X-Ray Network Flares of the Quiet Sun
Authors: Krucker, Säm; Benz, Arnold O.; Bastian, T. S.; Acton,
Loren W.
1997ApJ...488..499K Altcode:
Temporal variations in the soft X-ray (SXR) emission and the radio
emission above the solar magnetic network of the quiet corona are
investigated using Yohkoh SXR images with deep exposure and VLA
observations in the centimeter radio range. The SXR data show several
brightenings, with an extrapolated occurrence probability of one
brightening per 3 seconds on the total solar surface. During the roughly
10 minutes of enhanced flux, total radiative losses of the observed
plasma are around 10<SUP>25</SUP> ergs per event. These events are more
than an order of magnitude smaller than previously reported X-ray bright
points or active region transient brightenings. For all of the four
SXR events with simultaneous radio observations, a corresponding radio
source correlating in space and time can be found. There are several
similarities between solar flares and the SXR/radio events presented in
this paper. (1) Variations in temperature and emission measure during
the SXR enhancements are consistent with evaporation of cooler material
from the transition region and the chromosphere. (2) The ratio of the
total energies radiated in SXR and radio frequencies is similar to
that observed in flares. (3) At least one radio event shows a degree
of polarization as high as 35%. (4) In three out of four substructures
the centimeter radio emission peaks several tens of seconds earlier
than in the SXR emission. (5) The associated radio emission tends to
be more structured and to have faster rise times. These events thus
appear to be flare-like and are called network flares.
---------------------------------------------------------
Title: Chromospheric Events in the Quiet Network
Authors: Keller, C.; Bastian, T.; Benz, A.; Krucker, S.
1997SPD....28.1304K Altcode: 1997BAAS...29..917K
Time sequences of a quiet network region close to disk center have
been simultaneously recorded with the VLA, various instruments on SOHO,
and the solar telescopes on Kitt Peak. The analysis of the Hα spectra
obtained at the McMath-Pierce telescope revealed down-flows with
apparent velocities of more than 2.5 km/s associated with magnetic
field structures in the quiet network. During such events, the Hα
spectra show a pronounced asymmetry. The photospheric magnetic field was
determined from rapid scans in three iron lines with the Zurich Imaging
Stokes Polarimeter. Up- and down-flow velocity excursions outside of
magnetic field regions are compatible with chromospheric waves. We
describe the properties of these events as seen in the observations
of the visible part of the spectrum and their signatures at radio
and UV wavelengths. The final goal of this study is the construction
of a time-dependent 3-D picture of the quiet solar atmosphere and the
understanding of the dynamical coupling of photospheric magnetic fields
with the chromosphere and the corona.
---------------------------------------------------------
Title: Fine structure of the X-ray and radio emissions of the quiet
solar corona.
Authors: Benz, A. O.; Krucker, S.; Acton, L. W.; Bastian, T. S.
1997A&A...320..993B Altcode:
Two deep soft X-ray exposures of a quiet region on the Sun were made
with the SXT telescope on board the Yohkoh satellite on 20 Feb 1995. We
report on the spatial X-ray fine structure. Regions of enhanced X-ray
emission, more than two orders of magnitude fainter than previously
reported X-ray bright points, are loosely associated with bipolar
regions in the magnetic network. The power spectrum of quiet X-ray
images at small spatial scales is similar to that of active regions,
but exhibits a kink at a scale of =~25,000km, possibly connected to
the supergranular structure. The spatial X-ray structures in the
time averaged image amount to an rms amplitude which is 6% of the
mean value. The X-ray structures correlate with contemporaneous radio
maps obtained by the VLA at wavelengths of 1.3, 2.0, and 3.6cm. The
amplitude of the brightness variations in the images increases with
radio wavelength, i.e., with increasing height. The cross-correlation
coefficient with the absolute magnetic field strength, however,
generally decreases with height, consistent with the idea of bipolar
regions in the network and of the magnetic field deviating from
vertical in the upper chromosphere. The X-ray observations require an
enhanced pressure in the corona above the magnetic network, but suggest
similar temperatures. Model calculations show that, under a constant
temperature, an rms density increase (relative to that in the cell
interior) ranging from about 20% in the chromosphere to 60% in the low
corona is sufficient to explain the observed standard deviations due
to the spatial structures in radio waves and soft X-rays, respectively.
---------------------------------------------------------
Title: Coronal EUV and Radio Variability and Heating
Authors: Krucker, S.; Benz, A. O.; Delaboudinière, J. -P.
1997ESASP.404..465K Altcode: 1997cswn.conf..465K
No abstract at ADS
---------------------------------------------------------
Title: YOHKOH observation of the source regions of solar narrowband,
millisecond spike events.
Authors: Krucker, S.; Benz, A. O.; Aschwanden, M. J.
1997A&A...317..569K Altcode:
The source regions of metric spike events are investigated on Yohkoh
soft X-ray (SXR) maps. The spikes are identified by the spectrometer
Phoenix between 300MHz and 360MHz and are associated with groups of
type III bursts at lower frequencies reaching also the decametric
range. The Very Large Array (VLA) provides simultaneously spatial
information at 333MHz, 1446MHz and 4866MHz. Similar to the previous
VLA observation of a metric spike event, the new data are consistent
with a high altitude of the spike sources of about 5x10^10^cm above
the photosphere. The additionally available SXR data for one of the
presented events give the following new informations: (i) The spike
sources occur near open field lines and near regions of a slightly
enhanced SXR flux relative to the ambient plasma. (ii) Contrary to SXR
observations of type III bursts without metric spike activity, no SXR
jet is observed. (iii) At low altitude, a weak SXR enhancement occurs,
peaking about 60 s after the spike event. The SXR source and the spike
sources can be connected by potential field lines. The observations
corroborate a model in which a metric spike event is attributed to
an energy release region at high altitude, while upwards propagating
electrons produce type III bursts and downward moving electrons are
responsible for SXR emission of heated plasma.
---------------------------------------------------------
Title: X-Ray/Radio Network Flares of the Quiet Sun
Authors: Benz, Arnold O.; Krucker, Sam; Acton, Loren W.; Bastian, T. S.
1997IAUJD..19E...1B Altcode:
The temporal variations in the soft X-ray (SXR) emission and the
radio emission above the solar magnetic network of the quiet corona
have been investigated using Yohkoh SXR images with deep exposure and
VLA observations in the centimetric radio range. The SXR data show
several brightenings with an extrapolated occurrence probability of
one brightening per 3 seconds on the total solar surface. During the
roughly 10 minutes of enhanced flux, the total radiative losses of
the observed plasma are betwee () n 0.6 and 2.4 cdot 10<SUP>26</SUP>
erg per event. These events are more than an order of magnitude smaller
than previously reported X-ray bright points or active region transient
brightenings. For all of the four SXR events with simultaneous radio
observations, a corresponding radio source correlating in space
and time can be found. There are several similarities between these
SXR/radio events and regular solar flares. These events thus appear
to be flare-like and are called network flares. We will report also
on very recent work using SOHO's EIT and CDS experiments combined with
VLA and Kitt Peak observations.
---------------------------------------------------------
Title: Soft X-Ray (Yohkoh) and Radio (VLA) Observations of Solar
Narrowband, Millisecond Spike Events
Authors: Krucker, Sam; Benz, Arnold O.; Aschwanden, Markus J.
1996ASPC..111..129K Altcode: 1997ASPC..111..129K
The source regions of metric spike events are investigated on Yohkoh
soft X-ray (SXR) maps. The spikes are identified by the spectrometer
Phoenix between 300 MHz and 360 MHz and are associated with groups
of type III bursts at lower frequencies reaching also the decametric
range. The Very Large Array (VLA) provides simultaneously spatial
information at 333 MHz and 1445 MHz. Similar to the previous VLA
observation of a metric spike event, the new data are consistent with a
high altitude of the spike sources of about 5×10<SUP>10</SUP>cm above
the photosphere. The additionally available SXR data for one of the
presented events give the following new information: (i) The spike
sources occur near open field lines and near regions of a slightly
enhanced SXR flux relative to the ambient plasma. (ii) Contrary to SXR
observations of type III bursts without metric spike activity, no SXR
jet is observed. (iii) At low altitude, a weak SXR enhancement occurs,
peaking about 60 s after the spike event. The SXR source and the spike
sources can be connected by potential field lines. The observations
corroborate a model in which a metric spike event is attributed to
an energy release region at high altitude, while upwards propagating
electrons produce type III bursts and downward moving electrons are
responsible for SXR emission of heated plasma.
---------------------------------------------------------
Title: Small solar flares in radio and x-rays: microflares and
radio bursts
Authors: Krucker, Säm
1996PhDT.......127K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Location of Type I Radio Continuum and Bursts on YOHKOH Soft
X-ray Maps
Authors: Krucker, S.; Benz, A. O.; Aschwanden, M. J.; Bastian, T. S.
1996mpsa.conf..441K Altcode: 1996IAUCo.153..441K
No abstract at ADS
---------------------------------------------------------
Title: First VLA observation of a solar narrowband, millisecond
spike event.
Authors: Krucker, S.; Aschwanden, M. J.; Bastian, T. S.; Benz, A. O.
1995A&A...302..551K Altcode:
The first spatially resolved observation of solar, narrowband spikes
in two dimensions is presented. The 'metric' spikes around 333MHz
are classified by the broadband spectrometer Phoenix (ETH Zuerich),
whereas the simultaneously observing Very Large Array (VLA) provides
high angular resolution images of the solar disc. At lower frequencies,
a group of associated type III bursts is detected. The spikes occur
at high altitude (=~4.5x10^10^cm above the photosphere), and at least
3 separated locations of emission can be identified. The different
spike sources are separated by up to 130" and show different degrees of
polarization. Spikes and type III bursts have the same sense of circular
polarization, and according to the extrapolated potential field lines,
the polarization of the different spike sources is in x-mode. With
a delay of 42s, a thermal source appears on the same extrapolated
potential field lines as the spikes at the second frequency of the
VLA (1446MHz). The location of the energy release relative to the
spikes source is discussed. A scenario is proposed where the energy
is released in or near the spike source, and in which the spikes,
the type III bursts and the thermal source originate from the same
energy release. Hot electrons expanding along the field lines generate
a type III burst (upward direction) and heat the underlying dense plasma
(thermal source).
---------------------------------------------------------
Title: Location of Type I Radio Continuum and Bursts on YOHKOH Soft
X-Ray Maps
Authors: Krucker, S.; Benz, A. O.; Aschwanden, M. J.; Bastian, T. S.
1995SoPh..160..151K Altcode:
A solar type I noise storm was observed on 30 July, 1992 with the
radio spectrometer Phoenix of ETH Zürich, the Very Large Array (VLA)
and the soft X-ray (SXR) telescope on board theYohkoh satellite. The
spectrogram was used to identify the type I noise storm. In the VLA
images at 333 MHz a fully left circular polarized (100% LCP) continuum
source and several highly polarized (70% to 100% LCP) burst sources
have been located. The continuum and the bursts are spatially separated
by about 100″ and apparently lie on different loops as outlined
by the SXR. Continuum and bursts are separated in the perpendicular
direction to the magnetic field configuration. Between the periods of
strong burst activities, burst-like emissions are also superimposed
on the continuum source. There is no obvious correlation between the
flux density of the continuum and the bursts. The burst sources have
no systematic motion, whereas the the continuum source shows a small
drift of ≈ 0.2″ min<SUP>−1</SUP> along the X-ray loop in the
long-time evolution. The VLA maps at higher frequency (1446 MHz) show
no source corresponding to the type I event. The soft X-ray emission
measure and temperature were calculated. The type I continuum source
is located (in projection) in a region with enhanced SXR emission,
a loop having a mean density of «n<SUB>e</SUB>» = (1.5 ± 0.4) ×
10<SUP>9</SUP> cm<SUP>−3</SUP> and a temperature ofT = (2.1 ± 0.1)
× 10<SUP>6</SUP> K. The centroid positions of the left and right
circularly polarized components of the burst sources are separated
by 15″-50″ and seem to be on different loops. These observations
contradict the predictions of existing type I theories.
---------------------------------------------------------
Title: Multiple frequency spike emission during solar flares
Authors: Krucker, S.; Benz, A. O.
1994SSRv...68..247K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The frequency ratio of bands of microwave spikes during
solar flares
Authors: Krucker, S.; Benz, A. O.
1994A&A...285.1038K Altcode:
The frequency ratio of narrowband millisecond spikes during 17
solar flares has been measured and analyzed. The observations have
been made in the frequency range 0.3-3 GHz with the ETH Zuerich
spectrometers. The events have been selected from spectrograms for
their harmonic structure. Auto-correlation in frequency yields the
following major results: (i) The ratio of the harmonic spike groups
is not integer, and therefore the term 'harmonic' is strictly speaking
inappropriate. (ii) The ratios are independent of frequency. (iii) The
ratios range from 1.06 to 1.54 with a prominent peak at 1.39+/-0.01. The
cross-correlation of the time profile of harmonic groups peaks at
zero lag, demonstrating a tight relation between individual spikes in
associated groups. The results require an emission mechanism that can
produce harmonic emission at a ratio 5:7 or 1:1.4 and vary significantly
from this value. In several cases 3, and in one case 4 harmonic bands
have been observed. Previously proposed interpretations of harmonic
spike emission are discussed in view of the new observations.