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Author name code: gary-dale
ADS astronomy entries on 2022-09-14
author:"Gary, Dale E."
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Title: Hard X-ray solar flare observed by Solar Orbiter/STIX
associated with escaping electron beams
Authors: Battaglia, Andrea Francesco; Warmuth, Alexander; Krucker,
Samuel; Gary, Dale; Veronig, Astrid; Purkhart, Stefan; Saqri, Jonas;
Collier, Hannah; Wang, Wen
2022cosp...44.1542B Altcode:
Dramatic changes of the magnetic field configuration in the solar
corona may reduce the magnetic energy in favor of the kinetic energy
of high-energy particles. Such a phenomenon, known as magnetic
reconnection, can lead to solar eruptions in the form of flares and
coronal mass ejections. On the one hand, the accelerated particles heat
the ambient plasma to temperatures of the order of tens of MK, while,
on the other hand, particles can escape along open magnetic field lines
into the heliosphere. Combining remote sensing and in-situ observations
closer to the Sun, Solar Orbiter is able to study the acceleration
region, the escape and the transport of solar energetic particles into
the heliosphere. We analyze images obtained with remote-sensing hard
X-ray observations by Solar Orbiter/STIX (Spectrometer/Telescope
for Imaging X-rays) to investigate the acceleration region and
escape of electron beams detected with in-situ measurements in the
heliosphere. The prompt electron events show temporal correlation
with the hard X-ray nonthermal emissions observed by STIX, favoring
the idea of eruptive solar flares at the origin of the beams under
investigation. For the SOL2021-08-26 event, the presence of accelerated
electrons is further confirmed by the synchrotron emission measured
by the EOVSA (Expanded Owens Valley Solar Array), which originate from
electrons being accelerated along magnetic field lines.
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Title: Erratum: "A Study of Sunspot 3 Minute Oscillations Using ALMA
and GST" (2022, ApJ, 924, 100)
Authors: Chai, Yi; Gary, Dale E.; Reardon, Kevin P.; Yurchyshyn, Vasyl
2022ApJ...933..247C Altcode:
No abstract at ADS
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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>
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Title: Solar flare accelerates nearly all electrons in a large
coronal volume
Authors: Fleishman, Gregory D.; Nita, Gelu M.; Chen, Bin; Yu, Sijie;
Gary, Dale E.
2022Natur.606..674F Altcode:
Solar flares, driven by prompt release of free magnetic energy in
the solar corona<SUP>1,2</SUP>, are known to accelerate a substantial
portion (ten per cent or more)<SUP>3,4</SUP> of available electrons
to high energies. Hard X-rays, produced by high-energy electrons
accelerated in the flare<SUP>5</SUP>, require a high ambient
density for their detection. This restricts the observed volume to
denser regions that do not necessarily sample the entire volume of
accelerated electrons<SUP>6</SUP>. Here we report evolving spatially
resolved distributions of thermal and non-thermal electrons in a
solar flare derived from microwave observations that show the true
extent of the acceleration region. These distributions show a volume
filled with only (or almost only) non-thermal electrons while being
depleted of the thermal plasma, implying that all electrons have
experienced a prominent acceleration there. This volume is isolated
from a surrounding, more typical flare plasma of mainly thermal
particles with a smaller proportion of non-thermal electrons. This
highly efficient acceleration happens in the same volume in which the
free magnetic energy is being released<SUP>2</SUP>.
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Title: Revisiting the Solar Research Cyberinfrastructure Needs:
A White Paper of Findings and Recommendations
Authors: Nita, Gelu; Ahmadzadeh, Azim; Criscuoli, Serena;
Davey, Alisdair; Gary, Dale; Georgoulis, Manolis; Hurlburt, Neal;
Kitiashvili, Irina; Kempton, Dustin; Kosovichev, Alexander; Martens,
Piet; McGranaghan, Ryan; Oria, Vincent; Reardon, Kevin; Sadykov,
Viacheslav; Timmons, Ryan; Wang, Haimin; Wang, Jason T. L.
2022arXiv220309544N Altcode:
Solar and Heliosphere physics are areas of remarkable data-driven
discoveries. Recent advances in high-cadence, high-resolution
multiwavelength observations, growing amounts of data from realistic
modeling, and operational needs for uninterrupted science-quality data
coverage generate the demand for a solar metadata standardization and
overall healthy data infrastructure. This white paper is prepared as
an effort of the working group "Uniform Semantics and Syntax of Solar
Observations and Events" created within the "Towards Integration of
Heliophysics Data, Modeling, and Analysis Tools" EarthCube Research
Coordination Network (@HDMIEC RCN), with primary objectives to discuss
current advances and identify future needs for the solar research
cyberinfrastructure. The white paper summarizes presentations and
discussions held during the special working group session at the
EarthCube Annual Meeting on June 19th, 2020, as well as community
contribution gathered during a series of preceding workshops and
subsequent RCN working group sessions. The authors provide examples
of the current standing of the solar research cyberinfrastructure, and
describe the problems related to current data handling approaches. The
list of the top-level recommendations agreed by the authors of the
current white paper is presented at the beginning of the paper.
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Title: A solar flare driven by thermal conduction observed in
mid-infrared
Authors: López, Fernando M.; Giménez de Castro, Carlos Guillermo;
Mandrini, Cristina H.; Simões, Paulo J. A.; Cristiani, Germán D.;
Gary, Dale E.; Francile, Carlos; Démoulin, Pascal
2022A&A...657A..51L Altcode: 2021arXiv211015751L
Context. The mid-infrared (mid-IR) range has been mostly unexplored
for the investigation of solar flares. It is only recently that new
mid-IR flare observations have begun opening a new window into the
response and evolution of the solar chromosphere. These new observations
have been mostly performed by the AR30T and BR30T telescopes that are
operating in Argentina and Brazil, respectively. <BR /> Aims: We present
the analysis of SOL2019-05-15T19:24, a GOES class C2.0 solar flare
observed at 30 THz (10 μm) by the ground-based telescope AR30T. Our
aim is to characterize the evolution of the flaring atmosphere and
the energy transport mechanism in the context of mid-IR emission. <BR
/> Methods: We performed a multi-wavelength analysis of the event by
complementing the mid-IR data with diverse ground- and space-based data
from the Solar Dynamics Observatory (SDO), the H-α Solar Telescope
for Argentina, and the Expanded Owens Valley Solar Array (EOVSA). Our
study includes the analysis of the magnetic field evolution of the
flaring region and of the development of the flare. <BR /> Results:
The mid-IR images from AR30T show two bright and compact flare sources
that are spatially associated with the flare kernels observed in
ultraviolet (UV) by SDO. We confirm that the temporal association
between mid-IR and UV fluxes previously reported for strong flares
is also observed for this small flare. The EOVSA microwave data
revealed flare spectra consistent with thermal free-free emission,
which lead us to dismiss the existence of a significant number of
non-thermal electrons. We thus consider thermal conduction as the
primary mechanism responsible for energy transport. Our estimates
for the thermal conduction energy and total radiated energy fall
within the same order of magnitude, reinforcing our conclusions. <P
/>Movies associated to Figs. 1, 2 and 4 are available at <A
href="https://www.aanda.org/10.1051/0004-6361/202141967/olm">https://www.aanda.org</A>
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Title: A Study of Sunspot 3 Minute Oscillations Using ALMA and GST
Authors: Chai, Yi; Gary, Dale E.; Reardon, Kevin P.; Yurchyshyn, Vasyl
2022ApJ...924..100C Altcode: 2021arXiv211105812C
Waves and oscillations are important solar phenomena, not only because
they can propagate and dissipate energy in the chromosphere, but also
because they carry information about the structure of the atmosphere
in which they propagate. The nature of the 3 minute oscillations
observed in the umbral region of sunspots is considered to be an
effect of propagation of magnetohydrodynamic waves upward from below
the photosphere. We present a study of sunspot oscillations and wave
propagation in NOAA Active Region 12470 using an approximately 1
hr long data set acquired on 2015 December 17 by the Atacama Large
Millimeter/submillimeter Array (ALMA), the Goode Solar Telescope
(GST) operating at the Big Bear Solar Observatory, the Atmospheric
Imaging Assembly on board the Solar Dynamics Observatory, and the
Interface Region Imaging Spectrograph. The ALMA data are unique in
providing a time series of direct temperature measurements in the
sunspot chromosphere. The 2 s cadence of ALMA images allows us to well
resolve the 3 minute periods typical of sunspot oscillations in the
chromosphere. Fourier analysis is applied to ALMA Band 3 (~100 GHz, ~3
mm) and GST Hα data sets to obtain power spectra as well as oscillation
phase information. We analyzed properties of the wave propagation by
combining multiple wavelengths that probe physical parameters of solar
atmosphere at different heights. We find that the ALMA temperature
fluctuations are consistent with that expected for a propagating
acoustic wave, with a slight asymmetry indicating nonlinear steepening.
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Title: Large Microwave Flare Sources observed by EOVSA Imaging
Spectroscopy
Authors: Shaik, Shaheda Begum; Gary, Dale
2021AGUFMSH24B..08S Altcode:
We present the imaging spectroscopy of gyrosynchrotron emission from
C-class flare SOL2017-04-04 observed by the Expanded Owens Valley Solar
Array (EOVSA) in 3.44 to 18 GHz. At the low microwave frequencies,
we observe large sources that are almost ten times as large as the
associated high frequency and hard X-ray flare emission. The source area
seems to decrease steeply by more than an order of magnitude as we move
from low to high frequencies. Unlike a single and straightforward loop
standard solar model type flare, this event in the microwave emission
shows the contribution of the multiple flux loops in different sizes
with the three-dimensional loop-loop interaction scenario, resulting
in the flare eruption. The emission at other wavelengths barely shows
any sign of particle transport at the secondary sites where we see
the low-frequency extended sources. These high-resolution microwave
observations indicate that, after the main reconnection process,
the accelerated particles have access to a much larger volume of the
flaring region through the overlying loops.
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Title: An In Situ Type III Radio Burst Event Observed Jointly by
the Expanded Owens Valley Solar Array and the Parker Solar Probe
Authors: Wang, Meiqi; Chen, Bin; Yu, Sijie; Gary, Dale; Lee, Jeongwoo
2021AGUFMSH15C2046W Altcode:
Solar type III radio bursts are generated by beams of energetic
electrons that travel outward along open magnetic field lines through
the corona and the interplanetary space. Here we report a type III burst
event observed jointly by the Expanded Owens Valley Solar Array (EOVSA)
and the Parker Solar Probe (PSP) shortly after its second perihelion in
April 2019. This type III burst event is associated with a solar jet
near the western boundary of a solar active region, which manifests
in EOVSA 118 GHz dynamic spectrum as a group of impulsive microwave
bursts. The type III burst event continues to the interplanetary
space in the decameterkilometer wavelength range (300 kHz30 MHz)
observed by multiple spacecraft including PSP/FIELDS, Wind/WAVEs,
and STEREO/WAVES, and appears to reach the local plasma frequency at
the PSP spacecraft. The multi-point spacecraft measurements allow us
to constrain the source location of the bursts and their directivity
in the interplanetary space. In addition, the type III burst event
coincides with an enhanced suprathermal electron population with an
anti-sunward beam-like component as measured by PSP/SWEAP. We argue that
the type-III-burst-emitting energetic electrons observed in situ may
be associated with an electron beam produced during the jet event that
propagating upwards along open field lines reaching the PSP spacecraft.
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Title: Study of Type III Radio bursts in the Closed Corona and the
Solar Wind from Small-scale Reconnection: Observations
Authors: Chhabra, Sherry; Klimchuk, James; Gary, Dale
2021AGUFMSH24B..06C Altcode:
It is widely agreed that the ubiquitous presence of reconnection
events and the associated impulsive heating (nanoflares) are a strong
candidate for heating the magnetically closed corona. Whether nanoflares
accelerate energetic particles like fullsized flares is unknown. The
lack of strong emission in hard Xrays suggests that the quantity of
highly energetic particles is small. There could, however, be large
numbers of mildly energetic particles (~ 10 keV). Similarly, in the
context of the solar wind, these energetic particles can originate
from interchange reconnection, streamer tip reconnection, or turbulence
reconnection in the solar wind itself, in which case they stream away
from the Sun along the open field lines. To understand whether these
processes are efficient at accelerating particles, we search for the
type III radio bursts that they may produce. The timelag technique that
was developed to study subtle delays in light curves from different
EUV channels [Viall & Klimchuk 2012] can also be used to detect
subtle delays at different radio frequencies. We have modeled the
expected radio emission from mildly energetic particles propagating
in the closed corona and open corona/solar wind. The models were used
to test and calibrate the technique. We are currently applying the
technique to radio observations from VLA (Very Large Array), LOFAR
(LowFrequency Array), LWA, NM (Long Wavelength Array, New Mexico), and
the FIELDS experiment (encounters 1-6) to search for such signatures
of type IIIs. We also plan to investigate the relationship between
the bursts and activity on the Sun, such as the presence/absence of
active regions, relationship with their age etc. We will report the
results from our analysis.
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Title: Goldstone Apple Valley Radio Telescope (GAVRT) Solar Patrol as
a New Citizen Scientist Program in the Era of the Parker Solar Probe
(PSP) and a Gateway to NASA Heliophysics Missions
Authors: Lamb, Lisa; Thangasamy, Velusamy; Levin, Steven; Gary, Dale;
Dorcey, Ryan; Kuiper, Thomas; Lazio, Joseph; Chen, Bin; Yu, Sijie;
Desai, Mihir
2021AGUFMED55D0317L Altcode:
Goldstone Apple Valley Radio Telescope (GAVRT) Program operates a
decommissioned NASA Deep Space Network 34-m antenna for science
education under a unique partnership between the Jet Propulsion
Laboratory, California Institute of Technology (JPL), and Lewis
Center for Educational Research (LCER). GAVRT strives to inspire and
enrich student learning through active contribution to professional
science. For over 20 years, GAVRT has provided opportunities for
K-12 students and teachers to operate the telescope remotely and take
science data. In 2021, using ROSES 2020 Citizen Science seed funding,
GAVRT has developed Solar Patrol as a new opportunity to engage citizen
scientists outside of K-12 schools. GAVRT Solar Patrol conducts raster
scans to generate radio images of the Sun to provide information about
the structure of the solar atmosphere including the corona, transition
region, and upper chromosphere. These images help us understand how
magnetic fields on the Sun can interact and generate intense explosions,
which in turn produce strong radio emission, particularly in regions
associated with sunspots. GAVRT Solar Patrol conducts daily observations
of the Sun at four frequencies (wavelengths or colors) chosen to enable
the tracking of radio-emitting material as it rises or falls above
the Sun following magnetic field interactions. Citizen scientists,
including but not limited to K-12 students, can engage in GAVRT Solar
Patrol by taking data, constructing images, & analysis to be
used as a gateway to integration with NASAs Heliophysics missions. In
addition to being scientifically valuable in their own right, the Solar
Patrol data can be combined with measurements from other telescopes,
especially with EOVSA interferometer data at GAVRT frequencies, and
they complement measurements by the Parker Solar Probe. In particular,
data from the GAVRT Solar Patrol and Parker Solar Probe help illustrate
how a combination of ground-based and spacecraft measurements can be
combined to enhance understanding of the Sun.
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Title: Understanding the Initiation of the M2.4 Flare on 2017 July 14
Authors: Jing, Ju; Inoue, Satoshi; Lee, Jeongwoo; Li, Qin; Nita,
Gelu M.; Xu, Yan; Liu, Chang; Gary, Dale E.; Wang, Haimin
2021ApJ...922..108J Altcode:
We present both the observation and the magnetohydrodynamics (MHD)
simulation of the M2.4 flare (SOL2017-07-14T02:09) of NOAA active
region (AR) 12665 with a goal to identify its initiation mechanism. The
observation by the Atmospheric Image Assembly (AIA) on board the Solar
Dynamics Observatory (SDO) shows that the major topology of the AR
is a sigmoidal configuration associated with a filament/flux rope. A
persistent emerging magnetic flux and the rotation of the sunspot in
the core region were observed with Magnetic Imager (HMI) on board the
SDO on the timescale of hours before and during the flare, which may
provide free magnetic energy needed for the flare/coronal mass ejection
(CME). A high-lying coronal loop is seen moving outward in AIA EUV
passbands, which is immediately followed by the impulsive phase of the
flare. We perform an MHD simulation using the potential magnetic field
extrapolated from the measured pre-flare photospheric magnetic field
as initial conditions and adopting the observed sunspot rotation and
flux emergence as the driving boundary conditions. In our simulation,
a sigmoidal magnetic structure and an overlying magnetic flux rope
(MFR) form as a response to the imposed sunspot rotation, and the MFR
rises to erupt like a CME. These simulation results in good agreement
with the observation suggest that the formation of the MFR due to the
sunspot rotation and the resulting torus and kink instabilities were
essential to the initiation of this flare and the associated coronal
mass ejection.
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Title: A Solar-Dedicated Observing System for the OVRO Long Wavelength
Array
Authors: Gary, Dale; Chhabra, Sherry; O'Donnell, Brian; Chen, Bin
2021AGUFMSH52A..03G Altcode:
A new, solar-dedicated imaging and spectroscopy pipeline capability
is being added to the Owens Valley Long Wavelength Array (OVRO-LWA),
an all-sky radio imaging array operating in the metric frequency range
from 20-88 MHz. The first capability to go online is a solar-dedicated
beam that tracks the Sun and provides a continuous high frequency and
time resolution (< 100 kHz and 1 ms, respectively) dynamic spectrum
of solar activity simultaneously over this entire frequency range. The
sensitivity to weak, extended emission from the core of the array
will provide new insights into some of the weak activity missed by
many other instruments. Shortly thereafter (early 2022), an imaging
pipeline will begin operating that will produce images at up to 700
frequencies and 0.1 s time resolution, with spatial resolution down to
5 arcmin at 88 GHz, so that the sources can be spatially located with
respect to disturbances imaged in other wavelengths. This will allow
mapping of trajectories of electron beams (type III bursts) and the
outward-propagating source regions of shock-related coherent emission
(type II bursts), as well as tracking of weak incoherent emission from
within the body of CMEs. This is illustrated by a recently published
study (Chhabra et al. 2021, ApJ 906, 132) for a CME observed in 2015
with an earlier version of OVRO-LWA, but the capabilities of the
new instrument will be vastly superior. In addition to providing
near-real-time coverage of events with Space Weather impacts, the
imaging pipeline with its unprecidented sensitivity is sure to lead
to new science discoveries about high-energy particle acceleration
and transport over the 1.3-2.5 Rsun range of heights where the closed
corona interfaces with the open solar wind.
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Title: Signatures of Type III Solar Radio Bursts from Nanoflares:
Modeling
Authors: Chhabra, Sherry; Klimchuk, James A.; Gary, Dale E.
2021ApJ...922..128C Altcode: 2021arXiv210903355C
There is a wide consensus that the ubiquitous presence of magnetic
reconnection events and the associated impulsive heating (nanoflares)
are strong candidates for solving the solar coronal heating
problem. Whether nanoflares accelerate particles to high energies
like full-sized flares is unknown. We investigate this question by
studying the type III radio bursts that the nanoflares may produce on
closed loops. The characteristic frequency drifts that type III bursts
exhibit can be detected using a novel application of the time-lag
technique developed by Viall & Klimchuk (2012) even when there
are multiple overlapping events. We present a simple numerical model
that simulates the expected radio emission from nanoflares in an active
region, which we use to test and calibrate the technique. We find that
in the case of closed loops the frequency spectrum of type III bursts
is expected to be extremely steep such that significant emission is
produced at a given frequency only for a rather narrow range of loop
lengths. We also find that the signature of bursts in the time-lag
signal diminishes as: (1) the variety of participating loops within
that range increases; (2) the occurrence rate of bursts increases;
(3) the duration of bursts increases; and (4) the brightness of bursts
decreases relative to noise. In addition, our model suggests a possible
origin of type I bursts as a natural consequence of type III emission
in a closed-loop geometry.
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Title: Long-Lasting Intense Radio Bursts from a Sunspot
Authors: Yu, Sijie; Chen, Bin; Sharma, Rohit; Battaglia, Marina; Luo,
Yingjie; Gary, Dale
2021AGUFMSH52A..04Y Altcode:
We present the detection of a new type of long-lasting (>4 hours),
highly circularly polarized, intense radio bursts from a sunspot at
decimetric wavelengths. The temporal and spectral characteristics of
the radio bursts resemble certain planetary coherent radio emissions
in the solar system generated by the electron cyclotron maser (ECM)
instability. We track the morphology and dynamic evolution of the
radio source using high-cadence radio imaging spectroscopy provided
by Karl G. Jansky Very Large Array. We show that the radio emission
is located within the large-scale, converging magnetic field rooted
at the sunspot. The spatial-spectral distribution of the radio
source location agrees with that expected from the ECM emission by
using a non-linear force-free magnetic field extrapolation model. Our
observations suggest that the radio emission is powered by the injection
of energetic electrons accelerated in a series of flare activities
into the converging magnetic field above the sunspot, which drives
ECM instability in the source region.
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Title: Coronal Magnetic Field Measurements along a Partially Erupting
Filament in a Solar Flare
Authors: Wei, Yuqian; Chen, Bin; Yu, Sijie; Wang, Haimin; Jing, Ju;
Gary, Dale E.
2021ApJ...923..213W Altcode: 2021arXiv211006414W
Magnetic flux ropes are the centerpiece of solar eruptions. Direct
measurements for the magnetic field of flux ropes are crucial for
understanding the triggering and energy release processes, yet
they remain heretofore elusive. Here we report microwave imaging
spectroscopy observations of an M1.4-class solar flare that occurred
on 2017 September 6, using data obtained by the Expanded Owens Valley
Solar Array. This flare event is associated with a partial eruption
of a twisted filament observed in Hα by the Goode Solar Telescope
at the Big Bear Solar Observatory. The extreme ultraviolet (EUV)
and X-ray signatures of the event are generally consistent with the
standard scenario of eruptive flares, with the presence of double
flare ribbons connected by a bright flare arcade. Intriguingly, this
partial eruption event features a microwave counterpart, whose spatial
and temporal evolution closely follow the filament seen in Hα and
EUV. The spectral properties of the microwave source are consistent
with nonthermal gyrosynchrotron radiation. Using spatially resolved
microwave spectral analysis, we derive the magnetic field strength
along the filament spine, which ranges from 600 to 1400 Gauss from
its apex to the legs. The results agree well with the nonlinear
force-free magnetic model extrapolated from the preflare photospheric
magnetogram. We conclude that the microwave counterpart of the erupting
filament is likely due to flare-accelerated electrons injected into
the filament-hosting magnetic flux rope cavity following the newly
reconnected magnetic field lines.
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Title: High-frequency Wave Power Observed in the Solar Chromosphere
with IBIS and ALMA
Authors: Molnar, Momchil E.; Reardon, Kevin P.; Cranmer, Steven R.;
Kowalski, Adam F.; Chai, Yi; Gary, Dale
2021ApJ...920..125M Altcode: 2021arXiv210708952M
We present observational constraints on the chromospheric heating
contribution from acoustic waves with frequencies between 5 and 50
mHz. We use observations from the Dunn Solar Telescope in New Mexico,
complemented with observations from the Atacama Large Millimeter Array
collected on 2017 April 23. The properties of the power spectra of the
various quantities are derived from the spectral lines of Ca II 854.2
nm, H I 656.3 nm, and the millimeter continuum at 1.25 and 3 mm. At
the observed frequencies, the diagnostics almost all show a power-law
behavior, whose particulars (slope, peak, and white-noise floors)
are correlated with the type of solar feature (internetwork, network,
and plage). In order to disentangle the vertical versus transverse
Alfvénic plasma motions, we examine two different fields of view: one
near disk center, and the other close to the limb. To infer the acoustic
flux in the middle chromosphere, we compare our observations with
synthetic observables from the time-dependent radiative hydrodynamic
RADYN code. Our findings show that acoustic waves carry up to about
1 kW m<SUP>-2</SUP> of energy flux in the middle chromosphere, which
is not enough to maintain the quiet chromosphere. This is in contrast
to previous publications.
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Title: Implications of Flat Optically Thick Microwave Spectra in
Solar Flares for Source Size and Morphology
Authors: Shaik, Shaheda Begum; Gary, Dale E.
2021ApJ...919...44S Altcode: 2021arXiv210700192B; 2021arXiv210700192S
The study aims to examine the spectral dynamics of the low-frequency,
optically thick gyrosynchrotron microwave emission in solar flares to
determine the characteristics of the emitting source. We present the
high-resolution spectra of a set of microwave bursts observed by the
Expanded Owens Valley Solar Array (EOVSA) during its commissioning phase
in the 2.5-18 GHz frequency range with 1 second time resolution. Out
of the 12 events analyzed in this study, nine bursts exhibit a direct
decrease with time in the optically thick spectral index α<SUB>l</SUB>,
an indicator of source morphology. Particularly, five bursts display
a "flat" spectrum (α<SUB>l</SUB> ≤ 1.0) compared to that expected
for a homogeneous/uniform source (α<SUB>l</SUB> ≍ 2.9). These flat
spectra at low frequencies (<10 GHz) can be defined as the emission
from a spatially inhomogeneous source with a large area and/or with
multiple emission components. In a subset of six events with partial
cross-correlation data, both the events with flat spectra show a source
size of ~120″ at 2.6-3 GHz. Modeling based on inhomogeneity supports
the conclusion that multiple discrete sources can only reproduce a
flat spectrum. We report that these flat spectra appear predominantly
in the decay phase and typically grow flatter over the duration in
most of the bursts, which indicates an increasing inhomogeneity and
complexity of the emitting volume as the flare progresses. This large
volume of flare emission filled with the trapped energetic particles
is often invisible in other wavelengths, like hard X-rays, presumably
due to the collisionless conditions in these regions of low ambient
density and magnetic field strength.
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Title: Multiple Sources of Solar High-energy Protons
Authors: Kocharov, Leon; Omodei, Nicola; Mishev, Alexander;
Pesce-Rollins, Melissa; Longo, Francesco; Yu, Sijie; Gary, Dale E.;
Vainio, Rami; Usoskin, Ilya
2021ApJ...915...12K Altcode:
During the 24th solar cycle, the Fermi Large Area Telescope (LAT)
has observed a total of 27 solar flares possessing delayed γ-ray
emission, including the exceptionally well-observed flare and coronal
mass ejection (CME) on 2017 September 10. Based on the Fermi/LAT data,
we plot, for the first time, maps of possible sources of the delayed
>100 MeV γ-ray emission of the 2017 September 10 event. The
long-lasting γ-ray emission is localized under the CME core. The
γ-ray spectrum exhibits intermittent changes in time, implying that
more than one source of high-energy protons was formed during the
flare-CME eruption. We find a good statistical correlation between
the γ-ray fluences of the Fermi/LAT-observed delayed events and
the products of corresponding CME speed and the square root of the
soft X-ray flare magnitude. Data support the idea that both flares
and CMEs jointly contribute to the production of subrelativistic and
relativistic protons near the Sun.
---------------------------------------------------------
Title: Signatures of Type III Radio Bursts from Small-scale
Reconnection Events in the Solar Wind
Authors: Chhabra, S.; Klimchuk, J.; Gary, D.; Psp/Fields Team
2021AAS...23812307C Altcode:
We look for evidence of energetic particles in the solar wind,
that could be produced by reconnection in the solar wind itself or
reconnection in the lower corona, where particles escape on open field
lines, from e.g. interchange reconnection.We expect reconnection to be
common at the current sheets that separate the thin magnetic strands
that make up the corona and solar wind, but whether it is efficient
at accelerating particles is an open question. Type III radio bursts
have been very frequently observed in the solar wind over the past
few decades. Energetic electron beams propagating along magnetic
field lines cause a bump-on-tail instability generating Langmuir
waves. Produced by their interaction with other particles and waves,
type III bursts exhibit a characteristic drift in frequency as they
propagate through the density gradient in the field. An interesting
question is whether there is a ubiquitous presence of type IIIs in the
radio 'background' observed. The radio background outside of clearly
identified bursts may actually be comprised of multiple overlapping
events. The time-lag technique that was developed to study subtle delays
in light curves from different EUV channels [Viall & Klimchuk 2012]
can also be used to detect subtle delays at different frequencies in
the radio background even when there is no hint in the individual light
curves. The FIELDS instrument onboard the Parker Solar Probe (PSP) is
utilized to investigate the solar wind for these signatures. We perform
a systematic study of the observed type III storms in Encounters 1-5,
to understand the signatures that can identify their presence and
the different features observed using the technique. Our findings are
then employed to study the times where no activity is visibly detected
by the instrument. We are currently analyzing multiple periods of no
visible activity and will report our findings.
---------------------------------------------------------
Title: Magnetic Field Measurements Of A Twisted Flux Rope In A Failed
Solar Eruption
Authors: Wei, Y.; Chen, B.; Yu, S.; Wang, H.; Jing, J.; Gary, D.
2021AAS...23821311W Altcode:
Magnetic flux ropes are the centerpiece of solar eruptions. Direct
measurements for the magnetic field of flux ropes are crucial for
understanding the triggering and energy release processes, yet
they remain heretofore elusive. Here we report microwave imaging
spectroscopy observations of an M1.4-class solar flare that occurred
on 2017 September 6, using data obtained by the Expanded Owens Valley
Solar Array. This flare event is associated with a failed eruption
of a twisted filament observed in Hɑ by the Goode Solar Telescope at
the Big Bear Solar Observatory. The filament, initially located along
the magnetic polarity inversion line prior to the event, undergoes a
failed eruption during the course of the flare. The upper portion of
the erupting filament has a counterpart in microwaves, whose spectral
properties indicate gyrosynchrotron radiation from flare-accelerated
nonthermal electrons. Using spatially resolved microwave spectral
analysis, we derive the magnetic field strength along the filament
spine, which ranges from 600-1400 Gauss from its apex to the legs. The
results agree well with the non-linear force-free magnetic model
extrapolated from the pre-flare photospheric magnetogram. The
multi-wavelength signatures of the event are consistent with the
standard scenario of eruptive flares, except that the eruption failed
to fully develop and escape as a coronal mass ejection. We conclude
that the failed eruption is likely due to the strong strapping coronal
magnetic field above the filament.
---------------------------------------------------------
Title: Characterizing a "Solar FRB"
Authors: Hudson, H.; Briggs, M.; Chitta, L.; Fletcher, L.; Gary, D.;
Monstein, C.; Nimmo, K.; Saint-Hilaire, P.; White, S.
2021AAS...23812716H Altcode:
A remarkable solar microwave (1.4 GHz) burst,
SOL2019-05-06T17:47:35.385, has been reported by the STARE2 fast cosmic
transient survey (Bochenek et al., 2020). Its behavior resembles
that of the Fast Radio Burst (FRB) extragalactic events in having a
relatively broad spectral bandwidth and brief (19-msec) duration. It
also had no measureable dispersion. The associated flare, GOES class
C1, had a relatively hard X-ray spectrum as observed by Fermi/GBM,
but no temporal association at the msec time scale suggested by the
microwaves. Although msec variability in the microwave domain has
been known to solar radio astronomy since the 1970s, the brightness
and isolation of this event (both spatial and temporal) suggests
novelty. Accordingly we survey the available correlative data from
many sources and discuss possible interpretations in terms of type
III-like behavior, electron cyclotron masering, and gyrosynchrotron
emission. We note that the radio data (e-Callisto and EOVSA) revealed
abundant type III activity in the vicinity, and the related flares
as observed by GOES had exceptionally short time scales, suggesting
burst origins in the lower solar atmosphere.
---------------------------------------------------------
Title: Diagnostics of Magnetic Field and Electron Densities in a
Solar Flare using Microwave Imaging Spectroscopy
Authors: O'Donnell, B.; Gary, D.; Chen, B.; Nita, G.; Yu, S.
2021AAS...23812704O Altcode:
Here we report on the multi-frequency observations made of the C2.9
class solar flare (21:04 UT on 2017 August 18) by the Expanded Owen's
Valley Solar Array. The resolution in space and frequency allows for
multiparameter fitting to probe the magnetic field strength, the
thermal and non-thermal electron densities, and electron spectral
index delta in the microwave source throughout the flare. Evidence
for two interacting loops is seen in microwaves, allowing for analysis
of those parameters in the separate individual loops, including some
along overlapping lines of sight.
---------------------------------------------------------
Title: Understanding The Initiation Of The M2.4 Flare On 2017 July 14
Authors: Jing, J.; Inoue, S.; Lee, J.; Li, Q.; Nita, G.; Xu, Y.; Liu,
C.; Gary, D.; Wang, H.
2021AAS...23812706J Altcode:
We present both the observation and the magnetohydrodynamics (MHD)
simulation of the M2.4 flare (SOL2017-07-14T02:09) of NOAA active
region (AR) 12665 to understand the initiation mechanism of this
eruptive flare, which is associated with a coronal mass ejection
(CME) and produces a storm of Solar Energetic Particles (SEPs). The
observation by the Atmospheric Image Assembly (AIA) on the Solar
Dynamics Observatory (SDO) shows that the major topology of the AR
is a sigmoidal configuration associated with a filament/flux rope,
which could be well reconstructed in the nonlinear force-free (NLFF)
field model. Persistent emerging magnetic flux and the rotation of
the sunspot are observed with Magnetic Imager (HMI) on the SDO in
the core region, in the timescale of hours before and during the
flare, providing sufficient free magnetic energy accounting for the
flare/CME. A high-lying coronal loop is seen moving outward in AIA EUV
passbands, which is immediately followed by the impulsive phase of the
flare. We perform the MHD simulation that uses the potential magnetic
field extrapolated from the measured pre-flare photospheric magnetic
field as initial conditions and adopts the sunspot rotation as the
driving force. In our simulation, a sigmoidal magnetic structure
and an overlying magnetic flux rope (MFR) form as a response to
the imposed sunspot rotation. The MFR rises up and erupts like a
CME. These simulation results are in good agreement with observation,
and suggests that the the loss of MHD stability might be the main
cause for this eruption.
---------------------------------------------------------
Title: A Type III Radio Burst Event Observed Jointly by the Expanded
Owens Valley Solar Array and the Parker Solar Probe
Authors: Wang, M.; Chen, B.; Yu, S.; Gary, D.; Lee, J.
2021AAS...23832811W Altcode:
Solar type III radio bursts are generated by beams of energetic
electrons that travel outward along open magnetic field lines through
the corona and the interplanetary space. Here we report a type III
burst event observed jointly by the Expanded Owens Valley Solar Array
(EOVSA) and the Parker Solar Probe (PSP) near its second perihelion
in April 2019. This type III burst event is associated with a solar
jet near the boundary of a solar active region, which manifests in
EOVSA 1-18 GHz dynamic spectrum as a group of impulsive microwave
bursts. The type III burst event continues to the interplanetary space
in the decameter-kilometer wavelength range (300kHz-30MHz) observed by
multiple spacecraft including PSP/FIELDS, and appears to reach the local
plasma frequency at the spacecraft. In addition, the type III burst
event coincides with an enhanced suprathermal electron population with
an anti-sunward beam-like component as measured by PSP/SWEAP. We discuss
the source region of the type-III-burst-emitting energetic electrons and
their transport from near the solar surface to the interplanetary space.
---------------------------------------------------------
Title: Acceleration without significant heating in the late impulsive
phase of large solar flares
Authors: Alaoui, M.; Gary, D.; Hayes, L.; Dennis, B.; Allred, J.;
Tolbert, K.; Holman, G.
2021AAS...23812715A Altcode:
We investigate characteristics of electron acceleration during
the late impulsive phase of large flares. Specifically, whether
characteristic parameters of the acceleration process change during
the late phase acceleration during the same event. We present X-ray
(HXR) and microwave (MW) observations of the late impulsive phase from
seven M- and X-class flares with insignificant heating. All these
peaks are consistent with the absence of Neupert effect where the
derivative of the thermal emission is flat, similarly to the flare
reported by Warmuth et al. 2009. The X-ray spectral fits from RHESSI
are consistent with a higher-than-usual low-energy cutoff value
(> 70 keV). This is inferred from RHESSI spatially integrated
spectra and spectroscopic imaging using an extended collisional thick
target model (ECTTM), where electron propagation mechanisms such as
the co-spatial return current, warm target and magnetic mirroring
are also considered. Radio/MW emission from these peaks reveals an
additional sub-group where 4/7 flares are associated with an increase
and 3/7 with a decrease in magnetic field strengths compared to the
main impulsive phase. The gradual (impulsive) peaks have a higher
(lower) emission and are associated with increased (decreased) magnetic
field strengths compared to earlier HXR peaks, except one flare which
exhibits characteristics of gradual and impulsive events. Magnetograms
further show that the HXR footpoints move into a region of increased
magnetic field for gradual events. No such evidence is observed for
the impulsive peaks. The main result is that these late peaks are
consistent with a higher-than-usual injected low-energy cutoff and
a total injected flux density insufficient to significantly heat the
solar atmosphere compared to the initial HXR impulsive phase.
---------------------------------------------------------
Title: Solar Radio Burst Effects on Radio- and Radar-Based Systems
Authors: Gary, Dale E.; Bastian, Timothy S.
2021GMS...262..141G Altcode:
Radio emission from solar flares can attain such high flux density that
the Sun becomes the dominant source of broadband radio noise in the
terrestrial environment. The effects of this radio noise on wireless
communication and navigation systems can take many forms, depending on
the design and operation of the affected system. These effects can be of
special concern for regional or global systems, since the effects can
occur simultaneously over the entire sunlit hemisphere of Earth. This
chapter reviews the origin of solar radio bursts, the threat they pose
based on statistics of the flux-density distribution of such events
vs. frequency, and some of the effects that have been documented in the
literature. The chapter concludes with a discussion of the potential
impacts on current and future technology and how these impacts can
be mitigated (1) through improved radio monitoring of the Sun in
both circular polarizations to supply meaningful real-time warnings,
(2) through improved scientific understanding of the solar phenomena
underlying the radio bursts, and (3) through improved system design
that takes account of solar radio noise.
---------------------------------------------------------
Title: Imaging Spectroscopy of CME-associated Solar Radio Bursts
using OVRO-LWA
Authors: Chhabra, Sherry; Gary, Dale E.; Hallinan, Gregg; Anderson,
Marin M.; Chen, Bin; Greenhill, Lincoln J.; Price, Danny C.
2021ApJ...906..132C Altcode: 2020arXiv201106073C
We present the first results of a solar radio event observed with
the Owens Valley Radio Observatory Long Wavelength Array at metric
wavelengths. We examine a complex event consisting of multiple radio
sources/bursts associated with a fast coronal mass ejection (CME)
and an M2.1 GOES soft X-ray flare from 2015 September 20. Images of 9
s cadence are used to analyze the event over a 120 minute period, and
solar emission is observed out to a distance of ≍3.5 R<SUB>⊙</SUB>,
with an instantaneous bandwidth covering 22 MHz within the frequency
range of 40-70 MHz. We present our results from the investigation
of the radio event, focusing particularly on one burst source that
exhibits outward motion, which we classify as a moving type IV burst. We
image the event at multiple frequencies and use the source centroids
to obtain the velocity for the outward motion. Spatial and temporal
comparison with observations of the CME in white light from the C2
coronagraph of the Large Angle and Spectrometric COronagraph, indicates
an association of the outward motion with the core of the CME. By
performing graduated-cylindrical-shell reconstruction of the CME, we
constrain the density in the volume. The electron plasma frequency
obtained from the density estimates do not allow us to completely
dismiss plasma emission as the underlying mechanism. However, based
on source height and smoothness of the emission in frequency and time,
we argue that gyrosynchrotron is the more plausible mechanism. We use
gyrosynchrotron spectral-fitting techniques to estimate the evolving
physical conditions during the outward motion of this burst source.
---------------------------------------------------------
Title: Radio Measurements of the Magnetic field in the Solar
Chromosphere and the Corona
Authors: Alissandrakis, Costas E.; Gary, Dale E.
2021FrASS...7...77A Altcode:
The structure of the upper solar atmosphere, on all observable scales,
is intimately governed by the magnetic field. The same holds for a
variety of solar phenomena that constitute solar activity, from tiny
transient brightening to huge Coronal Mass Ejections. Due to inherent
difficulties in measuring magnetic field effects on atoms (Zeeman and
Hanle effects) in the corona, radio methods sensitive to electrons
are of primary importance in obtaining quantitative information about
its magnetic field. In this review we explore these methods and point
out their advantages and limitations. After a brief presentation of
the magneto-ionic theory of wave propagation in cold, collisionless
plasmas, we discuss how the magnetic field affects the radio emission
produced by incoherent emission mechanisms (free-free, gyroresonance
and gyrosynchrotron processes) and give examples of measurements of
magnetic filed parameters in the quiet sun, active regions and radio
CMEs. We proceed by discussing how the inversion of the sense of
circular polarization can be used to measure the field above active
regions. Subsequently we pass to coherent emission mechanisms and
present results of measurements from fiber bursts, zebra patterns
and type II burst emission. We close this review with a discussion
of the variation of the magnetic field, deduced by radio measurement,
from the low corona up to 10 solar radii and with some thoughts about
future work.
---------------------------------------------------------
Title: EOVSA Full-Disk Imaging of the Sun at Microwave Frequencies:
Techniques and Updates
Authors: Wang, M.; Chen, B.; Yu, S.; Gary, D. E.
2020AGUFMSH0480001W Altcode:
The Expanded Owens Valley Solar Array (EOVSA), which consists of
thirteen 2.1-meter antennas, is a recently commissioned radio
interferometer dedicated to solar observing. It provides daily
observations of the full solar disk in the microwave range 1-18 GHz,
with up to 451 frequency channels at 1-s time cadence. Despite the
small number of antennas, spectral imaging of the full solar disk can be
achieved by taking advantage of the Earth-rotation synthesis technique
to integrate the visibilities over an entire day (typically 8 hours)
to improve the uv coverage. Here we discuss techniques and recent
updates to our full-disk imaging pipeline, with a particular focus
on the self-calibration techniques we developed to improve the image
quality and fidelity. We present examples of the updated full-disk
imaging results for both the cases when the disk is spotless and
when it has sunspots/active regions present. These images reveal a
rich variety of features of the quiescent Sun at different microwave
frequencies, including active regions, filaments, coronal holes, and
network structures, providing a unique dataset to the solar community.
---------------------------------------------------------
Title: Next-Generation Solar Radio Imaging Spectroscopy
Authors: Gary, D. E.; Bastian, T.; Chen, B.; Saint-Hilaire, P.; White,
S. M.
2020AGUFMSH056..07G Altcode:
Recent progress using instruments such as the Expanded Owens Valley
Solar Array (EOVSA) and the Jansky Very Large Array (JVLA) have amply
demonstrated the power of radio imaging spectroscopy at centimeter and
decimeter wavelengths for quantitative diagnostics of both flaring
and non-flaring solar plasma. For example, the unique sensitivity
of radio emission to the flaring coronal magnetic field has been
dramatically shown in a series of recent EOVSA papers, along with
accelerated electron diagnostics in the same volume. JVLA observations
have been used to probe particle acceleration in a termination shock
during a flare. In addition, multi-frequency full-disk imaging of
the non-flaring Sun with EOVSA is showing promise for quantitative
diagnostics of electron-based emission measure and active region
magnetic field and temperature structure. But EOVSA and the JVLA are
mere demonstrators for a far more advanced solar radio instrument,
the Frequency Agile Solar Radiotelescope (FASR), that is designed to
address a much more comprehensive science program with much greater
precision than is possible with EOVSA or non-solar-dedicated instruments
like the JVLA. This includes direct imaging of Coronal Mass Ejections
(CMEs) and their associated energetic particles both on and off
the solar disk, routine coronal magnetic field measurements at high
cadence, and continuous sampling of the full-Sun coronal temperature
and emission measure. Here we use examples from EOVSA and the JVLA to
envision what FASR's advanced design will be capable of. When paired
with highly complementary new X-ray-based diagnostics from space,
the remote sensing of high-energy particles through radio imaging
spectroscopy from the ground provides a far more complete picture of
the broad range of energetic phenomena that occur on the Sun than
either alone. Scientists interested in high-energy solar phenomena
have ample incentive to coordinate their efforts to ensure that space-
and ground-based capabilities advance together.
---------------------------------------------------------
Title: Signatures of Type III Solar Radio Bursts from Nanoflares:
Final Results
Authors: Chhabra, S.; Klimchuk, J. A.; Gary, D. E.; Viall, N. M.
2020AGUFMSH0430016C Altcode:
The heating mechanisms responsible for the million degree solar corona
remain one of the most intriguing problems in space science. It is
widely agreed, that the ubiquitous presence o f reconnection events and
the associated impulsive heating (nanoflares) are a strong candidate in
solving this problem [Klimchuk J.A., 2015 and references therein]. <P
/>Whether nanoflares accelerate energetic particles like full sized
flares is unknown. The lack of strong emission in hard X rays suggests
that the quantity of highly energetic particles is small. There could,
however, be large numbers of mildly energetic particles (~ 10 keV). We
investigate such particles by searching for the type III radio bursts
that they may produce. If energetic electron beams propagating along
magnetic field lines generate a bump on tail instability, they will
produce Langmuir waves, which can then interact with other particles
and waves to give rise to emission at the local plasma frequency and
its first harmonic. Type III radio bursts bursts are characteristically
known to exhibit high frequency drifts as the beam propagates through
a density gradient. The time lag technique that was developed to study
subtle delays in light curves from different EUV channels [Viall &
Klimchuk 2012] can also be used to detect subtle delays at different
radio frequencies. We have modeled the expected radio emission from
nanoflares, which we used to test and calibrate the technique. We will
present the final results of our modeling efforts along with results
from application of the technique to actual radio observations from VLA
(Very Large Array), MWA (Murchison Widefield Array) and seeking data
from LOFAR (Low Frequency Array) as well.We are also using data from the
PSP (Parker Solar Probe) to look for similar reconnection signatures in
the Solar Wind. Our goal is to determine whether nanoflares accelerate
energetic particles and to determine their properties. The results
will have important implications for both the particle acceleration
and reconnection physics.
---------------------------------------------------------
Title: Untangling the global coronal magnetic field with
multiwavelength observations
Authors: Gibson, S. E.; Malanushenko, A.; de Toma, G.; Tomczyk, S.;
Reeves, K.; Tian, H.; Yang, Z.; Chen, B.; Fleishman, G.; Gary, D.;
Nita, G.; Pillet, V. M.; White, S.; Bąk-Stęślicka, U.; Dalmasse,
K.; Kucera, T.; Rachmeler, L. A.; Raouafi, N. E.; Zhao, J.
2020arXiv201209992G Altcode:
Magnetism defines the complex and dynamic solar corona. Coronal
mass ejections (CMEs) are thought to be caused by stresses, twists,
and tangles in coronal magnetic fields that build up energy and
ultimately erupt, hurling plasma into interplanetary space. Even the
ever-present solar wind possesses a three-dimensional morphology shaped
by the global coronal magnetic field, forming geoeffective corotating
interaction regions. CME evolution and the structure of the solar
wind depend intimately on the coronal magnetic field, so comprehensive
observations of the global magnetothermal atmosphere are crucial both
for scientific progress and space weather predictions. Although some
advances have been made in measuring coronal magnetic fields locally,
synoptic measurements of the global coronal magnetic field are not yet
available. We conclude that a key goal for 2050 should be comprehensive,
ongoing 3D synoptic maps of the global coronal magnetic field. This will
require the construction of new telescopes, ground and space-based,
to obtain complementary, multiwavelength observations sensitive
to the coronal magnetic field. It will also require development of
inversion frameworks capable of incorporating multi-wavelength data,
and forward analysis tools and simulation testbeds to prioritize and
establish observational requirements on the proposed telescopes.
---------------------------------------------------------
Title: Magnetic Reconnection during the Post-impulsive Phase of
a Long-duration Solar Flare: Bidirectional Outflows as a Cause of
Microwave and X-Ray Bursts
Authors: Yu, Sijie; Chen, Bin; Reeves, Katharine K.; Gary, Dale E.;
Musset, Sophie; Fleishman, Gregory D.; Nita, Gelu M.; Glesener, Lindsay
2020ApJ...900...17Y Altcode: 2020arXiv200710443Y
Magnetic reconnection plays a crucial role in powering solar flares,
production of energetic particles, and plasma heating. However,
where the magnetic reconnections occur, how and where the released
magnetic energy is transported, and how it is converted to other
forms remain unclear. Here we report recurring bidirectional plasma
outflows located within a large-scale plasma sheet observed in
extreme-ultraviolet emission and scattered white light during the
post-impulsive gradual phase of the X8.2 solar flare on 2017 September
10. Each of the bidirectional outflows originates in the plasma sheet
from a discrete site, identified as a magnetic reconnection site. These
reconnection sites reside at very low altitudes (<180 Mm, or 0.26
R<SUB>⊙</SUB>) above the top of the flare arcade, a distance only
<3% of the total length of a plasma sheet that extends to at
least 10 R<SUB>⊙</SUB>. Each arrival of sunward outflows at the
loop-top region appears to coincide with an impulsive microwave and
X-ray burst dominated by a hot source (10-20 MK) at the loop top and a
nonthermal microwave burst located in the loop-leg region. We propose
that the reconnection outflows transport the magnetic energy released
at localized magnetic reconnection sites outward in the form of kinetic
energy flux and/or electromagnetic Poynting flux. The sunward-directed
energy flux induces particle acceleration and plasma heating in the
post-flare arcades, observed as the hot and nonthermal flare emissions.
---------------------------------------------------------
Title: Imaging Spectroscopy of CME-Associated Solar Radio Bursts
using OVRO-LWA
Authors: Chhabra, S.; Gary, D. E.; Hallinan, G.
2020SPD....5121009C Altcode:
Radio emission from the solar corona provides a unique perspective
on the physical properties of energetic phenomena, such as solar
flares and coronal mass ejections (CMEs). We present first results
of a solar radio event observed with the new state-of-the-art Owens
Valley Radio Observatory Long Wavelength Array (OVRO-LWA) at metric
wavelengths. We examine a complex event consisting of multiple
radio sources/bursts associated with a fast coronal mass ejection
and an M2.1 GOES soft X-ray flare from 2015 September 20, as well as
quiet Sun images before and after the bursts. Images of 9-s cadence
are used to analyze the event over a 120-minute period, and solar
emission is observed out to a distance of ≈3.5R, over the frequency
range of 40-70MHz available at that time. We present our results
from the investigation of the radio event, focusing particularly on
one burst source that exhibits outward motion, which we classify as
a moving type IV burst. We image the event at multiple frequencies
and use the source centroids to obtain the velocity for the outward
motion. A coalignment with the LASCO(C2) coronagraph allows a spatial
and temporal comparison with observations of the CME in white light,
indicating an association of the outward motion with the core of the
CME. By performing graduated-cylindrical-shell (GCS) reconstruction
of the CME, we constrain the density in the volume. We find that the
estimated density values yield an electron plasma frequency lower than,
but close to the observed emission frequency, rendering it difficult to
completely dismiss the possibility of plasma emission as the underlying
mechanism. However, based on source height and smoothness of the
emission in frequency and time, we argue that gyrosynchrotron is the
more plausible mechanism. We use gyrosynchrotron spectral fitting
techniques to estimate the evolving physical conditions during the
outward motion of this CME-associated radio burst source.
---------------------------------------------------------
Title: Magnetic Reconnection in a Solar Flare: Bi-Directional Outflows
vs. Microwave and X-ray Bursts
Authors: Yu, S.; Chen, B.; Reeves, K.; Gary, D.; Fleishman, G.
2020SPD....5121103Y Altcode:
Magnetic reconnection is fundamental for the astrophysical and
laboratory plasma. Its role is crucial in powering solar flares,
production of energetic particles, and plasma heating. However, where
the magnetic reconnections occur, how and where the released magnetic
energy is transported, and how it is converted to other forms remain
unclear. Here we report recurring bi-directional plasma outflows located
within a large-scale plasma sheet observed in extreme ultraviolet and
white light during the post-impulsive gradual phase of the X8.2 solar
flare on 2017 September 10. Each pair of the bi-directional outflows
originates in the plasma sheet from a discrete site, identified as
the magnetic reconnection site. These reconnection sites reside at
very low altitudes (< 180 Mm, or 0.26 solar radii) above the top of
the flare arcade, which is only 4% of the total length of the plasma
sheet that extends to at least 10 solar radii. Each arrival of sunward
outflows at the looptop region appears to coincide with an impulsive
microwave and X-ray burst dominated by a superhot source (10-20 MK)
at the looptop, which is immediately followed by a nonthermal microwave
burst located in the loop-leg region. We propose that the reconnection
outflows transport the magnetic energy released at localized magnetic
reconnection sites outward in the form of kinetic energy flux and/or
electromagnetic Poynting flux. The sunward-directing energy flux
induces particle acceleration and plasma heating in the post-flare
arcades observed as the superhot and nonthermal flare emissions.
---------------------------------------------------------
Title: Magnetic Reconnection During the Post-Impulsive Phase of a
Long-Duration Solar Flare: Bi-Directional Outflows vs. Microwave
and X-ray Bursts
Authors: Yu, S.; Chen, B.; Reeves, K.; Gary, D.; Musset, S.; Fleishman,
G.; Nita, G.; Glesener, L.
2020AAS...23611203Y Altcode:
Magnetic reconnection is fundamental for the astrophysical and
laboratory plasma. Its role is crucial in powering solar flares,
production of energetic particles, and plasma heating. However, where
the magnetic reconnections occur, how and where the released magnetic
energy is transported, and how it is converted to other forms remain
unclear. Here we report recurring bi-directional plasma outflows located
within a large-scale plasma sheet observed in extreme ultraviolet and
white light during the post-impulsive gradual phase of the X8.2 solar
flare on 2017 September 10. Each pair of the bi-directional outflows
originates in the plasma sheet from a discrete site, identified as the
magnetic reconnection site. These reconnection sites reside at very low
altitudes (< 180 Mm, or 0.26 R<SUB>sun</SUB>) above the top of the
flare arcade, which is only 4% of the total length of the plasma sheet
that extends to at least 10 R<SUB>sun</SUB>. Each arrival of sunward
outflows at the looptop region appears to coincide with an impulsive
microwave and X-ray burst dominated by a superhot source (10-20 MK)
at the looptop, which is immediately followed by a nonthermal microwave
burst located in the loop-leg region. We propose that the reconnection
outflows transport the magnetic energy released at localized magnetic
reconnection sites outward in the form of kinetic energy flux and/or
electromagnetic Poynting flux. The sunward-directing energy flux
induces particle acceleration and plasma heating in the post-flare
arcades observed as the superhot and nonthermal flare emissions.
---------------------------------------------------------
Title: Microwave Spectral Imaging of an Erupting Magnetic Flux Rope:
Implications for the Standard Solar Flare Model in Three Dimensions
Authors: Chen, Bin; Yu, Sijie; Reeves, Katharine K.; Gary, Dale E.
2020ApJ...895L..50C Altcode: 2020arXiv200501900C
We report microwave spectral imaging observations of an erupting
magnetic flux rope during the early impulsive phase of the X8.2-class
limb flare on 2017 September 10, obtained by the Expanded Owens Valley
Solar Array. A few days prior to the eruption, when viewed against
the disk, the flux rope appeared as a reverse S-shaped dark filament
along the magnetic polarity inversion line. During the eruption,
the rope exhibited a "hot channel" structure in extreme ultraviolet
and soft X-ray passbands sensitive to ∼10 MK plasma. The central
portion of the flux rope was nearly aligned with the line of sight,
which quickly developed into a teardrop-shaped dark cavity during
the early phase of the eruption. A long and thin plasma sheet formed
below the cavity, interpreted as the reconnection current sheet viewed
edge on. A nonthermal microwave source was present at the location
of the central current sheet, which extended upward encompassing the
dark cavity. A pair of nonthermal microwave sources were observed for
several minutes on both sides of the main flaring region. They shared a
similar temporal behavior and spectral property to the central microwave
source below the cavity, interpreted as the conjugate footpoints of the
erupting flux rope. These observations are broadly consistent with the
magnetic topology and the associated energy release scenario suggested
in the three-dimensional standard model for eruptive solar flares. In
particular, our detection of nonthermal emission at conjugate flux
rope footpoints provides solid evidence of particle transport along
an erupting magnetic flux rope.
---------------------------------------------------------
Title: Observation and modeling of the umbra of sunspot AR 12470
using ALMA and BBSO data
Authors: Chai, Y.; Gary, D.
2020AAS...23633005C Altcode:
Waves and oscillations are an enduring topic in the study of the solar
atmosphere since they have been considered an important candidate for
solving the coronal heating problem. For decades, numerous efforts
in observation and modeling have been put forth, yet the complexity
of the solar atmosphere has left many questions unanswered . Now
that the Atacama Large Millimeter/submillimeter Array (ALMA) has
become available, we have entered a new era when it is possible to
directly probe the chromospheric temperature variation on very short
(1-2 s) timescales. Combined with some solar dedicated instruments,
such as Goode Solar Telescope (GST) at the Big Bear Solar Observatory
(BBSO), we are able to carry out high spatial and temporal observations
that target the sunspot umbra, so that they can provide additional
guidance to sunspot modeling. We present a study of umbral waves
and oscillations in the sunspot AR12470 using the data from both
ALMA and BBSO that were taken on December 17, 2015. With the help
of fourier analysis, we found the locations of the maximum power of
the 3-min oscillation signal in the sunspot umbra at ALMA band 3 (3
mm) wavelength. We found a stable phase relation between ALMA band 3
temperature variations and Hα intensities at various wavelengths in
the line, taken with the GST. With the help of these data, we model
the waves as a perturbation of the existing 1D solar models by Fontenla
et al. and the numerical radiative transfer RH (Rybicki & Hummer)
code. The detailed information and discussion are in the poster.
---------------------------------------------------------
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: Machine Learning in Heliophysics and Space Weather Forecasting:
A White Paper of Findings and Recommendations
Authors: Nita, Gelu; Georgoulis, Manolis; Kitiashvili, Irina; Sadykov,
Viacheslav; Camporeale, Enrico; Kosovichev, Alexander; Wang, Haimin;
Oria, Vincent; Wang, Jason; Angryk, Rafal; Aydin, Berkay; Ahmadzadeh,
Azim; Bai, Xiaoli; Bastian, Timothy; Filali Boubrahimi, Soukaina; Chen,
Bin; Davey, Alisdair; Fereira, Sheldon; Fleishman, Gregory; Gary, Dale;
Gerrard, Andrew; Hellbourg, Gregory; Herbert, Katherine; Ireland,
Jack; Illarionov, Egor; Kuroda, Natsuha; Li, Qin; Liu, Chang; Liu,
Yuexin; Kim, Hyomin; Kempton, Dustin; Ma, Ruizhe; Martens, Petrus;
McGranaghan, Ryan; Semones, Edward; Stefan, John; Stejko, Andrey;
Collado-Vega, Yaireska; Wang, Meiqi; Xu, Yan; Yu, Sijie
2020arXiv200612224N Altcode:
The authors of this white paper met on 16-17 January 2020 at the New
Jersey Institute of Technology, Newark, NJ, for a 2-day workshop that
brought together a group of heliophysicists, data providers, expert
modelers, and computer/data scientists. Their objective was to discuss
critical developments and prospects of the application of machine and/or
deep learning techniques for data analysis, modeling and forecasting
in Heliophysics, and to shape a strategy for further developments in
the field. The workshop combined a set of plenary sessions featuring
invited introductory talks interleaved with a set of open discussion
sessions. The outcome of the discussion is encapsulated in this white
paper that also features a top-level list of recommendations agreed
by participants.
---------------------------------------------------------
Title: Characterization of turbulent magnetic reconnection in solar
flares with microwave imaging spectroscopy
Authors: Fleishman, Gregory; Gary, Dale; Chen, Bin; Yu, Sijie; Kuroda,
Natsuha; Nita, Gelu
2020EGUGA..22.2099F Altcode:
Magnetic reconnection plays a central role in highly magnetized
plasma, for example, in solar corona. Release of magnetic energy
due to reconnection is believed to drive such transient phenomena
as solar flares, eruptions, and jets. This energy release should be
associated with a decrease of the coronal magnetic field. Quantitative
measurements of the evolving magnetic field strength in the corona are
required to find out where exactly and with what rate this decrease
takes place. The only available methodology capable of providing such
measurements employs microwave imaging spectroscopy of gyrosynchrotron
emission from nonthermal electrons accelerated in flares. Here, we
report microwave observations of a solar flare, showing spatial and
temporal changes in the coronal magnetic field at the cusp region;
well below the nominal reconnection X point. The field decays at a rate
of ~5 Gauss per second for 2 minutes. This fast rate of decay implies
a highly enhanced, turbulent magnetic diffusivity and sufficiently
strong electric field to account for the particle acceleration that
produces the microwave emission. Moreover, spatially resolved maps of
the nonthermal and thermal electron densities derived from the same
microwave spectroscopy data set allow us to detect the very acceleration
site located within the cusp region. The nonthermal number density is
extremely high, while the thermal one is undetectably low in this region
indicative of a bulk acceleration process exactly where the magnetic
field displays the fast decay. The decrease in stored magnetic energy is
sufficient to power the solar flare, including the associated eruption,
particle acceleration, and plasma heating. We discuss implications of
these findings for understanding particle acceleration in solar flares
and in a broader space plasma context.
---------------------------------------------------------
Title: Evolution of Flare-accelerated Electrons Quantified by
Spatially Resolved Analysis
Authors: Kuroda, Natsuha; Fleishman, Gregory D.; Gary, Dale E.; Nita,
Gelu M.; Chen, Bin; Yu, Sijie
2020FrASS...7...22K Altcode: 2020arXiv200413155K
Nonthermal electrons accelerated in solar flares produce electromagnetic
emission in two distinct, highly complementary domains---hard X-rays
(HXRs) and microwaves (MWs). This paper reports MW imaging spectroscopy
observations from the Expanded Owens Valley Solar Array of an M1.2 flare
that occurred on 2017 September 9, from which we deduce evolving coronal
parameter maps. We analyze these data jointly with the complementary
Reuven Ramaty High-Energy Solar Spectroscopic Imager HXR data to reveal
the spatially-resolved evolution of the nonthermal electrons in the
flaring volume. We find that the high-energy portion of the nonthermal
electron distribution, responsible for the MW emission, displays a much
more prominent evolution (in the form of strong spectral hardening)
than the low-energy portion, responsible for the HXR emission. We
show that the revealed trends are consistent with a single electron
population evolving according to a simplified trap-plus-precipitation
model with sustained injection/acceleration of nonthermal electrons,
which produces a double-powerlaw with steadily increasing break energy.
---------------------------------------------------------
Title: Evolution of Flare-accelerated Electrons in the Solar Corona
and Chromosphere Revealed by Spatially Resolved Microwave and Hard
X-Ray Analysis
Authors: Kuroda, Natsuha; Fleishman, Gregory; Gary, Dale; Nita, Gelu;
Chen, Bin; Yu, Sijie
2020EGUGA..22.3145K Altcode:
Hard X-ray (HXR) and microwave (MW) observations are highly
complementary for studying electron acceleration and transport processes
in solar flares. In recent years, a new effort has been made in the MW
domain using new high-resolution, multifrequency data from The Expanded
Owens Valley Solar Array (EOVSA) and a breakthrough numerical modeling
infrastructure that enables us to study properties of high-energy
electrons in unprecedented cadence and quantitative detail. This study
introduces the observation of an M1.2 flare that occurred on 2017
September 9 and analyzes the evolution of the nonthermal electrons
in the corona based on EOVSA MW spectral imaging data. We find a
significant spectral hardening of the MWemitting nonthermal electron
population in the corona, using EOVSA lower-frequency (<7 GHz)
observations over a selected 4-minute window of the flare's impulsive
phase. We compare this spectral evolution with the evolution of the
spectral index of nonthermal electrons emitting in the chromosphere,
derived from HXR observations from the Reuven Ramaty High Energy Solar
Spectroscopic Imager. We discuss the general picture of the evolution
of the nonthermal electron population in this flare by incorporating
observations at the two complementary wavelengths. We also make an
estimate of the total energy of the nonthermal electrons contained in
the observed coronal low-frequency MW source and discuss its temporal
evolution.
---------------------------------------------------------
Title: Drifting Pulsation Structure at the Very Beginning of the
2017 September 10 Limb Flare
Authors: Karlický, Marian; Chen, Bin; Gary, Dale E.; Kašparová,
Jana; Rybák, Jan
2020ApJ...889...72K Altcode: 2019arXiv191212518K
Drifting pulsation structures (DPSs) are important radio fine structures
usually observed at the beginning of eruptive solar flares. It has been
suggested that DPSs carry important information on the energy release
processes in solar flares. We study DPS observed in an X8.2-class flare
on 2017 September 10 in the context of spatial and spectral diagnostics
provided by microwave, EUV, and X-ray observations. We describe DPS
and its substructures that were observed for the first time. We use a
new wavelet technique to reveal characteristic periods in DPS and their
frequency bands. Comparing the periods of pulsations found in this DPS
with those in previous DPSs, we found new very short periods in the
0.09-0.15 s range. We present Expanded Owens Valley Solar Array images
and spectra of microwave sources observed during the DPS. This DPS at
its very beginning has pulsations in two frequency bands (1000-1300 MHz
and 1600-1800 MHz), which are interconnected by fast drifting bursts. We
show that these double-band pulsations started just at the moment when
the ejected filament splits apart in a tearing motion at the location
where a signature of the flare current sheet later appeared. Using the
standard flare model and previous observations of DPSs, we interpret
these double-band pulsations as a radio signature of superthermal
electrons trapped in the rising magnetic rope and flare arcade at the
moment when the flare magnetic reconnection starts. The results are
discussed in a scenario with the plasmoid in the rising magnetic rope.
---------------------------------------------------------
Title: A new view of the solar atmosphere: daily full-disk
multifrequency radio images from EOVSA
Authors: Gary, D.; Yu, S.; Chen, B.; LaVilla, V.
2020AAS...23538501G Altcode:
A new pipeline processing system is producing unprecedented daily
full-disk images of the Sun from the Expanded Owens Valley Solar Array
(EOVSA) in 6 frequency bands from 1-14 GHz. The images are produced by
fitting a disk model to the measured visibilities and using the disk for
self-calibration of the radio imaging data, resulting in images that
faithfully show even the weakest of features in the solar atmosphere,
including clear limb brightening, coronal holes, weak plage regions,
filaments, and prominences. The multi-frequency data can be used to
investigate the physical properties of these features of the solar
corona, transition region, and upper chromosphere, but the data are
so new that the interpretation is still in progress. We highlight
some initial results on the frequency dependence of equatorial limb
brightening and polar coronal hole darkening.
---------------------------------------------------------
Title: Decay of the coronal magnetic field can release sufficient
energy to power a solar flare
Authors: Fleishman, Gregory D.; Gary, Dale E.; Chen, Bin; Kuroda,
Natsuha; Yu, Sijie; Nita, Gelu M.
2020Sci...367..278F Altcode:
Solar flares are powered by a rapid release of energy in the solar
corona, thought to be produced by the decay of the coronal magnetic
field strength. Direct quantitative measurements of the evolving
magnetic field strength are required to test this. We report microwave
observations of a solar flare, showing spatial and temporal changes
in the coronal magnetic field. The field decays at a rate of ~5 Gauss
per second for 2 minutes, as measured within a flare subvolume of
~10<SUP>28</SUP> cubic centimeters. This fast rate of decay implies
a sufficiently strong electric field to account for the particle
acceleration that produces the microwave emission. The decrease in
stored magnetic energy is enough to power the solar flare, including
the associated eruption, particle acceleration, and plasma heating.
---------------------------------------------------------
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: Imaging-Spectroscopy of CME-Associated Solar Radio Emission
using OVRO-LWA
Authors: Chhabra, S.; Gary, D. E.; Hallinan, G.; Anderson, M.; Chen, B.
2019AGUFMSH21B..03C Altcode:
Radio emission from the solar corona provides a unique perspective on
the physical properties of energetic phenomena, such as solar flares
and coronal mass ejections (CMEs). We use the new state-of-the-art
Owens Valley Radio Observatory - Long Wavelength Array (OVRO-LWA)
for the purpose. We examine a complex event consisting multiple radio
sources/bursts associated with a fast CME and a M2.1 GOES SXR flare
from 2015 Sep 20, as well as quiet Sun images before and after the
bursts. Images of 9-s cadence are used to analyze the event over a
90-minute period, out to a distance of ~3 R<SUB>sun</SUB>, over the
frequency range of 40-70 MHz available at that time. <P />We present
our results from the investigation of the radio sources, focusing
particularly on one burst source that exhibits outward motion. In order
to understand better the evolution of the burst, we image the event
at hundreds of frequencies and use the source centroids to obtain the
velocity for the outward motion. A coalignment with LASCO(C2) allows a
spatial and temporal comparison with observations of the CME in white
light indicating an association of the outward motion with the core of
the CME. We perform graduating-cylindrical-shell reconstruction of the
CME to constrain the density in the volume to verify that the emission
from the moving source is not consistent with plasma emission, and
therefore is likely due to gyrosynchrotron emission. This then allows
the use gyrosynchrotron spectral fitting techniques to estimate the
evolving physical conditions. The results suggest that both accelerated
electron density and magnetic field strength decline as the source
expands outward, while the powerlaw index of the electrons hardens.
---------------------------------------------------------
Title: Towards Next Generation Radio Imaging Spectroscopy: a Path
Forward to FASR
Authors: Bastian, T.; Chen, B.; Gary, D. E.
2019AGUFMSH31C3327B Altcode:
A long term goal of the solar physics community, one that has been
endorsed by several NRC decadal surveys, is the Frequency Agile
Solar Radiotelescope (FASR). FASR is designed to address a broad
program of fundamental solar physics, including coronal magnetic
fields, magnetic energy release, particle acceleration and transport,
coronal and chromospheric heating, and drivers of space weather such as
coronal mass ejections. Fundamental to the FASR concept is observing the
chromosphere and corona as a system using innovative measurements —
ultra-broadband imaging spectropolarimetry — that will yield unique
science data products that are complementary to those produced in,
e.g., the O/IR and EUV/X-ray wavelength regimes. The potential of the
instrument is already being demonstrated by pathfinder observations made
by the Expanded Owens Valley Solar Array (EOVSA) and by the Jansky Very
Large Array (JVLA). This talk discusses a step-by-step Implementation of
FASR to spread out the cost (the full FASR concept would be in excess
of $75M) and achieve some of the most compelling science in time for
the upcoming solar maximum. This "stepping stone" approach would first
build out the high frequency subsystem at Owens Valley (~1-20 GHz)
and rely on other initiatives such as the OVRO Long Wavelength Array
to cover the metric wavelength range, deferring the mid-frequency
subsystem (~0.2-2 GHz) for the following decade. Such an approach
allows a timely deployment of critical science infrastructure that
can meet the demands of the wider community while training students
and researchers to exploit this new asset. The talk will emphasize
the great gain in image quality and science capability of the larger
array relative to EOVSA and JVLA.
---------------------------------------------------------
Title: Measuring Coronal Magnetic Fields with the Jansky Very Large
Array and RATAN Telescopes
Authors: Bastian, T.; Gary, D. E.; Fleishman, G. D.; Nita, G. M.;
Chen, B.; Kaltman, T.; Bogod, V.
2019AGUFMSH41B..05B Altcode:
Quantitative knowledge of coronal magnetic fields is fundamental to
understanding essentially all solar phenomena above the photosphere,
including the structure and evolution of solar active regions,
magnetic energy release, charged particle acceleration, flares,
coronal mass ejections (CMEs), coronal heating, the solar wind and,
ultimately, space weather and its impact on Earth. Characterized as
the solar and space physics community's "dark energy" problem, useful
quantitative measurements of the coronal magnetic field have been
largely unavailable until recently. Although understood in principle for
many years, suitable instrumentation at radio wavelengths - requiring
the ability to perform wideband imaging spectropolarimetry - has not
been available in practice for making quantitative maps of coronal
magnetic fields. This has changed in recent years, with the advent of
the Jansky Very Large Array (JVLA) and the Expanded Owens Valley Solar
Array (EOVSA) which are being exploited to demonstrate the utility
of radio observations for measuring coronal magnetic fields. <P />We
report radio observations of a large solar active region by the Jansky
Very Large Array (JVLA). The active region, AR 12209, was mapped on
four days: 18, 20, 22, and 24 November 2014 in 56 spectral windows
spanning 1-8 GHz. At the lowest frequencies the emission is dominated
by thermal free-free emission but at frequencies >1.5 GHz thermal
gyroresonance (GR) emission at the second or third harmonic of the
electron gyrofrequency dominates. GR emission enables nested coronal
isogauss surfaces ranging from approximately 180 G to as high as 1400
G to be mapped. At the time these observations were obtained, the JVLA
was not yet fully commissioned for solar observing. In particular,
the switched-power flux calibration system was not yet implemented. We
therefore cross-calibrated the JVLA observations against well-calibrated
one-dimensional observations obtained by the RATAN 600 telescope in
Zelenchukskaya, Russia. We present coronal magnetograms obtained by
the JVLA and compare them to model calculations.
---------------------------------------------------------
Title: Study of Type III Solar Radio Bursts in Nanoflares
Authors: Chhabra, S.; Klimchuk, J. A.; Gary, D. E.; Viall, N. M.
2019AGUFMSH23C3337C Altcode:
The heating mechanisms responsible for the million-degree solar corona
remain one of the most intriguing problems in space science. It is
widely agreed, that the ubiquitous presence of reconnection events and
the associated impulsive heating (nanoflares) are a strong candidate in
solving this problem [Klimchuk J.A., 2015 and references therein]. <P
/>Whether nanoflares accelerate energetic particles like full-sized
flares is unknown. The lack of strong emission in hard X-rays suggests
that the quantity of highly energetic particles is small. There could,
however, be large numbers of mildly energetic particles (~ 10 keV). We
investigate such particles by searching for the type III radio bursts
that they may produce. If energetic electron beams propagating along
magnetic field lines generate a bump-on-tail instability, they will
produce Langmuir waves, which can then interact with other particles
and waves to give rise to emission at the local plasma frequency and
its first harmonic. Type III bursts are characteristically known
to exhibit high frequency drifts as the beam propagates through a
density gradient. The time-lag technique that was developed to study
subtle delays in light curves from different EUV channels [Viall &
Klimchuk 2012] can also be used to detect subtle delays at different
radio frequencies. We have modeled the expected radio emission from
nanoflares, which we used to test and calibrate the technique. We are
applying the technique to actual radio observations from VLA (Very Large
Array), MWA (Murchison Widefield Array) and seeking data from LOFAR
(Low-Frequency Array) as well. We also plan to use data from the PSP
(Parker Solar Probe) to look for similar reconnection signatures in
the Solar Wind. Our goal is to determine whether nanoflares accelerate
energetic particles and to determine their properties. The results
will have important implications for both the particle acceleration
and reconnection physics.
---------------------------------------------------------
Title: Probing the thermal structure of the solar chromosphere with
ALMA and optical/NIR observations
Authors: Gary, Dale
2019asrc.confE..13G Altcode:
ALMA is a powerful new instrument that allows an unambiguous
determination of the solar chromospheric temperature, and its rapid
evolution. When combined with multiwavelength observations in other
diagnostics, most notably strong optical/NIR spectral lines, this
allows us to probe the highly structured atmosphere throughout much
of its height and with high spatial and temporal resolution. We will
discuss several unique datasets that combine solar ALMA observations
in Bands 3 and 6 with simultaneous imaging spectroscopy from the
Interferometric Bidimensional Spectrometer (IBIS) operating at the
Dunn Solar Telescope/NSO. We find that parameters of ""classical""
chromospheric spectral lines of H-alpha and Ca II are in some cases
closely correlated with the ALMA brightness temperatures, posing strong
constraints to interpretation of the observed spectral intensities in
terms of dynamical properties. We will also present information on
the temporal evolution of dynamic events as seen by ALMA, comparing
those with the shocks and small-scale impulsive events seen in the
co-temporal ground-based optical data. Finally, we will describe the
opportunities and advantages of joint observations between ALMA and
the soon-to-be-operational, four-meter DKI Solar Telescope (DKIST).
---------------------------------------------------------
Title: Imaging Spectroscopic Observations of Type I Noise Storms
with Ultrahigh Temporal and Spectral Resolution
Authors: Yu, S.; Chen, B.; Bastian, T.; Gary, D. E.
2019AGUFMSH23C3336Y Altcode:
Type I noise storms are the most common, but perhaps the least
understood type of solar radio bursts in the decimeter-meter wavelength
range. Noise storms are non-flare-related radio phenomena. Their
existence indicates that energy release and particle energization
can continue in the corona without notable solar activities such as
flares or coronal mass ejections. Noise storms manifest themselves as
intermittent short-lived narrow-band bursts superposed on a broad-band,
long-lasting continuum (sometimes referred to as the noise storm
continuum). However, previous studies were either based on total-power
dynamic spectroscopy (without spatial resolution), or on imaging
observations at one or few frequency channels. During the recent Karl
G. Jansky Very Large Array (VLA) solar observing campaign, we performed
dynamic imaging spectroscopic observations with unprecedentedly high
temporal resolution,10 milliseconds, along with fine spectral resolution
(250 kHz) in the 290-450 MHz P band. We recorded several noise storm
events associated with quiescent, non-flaring solar active regions. For
the first time, we are able to image these type I bursts and fully
resolve them in the frequency-time domain, which are used to better
elucidate the origin of these bursts. We discuss the implications of
our results for understanding energy release and particle energization
in the seemly quiescent solar corona.
---------------------------------------------------------
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: Solar Chromospheric Temperature Diagnostics: A Joint ALMA-Hα
Analysis
Authors: Molnar, Momchil E.; Reardon, Kevin P.; Chai, Yi; Gary, Dale;
Uitenbroek, Han; Cauzzi, Gianna; Cranmer, Steven R.
2019ApJ...881...99M Altcode: 2019arXiv190608896M
We present the first high-resolution, simultaneous observations of
the solar chromosphere in the optical and millimeter wavelength
ranges, obtained with the Atacama Large Millimeter Array (ALMA)
and the Interferometric Bidimensional Spectrometer at the Dunn Solar
Telescope. In this paper we concentrate on the comparison between the
brightness temperature observed in ALMA Band 3 (3 mm; 100 GHz) and the
core width of the Hα 6563 Å line, previously identified as a possible
diagnostic of the chromospheric temperature. We find that in the area
of plage, network and fibrils covered by our field of view, the two
diagnostics are well correlated, with similar spatial structures
observed in both. The strength of the correlation is remarkable,
given that the source function of the millimeter radiation obeys local
thermodynamic equilibrium, while the Hα line has a source function that
deviates significantly from the local Planck function. The observed
range of ALMA brightness temperatures is sensibly smaller than the
temperature range that was previously invoked to explain the observed
width variations in Hα. We employ analysis from forward modeling
with the Rybicki-Hummer (RH) code to argue that the strong correlation
between Hα width and ALMA brightness temperature is caused by their
shared dependence on the population number n <SUB>2</SUB> of the first
excited level of hydrogen. This population number drives millimeter
opacity through hydrogen ionization via the Balmer continuum, and
Hα width through a curve-of-growth-like opacity effect. Ultimately,
the n <SUB>2</SUB> population is regulated by the enhancement or lack
of downward Lyα flux, which coherently shifts the formation height
of both diagnostics to regions with different temperature, respectively.
---------------------------------------------------------
Title: Modeling the 2017 September 10 Long Duration Gamma Ray Flare
Authors: Ryan, J.; De Nolfo, G. A.; Gary, D.
2019ICRC...36.1144R Altcode: 2019PoS...358.1144R
No abstract at ADS
---------------------------------------------------------
Title: Magnetic Energy Release in Solar Flares seen through Microwave
Eyes
Authors: Fleishman, Gregory D.; Gary, Dale E.; Chen, Bin; Yu, Sijie;
Nita, Gelu M.; Kuroda, Natsuha
2019AAS...23421602F Altcode:
Solar Coronal Magnetism includes emergence of the magnetic flux from
the sub-photospheric volume, evolution of the coronal magnetic field,
generation and accumulation of a free non-potential magnetic energy,
gradual and explosive release of this energy, generation and dissipation
of turbulence. This set of phenomena drives solar flares and other
forms of eruptive activity such as jets or coronal mass ejections. An
emerging new remote sensing window - Microwave Imaging Spectroscopy
- offers a science-transforming, entirely new look at the coronal
magnetism. The foundation of this new capability is the sensitivity of
the microwave emission to the magnetic field. A break-through, however,
comes only when this emission is measured at many frequencies with
a reasonably high spatial and temporal resolution, which is the case
of a new imaging instrument - the Expanded Owens Valley Solar Array
(EOVSA). In this talk we briefly present the key methodology needed
to obtain the target physical parameters, show a few examples of
solar flares observed with EOVSA, and discuss main physical findings
made with these new data and methodology, in particular - about rapid
decay of the magnetic field at the site of the primary energy release
in solar flares, associated electric field and accelerated particles,
plasma heating, and turbulent magnetic diffusivity. We discuss these
new findings in the context of the standard model of solar flare and
the contemporary ideas about the magnetic energy release, magnetic
reconnection, turbulence generation, and particle 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; 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: Fast plasma outflows associated with impulsive microwave and
hard X-ray bursts during the gradual phase of the 2017 September 10
X8.2 flare
Authors: Chen, Bin; Yu, Sijie; Musset, Sophie; Gary, Dale E.;
Fleishman, Gregory D.; Glesener, Lindsay; Reeves, Kathy; Nita, Gelu M.
2019AAS...23421601C Altcode:
The 2017 September 10 X8.2 flare is a spectacular long duration event
associated with a fast coronal mass ejection (CME). It features an
extended gradual phase associated with hard X-ray (HXR) and microwave
(MW) bursts that last for at least two hours after the flare peak (at
1600 UT). We examine the gradual phase using multi-wavelength data
recorded by the Expanded Owens Valley Solar Array (EOVSA), RHESSI,
Fermi/GBM, SDO/AIA, and MLSO/K-Cor. During the extended gradual phase
( 17-18 UT) when the CME already propagates to >10 solar radii,
an extremely long and thin plasma sheet is visible in white light
(WL) images that extends to at least 1.5 solar radii above the
solar surface. We find evidence of multitudes of fast bi-directional
plasma outflows ( 300-700 km/s) within the plasma sheet emanating
from localized sites at very low coronal heights (0.1-0.2 solar radii
above the surface), interpreted as magnetic reconnection occurring in
the low corona. The occurrence of the fast plasma outflows correlates
very well in time with the intermittent broadband MW emission and HXR
enhancements. MW spectroscopic imaging reveals a MW source located at
the looptop that coincides with a HXR looptop source. Another MW source
is present in the northern leg of the flare arcade, whose temporal
evolution seems to correlate with the looptop MW source but shows a
time lag characteristic of the Alfvén transit time. We examine the
temporal and spatial correlations among the plasma outflows, looptop
MW/HXR source, and loopleg MW source, and discuss implications for
magnetic energy release, plasma heating, and electron acceleration
during the extended gradual phase of the flare.
---------------------------------------------------------
Title: New Insights into the 10 September 2017 Mega-Eruption
Authors: Karpen, Judith T.; Kumar, Pankaj; Antiochos, Spiro K.; Gary,
Dale E.; Dahlin, Joel
2019AAS...23431702K Altcode:
The X8.2 flare on 10 September 2017 was part of a well-observed,
extremely energetic solar eruption that has been intensely studied. Much
attention has been devoted to the striking appearance and persistence
of a current sheet behind the explosively accelerating CME. We focus
here on the unusual appearance of prominent emission features on either
side of the flare arcade, which were detected in microwave emissions by
NJIT's EOVSA before the peak impulsive phase. Our analysis combines the
results of 3D numerical simulations with observations by SDO, EOVSA,
and IRIS to decipher the underlying magnetic structure of the erupting
region and the initiation mechanism. The event originated in a complex
active region with a large-scale quadrupolar magnetic field punctuated
by many intrusions of minority polarity. We interpret the observed
microwave features as evidence of electron acceleration due to breakout
reconnection, and present compelling evidence for this conclusion.
---------------------------------------------------------
Title: Statistical analysis of evolving flare parameters inferred
from spatially-resolved microwave spectra observed with the Expanded
Owens Valley Solar Array
Authors: Nita, Gelu M.; Fleishman, Gregory D.; Gary, Dale E.; Chen,
Bin; Yu, Sijie; Kuroda, Natsuha
2019AAS...23420602N Altcode:
The newly completed Expanded Owens Valley Solar Array (EOVSA) performed
microwave (MW) imaging spectroscopy observations during several flares
that occurred during the first half of September 2017. The unprecedented
high frequency and spatial resolution of these observations allowed us
for the first time to infer, with 2-arcsecond spatial resolution and
1-second temporal cadence, the spatial distribution and evolution of the
coronal magnetic field strength, the number density of the accelerated
electrons, the power-law index of their energy distribution, as well as
other associated flare parameters. Our methodology, which consists of
independently fitting the MW spectra corresponding to each individual
pixel of the evolving mult-frequency maps with uniform gyrosynchrotron
source models, generated a statistically significant collection of
evolving flare parameters whose generally smooth spatial and time
variation demonstrates a collective behavior of the neighboring
volume elements, and thus validates our approach. Here we report
on the statistical properties of these physical flare parameters,
their spatial distributions, and their temporal trends for significant
portions of the duration of each flare, and we interpret our results
in the context of the standard solar flare model involving magnetic
energy release, magnetic reconnection, and particle acceleration.
---------------------------------------------------------
Title: Probing the plasma sheet of the 2017 September 10 limb event
using microwave spectroscopy
Authors: Gary, Dale E.; Chen, Bin; Fleishman, Gregory D.; Nita,
Gelu M.; Yu, Sijie
2019AAS...23431001G Altcode:
In its later phases, the 2017 Sep 10 X8.2 limb flare produced
a dramatic, nearly radial plasma sheet that was the site of both
bi-directional radial flows (inward and outward) and 100 km/s turbulent
motions measured in highly ionized Fe lines (Warren et al. 2018). Below
the plasma sheet is a dense set of post-reconnection flare loops whose
upper loops emit strongly in thermal hard X-rays and hot EUV lines (T
20 MK), while the lower loops emit in much cooler lines such as Ca II
(Kuridze et al. 2019). Observations of gyrosynchrotron emission from
the Expanded Owens Valley Solar Array (EOVSA) show the presence of
high-energy (> 300 keV) electrons at still higher heights above
the EUV and hard X-ray loops. At microwave frequencies above about
14 GHz, EOVSA shows that the source shape is a nearly complete loop
while at frequencies below about 10 GHz, the source is split into
two lobes on either side of the plasma sheet. Using the spatially
resolved EOVSA brightness temperature spectra, we investigate the
cause of this bifurcated spatial structure. There are two competing
possibilities for the "missing" emission at lower frequencies: (a)
the emission may be absorbed by intervening high-density, relatively
cool material in the plasma sheet or (b) the emission may be suppressed
by the Razin effect due to a combination of hot high density material
and low magnetic field strength. We use diagnostics from microwave
imaging spectroscopy and AIA DEM analysis to determine the likely
plasma parameters and other characteristics of the plasma sheet,
and the relative placement of the microwave-emitting electrons and
the plasma sheet along the line of sight.
---------------------------------------------------------
Title: Multi-wavelength Multi-height Study of Super Strong Surface
and Coronal Magnetic Fields in Active Region 12673
Authors: Wang, Haimin; Chen, Bin; Jing, Ju; Yu, Sijie; Liu, Chang;
Yurchyshyn, Vasyl B.; Ahn, Kwangsu; Okamoto, Takenori; Toriumi, Shin;
Cao, Wenda; Gary, Dale E.
2019AAS...23440205W Altcode:
Using the joint observations of Goode Solar telescope (GST), Expanded
Owens Valley Solar Array (EOVSA), Solar Dynamics Observatory (SDO)
and Hinode, we study the Solar Active Region (AR) 12673 in September
2017, which is the most flare productive AR in the solar cycle 24. GST
observations show the strong photospheric magnetic fields (nearly
6000 G) in polarity inversion line (PIL) and apparent photospheric
twist. Consistent upward flows are also observed in Dopplergrams
of Hinode, HMI and GST at the center part of that section of PIL,
while the down flows are observed in two ends, indicating that the
structure was rising from subsurface. Combining Non-Linear Force Free
Extrapolation and EOVSA microwave imaging spectroscopy, we also look
into the coronal structure of magnetic fields in this unusual AR,
including the evolution before and after the X9.3 flare on September
6, 2017. Coronal fields between 1000 and 2000 gauss are found above
the flaring PIL at the height range between 8 and 4Mm, outlining the
structure of a fluxrope or sheared arcade.
---------------------------------------------------------
Title: Study of Type III Radio Bursts in Nanoflares
Authors: Chhabra, Sherry; Klimchuk, James A.; Viall, Nicholeen M.;
Gary, Dale E.
2019shin.confE..12C Altcode:
The heating mechanisms responsible for the million-degree solar corona
remain one of the most intriguing problems in space science. It is
widely agreed, that the ubiquitous presence of reconnection events and
the associated impulsive heating (nanoflares) are a strong candidate in
solving this problem [Klimchuk J.A., 2015 and references therein]. <P
/>Whether nanoflares accelerate energetic particles like full-sized
flares is unknown. The lack of strong emission in hard X-rays suggests
that the quantity of highly energetic particles is small. There could,
however, be large numbers of mildly energetic particles ( 10 keV). We
investigate such particles by searching for the type III radio bursts
that they may produce. If energetic electron beams propagating along
magnetic field lines generate a bump-on-tail instability, they will
produce Langmuir waves, which can then interact with other particles
and waves to give rise to emission at the local plasma frequency and
its first harmonic. Type III bursts are characteristically known
to exhibit high frequency drifts as the beam propagates through a
density gradient. The time-lag technique that was developed to study
subtle delays in light curves from different EUV channels [Viall &
Klimchuk 2012] can also be used to detect subtle delays at different
radio frequencies. We have modeled the expected radio emission from
nanoflares, which we used to test and calibrate the technique. We have
begun applying the technique to actual radio observations from VLA
(Very Large Array) and seeking data from MWA (Murchison Widefield Array)
as well. We also plan to use data from the PSP(Parker Solar Probe) to
look for similar reconnection signatures in the Solar Wind. Our goal is
to determine whether nanoflares accelerate energetic particles and to
determine their properties. The results will have important implications
for both the particle acceleration and reconnection physics."
---------------------------------------------------------
Title: Particle Acceleration and Transport, New Perspectives from
Radio, X-ray, and Gamma-Ray Observations
Authors: Gary, Dale; Bastian, Timothy S.; Chen, Bin; Drake, James F.;
Fleishman, Gregory; Glesener, Lindsay; Saint-Hilaire, Pascal; White,
Stephen M.
2019BAAS...51c.371G Altcode: 2019astro2020T.371G
Particle acceleration and particle transport are ubiquitous in
astrophysics. The Sun offers an astrophysical laboratory to study these
in minute detail, using radio dynamic imaging spectroscopy to measure
coronal magnetic fields, time and space evolution of the electron
distribution function.
---------------------------------------------------------
Title: Diagnostics of Space Weather Drivers Enabled by Radio
Observations
Authors: Bastian, Tim; Bain, Hazel; Chen, Bin; Gary, Dale E.;
Fleishman, Gregory D.; Glesener, Lindsay; Saint-Hilaire, Pascal;
Lonsdale, Colin; White, Stephen M.
2019BAAS...51c.323B Altcode: 2019astro2020T.323B; 2019arXiv190405817B
The Sun is an active star that can impact the Earth, its magnetosphere,
and the technological infrastructure on which modern society
depends. Radio emission from space weather drivers offers unique
diagnostics that complement those available at other wavelengths. We
discuss the requirements for an instrument to enable such diagnostics.
---------------------------------------------------------
Title: Radio, Millimeter, Submillimeter Observations of the Quiet Sun
Authors: Bastian, Tim; Chen, Bin; Gary, Dale E.; Fleishman, Gregory
D.; Glesener, Lindsay; Lonsdale, Colin; Saint-Hilaire, Pascal; White,
Stephen M.
2019BAAS...51c.493B Altcode: 2019arXiv190405826B; 2019astro2020T.493B
We point out the lack of suitable radio observations of the quiet Sun
chromosphere and corona and outline requirements for next generation
instrumentation to address the gap.
---------------------------------------------------------
Title: Probing Magnetic Reconnection in Solar Flares: New Perspectives
from Radio Dynamic Imaging Spectroscopy
Authors: Chen, Bin; Bastian, Tim; Dahlin, Joel; Drake, James F.;
Fleishman, Gregory; Gary, Dale; Glesener, Lindsay; Guo, Fan; Ji,
Hantao; Saint-Hilaire, Pascal; Shen, Chengcai; White, Stephen M.
2019BAAS...51c.507C Altcode: 2019astro2020T.507C; 2019arXiv190311192C
Magnetic reconnection is a fundamental physical process in many
laboratory, space, and astrophysical plasma contexts. In this white
paper we emphasize the unique power of remote-sensing observations
of solar flares at radio wavelengths in probing fundamental physical
processes in magnetic reconnection.
---------------------------------------------------------
Title: Modeling the 2017 September 10 LDGRF
Authors: Ryan, James Michael; de Nolfo, Georgia A.; Gary, Dale
2019shin.confE.104R Altcode:
The series of large flares from 2017 September 4 to 10 were significant
microwave events with revealing multi-wavelength images of the flare
environment. The event on September 10 was a large long-duration,
gamma-ray flare (LDGRF). The event also produced a Ground Level
Enhancement (GLE). Using the microwave imaging data from the Expanded
Owens Valley Solar Array (EVOSA) we interpret and model the behavior
of the energetic-flare protons of September 10 as measured with the
Large Area Telescope (LAT) on the Fermi mission. We do this in the
context of stochastic acceleration in a large coronal bipolar structure
to produce the high-energy long-duration gamma-ray emission. Our
preliminary analysis suggests that the acceleration of the GeV protons
takes place in a large structure about 1.4 solar radii in length. The
requirements for the magnetic field and turbulence in this structure
will be presented.
---------------------------------------------------------
Title: Solar Coronal Magnetic Fields: Quantitative Measurements at
Radio Wavelengths
Authors: Fleishman, Gregory; Bastian, Timothy S.; Chen, Bin; Gary,
Dale E.; Glesener, Lindsay; Nita, Gelu; Saint-Hilaire, Pascal; White,
Stephen M.
2019BAAS...51c.426F Altcode: 2019astro2020T.426F
Quantitative measurements of coronal and chromospheric magnetic field is
currently in its infancy. We describe a foundation of such observations,
which is a key input for MHD numerical models of the solar atmosphere
and eruptive processes, and a key link between lower layers of the
solar atmosphere and the heliosphere.
---------------------------------------------------------
Title: Cause and Extent of the Extreme Radio Flux Density Reached
by the Solar Flare of 2006 December 06
Authors: Gary, Dale E.
2019arXiv190109262G Altcode:
The solar burst of 2006 December 06 reached a radio flux density of
more than 1 million solar flux units (1 sfu = $10^{-22}$ W/m$^2$/Hz),
as much as 10 times the previous record, and caused widespread loss
of satellite tracking by GPS receivers. The event was well observed
by NJITs Owens Valley Solar Array (OVSA). This work concentrates on
an accurate determination of the flux density (made difficult due to
the receiver systems being driving into non-linearity), and discuss
the physical conditions on the Sun that gave rise to this unusual
event. At least two other radio outbursts occurred in the same region
(on 2006 December 13 and 14) that had significant, but smaller effects
on GPS. We discuss the differences among these three events, and
consider the implications of these events for the upcoming solar cycle.
---------------------------------------------------------
Title: ngVLA Observations of Coronal Magnetic Fields
Authors: Fleishman, G. D.; Nita, G. M.; White, S. M.; Gary, D. E.;
Bastian, T. S.
2018ASPC..517..125F Altcode:
Energy stored in the magnetic field in the solar atmosphere above
active regions is a key driver of all solar activity (e.g., solar
flares and coronal mass ejections), some of which can affect life
on Earth. Radio observations provide a unique diagnostic of the
coronal magnetic fields that make them a critical tool for the
study of these phenomena, using the technique of broadband radio
imaging spectropolarimetry. Observations with the ngVLA will provide
unique observations of coronal magnetic fields and their evolution,
key inputs and constraints for MHD numerical models of the solar
atmosphere and eruptive processes, and a key link between lower layers
of the solar atmosphere and the heliosphere. In doing so they will
also provide practical "research to operations" guidance for space
weather forecasting.
---------------------------------------------------------
Title: Radio Observations of Solar Flares
Authors: Gary, D. E.; Bastian, T. S.; Chen, B.; Fleishman, G. D.;
Glesener, L.
2018ASPC..517...99G Altcode:
Solar flares are due to the catastrophic release of magnetic energy
in the Sun's corona, resulting in plasma heating, mass motions,
particle acceleration, and radiation emitted from radio to γ-ray
wavelengths. They are associated with global coronal eruptions of plasma
into the interplanetary medium—coronal mass ejections—that can
result in a variety of “space weather” phenomena. Flares release
energy over a vast range of energies, from ∼10<SUP>23</SUP> ergs
(nanoflares) to more than 10<SUP>32</SUP> ergs. Solar flares are
a phenomenon of general astrophysical interest, allowing detailed
study of magnetic energy release, eruptive processes, shock formation
and propagation, particle acceleration and transport, and radiative
processes. Observations at radio wavelengths offer unique diagnostics of
the physics of flares. To fully exploit these diagnostics requires the
means of performing time-resolved imaging spectropolarimetry. Recent
observations with the Jansky Very Large Array (JVLA) and the Expanded
Owens Valley Solar Array (EOVSA), supported by extensive development
in forward modeling, have demonstrated the power of the approach. The
ngVLA has the potential to bring our understanding of flare processes
to a new level through its combination of high spatial resolution,
broad frequency range, and imaging dynamic range—especially when
used in concert with multi-wavelength observations and data at hard
X-ray energies.
---------------------------------------------------------
Title: The Coronal Volume of Energetic Particles in Solar Flares as
Revealed by Microwave Imaging
Authors: Fleishman, Gregory D.; Loukitcheva, Maria A.; Kopnina,
Varvara Yu.; Nita, Gelu M.; Gary, Dale E.
2018ApJ...867...81F Altcode: 2018arXiv180904753F
The spectrum of gyrosynchrotron emission from solar flares generally
peaks in the microwave range. Its optically thin, high-frequency
component, above the spectral peak, is often used for diagnostics
of the nonthermal electrons and the magnetic field in the radio
source. Under favorable conditions, its low-frequency counterpart
brings additional, complementary information about these parameters
as well as thermal plasma diagnostics, either through gyrosynchrotron
self-absorption, free-free absorption by the thermal plasma, or the
suppression of emission through the so-called Razin effect. However,
their effect on the low-frequency spectrum is often masked by spatial
nonuniformity. To disentangle the various contributions to low-frequency
gyrosynchrotron emission, a combination of spectral and imaging data
is needed. To this end, we have investigated Owens Valley Solar Array
(OVSA) multi-frequency images for 26 solar bursts observed jointly with
RHESSI during the first half of 2002. For each, we examined dynamic
spectra, time- and frequency-synthesis maps, RHESSI images with overlaid
OVSA contours, and a few representative single-frequency snapshot OVSA
images. We focus on the frequency dependence of microwave source sizes
derived from the OVSA images and their effect on the low-frequency
microwave spectral slope. We succeed in categorizing 18 analyzed
events into several groups. Four events demonstrate clear evidence of
being dominated by gyrosynchrotron self-absorption, with an inferred
brightness temperature of ≥10<SUP>8</SUP> K. The low-frequency
spectra in the remaining events are affected to varying degrees by
Razin suppression. We find that many radio sources are rather large
at low frequencies, which can have important implications for solar
energetic particle production and escape.
---------------------------------------------------------
Title: Science with an ngVLA: Radio Observations of Solar Flares
Authors: Gary, Dale E.; Bastian, Timothy S.; Chen, Bin; Fleishman,
Gregory D.; Glesener, Lindsay
2018arXiv181006336G Altcode:
Solar flares are due to the catastrophic release of magnetic energy
in the Sun's corona, resulting in plasma heating, mass motions,
particle acceleration, and radiation emitted from radio to $\gamma$-ray
wavelengths. They are associated with global coronal eruptions of plasma
into the interplanetary medium---coronal mass ejections---that can
result in a variety of "space weather" phenomena. Flares release energy
over a vast range of energies, from $\sim\!10^{23}$ ergs (nanoflares)
to more than $10^{32}$ ergs. Solar flares are a phenomenon of general
astrophysical interest, allowing detailed study of magnetic energy
release, eruptive processes, shock formation and propagation, particle
acceleration and transport, and radiative processes. Observations at
radio wavelengths offer unique diagnostics of the physics of flares. To
fully exploit these diagnostics requires the means of performing
time-resolved imaging spectropolarimetry. Recent observations with the
Jansky Very Large Array (JVLA) and the Expanded Owens Valley Solar Array
(EOVSA), supported by extensive development in forward modeling, have
demonstrated the power of the approach. The ngVLA has the potential
to bring our understanding of flare processes to a new level through
its combination of high spatial resolution, broad frequency range,
and imaging dynamic range---especially when used in concert with
multi-wavelength observations and data at hard X-ray energies.
---------------------------------------------------------
Title: Roadmap for Reliable Ensemble Forecasting of the Sun-Earth
System
Authors: Nita, Gelu; Angryk, Rafal; Aydin, Berkay; Banda, Juan;
Bastian, Tim; Berger, Tom; Bindi, Veronica; Boucheron, Laura; Cao,
Wenda; Christian, Eric; de Nolfo, Georgia; DeLuca, Edward; DeRosa,
Marc; Downs, Cooper; Fleishman, Gregory; Fuentes, Olac; Gary, Dale;
Hill, Frank; Hoeksema, Todd; Hu, Qiang; Ilie, Raluca; Ireland,
Jack; Kamalabadi, Farzad; Korreck, Kelly; Kosovichev, Alexander;
Lin, Jessica; Lugaz, Noe; Mannucci, Anthony; Mansour, Nagi; Martens,
Petrus; Mays, Leila; McAteer, James; McIntosh, Scott W.; Oria, Vincent;
Pan, David; Panesi, Marco; Pesnell, W. Dean; Pevtsov, Alexei; Pillet,
Valentin; Rachmeler, Laurel; Ridley, Aaron; Scherliess, Ludger; Toth,
Gabor; Velli, Marco; White, Stephen; Zhang, Jie; Zou, Shasha
2018arXiv181008728N Altcode:
The authors of this report met on 28-30 March 2018 at the New Jersey
Institute of Technology, Newark, New Jersey, for a 3-day workshop
that brought together a group of data providers, expert modelers, and
computer and data scientists, in the solar discipline. Their objective
was to identify challenges in the path towards building an effective
framework to achieve transformative advances in the understanding
and forecasting of the Sun-Earth system from the upper convection
zone of the Sun to the Earth's magnetosphere. The workshop aimed to
develop a research roadmap that targets the scientific challenge
of coupling observations and modeling with emerging data-science
research to extract knowledge from the large volumes of data (observed
and simulated) while stimulating computer science with new research
applications. The desire among the attendees was to promote future
trans-disciplinary collaborations and identify areas of convergence
across disciplines. The workshop combined a set of plenary sessions
featuring invited introductory talks and workshop progress reports,
interleaved with a set of breakout sessions focused on specific topics
of interest. Each breakout group generated short documents, listing
the challenges identified during their discussions in addition to
possible ways of attacking them collectively. These documents were
combined into this report-wherein a list of prioritized activities
have been collated, shared and endorsed.
---------------------------------------------------------
Title: Science with an ngVLA: ngVLA Observations of Coronal Magnetic
Fields
Authors: Fleishman, Gregory D.; Nita, Gelu M.; White, Stephen M.;
Gary, Dale E.; Bastian, Tim S.
2018arXiv181006622F Altcode:
Energy stored in the magnetic field in the solar atmosphere above
active regions is a key driver of all solar activity (e.g., solar
flares and coronal mass ejections), some of which can affect life
on Earth. Radio observations provide a unique diagnostic of the
coronal magnetic fields that make them a critical tool for the
study of these phenomena, using the technique of broadband radio
imaging spectropolarimetry. Observations with the ngVLA will provide
unique observations of coronal magnetic fields and their evolution,
key inputs and constraints for MHD numerical models of the solar
atmosphere and eruptive processes, and a key link between lower layers
of the solar atmosphere and the heliosphere. In doing so they will
also provide practical "research to operations" guidance for space
weather forecasting.
---------------------------------------------------------
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: Microwave Spectroscopic Imaging of the Decay Phase of the
X8.2 flare on 2017 Sep 10
Authors: Yu, Sijie; Chen, Bin; Musset, Sophie; Reeves, Kathy; Glesener,
Lindsay; Gary, Dale
2018shin.confE.213Y Altcode:
We present microwave imaging spectroscopic observation of the decay
phase of the X8.2 flare on 2017 September 10 recorded by the Expanded
Owens Valley Solar Array (EOVSA) at 2.5-18 GHz. The time period (
17-18 UT) under this study is preceded by two major microwave peaks
- one at 16:00 UT (which corresponds to the main hard X-ray peak)
and another at 16:40 UT - showing multiple short-duration, broadband
microwave bursts that reappear at an interval of 3-5 minutes. During
this period the flare, as imaged by SDO/AIA and Hinode/XRT in EUV and
soft X-ray, develops a thin plasma sheet extending from the top of
the post-flare arcades to 200" above the loop-top. EOVSA spectroscopic
imaging shows a nonthermal microwave source that appears to follow the
outer rim of the EUV post-flare arcades, which is consistent with the
classic picture of nonthermal electrons gyrating in newly reconnected,
closed arcades. In particular, the upper portion of the microwave
source coincides with a looptop RHESSI X-ray source located at bottom
of the current-sheet-like feature and the multitudes of fast plasma
downflows. We investigate the physical properties of the source by
combining the concurrent microwave, EUV, and X-ray data, and discuss
implications for particle acceleration during the flare decay phase.
---------------------------------------------------------
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: Microwave Imaging Spectroscopy of the 2017 Sep 10 X8.2 Flare
with EOVSA
Authors: Gary, Dale; Fleishman, Gregory; Nita, Gelu; Chen, Bin;
Kuroda, Natsuha
2018shin.confE.210G Altcode:
The Expanded Owens Valley Solar Array (EOVSA) became fully operational
just in time to record breakthrough multi-frequency microwave images,
on a 1-s cadence, of several of the events of 2017 September. The X8.2
flare on September 10 was particularly well observed, and offers an
opportunity to use imaging spectroscopy to create diagnostic parameter
maps of magnetic field, plasma, and energetic electron parameters as
a function of position and time. The EOVSA multi-frequency images and
movies show a dramatic change from the initial flux-rope eruption,
where microwave emission extends all along the plasma sheet below the
flux rope, to the later post-flare loop phase, where an extremely
bright microwave-emitting source located well above the EUV loops
rises slowly at the same rate as the growing EUV loops. We fit the
spatially-resolved microwave spectrum at each point in the emitting
source using a multi-parameter homogeneous gyrosynchrotron emission
model, and show the resulting parameter maps at several key times. We
discuss the findings, as well as the limitations of the results, in
terms of the expectations from the standard solar flare model. The
parameter maps are the starting point for further 3D forward-fit
modeling which we plan to undertake next.
---------------------------------------------------------
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: Microwave Imaging of Flares and Active Regions with the
Expanded Owens Valley Solar Array
Authors: Gary, Dale E.; Chen, Bin; Fleishman, Gregory D.; Kuroda,
Natsuha; Nita, Gelu M.; White, Stephen M.; Yu, Sijie
2018tess.conf21058G Altcode:
The Expanded Owens Valley Solar Array (EOVSA), operating in the
microwave range (2.5-18 GHz), is the first solar-dedicated radio
instrument to achieve true multi-frequency imaging of the Sun. As of
the time of this Triennial Earth-Sun Summit meeting, EOVSA will have
completed its first year of full operation. We present some highlights
of both active region and flare observations that demonstrate the
exciting new capabilities of the instrument, including excellent
coverage of several large flares that occurred in the 2017 September
period. We also provide information for public access to the data, and
for getting started with analysis. We are developing tools to simplify
working with the data, as well as modeling tools for interpreting
the results in the context of complementary multi-wavelength data
from space- and ground-based instruments. The progress and current
status of these efforts is given in this and a number of companion
presentations at the meeting.
---------------------------------------------------------
Title: Highlights of EOVSA Microwave Imaging Spectroscopy of the
Flares of September 2017.
Authors: Gary, Dale E.
2018tess.conf31501G Altcode:
The Expanded Owens Valley Solar Array (EOVSA) began full operations
only in April of 2017, just in time to cover the late surge of solar
activity in Cycle 24. The solar flare events of September 4 and 10 were
especially well observed, and the 2.5-18 GHz images provide outstanding
and never-before-obtained spatially-resolved microwave spectra produced
by high-energy electron of energy ~0.5 to a few MeV. The combination
of timing, position, and spectrum of the emission, compared with
multi-wavelength observations from other instruments, provides a new
view of the high-energy component of these important events, both of
which were associated with solar energetic particle (SEP) events. The
highlights of results from these spectacular events are presented.
---------------------------------------------------------
Title: Absorption Spectroscopy of Mercury's Exosphere During the
2016 Solar Transit
Authors: Schmidt, C. A.; Leblanc, F.; Reardon, K.; Killen, R. M.;
Gary, D. E.; Ahn, K.
2018LPICo2047.6022S Altcode:
Solar transits of Mercury provide a rare opportunity to study the
exosphere in absorption and a valuable analog to transiting exoplanet
studies. This presentation will characterize the sodium exosphere
during the 2016 transit.
---------------------------------------------------------
Title: Exploring the Sun with ALMA
Authors: Bastian, T. S.; Bárta, M.; Brajša, R.; Chen, B.; Pontieu,
B. D.; Gary, D. E.; Fleishman, G. D.; Hales, A. S.; Iwai, K.; Hudson,
H.; Kim, S.; Kobelski, A.; Loukitcheva, M.; Shimojo, M.; Skokić,
I.; Wedemeyer, S.; White, S. M.; Yan, Y.
2018Msngr.171...25B Altcode:
The Atacama Large Millimeter/submillimeter Array (ALMA) Observatory
opens a new window onto the Universe. The ability to perform continuum
imaging and spectroscopy of astrophysical phenomena at millimetre and
submillimetre wavelengths with unprecedented sensitivity opens up new
avenues for the study of cosmology and the evolution of galaxies, the
formation of stars and planets, and astrochemistry. ALMA also allows
fundamentally new observations to be made of objects much closer
to home, including the Sun. The Sun has long served as a touchstone
for our understanding of astrophysical processes, from the nature of
stellar interiors, to magnetic dynamos, non-radiative heating, stellar
mass loss, and energetic phenomena such as solar flares. ALMA offers
new insights into all of these processes.
---------------------------------------------------------
Title: Dressing the Coronal Magnetic Extrapolations of Active Regions
with a Parameterized Thermal Structure
Authors: Nita, Gelu M.; Viall, Nicholeen M.; Klimchuk, James A.;
Loukitcheva, Maria A.; Gary, Dale E.; Kuznetsov, Alexey A.; Fleishman,
Gregory D.
2018ApJ...853...66N Altcode:
The study of time-dependent solar active region (AR) morphology and
its relation to eruptive events requires analysis of imaging data
obtained in multiple wavelength domains with differing spatial and
time resolution, ideally in combination with 3D physical models. To
facilitate this goal, we have undertaken a major enhancement of our
IDL-based simulation tool, GX_Simulator, previously developed for
modeling microwave and X-ray emission from flaring loops, to allow it
to simulate quiescent emission from solar ARs. The framework includes
new tools for building the atmospheric model and enhanced routines
for calculating emission that include new wavelengths. In this paper,
we use our upgraded tool to model and analyze an AR and compare the
synthetic emission maps with observations. We conclude that the modeled
magneto-thermal structure is a reasonably good approximation of the
real one.
---------------------------------------------------------
Title: Transient rotation of photospheric vector magnetic fields
associated with a solar flare
Authors: Xu, Yan; Cao, Wenda; Ahn, Kwangsu; Jing, Ju; Liu, Chang;
Chae, Jongchul; Huang, Nengyi; Deng, Na; Gary, Dale E.; Wang, Haimin
2018NatCo...9...46X Altcode: 2018arXiv180103171X
As one of the most violent eruptions on the Sun, flares are believed to
be powered by magnetic reconnection. The fundamental physics involving
the release, transfer, and deposition of energy have been studied
extensively. Taking advantage of the unprecedented resolution provided
by the 1.6 m Goode Solar Telescope, here, we show a sudden rotation of
vector magnetic fields, about 12-20° counterclockwise, associated
with a flare. Unlike the permanent changes reported previously,
the azimuth-angle change is transient and cospatial/temporal with
Hα emission. The measured azimuth angle becomes closer to that in
potential fields suggesting untwist of flare loops. The magnetograms
were obtained in the near infrared at 1.56 μm, which is minimally
affected by flare emission and no intensity profile change was
detected. We believe that these transient changes are real and discuss
the possible explanations in which the high-energy electron beams or
Alfve'n waves play a crucial role.
---------------------------------------------------------
Title: Three-dimensional Forward-fit Modeling of the Hard X-Ray and
Microwave Emissions of the 2015 June 22 M6.5 Flare
Authors: Kuroda, Natsuha; Gary, Dale E.; Wang, Haimin; Fleishman,
Gregory D.; Nita, Gelu M.; Jing, Ju
2018ApJ...852...32K Altcode: 2017arXiv171207253K
The well-established notion of a “common population” of the
accelerated electrons simultaneously producing the hard X-ray (HXR)
and microwave (MW) emission during the flare impulsive phase has
been challenged by some studies reporting the discrepancies between
the HXR-inferred and MW-inferred electron energy spectra. The
traditional methods of spectral inversion have some problems that
can be mainly attributed to the unrealistic and oversimplified
treatment of the flare emission. To properly address this problem,
we use a nonlinear force-free field (NLFFF) model extrapolated from an
observed photospheric magnetogram as input to the three-dimensional,
multiwavelength modeling platform GX Simulator and create a unified
electron population model that can simultaneously reproduce the
observed HXR and MW observations. We model the end of the impulsive
phase of the 2015 June 22 M6.5 flare and constrain the modeled
electron spatial and energy parameters using observations made by the
highest-resolving instruments currently available in two wavelengths,
the Reuven Ramaty High Energy Solar Spectroscopic Imager for HXR and the
Expanded Owens Valley Solar Array for MW. Our results suggest that the
HXR-emitting electron population model fits the standard flare model
with a broken power-law spectrum ({E}<SUB>{break</SUB>}∼ 200 keV)
that simultaneously produces the HXR footpoint emission and the MW
high-frequency emission. The model also includes an “HXR-invisible”
population of nonthermal electrons that are trapped in a large volume of
magnetic field above the HXR-emitting loops, which is observable by its
gyrosynchrotron radiation emitting mainly in the MW low-frequency range.
---------------------------------------------------------
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: Three-Dimensional Forward-Fit Modeling of The Hard X-ray and
The Microwave Emissions of The 2015-06-22 M6.5 Flare
Authors: Kuroda, N.; Gary, D. E.; Wang, H.; Fleishman, G. D.; Nita,
G. M.; Jing, J.
2017AGUFMSH41A2753K Altcode:
The well-established notion of a "common population" of the
accelerated electrons simultaneously producing the hard X-ray (HXR)
and the microwave (MW) emission during the flare impulsive phase has
been challenged by some studies reporting the discrepancies between
the HXR-inferred and the MW-inferred electron energy spectra. The
traditional methods of their spectral inversion have some problems that
can be mainly attributed to the unrealistic and the oversimplified
treatment of the flare emission. To properly address this problem,
we use a Non-linear Force Free Field (NLFFF) model extrapolated from
an observed photospheric magnetogram as input to the threedimensional,
multi-wavelength modeling platform GX Simulator, and create a unified
electron population model that can simultaneously reproduce the observed
HXR and MW observations. We model the end of the impulsive phase of
the 2015-06-22 M6.5 flare, and constrain the modeled electron spatial
and energy parameters using observations made by the highest-resolving
instruments currently available in two wavelengths, the Reuven Ramaty
High Energy Solar Spectroscopic Imager (RHESSI) for HXR and the Expanded
Owens Valley Solar Array (EOVSA) for MW. Our results suggest that the
HXR-emitting electron population model fits the standard flare model
with a broken power-law spectrum that simultaneously produces the HXR
footpoint emission and the MW high frequency emission. The model also
includes an "HXR invisible" population of nonthermal electrons that
are trapped in a large volume of magnetic field above the HXR-emitting
loops, which is observable by its gyrosynchrotron (GS) radiation
emitting mainly in MW low frequency range.
---------------------------------------------------------
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: Solar Flare Dynamic Microwave Imaging with EOVSA
Authors: Gary, D. E.; Chen, B.; Nita, G. M.; Fleishman, G. D.; Yu,
S.; White, S. M.; Hurford, G. J.; McTiernan, J. M.
2017AGUFMSH41A2755G Altcode:
The Expanded Owens Valley Solar Array (EOVSA) is both an expansion
of our existing solar array and serves as a prototype for a much
larger future project, the Frequency Agile Solar Radiotelescope
(FASR). EOVSA is now complete, and is producing daily imaging of the
full solar disk, including active regions and solar radio bursts
at hundreds of frequencies in the range 2.8-18 GHz. We present
highlights of the 1-s-cadence dynamic imaging spectroscropy of
radio bursts we have obtained to date, along with deeper analysis
of multi-wavelength observations and modeling of a well-observed
burst. These observations are revealing the full life-cycle of the
trapped population of high-energy electrons, from their initial
acceleration and subsequent energy-evolution to their eventual decay
through escape and thermalization. All of our data are being made
available for download in both quick-look image form and in the form
of the community-standard CASA measurement sets for subsequent imaging
and analysis.
---------------------------------------------------------
Title: Dynamic Spectral Imaging of Decimetric Fiber Bursts in an
Eruptive Solar Flare
Authors: Wang, Zhitao; Chen, Bin; Gary, Dale E.
2017ApJ...848...77W Altcode: 2017arXiv170908137W
Fiber bursts are a type of fine structure that is often superposed
on type IV radio continuum emission during solar flares. Although
studied for many decades, its physical exciter, emission mechanism,
and association with the flare energy release remain unclear, partly
due to the lack of simultaneous imaging observations. We report
the first dynamic spectroscopic imaging observations of decimetric
fiber bursts, which occurred during the rise phase of a long-duration
eruptive flare on 2012 March 3, as obtained by the Karl G. Jansky Very
Large Array in 1-2 GHz. Our results show that the fiber sources are
located near and above one footpoint of the flare loops. The fiber
source and the background continuum source are found to be co-spatial
and share the same morphology. It is likely that they are associated
with nonthermal electrons trapped in the converging magnetic fields
near the footpoint, as supported by a persistent coronal hard X-ray
source present during the flare rise phase. We analyze three groups
of fiber bursts in detail with dynamic imaging spectroscopy and obtain
their mean frequency-dependent centroid trajectories in projection. By
using a barometric density model and magnetic field based on a potential
field extrapolation, we further reconstruct the 3D source trajectories
of fiber bursts, for comparison with expectations from the whistler
wave model and two MHD-based models. We conclude that the observed
fiber burst properties are consistent with an exciter moving at the
propagation velocity expected for whistler waves, or models that posit
similar exciter velocities.
---------------------------------------------------------
Title: Imaging Spectroscopy of CME-Associated Solar Radio Bursts
using OVRO-LWA
Authors: Chhabra, Sherry; Gary, Dale; Chen, Bin; Hallinan, Gregg;
Anderson, Marin
2017SPD....4820605C Altcode:
Energetic phenomenon on the Sun, such as solar flares and CMEs are a
dynamic laboratory to study radio emission. We use Owens Valley Long
Wavelength Array (OVRO-LWA) for the study. The new array with its 251
crossed broadband dipoles spread over a 200 m diameter core and 37
long baseline antennas extending to 1600 m baselines allows spatially
resolving the Sun in the frequency range 24-82 MHz, with high spectral
resolution.We examine coherent Type III and Type IV burst emission
associated with a CME from 2015 Sep 20, as well as quiet Sun images
before and after the bursts. Images of 9 s cadence are used to study
the event over a 100 minute period, out to a distance of about 2 solar
radii, over the frequency range of 40-70 MHz available at that time. In
order to understand better the behaviour and structural evolution
of the bursts, we image the event at hundreds of frequencies and
use the source centroids to obtain the velocity of outward motion. A
coalignment with LASCO(C2) and SWAP data allows spatial and temporal
comparison with observations of the CME in white light and EUV. We
also place the bursts in context of AIA-EUV, Fermi hard X-ray and
EOVSA Microwave emission associated with the event.
---------------------------------------------------------
Title: Science with the Expanded Owens Valley Solar Array
Authors: Nita, Gelu M.; Gary, Dale E.; Fleishman, Gregory D.; Chen,
Bin; White, Stephen M.; Hurford, Gordon J.; McTiernan, James; Hickish,
Jack; Yu, Sijie; Nelin, Kjell B.
2017SPD....4811009N Altcode:
The Expanded Owens Valley Solar Array (EOVSA) is a solar-dedicated
radio array that makes images and spectra of the full Sun on a daily
basis. Our main science goals are to understand the basic physics of
solar activity, such as how the Sun releases stored magnetic energy
on timescales of seconds, and how that solar activity, in the form
of solar flares and coronal mass ejections, influences the Earth and
near-Earth space environment, through disruptions of communication
and navigation systems, and effects on satellites and systems on the
ground. The array, which is composed out of thirteen 2.1 m dishes
and two 27 m dishes (used only for calibration), has a footprint of
1.1 km EW x 1.2 km NS and it is capable of producing, every second,
microwave images at two polarizations and 500 science channels spanning
the 1-18 GHz frequency range. Such ability to make multi-frequency
images of the Sun in this broad range of frequencies, with a frequency
dependent resolution ranging from ∼53” at 1 GHz to ∼3”at 18
GHz, is unique in the world. Here we present an overview of the EOVSA
instrument and a first set of science-quality active region and solar
flare images produced from data taken during April 2017.This research
is supported by NSF grant AST-1615807 and NASA grant NNX14AK66G to
New Jersey Institute of Technology.
---------------------------------------------------------
Title: Magnetic vector rotation in response to the energetic electron
beam during a flare
Authors: Xu, Yan; Cao, Wenda; Kwangsu, Ahn; Jing, Ju; Liu, Chang;
Chae, Jongchul; Huang, Nengyi; Deng, Na; Gary, Dale E.; Wang, Haimin
2017SPD....4810001X Altcode:
As one of the most violent forms of eruption on the Sun, flares are
believed to be powered by magnetic reconnection, by which stored
magnetic energy is released. The fundamental physical processes
involving the release, transfer and deposition of energy in multiple
layers of the solar atmosphere have been studied extensively with
significant progress. Taking advantage of recent developments in
observing facilities, new phenomena are continually revealed, bringing
new understanding of solar flares. Here we report the discovery of
a transient rotation of vector magnetic fields associated with a
flare observed by the 1.6-m New Solar Telescope at Big Bear Solar
Observatory. After ruling out the possibility that the rotation is
caused by line profile changes due to flare heating, our observation
shows that the transverse field rotateded by about 12-20 degrees
counterclockwise, and returned quickly to previous values after the
flare ribbons swept through. More importantly, as a consequence of
the rotation, the flare loops untwisted and became more potential. The
vector magnetograms were obtained in the near infrared at 1560 nm, which
is minimally affected by flare emission and no intensity profile change
was detected. Therefore, we believe that these transient changes are
real, and conclude the high energy electron beams play an crucial role
in the field changes. A straightforward and instructive explanation is
that the induced magnetic field of the electron beam superimposed on
the pre-flare field leads to a transient rotation of the overall field.
---------------------------------------------------------
Title: A Large-scale Plume in an X-class Solar Flare
Authors: Fleishman, Gregory D.; Nita, Gelu M.; Gary, Dale E.
2017ApJ...845..135F Altcode: 2017arXiv170706636F
Ever-increasing multi-frequency imaging of solar observations suggests
that solar flares often involve more than one magnetic fluxtube. Some
of the fluxtubes are closed, while others can contain open fields. The
relative proportion of nonthermal electrons among those distinct
loops is highly important for understanding energy release, particle
acceleration, and transport. The access of nonthermal electrons to
the open field is also important because the open field facilitates
the solar energetic particle (SEP) escape from the flaring site, and
thus controls the SEP fluxes in the solar system, both directly and
as seed particles for further acceleration. The large-scale fluxtubes
are often filled with a tenuous plasma, which is difficult to detect
in either EUV or X-ray wavelengths; however, they can dominate at low
radio frequencies, where a modest component of nonthermal electrons
can render the source optically thick and, thus, bright enough to be
observed. Here we report the detection of a large-scale “plume” at
the impulsive phase of an X-class solar flare, SOL2001-08-25T16:23,
using multi-frequency radio data from Owens Valley Solar Array. To
quantify the flare’s spatial structure, we employ 3D modeling
utilizing force-free-field extrapolations from the line of sight
SOHO/MDI magnetograms with our modeling tool GX_Simulator. We found that
a significant fraction of the nonthermal electrons that accelerated at
the flare site low in the corona escapes to the plume, which contains
both closed and open fields. We propose that the proportion between
the closed and open fields at the plume is what determines the SEP
population escaping into interplanetary space.
---------------------------------------------------------
Title: The analysis and the three-dimensional, forward-fit modeling
of the X-ray and the microwave emissions of major solar flares
Authors: Kuroda, Natsuha; Wang, Haimin; Gary, Dale E.
2017SPD....4840003K Altcode:
It is well known that the time profiles of the hard X-ray (HXR)
emission and the microwave (MW) emission during the impulsive phase of
the solar flare are well correlated, and that their analysis can lead
to the understandings of the flare-accelerated electrons. In this work,
we first studied the source locations of seven distinct temporal peaks
observed in HXR and MW lightcurves of the 2011-02-15 X2.2 flare using
the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and
Nobeyama Radioheliograph. We found that the seven emission peaks did
not come from seven spatially distinct sites in HXR and MW, but rather
in HXR we observed a sudden change in location only between the second
and the third peak, with the same pattern occurring, but evolving more
slowly in MW, which is consistent with the tether-cutting model of solar
flares. Next, we closely examine the widely-used notion of a "common
population" of the accelerated electrons producing the HXR and the MW,
which has been challenged by some studies suggesting the differences in
the inferred energy spectral index and emitting energies of the HXR-
and MW- producing electrons. We use the Non-linear Force Free Field
model extrapolated from the observed photospheric magnetogram in the
three-dimensional, multi-wavelength modeling platform GX Simulator,
and attempt to create a unified electron population model that can
simultaneously reproduce the observed X-ray and MW observations of
the 2015-06-22 M6.5 flare. We constrain the model parameters by the
observations made by the highest-resolving instruments currently
available in two wavelengths, the RHESSI for X-ray and the Expanded
Owens Valley Solar Array for MW. The results suggest that the X-ray
emitting electron population model fits to the standard flare model with
the broken, hardening power-law spectrum at ~300 keV that simultaneously
produces the HXR footpoint emission and the MW high frequency emission,
and also reveals that there could be a “X-ray invisible” population
of nonthermal electrons that are trapped in a large magnetic field
volume above the X-ray emitting loops, that emits gyrosynchrotron
radiation mainly in MW low frequency range.
---------------------------------------------------------
Title: Observing the Sun with the Atacama Large
Millimeter/submillimeter Array (ALMA): High-Resolution Interferometric
Imaging
Authors: Shimojo, M.; Bastian, T. S.; Hales, A. S.; White, S. M.;
Iwai, K.; Hills, R. E.; Hirota, A.; Phillips, N. M.; Sawada, T.;
Yagoubov, P.; Siringo, G.; Asayama, S.; Sugimoto, M.; Brajša, R.;
Skokić, I.; Bárta, M.; Kim, S.; de Gregorio-Monsalvo, I.; Corder,
S. A.; Hudson, H. S.; Wedemeyer, S.; Gary, D. E.; De Pontieu, B.;
Loukitcheva, M.; Fleishman, G. D.; Chen, B.; Kobelski, A.; Yan, Y.
2017SoPh..292...87S Altcode: 2017arXiv170403236S
Observations of the Sun at millimeter and submillimeter wavelengths
offer a unique probe into the structure, dynamics, and heating of the
chromosphere; the structure of sunspots; the formation and eruption
of prominences and filaments; and energetic phenomena such as jets
and flares. High-resolution observations of the Sun at millimeter and
submillimeter wavelengths are challenging due to the intense, extended,
low-contrast, and dynamic nature of emission from the quiet Sun,
and the extremely intense and variable nature of emissions associated
with energetic phenomena. The Atacama Large Millimeter/submillimeter
Array (ALMA) was designed with solar observations in mind. The
requirements for solar observations are significantly different from
observations of sidereal sources and special measures are necessary
to successfully carry out this type of observations. We describe the
commissioning efforts that enable the use of two frequency bands,
the 3-mm band (Band 3) and the 1.25-mm band (Band 6), for continuum
interferometric-imaging observations of the Sun with ALMA. Examples of
high-resolution synthesized images obtained using the newly commissioned
modes during the solar-commissioning campaign held in December 2015
are presented. Although only 30 of the eventual 66 ALMA antennas
were used for the campaign, the solar images synthesized from the
ALMA commissioning data reveal new features of the solar atmosphere
that demonstrate the potential power of ALMA solar observations. The
ongoing expansion of ALMA and solar-commissioning efforts will continue
to enable new and unique solar observing capabilities.
---------------------------------------------------------
Title: Observing the Sun with the Atacama Large
Millimeter/submillimeter Array (ALMA): Fast-Scan Single-Dish Mapping
Authors: White, S. M.; Iwai, K.; Phillips, N. M.; Hills, R. E.; Hirota,
A.; Yagoubov, P.; Siringo, G.; Shimojo, M.; Bastian, T. S.; Hales,
A. S.; Sawada, T.; Asayama, S.; Sugimoto, M.; Marson, R. G.; Kawasaki,
W.; Muller, E.; Nakazato, T.; Sugimoto, K.; Brajša, R.; Skokić, I.;
Bárta, M.; Kim, S.; Remijan, A. J.; de Gregorio, I.; Corder, S. A.;
Hudson, H. S.; Loukitcheva, M.; Chen, B.; De Pontieu, B.; Fleishmann,
G. D.; Gary, D. E.; Kobelski, A.; Wedemeyer, S.; Yan, Y.
2017SoPh..292...88W Altcode: 2017arXiv170504766W
The Atacama Large Millimeter/submillimeter Array (ALMA) radio
telescope has commenced science observations of the Sun starting
in late 2016. Since the Sun is much larger than the field of view
of individual ALMA dishes, the ALMA interferometer is unable to
measure the background level of solar emission when observing the
solar disk. The absolute temperature scale is a critical measurement
for much of ALMA solar science, including the understanding of energy
transfer through the solar atmosphere, the properties of prominences,
and the study of shock heating in the chromosphere. In order to provide
an absolute temperature scale, ALMA solar observing will take advantage
of the remarkable fast-scanning capabilities of the ALMA 12 m dishes
to make single-dish maps of the full Sun. This article reports on the
results of an extensive commissioning effort to optimize the mapping
procedure, and it describes the nature of the resulting data. Amplitude
calibration is discussed in detail: a path that uses the two loads in
the ALMA calibration system as well as sky measurements is described
and applied to commissioning data. Inspection of a large number of
single-dish datasets shows significant variation in the resulting
temperatures, and based on the temperature distributions, we derive
quiet-Sun values at disk center of 7300 K at λ =3 mm and 5900 K at
λ =1.3 mm. These values have statistical uncertainties of about 100
K, but systematic uncertainties in the temperature scale that may be
significantly larger. Example images are presented from two periods
with very different levels of solar activity. At a resolution of about
25<SUP>″</SUP>, the 1.3 mm wavelength images show temperatures on
the disk that vary over about a 2000 K range. Active regions and plages
are among the hotter features, while a large sunspot umbra shows up as
a depression, and filament channels are relatively cool. Prominences
above the solar limb are a common feature of the single-dish images.
---------------------------------------------------------
Title: The 2012 November 20 impulsive SEP event: likely sources and
their properties
Authors: Ireland, Jack; de Nolfo, Georgia; Fleishman, Gregory; Ryan,
James; Gary, Dale
2017shin.confE..84I Altcode:
Helium-3 (3He) enriched impulsive events are one of the most common
phenomena associated with eruptive events on the Sun. In most cases,
impulsive ions are associated with increases in electron intensity,
though not always. We examine an unusual event in which 3He ions were
abundant but there was no detectable accompanying electron signature. We
derive injections times from 3He ions observed with ACE/ULEIS and
LEMPT/WIND that are consistent with jets emanating from either of
two active regions on the Sun. Both active regions exhibit jetting
features but only one has type III radio emission associated with it. We
evaluate the potential for ion and electron acceleration and escape}
within these two regions using the Nonlinear Force-Free Field (NLFFF)
extrapolation of the underlying magnetic field and discuss scenarios
of ion acceleration with suppressed electron acceleration.
---------------------------------------------------------
Title: Investigating the Origins of Two Extreme Solar Particle Events:
Proton Source Profile and Associated Electromagnetic Emissions
Authors: Kocharov, Leon; Pohjolainen, Silja; Mishev, Alexander; Reiner,
Mike J.; Lee, Jeongwoo; Laitinen, Timo; Didkovsky, Leonid V.; Pizzo,
Victor J.; Kim, Roksoon; Klassen, Andreas; Karlicky, Marian; Cho,
Kyung-Suk; Gary, Dale E.; Usoskin, Ilya; Valtonen, Eino; Vainio, Rami
2017ApJ...839...79K Altcode:
We analyze the high-energy particle emission from the Sun in two
extreme solar particle events in which protons are accelerated to
relativistic energies and can cause a significant signal even in the
ground-based particle detectors. Analysis of a relativistic proton event
is based on modeling of the particle transport and interaction, from a
near-Sun source through the solar wind and the Earth’s magnetosphere
and atmosphere to a detector on the ground. This allows us to deduce
the time profile of the proton source at the Sun and compare it with
observed electromagnetic emissions. The 1998 May 2 event is associated
with a flare and a coronal mass ejection (CME), which were well
observed by the Nançay Radioheliograph, thus the images of the radio
sources are available. For the 2003 November 2 event, the low corona
images of the CME liftoff obtained at the Mauna Loa Solar Observatory
are available. Those complementary data sets are analyzed jointly
with the broadband dynamic radio spectra, EUV images, and other data
available for both events. We find a common scenario for both eruptions,
including the flare’s dual impulsive phase, the CME-launch-associated
decimetric-continuum burst, and the late, low-frequency type III
radio bursts at the time of the relativistic proton injection into
the interplanetary medium. The analysis supports the idea that the
two considered events start with emission of relativistic protons
previously accelerated during the flare and CME launch, then trapped
in large-scale magnetic loops and later released by the expanding CME.
---------------------------------------------------------
Title: High-resolution observations of flare precursors in the low
solar atmosphere
Authors: Wang, Haimin; Liu, Chang; Ahn, Kwangsu; Xu, Yan; Jing, Ju;
Deng, Na; Huang, Nengyi; Liu, Rui; Kusano, Kanya; Fleishman, Gregory
D.; Gary, Dale E.; Cao, Wenda
2017NatAs...1E..85W Altcode: 2017arXiv170309866W
Solar flares are generally believed to be powered by free magnetic
energy stored in the corona<SUP>1</SUP>, but the build up of
coronal energy alone may be insufficient to trigger the flare to
occur<SUP>2</SUP>. The flare onset mechanism is a critical but poorly
understood problem, insights into which could be gained from small-scale
energy releases known as precursors. These precursors are observed as
small pre-flare brightenings in various wavelengths<SUP>3-13</SUP>
and also from certain small-scale magnetic configurations such
as opposite-polarity fluxes<SUP>14-16</SUP>, where the magnetic
orientation of small bipoles is opposite to that of the ambient main
polarities. However, high-resolution observations of flare precursors
together with the associated photospheric magnetic field dynamics are
lacking. Here we study precursors of a flare using the unprecedented
spatiotemporal resolution of the 1.6-m New Solar Telescope, complemented
by new microwave data. Two episodes of precursor brightenings are
initiated at a small-scale magnetic channel<SUP>17-20</SUP> (a form of
opposite-polarity flux) with multiple polarity inversions and enhanced
magnetic fluxes and currents, lying near the footpoints of sheared
magnetic loops. Microwave spectra corroborate that these precursor
emissions originate in the atmosphere. These results provide evidence
of low-atmospheric small-scale energy release, possibly linked to the
onset of the main flare.
---------------------------------------------------------
Title: HeRO: A space-based low frequency interferometric observatory
for heliophysics enabled by novel vector sensor technology
Authors: Knapp, M.; Gary, D. E.; Hecht, M. H.; Lonsdale, C.; Lind,
F. D.; Robey, F. C.; Fuhrman, L.; Chen, B.; Fenn, A. J.; HeRO Team
2017pre8.conf..411K Altcode:
HeRO (Heliophysics Radio Observer) is a hybrid ground and space
mission concept for radio interferometry of solar radio bursts. The
space segment (HeRO-S) covers low frequencies, 100 kHz-20 MHz,
and is composed of 6 free-flying CubeSats equipped with vector
sensors. The ground segment (HeRO-G), covers higher frequencies,
15 MHz-300 MHz. HeRO will explore conditions and disturbances in
a key region of the heliosphere, from two to tens of solar radii,
using interferometric observations of solar radio bursts over three
decades in frequency. Spot mapping across the full frequency range
will provide precise positions and basic structural information about
type II and III radio bursts. The morphology of CME shock fronts
will be traced via type II burst emissions, and heliospheric magnetic
field geometries will be probed by measuring precise trajectories of
type III bursts. Refraction in the heliospheric plasma on large and
intermediate scales will be investigated throughout large volumes via
the frequency dependence of accurate interferometric positional data
on bursts. The HeRO data will be information rich with high resolution
in time, frequency and spatial position, and high SNR, creating fertile
ground for discovery of new phenomena.
---------------------------------------------------------
Title: Exploring impulsive solar magnetic energy release and particle
acceleration with focused hard X-ray imaging spectroscopy
Authors: Christe, Steven; Krucker, Samuel; Glesener, Lindsay; Shih,
Albert; Saint-Hilaire, Pascal; Caspi, Amir; Allred, Joel; Battaglia,
Marina; Chen, Bin; Drake, James; Dennis, Brian; Gary, Dale; Gburek,
Szymon; Goetz, Keith; Grefenstette, Brian; Gubarev, Mikhail; Hannah,
Iain; Holman, Gordon; Hudson, Hugh; Inglis, Andrew; Ireland, Jack;
Ishikawa, Shinosuke; Klimchuk, James; Kontar, Eduard; Kowalski, Adam;
Longcope, Dana; Massone, Anna-Maria; Musset, Sophie; Piana, Michele;
Ramsey, Brian; Ryan, Daniel; Schwartz, Richard; Stęślicki, Marek;
Turin, Paul; Warmuth, Alexander; Wilson-Hodge, Colleen; White, Stephen;
Veronig, Astrid; Vilmer, Nicole; Woods, Tom
2017arXiv170100792C Altcode:
How impulsive magnetic energy release leads to solar eruptions and how
those eruptions are energized and evolve are vital unsolved problems
in Heliophysics. The standard model for solar eruptions summarizes
our current understanding of these events. Magnetic energy in the
corona is released through drastic restructuring of the magnetic
field via reconnection. Electrons and ions are then accelerated by
poorly understood processes. Theories include contracting loops,
merging magnetic islands, stochastic acceleration, and turbulence at
shocks, among others. Although this basic model is well established,
the fundamental physics is poorly understood. HXR observations
using grazing-incidence focusing optics can now probe all of the key
regions of the standard model. These include two above-the-looptop
(ALT) sources which bookend the reconnection region and are likely
the sites of particle acceleration and direct heating. The science
achievable by a direct HXR imaging instrument can be summarized by the
following science questions and objectives which are some of the most
outstanding issues in solar physics (1) How are particles accelerated
at the Sun? (1a) Where are electrons accelerated and on what time
scales? (1b) What fraction of electrons is accelerated out of the
ambient medium? (2) How does magnetic energy release on the Sun lead
to flares and eruptions? A Focusing Optics X-ray Solar Imager (FOXSI)
instrument, which can be built now using proven technology and at modest
cost, would enable revolutionary advancements in our understanding of
impulsive magnetic energy release and particle acceleration, a process
which is known to occur at the Sun but also throughout the Universe.
---------------------------------------------------------
Title: Absorption by Mercury's Exosphere During the May 9th, 2016
Solar Transit.
Authors: Schmidt, C.; Reardon, K.; Killen, R. M.; Gary, D. E.; Ahn, K.;
Leblanc, F.; Baumgardner, J. L.; Mendillo, M.; Beck, C.; Mangano, V.
2016AGUFM.P53B2198S Altcode:
Observations of Mercury during a solar transit have the unique property
that line absorption may be used to retrieve the exosphere's column
density at all points above the terminator simultaneously. We report
on measurements during the 9 May 2016 transit with the Dunn Solar
Telescope (Interferometric BIdimensional Spectropolarimeter: IBIS &
Horizontal Spectrograph: HSG) and the Big Bear Solar Observatory (Fast
Imaging Solar Spectrograph: FISS). The sodium exosphere was observed
via Fabry-Perot imaging with IBIS in 9 mA increments, and with FISS at
a dispersion of 17 mA/pixel by scanning the spectrograph slit over the
planet's disk. A search for potassium D line absorption was performed
using slit spectroscopy with HSG at a resolution of R 270,000. In each
instrument, exposures of 20-40 ms and adaptive optics enable spatial
structure to be resolved on sub-arcsecond scales. The line profiles at
every spatial bin are divided by a shifted and scaled reference spectrum
in order to isolate the exosphere's absorption from line absorption in
the solar atmosphere and structures inherent to granulation. Analysis
of these data sets is ongoing, but preliminary findings clearly show
the densest column of sodium near the poles and the content at dawn
enhanced several times with respect to dusk. Such is consistent with
2003 transit results taken at the same Mercury season (Schleicher et
al., 2004), however the data volumes herein permit a more in-depth
study in which time-dependence of the exosphere may be considered.
---------------------------------------------------------
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: EOVSA Implementation of a Spectral Kurtosis Correlator for
Transient Detection and Classification
Authors: Nita, Gelu M.; Hickish, Jack; MacMahon, David; Gary, Dale E.
2016JAI.....541009N Altcode: 2017arXiv170205391N
We describe in general terms the practical use in astronomy of a
higher-order statistical quantity called spectral kurtosis (SK),
and describe the first implementation of SK-enabled firmware in the
Fourier transform-engine (F-engine) of a digital FX correlator for
the Expanded Owens Valley Solar Array (EOVSA). The development of the
theory for SK is summarized, leading to an expression for generalized SK
that is applicable to both SK spectrometers and those not specifically
designed for SK. We also give the means for computing both the SK̂
estimator and thresholds for its application as a discriminator of RFI
contamination. Tests of the performance of EOVSA as an SK spectrometer
are shown to agree precisely with theoretical expectations, and the
methods for configuring the correlator for correct SK operation are
described.
---------------------------------------------------------
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: Imaging Spectroscopy of CME-Associated Solar Radio Bursts
With OVRO-LWA
Authors: Chhabra, S.; Gary, D. E.; Chen, B.; Hallinan, G.; Anderson, M.
2016AGUFMSH33A..05C Altcode:
Energetic phenomenon on the Sun, such as solar flares and CMEs are a
dynamic laboratory to study radio emission. We use Owens Valley Long
Wavelength Array (OVRO-LWA) for the study. The new array with its 251
crossed broadband dipoles spread over a 200 m diameter core and 37
long baseline antennas extending to 1600 m baselines allows spatially
resolving the Sun in the frequency range 24-82 MHz, with high spectral
resolution. We examine coherent Type III and Type IV burst emission
associated with a CME from 2015 Sep 20, as well as quiet Sun images
before and after the bursts. Images of 9 s cadence are used to study
the event over a 100 minute period, out to a distance of about 2 solar
radii, over the frequency range of 40-70 MHz available at that time. In
order to understand better the behaviour and structural evolution
of the bursts, we image the event at hundreds of frequencies and
use the source centroids to obtain the velocity of outward motion. A
co-alignment with LASCO (C2) and SWAP data allows spatial and temporal
comparison with observations of the CME in white light and EUV. We
also place the bursts in context of AIA EUV, Fermi hard X-ray, and
EOVSA microwave emission associated with the event.
---------------------------------------------------------
Title: The 2016 Transit of Mercury Observed from Major Solar
Telescopes and Satellites
Authors: Pasachoff, Jay M.; Schneider, Glenn; Gary, Dale; Chen, Bin;
Sterling, Alphonse C.; Reardon, Kevin P.; Dantowitz, Ronald; Kopp,
Greg A.
2016DPS....4811705P Altcode:
We report observations from the ground and space of the 9 May 2016
transit of Mercury. We build on our explanation of the black-drop
effect in transits of Venus based on spacecraft observations of the 1999
transit of Mercury (Schneider, Pasachoff, and Golub, Icarus 168, 249,
2004). In 2016, we used the 1.6-m New Solar Telescope at the Big Bear
Solar Observatory with active optics to observe Mercury's transit at
high spatial resolution. We again saw a small black-drop effect as 3rd
contact neared, confirming the data that led to our earlier explanation
as a confluence of the point-spread function and the extreme solar
limb darkening (Pasachoff, Schneider, and Golub, in IAU Colloq. 196,
2004). We again used IBIS on the Dunn Solar Telescope of the Sacramento
Peak Observatory, as A. Potter continued his observations, previously
made at the 2006 transit of Mercury, at both telescopes of the sodium
exosphere of Mercury (Potter, Killen, Reardon, and Bida, Icarus 226,
172, 2013). We imaged the transit with IBIS as well as with two RED
Epic IMAX-quality cameras alongside it, one with a narrow passband. We
show animations of our high-resolution ground-based observations along
with observations from XRT on JAXA's Hinode and from NASA's Solar
Dynamics Observatory. Further, we report on the limit of the transit
change in the Total Solar Irradiance, continuing our interest from
the transit of Venus TSI (Schneider, Pasachoff, and Willson, ApJ 641,
565, 2006; Pasachoff, Schneider, and Willson, AAS 2005), using NASA's
SORCE/TIM and the Air Force's TCTE/TIM. See http://transitofvenus.info
and http://nicmosis.as.arizona.edu.Acknowledgments: We were glad for
the collaboration at Big Bear of Claude Plymate and his colleagues of
the staff of the Big Bear Solar Observatory. We also appreciate the
collaboration on the transit studies of Robert Lucas (Sydney, Australia)
and Evan Zucker (San Diego, California). JMP appreciates the sabbatical
hospitality of the Division of Geosciences and Planetary Sciences of
the California Institute of Technology, and of Prof. Andrew Ingersoll
there. The solar observations lead into the 2017 eclipse studies,
for which JMP is supported by grants from the NSF AGS and National
Geographic CRE.
---------------------------------------------------------
Title: Flare differentially rotates sunspot on Sun's surface
Authors: Liu, Chang; Xu, Yan; Cao, Wenda; Deng, Na; Lee, Jeongwoo;
Hudson, Hugh S.; Gary, Dale E.; Wang, Jiasheng; Jing, Ju; Wang, Haimin
2016NatCo...713104L Altcode: 2016arXiv161002969L
Sunspots are concentrations of magnetic field visible on the solar
surface (photosphere). It was considered implausible that solar flares,
as resulted from magnetic reconnection in the tenuous corona, would
cause a direct perturbation of the dense photosphere involving bulk
motion. Here we report the sudden flare-induced rotation of a sunspot
using the unprecedented spatiotemporal resolution of the 1.6 m New
Solar Telescope, supplemented by magnetic data from the Solar Dynamics
Observatory. It is clearly observed that the rotation is non-uniform
over the sunspot: as the flare ribbon sweeps across, its different
portions accelerate (up to ~50° h<SUP>-1</SUP>) at different times
corresponding to peaks of flare hard X-ray emission. The rotation may be
driven by the surface Lorentz-force change due to the back reaction of
coronal magnetic restructuring and is accompanied by a downward Poynting
flux. These results have direct consequences for our understanding of
energy and momentum transportation in the flare-related phenomena.
---------------------------------------------------------
Title: Measurement of duration and signal-to-noise ratio of
astronomical transients using a Spectral Kurtosis spectrometer
Authors: Nita, Gelu M.; Gary, Dale E.
2016JGRA..121.7353N Altcode:
Following our prior theoretical and instrumental work addressing
the problem of automatic real-time radio frequency interference
(RFI) detection and excision from astronomical signals, the wideband
Spectral Kurtosis (SK) spectrometer design we proposed is currently
being considered as an alternative to the traditional spectrometers
when building the new generation of radio instruments. The unique
characteristic of an SK spectrometer is that it accumulates both power
and power-squared, which are then used to compute an SK statistical
estimator proven to be very effective in detecting and excising certain
types of RFI signals. In this paper we introduce a novel measurement
technique that exploits the power and power square statistics of an
SK spectrometer to determine durations and signal-to-noise ratios of
transient signals, whether they are RFI or natural signals, even when
they are below the time resolution of the instrument. We demonstrate
this novel experimental technique by analyzing a segment of data
recorded by the Expanded Owens Valley Solar Array Subsystem Testbed
(EST) during a solar radio burst in which microwave spike bursts
occurred with durations shorter than the 20 ms time resolution of the
instrument. The duration of one well-observed spike is quantitatively
shown to be within a few percent of 8 ms despite the 20 ms resolution
of the data.
---------------------------------------------------------
Title: Narrowband Gyrosynchrotron Bursts: Probing Electron
Acceleration in Solar Flares
Authors: Fleishman, Gregory D.; Nita, Gelu M.; Kontar, Eduard P.;
Gary, Dale E.
2016ApJ...826...38F Altcode: 2016arXiv160500948F
Recently, in a few case studies we demonstrated that gyrosynchrotron
microwave emission can be detected directly from the acceleration
region when the trapped electron component is insignificant. For the
statistical study reported here, we have identified events with steep
(narrowband) microwave spectra that do not show a significant trapped
component and, at the same time, show evidence of source uniformity,
which simplifies the data analysis greatly. Initially, we identified
a subset of more than 20 radio bursts with such narrow spectra, having
low- and high-frequency spectral indices larger than three in absolute
value. A steep low-frequency spectrum implies that the emission is
nonthermal (for optically thick thermal emission, the spectral index
cannot be steeper than two), and the source is reasonably dense and
uniform. A steep high-frequency spectrum implies that no significant
electron trapping occurs, otherwise a progressive spectral flattening
would be observed. Roughly half of these radio bursts have RHESSI data,
which allow for detailed, joint diagnostics of the source parameters and
evolution. Based on an analysis of radio-to-X-ray spatial relationships,
timing, and spectral fits, we conclude that the microwave emission
in these narrowband bursts originates directly from the acceleration
regions, which have a relatively strong magnetic field, high density,
and low temperature. In contrast, the thermal X-ray emission comes
from a distinct loop with a smaller magnetic field, lower density,
but higher temperature. Therefore, these flares likely occurred due
to interaction between two (or more) magnetic loops.
---------------------------------------------------------
Title: Ultra-Narrow Negative Flare Front Observed in Helium-10830
Å Using the1.6m New Solar Telescope
Authors: Xu, Yan; Cao, Wenda; Ding, Mingde; Kleint, Lucia; Su,
Jiangtao; Liu, Chang; Ji, Haisheng; Chae, Jongchul; Jing, Ju; Cho,
Kyuhyoun; Cho, Kyung-Suk; Gary, Dale E.; Wang, Haimin
2016SPD....47.0633X Altcode:
Solar flares are sudden flashes of brightness on the Sun and are often
associated with coronal mass ejections and solar energetic particles
that have adverse effects on the near-Earth environment. By definition,
flares are usually referred to as bright features resulting from excess
emission. Using the newly commissioned 1.6-m New Solar Telescope at
Big Bear Solar Observatory, we show a striking “negative” flare
with a narrow but unambiguous “dark” moving front observed in He I
10830 Å, which is as narrow as 340 km and is associated with distinct
spectral characteristics in Hα and Mg II lines. Theoretically, such
negative contrast in He I 10830 Å can be produced under special
circumstances by nonthermal electron collisions or photoionization
followed by recombination. Our discovery, made possible due to
unprecedented spatial resolution, confirms the presence of the required
plasma conditions and provides unique information in understanding
the energy release and radiative transfer in solar flares.
---------------------------------------------------------
Title: Radio Spectroscopic Imaging of Bi-directional Electron Beam
Pairs in a Solar Flare
Authors: Chen, Bin; Wang, Zhitao; Gary, Dale E.
2016SPD....4730201C Altcode:
In solar flares, energetic electrons are believed to be accelerated
at or near the magnetic reconnection site. They propagate outward
along newly reconnected field lines usually in the form of electron
beams. These beams can emit radio waves commonly known as type III
radio bursts. An important feature of these bursts is that they are
emitted near the local plasma frequency or its harmonic, which is only a
function of the ambient plasma density. In particular, an electron beam
propagating upward in the corona encounters plasma with lower and lower
density, producing a radio burst with a “normal” frequency slope
(whose frequency decreases in time). Similarly, a downward propagating
beam produces a reverse-slope burst. Sometimes both the normal- and
reverse-slope type III bursts are observed simultaneously. These type
III burst with opposite slopes have been considered to be the signature
of a pair of bi-directional electron beams emerging from a common
acceleration site. However, previous studies had no imaging capability
to locate these bursts and put them in the flare context. Here we
report observations of decimetric type III burst pairs by the Karl
G. Jansky Very Large Array (VLA) during the impulsive phase of a C5.6
flare. Using VLA’s unprecedented ultra-high-cadence spectroscopic
imaging capability, we demonstrate that the type III burst pairs indeed
correspond to high speed (~0.1c), bi-directional electron beams emerging
from a common site in the corona where post-flare loops appeared later
on. Implications of our results on magnetic reconnection and particle
acceleration will be briefly discussed.
---------------------------------------------------------
Title: Early Observations with the Expanded Owens Valley Solar Array
Authors: Gary, Dale E.
2016SPD....4730101G Altcode:
The Expanded Owens Valley Solar Array (EOVSA) is a newly expanded
and upgraded, solar-dedicated radio array consisting of 13 antennas
of 2.1 m diameter equipped with receivers designed to cover the
1-18 GHz frequency range. Two large (27-m diameter) dishes are being
outfitted with He-cooled receivers for use in calibration of the small
dishes. During 2015, the array obtained observations from dozens
of flares in total power mode on 8 antennas. Since February 2016,
it has begun taking solar data on all 13 small antennas with full
interferometric correlations, as well as calibration observations
with the first of the two large antennas equipped with its He-cooled
receiver. The second He-cooled receiver is nearly complete, and will
be available around the time of the meeting. We briefly review the
commissioning activities leading up to full operations, including
polarization and gain measurements and calibration methods, and
resulting measures of array performance. We then present some early
imaging observations with the array, emphasizing the remarkable temporal
and spectral resolution of the instrument, together with joint RHESSI
hard X-ray and SDO EUV observations.
---------------------------------------------------------
Title: BBSO/NST Observations of the Sudden Differential Rotation of
a Sunspot Caused by a Major Flare
Authors: Liu, Chang; Xu, Yan; Deng, Na; Cao, Wenda; Lee, Jeongwoo;
Hudson, Hugh S.; Gary, Dale E.; Wang, Jiasheng; Jing, Ju; Wang, Haimin
2016SPD....47.0615L Altcode:
Sunspots are concentrations of magnetic field visible on the solar
surface (photosphere), from which the field extends high into the
corona. Complex plasma motions that drag field in the photosphere can
build up free energy in the corona that powers solar eruptions. It
is known that solar flares and the often associated coronal ejections
(CMEs) can produce various radiations in the low atmosphere. However,
it was considered implausible that disturbances created in the tenuous
corona would cause a direct perturbation of the dense photosphere
involving bulk motion. Here we report the sudden rotational motion of
a sunspot clearly induced by a major solar flare (SOL2015-06-22T18:23
M6.6), using the unprecedented spatiotemporal resolution of the 1.6 m
New Solar Telescope (NST) at Big Bear Solar Observatory (BBSO). It is
particularly striking that the rotation is not uniform over the sunspot:
as the flare ribbon sweeps across, its different portions accelerate
(up to ~50 degree per hour) at different times corresponding to peaks
of flare hard X-ray emission. The intensity and magnetic field of
the sunspot also change significantly associated with the flare. Our
results reveal an intrinsic relationship between the photospheric
plasma bulk motions and coronal energy release, with direct consequences
for our understanding of energy and momentum balance in the flare/CME
phenomenon. This work is mainly supported by NASA grants NNX13AF76G
and NNX13AG13G (LWS), and NNX16AF72G, and NSF grants AGS 1250818
and 1408703.
---------------------------------------------------------
Title: Hard X-ray and Microwave Simulation of 2015-06-22 M6.6 flare
Authors: Kuroda, Natsuha; Wang, Haimin; Gary, Dale E.; Fleishman,
Gregory D.; Nita, Gelu M.; Chen, Bin; Xu, Yan; Jing, Ju
2016SPD....47.0614K Altcode:
It is well known that the time profiles of the hard X-ray (HXR) emission
and the microwave (MW) emission during the impulsive phase of the solar
flare are well correlated, and this has led to the expectation that
these emissions come from a common population of flare-accelerated
electrons. However, the energy ranges of the electrons producing two
emissions are believed to be different (below and above several hundred
keV for HXR-producing and MW-producing electrons, respectively), and
some studies have shown that the indices of their energy spectra may
differ as well. To better understand the energy distributions of the
electrons producing these emissions, we present realistic forward-fit
simulations of the HXR and the MW emissions of 2015 June 22, M6.6 flare
using the newly developed, IDL-based platform GX simulator. We use the
3D magnetic field model extrapolated from magnetogram data from the
Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics
Observatory (SDO), the images and the electron energy distribution
parameters deduced from the photon spectrum from the Reuven Ramaty
High Energy Solar Spectroscopic Imager (RHESSI), and the spatially
integrated MW spectrum and the cross-correlated amplitude data from the
Expanded Owens Valley Solar Array (EOVSA) to guide the modeling. We have
observed a possible above the-loop-top HXR source in 20-25 keV image,
well separated from the source seen in 6-12 keV that is typically
interpreted as a thermal loop-top source. Therefore, we simulate
the HXR emissions by combining two flux tubes at different heights:
the lower loop dominated by thermal electrons and the higher loop
dominated by nonthermal electrons. The MW and HXR emissions produced
from the forward-fit model are compared with observations to investigate
possible differences in the energy spectra of the HXR-producing and
the MW-producing electrons and what they can tell us about particle
acceleration.
---------------------------------------------------------
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: High-Resolution Observations of Flare Precursors and Their
Relationship with Magnetic Channels
Authors: Wang, Haimin; Xu, Yan; Ahn, Kwangsu; Jing, Ju; Liu, Chang;
Deng, Na; Huang, Nengyi; Gary, Dale E.; Cao, Wenda
2016SPD....4720501W Altcode:
The study of precursors of flares is important for understanding
the basic magnetic instability leading to solar flares, which can
aid the forecasting of eruptions potentially related to severe
space weather effects. Although literatures reported many important
clues, high-resolution observations of pre-flare activities before a
well-observed solar flare have been rare. Even rarely, the associated
magnetic structures in fine scale (below 1") were also observed. In
this study we take advantage of multiwavelength high-resolution
observations completely covering the 2015 June 22 M6.6 flare, which
were obtained under excellent seeing condition with the 1.6 m New Solar
Telescope (NST) at Big Bear Solar Observatory. The NST data includes
observations of the H-alpha line in five spectral positions at a spatial
resolution of 0.1" and magnetograms at a resolution of 0.25". These
are complemented by IRIS UV observations with a resolution of 0.25". We
find that there are two episodes of pre-flare brightenings (precursors),
which are spatially associated with magnetic channels, i.e., elongated
structures comprising alternating magnetic polarity inversion lines
(Zirin & Wang, 1993, Nature, 363, 426). The pre-flare chromospheric
and coronal features reflect an extremely sheared magnetic topology,
while the initiation of main flare brightenings correspond to a much
less sheared configuration. RHESSI HXR observations reveal that the
precursors have both thermal and nonthermal components, and the latter
is further evidenced by the microwave observations of the newly expanded
Solar Radio Array at Owens Valley.We further investigate the electric
current system above the magnetic channels using NLFFF extrapolations,
which show strong current sheets above the channel structure. This is
consistent with the MHD modeling of Kusano et al (2012, Ap.J., 760, 31),
who noted the importance of localized small-scale magnetic structure
in triggering the eruption of the whole active region. We suggest that
small-scale magnetic reconnection along the channels destabilizes the
magnetic structure of the active region and subsequently triggers the
main M6.6 flare and the associated halo CME.
---------------------------------------------------------
Title: Source dynamics of the microwave emission during a solar flare
Authors: Shaik, Shaheda Begum; Gary, Dale E.; Nita, Gelu M.
2016SPD....4730702S Altcode: 2016SPD....4730702B
Determining the microwave burst source characteristics is important
to understand the parameters of the flare process which produce the
microwave emission. Previous studies show that the microwave solar
bursts do typically exhibit a single source of emission but also
often show inhomogeneous sources as a function of frequency at some
periods during the burst. This study focuses on the spectral and spatial
dynamics of the microwave gyrosynchrotron source through the microwave
spectral and imaging analysis. We report the source characteristics
of few impulsive flare events observed by the newly upgraded Expanded
Owens Valley Solar Array (EOVSA) in the frequency range of 2.5 to 18 GHz
and from the complimentary data of (Nobeyama Radioheliograph / Nobeyama
Radio Polarimeters) NoRH / NoRP. The low frequency optically thick part
of the microwave spectrum is an indicator of spatial inhomogeneity and
complexity of the sources. We concentrate in the dynamics of the low
frequency spectrum (intensity and spectral index) measured by EOVSA,
and compare it to the corresponding spatial propoerties of the NoRH
sources observed at 17 GHz and to the loop structures seen in the EUV
(Extreme Ultraviolet) images with SDO (Solar Dynamics Observatory).
---------------------------------------------------------
Title: Unprecedented Fine Structure of a Solar Flare Revealed by
the 1.6~m New Solar Telescope
Authors: Jing, Ju; Xu, Yan; Cao, Wenda; Liu, Chang; Gary, Dale E.;
Wang, Haimin
2016SPD....47.0601J Altcode:
Solar flares signify the sudden release of magnetic energy and are
sources of so called space weather. The fine structures (below 500
km) of flares are rarely observed and are accessible to only a few
instruments world-wide. Here we present observation of a solar flare
using exceptionally high resolution images from the 1.6~m New Solar
Telescope (NST) equipped with high order adaptive optics at Big Bear
Solar Observatory (BBSO). The observation reveals the process of the
flare in unprecedented detail, including the flare ribbon propagating
across the sunspots, coronal rain (made of condensing plasma) streaming
down along the post-flare loops, and the chromosphere's response to
the impact of coronal rain, showing fine-scale brightenings at the
footpoints of the falling plasma. Taking advantage of the resolving
power of the NST, we measure the cross-sectional widths of flare
ribbons, post-flare loops and footpoint brighenings, which generally lie
in the range of 80-200 km, well below the resolution of most current
instruments used for flare studies. Confining the scale of such fine
structure provides an essential piece of information in modeling the
energy transport mechanism of flares, which is an important issue in
solar and plasma physics.
---------------------------------------------------------
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: Solar Science with the Atacama Large Millimeter/Submillimeter
Array—A New View of Our Sun
Authors: Wedemeyer, S.; Bastian, T.; Brajša, R.; Hudson, H.;
Fleishman, G.; Loukitcheva, M.; Fleck, B.; Kontar, E. P.; De Pontieu,
B.; Yagoubov, P.; Tiwari, S. K.; Soler, R.; Black, J. H.; Antolin,
P.; Scullion, E.; Gunár, S.; Labrosse, N.; Ludwig, H. -G.; Benz,
A. O.; White, S. M.; Hauschildt, P.; Doyle, J. G.; Nakariakov, V. M.;
Ayres, T.; Heinzel, P.; Karlicky, M.; Van Doorsselaere, T.; Gary,
D.; Alissandrakis, C. E.; Nindos, A.; Solanki, S. K.; Rouppe van
der Voort, L.; Shimojo, M.; Kato, Y.; Zaqarashvili, T.; Perez, E.;
Selhorst, C. L.; Barta, M.
2016SSRv..200....1W Altcode: 2015SSRv..tmp..118W; 2015arXiv150406887W
The Atacama Large Millimeter/submillimeter Array (ALMA) is a new
powerful tool for observing the Sun at high spatial, temporal, and
spectral resolution. These capabilities can address a broad range
of fundamental scientific questions in solar physics. The radiation
observed by ALMA originates mostly from the chromosphere—a complex
and dynamic region between the photosphere and corona, which plays a
crucial role in the transport of energy and matter and, ultimately,
the heating of the outer layers of the solar atmosphere. Based on
first solar test observations, strategies for regular solar campaigns
are currently being developed. State-of-the-art numerical simulations
of the solar atmosphere and modeling of instrumental effects can help
constrain and optimize future observing modes for ALMA. Here we present
a short technical description of ALMA and an overview of past efforts
and future possibilities for solar observations at submillimeter and
millimeter wavelengths. In addition, selected numerical simulations
and observations at other wavelengths demonstrate ALMA's scientific
potential for studying the Sun for a large range of science cases.
---------------------------------------------------------
Title: Unprecedented Fine Structure of a Solar Flare Revealed by
the 1.6 m New Solar Telescope
Authors: Jing, Ju; Xu, Yan; Cao, Wenda; Liu, Chang; Gary, Dale;
Wang, Haimin
2016NatSR...624319J Altcode: 2016arXiv160408562J
Solar flares signify the sudden release of magnetic energy and
are sources of so called space weather. The fine structures (below
500 km) of flares are rarely observed and are accessible to only a few
instruments world-wide. Here we present observation of a solar flare
using exceptionally high resolution images from the 1.6 m New Solar
Telescope (NST) equipped with high order adaptive optics at Big Bear
Solar Observatory (BBSO). The observation reveals the process of the
flare in unprecedented detail, including the flare ribbon propagating
across the sunspots, coronal rain (made of condensing plasma) streaming
down along the post-flare loops, and the chromosphere’s response to
the impact of coronal rain, showing fine-scale brightenings at the
footpoints of the falling plasma. Taking advantage of the resolving
power of the NST, we measure the cross-sectional widths of flare
ribbons, post-flare loops and footpoint brighenings, which generally lie
in the range of 80-200 km, well below the resolution of most current
instruments used for flare studies. Confining the scale of such fine
structure provides an essential piece of information in modeling the
energy transport mechanism of flares, which is an important issue in
solar and plasma physics.
---------------------------------------------------------
Title: Ultra-narrow Negative Flare Front Observed in Helium-10830
Å Using the 1.6 m New Solar Telescope
Authors: Xu, Yan; Cao, Wenda; Ding, Mingde; Kleint, Lucia; Su,
Jiangtao; Liu, Chang; Ji, Haisheng; Chae, Jongchul; Jing, Ju; Cho,
Kyuhyoun; Cho, Kyungsuk; Gary, Dale; Wang, Haimin
2016ApJ...819...89X Altcode: 2016arXiv160104729X
Solar flares are sudden flashes of brightness on the Sun and are often
associated with coronal mass ejections and solar energetic particles
that have adverse effects on the near-Earth environment. By definition,
flares are usually referred to as bright features resulting from excess
emission. Using the newly commissioned 1.6 m New Solar Telescope at
Big Bear Solar Observatory, we show a striking “negative” flare
with a narrow but unambiguous “dark” moving front observed in He I
10830 Å, which is as narrow as 340 km and is associated with distinct
spectral characteristics in Hα and Mg II lines. Theoretically, such
negative contrast in He I 10830 Å can be produced under special
circumstances by nonthermal electron collisions or photoionization
followed by recombination. Our discovery, made possible due to
unprecedented spatial resolution, confirms the presence of the required
plasma conditions and provides unique information in understanding
the energy release and radiative transfer in astronomical objects.
---------------------------------------------------------
Title: Validation of the Coronal Thick Target Source Model
Authors: Fleishman, Gregory D.; Xu, Yan; Nita, Gelu N.; Gary, Dale E.
2016ApJ...816...62F Altcode: 2015arXiv151106947F
We present detailed 3D modeling of a dense, coronal thick-target X-ray
flare using the GX Simulator tool, photospheric magnetic measurements,
and microwave imaging and spectroscopy data. The developed model offers
a remarkable agreement between the synthesized and observed spectra and
images in both X-ray and microwave domains, which validates the entire
model. The flaring loop parameters are chosen to reproduce the emission
measure, temperature, and the nonthermal electron distribution at low
energies derived from the X-ray spectral fit, while the remaining
parameters, unconstrained by the X-ray data, are selected such as
to match the microwave images and total power spectra. The modeling
suggests that the accelerated electrons are trapped in the coronal part
of the flaring loop, but away from where the magnetic field is minimal,
and, thus, demonstrates that the data are clearly inconsistent with
electron magnetic trapping in the weak diffusion regime mediated by
the Coulomb collisions. Thus, the modeling supports the interpretation
of the coronal thick-target sources as sites of electron acceleration
in flares and supplies us with a realistic 3D model with physical
parameters of the acceleration region and flaring loop.
---------------------------------------------------------
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: On Using Solar Radio Emission to Probe Interiors of Asteroids
and Comets
Authors: Winebrenner, D. P.; Gary, D. E.; Sahr, J. D.; Asphaug, E. I.
2015AGUFM.P51C2083W Altcode:
Asteroids, comets and other primitive solar system bodies are key
sources of information on the early solar system, on volatiles and
organics delivered to the terrestrial planets, and on processes
of planetary formation now observed in operation around other
stars. Whether asteroids (in various size classes) are rubble
piles or monolithic, and whether any porosity or internal voids
contain volatiles, are first-order questions for understanding the
delivery of volatiles to the early Earth, and for assessing impact
hazards. Information on bulk composition aids discrimination between
types and origins of primitive bodies, .e.g., the degree of aqueous
alteration and bound-water content of carbonaceous chondrite bodies,
and the volatile mass fraction of comets. Radar and radio methods
can provide direct information on bulk composition, micro- and
macro-porosity, body-scale internal structure, and on whether voids in
rocky materials are volatile- or vacuum-filled. Such methods therefore
figure prominently in current missions to primitive bodies (e.g.,
CONSERT) and in a variety of proposed missions. Radio transmitters
necessary for conventional methods, however, add considerably to
spacecraft mass and power requirements. Moreover, at many wavelengths
most useful for radio sounding, powerful radio emission from the
Sun strongly interferes with conventional signals. Here we present
initial results from an investigation of how solar radio emission
could serve as a natural resource for probing interiors of primitive
bodies, rather than as interference. We briefly review methods
for using stochastic radio illumination (aka noise radar methods),
and illustrate the characteristics of solar radio emission relevant
to mission design (e.g., observed intervals between emission events
of specified intensity for different points in the solar cycle). We
then discuss methods for selecting and interpreting observations in
terms of interior properties, for bodies is different size classes
(relative to the radio wavelength). Our preliminary results indicate
considerable promise, certainly for bodies near 1 AU, and possibly
also for bodies farther from the Sun.
---------------------------------------------------------
Title: The Atacama Large Millimeter/Submillimeter Array: a New Asset
for Solar and Heliospheric Physics
Authors: Bastian, Timothy S.; Barta, Miroslav; Brajsa, Roman; Chen,
Bin; De Pontieu, Bart; Fleishman, Gregory; Gary, Dale; Hales, Antonio;
Hills, Richard; Hudson, Hugh; Iwai, Kazamasu; Shimojo, Masumi; White,
Stephen; Wedemeyer, Sven; Yan, Yihua
2015IAUGA..2257295B Altcode:
The Atacama Large Millimeter/Submillimeter Array (ALMA) is a joint
North American, European, and East Asian interferometric array that
opens the mm-submm wavelength part of the electromagnetic spectrum
for general astrophysical exploration, providing high-resolution
imaging in frequency bands ranging from 86 to 950 GHz. 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 energy and matter and the heating of the
outer layers of the solar atmosphere. In this paper we describe
recent efforts to ensure that ALMA can be usefully exploited by
the scientific community to address outstanding questions in solar
physics. We summarize activities under North American and European
ALMA development studies, including instrument testing, calibration
and imaging strategies, a science simulations. With the support of
solar observations, ALMA joins next-generation groundbased instruments
that can be used alone or in combination with other ground-based and
space-based instruments to address outstanding questions in solar
and heliospheric physics. Opportunities for the wider community to
contribute to these efforts will be highlighted.
---------------------------------------------------------
Title: Observation of the 2011-02-15 X2.2 Flare in the Hard X-Ray
and Microwave
Authors: Kuroda, Natsuha; Wang, Haimin; Gary, Dale E.
2015ApJ...807..124K Altcode: 2015arXiv150601424K
Previous studies have shown that the energy release mechanism of some
solar flares follow the Standard magnetic-reconnection model, but the
detailed properties of high-energy electrons produced in the flare are
still not well understood. We conducted a unique, multi-wavelength
study that discloses the spatial, temporal and energy distributions
of the accelerated electrons in the X2.2 solar flare on 2011 February
15. We studied the source locations of seven distinct temporal peaks
observed in hard X-ray (HXR) and microwave (MW) light curves using the
RHESSI in 50-75 keV channels and Nobeyama Radioheliograph in 34 GHz,
respectively. We found that the seven emission peaks did not come
from seven spatially distinct sites in HXR and MW, but rather in HXR
we observed a sudden change in location only between the second and
the third peak, with the same pattern occurring, but evolving more
slowly in MW. Comparison between the HXR light curve and the temporal
variations in intensity in the two MW source kernels also confirmed that
the seven peaks came predominantly from two sources, each with multiple
temporal peaks. In addition, we studied the polarization properties of
MW sources, and time delay between HXR and MW. We discuss our results
in the context of the tether-cutting model.
---------------------------------------------------------
Title: Coronal Magnetography of a Simulated Solar Active Region from
Microwave Imaging Spectropolarimetry
Authors: Wang, Zhitao; Gary, Dale E.; Fleishman, Gregory D.; White,
Stephen M.
2015ApJ...805...93W Altcode: 2015arXiv150305239W
We have simulated the Expanded Owens Valley Solar Array (EOVSA)
radio images generated at multiple frequencies from a model solar
active region, embedded in a realistic solar disk model, and explored
the resulting data cube for different spectral analysis schemes to
evaluate the potential for realizing one of EOVSA’s most important
scientific goals—coronal magnetography. In this paper, we focus on
modeling the gyroresonance and free-free emission from an on-disk solar
active region model with realistic complexities in electron density,
temperature and magnetic field distribution. We compare the magnetic
field parameters extrapolated from the image data cube along each line
of sight after folding through the EOVSA instrumental profile with the
original (unfolded) parameters used in the model. We find that even
the most easily automated, image-based analysis approach (Level-0)
provides reasonable quantitative results, although they are affected
by systematic effects due to finite sampling in the Fourier (UV)
plane. Finally, we note the potential for errors due to misidentified
harmonics of the gyrofrequency, and discuss the prospects for applying
a more sophisticated spectrally based analysis scheme (Level-1) to
resolve the issue in cases where improved UV coverage and spatial
resolution are available.
---------------------------------------------------------
Title: Radio Coronal Magnetography of a Large Active Region
Authors: Bastian, Timothy S.; Gary, Dale E.; White, Stephen; Fleishman,
Gregory; Chen, Bin
2015TESS....111301B Altcode:
Quantitative knowledge of coronal magnetic fields is fundamental to
understanding energetic phenomena such as solar flares. Flares occur
in solar active regions where strong, non-potential magnetic fields
provide free energy. While constraints on the coronal magnetic field
topology are readily available through high resolution SXR and EUV
imaging of solar active regions, useful quantitative measurements of
coronal magnetic fields have thus far been elusive. Recent progress has
been made at infrared (IR) wavelengths in exploiting both the Zeeman
and Hanle effects to infer the line-of-sight magnetic field strength
or the orientation of the magnetic field vector in the plane of the
sky above the solar limb. However, no measurements of coronal magnetic
fields against the solar disk are possible using IR observations. Radio
observations of gyroresonance emission from active regions offer the
means of measuring coronal magnetic fields above the limb and on the
solar disk. In particular, for plasma plasma conditions in the solar
corona, active regions typically become optically thick to emission
over a range of radio frequencies through gyroresonance absorption
at a low harmonic of the electron gyrofrequency. The specific range
of resonant frequencies depends on the range of coronal magnetic
field strengths present in the active region.The Karl G. Jansky Very
Large Array was used in November 2014 to image NOAA/USAF active region
AR12209 over a continuous frequency range of 1-8 GHz, corresponding to
a wavelength range of 3.75-30 cm. This frequency range is sensitive to
coronal magnetic field strengths ranging from ~120-1400G. The active
region was observed on four different dates - November 18, 20, 22,
and 24 - during which the active region longitude ranged from -15 to
+70 degrees, providing a wide range of aspect angles. In this paper
we provide a preliminary description of the coronal magnetic field
measurements derived from the radio observations.
---------------------------------------------------------
Title: Energy Partitions and Evolution in a Purely Thermal Solar Flare
Authors: Fleishman, Gregory D.; Nita, Gelu M.; Gary, Dale E.
2015ApJ...802..122F Altcode: 2015arXiv150200315F
This paper presents a solely thermal flare, which we detected in
the microwave range from the thermal gyro- and free-free emission
it produced. An advantage of analyzing thermal gyro emission is its
unique ability to precisely yield the magnetic field in the radiating
volume. When combined with observationally deduced plasma density and
temperature, these magnetic field measurements offer a straightforward
way of tracking evolution of the magnetic and thermal energies in the
flare. For the event described here, the magnetic energy density in the
radio-emitting volume declines over the flare rise phase, then stays
roughly constant during the extended peak phase, but recovers to the
original level over the decay phase. At the stage where the magnetic
energy density decreases, the thermal energy density increases; however,
this increase is insufficient, by roughly an order of magnitude, to
compensate for the magnetic energy decrease. When the magnetic energy
release is over, the source parameters come back to nearly their
original values. We discuss possible scenarios to explain this behavior.
---------------------------------------------------------
Title: Tracing Solar Fiber Bursts Spatially and Spectrally with
Microwave Imaging Spectroscopy
Authors: Wang, Zhitao; Chen, Bin; Gary, Dale E.
2015TESS....120321W Altcode:
We report observations of fiber fine structures during type
IV decimetric radio bursts on March 3, 2012 using the recently
upgraded Karl G. Jansky Very Large Array (VLA). For the first time,
the technique of microwave imaging spectroscopy with high temporal
and spectral resolution allowed us to image the fiber bursts over a
continuous frequency range from 1-2 GHz, and precisely measure the
centroid trajectories in the heliocentric coordinate. We found that
the burst source originates at lower height near the footpoint, and
streams upward through the corona along some field lines, whose trend
is similar to some coronal loops seen in extreme ultra violet (EUV)
wavelengths. In the dynamic spectrum, we also studied the average
drift rates of different groups. Using an automatic fiber-tracing
algorithm, we obtained the normalized drift rates between -0.1 and
-0.025 s<SUP>-1</SUP> from over 2000 fiber samples. Together with an
appropriate coronal density model, we determined a subset of 3D field
lines in the extrapolated coronal magnetic field, provided by the
potential field source surface model (PFSS). As a result, we obtained
the 3D source velocity of the fiber bursts at more than 2000 km/s above
1.1 GHz. The derived velocity can be used to discriminate between the
two competing theories, the whistler wave model and Alfvén wave model.
---------------------------------------------------------
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: Energy partitions and evolution in a purely thermal solar flare
Authors: Fleishman, Gregory; Nita, Gelu M.; Gary, Dale E.
2015TESS....130703F Altcode:
A conventional way of producing a hot plasma in the flaring loops is
via the impact of the nonthermal particles accelerated in flares due to
release of the excessive magnetic energy, which has come to be known as
the Neupert effect. We know, however, that in many events the heating
starts clearly before the particle acceleration, which implies that no
accelerated particles may be required for this heating. To this end,
we present here a flare whose microwave emission is consistent with
a purely thermal distribution of electrons, based on the gyro- and
free-free emission it produced. An advantage of analyzing thermal gyro
emission is its unique ability to precisely yield the magnetic field
in the radiating volume. When combined with observationally-deduced
plasma density and temperature, these magnetic field measurements
offer a straightforward way of tracking evolution of the magnetic
and thermal energies in the flare. For the event described here, the
magnetic energy density in the radio-emitting volume declines over the
flare rise phase, then stays roughly constant during the extended peak
phase, but recovers to the original level over the decay phase. At the
stage where the magnetic energy density decreases, the thermal energy
density increases; however, this increase is insufficient, by roughly an
order of magnitude, to compensate for the magnetic energy decrease. We
conclude that the apparent decrease of the magnetic field in the radio
source over the rise phase of the flare requires an upward propagating
magnetic reconnection/plasma heating process, such as in the standard
flare scenario, but with one remarkable difference: the absence of any
significant nonthermal electron generation. We expect that the study of
these rare thermal flares will better clarify the origin of such purely
thermal events, characterized by significant energy release observed
through the plasma heating, but without any measurable acceleration of
the charged particles.This work was partially supported by NSF grants
AGS-1250374 and AGS-1262772, and NASA grant NNX14AC87G.
---------------------------------------------------------
Title: Initial Observations of Solar Bursts with the Expanded Owens
Valley Solar Array
Authors: Gary, Dale E.; Kuroda, Natsuha; Fleishman, Gregory; Nita,
Gelu M.; White, Stephen; Hurford, Gordon; McTiernan, James
2015TESS....130706G Altcode:
The Expanded Owens Valley Solar Array (EOVSA) is a newly expanded
and upgraded, solar-dedicated radio array consisting of 13 antennas
equipped with receivers designed to cover the 1-18 GHz frequency
range. Beginning in the fall of 2014, it began taking data on four
antennas in total power mode, and observed a number of solar flares
ranging from the X3.1 flare of 2014 Oct 24 to small events of low
C-class, with 1-s time resolution at more than 300 frequencies in
the range 2.5-18 GHz. The array is now (Feb. 2015) operating with 8
antennas in both total power and interferometry mode, and is rapidly
being commissioned for full operation with all 13 antennas. Here
we present some initial observations with the array, emphasizing
the remarkable temporal and spectral resolution of the instrument,
together with joint RHESSI hard X-ray and SDO EUV observations.
---------------------------------------------------------
Title: Synthetic 3D modeling of active regions and simulation of
their multi-wavelength emission
Authors: Nita, Gelu M.; Fleishman, Gregory; Kuznetsov, Alexey A.;
Loukitcheva, Maria A.; Viall, Nicholeen M.; Klimchuk, James A.; Gary,
Dale E.
2015TESS....131204N Altcode:
To facilitate the study of solar active regions, we have created a
synthetic modeling framework that combines 3D magnetic structures
obtained from magnetic extrapolations with simplified 1D thermal
models of the chromosphere, transition region, and corona. To handle,
visualize, and use such synthetic data cubes to compute multi-wavelength
emission maps and compare them with observations, we have
undertaken a major enhancement of our simulation tools, GX_Simulator
(ftp://sohoftp.nascom.nasa.gov/solarsoft/packages/gx_simulator/),
developed earlier for modeling emission from flaring loops. The greatly
enhanced, object-based architecture, which now runs on Windows, Mac,
and UNIX platform, offers important new capabilities that include the
ability to either import 3D density and temperature distribution models,
or to assign to each individual voxel numerically defined coronal
or chromospheric temperature and densities, or coronal Differential
Emission Measure distributions. Due to these new capabilities, the
GX_Simulator can now apply parametric heating models involving average
properties of the magnetic field lines crossing a given voxel volume,
as well as compute and investigate the spatial and spectral properties
of radio (to be compared with VLA or EOVSA data), (sub-)millimeter
(ALMA), EUV (AIA/SDO), and X-ray (RHESSI) emission calculated from the
model. The application integrates shared-object libraries containing
fast free-free, gyrosynchrotron, and gyroresonance emission codes
developed in FORTRAN and C++, and soft and hard X-ray and EUV codes
developed in IDL. We use this tool to model and analyze an active
region and compare the synthetic emission maps obtained in different
wavelengths with observations.This work was partially supported
by NSF grants AGS-1250374, AGS-1262772, NASA grant NNX14AC87G, the
Marie Curie International Research Staff Exchange Scheme "Radiosun"
(PEOPLE-2011-IRSES-295272), RFBR grants 14-02-91157, 15-02-01089,
15-02-03717, 15-02-03835, 15-02-08028.
---------------------------------------------------------
Title: Three-dimensional Radio and X-Ray Modeling and Data Analysis
Software: Revealing Flare Complexity
Authors: Nita, Gelu M.; Fleishman, Gregory D.; Kuznetsov, Alexey A.;
Kontar, Eduard P.; Gary, Dale E.
2015ApJ...799..236N Altcode: 2014arXiv1409.0896N
Many problems in solar physics require analysis of imaging data
obtained in multiple wavelength domains with differing spatial
resolution in a framework supplied by advanced three-dimensional
(3D) physical models. To facilitate this goal, we have undertaken a
major enhancement of our IDL-based simulation tools developed earlier
for modeling microwave and X-ray emission. The enhanced software
architecture allows the user to (1) import photospheric magnetic field
maps and perform magnetic field extrapolations to generate 3D magnetic
field models; (2) investigate the magnetic topology by interactively
creating field lines and associated flux tubes; (3) populate the flux
tubes with user-defined nonuniform thermal plasma and anisotropic,
nonuniform, nonthermal electron distributions; (4) investigate the
spatial and spectral properties of radio and X-ray emission calculated
from the model; and (5) compare the model-derived images and spectra
with observational data. The package integrates shared-object libraries
containing fast gyrosynchrotron emission codes, IDL-based soft and hard
X-ray codes, and potential and linear force-free field extrapolation
routines. The package accepts user-defined radiation and magnetic
field extrapolation plug-ins. We use this tool to analyze a relatively
simple single-loop flare and use the model to constrain the magnetic
3D structure and spatial distribution of the fast electrons inside this
loop. We iteratively compute multi-frequency microwave and multi-energy
X-ray images from realistic magnetic flux tubes obtained from pre-flare
extrapolations, and compare them with imaging data obtained by SDO,
NoRH, and RHESSI. We use this event to illustrate the tool's use
for the general interpretation of solar flares to address disparate
problems in solar physics.
---------------------------------------------------------
Title: Reduced Coronal Emission Above Large Isolated Sunspots
Authors: Ryabov, B. I.; Gary, D. E.; Peterova, N. G.; Shibasaki, K.;
Topchilo, N. A.
2015SoPh..290...21R Altcode: 2014SoPh..tmp..192R
We analysed specific regions of reduced soft X-ray and microwave
emission in five large isolated sunspots. The Nobeyama Radioheliograph
17 GHz observations reveal a local depression of microwave brightness
in the peripheral area of the sunspots. The depression regions appear
light (weak absorption) in the He 10830 Å line in areas with extended
(open) field lines, as indicated by potential field source surface
model (PFSS) extrapolations up to 1.5 R<SUB>⊙</SUB>. The observed
depressions of 3 - 8 % in ordinary mode at 17 GHz are interpreted as
resulting from free-free emission when the plasma density is lower
by 5 - 10 %. Our model estimates show that the decrease in density
in both the coronal and the lower layers above the depression region
accounts for the depression. These depression regions lend themselves
well to marking the location of outward plasma motions.
---------------------------------------------------------
Title: Constraining Solar Coronal Magnetic Fields with New Radio
Observing Techniques
Authors: Chen, B.; Gary, D. E.; Bastian, T.
2014AGUFMSH23C..07C Altcode:
Solar radio emission, produced by energetic electrons in the low-beta
solar corona, is highly dependent on coronal magnetic field strength
and direction, hence offering a rich potential for constraining coronal
magnetic fields. However, the observed radio intensity is contributed
by several different emission mechanisms and moreover, is known to
be sensitive to parameters other than the magnetic field, such as the
electron distribution function. Collectively, they introduce ambiguities
that are difficult to resolve in the absence of high-quality, broadband
radio dynamic imaging spectroscopic observations with sufficiently high
spatial, spectral, and temporal resolution. Such observations have
not been possible until very recently, thanks to the newly developed
radio instruments such as the Karl G. Jansky Very Large Array (VLA)
and the Expanded Owens Valley Solar Array (EOVSA). In this talk, we
will present first results obtained from the Jansky VLA demonstrating
their ability of constraining the coronal magnetic fields in active
regions. We will also introduce a forward-modeling tool currently under
development, which can be used to reconstruct the 3D coronal magnetic
fields based on the newly available radio observations.
---------------------------------------------------------
Title: VLA Observations of Solar Decimetric Spike Bursts: Direct
Signature of Accelerated Electrons in Reconnection Outflow Region
Authors: Chen, B.; Bastian, T.; Gary, D. E.
2014AGUFMSH23A4149C Altcode:
Solar decimetric spike bursts, which appear in a radio dynamic
spectrum as a cluster of short-lived and narrowband brightenings,
have been suggested as a possible signature of many, "elementary"
particle accelerations at or near a magnetic reconnection site. Their
dynamic spectral feature can be potentially used to diagnose important
parameters of the reconnection site such as plasma density and
spatial size of the fragmentation. Yet direct observational evidence
supporting this scenario has been elusive mainly due to the lack of
imaging observations. The upgraded Karl G. Jansky Very Large Array
(VLA) provides the first opportunity of performing simultaneous radio
imaging and dynamic spectroscopy, which allows radio sources to be
imaged at every spatio-temporal pixel in the dynamic spectrum. Here
we report Jansky VLA observations of decimetric spike bursts recorded
during an eruptive solar limb flare. Combined with EUV and X-ray data
from SDO and RHESSI, we show that the spike bursts coincide spatially
with a loop-top hard X-ray source, which are located in a region where
supra-arcade downflows meet the underlying hot, EUV/X-ray loops. We
interpret the observed spike bursts as a direct signature of non-thermal
electrons accelerated by turbulences and/or shocks in the reconnection
outflow region.
---------------------------------------------------------
Title: The South Pole, Antarctica, Solar Radio Telescope (SPASRT)
System
Authors: Gerrard, A. J.; Weatherwax, A. T.; Gary, D. E.; Kujawski,
J. T.; Nita, G. M.; Melville, R.; Stillinger, A.; Jeffer, G.
2014AGUFMSA13B3990G Altcode:
The study of the sun in the radio portion of the electromagnetic
spectrum furthers our understanding of fundamental solar processes
observed in the X-ray, UV, and visible regions of the spectrum. For
example, the study of solar radio bursts, which have been shown to
cause serious disruptions of technologies at Earth, are essential for
advancing our knowledge and understanding of solar flares and their
relationship to coronal mass ejections and solar energetic particles, as
well as the underlying particle acceleration mechanisms associated with
these processes. In addition, radio coverage of the solar atmosphere
could yield completely new insights into the variations of output solar
energy, including Alfven wave propagation through the solar atmosphere
and into the solar wind, which can potentially modulate and disturb the
solar wind and Earth's geospace environment. In this presentation we
discuss the development, construction, and testing of the South Pole,
Antarctica, Solar Radio Telescope that is planned for installation at
South Pole. The system will allow for 24-hour continuous, long-term
observations of the sun across the 1-18 GHz frequency band and allow
for truly continuous solar observations. We show that this system will
enable unique scientific investigations of the solar atmosphere.
---------------------------------------------------------
Title: Observation of 2011-02-15 X2.2 Flare in Hard X-Ray and
Microwave
Authors: Kuroda, N.; Wang, H.; Gary, D. E.
2014AGUFMSH41C4153K Altcode:
Previous studies have shown that the energy release mechanism of some
solar flares follow the Standard magnetic-reconnection model, but the
detailed properties of high-energy electrons produced in the flare
are still not well understood. We conducted a unique, multi-wavelength
study that discloses the spatial, temporal and energy distributions of
the accelerated electrons in the X2.2 solar flare on Feb. 15, 2011. We
studied the source locations of the observed seven temporal peaks in
hard X-ray (HXR) and microwave (MW) lightcurves using the Reuven Ramaty
High Energy Solar Spectroscopic Imager (RHESSI) in 50-75 keV channels
and Nobeyama Radioheliograph (NoRH) in 34 GHz, respectively. We found
that peak emissions were coming from two spatially distinct sites
in HXR and MW, and in HXR we observed a sudden change between these
sites over the second and the third peak. Comparison between the HXR
lightcurve and the temporal variations in MW source kernel intensity
also confirmed that seven peak emissions were actually coming from
two sources, each with their own multiple peaks.
---------------------------------------------------------
Title: Direct Evidence of an Eruptive, Filament-hosting Magnetic
Flux Rope Leading to a Fast Solar Coronal Mass Ejection
Authors: Chen, Bin; Bastian, T. S.; Gary, D. E.
2014ApJ...794..149C Altcode: 2014arXiv1408.6473C
Magnetic flux ropes (MFRs) are believed to be at the heart of solar
coronal mass ejections (CMEs). A well-known example is the prominence
cavity in the low corona that sometimes makes up a three-part
white-light (WL) CME upon its eruption. Such a system, which is
usually observed in quiet-Sun regions, has long been suggested to
be the manifestation of an MFR with relatively cool filament material
collecting near its bottom. However, observational evidence of eruptive,
filament-hosting MFR systems has been elusive for those originating
in active regions. By utilizing multi-passband extreme-ultraviolet
(EUV) observations from Solar Dynamics Observatory/Atmospheric Imaging
Assembly, we present direct evidence of an eruptive MFR in the low
corona that exhibits a hot envelope and a cooler core; the latter is
likely the upper part of a filament that undergoes a partial eruption,
which is later observed in the upper corona as the coiled kernel of a
fast, WL CME. This MFR-like structure exists more than 1 hr prior to
its eruption, and displays successive stages of dynamical evolution, in
which both ideal and non-ideal physical processes may be involved. The
timing of the MFR kinematics is found to be well correlated with the
energy release of the associated long-duration C1.9 flare. We suggest
that the long-duration flare is the result of prolonged energy release
associated with the vertical current sheet induced by the erupting MFR.
---------------------------------------------------------
Title: Fitting FFT-derived Spectra: Theory, Tool, and Application
to Solar Radio Spike Decomposition
Authors: Nita, Gelu M.; Fleishman, Gregory D.; Gary, Dale E.; Marin,
William; Boone, Kristine
2014ApJ...789..152N Altcode: 2014arXiv1406.2280N
Spectra derived from fast Fourier transform (FFT) analysis of
time-domain data intrinsically contain statistical fluctuations whose
distribution depends on the number of accumulated spectra contributing
to a measurement. The tail of this distribution, which is essential
for separating the true signal from the statistical fluctuations,
deviates noticeably from the normal distribution for a finite number
of accumulations. In this paper, we develop a theory to properly
account for the statistical fluctuations when fitting a model to a
given accumulated spectrum. The method is implemented in software for
the purpose of automatically fitting a large body of such FFT-derived
spectra. We apply this tool to analyze a portion of a dense cluster
of spikes recorded by our FASR Subsystem Testbed instrument during a
record-breaking event that occurred on 2006 December 6. The outcome
of this analysis is briefly discussed.
---------------------------------------------------------
Title: WIMAGR: An Interactive SSW IDL Tool for Mapping OVSA Legacy
Microwave Interferometry Data
Authors: Nita, Gelu M.; Fleishman, Gregory D.; Gary, Dale E.
2014AAS...22421845N Altcode:
The Owens Valley Solar Array (OVSA), which is currently the subject
of a major upgrade leading to the new Expanded Owens Valley Solar
Array (EOVSA), has operated between the years 2000-2007 as a 5-7
antenna solar-dedicated radio interferometer, with daily observations
at typically 40 frequencies in the microwave frequency range, 1-18
GHz. Given the importance of these unique data for complementary studies
with data taken during the same period by Yohkoh, RHESSI, SoHO, TRACE,
Hinode and other NASA spacecraft, we have undertaken an effort to
maximize their usefulness, ease of use, and longevity by creating
a uniform, calibrated OVSA legacy database and community-friendly,
SSW-based software, compatible with the RHESSI and EOVSA software
packages. With these efforts, we can anticipate that the data
will continue serving the community well into the future. In this
presentation, we will introduce one of the recent upgrades of the
OVSA SSW software package, WIMAGR, whose interface allows the user
to generate OVSA radio maps in intensity and polarization at many
available frequencies with a spatial resolution about 3” at 18
GHz, which is comparable with the spatial resolution of other imaging
instruments. To illustrate the main capabilities of this software tool
and its potential for promoting scientific discovery, we will present a
real-time computation of a sequence of multi-frequency OVSA microwave
maps and compare them with images obtained by other instruments. This
work was supported in part by NSF grants AGS-1250374, and NASA grants
NNX11AB49G and NNX14AC87G to New Jersey Institute of Technology
---------------------------------------------------------
Title: Coronal Magnetography of a Simulated Solar Active Region from
Microwave Imaging Spectropolarimetry
Authors: Wang, Zhitao; Gary, Dale E.; White, Stephen M.
2014AAS...22432345W Altcode:
We have simulated the Expanded Owens Valley Solar Array (EOVSA) radio
images generated at multiple frequencies from a model solar active
region, embedded in a realistic solar disk model, and evaluated
the resulting datacube for different spectral analysis schemes to
evaluate the potential for realizing one of EOVSA's most important
scientific goals — coronal magnetography. In this paper, we focus on
modeling the gyroresonance and free-free emission from an on-disk solar
active region model with realistic complexities in electron density,
temperature and magnetic field distribution. We compare the magnetic
field parameters extrapolated from the image datacube along each line
of sight after folding through the EOVSA instrumental profile with the
original (unfolded) parameters used in the model. We find that even
the most easily automated, image-based analysis approach (Level 0)
provides reasonable quantitative results, although they are affected
by systematic effects due to finite sampling in the Fourier (uv) plane.
---------------------------------------------------------
Title: Observation of 2011-02-15 X2.2 solar flare in Hard X-ray
and microwave
Authors: Kuroda, Natsuha; Wang, Haimin; Gary, Dale E.
2014AAS...22412332K Altcode:
Previous studies have shown that the energy release mechanism of some
solar flares follow the Standard magnetic-reconnection model, but the
detailed properties of high-energy electrons produced in the flare
are still not well understood. We conducted a unique, multi-wavelength
study that discloses the spatial, temporal and energy distributions of
the accelerated electrons in the X2.2 solar flare on Feb. 15, 2011. We
studied the source locations of the observed seven temporal peaks
in hard X-ray (HXR) and microwave (MW) lightcurves using the Reuven
Ramaty High Energy Solar Spectroscopic Imager (RHESSI) in 50-75 keV
channels and Nobeyama Radioheliograph (NoRH) in 34 GHz, respectively. We
confirmed that peak emissions were coming from two spatially distinct
sites in HXR and MW, and in HXR we observed a sudden change between
these sites over the second and the third peak. Comparison between
the HXR lightcurve and the temporal variations in MW source kernel
intensity also confirmed that seven peak emissions were actually coming
from two sources, each with their own multiple peaks.
---------------------------------------------------------
Title: The Expanded Owens Valley Solar Array (EOVSA)
Authors: Gary, Dale E.; Hurford, Gordon J.; Nita, Gelu M.; White,
Stephen M.; McTiernan, James; Fleishman, Gregory D.
2014AAS...22412360G Altcode:
The Expanded Owens Valley Solar Array (EOVSA) near Big Pine, CA is
undergoing commissioning as a solar-dedicated microwave imaging array
operating in the frequency range 2.5-18 GHz. The solar science to be
addressed focuses on the 3D structure of the solar corona (magnetic
field, temperature and density), and on the particle acceleration,
transport and heating in solar flares. The project will support
the scientific community by providing open data access and software
tools for analysis and modeling of the data, to exploit synergies with
on-going solar research in other wavelengths. The array consists of a
total of 15 antennas, including the two 27-m antennas with He-cooled
receivers for sensitive calibration, and thirteen 2.1-m antennas that
each view the entire disk of the Sun. The system includes a completely
new control system, broadband signal transmission, and high-speed
digital signal processing, using new technology developed for the
Frequency Agile Solar Radiotelescope (FASR). We present an overview
of the instrument, the current status of commissioning activities,
and some initial observations to assess performance.This research is
supported by NSF grants AST-1312802, and NASA grants NNX11AB49G and
NNX10AF27G to New Jersey Institute of Technology.
---------------------------------------------------------
Title: Acceleration Regions Jointly Observed with Microwave and
X-Ray Imaging Spectroscopy in a Number of Solar Flares
Authors: Fleishman, Gregory D.; Kontar, Eduard; Nita, Gelu M.; Gary,
Dale E.
2014AAS...22412312F Altcode:
Detection of acceleration regions in solar flares has proved challenging
for many reasons, in particular, because the X-ray emission is weighted
by denser regions of the flare volume, although the acceleration can
take place in a tenuous region, while the microwave emission, which
can be significant even from the tenuous regions, is often dominated
by a (looptop) trapped population, rather than the acceleration region
itself. For these reasons we undertook a systematic database search
to identify events that do not show a significant trapped component
and at the same time show evidence of the source uniformity, which
simplifies the data analysis greatly. Initially, we identified a
subset of more than 20 radio bursts with a relatively narrow spectrum,
having the low- and high- frequency spectral indices larger than 3 by
the absolute value. That steep low-frequency spectrum implies that the
emission is nonthermal (for the thermal emission the spectral index is
supposed to be 2 or flatter), and the source is reasonably dense and
uniform. The steep high-frequency spectrum implies that no significant
electron trapping occurs; otherwise a progressive spectral flattening
would be observed. Roughly half of these radio bursts have RHESSI data,
which allows for a detail joint diagnostics of the source parameters and
evolution. Based on the studied radio-to-X-ray spatial relationships,
timing, and spectral fits we do conclude that we deal here with emission
from directly the acceleration regions. We discuss the implications
of these observations for the acceleration mechanism involved. We also
discuss further strategy of how to detect the acceleration region with
the currently available observational means. This work was supported
in part by NSF grants AGS-1250374, and NASA grants NNX11AB49G and
NNX14AC87G to New Jersey Institute of Technology.
---------------------------------------------------------
Title: Solar Activity and Motions in the Solar Chromosphere and
Corona at the 2012 and 2013 Total and Annular Eclipses in the U.S.,
Australia, and Africa
Authors: Pasachoff, Jay M.; Babcock, B. A.; Davis, A. B.; Demianski,
M.; Lucas, R.; Lu, M.; Dantowitz, R.; Rusin, V.; Saniga, M.; Seaton,
D. B.; Gaintatzis, P.; Voulgaris, A.; Seiradakis, J. H.; Gary, D. E.;
Shaik, S. B.
2014AAS...22311801P Altcode:
Our studies of the solar chromosphere and corona at the 2012 and
2013 eclipses shortly after cycle maximum 24 (2011/2012) of solar
activity (see: http://www.swpc.noaa.gov/SolarCycle/) involved radio
observations of the 2012 annular eclipse with the Jansky Very Large
Array, optical observations of the 2012 total eclipse from Australia,
optical observations of the 2013 annular eclipse from Tennant Creek,
Australia, and the 3 November 2013 total solar eclipse from Gabon. Our
observations are coordinated with those from solar spacecraft: Solar
Dynamics Observatory AIA and HMI, Hinode XRT and SOT, SOHO LASCO and
EIT, PROBA2 SWAP, and STEREO SECCHI. Our 2012 totality observations
include a CME whose motion was observed with a 37-minute interval. We
include first results from the expedition to Gabon for the 3 November
2013 eclipse, a summary of eclipse results from along the path of
totality across Africa, and a summary of the concomitant spacecraft
observations. The Williams College 2012 expeditions were supported in
part by NSF grant AGS-1047726 from Solar Terrestrial Research/NSF AGS,
and by the Rob Spring Fund and Science Center funds at Williams. The
JVLA is supported by the NSF. The Williams College 2013 total-eclipse
expedition was supported in part by grant 9327-13 from the Committee
for Research and Exploration of the National Geographic Society. ML was
also supported in part by a Grant-In-Aid of Research from the National
Academy of Sciences, administered by Sigma Xi, The Scientific Research
Society (Grant ID: G20120315159311). VR and MS acknowledge support
for 2012 from projects VEGA 2/0003/13 and NGS-3139-12 of the National
Geographic Society. We are grateful to K. Shiota (Japan) for kindly
providing us with some of his 2012 eclipse coronal images. We thank
Alec Engell (Montana State U) for assistance on site, and Terry Cuttle
(Queensland Amateur Astronomers) for help with site arrangements. We
thank Aram Friedman (Ansible Technologies), Michael Kentrianakis,
and Nicholas Weber (Dexter Southfield School) for collaboration on
imaging at the Australian total eclipse.
---------------------------------------------------------
Title: Probing Magnetic Energy Release in a Solar Flare with Radio
Dynamic Imaging Spectroscopy
Authors: Chen, Bin; Bastian, T. S.; Gary, D. E.; White, S. M.
2014AAS...22311804C Altcode:
Solar flares involve sudden release of magnetic energy that is
previously stored in the Sun's corona. Yet details of the flare
energy release processes are still poorly understood. Solar radio
bursts are intense and short-lived radio emissions that occur in
solar flares. They are believed to be intimately related to flare
energy release processes. However, their potential in diagnosing flare
energy release has been greatly limited by the lack of simultaneous
spatial information. The upgraded Karl G. Jansky Very Large Array (VLA)
provides the first opportunity of radio synthesis imaging along with
high spectral and temporal resolution, making the new technique of radio
dynamic imaging spectroscopy possible. We report VLA observations of a
solar flare event using this new technique, during which a rich variety
of radio bursts are recorded. With the help of concurrent data in
extreme ultra-violet and X-ray wavelengths, these observations allow us
to establish the relation between the bursts and flare energy release,
and use them to probe physical properties of the energy release site.
---------------------------------------------------------
Title: Advanced approaches to modeling and analysis of radio
imaging data
Authors: Fleishman, Gregory; Nita, Gelu; Gary, Dale
2014cosp...40E.878F Altcode:
High-resolution imaging radio observations have an amazingly strong,
but yet largely unexploited, potential for probing solar corona and
chromosphere. The rich variety of radio emission processes offers
multiple ways of detecting and tracking electron beams, studying
chromospheric and coronal thermal structure, plasma processes, particle
acceleration and transport, and measuring magnetic fields. Currently
existing, real radio instruments have various kinds of imperfections
that severely limit their diagnostics ability. Examples of the
imperfections include a small number of spectral channels (low frequency
resolution) or low (and frequency-dependent) spatial resolution. These
and other issues call the practicality of the precise imaging
diagnostics into question. To clarify this question the performance
of a given radio instrument for quantitative diagnostics has to be
properly evaluated, which can be done via realistic 3D modeling. This
report presents advanced modeling and forward fitting tools, which are
based on realistic (extrapolated) 3D magnetic structure and powerful,
recently developed fast gyrosynchrotron and gyroresonant computing
codes. We are going to demonstrate creation of a 3D model in real
time and compute different emissions from this model—radio, EUV,
and X-ray. Then, we show how this ‘ideal’ imaging data will look
when folded with a point spread function of a given radio interferometer
(Expanded Owens Valley Solar Array, EOVSA, to give a specific example),
and finally apply pixel-by-pixel forward fitting to give a quantitative
assessment of the EOVSA ability to derive the magnetic field, fast
electron, and thermal plasma data from the imaging spectropolarimetry
measurements. To conclude, we also present the diagnostics derived
from the use of real OVSA/EOVSA imaging spectroscopy data and discuss
the microwave diagnostics perspectives in a broader context of modern
solar physics.
---------------------------------------------------------
Title: Mapping the 3D Coronal Magnetic Field and Temperature Structure
Using Radio Techniques
Authors: Gary, Dale; Fleishman, Gregory; Nita, Gelu; Wang, Zhitao
2014cosp...40E.945G Altcode:
Solar radio emission is highly dependent on the magnetic field
strength and direction in the corona, and at the same time provides a
direct measure of electron temperature/energy. A variety of radiation
mechanisms contribute to the emission, each with their own dependences,
and which mechanism dominates in a particular circumstance is determined
by electron energetics, magnetic field strength and direction,
and density. The key to accessing the inherent diagnostic power of
radio emission is to have multi-frequency radio images of sufficient
quality, resolution, and polarization purity, with which to confidently
identify and separate the different emission mechanisms, to remove the
ambiguities that have limited many past radio studies. The technical
challenge of obtaining high-quality, multi-frequency images is being
addressed for the first time by a number of new radio instruments such
as EOVSA, JVLA, CSRH, USSRT, and FASR, each of which are designed
specifically for this purpose. This report emphasizes one of the
more important applications—determining the three-dimensional
coronal magnetic field and temperature structure of solar active
regions. We illustrate the talk with new and accurate 3D modeling
of radio emission and radiative transfer, based on the nonlinear
force-free field extrapolation of vector magnetic field measurements,
and describe the potent combination of new observations with forward
fitting tools in development.
---------------------------------------------------------
Title: Future Trends in Solar Radio Astronomy and Coronal
Magnetic-Field Measurements
Authors: Fleishman, Gregory; Nita, Gelu; Gary, Dale
2014cosp...40E.877F Altcode:
Solar radio astronomy has an amazingly rich, but yet largely
unexploited, potential for probing the solar corona and
chromosphere. Radio emission offers multiple ways of detecting and
tracking electron beams, studying chromospheric and coronal thermal
structure, plasma processes, particle acceleration, and measuring
magnetic fields. To turn the mentioned potential into real routine
diagnostics, two major components are needed: (1) well-calibrated
observations with high spatial, spectral, and temporal resolutions
and (2) accurate and reliable theoretical models and fast numerical
tools capable of recovering the emission source parameters from the
radio data. This report gives a brief overview of the new, expanded,
and planned radio facilities, such as Expanded Owens Valley Solar
Array (EOVSA), Jansky Very Large Array (JVLA), Chinese Solar Radio
Heliograph (CSRH), Upgraded Siberian Solar Radio Telescope (USSRT),
and Frequency Agile Solar Radiotelescope (FASR) with the emphasis on
their ability to measure the coronal magnetic fields in active regions
and flares. In particular, we emphasize the new tools for 3D modeling
of the radio emission and forward fitting tools in development needed
to derive the magnetic field data from the radio measurements.
---------------------------------------------------------
Title: Magnetography of Solar Flaring Loops with Microwave Imaging
Spectropolarimetry
Authors: Gary, D. E.; Fleishman, G. D.; Nita, G. M.
2013SoPh..288..549G Altcode: 2013arXiv1303.1573G
We have developed a general framework for modeling gyrosynchrotron
and free-free emission from solar flaring loops and used it to test
the premise that 2D maps of source parameters, particularly the
magnetic field, can be deduced from spatially resolved microwave
spectropolarimetry data. We show quantitative results for a
flaring loop with a realistic magnetic geometry, derived from a
magnetic-field extrapolation, and containing an electron distribution
with typical thermal and nonthermal parameters, after folding through
the instrumental profile of a realistic interferometric array. We
compare the parameters generated from forward-fitting a homogeneous
source model to each line of sight through the folded image data cube
both with the original parameters used in the model and with parameters
generated from forward-fitting a homogeneous source model to the
original (unfolded) image data cube. We find excellent agreement in
general, but with systematic effects that can be understood as due
to the finite resolution in the folded images and the variation of
parameters along the line of sight, which are ignored in the homogeneous
source model. We discuss the use of such 2D parameter maps within a
larger framework of 3D modeling, and the prospects for applying these
methods to data from a new generation of multifrequency radio arrays
now or soon to be available.
---------------------------------------------------------
Title: Probing Dynamics of Electron Acceleration with Radio and X-Ray
Spectroscopy, Imaging, and Timing in the 2002 April 11 Solar Flare
Authors: Fleishman, Gregory D.; Kontar, Eduard P.; Nita, Gelu M.;
Gary, Dale E.
2013ApJ...768..190F Altcode: 2013arXiv1303.4098F
Based on detailed analysis of radio and X-ray observations of a flare
on 2002 April 11 augmented by realistic three-dimensional modeling,
we have identified a radio emission component produced directly
at the flare acceleration region. This acceleration region radio
component has distinctly different (1) spectrum, (2) light curves, (3)
spatial location, and, thus, (4) physical parameters from those of the
separately identified trapped or precipitating electron components. To
derive evolution of physical parameters of the radio sources we
apply forward fitting of the radio spectrum time sequence with the
gyrosynchrotron source function with five to six free parameters. At the
stage when the contribution from the acceleration region dominates the
radio spectrum, the X-ray- and radio-derived electron energy spectral
indices agree well with each other. During this time the maximum energy
of the accelerated electron spectrum displays a monotonic increase with
time from ~300 keV to ~2 MeV over roughly one minute duration indicative
of an acceleration process in the form of growth of the power-law tail;
the fast electron residence time in the acceleration region is about
2-4 s, which is much longer than the time of flight and so requires a
strong diffusion mode there to inhibit free-streaming propagation. The
acceleration region has a relatively strong magnetic field, B ~ 120 G,
and a low thermal density, n<SUB>e</SUB> <~ 2 × 10<SUP>9</SUP>
cm<SUP>-3</SUP>. These acceleration region properties are consistent
with a stochastic acceleration mechanism.
---------------------------------------------------------
Title: Time delays between radio and HXR observed during the
2011-02-15 X2.2 solar flare
Authors: Kuroda, Natsuha; Wang, Haimin; Gary, Dale
2013enss.confE..86K Altcode:
We report the second-scale delays observed between radio and HXR during
the X2.2 solar flare on February 15, 2011. The delays between different
energy channels from various instruments were previously reported by
Dolla et al. in 2012; they reported that the Reuven Ramaty High Energy
Solar Spectroscopic Imager (RHESSI) channels in the 25-100 keV range
lead all the other channels including the Nobeyama RadioPolarimeters
at 9 and 17 GHz and the Extreme Ultra-Violet (EUV) channels of the Euv
SpectroPhotometer (ESP) onboard the Solar Dynamic Observatory (SDO). We
looked at RHESSI channels in 25-250 keV and Nobeyama channels in 1-35
GHz and found that, in four data pairs that showed more than 80 %
correlation, the same several RHESSI peaks lead Nobeyama peaks by few
seconds. We discuss the possible physical meaning of this delay.
---------------------------------------------------------
Title: Tracing Electron Beams in the Sun's Corona with Radio Dynamic
Imaging Spectroscopy
Authors: Chen, Bin; Bastian, T. S.; White, S. M.; Gary, D. E.; Perley,
R.; Rupen, M.; Carlson, B.
2013ApJ...763L..21C Altcode: 2012arXiv1211.3058C
We report observations of type III radio bursts at decimeter wavelengths
(type IIIdm bursts)—signatures of suprathermal electron beams
propagating in the low corona—using the new technique of radio dynamic
imaging spectroscopy provided by the recently upgraded Karl G. Jansky
Very Large Array. For the first time, type IIIdm bursts were imaged with
high time and frequency resolution over a broad frequency band, allowing
electron beam trajectories in the corona to be deduced. Together with
simultaneous hard X-ray and extreme ultraviolet observations, we show
that these beams emanate from an energy release site located in the
low corona at a height below ~15 Mm, and propagate along a bundle of
discrete magnetic loops upward into the corona. Our observations enable
direct measurements of the plasma density along the magnetic loops,
and allow us to constrain the diameter of these loops to be less than
100 km. These overdense and ultra-thin loops reveal the fundamentally
fibrous structure of the Sun's corona. The impulsive nature of the
electron beams, their accessibility to different magnetic field lines,
and the detailed structure of the magnetic release site revealed by
the radio observations indicate that the localized energy release is
highly fragmentary in time and space, supporting a bursty reconnection
model that involves secondary magnetic structures for magnetic energy
release and particle acceleration.
---------------------------------------------------------
Title: Observing the Sun at Radio Wavelengths: Current Status and
Future Prospects
Authors: Bastian, T.; Gary, D.
2012IAUSS...6E.215B Altcode:
Radio emission from the Sun offers the means of probing thermal
and non-thermal processes in the chromosphere and corona using
diagnostics that are largely complementary to those available at optical
wavelengths. This talk briefly reviews radio diagnostics and observing
techniques and several of the science objectives motivating new and
planned instrumentation, including coronal magnetography, magnetic
energy release and particle acceleration, drivers of space weather, and
the quiet solar atmosphere. New instrumentation includes ALMA, opening
a new spectral window at millimeter and submillimeter wavelengths; and
the Jansky Very Large Array (JVLA), a powerful new instrument operating
at centimeter and decimeter wavelengths. A solar-dedicated instrument
under construction is the Expanded Owens Valley Solar Array (EOVSA);
and a planned, new, solar-dedicated facility is the Frequency Agile
Solar Radiotelescope (FASR). EOVSA will soon pioneer dynamic imaging
spectroscopy techniques. FASR will fully exploit these techniques as
a general purpose radioheliograph designed to perform ultra-wideband
dynamic imaging spectroscopy.
---------------------------------------------------------
Title: Integrated Idl Tool For 3d Modeling And Imaging Data Analysis
Authors: Nita, Gelu M.; Fleishman, G. D.; Gary, D. E.; Kuznetsov,
A. A.; Kontar, E. P.
2012AAS...22020451N Altcode:
Addressing many key problems in solar physics requires detailed analysis
of non-simultaneous imaging data obtained in various wavelength domains
with different spatial resolution and their comparison with each
other supplied by advanced 3D physical models. To facilitate achieving
this goal, we have undertaken a major enhancement and improvements of
IDL-based simulation tools developed earlier for modeling microwave
and X-ray emission. The greatly enhanced object-based architecture
provides interactive graphic user interface that allows the user
i) to import photospheric magnetic field maps and perform magnetic
field extrapolations to almost instantly generate 3D magnetic field
models, ii) to investigate the magnetic topology of these models by
interactively creating magnetic field lines and associated magnetic
field tubes, iii) to populate them with user-defined nonuniform thermal
plasma and anisotropic nonuniform nonthermal electron distributions;
and iv) to calculate the spatial and spectral properties of radio and
X-ray emission. The application integrates DLL and Shared Libraries
containing fast gyrosynchrotron emission codes developed in FORTRAN and
C++, soft and hard X-ray codes developed in IDL, and a potential field
extrapolation DLL produced based on original FORTRAN code developed
by V. Abramenko and V. Yurchishin. The interactive interface allows
users to add any user-defined IDL or external callable radiation code,
as well as user-defined magnetic field extrapolation routines. To
illustrate the tool capabilities, we present a step-by-step live
computation of microwave and X-ray images from realistic magnetic
structures obtained from a magnetic field extrapolation preceding a
real event, and compare them with the actual imaging data produced
by NORH and RHESSI instruments. <P />This work was supported in part
by NSF grants AGS-0961867, AST-0908344, AGS-0969761, and NASA grants
NNX10AF27G and NNX11AB49G to New Jersey Institute of Technology, by a
UK STFC rolling grant, the Leverhulme Trust, UK, and by the European
Commission through the Radiosun and HESPE Networks.
---------------------------------------------------------
Title: Expanded Owens Valley Solar Array (EOVSA) Testbed and Prototype
Authors: Gary, Dale E.; Nita, G. M.; Sane, N.
2012AAS...22020430G Altcode:
NJIT is engaged in constructing a new solar-dedicated radio array,
the Expanded Owens Valley Solar Array (EOVSA), which is slated for
completion in late 2013. An initial 3-antenna array, the EOVSA Subsystem
Testbed (EST), is now in operation from 1-9 GHz based on three of the
old OVSA antennas, to test certain design elements of the new array. We
describe this instrument and show some results from recent solar flares
observed with it. We also describe plans for an upcoming prototype of
EOVSA, which will use three antennas of the new design over the full
1-18 GHz signal chain of the entirely new system. The EOVSA prototype
will be in operation by late 2012. Highlights of the new design are
ability to cover the entire 1-18 GHz in less than 1 s, simultaneous
dual polarization, and improved sensitivity and stability. We discuss
what can be expected from the prototype, and how it will compare
with the full 13-antenna EOVSA. This work was supported by NSF grants
AGS-0961867 and AST-0908344, and NASA grant NNX11AB49G to New Jersey
Institute of Technology.
---------------------------------------------------------
Title: Novel 3D Approach to Flare Modeling via Interactive IDL
Widget Tools
Authors: Nita, G. M.; Fleishman, G. D.; Gary, D. E.; Kuznetsov, A.;
Kontar, E. P.
2011AGUFMSH44A..07N Altcode:
Currently, and soon-to-be, available sophisticated 3D models of particle
acceleration and transport in solar flares require a new level of
user-friendly visualization and analysis tools allowing quick and
easy adjustment of the model parameters and computation of realistic
radiation patterns (images, spectra, polarization, etc). We report the
current state of the art of these tools in development, already proved
to be highly efficient for the direct flare modeling. We present an
interactive IDL widget application intended to provide a flexible tool
that allows the user to generate spatially resolved radio and X-ray
spectra. The object-based architecture of this application provides
full interaction with imported 3D magnetic field models (e.g., from an
extrapolation) that may be embedded in a global coronal model. Various
tools provided allow users to explore the magnetic connectivity of the
model by generating magnetic field lines originating in user-specified
volume positions. Such lines may serve as reference lines for creating
magnetic flux tubes, which are further populated with user-defined
analytical thermal/non thermal particle distribution models. By
default, the application integrates IDL callable DLL and Shared
libraries containing fast GS emission codes developed in FORTRAN
and C++ and soft and hard X-ray codes developed in IDL. However, the
interactive interface allows interchanging these default libraries
with any user-defined IDL or external callable codes designed to
solve the radiation transfer equation in the same or other wavelength
ranges of interest. To illustrate the tool capacity and generality,
we present a step-by-step real-time computation of microwave and X-ray
images from realistic magnetic structures obtained from a magnetic
field extrapolation preceding a real event, and compare them with the
actual imaging data obtained by NORH and RHESSI instruments. We discuss
further anticipated developments of the tools needed to accommodate
temporal evolution of the magnetic field structure and/or fast electron
population implied by the electron acceleration and transport. This work
was supported in part by NSF grants AGS-0961867, AST-0908344, and NASA
grants NNX10AF27G and NNX11AB49G to New Jersey Institute of Technology,
by a UK STFC rolling grant, STFC/PPARC Advanced Fellowship, and the
Leverhulme Trust, UK. Financial support by the European Commission
through the SOLAIRE and HESPE Networks is gratefully acknowledged.
---------------------------------------------------------
Title: Three-dimensional Structure of Microwave Sources from Solar
Rotation Stereoscopy Versus Magnetic Extrapolations
Authors: Nita, Gelu M.; Fleishman, Gregory D.; Jing, Ju; Lesovoi,
Sergey V.; Bogod, Vladimir M.; Yasnov, Leonid V.; Wang, Haimin; Gary,
Dale E.
2011ApJ...737...82N Altcode: 2011arXiv1106.0262N
We use rotation stereoscopy to estimate the height of a steady-state
solar feature relative to the photosphere, based on its apparent motion
in the image plane recorded over several days of observation. The
stereoscopy algorithm is adapted to work with either one-
or two-dimensional data (i.e., from images or from observations
that record the projected position of the source along an arbitrary
axis). The accuracy of the algorithm is tested on simulated data, and
then the algorithm is used to estimate the coronal radio source heights
associated with the active region NOAA 10956, based on multifrequency
imaging data over seven days from the Siberian Solar Radio Telescope
near 5.7 GHz, the Nobeyama Radio Heliograph at 17 GHz, as well as
one-dimensional scans at multiple frequencies spanning the 5.98-15.95
GHz frequency range from the RATAN-600 instrument. The gyroresonance
emission mechanism, which is sensitive to the coronal magnetic field
strength, is applied to convert the estimated radio source heights
at various frequencies, h(f), to information about magnetic field
versus height B(h), and the results are compared to a magnetic field
extrapolation derived from photospheric magnetic field observations
obtained by Hinode and Michelson Doppler Imager. We found that the
gyroresonant emission comes from heights exceeding the location of the
third gyrolayer irrespective of the magnetic extrapolation method;
implications of this finding for coronal magnetography and coronal
plasma physics are discussed.
---------------------------------------------------------
Title: Spatially and Spectrally Resolved Observations of a Zebra
Pattern in a Solar Decimetric Radio Burst
Authors: Chen, Bin; Bastian, T. S.; Gary, D. E.; Jing, Ju
2011ApJ...736...64C Altcode: 2011arXiv1105.0715C
We present the first interferometric observation of a zebra-pattern
radio burst with simultaneous high spectral (≈1 MHz) and high time
(20 ms) resolution. The Frequency-Agile Solar Radiotelescope Subsystem
Testbed (FST) and the Owens Valley Solar Array (OVSA) were used in
parallel to observe the X1.5 flare on 2006 December 14. By using
OVSA to calibrate the FST, the source position of the zebra pattern
can be located on the solar disk. With the help of multi-wavelength
observations and a nonlinear force-free field extrapolation, the zebra
source is explored in relation to the magnetic field configuration. New
constraints are placed on the source size and position as a function
of frequency and time. We conclude that the zebra burst is consistent
with a double-plasma resonance model in which the radio emission occurs
in resonance layers where the upper-hybrid frequency is harmonically
related to the electron cyclotron frequency in a coronal magnetic loop.
---------------------------------------------------------
Title: New interactive solar flare modeling and advanced radio
diagnostics tools
Authors: Fleishman, Gregory D.; Nita, Gelu M.; Gary, Dale E.
2011IAUS..274..280F Altcode: 2010arXiv1011.0237F
The coming years will see routine use of solar data of unprecedented
spatial and spectral resolution, time cadence, and completeness in
the wavelength domain. To capitalize on the soon to be available radio
facilities such as the expanded OVSA, SSRT and FASR, and the challenges
they present in the visualization and synthesis of the multi-frequency
datasets, we propose that realistic, sophisticated 3D active region
and flare modeling is timely now and will be a forefront of coronal
studies over the coming years. Here we summarize our 3D modeling
efforts, aimed at forward fitting of imaging spectroscopy data, and
describe currently available 3D modeling tools. We also discuss plans
for future generalization of our modeling tools.
---------------------------------------------------------
Title: Detection of the Acceleration Site in a Solar Flare
Authors: Fleishman, Gregory D.; Kontar, E. P.; Nita, G. M.; Gary, D. E.
2011SPD....42.1203F Altcode: 2011BAAS..43S.1203F
We report the observation of an unusual cold, tenuous solar flare (ApJL,
v. 731, p. L19, 2011), which reveals itself via numerous and prominent
non-thermal manifestations, while lacking any noticeable thermal
emission signature. RHESSI hard X-rays and 0.1-18 GHz radio data from
OVSA and Phoenix-2 show copious electron acceleration (10<SUP>35</SUP>
electrons per second above 10 keV) typical for GOES M-class flares with
electrons energies up to 100 keV, but GOES temperatures not exceeding
6.1 MK. The HXR footpoints and coronal radio sources belong, supposedly,
to a single magnetic loop, which departs strongly from the corresponding
potential loop (obtained from a photospheric extrapolation) in agreement
with the apparent need of a non-potential magnetic field structure to
produce a flare. The imaging, temporal, and spectral characteristics
of the flare have led us to a firm conclusion that the bulk of the
microwave continuum emission from this flare was produced directly
in the acceleration region. We found that the electron acceleration
efficiency is very high in the flare, so almost all available thermal
electrons are eventually accelerated. However, given a relatively small
flaring volume and rather low thermal density at the flaring loop, the
total energy release turned out to be insufficient for a significant
heating of the coronal plasma or for a prominent chromospheric response
giving rise to chromospheric evaporation. Some sort of stochastic
acceleration process is needed to account for an approximately
energy-independent lifetime of about 3 s for the electrons in the
acceleration region. <P />This work was supported in part by NSF grants
AGS-0961867, AST-0908344, and NASA grants NNX10AF27G and NNX11AB49G
to New Jersey Institute of Technology. This work was supported by a UK
STFC rolling grant, STFC/PPARC Advanced Fellowship, and the Leverhulme
Trust, UK. Financial support by the European Commission through the
SOLAIRE and HESPE Networks is gratefully acknowledged.
---------------------------------------------------------
Title: HXR and Microwave Spectroscopy of the X2.2 flare on February
15 2011
Authors: Xu, Yan; Park, S.; Gary, D. E.; Bong, S.; Jing, J.; Wang, H.
2011SPD....42.2203X Altcode: 2011BAAS..43S.2203X
We present a spectroscopic study of an X-class flare observed on 2011
Feb. 15, which is confirmed as a white-light flare by HINODE/SOT
continuum images. HXR observation of this flare was taken by the
Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI)
and radio observation was taken by the Korean Solar Radio Burst
Locator (KSRBL). <P />During the 10 minute lifetime of this flare,
four major temporal peaks have <P />been identified in both HXR and
radio emissions. Spectra at each individual peak are obtained with a
frequency range in microwave from 5 to 18 GHz and an energy range in
HXR from 35 to 150 keV, respectively. The high cadence observations in
HXR and microwave provide an unique tool to diagnose the properties
of energetic electrons and their temporal evolution. In particular,
we will present the result of electron power index as derived from
these two wavelengths, and its peak-to-peak variation.
---------------------------------------------------------
Title: Three-dimensional Mapping of the Lower Corona and Transition
Region
Authors: Tun, Samuel D.; Gary, D. E.
2011SPD....42.0704T Altcode: 2011BAAS..43S.0704T
Numerous studies have shown that temperature in the solar atmosphere
rises abruptly from chromospheric to coronal values through a spatially
thin transition region. Through the use of a novel method by which to
obtain three-dimensional temperature and magnetic field strength maps of
the lower corona and transition region from microwave observations [Tun,
Gary & Georgoulis, 2011, ApJ 728, 1], a region of relatively cool
material is found to extend to greater heights over the sunspot's umbra
than in surrounding regions. While previous studies have occasionally
pointed toward the existence of such material, this mapping represents
a direct observation of this feature and of its three dimensional
distribution above the sunspot. This distribution, in turn, has a
direct application to the investigation of heat distribution in the
higher corona, as evidence is presented that temperatures in different
loop systems are correlated to the location of their footpoints in
the sunspot. The mapping method itself also represents a test of
the accuracy of magnetic field extrapolations. <P />This work was
supported in part by NSF grants AGS-0961867, AST-0908344, and NASA
grants NNX10AF27G and NNX11AB49G to New Jersey Institute of Technology.
---------------------------------------------------------
Title: Study of Magnetic Helicity Injection in the Active Region
NOAA 11158 Associated with the X-class Flare of 2011 February 15
Authors: Park, Sung-Hong; Cho, K.; Kim, Y.; Bong, S.; Gary, D. E.;
Park, Y.
2011SPD....42.2227P Altcode: 2011BAAS..43S.2227P
The main objective of this study is to examine a long-term (a few
days) precondition and a trigger mechanism for an X2.2 flare peaking
at 01:56 UT on 2011 February 15 in GOES soft X-ray flux. For this,
we investigated the variation of magnetic helicity injection through
the photospheric surface of the flare-productive active region NOAA
11158 during (1) the long-term period of February 11 to 15 with a
1-hour cadence and (2) the short-term period of 01:26 to 02:10 UT
on February 15 with a 45-second cadence. The helicity injection was
determined using line-of-sight magnetograms with high spatial and
temporal resolution taken by the Helioseismic and Magnetic Imager
(HMI) onboard the Solar Dynamics Observatory (SDO). As a result, we
found two characteristic phases of helicity injection related to the
X2.2 flare. A large amount of positive helicity was first injected
over 2 days with a phase of monotonically increasing helicity. And
then the flare started simultaneously with a significant injection of
the opposite (negative) sign of helicity around the flaring magnetic
polarity inversion line. This observational finding clearly supports
the previous studies that there is a continuous injection of helicity
a few days before flares and a rapid injection of the helicity in the
opposite sign into an existing helicity system triggers flares.
---------------------------------------------------------
Title: The Expanded Owens Valley Solar Array
Authors: Gary, Dale E.; Hurford, G. J.; Nita, G. M.; White, S. M.;
Tun, S. D.; Fleishman, G. D.; McTiernan, J. M.
2011SPD....42.0102G Altcode: 2011BAAS..43S.0102G
The Expanded Owens Valley Solar Array (EOVSA) is now under construction
near Big Pine, CA as a solar-dedicated microwave imaging array
operating in the frequency range 1-18 GHz. The solar science to be
addressed focuses on the 3D structure of the solar corona (magnetic
field, temperature and density), on the sudden release of energy and
subsequent particle acceleration, transport and heating, and on space
weather phenomena. The project will support the scientific community
by providing open data access and software tools for analysis of the
data, to exploit synergies with on-going solar research in other
wavelengths. <P />The New Jersey Institute of Technology (NJIT)
is expanding OVSA from its previous complement of 7 antennas to
a total of 15 by adding 8 new antennas, and will reinvest in the
existing infrastructure by replacing the existing control systems,
signal transmission, and signal processing with modern, far more
capable and reliable systems based on new technology developed for
the Frequency Agile Solar Radiotelescope (FASR). The project will be
completed in time to provide solar-dedicated observations during the
upcoming solar maximum in 2013 and beyond. We provide an update on
current status and our preparations for exploiting the data through
modeling and data analysis tools. <P />This research is supported by
NSF grants AST-0908344, and AGS-0961867 and NASA grant NNX10AF27G to
New Jersey Institute of Technology.
---------------------------------------------------------
Title: GX_Simulator: An Interactive Idl Widget Tool For Visualization
And Simulation Of Imaging Spectroscopy Models And Data
Authors: Nita, Gelu M.; Fleishman, G. D.; Gary, D. E.; Kuznetsov,
A. A.; Kontar, E. P.
2011SPD....42.1811N Altcode: 2011BAAS..43S.1811N
An interactive IDL widget application intended to provide a flexible
tool that allows the user to generate spatially resolved radio
and/or X-ray spectra is presented. The object-based architecture of
this application provides full interaction with local 3D magnetic
field models (e.g., from an extrapolation) that may be embedded in a
global coronal model. Various tools provided allow users to explore
the magnetic connectivity of the model by generating magnetic field
lines originating in user-specified volume positions. Such lines may
serve as reference lines for creating magnetic flux tubes, which are
further populated with user-defined analytical thermal/non thermal
particle distribution models. By default, the application integrates
IDL callable DLL and Shared libraries containing fast GS emission codes
developed in FORTRAN and C++ and soft and hard X-ray codes developed
in IDL. However, the interactive interface allows interchanging these
default libraries with any user-defined IDL or external callable
codes designed to solve the radiation transfer equation in the same or
other wavelength ranges of interest. We illustrate the tool capacity
and generality by a real-time computation of microwave and X-ray
images from realistic magnetic structures obtained from nonlinear
force-free field extrapolations. <P />This work was supported in part
by NSF grants AGS-0961867, AST-0908344, and NASA grants NNX10AF27G
and NNX11AB49G to New Jersey Institute of Technology, by a UK STFC
rolling grant, STFC/PPARC Advanced Fellowship, and the Leverhulme Trust,
UK. Financial support by the European Commission through the SOLAIRE
and HESPE Networks is gratefully acknowledged.
---------------------------------------------------------
Title: A Cold, Tenuous Solar Flare: Acceleration Without Heating
Authors: Fleishman, Gregory D.; Kontar, Eduard P.; Nita, Gelu M.;
Gary, Dale E.
2011ApJ...731L..19F Altcode: 2011arXiv1103.2705F
We report the observation of an unusual cold, tenuous solar flare, which
reveals itself via numerous and prominent non-thermal manifestations,
while lacking any noticeable thermal emission signature. RHESSI hard
X-rays and 0.1-18 GHz radio data from OVSA and Phoenix-2 show copious
electron acceleration (10<SUP>35</SUP> electrons s<SUP>-1</SUP> above
10 keV) typical for GOES M-class flares with electrons energies up
to 100 keV, but GOES temperatures not exceeding 6.1 MK. The imaging,
temporal, and spectral characteristics of the flare have led us to
a firm conclusion that the bulk of the microwave continuum emission
from this flare was produced directly in the acceleration region. The
implications of this finding for the flaring energy release and particle
acceleration are discussed.
---------------------------------------------------------
Title: Three-dimensional Structure of a Solar Active Region from
Spatially and Spectrally Resolved Microwave Observations
Authors: Tun, Samuel D.; Gary, Dale E.; Georgoulis, Manolis K.
2011ApJ...728....1T Altcode:
We report on the structure of the solar atmosphere above active
region (AR) 10923, observed on 2006 November 10, as deduced from
multi-wavelength studies including combined microwave observations
from the Very Large Array (VLA) and the Owens Valley Solar Array
(OVSA). The VLA observations provide excellent image quality
at a few widely spaced frequencies, while the OVSA data provide
information at many intermediate frequencies to fill in the spectral
coverage. Images at 25 distinct frequencies are used to provide
spatially resolved spectra along many lines of sight in the AR,
from which microwave spectral diagnostics are obtained for deducing
maps of temperature, magnetic field, and column density. The derived
quantities are compared with multiwavelength observations from the
Solar and Heliospheric Observatory and Hinode spacecraft, and with a
current-free magnetic field extrapolation. We find that a two-component
temperature model is required to fit the data, in which a hot (>2
MK) lower corona above the strong-field plage and sunspot regions
(emitting via the gyroresonance process) is overlaid with somewhat
cooler (~1 MK) coronal loops that partially absorb the gyroresonance
emission through the free-free (Bremsstrahlung) process. We also find
that the extrapolated potential magnetic fields can quantitatively
account for the observed gyroresonance emission over most of the AR,
but in a few areas a higher field strength is required. The results
are used to explore the coronal configuration needed to explain the
observations. These results show that the bulk of free-free emission
in both radio and X-rays emanates from two loop systems, distinguished
by the location of their loop footpoints. We discuss the implications
of such comparisons for studies of AR structure when better microwave
spectral imaging becomes available in the future.
---------------------------------------------------------
Title: Uncovering Mechanisms of Coronal Magnetism via Advanced 3D
Modeling of Flares and Active Regions
Authors: Fleishman, Gregory; Gary, Dale; Nita, Gelu; Alexander,
David; Aschwanden, Markus; Bastian, Tim; Hudson, Hugh; Hurford,
Gordon; Kontar, Eduard; Longcope, Dana; Mikic, Zoran; DeRosa, Marc;
Ryan, James; White, Stephen
2010arXiv1011.2800F Altcode:
The coming decade will see the routine use of solar data of
unprecedented spatial and spectral resolution, time cadence, and
completeness. To capitalize on the new (or soon to be available)
facilities such as SDO, ATST and FASR, and the challenges they present
in the visualization and synthesis of multi-wavelength datasets,
we propose that realistic, sophisticated, 3D active region and flare
modeling is timely and critical, and will be a forefront of coronal
studies over the coming decade. To make such modeling a reality, a
broad, concerted effort is needed to capture the wealth of information
resulting from the data, develop a synergistic modeling effort, and
generate the necessary visualization, interpretation and model-data
comparison tools to accurately extract the key physics.
---------------------------------------------------------
Title: The generalized spectral kurtosis estimator
Authors: Nita, G. M.; Gary, D. E.
2010MNRAS.406L..60N Altcode: 2010arXiv1005.4371N; 2010MNRAS.tmpL..83N
Due to its conceptual simplicity and its proven effectiveness in
real-time detection and removal of radio frequency interference (RFI)
from radio astronomy data, the spectral kurtosis (SK) estimator
is likely to become a standard tool of a new generation of radio
telescopes. However, the SK estimator in its original form must
be developed from instantaneous power spectral density estimates,
and hence cannot be employed as an RFI excision tool downstream of
the data pipeline in existing instruments where any time averaging is
performed. In this Letter, we develop a generalized estimator with wider
applicability for both instantaneous and averaged spectral data, which
extends its practical use to a much larger pool of radio instruments.
---------------------------------------------------------
Title: Statistics of the Spectral Kurtosis Estimator
Authors: Nita, G.; Gary, D.
2010rfim.workE..19N Altcode: 2010PoS...107E..19N
No abstract at ADS
---------------------------------------------------------
Title: Statistics of the Spectral Kurtosis Estimator
Authors: Nita, Gelu M.; Gary, Dale E.
2010PASP..122..595N Altcode:
Spectral kurtosis (SK) is a statistical approach for detecting
and removing radio-frequency interference (RFI) in radio astronomy
data. In this article, the statistical properties of the SK estimator
are investigated and all moments of its probability density function
are analytically determined. These moments provide a means to determine
the tail probabilities of the estimator that are essential to defining
the thresholds for RFI discrimination. It is shown that, for a number
of accumulated spectra M≥24, the first SK standard moments satisfy
the conditions required by a Pearson type IV probability density
function (pdf), which is shown to accurately reproduce the observed
distributions. The cumulative function (CF) of the Pearson type IV
is then found, in both analytical and numerical forms, suitable for
accurate estimation of the tail probabilities of the SK estimator. This
same framework is also shown to be applicable to the related time-domain
kurtosis (TDK) estimator, whose pdf corresponds to Pearson type IV
when the number of time-domain samples is M≥46. The pdf and CF also
are determined for this case.
---------------------------------------------------------
Title: A Wideband Spectrometer with RFI Detection
Authors: Gary, Dale E.; Liu, Zhiwei; Nita, Gelu M.
2010PASP..122..560G Altcode:
We report on the design and construction of a wideband spectrometer
of 500 MHz instantaneous bandwidth that includes automatic radio
frequency interference (RFI) detection. The implementation is
based on hardware developed at the Center for Astronomical Signal
Processing and Electronics Research (CASPER). The unique aspect of
the spectrometer is that it accumulates both power and power-squared,
which are then used to develop a spectral kurtosis (SK) estimator. The
SK estimator statistics are used for real-time detection and excision
of certain types of RFI embedded in the received signal. We report on
the use of this spectrometer in the Korean Solar Radio Burst Locator
(KSRBL). This instrument utilizes four of these 500 MHz bandwidth SK
spectrometers in parallel, to achieve a 2 GHz instantaneous bandwidth
that is time multiplexed over the entire 0.24-18 GHz radio frequency
range, to study solar bursts. The performance of the spectrometers
for excising RFI over this range is presented. It is found that the
algorithm is especially useful for excising highly intermittent RFI
but is less successful for RFI due to digital signals. A method we
call multiscale SK is presented that addresses the known blindness of
Kurtosis-based estimators to 50% duty-cycle RFI. The SK algorithm can
also be applied to spectral channels prior to correlation to remove
unwanted RFI from interferometer data.
---------------------------------------------------------
Title: Erratum: A Wideband Spectrometer with RFI Detection by Dale
E. Gary, Zhiwei Liu, and Gelu M. Nita (PASP, 122, 560, [2010])
Authors: Gary, Dale E.; Liu, Zhiwei; Nita, Gelu, M.
2010PASP..122..743G Altcode: 2010PASP..122..743.
In the paper “A Wideband Spectrometer with RFI Detection” by Dale
E. Gary, Zhiwei Liu, and Gelu M. Nita (PASP, 122, 560, [2010]), Figure
9 was omitted and Figure appeared twice as a result of an error in
the production process. The correct figures and their legends appear
below. <P />The Press sincerely regrets this error.
---------------------------------------------------------
Title: Hardware Implementation of an SK Spectrometer
Authors: Gary, D.; Li, Z.; Nita, G.
2010rfim.workE..20G Altcode: 2010PoS...107E..20G
No abstract at ADS
---------------------------------------------------------
Title: Expanded Owens Valley Solar Array Science and Data Products
Authors: Gary, Dale E.; Hurford, G. J.; Nita, G. M.; Fleishman, G. D.;
McTiernan, J. M.
2010AAS...21640520G Altcode: 2010BAAS...41..892G
The Owens Valley Solar Array (OVSA) has been funded for major expansion,
to create a university-based facility serving a broad scientific
community, to keep the U.S. competitive in the field of solar radio
physics. The project, funded by the National Science Foundation through
the MRI-Recovery and Reinvestment program, will result in a world-class
facility for scientific research at microwave radio frequencies (1-18
GHz) in solar and space weather physics. The project also includes an
exciting program of targeted astronomical science. The solar science
to be addressed focuses on the magnetic structure of the solar corona,
on transient phenomena resulting from magnetic interactions, including
the sudden release of energy and subsequent particle acceleration and
heating, and on space weather phenomena. The project will support the
scientific community by providing open data access and software tools
for analysis of the data, to exploit synergies with on-going solar
research in other wavelength bands. <P />The New Jersey Institute of
Technology (NJIT) will upgrade OVSA from its current complement of 7
antennas to a total of 15 by adding 8 new antennas, and will reinvest
in the existing infrastructure by replacing the existing control
systems, signal transmission, and signal processing with modern, far
more capable and reliable systems based on new technology developed
for the Frequency Agile Solar Radiotelescope (FASR). The project will
be completed in time to provide solar-dedicated observations during
the upcoming solar maximum in 2013 and beyond. We will detail the new
science addressed by the expanded array, and provide an overview of
the expected data products.
---------------------------------------------------------
Title: Spatially and Spectrally Resolved Observations of a "Zebra”
Solar Radio Burst
Authors: Bastian, Timothy S.; Chen, B.; Gary, D. E.
2010AAS...21542201B Altcode: 2010BAAS...42..291B
The FASR Subsystems Testbed (FST) is a frequency-agile three-element
interferometer located at the Owens Valley Radio Observatory in
California. A frequency band of 500 MHz can be dynamically selected
within the 1-9 GHz frequency FST operating range. The signal from each
antenna is sampled at 1 Gsps and written to disk. The full-resolution
time-domain data are then correlated offline to produce amplitude
and phase spectra on three interferometric baselines. The FST was
used on 14 December 2006 to observe the GOES X1.5 soft X-ray flare in
NOAA/USAF active region 10930 at S06W46. The FST observed the event
between 1.0-1.5 GHz with a time resolution of 20 ms and a frequency
resolution of approximately 1 MHz, time sharing between observations
sensitive to right- and left-circularly polarized radiation. A variety
of coherent radio bursts was observed, including a highly circularly
polarized "Zebra” burst characterized by 7-10 regularly spaced bands
of emission in the dynamic spectrum. With new constraints available
on the source size and the relative source position as a function
of frequency, the double-plasma resonance model is explored, wherein
emission in a given band occurs at the upper hybrid frequency that is,
in turn, harmonically related to the local electron cyclotron frequency.
---------------------------------------------------------
Title: Statistical Hypothesis Testing and Variance Analysis for
Radio Frequency Interference Identification in Solar Data
Authors: Wang, Xiaoli; Ge, Hongya; Gary, Dale E.; Nita, Gelu M.
2009PASP..121.1139W Altcode:
This work presents an effective algorithm for radio frequency
interference (RFI) identification using dynamic power spectrum
statistics in the frequency domain. Statistical signal processing
techniques such as hypothesis testing and variance analysis are
utilized to derive a test statistic for effective and efficient
RFI identification. Starting from the generalized likelihood ratio
test (GLRT), we formulate the problem systematically and propose a
practical test statistic T(x f), shown to be distributed, for RFI
identification. A threshold approach working on this test statistic
is developed to identify the presence of narrowband RFI in the power
spectrum with additive Gaussian noise and/or solar flare background,
corresponding to a desired constant false alarm rate (CFAR). Detailed
analysis on detector performance and effect of RFI duty cycle are also
provided. The proposed statistical test is applied to experimental
solar data collected by our frequency-agile solar radio telescope (FASR)
subsystem testbed (FST) to demonstrate the robustness and scalability of
the algorithm, as well as its capability for real-time implementation.
---------------------------------------------------------
Title: Global Positioning System and solar radio burst forensics
Authors: Kintner, P. M.; O'Hanlon, B.; Gary, D. E.; Kintner, P. M. S.
2009RaSc...44.0A08K Altcode:
On 6 December 2006, a solar radio burst associated with a class X6
solar flare demonstrated that GPS receiver operation is vulnerable to
solar radio burst noise at 1.2 GHz and 1.6 GHz. Within 8 days, two more
solar radio bursts confirmed the initial results. These solar radio
bursts occurred at solar minimum when they were least expected. Given
that measurements of solar radio bursts extend back to at least 1960,
why did 40 years pass before anyone realized that solar radio bursts
could be so intense or pose a potential threat to the continuous
availability of GPS operations? An investigation has been conducted to
see if archived solar radio burst data or GPS data could be used to
detect intense solar radio bursts. With the exception of the intense
solar radio bursts of December 2006, we find that when both GPS data
and Radio Solar Telescope Network (RSTN) data are available, they agree
within the limits presented by differing reception frequencies and
unknown polarization. However, inconsistencies and lapses within the
RSTN data set were also discovered, making it unlikely that we will
ever know the true number of intense (>150,000 solar flux unit)
solar radio bursts that may have occurred during the last 40 years.
---------------------------------------------------------
Title: Dynamic Magnetography of Solar Flaring Loops
Authors: Fleishman, Gregory D.; Nita, Gelu M.; Gary, Dale E.
2009ApJ...698L.183F Altcode: 2009arXiv0906.0192F
We develop a practical forward fitting method based on the SIMPLEX
algorithm with shaking, which allows the derivation of the magnetic
field and other parameters along a solar flaring loop using microwave
imaging spectroscopy of gyrosynchrotron emission. We illustrate the
method using a model loop with spatially varying magnetic field,
filled with uniform ambient density and an evenly distributed fast
electron population with an isotropic, power-law energy distribution.
---------------------------------------------------------
Title: Dynamic Magnetography of Solar Flaring Loops
Authors: Fleishman, Gregory D.; Nita, G. M.; Gary, D. E.
2009SPD....40.1522F Altcode:
The coronal magnetic field is a key parameter controlling most solar
flaring activity, particle acceleration and transport. However, unlike
photospheric and chromospheric magnetography data, there is currently a
clear lack of quantitative information on the coronal magnetic field in
the dynamically flaring region, which complicates the detailed modeling
of fundamental physical processes occurring in the corona. It has been
understood, and often proposed, that the coronal magnetic field can in
principle be evaluated from the microwave gyrosynchrotron radiation,
which is indeed sensitive to the instantaneous magnetic field strength
and orientation relative to the line of sight. <P />Anticipating a
large breakthrough in the radio imaging spectroscopy observations,
which will become possible soon due to the next generation of the
radio instruments, we develop a practical forward fitting method based
on the SIMPLEX algorithm with shaking that allows reliable derivation
of the magnetic field and other parameters along a solar flaring loop
using microwave imaging spectroscopy of gyrosynchrotron emission. We
illustrate the method using a model loop (the subject of another
presentation at this meeting, Nita et al., abstract # 09-A-57-SPD40)
with spatially varying magnetic field, filled with uniform ambient
density and an evenly distributed fast electron population with an
isotropic, power-law energy distribution. <P />This work was supported
in part by NSF grants AST-0607544 and ATM-0707319 and NASA grant
NNG06GJ40G to New Jersey Institute of Technology
---------------------------------------------------------
Title: Electron Energy and Magnetic Field Derived from Solar Microwave
Burst Spectra
Authors: Lee, Jeongwoo; Nita, Gelu M.; Gary, Dale E.
2009ApJ...696..274L Altcode:
Microwave bursts during solar flares are known to be sensitive
to high-energy electrons and magnetic field, both of which are
important ingredients of solar flare physics. This paper presents
such information derived from the microwave bursts of the 412 flares
that were measured with the Owens Valley Solar Array. We assumed
that these bursts are predominantly due to gyrosynchrotron radiation
by nonthermal electrons in a single power-law energy distribution to
use the simplified formulae for gyrosynchrotron radiation in the data
analysis. A second major assumption was that statistical properties
of flare electrons derived from this microwave database should agree
with an earlier result based on the hard X-ray burst spectrometer on
Solar Maximum Mission. Magnetic field information was obtained in the
form of a scaling law between the average magnetic field and the total
source area, which turns out to be a narrow distribution around ~400
G. The derived nonthermal electron energy is related to the peak flux,
peak frequency, and spectral index, through a multistep regression fit,
which can be used for a quick estimate for the nonthermal electron
energy from spatially integrated microwave spectral observations.
---------------------------------------------------------
Title: Spatial Locations of Solar Decimetric Bursts at Spectrographic
Resolution
Authors: Gary, Dale E.; Naqvi, M.
2009SPD....40.1927G Altcode:
Several large (X-class) solar flares occurred in 2006 December that
included strong decimetric components displaying several burst types,
including zebra bursts, spike bursts, fiber bursts, pulsations,
and others. The bursts were observed with the FASR Subsystem Testbed
(FST---a prototype system for the Frequency Agile Solar Radiotelescope),
which is operating at Owens Valley Radio Observatory. The FST,
described in Liu et al. (2007), PASP 119, 303, is a three-element
interferometer with spectrograph-like frequency and time resolution,
and so is able to locate burst centroids with unprecedented temporal
and spectral resolution. Using FST, with some assumptions, we are
able to place different types of burst spatially in context with
Hinode and TRACE data taken at the same time. The spatial placement
of burst types within the magnetic structure extrapolated from Hinode
SOT-SP vector field measurements for 2006 December 14 constrains the
physical mechanisms responsible for them. We discuss the implications
of these results in the context of the true spectral imaging that will
be available with the full FASR.
---------------------------------------------------------
Title: Evaluating Mean Magnetic Field in Flare Loops
Authors: Qiu, Jiong; Gary, D. E.; Fleishman, G. D.
2009SPD....40.1906Q Altcode:
We apply a new method to measure the mean magnetic field in flare
loops by analyzing multiple-wavelength observations of a two-ribbon
flare. The flare exhibits apparent expansion motion of the ribbons in
the lower-atmosphere and rising motion of the top of newly formed flare
loops observed in X-rays. These apparent motion patterns are signatures
of progressive magnetic reconnection proceeding in a macroscopically
organized manner, which may be approximated by a 2D model. Therefore,
the reconnection rate is expressed in terms of the reconnection electric
field E. As the reconnection flux along the newly formed flare loop is
conserved, the relation holds that, when averaged over time, E = VrBr =
VtBt, where Vr and Br refer to the ribbon expansion speed and magnetic
field swept by the ribbons, and Vt and Bt denote the apparent rising
speed of the X-ray source and mean magnetic field at the loop top. By
directly measuring Vr, Br, and Vt, we find Bt to be 120 and 60 G,
respectively, during two emission peaks 5 min apart in the impulsive
phase. An estimate of magnetic field in flare loops is also achieved
by analyzing the microwave and hard X-ray spectral observations,
yielding B = 250, 120 G at the two emission peaks, respectively. The
measured B from the microwave spectrum is an appropriately weighted
value of magnetic field from the loop top to the loop leg. Therefore,
the two methods to evaluate magnetic field in flaring loops produce
fully consistent results, which provides evidence that parameters
of reconnection rate derived from ribbon motions are quantitatively
valid. <P />This work is supported by NSF grant and by NSF grant
ATM-0748428 and NASA grant NNX08AE44G to Montana State University and
NSF grants AST-0607544 and ATM-0707319 and NASA grant NNG06GJ40G to
New Jersey Institute of Technology.
---------------------------------------------------------
Title: A Kinetic Model for the Radio CME
Authors: Lee, Jeongwoo; Gary, D. E.
2009SPD....40.2209L Altcode:
Current studies on Coronal Mass Ejections (CMEs) are mostly concentrated
on their macroscopic properties as measured on White-Light images. On
the other hand, radio emissions from CMEs carry the information of
high energy particles associated with them, but usually without spatial
information. In this regard, the rare radio maps of the 1998 April 20
CME obtained with the Nancay radioheliograph between 164 and 432 MHz
(called a radio CME by Bastian et al. in 2001) offer an exceptional
opportunity to explore the spatial distribution of high energy electrons
inside the CME loop. We present a detailed kinetic model for the radio
CME employing the lower hybrid (LH) waves excited by the CME shock as
the primary electron acceleration mechanism, and magnetic mirroring
and Coulomb collisions as the propagation effects inside the expanding
loop. The main constraint in this modeling comes from the fact that
the LH waves accelerate electrons parallel to the magnetic field and
the accelerated electrons should gain, during propagation, sufficient
amount of the perpendicular momentum to emit the synchrotron radiation
as observed. The relative magnetic field variation responsible for the
magnetic mirroring is inferred from the geometrical shape of the CME on
the images of the Large Angle and Spectrometric Coronagraph Experiment
(LASCO), and the field strength and the amplitude of the LH waves are
determined from the observed radio spectra. The modeling is focused on
the spatial distribution of the LH waves most plausible to explain the
radio maps, and the result is discussed in relation to the associated
shock property.
---------------------------------------------------------
Title: GS-3D Simulator: An Interactive IDL Widget Tool for Simulating
Spatially Resolved Gyrosynchrotron Spectra Emitted by Solar Radio
Bursts
Authors: Nita, Gelu M.; Fleishman, G. D.; Gary, D. E.
2009SPD....40.1524N Altcode:
An interactive IDL widget application intended to provide a flexible
tool that allows the user to generate spatially resolved gyrosynchrotron
spectra is presented. The object-based architecture of this application
provides full 3D interaction with a user-specified magnetic loop
geometry. Alternatively, the user may define and pass to the same
interface arbitrary analytical or numerical models, including those
derived from magnetic field extrapolation, provided that they inherit
the generic properties of the base class defined in this package. The
default code generating the GS emission from the input geometrical model
was developed in FORTRAN based on the Petrosian-Klein approximation, and
compiled as a DLL callable by IDL. However, the interactive interface
allows interchanging this default library with any user-defined callable
code. To illustrate the concept, a simple dipole magnetic loop object
is analytically defined, and GS radio maps at 100 frequencies in the
1-100 GHz frequency range are produced. Similar maps produced by this
tool were used as input test data in a forward-fitting algorithm that
makes the subject of another presentation at this meeting (Fleishman
et al. 09-D-83-SPD40). <P />This work was supported by NSF grants
AST-0607544 and ATM-0707319 and NASA grant NNG06GJ40G to New Jersey
Institute of Technology.
---------------------------------------------------------
Title: The Korean Solar Radio Burst Locator (KSRBL)
Authors: Dou, Yujiang; Gary, Dale E.; Liu, Zhiwei; Nita, Gelu M.;
Bong, Su-Chan; Cho, Kyung-Suk; Park, Young-Deuk; Moon, Yong-Jae
2009PASP..121..512D Altcode:
This paper describes the design and operation of the Korean Solar Radio
Burst Locator (KSRBL). The KSRBL is a radio spectrometer designed to
observe solar decimeter and microwave bursts over a wide band (0.245-18
GHz) as well as to detect the burst locations without interferometry
or mechanical sweeping. As a prototype, it is temporarily observing
at the Owens Valley Radio Observatory (OVRO), California, USA, and
after commissioning will be operated at the Korea Astronomy and Space
Science Institute (KASI), Daejeon, Republic of Korea. The control
system can agilely choose four 500 MHz intermediate frequency (IF)
bands (2 GHz instantaneous bandwidth) from the entire 0.245-18 GHz
band, with a standard time resolution of 100 ms, although higher time
resolution is possible subject to data-rate constraints. To cover the
entire band requires 10 tunings, which are therefore completed in 1
s. Each 500 MHz band is sampled at a 1 GS s<SUP>-1</SUP> (gigasample per
second) rate, and 4096 time samples are Fast Fourier transformed (FFT)
to 2048 subchannels for a frequency resolution of 0.24 MHz. To cover
the entire range also requires two different feeds, a dual-frequency
Yagi centered at 245 and 410 MHz, and a broadband spiral feed covering
0.5-18 GHz. The dynamic range is 35 dB over the 0.5-18 GHz band, and 55
dB in the 245 and 410 MHz bands, set by using switchable attenuators in
steps of 5 dB. Each 500 MHz IF has a further 63 dB of settable analog
attenuation. The characteristics of the spiral feed provide the ability
to locate flaring sources on the Sun to typically 2‧. The KSRBL will
provide a broadband view of solar bursts for the purposes of studying
solar activity for basic research, and for monitoring solar activity
as the source of Space Weather and solar-terrestrial effects.
---------------------------------------------------------
Title: Evaluating Mean Magnetic Field in Flare Loops
Authors: Qiu, Jiong; Gary, Dale E.; Fleishman, Gregory D.
2009SoPh..255..107Q Altcode:
We analyze multiple-wavelength observations of a two-ribbon flare
exhibiting apparent expansion motion of the flare ribbons in the
lower atmosphere and rising motion of X-ray emission at the top of
newly-formed flare loops. We evaluate magnetic reconnection rate in
terms of V<SUB>r</SUB>B<SUB>r</SUB> by measuring the ribbon-expansion
velocity (V<SUB>r</SUB>) and the chromospheric magnetic field
(B<SUB>r</SUB>) swept by the ribbons. We also measure the velocity
(V<SUB>t</SUB>) of the apparent rising motion of the loop-top X-ray
source, and estimate the mean magnetic field (B<SUB>t</SUB>)
at the top of newly-formed flare loops using the relation
«V<SUB>t</SUB>B<SUB>t</SUB>»≈«V<SUB>r</SUB>B<SUB>r</SUB>», namely,
conservation of reconnection flux along flare loops. For this flare,
B<SUB>t</SUB> is found to be 120 and 60 G, respectively, during two
emission peaks five minutes apart in the impulsive phase. An estimate
of the magnetic field in flare loops is also achieved by analyzing
the microwave and hard X-ray spectral observations, yielding B=250
and 120 G at the two emission peaks, respectively. The measured B
from the microwave spectrum is an appropriately-weighted value of
magnetic field from the loop top to the loop leg. Therefore, the two
methods to evaluate coronal magnetic field in flaring loops produce
fully-consistent results in this event.
---------------------------------------------------------
Title: Digital Instrumentation for the Radio Astronomy Community
Authors: Parsons, Aaron; Werthimer, Dan; Backer, Donald; Bastian, Tim;
Bower, Geoffrey; Brisken, Walter; Chen, Henry; Deller, Adam; Filiba,
Terry; Gary, Dale; Greenhill, Lincoln; Hawkins, David; Jones, Glenn;
Langston, Glen; Lasio, Joseph; Van Leeuwen, Joeri; Mitchell, Daniel;
Manley, Jason; Siemion, Andrew; So, Hayden Kwok-Hay; Whitney, Alan;
Woody, Dave; Wright, Melvyn; Zarb-Adami, Kristian
2009astro2010T..21P Altcode: 2009arXiv0904.1181P
Time-to-science is an important figure of merit for digital
instrumentation serving the astronomical community. A digital
signal processing (DSP) community is forming that uses shared
hardware development, signal processing libraries, and instrument
architectures to reduce development time of digital instrumentation and
to improve time-to-science for a wide variety of projects. We suggest
prioritizing technological development supporting the needs of this
nascent DSP community. After outlining several instrument classes
that are relying on digital instrumentation development to achieve
new science objectives, we identify key areas where technologies
pertaining to interoperability and processing flexibility will reduce
the time, risk, and cost of developing the digital instrumentation for
radio astronomy. These areas represent focus points where support of
general-purpose, open-source development for a DSP community should
be prioritized in the next decade. Contributors to such technological
development may be centers of support for this DSP community, science
groups that contribute general-purpose DSP solutions as part of their
own instrumentation needs, or engineering groups engaging in research
that may be applied to next-generation DSP instrumentation.
---------------------------------------------------------
Title: A Wideband Spectrometer with Automatic RFI Detection
Authors: Gary, Dale E.; Liu, Zhiwei; Nita, Gelu M.
2009ursi.confE..30G Altcode:
No abstract at ADS
---------------------------------------------------------
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: Erratum: Peak Frequency Dynamics in Solar Microwave Bursts
Authors: Melnikov, V. F.; Gary, Dale E.; Nita, Gelu M.
2008SoPh..253...75M Altcode: 2008SoPh..tmp..187M
No abstract at ADS
---------------------------------------------------------
Title: Peak Frequency Dynamics in Solar Microwave Bursts
Authors: Melnikov, V. F.; Gary, Dale E.; Nita, Gelu M.
2008SoPh..253...43M Altcode: 2008SoPh..tmp..180M
We analyze the dynamics of the broadband frequency spectrum of 338
microwave bursts observed in the years 2001 - 2002 with the Owens
Valley Solar Array. A subset of 38 strong microwave bursts that show
a single spectral maximum are studied in detail. Our main goal is to
study changes in spectral peak frequency ν<SUB>pk</SUB> with time. We
show that, for a majority of these simple bursts, the peak frequency
shows a high positive correlation with flux density - it increases
on the rise phase in ≈83% of 24 bursts where it could be cleanly
measured, and decreases immediately after the peak time in ≈62% of
34 bursts. This behavior is in qualitative agreement with theoretical
expectations based on gyrosynchrotron self-absorption. However,
for a significant number of events (≈30 - 36%) the peak frequency
variation is much smaller than expected from self-absorption, or may
be entirely absent. The observed temporal behavior of ν<SUB>pk</SUB>
is compared with a simple model of gyrosynchrotron radio emission. We
show that the anomalous behavior is well accounted for by the effects
of Razin suppression, and further show how an analysis of the temporal
evolution of ν<SUB>pk</SUB> can be used to uniquely determine the
relative importance of self-absorption and Razin suppression in a given
burst. The analysis technique provides a new, quantitative diagnostic
for the gyrosynchrotron component of solar microwave bursts. Applying
this analysis technique to our sample of bursts, we find that in most
of the bursts (60%) the spectral dynamics of ν<SUB>pk</SUB> around
the time of peak flux density is caused by self-absorption. On the
other hand, for a significant number of events (≈70%), the Razin
effect may play the dominant role in defining the spectral peak and
dynamics of ν<SUB>pk</SUB>, especially on the early rise phase and
late decay phase of the bursts.
---------------------------------------------------------
Title: Spike Decomposition Technique: Modeling and Performance Tests
Authors: Nita, Gelu M.; Fleishman, Gregory D.; Gary, Dale E.
2008ApJ...689..545N Altcode:
We develop an automated technique for fitting the spectral components
of solar microwave spike bursts, which are characterized by narrowband
spectral features. The algorithm is especially useful for periods when
the spikes occur in densely packed clusters, where the algorithm is
capable of decomposing overlapping spike structures into individual
spectral components. To test the performance and applicability limits
of this data reduction tool, we perform comprehensive modeling of spike
clusters characterized by various typical bandwidths, spike densities,
and amplitude distributions. We find that, for a wide range of favorable
combinations of input parameters, the algorithm is able to recover the
characteristic features of the modeled distributions within reasonable
confidence intervals. Having model-tested the algorithm against spike
overlap, broadband spectral background, noise contamination, and
possible malfunction of some spectral channels, we apply the technique
to a spike cluster recorded by the Chinese Purple Mountain Observatory
(PMO) spectrometer, operating above 4.5 GHz. We study the variation
of the spike distribution parameters, such as amplitude, bandwidth,
and related derived physical parameters, as a function of time. The
method can be further applied to observations from other instruments
and to other types of fine structures.
---------------------------------------------------------
Title: Broadband Quasi-periodic Radio and X-Ray Pulsations in a
Solar Flare
Authors: Fleishman, Gregory D.; Bastian, T. S.; Gary, Dale E.
2008ApJ...684.1433F Altcode: 2008arXiv0804.4037F
We describe microwave and hard X-ray observations of strong
quasi-periodic pulsations from the GOES X1.3 solar flare on 2003
June 15. The radio observations were made jointly by the Owens
Valley Solar Array (OVSA), the Nobeyama Polarimeter (NoRP), and
the Nobeyama Radioheliograph (NoRH). Hard X-ray observations were
made by RHESSI. Using Fourier analysis, we study the frequency-
and energy-dependent oscillation periods, differential phase, and
modulation amplitudes of the radio and X-ray pulsations. Focusing on
the more complete radio observations, we also examine the modulation
of the degree of circular polarization and of the radio spectral
index. The observed properties of the oscillations are compared
with those derived from two simple models for the radio emission. In
particular, we explicitly fit the observed modulation amplitude data
to the two competing models. The first model considers the effects of
MHD oscillations on the radio emission. The second model considers
the quasi-periodic injection of fast electrons. We demonstrate that
quasi-periodic acceleration and injection of fast electrons is the
more likely cause of the quasi-periodic oscillations observed in the
radio and hard X-ray emission, which has important implications for
particle acceleration and transport in the flaring sources.
---------------------------------------------------------
Title: Parallel Motions of Coronal Hard X-Ray Source and Hα Ribbons
Authors: Lee, Jeongwoo; Gary, Dale E.
2008ApJ...685L..87L Altcode:
During solar flares Hα ribbons form and often move away from the local
magnetic polarity inversion line (PIL). While the motion perpendicular
to the PIL has been taken as evidence for coronal magnetic reconnection
in the so-called CSHKP standard model, the other velocity component
parallel to the PIL is much less adopted as a property of the magnetic
reconnection process. In this Letter we report an event in which
the motion parallel to the PIL is found in both Hα ribbons and a
thermal hard X-ray source. Such commonality would indicate a link
between the coronal magnetic reconnection and footpoint emissions as
in the standard solar flare model. However, its direction implies a
reconnection region that is increasing in length, a feature missing from
the standard two-dimensional model. We present a modified framework in
which the variation along the third dimension is allowed, in order to
assess the effect of such a proper motion on estimation of the magnetic
reconnection rate. Data used are hard X-ray maps from the Reuven Ramaty
High Energy Solar Spectroscopic Imager (RHESSI), Hα filtergrams of Big
Bear Solar Observatory (BBSO), and the SOHO Michelson Doppler Imager
(MDI) magnetogram obtained for the 2004 March 30 flare.
---------------------------------------------------------
Title: Positions of Fiber Bursts in the 2006 December 6 Flare
Authors: White, S. M.; Liu, Z.; Gary, D. E.
2008AGUSMSP44A..07W Altcode:
The large flare of 2006 December 6 produced one of the brightest radio
bursts ever recorded. The burst was observed with very rapid sampling
by the FASR Subsystem Testbed, operating on 3 antennas of the OVSA
array at Owens Valley. The burst contained a range of emission types
within the FST band (1.0-1.5 GHz), including continuum and "fiber"
bursts. This is the first time that fiber bursts have been observed
over a significant frequency range with an interferometer. Data
are processed into visibilities using a software correlator with
positional information retained. We present an analysis of the fiber
bursts relative to the continuum, in order to investigate whether the
projected position of the bursts varies with frequency. This information
is used to test models for fiber bursts, in particular the model that
they represent propagating whistler wave trains.
---------------------------------------------------------
Title: Detection of explosives by Terahertz synthetic aperture
imaging—focusing and spectral classification
Authors: Sinyukov, Alexander; Zorych, Ivan; Michalopoulou, Zoi-Heleni;
Gary, Dale; Barat, Robert; Federici, John F.
2008CRPhy...9..248S Altcode:
In the adaptation of Terahertz (THz) synthetic aperture imaging to
stand-off screening of concealed weapons and explosives, the incoming
THz wavefronts exhibit significant curvature that must be considered
in the image reconstruction. Consequently, the imaging array must
be focused at a specific distance to correct for the wavefront
curvature. In addition to the focusing correction, detection of
explosives requires spectral analysis of the reconstructed THz
image. Kohonen self-organizing maps are shown to be promising tools
for differentiating the spectral signature of C4 explosive from the
reflection spectra of metal and semi-transparent barrier materials. To
cite this article: A. Sinyukov et al., C. R. Physique 9 (2008).
---------------------------------------------------------
Title: The Spatial Distribution of the Hard X-Ray Spectral Index
and the Local Magnetic Reconnection Rate
Authors: Liu, Chang; Lee, Jeongwoo; Jing, Ju; Gary, Dale E.; Wang,
Haimin
2008ApJ...672L..69L Altcode: 2007arXiv0711.1370L
The rare phenomenon of ribbon-like hard X-ray (HXR) sources up to 100
keV found in the 2005 May 13 M8.0 flare observed with the Reuven Ramaty
High Energy Solar Spectroscopic Imager provides detailed information
on the spatial distribution of flare HXR emission. In this Letter, we
further investigate the characteristics of HXR emission in this event
using imaging spectroscopy, from which we obtain spatially resolved HXR
spectral maps during the flare impulsive phase. As a result we found,
along a flare ribbon, an anticorrelation relationship between the local
HXR flux and the local HXR spectral index. We suggest that this can be
regarded as a spatial analog of the well-known temporal soft-hard-soft
spectral evolution pattern of the integrated HXR flux. We also found an
anticorrelation between the HXR spectral index and the local electric
field along the ribbon, which suggests electron acceleration by the
electric field during flares.
---------------------------------------------------------
Title: Transition radiation in turbulent astrophysical
medium. Application to solar radio bursts
Authors: Fleishman, Gregory D.; Gary, Dale E.; Nita, Gelu M.
2007arXiv0710.0351F Altcode:
Modern observations and models of various astrophysical objects suggest
that many of their physical parameters fluctuate substantially at
different spatial scales. The rich variety of the emission processes,
including Transition Radiation but not limited to it, arising in
such turbulent media constitutes the scope of Stochastic Theory of
Radiation. We review general approaches applied in the stochastic theory
of radiation and specific methods used to calculate the transition
radiation produced by fast particles in the magnetized randomly
inhomogeneous plasma. The importance of the theory of transition
radiation for astrophysics is illustrated by one example of its
detailed application to a solar radio burst, including specially
designed algorithms of the spectral forward fitting.
---------------------------------------------------------
Title: Radio Spectral Evolution of an X-Ray-poor Impulsive Solar
Flare: Implications for Plasma Heating and Electron Acceleration
Authors: Bastian, T. S.; Fleishman, G. D.; Gary, D. E.
2007ApJ...666.1256B Altcode: 2007arXiv0704.2413B
We present radio and X-ray observations of an impulsive solar flare
that was moderately intense in microwaves, yet showed very meager EUV
and X-ray emission. The flare occurred on 2001 October 24 and was well
observed at radio wavelengths by the Nobeyama Radioheliograph (NoRH),
the Nobeyama Radio Polarimeters (NoRP), and the Owens Valley Solar Array
(OVSA). It was also observed in EUV and X-ray wavelength bands by the
TRACE, GOES, and Yohkoh satellites. We find that the impulsive onset of
the radio emission is progressively delayed with increasing frequency
relative to the onset of hard X-ray emission. In contrast, the time
of flux density maximum is progressively delayed with decreasing
frequency. The decay phase is independent of radio frequency. The
simple source morphology and the excellent spectral coverage at radio
wavelengths allowed us to employ a nonlinear χ<SUP>2</SUP>-minimization
scheme to fit the time series of radio spectra to a source model that
accounts for the observed radio emission in terms of gyrosynchrotron
radiation from MeV-energy electrons in a relatively dense thermal
plasma. We discuss plasma heating and electron acceleration in view
of the parametric trends implied by the model fitting. We suggest
that stochastic acceleration likely plays a role in accelerating the
radio-emitting electrons.
---------------------------------------------------------
Title: Hard X-Ray Intensity Distribution along Hα Ribbons
Authors: Jing, Ju; Lee, Jeongwoo; Liu, Chang; Gary, Dale E.; Wang,
Haimin
2007ApJ...664L.127J Altcode:
Unusual ribbon-like hard X-ray sources were found with the RHESSI
observation of a 2B/M8.0 flare on 2005 May 13. We use this unique
observation to investigate the spatial distribution of the hard
X-ray intensity along the ribbons and compare it with the local
magnetic reconnection rate and energy release rate predicted by
the standard magnetic reconnection model for two ribbon flares. In
the early phase of the flare, the hard X-ray sources appear to be
concentrated in strong field regions within the Hα ribbons, which
is explicable by the model. At and after the flare maximum phase,
the hard X-ray sources become spatially extended to resemble Hα
ribbons in morphology, during which the spatial distribution of hard
X-ray intensity lacks a correlation with that of the local magnetic
reconnection rate and energy release rate predicted by the model. We
argue that the magnetic reconnection during this event may involve
the rearrangement of magnetic field along the magnetic arcade axis
that is inevitably overlooked by the two-dimensional model and suggest
that this type of three-dimensional reconnection will be best seen in
so-called sigmoid-to-arcade transformations.
---------------------------------------------------------
Title: Magnetic Field Strength in the Solar Corona from Type II
Band Splitting
Authors: Cho, K. -S.; Lee, J.; Gary, D. E.; Moon, Y. -J.; Park, Y. D.
2007ApJ...665..799C Altcode:
The phenomenon of band splitting in type II bursts can be a unique
diagnostic for the magnetic field in the corona, which is, however,
inevitably sensitive to the ambient density. We apply this diagnostic
to the CME-flare event on 2004 August 18, for which we are able to
locate the propagation of the type II burst and determine the ambient
coronal electron density by other means. We measure the width of the
band splitting on a dynamic spectrum of the bursts observed with the
Green Bank Solar Radio Burst Spectrometer (GBSRBS), and convert it
to the Alfvén Mach number under the Rankine-Hugoniot relation. We
then determine the Alfvén speed and magnetic field strength using the
coronal background density and shock speed measured with the MLSO/MK4
coronameter. In this way we find that the shock compression ratio is
in the range of 1.5-1.6, the Alfvénic Mach number is 1.4-1.5, the
Alfvén speed is 550-400 km s<SUP>-1</SUP>, and finally the magnetic
field strength decreases from 1.3 to 0.4 G while the shock passes
from 1.6 to 2.1 R<SUB>solar</SUB>. The magnetic field strength derived
from the type II spectrum is finally compared with the potential field
source surface (PFSS) model for further evaluation of this diagnostic.
---------------------------------------------------------
Title: Radio Frequency Interference Excision Using Spectral-Domain
Statistics
Authors: Nita, Gelu M.; Gary, Dale E.; Liu, Zhiwei; Hurford, Gordon
J.; White, Stephen M.
2007PASP..119..805N Altcode:
A radio frequency interference (RFI) excision algorithm based on
spectral kurtosis, a spectral variant of time-domain kurtosis, is
proposed and implemented in software. The algorithm works by providing
a robust estimator for Gaussian noise that, when violated, indicates
the presence of non-Gaussian RFI. A theoretical formalism is used that
unifies the well-known time-domain kurtosis estimator with past work
related to spectral kurtosis, and leads naturally to a single expression
encompassing both. The algorithm accumulates the first two powers of M
power spectral density (PSD) estimates, obtained via Fourier transform,
to form a spectral kurtosis (SK) estimator whose expected statistical
variance is used to define an RFI detection threshold. The performance
of the algorithm is theoretically evaluated for different time-domain
RFI characteristics and signal-to-noise ratios η. The theoretical
performance of the algorithm for intermittent RFI (RFI present in R
out of M PSD estimates) is evaluated and shown to depend greatly on the
duty cycle, d=R/M. The algorithm is most effective for d=1/(4+η), but
cannot distinguish RFI from Gaussian noise at any η when d=0.5. The
expected efficiency and robustness of the algorithm are tested using
data from the newly designed FASR Subsystem Testbed radio interferometer
operating at the Owens Valley Solar Array. The ability of the algorithm
to discriminate RFI against the temporally and spectrally complex
radio emission produced during solar radio bursts is demonstrated.
---------------------------------------------------------
Title: Positional Properties of Solar Decimetric Fiber Bursts
Authors: Liu, Zhiwei; Gary, D. E.; Nita, G. M.; White, S. M.; Hurford,
G. J.
2007AAS...210.9328L Altcode: 2007BAAS...39..215L
The large solar flare of 2006 December 6 was detected by the newly
constructed FASR Subsystem Testbed (FST; Liu et al. 2007, PASP,
119), which is operating on three antennas of Owens Valley Solar
Array. This record-setting burst produced an especially fine set of
fibers bursts--so-called intermediate-drift bursts that drift from
high to low frequencies over 6-10s. According to a leading theory, the
fibers are generated by packets of whistler waves propagating along a
magnetic loop, which coalesce with Langmuir waves to produce escaping
electromagnetic radiation in the decimeter band. With this three element
interferometer, for the first time fiber burst source locations can be
determined. We use the radio information over a 500 MHz band (1.0-1.5
GHz) to determine the trajectories of the bursts. We attempt to define
three-dimensional trajectories by combining the two-dimensional radio
positions with height of the fiber source determined from a coronal
density model.
---------------------------------------------------------
Title: Interferometric Observations of the Record-Breaking Decimetric
Solar Radio Event of 2006 December 06
Authors: Gary, Dale E.; White, S. M.; Hurford, G. J.; Nita, G. M.;
Liu, Z.
2007AAS...210.9329G Altcode: 2007BAAS...39..216G
The radio burst associated with the 2006 Dec 06, X6.5 solar
flare attained a record-breaking radio flux density of nearly 1
million solar flux units in the decimetric ( 1 GHz) frequency band,
exceeding the previous record by nearly a factor of 10. The event was
so strong in the range 1-2 GHz that it caused loss of lock of Global
Positioning System (GPS) receivers everywhere in the sunlit hemisphere
of Earth. The event was observed with a new instrument operating at
the Owens Valley Solar Array (OVSA), the FASR Subsystem Testbed (FST),
which features not only unprecedented time and frequency resolution (
1 microsecond and < 1 MHz, respectively), but also interferomety
on three baselines to provide corresponding spatial information on
a 10 arcsecond scale. The event produced about 10 million individual
decimetric spike bursts produced by the Electron Cyclotron Maser (ECM)
mechanism over a period of nearly 1 hour. We discuss the spectral,
temporal, and positional characteristics of individual spike bursts,
their statistical properties and their relation to the flare observed
at other wavelengths. This work is supported by NSF grant AST-0352915
to the New Jersey Institute of Technology. We acknowledge additional
support for the Owens Valley Solar Array through NSF grant AST-0607544
and NASA grant NNG06GJ40G.
---------------------------------------------------------
Title: Fiber Bursts in Solar Flares at High Time and Frequency
Resolution
Authors: White, Stephen M.; Liu, Z.; Gary, D. E.; Nita, G. M.; Hurford,
G. J.
2007AAS...210.6806W Altcode: 2007BAAS...39..176W
The FASR Subsystem Testbed (FST), operating on three dishes of the
Owens Valley Solar Array, detected radio fiber bursts in two large
solar flares in 2006. The frequencies of observation correspond to
densities of order 10 to the 10 per cc, so the bursts are being emitted
low in the corona, presumably near the energy release region. The
fiber bursts drift in frequency with time, and appear to be travelling
outwards. In one model, fiber bursts can be used to infer the magnetic
field in the corona in the emission site. The FST data provide radio
spectra of the bursts with a time resolution of milliseconds: using
this exceptional data, we present the properties of the bursts and
discuss their physical interpretation.
---------------------------------------------------------
Title: Solar Burst Submillimeter Wave Emission Components Associated
To Microwaves, Uv, X- And γ- Rays Continuum In Time And Space
Authors: Kaufmann, Pierre; Trottet, G.; Giménez de Castro, C.;
Raulin, J.; Gary, D.; Fernandez, G.; Godoy, R.; Levato, H.; Marun,
A.; Pereyra, P.
2007AAS...210.9330K Altcode: 2007BAAS...39..216K
The solar burst spectral component peaking somewhere in the terahertz
(THz) range, along with, but distinct from, the well-known microwave
component, bring new observational and theoretical possibilities to
explore the flaring physical processes. The solar event of December 6,
2006, 18:30 UT, exhibited a particularly well-defined double spectral
structure, with the THz spectral component detected at 212 and 405 GHz
by SST and microwaves (1-18 GHz) by OVSA. The burst was observed by
instruments in satellites at high energies, UV by TRACE, soft X-rays by
GOES, X- to γ- rays by RHESSI. Although the event occupied an extended
area at optical and UV wavelengths, showing various brightnings along
several arcminutes, the hard X-ray emission region is restricted (within
a region 30” x 50”) showing three sources at low energies (<
150 keV) and a single source above 300 keV. At submillimeter-waves,
a precursor was observed, followed by a rapid impulsive event and a
post-burst long-enduring component. This post-burst component was also
accompanied by the largest flux-density decimeter burst ever reported,
reaching 1 million solar flux units. The submillimeter impulsive burst
centroid position at 212 GHz was clearly displaced from the precursor
component by almost 1 arc-minute. The maximum limit sizes, estimated at
212 GHz, were of the order or smaller than the beam-sizes (4’). The
microwave spectra for the precursor and long-enduring burst components
peak at about 5-10 GHz. The submillimeter precursor spectrum might be
optically thick emission of the cold chromospheric plasma. Despite the
complexity in space, time and spectra of the superimposed impulsive
and post-impulsive emission, it was remarkable that the THz impulsive
component had its closer counterpart only in the higher energy X- and
γ- rays ranges, suggesting that they are part of the same physical
process, produced by a source of continuously accelerated high-energy
particles.
---------------------------------------------------------
Title: The Ribbon-like Hard X-Ray Emission in a Sigmoidal Solar
Active Region
Authors: Liu, Chang; Lee, Jeongwoo; Gary, Dale E.; Wang, Haimin
2007ApJ...658L.127L Altcode: 2007astro.ph..2326L
Solar flare emissions at Hα and EUV/UV wavelengths often appear in
the form of two ribbons, and this has been regarded as evidence of a
typical configuration of solar magnetic reconnection. However, such
a ribbon structure has rarely been observed in hard X-rays (HXRs),
although it is expected. We report a ribbon-like HXR source observed
with the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI)
at energies as high as 25-100 keV during the 2005 May 13 flare. For
a qualitative understanding of this unusual HXR morphology, we also
note that the source active region appeared in a conspicuous sigmoid
shape before the eruption and changed to an arcade structure afterward
as observed with the Transition Region and Coronal Explorer (TRACE) at
171 Å. We suggest that the ribbon-like HXR structure is associated with
the sigmoid-to-arcade transformation during this type of reconnection.
---------------------------------------------------------
Title: Minor Planet Observations [H93 Berkeley Heights]
Authors: Gary, D.
2007MPC..59317..16G Altcode:
No abstract at ADS
---------------------------------------------------------
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: A Subsystem Test Bed for the Frequency-Agile Solar
Radiotelescope
Authors: Liu, Zhiwei; Gary, Dale E.; Nita, Gelu M.; White, Stephen M.;
Hurford, Gordon J.
2007PASP..119..303L Altcode:
This paper describes the design and operation of the Frequency-Agile
Solar Radiotelescope (FASR) Subsystem Testbed (FST) and the first
observational results. Three antennas of the Owens Valley Solar
Array have been modified so that each sends a 1-9 GHz band of radio
frequency to a central location using a broadband analog optical fiber
link. A dynamically selected 500 MHz subset of this frequency range is
digitized at 1 Gsps (gigasample per second) and recorded to disk. The
full-resolution time-domain data thus recorded are then correlated
through offline software to provide interferometric phase and amplitude
spectra on three baselines. An important feature of this approach is
that the data can be reanalyzed multiple times with different digital
signal-processing techniques (e.g., different bit-sampling, windowing,
and radio frequency interference [RFI] excision methods) to test
the effect of different designs. As a prototype of the FASR system,
FST provides the opportunity to study the design, calibration, and
interference-avoidance requirements of FASR. In addition, FST provides,
for the first time, the ability to perform broadband spectroscopy of
the Sun with spectrograph-like spectral and temporal resolution, while
providing locating ability for simple sources. Initial observations of
local RFI, geostationary satellite signals, global positioning system
(GPS) satellite signals, and the Sun are presented to illustrate the
system performance.
---------------------------------------------------------
Title: Ground-based solar facilities in the U.S.A.
Authors: Denker, C.; Gary, D. E.; Rimmele, T. R.
2007msfa.conf...31D Altcode:
In this review, we present the status of new ground-based facilities
for optical and radio observations of the Sun in the United States. The
4-meter aperture Advanced Technology Solar Telescope (ATST) under the
stewardship of the National Solar Observatory (NSO) has successfully
completed its design phase and awaits funding approval. The 1.6-meter
aperture New Solar Telescope (NST) at Big Bear Solar Observatory
(BBSO) is currently under construction. Complementing these optical
telescopes is the Frequency Agile Solar Radiotelescope (FASR)
an instrument for dynamic broadband imaging spectroscopy covering
a multitude of radio frequencies from 50 MHz to 20 GHz. Imaging
spectroscopy and polarimetry are common features of these telescopes,
which will provide new insight regarding the evolution and nature of
solar magnetic fields. High-resolution observations of solar activity,
bridging the solar atmosphere from the photosphere to the corona, will
be obtained with a dedicated suite of instruments. Special emphasis
of this review will be put on the interplay between instrumentation
and scientific discovery.
---------------------------------------------------------
Title: Minor Planet Observations [H93 Berkeley Heights]
Authors: Gary, D.
2006MPC..58116...9G Altcode:
No abstract at ADS
---------------------------------------------------------
Title: FASR Flare Science: Lessons from the Nobeyama Radioheliograph
Authors: Gary, D. E.
2006spnr.conf..121G Altcode:
We compare and contrast the instrumental parameters of the
Nobeyama Radioheliograph (NoRH) and the future Frequency Agile
Solar Radiotelescope (FASR) with an emphasis on flare research. The
primary results and discoveries of NoRH in the realm of flare science
are presented and used to motivate many of the choices for design
parameters and science goals for FASR. Key parameters of NoRH for flare
science are its dual-frequency, dual-polarization, excellent image
quality, solar-dedicated and solar-optimized design, and high time
resolution. Its main limitations are spatial resolution, frequency
coverage, and frequency resolution. FASR's design makes use of the
strengths of NoRH, and adds these three missing elements: a factor
of 10 improvement in spatial resolution, broad frequency coverage
and high frequency resolution. We discuss the manner in which these
improvements extend the legacy of NoRH for flare science.
---------------------------------------------------------
Title: Observed solar radio burst effects on GPS/Wide Area
Augmentation System carrier-to-noise ratio
Authors: Cerruti, Alessandro P.; Kintner, Paul M.; Gary, Dale E.;
Lanzerotti, Louis J.; de Paula, Eurico R.; Vo, Hien B.
2006SpWea...410006C Altcode:
The first direct observations of Global Positioning System (GPS)
L1 (1.57542 GHz) carrier-to-noise ratio degradation due to a solar
radio burst are presented for an event that occurred on 7 September
2005. Concurrent carrier-to-noise ratio data from GPS satellites are
available from receivers at Arecibo Observatory, Puerto Rico; San Juan,
Puerto Rico; and also from Anderson, South Carolina, United States. The
right-hand circularly polarized (RHCP) signals from this solar radio
burst caused a corresponding decrease in the carrier-to-noise ratio of
about 2.3 dB across all visible satellites. The maximum solar radio
burst power associated with this event was 8700 solar flux units (1
SFU = 10<SUP>-22</SUP> W/m<SUP>2</SUP>/Hz) RHCP at 1600 MHz. Direct
observations of GPS semicodeless L2 carrier-to-noise ratio degradation
from receivers in Brazil are also presented for a solar radio burst
that occurred on 28 October 2003. The maximum degradation at GPS L1
was about 3.0 dB, and a degradation of 10.0 dB was observed on the
semicodeless L2 signal. Scaling to historic solar radio burst records
suggests that GPS L1 receivers could fail to produce a navigation
solution and that semicodeless L1/L2 receivers will fail.
---------------------------------------------------------
Title: The Frequency Agile Solar Radiotelescope (FASR)
Authors: Gary, D. E.
2006IAUJD..12E..19G Altcode:
The Frequency Agile Solar Radiotelescope (FASR) is a solar-optimized,
solar-dedicated radio array that is expected to be the world's premier
solar radio facility for two decades after completion in 2012. This
talk describes the instrument and focuses on the broad science it is to
address. The key science objectives are: 1) the nature and evolution
of coronal magnetic fields; 2) the physics of flares; 3) drivers of
space weather; and 4) physics of the quiet solar atmosphere. Science
goals include coronal magnetography, electron energy and transport,
initiation and acceleration of Coronal Mass Ejections, and coronal
and chromospheric heating, among many others. The instrument, made
up of hundreds of individual antenna elements, will provide solar
images of the highest quality and spatial resolution ever obtained,
at hundreds of frequencies ranging from 50 MHz - 20 GHz, with less
optimized coverage to 30 GHz. The talk will also discuss the key
instrumental design characteristics, and will give the current status
of the project, as well as describing prototyping and study activities
leading to the final proposal for construction.
---------------------------------------------------------
Title: Magnetic Energy Release during the 2002 September 9 Solar Flare
Authors: Lee, Jeongwoo; Gary, Dale E.; Choe, G. S.
2006ApJ...647..638L Altcode:
We study the magnetic energy release during the 2002 September 9
flare using the high-cadence (40 ms) Hα filtergram at Big Bear Solar
Observatory (BBSO), along with hard X-ray and microwave data from
the Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI)
and the Owens Valley Solar Array (OVSA), respectively. We take the
Poynting vector approach with the standard two-dimensional geometry
of the reconnecting current sheet (RCS) but suggest a new technique
to infer the area of the RCS, in order to complete the magnetic energy
calculation entirely with observed quantities. We found five peaks of
impulsive magnetic energy release, concentrated within 10-30 s periods,
that are episodic with the peaks of the hard X-ray light curve. The
maximum amount of energy released per peak reaches ~2.6×10<SUP>30</SUP>
ergs s<SUP>-1</SUP>, and the electron energy deposition rate derived
from the RHESSI spectra falls into the range of 10%-80% of the magnetic
energy release rate. We briefly discuss this result in comparison
with other studies thus far made toward understanding of the magnetic
reconnection in solar flares and suggest the pulsating current sheet
model as the most plausible interpretation of our result.
---------------------------------------------------------
Title: Quasi-periodic Radio And X-ray Pulsations In A Solar Flare
Authors: Bastian, Timothy S.; Fleishman, G.; Gary, D. E.
2006SPD....37.1309B Altcode: 2006BAAS...38..242B
We describe microwave and hard X-ray observations of strong
quasiperiodic pulsations from the GOES X1.3 solar flare on 15
June 2003. Using Fourier analysis, we study the frequency- and
energy-dependent oscillation periods, phase differences, and modulation
depths of the radio and X-ray pulsations. Focusing on the more complete
radio observations, the observational properties of the oscillations
are compared with those derived from models for the radio emission. Two
simple models are considered: i) gyrosynchrotron emission modulated
by the quasiperiodic injection of fast electrons; ii) gyrosynchrotron
emission modulated by an MHD oscillation of the magnetic field. We
demonstrate that quasiperiodic injection of fast electrons is the more
likely cause of the observed quasiperiodic oscillations observed in
the radio and hard X-ray emission. We discuss the implications of this
finding for particle acceleration and transport in the flaring sources.
---------------------------------------------------------
Title: Sigmoid, Type II Precursor, and Coronal Mass Ejection
Authors: Lee, Jeongwoo; Liu, C.; Cho, K.; Gary, D. E.; Yurchyshyn,
V.; Deng, N.; Wang, H.
2006SPD....37.0907L Altcode: 2006BAAS...38..236L
The 2005 May 13 flare occurred in the sigmoidal active region, NOAA
10759, and its time dependent change on the TRACE, EIT, and SXI images
suggests an eruption process as envisioned by the runaway tether-cutting
model. However, the onset of the eruption in the low corona and the
final explosion of the magnetic field in the high corona are not
directly observable at these wavelengths and we infer them indirectly
from the radio data obtained with the Owens Valley Solar Array (OVSA),
Green Bank Solar Radio Burst Spectrometer (GBSRBS), PHOENIX-spectrograph
of ETH Zurich, and the Potsdam-Tremsdorf Spectrograph. The Potsdam
spectrograph shows a so-called Type II Precursor in the early phase
of loop expansion, indicating a coronal shock formed near the flare
site. The GBSRBS spectra show a type III burst followed by type II
bursts in the maximum phase, which implies opening of field lines
and strong shock formation in the high corona. Finally, the radio
pulsations are detected on the OVSA and PHOENIX spectrographs, which
we interpret as due to the magnetic loop oscillation as a dynamic
response to the mass ejection. These radio data along with the EUV
and X-ray images are used to infer the runaway tether-cutting process
during this event in all coronal heights.This work is supported by
NSF/SHINE grant ATM-0548952 and NSF grant AST-0307670 to NJIT.
---------------------------------------------------------
Title: FASR Subsystem Testbed
Authors: Liu, Zhiwei; Gary, D. E.; Nita, G. M.; White, S. M.; Hurford,
G. J.
2006SPD....37.0602L Altcode: 2006BAAS...38..225L
The construction of the Frequency Agile Solar Radiotelescope (FASR)
Subsystem Testbed (FST) and first results are described. Three
antennas of Owens Valley Solar Array (OVSA) are upgraded with the
newly designed, state of art technology. The 1-9 GHz RF signal from
the feed is transmitted through the fiber optical system to the control
room. Then it is downconverted to a 500 MHz single-sideband signal that
can be tuned across the 1-9 GHz RF band. The data are sampled with an
8-bit, 1 GHz sampling-rate digitizer, and further saved to the hard
disk. The correlated (phase and amplitude) spectra are derived through
offline software. As a prototype of the FASR system, FST provides the
opportunity to study the design, calibration, and interference-avoidance
requirements of FASR. FST provides, for the first time, the ability to
perform broadband imaging spectroscopy with high spectral, temporal and
moderate spatial resolution. With this three element interferometer, we
have the ability to determine the location of simple source structures
with very high time resolution (20 ms) and frequency resolution ( <1
MHz) as well as the dynamic spectrum. Initial examples of geostationary
satellite, GPS satellite and solar observations are presented.
---------------------------------------------------------
Title: First Solar Results with the FASR Subsystem Testbed
Authors: Gary, Dale E.; Hurford, G. J.; Liu, Z.; Nita, G. M.; White,
S. M.
2006SPD....37.2502G Altcode: 2006BAAS...38..252G
The Frequency Agile Solar Radiotelescope (FASR) Subsystem Testbed
(FST) is a new, three-element interferometer, utilizing three antennas
of the Owens Valley Solar Array. FST is the first radio instrument
capable of measuring solar burst locations with spectrograph-like
time and frequency resolution. Operating with 500 MHz instantaneous
bandwidth, tunable anywhere in the 1-9 GHz range, FST can measure
centroid locations of typical bursts on millisecond timescales and
MHz frequency resolution. The system is designed as a prototype for
FASR, for several purposes including the study of radio frequency
interference (RFI) mitigation techniques, a testbed for the design of
FASR's digitial signal processing, and investigating use of satellite
signals for calibration. In addition, however, FST is unique in its
ability to locate solar bursts on the extremely fine frequency and
time scales on which their emission varies.A technical description of
the instrument can be found in an accompanying poster (Liu et al.),
along with first results on the several topics mentioned above. In
this talk we focus on the solar applications of FST. We describe the
operation of the system for solar observations, the science goals of the
instrument, and some first results of observations of solar bursts. The
first recorded burst, obtained on the first day of solar observations,
was a group of type III bursts associated with a B5.1 X-ray burst, also
observed with RHESSI. Individual type IIIs show downward propagation
(reverse-frequency slope), with significant positional differences
from one type III to another, while within a type III are seen smaller
positional changes as a function of frequency. This suggests smooth
trajectories for individual bursts, but widely diverging locations
for separate bursts. We describe joint RHESSI/FST observations and
findings for those bursts observed so far. This work is supported by
NSF grant AST-0352915 to NJIT.
---------------------------------------------------------
Title: Analysis of a Flare Producing Active Region Using Data from
the Upgraded OVSA
Authors: Tun, Samuel D.; Gary, D. E.; Nita, G. M.; Lee, J.
2006SPD....37.0117T Altcode: 2006BAAS...38..218T
The Owens Valley Solar Array (OVSA) provides spectral imaging data
in the microwave radio range, 1-18 GHz. In August 2004, work was
completed on adding two 2-m antennas to the existing array, for a
total of seven elements. In addition, the five 2-m antennas were
upgraded to record right and left circular polarization, a feature
previously available only on the two 27-m dishes. The increased number
of baselines improves the instrument's imaging capability and, thus,
allows a more accurate analysis of solar features. We have embarked
on a project to study active regions that were well observed by OVSA
from August 2004 through the end of 2005. This poster concentrates on
the period in September 2005 when NOAA active region AR 10808 crossed
the disk (and produced several strong flares). The present work is an
analysis of the AR 10808's structure during non-flaring times in order
to determine coronal parameters such as temperature, magnetic field
strength, and column emission measure by means of the radio brightness
temperature spectra obtained from radio maps of the region. Polarization
information is used to determine these parameters more precisely,
using the magnetic-field-induced polarization as a constraint. The
results are compared to those inferred from concurrent observations
by BBSO and MDI. This is part of a work in progress to make similar
analyses of the active region over several days in order to determine
changes in the active region structure around flare times, as well as
changes in the active region during its passage across the solar disk.
---------------------------------------------------------
Title: The Eruption from a Sigmoid Active Region on 2005 May 13
Authors: Liu, Chang; Lee, J.; Yurchyshyn, V.; Cho, K.; Deng, N.;
Gary, D. E.; Wang, H.
2006SPD....37.0821L Altcode: 2006BAAS...38..234L
A sigmoidal structure of active regions has been of interest as is
believed to lead to magnetic eruption. We found from TRACE EUV images
that NOAA AR 10759 exhibited a typical sigmoid shape just before the
M8.0 flare and a fast halo CME on 2005 May 13, and reduced to a more
confined arcade after the eruption. We have thus examined these time
dependent changes during the flare by utilizing a multiwavelength
data set: EUV (TRACE, EIT), soft X-rays (SXI), H-alpha (BBSO,ISOON),
radio spectra (OVSA, Potsdam-Tremsdorf, GBSRBS, Phoenix, RSTN),
and CME (LASCO). Several similarities of this event with the runaway
tether-cutting scenario elaborated by Moore et al. (2001) has been
found. Before the maximum phase, the flare brightening first occurred in
the core of the active region, and then two elbows in the outer region
gradually expanded, which we compare to the tether-cutting in the
sigmoid center and the ejective eruption, respectively. At the flare
maximum, the large-scale disturbances such as Moreton waves, type II,
and III radio bursts were observed, which we suggest as evidence for
the blow-out of the envelope field and particle acceleration. Finally
the flare arcade formed along the neutral line as the opened legs of
the envelop field reconnect with each other. This dataset therefore
supports the idea that the eruption may start in the sheared core
field and proceeds outward with the rising plasmoid via the runaway
tether-cutting reconnection and finally becomes a CME.This work is
supported by NSF/SHINE grant ATM-0548952, NSF grant ATM-0536921,
and NASA grant NNG0-4GJ51G.
---------------------------------------------------------
Title: Direct Observations of GPS L1 Signal-to-Noise Degradation
due to Solar Radio Bursts
Authors: Cerruti, A. P.; Kintner, P. M.; Gary, D.; Lanzerotti, L.
2006AGUSMSH53A..07C Altcode:
GPS signals, systems, and navigation accuracy are vulnerable to a
variety of space weather effects mostly caused by the ionosphere. This
paper considers a different class of space weather effects on GPS
signals: solar radio bursts. We present the first direct observations
of GPS L1 (1.6 GHz) carrier-to-noise degradation on two different
models of GPS receivers due to the solar radio burst associated with
the 7 September, 2005 solar flare. The solar radio burst consisted of
two periods of 1.6 GHz activity at approximately 17:40 UT and again
at 18:30 UT. All the receivers that were affected by the solar radio
burst were in the sun-lit hemisphere: three identical receivers were
collocated at the Arecibo Observatory, and four identical receivers
of a different model were located in Brazil. For both models, all GPS
satellites in view were affected similarly. In some cases the decrease
in the GPS L1 signal-to-noise agreed perfectly with the solar radio
burst amplitudes, while in other cases there was no association. Further
analysis indicated that only the right hand circularly polarized (RHCP)
emissions affected the GPS signals. Since GPS signals are RHCP and GPS
antennas are also RHCP, the null effect of the LHCP power confirms our
hypothesis that the solar radio bursts are causal. The maximum solar
radio burst power associated with the 7 September 2005 flare had a
peak intensity of about 8,700 solar flux units (SFU) RHCP at 1,600 MHz,
which caused a corresponding decrease in the signal- to-noise of about
2.5 dB across all visible satellites. Furthermore, an event with a peak
intensity of 5,000 SFU RHCP at 1,600 MHz caused a 2 dB fade for nearly
15 minutes. To further investigate the effect of solar radio bursts,
we also examined the emissions associated with the 28 October 2003
flare. Although polarization data was not available for this even,
a similar association was found between 1,400 MHz solar radio power
and GPS signal-to-noise degradation. The maximum degradation at GPS
L1 was about 3 dB, and a degradation of 10 dB was observed on the
semi-codeless L2 signal. This is larger than previously estimated
by (Klobuchar et al. 1999). Although the events shown are small,
scaling to historical solar radio bursts of 80,000 SFU imply a 12-15
dB drop in the signal-to-noise ratio. Furthermore, solar radio bursts
affect all satellites in view of a receiver and all receivers in the
sun-lit hemisphere. The implications are clear that the largest solar
radio bursts could cause loss-of-tracking for inadequately designed
GPS receivers. Solar radio bursts will also affect the new Galileo
navigation system and all space-based augmentation systems such as
WAAS and EGNOS. Kobuchar, J.A., J.M. Kunches, and A.J. VanDierendonck
(1999), Eye on the Ionosphere: Potential Solar Radio Burst Effects on
GPS Signal to Noise, GPS Solutions, 3(2), 69-71
---------------------------------------------------------
Title: The FASR Reference Instrument
Authors: Bastian, T. S.; Gary, D. E.; Hurford, G. J.; Kawakubo, H.;
Ruf, C.; White, S. M.; Zurbuchen, T.
2006AGUSMSH33A..08B Altcode:
The Frequency Agile Solar Radiotelescope (FASR) is a unique,
solar-dedicated radio facility slated for completion by 2012. The
instrument will address an extremely broad range of solar and
space weather science, including routine measurement of coronal
magnetic fields, imaging coronal mass ejections near the solar
surface, quantitative diagnostics of energy release and particle
acceleration in flares, and the extension of the solar corona into
the heliosphere. Although the precise details of the FASR design
are still being developed, we present for the first time a complete
high-level design referred to as the FASR Reference Instrument. The
Reference Instrument meets the science requirements and will serve
as the basis for cost estimates for construction and operation of
the instrument. This paper gives an overview of the FASR Reference
Instrument, describes the science goals and objectives, and gives the
flowdown of science goals to engineering specifications. The innovative
aspects of the FASR design are highlighted, and a complete, end-to-end
description of the instrument is given. The instrument operations plan
is also discussed.
---------------------------------------------------------
Title: Large-Scale Activities Associated with the 2003 October 29
X10 Flare
Authors: Liu, Chang; Lee, Jeongwoo; Deng, Na; Gary, Dale E.; Wang,
Haimin
2006ApJ...642.1205L Altcode:
We present a multiwavelength study focusing on the large-scale
activities associated with the 2003 October 29 X10 flare and a halo
CME. This event was strong enough to clearly show several large-scale
activities, such as remote brightenings, Moreton waves at Hα off-bands,
and type II radio bursts, which offers an excellent opportunity to
clarify the relationship among them. The remote brightenings were found
near two coronal holes more than 2×10<SUP>5</SUP> km from the main
flare in eastern and southern directions, respectively. Coronal dimmings
were seen at the locus of the remote brightenings right after the flare
at both EUV and soft X-ray wavelengths. The Moreton waves propagated
both northeastward and southward, toward the aforementioned remote
regions, at speeds of approximately 1100 and 1900 km s<SUP>-1</SUP>,
respectively. Our analyses show that the Moreton waves, the type II
radio bursts, and the CME started almost simultaneously, but were not
cotemporal with the remote brightenings. The remote brightenings are
rather consistent with the flare hard X-ray emissions within the active
region, although they have much smaller scales. We therefore conclude
that the two remote brightening regions were magnetically connected to
the flaring active region, and that the remote brightenings, as well
as all other activities, were due to the interaction of an erupting
flux rope at the core of the flare with magnetic field overlying the
region. In this scenario, the large overlying loops should open to
allow the flare activity underneath them, which points to a picture
similar to the magnetic break-out process in such a large scale.
---------------------------------------------------------
Title: Comet Observations [H93 Berkeley Heights]
Authors: Gary, D.
2006MPC..56559...6G Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Observations of Comets
Authors: Tichy, M.; Kadota, K.; Abe, H.; Wakuda, S.; Herald, D.;
Sanchez, A.; Stevens, B. L.; Skiff, B. A.; Kowalski, R. A.; Beshore,
E. C.; Christensen, E. J.; Garradd, G. J.; Grauer, A. D.; Hill, R. E.;
Larson, S. M.; McNaught, R. H.; Bezpalko, M.; Manguso, L.; Torres,
D.; Kracke, R.; Blythe, M.; Love, H.; Stuart, J.; Kommers, J.; Durig,
D. T.; Castellano, J.; Ferrando, R.; Garcia, F.; Cortes, E.; Takbou,
S.; Kocher, P.; Brinkmann, B.; Denzau, H.; Oksanen, A.; Yang, M.;
Ye, Q.; Sugiyama, Y.; Nicholson, M.; Bell, C.; Levin, K.; Pelle,
J. C.; Gary, D.; Mendicini, D.; Morales, R.; Doreste, J. L.; Arce,
E.; Navarro Pina, J. Pablo; Pastor, S.; Reyes, J. A.; Birtwhistle,
P.; Climent, T.; Marsden, B. G.
2006MPEC....F...16T Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Minor Planet Observations [H93 Berkeley Heights]
Authors: Gary, D.
2006MPC..56167...9G Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Sunspot Gyroresonance Emission at 17 GHz: A Statistical Study
Authors: Vourlidas, Angelos; Gary, Dale E.; Shibasaki, Kiyoto
2006PASJ...58...11V Altcode:
We investigate the sunspot gyroresonance emission at 17 GHz using the
synoptic database of the Nobeyama Radioheliograph. Our statistical
study is based on full disk observations obtained during the maximum
of Cycle 22 (1992-94). We study the center-to-limb variation of the
brightness and polarization of the sunspot radio emission and present
some cases of polarization reversal. We find that the radio emission
is most likely 3rd-harmonic gyroresonance emission arising from 2000
G fields in transition region, or low corona temperatures.
---------------------------------------------------------
Title: Spatio-spectral Maximum Entropy Method. I. Formulation and Test
Authors: Bong, Su-Chan; Lee, Jeongwoo; Gary, Dale E.; Yun, Hong Sik
2006ApJ...636.1159B Altcode:
The spatio-spectral maximum entropy method (SSMEM) has been developed
by Komm and coworkers in 1997 for use with solar multifrequency
interferometric observation. In this paper we further improve the
formulation of the SSMEM to establish it as a tool for astronomical
imaging spectroscopy. We maintain their original idea that spectral
smoothness at neighboring frequencies can be used as an additional
a priori assumption in astrophysical problems and that this can be
implemented by adding a spectral entropy term to the usual maximum
entropy method (MEM) formulation. We, however, address major technical
difficulties in introducing the spectral entropy into the imaging
problem that are not encountered in the conventional MEM. These include
calculation of the spectral entropy in a generally frequency-dependent
map grid, simultaneous adjustment of the temperature variables and
Lagrangian multipliers in the spatial and spectral domain, and matching
the solutions to the observational constraints at a large number of
frequencies. We test the performance of the SSMEM in comparison with
the conventional MEM.
---------------------------------------------------------
Title: NoRH and RHESSI Observations of Quasiperiodic Radio and X-ray
Oscillations in a Solar Flare
Authors: Bastian, T.; Fleishman, G.; Gary, D.
2006cosp...36.3251B Altcode: 2006cosp.meet.3251B
We describe microwave and hard X-ray observations of strong
quasiperiodic pulsations from the GOES X1 3 solar flare of 15 June 2003
Using Fourier analysis we study the frequency- and energy-dependent
oscillation periods phase differences and modulation depths of the radio
and X-ray pulsations Focusing on the more complete radio observations
the observational properties of the oscillations are compared with
those derived from models for the radio emission Two simple models are
considered i gyrosynchrotron emission modulated by the quasiperiodic
injection of fast electrons ii gyrosynchrotron emission modulated
by an MHD oscillation of the magnetic field We demonstrate that
quasiperiodic injection of fast electrons is the more likely cause
of the observed quasiperiodic oscillations observed in the radio and
hard X-ray emission We discuss the implications of this finding for
particle acceleration and transport in the flaring sources
---------------------------------------------------------
Title: An Evaluation of the Solar Radio Burst Locator (SRBL) at OVRO
Authors: HwangBo, J. E.; Bong, Su-Chan; Cho, K. S.; Moon, Y. J.; Lee,
D. Y.; Park, Y. D.; Gary, Dale E.; Dougherty, Brian L.
2005JKAS...38..437H Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Spatio-Spectral Maximum Entropy Method: II. Solar Microwave
Imaging Spectroscopy
Authors: Bong, Su-Chan; Lee, Jeong-Woo; Gary, Dale E.; Yun, Hong-Sik;
Chae, Jong-Chul
2005JKAS...38..445B Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Low Frequency Solar Radiophysics and Next Generation
Instrumentation
Authors: Bastian, T. S.; Gary, D. E.
2005ASPC..345..142B Altcode:
Radio astronomy and solar radio astronomy developed rapidly together
in the years following the Second World War. Much of this development
occurred at low frequencies. In more recent decades, the emphasis has
been on centimeter and millimeter wavelengths. However, motivated
by both computational advances and new science drivers, there is
a strong desire on the part of both the solar and astronomical
communities to build modern low frequency radio telescopes. Here,
some of the early work in solar radio astronomy is briefly reviewed
and recent developments in solar and heliospheric physics are
noted. Two next-generation radio telescopes, the Frequency Agile Solar
Radiotelescope and the Long Wavelength Array, are described. Possible
synergies between the two projects are discussed.
---------------------------------------------------------
Title: Our Solar Connection: A themed Set of Activities for Grades
5-12
Authors: van der Veen, W. E.; Gary, D. E.; Gallagher, A. C.; Vinski,
J. M.
2005AAS...207.6505V Altcode: 2005BAAS...37.1260V
The project is a partnership between the Center for Solar-Terrestrial
Research at New Jersey Institute of Technology (NJIT), and the
New Jersey Astronomy Center for Education (NJACE) at Raritan Valley
Community College. It was supported by a NASA Education/Public Outreach
grant from the Office of Space Science. <P />The project involved the
development of a set of seven activities connected by the theme of
solar magnetism and designed to meet the New Jersey Science Process
Standards and the Science Core Curriculum Content Standards in Physics
and Astronomy. The products include a 70-page teacher guide and an
integrated CD-ROM with video clips, internet links, image sets used
in the activities, and worksheets. The activities were presented at
a series of teacher workshops. The teachers performed the activities
themselves, learned additional background information on the Sun,
solar magnetism, and the Sun-Earth connection, and were trained to use
several items of equipment, which were made available in two "resource
centers," one at NJIT and one at NJACE. <P />In all, 81 teachers have
been exposed to some or all of the activities. After the training,
the teachers took the activities back to their classrooms, and 15
equipment to use with their students. Some teachers had access to,
or had their schools purchase, Sunspotters and spectrometers rather
than borrow the equipment. The success of the teacher training was
assessed by questionnaires at the end of the workshops, by evaluation
forms that the teachers filled out on returning the borrowed equipment.
---------------------------------------------------------
Title: An RFI Survey at the Site of the Long Wavelength Deomonstration
Array (LWDA)
Authors: Stewart, K. P.; Crane, P. C.; Paravastu, N.; Hicks, B. C.;
Theodorou, A.; Price, R. M.; Pihlstrom, Y. M.; Bastian, T. S.; Gary,
D. E.
2005AAS...20713604S Altcode: 2005BAAS...37.1389S
An initial survey of the radio-frequency environment at the site of the
Long Wavelength Demonstration Array (LWDA) has been performed using a
measurement protocol developed for both the Long Wavelength Array (LWA)
and the Frequency Agile Solar Radiotelescope (FASR). The measurements
cover the frequency range from 25 MHz to 18 GHz to include the LWA
(23-80 MHz) and FASR (30 MHz-30 GHz). Measurements were obtained
nearly continuously for a week to characterize the day/night and
weekday/weekend variations expected for many sources of RFI. The
equipment, antennas, and protocols are suitable for measuring strong
RFI that potentially threatens the linearity of radio-astronomical
receivers and may therefore rule out possible sites or influence the
design of the receivers. (Weak RFI which may obscure weak signals
of interest is beyond the capabilities of these measurements.) The
protocol seeks only to identify RFI originating from terrestrial
sources; RFI from satellites and astrophysical sources is assumed to
be site independent and not a factor in site selection. Therefore,
the receiving antennas provide azimuthal coverage in the direction of
the horizon. This first survey, conducted prior to the start of LWDA
construction, establishes a baseline for the later identification of
any self-generated interference from the LWDA and its mitigation to
ensure no adverse effect on the operations of the VLA.
---------------------------------------------------------
Title: Minor Planet Observations [H93 Berkeley Heights]
Authors: Gary, D.
2005MPC..54984...3G Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Spatial Evidence for Transition Radiation in a Solar Radio
Burst
Authors: Nita, Gelu M.; Gary, Dale E.; Fleishman, Gregory D.
2005ApJ...629L..65N Altcode: 2005astro.ph..7230N
Microturbulence, i.e., enhanced fluctuations of plasma density and
electric and magnetic fields, is of great interest in astrophysical
plasmas, but occurs on spatial scales far too small to resolve by
remote sensing, e.g., at ~1-100 cm in the solar corona. This Letter
reports spatially resolved observations that offer strong support for
the presence in solar flares of a suspected radio emission mechanism,
resonant transition radiation, which is tightly coupled to the level of
microturbulence and provides direct diagnostics of the existence and
level of fluctuations on decimeter spatial scales. Although the level
of the microturbulence derived from the radio data is not particularly
high, <Δn<SUP>2</SUP>>/n<SUP>2</SUP>~10<SUP>-5</SUP>, it is
large enough to affect the charged particle diffusion and give rise
to effective stochastic acceleration. This finding has exceptionally
broad astrophysical implications, since modern sophisticated numerical
models predict generation of much stronger turbulence in relativistic
objects, e.g., in gamma-ray burst sources.
---------------------------------------------------------
Title: Minor Planet Observations [H93 Berkeley Heights]
Authors: Gary, D.
2005MPC..54362...7G Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Comet Observations [H93 Berkeley Heights]
Authors: Gary, D.
2005MPC..54306..23G Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Comet P/2005 JY126 (catalina)
Authors: Dawson, M.; Lopez, A.; Pacheco, R.; Galli, G.; Seki, T.;
Herald, D.; Blythe, M.; Shelly, F.; Bezpalko, M.; Huber, R.; Manguso,
L.; Torres, D.; Kracke, R.; McCleary, M.; Stange, H.; Milner, A.;
Stuart, J.; Sayer, R.; Evans, J.; Kommers, J.; Emmerich, M.; Melchert,
S.; Sherrod, P. C.; Gary, D.; Sanchez, J.; Marsden, B. G.
2005MPEC....L...56D Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Terahertz imaging using an interferometric array
Authors: Federici, John F.; Gary, Dale; Barat, Robert; Zimdars, David
2005SPIE.5790...11F Altcode:
It has been suggested that interferometric/ synthetic aperture imaging
techniques, when applied to the THz regime, can provide sufficient
imaging resolution and spectral content to detect concealed explosives
and other weapons from a standoff distance. The interferometric imaging
method is demonstrated using CW THz generation and detection. Using
this hardware, the reconstruction of THz images from a point source
is emphasized and compared to theoretical predictions.
---------------------------------------------------------
Title: The Spatial Association of OVSA and RHESSI Sources with
H-alpha Ribbons
Authors: Lee, J.; Gary, D. E.
2005AGUSMSP52A..03L Altcode:
We present detailed comparisons of microwave and hard X-ray maps
with H-alpha ribbons that are obtained with the Owens Valley Solar
Array (OVSA), Reuven Ramaty High Energy Solar Spectroscopic Imager
(RHESSI), and H-alpha filtergrams at Big Bear Solar Observatory
(BBSO), respectively, for the 2002 September 9 flare. Since microwave
emission can be sensitive to nonthermal electrons residing in any part
of the flaring volume depending on local magnetic field and observing
frequency, our investigation is motivated to see whether the OVSA's
imaging spectroscopy will reveal a link between typically confined
hard X-ray sources and rather extended H-alpha kernels. For this
event, we observe single hard X-ray source, three kernels at H-alpha
blue wing, and more than five kernels at H-alpha line center. The
microwave morphology changes from a large arcade-like structure at low
frequencies to single dominant footpoint source at high frequencies, and
indeed indicates the overall connection between the hard X-ray source
and H-alpha kernels. However, one H-alpha kernel lying in an outer
weak-field region has neither hard X-ray nor microwave counterparts, and
the energetic particle transport to this kernel is yet to be explained
by other means. Another non-standard feature of this event is that
the H-alpha kernels remained fixed in space rather than separating
away from the magnetic neutral line while the flare energy release
was going on. We still argue, based on the multifrequency lightcurves,
that all these separate components are energetically related, and the
wavelength dependent morphology is perhaps due to energy threshold
associated with each radiation.
---------------------------------------------------------
Title: Antenna Configurations for the FASR B Array
Authors: Tun, S. D.; Gary, D.; White, S. M.
2005AGUSMSP43A..09T Altcode:
The proposed Frequency-Agile Solar Radiotelescope (FASR) is to
produce high space, time, and frequency resolution full disk solar
images. These will allow the reconstructions of the 3-D structure
of the Sun's atmosphere from chromosphere to mid-corona. The
high resolutions are required to carry out detailed diagnostics
of fast-moving phenomena, such as flares and CMEs, as well as of
quasi-static structures above active and quiet regions. The full disk
images will capture all activity in the corona, even when there are
many active regions at solar maximum. One of the tasks necessary to
ensure optimal imaging for the array is to determine the best antenna
configuration. The current poster presents work done towards this
goal. We focus on the FASR-B array, which will cover the 200-3000 MHz
range with 60 to 90 antennas of 6-m diameter. We have examined the
imaging characteristics for a 3 arm, log-spiral configuration with
radius of about 3 km. Multi-frequency observations are simulated for
arrays varying in initial antenna spacing, number of antenna elements,
and number of turns for each arm. The images reconstructed from the
simulated snapshot observations are compared to the original model,
convolved with the appropriate beam, to see which configuration yields
the most faithful images. We discuss the implications of the findings
for the planning of the FASR array configurations.
---------------------------------------------------------
Title: Imaging Type IIIdm Burst Trajectories
Authors: Liu, Z.; Gary, D. E.; Nita, G. M.; White, S. M.; Hurford,
G. J.
2005AGUSMSP21B..06L Altcode:
Type IIIdm radio bursts are important diagnostics of the upward and
downward directed beams of nonthermal electrons that originate in the
energy release site. Because these bursts have a very high frequency
drift rate ( |df/dt|=(0.09±0.03)× f1.35±0.10, Melendez et al.,
1999), high time and frequency resolution is a necessary for their
observation. Since existing interferometers do not have this combination
of spectral and time resolution in the decimetric range, we still do
not have detailed information about the location and trajectory of
Type IIIdm bursts. We are developing the new FASR Subsystem Testbed
(FST) with three existing antennas of Owens Valley Solar Array (OVSA)
that will permit imaging with high time and spectral resolution
over the 1-9 GHz band. To accomplish this, the received signal is
downconverted to 500 MHz bandwidth, then digitized with 1 GHz sampling
rate, with digital correlation performed offline. With this three
element interferometer, we have the ability to determine the centroid
of simple source structures with very high time resolution (10 ms)
and frequency resolution (<1 MHz). Such centroids as a function of
frequency may outline the trajectories of nonthermal electron beams
and so may show the magnetic topology at the energy release site. The
FST is expected to be operational in early 2006. This poster presents
the FST system configuration and the results of simulations of such
trajectories, using a loop model with a simple magnetic geometry,
density and temperature profiles given by hydrostatic equilibrium,
for different loop heating functions (Aschwanden 2004, Physics of The
Solar Corona, Chapter 3).
---------------------------------------------------------
Title: Our Solar Connection: A Themed Set of Activities for Grades
5-12
Authors: Gary, D. E.; van der Veen, W.; Gallagher, A.
2005AGUSMSH11A..04G Altcode:
We present a set of 7 interconnected activities on the theme of
Sun-Earth connections that we have developed and presented at teacher
workshops as part of a NASA-funded Education and Public Outreach
project. Associated with the activities are two resource centers that
are maintained locally at NJIT in Newark, NJ, and at Raritan Valley
Community College (RVCC) in North Branch, NJ. We describe each of the
activities and the resource equipment, and present some statistics of
how successful the teachers have been in using the activities in their
classrooms. A resource CD containing the text of a teacher's guide is
available that explains each activity, lists the associated materials
needed for each activity, gives estimates for the time required for
each activity, and points out specific New Jersey Science Standards
met by each activity. Additional resources including movies and photos
are included on the CD, and links to this material are contained in
the teacher's guide.
---------------------------------------------------------
Title: Quasiperiodic Electron Acceleration in the 15 June 2003
Solar Flare
Authors: Fleishman, G. D.; Bastian, T. S.; Gary, D. E.
2005AGUSMSP41C..04F Altcode:
We report a comprehensive analysis of strong quasiperiodic radio-
and X-ray pulsations observed from the X1.3-class flare which
occurred at S06E78 on 23:42-23:50 UT, 15 June 2003. Because of the
favorable time of the flare, it was jointly observed by the Owens
Valley Solar Array (OVSA) and the Nobeyama Solar Radio Observatory,
which allowed us the advantage of combining high spectral, temporal,
and spatial resolution radio observations. In addition, the part of
this event displaying the strongest pulsations was also observed
in hard X-rays with RHESSI. We study the frequency dependence of
quantitative measures of the pulsations, including Fourier spectra,
phase differences, modulation depth, as well as the degree of (radio)
polarization. We compare these measures with the expectations of
competing radio pulsation models, i.e., MHD loop oscillations and
quasiperiodic electron injection. Although the Fourier spectra display
a few significant peaks at each observing frequency, we found that
none of them can be explained by the MHD-oscillations. In contrast,
the model of quasiperiodic particle acceleration/injection is capable
of explaining all quantitative measures observed for this event in a
natural way. We discuss implications of these findings for electron
acceleration and transport in solar flares.
---------------------------------------------------------
Title: Spatially Resolved Observations Confirm Transition Radiation
in a Solar Radio Burst
Authors: Nita, G. M.; Gary, D. E.; Fleishman, G. D.
2005AGUSMSP52A..01N Altcode:
We report the first confirmed spatially resolved observation of the
decimetric Resonant Transition Radiation (RTR) in a solar radio burst,
which is an unavoidable by-product of microturbulences present in
dense enough astrophysical plasmas. A number of recent publications,
based mainly on studies of individual events, found some indication
that RTR may be produced in solar radio bursts. Most recently, we
have described the observational characteristics expected for RTR
in the case of solar radio bursts (Fleishman, Nita, and Gary, 2005,
ApJ, 620, 506), and found that the correlations and associations
predicted for total power data are indeed present in the decimetric
components of a statistical sample of two-component solar continuum
radio bursts. However, interpretations based on non-imaging total
power data remain indirect (and, thus, ambiguous) until they can be
combined with direct imaging evidence from multi-wavelength spatially
resolved observations, which so far have been missing in the previous
studies. The spectral components of such RTR candidate bursts (one at
centimeter wavelengths due to the usual gyrosynchrotron mechanism, and
one at decimeter wavelengths suspected as RTR), must be co-spatial
to allow an unambiguous RTR interpretation. This study presents
comprehensive (radio, optical, and soft X-ray) spatially resolved
observations for one of these bursts, which, together with the already
demanding spectral and polarization correlations found previously,
provide direct evidence for the presence of RTR.
---------------------------------------------------------
Title: Evidence for Resonant Transition Radiation in Decimetric
Continuum Solar Bursts
Authors: Fleishman, Gregory D.; Nita, Gelu M.; Gary, Dale E.
2005ApJ...620..506F Altcode:
We investigate the properties of radio bursts observed in the 1-18
GHz frequency range that display two distinct spectral components
peaking at decimetric (dm) and centimetric (cm) ranges. The dm
emission is relatively smooth in both frequency and time, with
timescales comparable to those of the cm component. The two spectral
components display specific correlations in their temporal and spectral
behavior. Through detailed analysis, we find the following: (1) A large
ratio of plasma frequency to gyrofrequency is a characteristic of all of
these events. (2) The dm component generally displays a high degree of
o-mode polarization. (3) The dm component is produced by an incoherent
emission mechanism. (4) The dm and cm components are generated by
the same electron distribution. (5) The characteristic energy of the
fast electrons producing the dm continuum is significantly lower than
the energy of the electrons generating the microwave gyrosynchrotron
component. (6) The spectral shape of the dm component is not well
correlated with the fast electron distribution over energy. These
findings cannot easily be explained with standard mechanisms for dm
solar burst emission but agree well with the predictions of the theory
of resonant transition radiation. We conclude that these two-component
bursts represent a distinctive subclass of events, comprising about
10% of all bursts, with the dm continuum component generated most
probably by resonant transition radiation produced by interaction
of fast electrons with small-scale inhomogeneities of the background
plasma. We discuss new possibilities for flaring plasma diagnostics
using these two-component bursts.
---------------------------------------------------------
Title: Minor Planet Observations [H93 Berkeley Heights]
Authors: Gary, D.
2005MPC..53647..13G Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Noise in wireless systems from solar radio bursts
Authors: Lanzerotti, L. J.; Gary, D. E.; Nita, G. M.; Thomson, D. J.;
Maclennan, C. G.
2005AdSpR..36.2253L Altcode:
Solar radio bursts were first discovered as result of their
interference in early defensive radar systems during the Second World
War (1942). Such bursts can still affect radar systems, as well as
new wireless technologies. We have investigated a forty-year record of
solar radio burst data (1960-1999) as well as several individual radio
events in the 23rd solar cycle. This paper reviews the results of a
portion of this research. Statistically, for frequencies f ∼ 1 GHz
(near current wireless bands), there can be a burst with amplitudes
>10<SUP>3</SUP> solar flux units (SFU; 1 SFU = 10<SUP>-22</SUP>
W/m<SUP>2</SUP>) every few days during solar maximum conditions, and
such burst levels can produce problems in contemporary wireless systems.
---------------------------------------------------------
Title: Solar system science with SKA
Authors: Butler, B. J.; Campbell, D. B.; de Pater, I.; Gary, D. E.
2004NewAR..48.1511B Altcode: 2004astro.ph..9220B
Radio wavelength observations of solar system bodies reveal unique
information about them, as they probe to regions inaccessible by nearly
all other remote sensing techniques and wavelengths. As such, the SKA
will be an important telescope for planetary science studies. With
its sensitivity, spatial resolution, and spectral flexibility and
resolution, it will be used extensively in planetary studies. It will
make significant advances possible in studies of the deep atmospheres,
magnetospheres and rings of the giant planets, atmospheres, surfaces,
and subsurfaces of the terrestrial planets, and properties of small
bodies, including comets, asteroids, and KBOs. Further, it will allow
unique studies of the Sun. Finally, it will allow for both indirect
and direct observations of extrasolar giant planets.
---------------------------------------------------------
Title: Statistics of solar microwave radio burst spectra with
implications for operations of microwave radio systems
Authors: Nita, Gelu M.; Gary, Dale E.; Lanzerotti, Louis J.
2004SpWea...211005N Altcode:
Analysis of 412 solar radio burst events in the microwave range [1.2-18]
GHz that were measured at the New Jersey Institute of Technology Owens
Valley Solar Array over 2 years (2001-2002) during the peak of the 23rd
solar cycle is conducted in the context of the possible interference
of such bursts with radio-receiving and radar systems that operate
at these frequencies. It is found that there is a small probability
for solar interference for systems operating at a frequency ν >
2.6 GHz and with a typical noise floor, but the probability increases
significantly at lower frequencies. The probability is greater for
more sensitive systems operating with higher gain, but the likelihood
of interference can be partially ameliorated by an increase in the
operating frequency. Finally, our statistics indicate that it could be
beneficial for radio and radar systems operating below 2.6 GHz to employ
an adaptable mode wherein, if the prediction of solar activity suggested
the possible occurrence of a large radio burst, system operators could
switch operations to a less susceptible frequency band above 2.6 GHz.
---------------------------------------------------------
Title: Terahertz near-field interferometric and synthetic aperture
imaging
Authors: Walsh, Kenneth P.; Schulkin, Brian; Gary, Dale; Federici,
John F.; Barat, Robert; Zimdars, David
2004SPIE.5411....9W Altcode:
The imaging properties of planar, spherical, and circular
interferometric imaging arrays are examined in the near-field region
limit. In this region, spherical and circular array architectures can
compensate for near-field distortions and increase the field of view
and depth of focus. The application of near-field interferometric
imaging to the Terahertz frequency range for detection of concealed
objects is emphasized.
---------------------------------------------------------
Title: Analysis of terahertz spectral images of explosives and
bioagents using trained neural networks
Authors: Oliveira, Filipe; Barat, Robert; Schulkin, Brian; Huang,
Feng; Federici, John F.; Gary, Dale; Zimdars, David
2004SPIE.5411...45O Altcode:
A non-invasive means to detect and characterize concealed agents of
mass destruction in near real-time with a wide field-of-view is under
development. The method employs spatial interferometric imaging of
the characteristic transmission or reflection frequency spectrum in
the Terahertz range. However, the successful (i.e. low false alarm
rate) analysis of such images will depend on correct distinction of
the true agent from non-lethal background signals. Neural networks
are being trained to successfully distinguish images of explosives
and bioagents from images of harmless items. Artificial neural
networks are mathematical devices for modeling complex, non-linear
relationships. Both multilayer perceptron and radial basis function
neural network architectures are used to analyze these spectral
images. Positive identifications are generally made, though, neural
network performance does deteriorate with reduction in frequency
information. Internal tolerances within the identification process
can affect the outcome.
---------------------------------------------------------
Title: Radio Spectral Diagnostics
Authors: Gary, Dale E.; Hurford, G. J.
2004ASSL..314...71G Altcode:
We review solar radio emission from the perspective of imaging
spectroscopy. Radio emission mechanisms differ in their spectral
properties, which can be used to determine parameters of the emitting
particles and the ambient solar atmosphere in the emitting volume. After
generation, the propagation of radiation in the solar atmosphere
leads to changes in the spectrum that can be exploited to gain further
information about the atmospheric parameters along the line of sight.
---------------------------------------------------------
Title: Analysis of the impulsive phase of a solar flare at
submillimeter wavelengths
Authors: Raulin, Jean Pierre; Makhmutov, Vladimir S.; Kaufmann, Pierre;
Pacini, Alessandra Abe; Lüthi, Thomas; Hudson, Hugh S.; Gary, Dale E.
2004SoPh..223..181R Altcode:
We present a report on the strong X5.3 solar flare which occurred
on 25 August 2001, producing high-level γ-ray activity, nuclear
lines and a dramatic long-duration white-light continuum. The bulk
of millimeter radio fluxes reached a peak of ∼100 000 solar flux
units at 89.4 GHz, and a few thousands of solar flux units were
detected in the submillimeter range during the impulsive phase. In
this paper we focus on and discuss (i) the implications inferred
from high frequency radio observations during the impulsive phase;
(ii) the dynamics of the low corona active region during the impulsive
phase. In particular we found that 4-5 × 10<SUP>36</SUP> accelerated
(>20 keV) electrons s<SUP>−1</SUP> radiating in a 1000-1100 G
region, are needed to explain the millimeter to submillimeter-wave
emissions. We present evidence that the magnetic field in the active
region was very dynamic, and that strong non-thermal processes were
triggered by the appearance of new, compact, low-lying (few thousand
kilometers) loop systems, suggesting the acceleration site(s) were
also located in the low solar atmosphere.
---------------------------------------------------------
Title: Solar and Space Weather Radiophysics - Current Status and
Future Developments
Authors: Gary, D. E.; Keller, C. U.
2004ASSL..314.....G Altcode: 2004sswr.book.....G
The book explores what can be learned about the Sun and interplanetary
space using present-day and future radio observations and
techniques. The emphasis is on interpretation of radio data with high
spatial and spectral resolution, motivated by the planned construction
of a new, powerful, solar-dedicated radio array called the Frequency
Agile Solar Radiotelescope (FASR). The book is unique in exploring a
broad frequency range, which corresponds to heights ranging from the
low solar atmosphere out to the Earth. The book contains a thorough
review of the entire field of solar and Space Weather radio research;
gives background information suitable for advanced undergraduates,
graduates, and researchers in solar and Space Weather research and
related fields; and looks at what new results may be expected in
the next two decades with FASR and other new instruments now under
development. The individual chapters are written by international
experts in each topic, and although each chapter may be read as a
stand-alone article, the ordering of the chapters and the topical
development makes the book readable from beginning to end, to provide
an excellent understanding of the field as a whole.
---------------------------------------------------------
Title: Hard X-Ray and Microwave Observations of Microflares
Authors: Qiu, Jiong; Liu, Chang; Gary, Dale E.; Nita, Gelu M.;
Wang, Haimin
2004ApJ...612..530Q Altcode:
In this paper, we study solar microflares using the coordinated hard
X-ray and microwave observations obtained by the Reuven Ramaty High
Energy Solar Spectroscopic Imager (RHESSI) during its open-shutter
operation mode and the Owens Valley Solar Array (OVSA). The events
in our study are selected in the energy range of 12-25 keV and are
relatively large microflares with an average GOES soft X-ray incremental
flux at the B2.0 level. A total of 760 microflares are identified from
the RHESSI burst catalog. Of the 200 microflares that fall into the
OVSA observing window, about 40% are detected in microwaves. Using
these hundreds of events as samples, we study the event distribution
with respect to the flux, the solar activity, and active regions,
in comparison with flares of larger scales. Nonthermal properties
of microflares are investigated through spectral analysis of X-rays
and microwaves. (1) We find that the event frequency distribution
with respect to the RHESSI peak count rates at 12-25 keV can be
accurately described with a power-law function down to 8 counts
s<SUP>-1</SUP>, the power-law index being 1.75+/-0.03, consistent with
previous studies. (2) Similar to large flares, the occurrence rate of
microflares is correlated with solar activity. The studied samples
of microflares are mostly produced by active regions, as suggested
by the large percentage of events detected by OVSA, which observes
target active regions. However, all active regions do not have equal
productivity, and certain active regions are a lot more productive
than other regions. (3) While some large and complex active regions are
predominantly productive in both very weak and strong events, we also
find an active region that produces many microflares and C-class events
but does not produce powerful events. (4) Analysis of energy-dependent
time profiles suggests that there is a pronounced temporal correlation
between the time derivative of soft X-rays and 14-20 keV hard X-rays,
i.e., the Neupert effect, in about one-half the studied events. (5)
Albeit small, many microflares exhibit hard X-ray emission at over
10 keV and microwave emission at around 10 GHz. Spectral analysis in
these two wavelengths corroborates the nonthermal nature of these
emissions. (6) In a limited number of samples, the RHESSI spectral
fitting yields a photon spectral index of 4.5-7, and microwave
spectral analysis on the same events shows that the power-law index
of the electron spectrum is in the range of 2-5. The discrepancy in
the electron spectrum index derived from hard X-rays and microwaves is
substantially greater than previously reported in big flares, hinting
at the existence of high-energy, microwave-emitting electrons that have
a much hardened spectrum compared with electrons emitting hard X-rays.
---------------------------------------------------------
Title: FASR Simulation of Solar Flare Microwave Spectrum
Authors: Lee, J.; Gary, D. E.; White, S. M.
2004AAS...204.5414L Altcode: 2004BAAS...36..760L
We investigate microwave diagnostics on solar flare electrons at the
quality of imaging spectroscopy expected from the proposed Frequency
Agile Solar Radiotelescope (FASR). We mainly concern ourselves with
influence of the spatial inhomogeneities upon microwave spectral
features and realistic magnetic configurations of flare active
regions. Solar flare microwave radiations are calculated by using
nonthermal electrons distributed along a set of reconnected field
lines, and by sampling the resulting gyrosynchrotron intensities
at the resolution of the FASR. The simulated observations are then
compared with reference spectra of homogeneous source models to find
a strategy for inversion of the observed spectrum to the physical
parameters of flare electrons. <P />This work is supported by the NSF
grant AST-0138317 and NASA grant NAG5-10891 to New Jersey Institute
of Technology.
---------------------------------------------------------
Title: Decimetic Continuum Bursts Observed with the Owens Valley
Solar Array
Authors: Nita, G. M.; Fleishman, G. D.; Gary, D. E.
2004AAS...204.3805N Altcode: 2004BAAS...36..713N
A recent statistical study of 412 microwave burst spectra observed with
OVSA during 2001-2002 (Nita, Gary and Lee 2004) revealed that about
16% of these spectra display a decimetric (dm) spectral component
accompanying the centimetric (cm) emission. Rather surprisingly,
most of the dm components display smooth spectra, with time scales
comparable with those of their cm counterparts. In all considered
events, the dm and cm spectral components are correlated in different
aspects, so related to each other. We find no indication for these
events of an anisotropic pitch-angle distribution, neither loss-cone
nor beam-like, so no condition for any kinetic instability producing
coherent radio emission is found. The dm component is likely produced
by an incoherent emission mechanism, since its flux is proportional
to the number of emitting electrons. The dm component is O-polarized,
the degree of polarization being very large in some cases. We also
find that the characteristic energy of fast electrons producing the
dm continuum is significantly lower than the energy of the electrons
generating the microwave gyrosynchrotron component. It appears that
the spectral shape of the dm component is fairly independent of the
fast electron distribution over energy. These findings agree well
with the theoretical predictions for resonant transition radiation
arising as a result of interaction of fast electrons and small-scale
inhomogeneities of the background plasma, while they can hardly be
interpreted within other known mechanisms of incoherent emission
(like gyrosynchrotron, bremsstrahlung or inverse Compton). We discuss
new possibilities of flaring plasma diagnostics with the use of these
two-component bursts. This work is supported by NSF grant AST-0307670
to New Jersey Institute of Technology.
---------------------------------------------------------
Title: A New Radio Diagnostic for Razin Suppression
Authors: Gary, D. E.; Melnikov, V. F.; Nita, G. M.
2004AAS...204.8806G Altcode: 2004BAAS...36..820G
We have investigated the dynamical behavior of the radio spectrum
of more than 300 microwave bursts observed with the Owens Valley
Solar Array in the range 1.2-18 GHz. For this talk, we concentrate
on the evolution of the peak frequency of the spectrum as measured
throughout the bursts. We find that in most bursts the evolution
fits the expected behavior for gyrosynchrotron emission, moving to
higher frequencies on the rise and falling back on the decay of the
burst. However, for some 35% of bursts we find that this expected
behavior is modified during part or all of the burst by the presence
of Razin Suppression, which is expected in sources with higher than
usual ambient density. Looking at other aspects of the spectral
evolution, we deduce several measures that occur together to uniquely
distinguish the presence of Razin Suppression. The presence of Razin
Suppression has historically been associated only with the presence of
a steeper than usual low frequency spectral slope, but this traditional
diagnostic is valid only for homogeneous sources and can be completely
masked in the typical case of inhomogeneous or multiple sources. Our
new diagnostics of Razin Suppression reveal its presence even for
inhomogeneous sources, and hence has a wider range of validity. The
new diagnostics will be especially useful with the high resolution
imaging microwave spectroscopy that will be provided by the proposed
FASR array. This work is supported by NSF grant AST-0307670 to New
Jersey Institute of Technology.
---------------------------------------------------------
Title: Simulation of Microwave Emissions from Helmet Streamer
Authors: Tun, S. D.; Gary, D. E.; Lee, J.
2004AAS...204.7102T Altcode: 2004BAAS...36R.797T
Helmet streamers are an important magnetic structure on the sun
in relation to prominence formation and coronal mass ejections. We
investigate some methods by which the magnetic field configuration in
a streamer can be reconstructed from observations with the proposed
Frequency Agile Solar Radio telescope (FASR). We begin with the global
coronal magnetic field model for the forward problem proposed by Judge
and Low in 2004. We add a realistic coronal temperature and density
model that obeys hydrostatic equilibrium and allows gyroresonance
emission to dominate the free-free emission at a selected observing
frequency. Theoretical brightness temperature maps are created from the
model at multi-frequencies and are then sampled using AIPS to simulate
observations with the FASR. The magnetic field structure reconstructed
from the simulated observations is compared with the original input
configuration as a test of our diagnostics on the magnetic field. <P
/>This work is supported by the NSF grants AST-0138317 and ATM-0077273
to New Jersey Institute of Technology.
---------------------------------------------------------
Title: Magnetic Reconnection and Mass Acceleration in Flare-Coronal
Mass Ejection Events
Authors: Qiu, Jiong; Wang, Haimin; Cheng, C. Z.; Gary, Dale E.
2004ApJ...604..900Q Altcode:
An observational relationship has been well established among
magnetic reconnection, high-energy flare emissions and the rising
motion of erupting flux ropes. In this paper, we verify that the rate
of magnetic reconnection in the low corona is temporally correlated
with the evolution of flare nonthermal emissions in hard X-rays and
microwaves, all reaching their peak values during the rising phase of
the soft X-ray emission. In addition, however, our new observations
reveal a temporal correlation between the magnetic reconnection rate
and the directly observed acceleration of the accompanying coronal
mass ejection (CME) and filament in the low corona, thus establishing
a correlation with the rising flux rope. These results are obtained
by examining two well-observed two-ribbon flare events, for which we
have good measurements of the rise motion of filament eruption and
CMEs associated with the flares. By measuring the magnetic flux swept
through by flare ribbons as they separate in the lower atmosphere,
we infer the magnetic reconnection rate in terms of the reconnection
electric field E<SUB>rec</SUB> inside the reconnecting current sheet
(RCS) and the rate of magnetic flux convected into the diffusion
region. For the X1.6 flare event, the inferred E<SUB>rec</SUB> is ~5.8 V
cm<SUP>-1</SUP> and the peak mass acceleration is ~3 km s<SUP>-2</SUP>,
while for the M1.0 flare event E<SUB>rec</SUB> is ~0.5 V cm<SUP>-1</SUP>
and the peak mass acceleration is 0.2-0.4 km s<SUP>-2</SUP>.
---------------------------------------------------------
Title: Statistical Study of Two Years of Solar Flare Radio Spectra
Obtained with the Owens Valley Solar Array
Authors: Nita, Gelu M.; Gary, Dale E.; Lee, Jeongwoo
2004ApJ...605..528N Altcode:
We present results of analysis of 412 flares during 2001-2002 as
detected by the Owens Valley Solar Array (OVSA). This is an in-depth
study to investigate some results suggested by a previous study of
solar bursts (Nita et al. 2002), which was limited to the peak time
of the bursts at a few frequency bands. The new study includes the
temporal dependence, at 4 s time resolution, of parameters measured
at 40 frequencies in the range 1-18 GHz. We investigate distributions
of burst parameters such as maximum flux density in the spectra,
peak frequency, spectral slopes below and above the peak frequency
(optically thick and thin slopes, respectively), and burst durations. We
classify the microwave bursts according to their spectral properties
and provide tables of averaged spectral parameters for each spectral
type and for different frequency and intensity ranges.
---------------------------------------------------------
Title: Impulsive and Gradual Nonthermal Emissions in an X-Class Flare
Authors: Qiu, Jiong; Lee, Jeongwoo; Gary, Dale E.
2004ApJ...603..335Q Altcode:
In this paper we present a comprehensive case study of an X-class
flare observed on 2001 April 6. The flare consists of two episodes, the
first characterized by impulsive spiky bursts and the second by gradual
smooth emission at hard X-ray and microwave wavelengths. Emissions in
the two episodes are regarded as the impulsive and gradual components,
respectively. We compare the temporal, spatial, and spectral evolution
of the two components in hard X-rays and microwaves. For this event,
the most important finding is that both the impulsive and gradual
hard X-rays at >=50 keV are thick-target emissions at conjugate
footpoints. Evolution of hard X-rays and microwaves during the gradual
phase exhibits a separation motion between two footpoint sources,
which reflects progressive magnetic reconnection. Observations further
reveal distinct spectral properties of the gradual component. It is
most prominently observed in the high-energy (>20 keV) range,
and the gradual hard X-rays have a harder and hardening spectrum
compared with the impulsive component. The gradual component is also
a microwave-rich event, with the microwaves lagging the hard X-rays
by tens of seconds. A correlation analysis of the hard X-ray light
curves shows energy-dependent time delays, with the 200 keV hard X-rays
lagging the 40 keV emission by 20 s. The observations and analyses
suggest that magnetic reconnection occurs during both the impulsive
and gradual phases that generate nonthermal electrons, primarily
precipitating at the footpoints. However, the temporal and spectral
properties of the gradual component must be produced by an acceleration
mechanism different from that of the impulsive phase. We propose that
the “collapsing-trap” effect, as a product of progressive magnetic
reconnection in bipolar magnetic fields, is a viable mechanism that
continuously accelerates the gradual-phase electrons in a low-density
trap before they precipitate into the footpoints.
---------------------------------------------------------
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: A New Solar Burst Spectral Component Emitting Only in the
Terahertz Range
Authors: Kaufmann, Pierre; Raulin, Jean-Pierre; de Castro,
C. G. Giménez; Levato, Hugo; Gary, Dale E.; Costa, Joaquim E. R.;
Marun, Adolfo; Pereyra, Pablo; Silva, Adriana V. R.; Correia, Emilia
2004ApJ...603L.121K Altcode:
Solar flare energy manifestations were believed to be the result of
the same kind of particle acceleration. It is generally accepted that a
population of relativistic electrons accelerated during the impulsive
phase of solar flares produces microwaves by synchrotron losses in
the solar magnetic field and X-rays by collisions in denser regions
of the solar atmosphere. We report the discovery of a new intense
solar flare spectral radiation component, peaking somewhere in the
shorter submillimeter to far-infrared range, identified during the
2003 November 4 large flare. The new solar submillimeter telescope,
designed to extend the frequency range to above 100 GHz, detected this
new component with increasing fluxes between 212 and 405 GHz. It appears
along with, but is separated from, the well-known gyrosynchrotron
emission component seen at microwave frequencies. The novel emission
component had three major peaks with time, originated in a compact
source whose position remained remarkably steady within 15". Intense
subsecond pulses are superposed with excess fluxes also increasing
with frequency and amplitude increasing with the pulse repetition
rate. The origin of the terahertz emission component during the flare
impulsive phase is not known. It might be representative of emission
due to electrons with energies considerably larger than the energies
assumed to explain emission at microwaves. This component can attain
considerably larger intensities in the far-infrared, with a spectrum
extending to the white-light emission observed for that flare.
---------------------------------------------------------
Title: Flare-related Magnetic Anomaly with a Sign Reversal
Authors: Qiu, Jiong; Gary, Dale E.
2003ApJ...599..615Q Altcode:
In this paper we report a significant magnetic anomaly, specifically
an apparent sign reversal of magnetic polarities in small areas of
Michelson Doppler Imager (MDI) magnetograms during the impulsive phase
of an X5.6 flare on 2001 April 6. Three flare kernels were observed
to emit >=50 keV hard X-rays, which are located in strong magnetic
fields of order +/-1000-1500 G. We find that the apparent sign reversal
began and persisted for a few minutes in all three kernels, in precise
temporal and spatial correspondence with the hard X-ray sources. We
search for a combination of instrumental and flare-induced line profile
effects that can account for this behavior. Our studies provide a
viable scenario that the observed transient sign reversal is likely to
be produced by distorted measurements when the Ni I 6768 Å line comes
into emission or strong central reversal as a result of nonthermal
beam impact on the atmosphere in regions of strong magnetic fields.
---------------------------------------------------------
Title: Radio Coverage from Chromosphere to Earth: FASR-LOFAR-SIRA
Synergy
Authors: Gary, D. E.; Kassim, N.; Gopalswamy, N.; Aschwanden, M. J.
2003AGUFMSH42E..02G Altcode:
Radio emission is uniquely sensitive to a number of key plasma
parameters (magnetic field, temperature, density, high-energy
electrons, and various plasma waves) over heights ranging without
gaps from the chromosphere, throughout the corona and heliosphere, to
the Earth. Two ground-based radio arrays, the Frequency Agile Solar
Radiotelescope (FASR) and the Low Frequency Array (LOFAR), together
with the space-based Solar Imaging Radio Array (SIRA) are planned
that will for the first time provide direct imaging of disturbances
over this vast height range through interferometric imaging over their
equally impressive frequency range of 24 GHz to 30 kHz. We describe the
science goals of these instruments, focusing especially on the science
addressed jointly by all three instruments. Among the examples are
(1) simultaneous imaging of CMEs, flaring loops, and shock-associated
(type II) emission and (2) imaging the propagation of electrons on
open field lines (type III), from their acceleration point through
the corona and heliosphere to the point where they are measured in
situ by near-Earth spacecraft. In addition to spatially relating the
different phenomena, the spectral information is rich in quantitative
diagnostics. We give some examples of the revolutionary results we
can expect from the combined instruments.
---------------------------------------------------------
Title: Coronal Magnetic Diagnostics With FASR
Authors: White, S. M.; Gary, D. E.; Lee, J.; Giordano, G.
2003AGUFMSH42D..04W Altcode:
Coronal magnetography is one of the main scientific drivers for the
proposed Frequency Agile Solar Radiotelescope (FASR). Radio emission
is particularly valuable as a diagnostic of coronal magnetic fields
because (a) the emission mechanisms all depend on magnetic field,
and (b) typical values of the electron gyroresonance frequency f_B
for coronal field strengths lie in the radio domain. The microwave
emission from active regions is dominated by thermal gyroresonance
emission at low harmonics of f_B and this provides a well-understood
diagnostic. Since f_B is proportional to magnetic field strength,
there is a simple mapping between frequency and magnetic field. A
wide range of coronal magnetic field strengths can be sampled by
observing across a wide range of radio frequencies simultaneously,
and FASR is designed to do this quickly enough to follow changes in
coronal fields. We demonstrate the ability to measure coronal fields
with this technique by simulating a FASR observation of a realistic
three-dimensional model of an active region and then determining the
coronal magnetic field at the base of the corona from the simulated
images. Comparison with radio images of gyroresonance emission from
active regions is also a valuable tool for assessing extrapolations
of surface magnetic field measurements into the corona, and we discuss
several applications of this comparison. Gyrosynchrotron radio emission
from nonthermal electrons accelerated by solar flares also can reveal
the magnetic topology of the flare source and we discuss this briefly.
---------------------------------------------------------
Title: Probing Solar Energetic Particles with SIRA
Authors: Aschwanden, M. J.; Nitta, N.; Lemaster, E.; Byler, E.; Gary,
D.; Kassim, N.; Gopalswamy, N.
2003AGUFMSH42C0555A Altcode:
The space-based SIRA (Solar Imaging Radio Array) will provide a powerful
capability to track high energy particles from solar flare and CME sites
through interplanetary/heliospheric space all the way to Earth. Together
with two other overlapping planned radio interferometers, i.e., FASR
(Frequency-Agile Solar Radiotelescope) and LOFAR (Low-Frequency Array)
the entire plasma frequency range from 30 GHz all the way down to
the plasma frequency cutoff of 30 kHz at 1 AU will be covered. These
instruments will track the magnetic trajectory of high energy particles,
beam-driven radio emission, and localize the acceleration sites in
the corona or interplanetary shocks. We simulate some CME and type III
events, as they will be mapped with these instruments, using realistic
scattering functions of radio waves on coronal and heliospheric density
inhomogeneities.
---------------------------------------------------------
Title: Cutoff of non-thermal electrons responsible for solar microwave
and hard X-ray bursts
Authors: Huang, G. L.; Zhang, J.; Gary, D.
2003ESASP.535..663H Altcode: 2003iscs.symp..663H
The method to estimate the low cutoff energy of non-thermal electrons
in solar microwave and hard X-ray bursts is discussed in this paper. At
first, for a given radiation mechanism, the ratio between two frequency
(or energy) channels is calculated with different value of the cutoff
to fit the observational data. Secondly, the cross point of the spectral
profiles at different time for a given burst may provide the information
of the low or high cutoff energy of non-thermal electrons. Two events
on June 3 and 10, 2000 are studied with data of OVSA and YOHKOH.
---------------------------------------------------------
Title: Detecção da fase impulsiva de uma explosão solar gigante
até 405 GHz
Authors: Raulin, J. -P.; Makhmutov, V.; Kaufmann, P.; Pacini, A. A.;
Luethi, T.; Hudson, H. S.; Gary, D. E.; Yoshimori, M.
2003BASBr..23..179R Altcode:
A explosão ocorrida no dia 25/08/2001 foi uma das mais
intensas do presente ciclo solar em ondas de rádio de altas
frequências. Foram medidas em ondas milimétricas e submilimétricas,
aproximadamente, 105 e vários milhares de unidades de fluxo solar,
respectivamente. Apresentamos um estudo deste evento em múltiplas
frequências, desde microondas (1GHz), até ondas submilimétricas
(405 GHz) detectadas pelo Telescópio Solar para ondas Submilimétricas
(SST). Esta base de dados foi complementada utilizando-se o experimento
Yohkoh, incluindo a emissão em raios-X duros e raios-g (até 100 MeV),
e imagens em raios-X moles da região ativa envolvida. Enfocamos e
discutimos principalmente os seguintes aspectos da fase impulsiva do
evento: (i) as implicações deduzidas do espectro eletromagnético,
obtido pela primeira vez até 405 GHz; (ii) a dinâmica da região
ativa. Os resultados mostram que para explicar o espectro rádio
observado, são necessários entre 3.5×1037 e 1.5×1039 elétrons
acelerados acima de 20 keV em uma região de campo magnético entre
300 e 800 Gauss. A estimativa do fluxo de fótons que seria produzido
por estes elétrons, mostra que grande parte deles não precipitou
na baixa atmosfera. A evolução temporal da emissão em raios-X
moles revela que a configuração magnética da região ativa foi
muito dinâmica durante a fase impulsiva da explosão. Em particular,
mostramos que a produção dos elétrons altamente energéticos foi
iniciada junto com a aparição, na baixa coroa solar, de um novo
sistema compacto de estruturas magnéticas. Este fato sugere que os
locais de aceleração estão localizados na baixa atmosfera do Sol,
como resultado da interação entre o novo sistema compacto e o campo
magnético ambiente da região ativa.
---------------------------------------------------------
Title: Decimetric Spike Bursts versus Microwave Continuum
Authors: Fleishman, Gregory D.; Gary, Dale E.; Nita, Gelu M.
2003ApJ...593..571F Altcode:
We analyze properties of decimetric spike bursts occurring
simultaneously with microwave gyrosynchrotron continuum bursts. We
found that all of the accompanying microwave bursts were highly
polarized in the optically thin range. The sense of polarization of
the spike clusters is typically the same as that of the optically thin
gyrosynchrotron emission, implying preferential extraordinary wave-mode
spike polarization. Optically thick spectral indices of the continuum
in spike-producing events were not observed to be larger than 2.5,
suggesting low or absent Razin suppression. This implies that the
plasma frequency-to-gyrofrequency ratio is systematically lower in the
spike-producing bursts than in other bursts. The spike cluster flux
density is found to be tightly correlated with the high-frequency
spectral index of the microwave continuum for each event, while the
flux-to-flux correlation may not be present. We discovered strong
evidence that the trapped fast electrons producing the microwave
gyrosynchrotron continuum have an anisotropic pitch-angle distribution
of the loss cone type in the spike-producing bursts. The spike clusters
are mainly generated when the trapped electrons have the hardest and
the most anisotropic distributions. The new properties are discussed
against the currently available ideas about emission processes and
models for spike generation. We conclude that the findings strongly
support the electron cyclotron maser mechanism of spike emission, with
characteristics agreeing with expectations from the local-trap model.
---------------------------------------------------------
Title: The Frequency Agile Solar Radiotelescope
Authors: GARY, DALE E.
2003JKAS...36S.135G Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Solar Active Region Study Using Microwave Maps
Authors: Bong, Su-Cran; Lee, Jeongwoo; Gary, Dale E.; Yun, Hong Sik
2003JKAS...36S..29B Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Small Radio Interferometer Arrays in Solar Physics
Authors: Gary, Dale E.
2003ASSL..288..309G Altcode: 2003ASSL..287..609G
No abstract at ADS
---------------------------------------------------------
Title: Coronal Magnetography with the Frequency Agile Solar
Radiotelescope
Authors: Gary, D. E.; Lee, J.
2003SPD....34.2016G Altcode: 2003BAAS...35..847G
We investigate the expected performance of the proposed Frequency
Agile Solar Radiotelescope (FASR) for coronal magnetography. FASR is
expected to produce high quality radio images at hundreds of radio
frequencies in the appropriate frequency range (1-24 GHz) to measure
active region coronal magnetic fields with exquisite precision. We
start with a realistic coronal active region model provided by Y. Mok
(UC-Irvine), and calculate the radio emission expected from the
region due to both the gyroresonance and free-free mechanisms. We then
sample the model images with a candidate FASR antenna configuration
and add noise, to obtain a set of multifrequency images of the same
spatial resolution and quality expected from the instrument. <P />The
resulting data-cube of two spatial and one spectral dimensions yields
microwave spectra at each point in the active region, which are then
inverted to obtain the active region plasma parameters (temperature,
emission measure, and magnetic field strength and direction). These
are finally compared to the input model, to determine the quantitative
precision of the inversion. We try several inversion techniques of
varying sophistication, to simulate both automated quick-look coronal
magnetograms and more time-consuming, research-quality magnetograms. <P
/>This work is supported by the National Science Foundation under
grant AST-0138317 to New Jersey Institute of Technology.
---------------------------------------------------------
Title: Spectral Dynamics of Solar Broadband Microwave Bursts.
Authors: Melnikov, V. F.; Gary, D. E.; Nita, G.
2003SPD....34.1505M Altcode: 2003BAAS...35..831M
Microwave spectral observations provide important information on
acceleration and transport of mildly relativistic electrons as well
as on magnetoactive plasma in solar flaring loops. In this study
we analyze dynamics of the frequency spectrum of a sample of broad
band microwave bursts selected to have only a single spectral maximum
and simple time profiles, using data obtained in 2001 with the OVSA
solar-dedicated frequency-agile interferometer. In particular, we study
the evolution during a burst of such parameters as the spectral peak
frequency f<SUB>p</SUB>, the ratio of fluxes at f > f<SUB>p</SUB>
and f < f<SUB>p</SUB>, the average spectral indices above and below
f<SUB>p</SUB>, as well as the frequency distribution of local spectral
indices. <P />The most common dynamic properties of the parameters
are as follows. Peak frequency changes with time, increasing on the
rising phase and decreasing on the decay phase of bursts. The ratio of
fluxes at f > f<SUB>p</SUB> and f < f<SUB>p</SUB> in most of cases
increases on the rise phase and decreases on the decay phase. Spectral
indices above and below f<SUB>p</SUB> decrease their absolute value
on the decay phase. However this general time behavior may change to
opposite one in some events. <P />The spectral peculiarities found
are then compared with relevant theoretical expectations derived from
model simulations of radio emission that take into account some effects
of energetic electron spectral and spatial evolution in flaring loops
and also their pitch-angle anisotropy. We also consider the influence
of self-absorption and dense plasma inside the loop (Razin effect) on
the spectral characteristics of the gyrosynchrotron emission. Finally,
results of the comparisons are used to get new information on the
high energy electron kinematics and temporal evolution of plasma
parameters inside flaring loops. <P />This work is supported by NASA
grants NAG5-10212 and NAG5-10891. OVSA/NJIT is supported by NSF under
grant AST-9987366.
---------------------------------------------------------
Title: Hα , EUV, and Microwave Observations of a Large Flare as
Evidence for Spontaneous Magnetic Reconnection
Authors: Lee, J.; Gallagher, P. T.; Gary, D. E.; Nita, G. M.; Choe,
G. S.
2003SPD....34.1604L Altcode: 2003BAAS...35Q.833L
The large solar flare with GOES class X1.1 occurred on 2000 March 22
is observed with the Owens Valley Solar Array (OVSA), Hα filtergram
of Big Bear Solar observatory (bbso), the Transition region and Coronal
Explorer (trace), and the Michelson Doppler Imager (mdi) onboard Solar
and Heliospheric Observatory (SOHO). During the impulsive phase a set of
EUV loops are visible in a small volume confined to the center of the
large β γ δ -type active region. Radio emission at 5 GHz appears as
a single source encompassing multiple Hα ribbons, and radio emissions
at other frequencies also appear within the central core region. We
interpret these observations under the idea of the confined flare in
contrast with the more commonly cited, eruptive flare, using a schematic
magnetic reconnection geometry based on the mdi magnetogram. It is
suggested that the EUV loops represent a separatrix in part, and that
the radio and Hα sources coincide with the whole part of the separatrix
and its footpoints, respectively. In addition, a Coronal Mass Ejection
(CME) as detected in the LASCO coronagraph after this flare is briefly
discussed in relation to the above idea of magnetic reconnection. <P
/>This work has been supported by NASA grants NAG 5-10891 and NAG-11875.
---------------------------------------------------------
Title: Statistical study of flare radio spectra obtained with OVSA
Authors: Nita, G. M.; Gary, D. E.; Lee, J.
2003SPD....34.1506N Altcode: 2003BAAS...35..831N
We present results of analysis of 397 flares during 2001 as detected by
the Owens Valley Solar Array (OVSA). This is a more in-depth study to
investigate some results indicated by a previous study of solar bursts
(Nita, Gary, Lanzerotti & Thomson 2002, ApJ, 570, 423.), which
was limited at the peak time of the bursts at a few frequencies. <P
/>The new study involves temporal dependence of parameters and the more
complete frequency coverage available with OVSA, which measures solar
fluxes at 40 frequencies in 1-18 GHz range with 4 s time resolution. <P
/>We investigate distributions of the burst parameters such as maximum
flux density in the spectra, peak frequency, spectral slopes below
and above peak frequency, duration in both rise and decay phases and
total energy at selected frequencies. We discuss correlations between
these parameters, which help us to understand particle acceleration
and flare energetics.
---------------------------------------------------------
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: Study of Microflares at Hard X-ray and Microwave Wavelengths
Authors: Qiu, J.; Gary, D. E.; Liu, C.; Wang, H.
2003SPD....34.1504Q Altcode: 2003BAAS...35..831Q
We present the preliminary results of studying microflares at hard X-ray
and microwave wavelengths which are observed by Ramaty High Energy Solar
Spectroscopic Imager (RHESSI) and Owens Valley Solar Array (OVSA). Among
over 700 microflare bursts that are observed by RHESSI during the
open-shutter mode from May to September in 2002, more than 200 fall
into the OVSA observing time window. In this study, we analyze the first
set of about 130 events jointly observed by the two instruments. These
events are predominantly detected by RHESSI in the energy range below
15 keV. They are mostly accompanied by a minor increase in GOES flux
below B5 level, but the time profiles at >10keV peak earlier than
GOES flux by tens of seconds. By examining the lightcurves and dynamic
spectra at hard X-ray and radio wavelengths, we find that more than 40%
of the RHESSI bursts are unambiguously identified as microwave bursts,
with the peak flux ranging from a few to several tens of solar flux
units and the peak frequency at greater than 5 GHz. These results are
indications of non-thermal features of the microflare events. We also
discuss the distribution of microflares in quiet-Sun and active regions,
the temporal and spatial relationship between microflares and major
flares, and the spectral properties of microflares in comparison with
major flares. <P />This work is supported by the NASA grant NAG5-10212.
---------------------------------------------------------
Title: A Multi-wavelength Analysis of a Solar Limb Flare on 2002
April 15
Authors: Sui, L.; Holman, G. D.; Gary, D. E.; Shibasaki, K.
2003SPD....34.1807S Altcode: 2003BAAS...35..840S
A detailed analysis of a limb flare on 2002 April 15 was carried out
using RHESSI X-ray images and spectra, Nobeyama microwave images,
Owens Valley Solar Array (OVSA) microwave spectra, and SOHO EIT &
LASCO images. This M1.2 flare lasted about 40 minutes in GOES soft
X-rays. RHESSI missed part of the decay phase of the flare. RHESSI
images show a thermal flare loop at 6-25 keV and a bright thermal
looptop source within the loop throughout the flare. At the peak of
the flare, a coronal source 6 arcseconds above the flare loop and
two clear-cut footpoints appear at 25-50 keV. Another coronal source
above the loop at 10-25 keV ejected outward at about 600 km/s after the
peak. By fitting the RHESSI spectra, we found that a low-energy cut-off
at about 22 keV is required in order to get a consistent evolution of
the thermal plasma parameters with time. After the hard X-ray peak,
the flare loop moves outward in both the SOHO/EIT 195 Å band and the
Nobeyama microwave images. It precedes an expanding coronal anomaly
observed in SOHO/LASCO C2 images. We will show images and movies of
our results, and a comparison of the microwave images and spectra with
the RHESSI observations. <P />This project was supported in part by
the RHESSI Project and the NASA Sun-Earth Connection Program.
---------------------------------------------------------
Title: The Frequency Agile Solar Radiotelescope (FASR)
Authors: White, S. M.; Gary, D. E.; Bastian, T. S.; Hurford, G. J.;
Lanzerotti, L. J.
2003EAEJA....11021W Altcode:
The Frequency Agile Solar Radiotelescope (FASR) is a radio
interferometer designed to make high spatial resolution images of the
Sun across a broad range of radio wavelengths simultaneously, allowing
the technique of imaging spectroscopy to be exploited on a routine
basis. The telescope will cover the frequency range 0.1-30 GHz using
several sets of receiving elements that provide full-disk imaging,
with of order 100 antennas at highest frequency range. FASR will be
optimized for solar radio phenomena and will be the most powerful
and versatile radioheliograph ever built, providing an improvement of
orders of magnitude in image quality over existing instruments. FASR
recently received the top ranking amongst all small projects considered
by the decadal survey of the National Academy of Science Committee on
Solar and Space Physics. FASR will probe all phenomena in the solar
atmosphere from the mid-chromosphere outwards. In particular, FASR
will provide direct measurement of coronal magnetic field strengths,
will image the nonthermal solar atmosphere and show directly the
locations of electrons accelerated by solar flares, will provide
images of coronal mass ejections travelling outwwards through the
solar corona, and supply extensive data products for forecasting and
synoptic studies. A major emphasis in the project is to make FASR data
as widely and easily used as possible, i.e., providing the general
user with processed, fully-calibrated high-quality images that do not
need particular knowledge of radio astronomy for interpretation. This
paper will describe the telescope and its science goals, and summarize
its current status.
---------------------------------------------------------
Title: Hα, Extreme-Ultraviolet, and Microwave Observations of the
2000 March 22 Solar Flare and Spontaneous Magnetic Reconnection
Authors: Lee, Jeongwoo; Gallagher, Peter T.; Gary, Dale E.; Nita,
Gelu M.; Choe, G. S.; Bong, Su-Chan; Yun, Hong Sik
2003ApJ...585..524L Altcode:
The evolution of a GOES class X1.1 solar flare, which occurred
in NOAA Active Region 8910 on 2000 March 22, is discussed using
observations from the Owens Valley Solar Array (OVSA), Big Bear Solar
observatory (BBSO), Transition Region and Coronal Explorer (TRACE),
and the Michelson Doppler Imager (MDI) on board Solar and Heliospheric
Observatory (SOHO). During the impulsive phase, a set of coronal loops
are visible in the TRACE 171 Å (~1×10<SUP>6</SUP> K) wavelength
band, which is confined to a small volume in the center of the large
βγδ-type active region. This is rapidly followed by the emergence of
bright Hα ribbons that coincide with the EUV emission. Radio images
show a single source encompassing the Hα ribbons at 5 GHz, but at
higher frequencies a double source is seen within the area bounded by
the compact Hα and EUV emissions. We interpret the observation under
the idea of the confined flare in contrast with the more commonly cited,
eruptive flare. We use a schematic magnetic reconnection geometry based
on the MDI magnetogram to suggest that the EUV loops show some parts
of a separatrix, and that the radio and Hα sources coincide with the
whole part of the separatrix and its footpoints, respectively. First
of all, it explains why this flare lacks the separating motion of
Hα ribbons, a signature for eruptive flares. Second, the very short
duration of microwave bursts in spite of the large amount of soft
X-ray flux is explicable under this scenario, since energy release
via spontaneous reconnection in a confined magnetic structure can be
very rapid. Third, the confined magnetic geometry is also considered
favorable for preserving chromospheric evaporation and plasma turbulence
as inferred from the OVSA microwave spectrum. In addition, a coronal
mass ejection as detected in the LASCO coronagraph after this flare
is briefly discussed in relation to the above flare model.
---------------------------------------------------------
Title: Site testing issues for the frequency agile solar
radiotelescope (FASR)
Authors: Gary, Dale E.; Keller, Christoph U.
2003SPIE.4853..523G Altcode:
The Frequency Agile Solar Radiotelescope (FASR) will be a broadband
synthesis-imaging array with 3-km or larger baselines, operating over
a broad frequency range of 0.1-30 GHz. The instrument demands a site
with low levels of Radio Frequency Interference (RFI) over this entire
band. The site also must be large enough to accommodate the expected
size of the array configuration and ideally would provide room to grow
with future upgrades. The site must have a benign environment in which
at least 100 separate elements will operate with little degradation
and weather-related downtime. Several sites in the U.S. are being
considered. We discuss what criteria are being used to assess the sites,
and give some initial results of testing some of the sites.
---------------------------------------------------------
Title: Solar Submillimeter and Gamma-Ray Burst Emission
Authors: Kaufmann, P.; Raulin, J. -P.; Melo, A. M.; Correia, E.; Costa,
J. E. R.; de Castro, C. G. Giménez; Silva, A. V. R.; Yoshimori, M.;
Hudson, H. S.; Gan, W. Q.; Gary, D. E.; Gallagher, P. T.; Levato,
H.; Marun, A.; Rovira, M.
2002ApJ...574.1059K Altcode:
Solar flare emission was measured at 212 GHz in the submillimeter
range by the Submillimeter Solar Telescope in the 1.2-18 GHz microwave
range by the Owens Valley Solar Array and in the gamma-ray energy
range (continuum) by experiments on board the Yohkoh (>1.2 MeV)
and Shenzhou 2 (>0.2 MeV) satellites. At the burst onset, the
submillimeter and microwave time profiles were well correlated with
gamma rays to the limit of the temporal resolution (<=10 s). At 212
GHz, fast pulses (<1 s), defined as time structures in excess of the
bulk emission, were identified as the flux increased. Their spatial
positions were scattered by tens of arcseconds with respect to the
main burst emission position. Correlation of submillimeter emission
with gamma-ray fast time structures shorter than 500 ms is suggested
at the gamma-ray maximum. The time variation of the rate of occurrence
of the submillimeter rapid pulses was remarkably well correlated with
gamma-ray intensities in the energy range (>1.2 MeV), attaining
nearly 50 pulses per minute at the maximum. These results suggest that
gamma rays might be the response to multiple rapid pulses at 212 GHz
and might be produced at different sites within the flaring region.
---------------------------------------------------------
Title: Electron Transport during the 1999 August 20 Flare Inferred
from Microwave and Hard X-Ray Observations
Authors: Lee, Jeongwoo; Gary, Dale E.; Qiu, Jiong; Gallagher, Peter T.
2002ApJ...572..609L Altcode:
We discuss injection and transport of high-energy electrons during
a GOES X-ray class M9.8 flare observed in microwaves with the Owens
Valley Solar Array (OVSA) and in hard X-rays (HXRs) with the hard
X-ray telescope (HXT) on board Yohkoh. Observed at 1 s timescales
or better in both wavelength regimes, the event shows (1) a large
difference in scale between the microwave source and the HXR source;
(2) an unusually hard HXR spectrum (maximum spectral index ~-1.6),
followed by rapid spectral softening; and (3) a microwave light curve
containing both impulsive peaks (3 s rise time) simultaneous with
those of the HXRs and a long, extended tail with a uniform decay rate
(2.3 minutes). We analyze the observations within the framework of
the electron trap-and-precipitation model, allowing a time-dependent
injection energy spectrum. Assuming thick-target bremsstrahlung
for the HXRs, we infer the electron injection function in the form
Q(E,t)~(E/E<SUB>0</SUB>)<SUP>-δ(t)</SUP>, where the timescale for
δ(t) to change by unity is ~7 s. This injection function can account
for the characteristics of the impulsive part of the microwave burst
by considering the bulk of the electrons to be directly precipitating
without trapping. The same injection function also accounts for the
gradual part of the microwave emission by convolving the injection
function with a kernel representing the trapping process, which at late
times gives N(E,t)~e<SUP>-νt</SUP>(E/E<SUB>0</SUB>)<SUP>-b</SUP>. We
require b~1.4 and ν~6×10<SUP>-3</SUP>β s<SUP>-1</SUP>, where β
is the electron speed divided by the speed of light. Therefore, the
derived form of the precipitation rate ν itself indicates strong
pitch-angle diffusion, but the slow decay of the microwave radiation
requires a small loss cone (~4°) and a low ambient density in the
coronal trap. Also, the numbers of electrons needed to account for
the two components of the microwave emission differ by an order of
magnitude. We estimate that the >=100 keV number of the directly
precipitating electrons is ~10<SUP>33</SUP>, while the trapped
population requires ~10<SUP>32</SUP> electrons. This leads us to
a model of two interacting loops, the larger of which serves as an
efficient trap while the smaller provides the impulsive source. These
characteristics are consistent with the spatially resolved observations.
---------------------------------------------------------
Title: The Peak Flux Distribution of Solar Radio Bursts
Authors: Nita, Gelu M.; Gary, Dale E.; Lanzerotti, L. J.; Thomson,
D. J.
2002ApJ...570..423N Altcode:
We have investigated the peak flux distribution of 40 years of solar
radio burst data as a function of frequency and time over a wide range
of frequencies. The bursts were reported by observing stations around
the world during 1960-1999, as compiled by the National Geophysical Data
Center (NGDC) of the National Oceanic and Atmospheric Administration
(NOAA). This period covers three full and two partial solar cycles. We
have analyzed the data set to find correction factors for missed
events, and find evidence that nearly half of the events were missed
by the worldwide network. We obtain power-law fits to the differential
(density) (dN/dS in events sfu<SUP>-1</SUP>) and cumulative [N(>S)
in events] distributions as a function of frequency, time, and phase
of the solar cycle. The typical power-law index, ~-1.8, is similar
to that found in many hard X-ray studies. The average waiting time
between bursts with flux density exceeding 1000 sfu was found to be
6 days at solar maximum, and 33 days at solar minimum. Taking account
of missed events, the expected waiting time decreases to 3.5 and 18.5
days, respectively. Bursts of this flux level can cause problems with
wireless communication systems. We present tables of fit parameters that
can be used to find burst occurrence rates in a number of frequency
ranges. We find no significant variation of power-law index from one
solar cycle to the next, or with phase of the solar cycle, but we do
find significant changes of power-law index with frequency.
---------------------------------------------------------
Title: Solar Radio Bursts and Their Effects on Wireless Systems
Authors: Gary, D. E.; Lanzerotti, L. J.; Nita, G. M.; Thomson, D. J.
2002AAS...200.4911G Altcode: 2002BAAS...34..723G
We review the state of current understanding of the potential for
interference and interruption of service of wireless communications
systems due to solar radio bursts. There have been several reported
instances of an enhanced rate of dropped cell-phone calls during solar
bursts, and the design of current base station systems make them
vulnerable to problems near sunrise and sunset for antennas facing
in the direction of the Sun during outbursts. It is likely that many
cases of interference have gone unreported and perhaps unrecognized. We
determine the level of radio noise that can cause potential problems,
and then discuss how often bursts of the required magnitude might
happen. We find that bursts that can cause potential problems occur
on average once every 3.5 days at solar maximum, but also occur
at a reduced rate of 18.5 days between events at solar minimum. We
investigate the rate of occurrence as a function of frequency, which
is relevant for future wireless systems that will operate at higher
frequencies than the present systems. This work is supported by NSF
grant ATM-0077273 to New Jersey Institute of Technology.
---------------------------------------------------------
Title: Impulsive and gradual energetic emissions in a flare
Authors: Qiu, J.; Lee, J.; Gary, D. E.
2002AAS...200.2901Q Altcode: 2002BAAS...34..680Q
We present a comprehensive case study of an X-class flare observed on
April 6, 2001. The flare consists of two peaks, the first characterized
by an impulsive spiky emission and the second by a gradual smooth
emission in the same magnetic environment. We compare the temporal
and spectral evolution of the two components in hard X-rays and
microwaves. Images at both wavelengths are obtained to locate the
emission source as it evolves during the flare. We find the following
results: (1) The impulsive hard X-ray and microwave emissions are
from complex loop structures, primarily in low-lying loops, while the
gradual energetic emissions occur in a simple and over-lying loop. Both
the impulsive and gradual hard X-rays at > 50keV are thick-target
emission. We also find a microwave source that evolves from the loop top
toward the foot-point during the gradual phase. (2) A time lag analysis
reveals energy-dependent time delays in the impulsive hard X-rays, with
the 200 keV hard X-rays lagging the 40keV emission by about 5s. (3)
The gradual component is a microwave-rich event. Large delays are seen
in the gradual hard X-ray emissions, and the gradual microwaves further
lag the hard X-rays by tens of seconds. The correlation study suggests
that the gradual microwaves are emitted by > 200keV hard X-ray
electrons, possibly MeV electrons. The gradual hard X-rays exhibit
a harder and hardening spectrum than the impulsive component, while
the electron spectrum derived from the gradual microwaves is still
harder. Based on the observations, we suggest that magnetic energy
release primarily occurs during the impulsive phase in the complex loop
system involving both low-lying and over-lying magnetic loops. Electrons
injected into the low-lying loops rapidly escape the coronal trap and
precipitate, giving rise to the impulsive energetic bursts at both
wavelengths. Electrons that are injected into the over-lying loop are
well trapped, which soon undergo a new acceleration, most probably in
the form of a stochastic mechanism. It is likely that the secondary
acceleration produces a pitch angle distribution in favor of electron
trapping, and an evolving trapping condition may result in the moving
microwave source along the magnetic loop. This work is supported by
NASA grants NAG5-10212 and NAG5-10891. OVSA/NJIT is supported by NSF
under grant AST-9987366.
---------------------------------------------------------
Title: First Results of Joint OVSA and HESSI Observations
Authors: Gary, D. E.; Schmahl, E. J.; Nita, G. M.
2002AAS...200.6904G Altcode: 2002BAAS...34..757G
We report on an initial study of several solar bursts observed
with the Owens Valley Solar Array (OVSA) and the new RHESSI X-ray
spacecraft. Six M-class soft X-ray bursts were observed jointly in
the first month of RHESSI operations, with a wide range of radio and
hard X-ray fluxes. Both the hard X-rays and microwaves are well known
to arise from electrons of similar energies, so spatial and spectral
comparisons of jointly observed events are fruitful for investigating
the initial energy release and subsequent evolution of the thermal
plasma. The new capabilities of RHESSI to image both high and low
energy X-rays with high spectral resolution offer the possibility
to learn about this process in unprecedented detail. We compare and
contrast the radio and hard X-ray spectra for the 6 events, and show
the results of an initial investigation of the spatial characteristics
as observed in the two wavelength regimes.
---------------------------------------------------------
Title: Noise in wireless systems produced by solar radio bursts
Authors: Bala, B.; Lanzerotti, L. J.; Gary, D. E.; Thomson, D. J.
2002RaSc...37.1018B Altcode: 2002RaSc...37b...2B
We have carried out an investigation of 40 years of solar radio
burst data in a wide range of frequencies that have been reported
by observing stations around the world during 1960-1999. The data
were compiled by the National Geophysical Data Center of the National
Oceanic and Atmospheric Administration. This period covers three full
and two partial solar cycles. We find that the number of bursts per day
with amplitudes >10<SUP>3</SUP> solar flux units (SFU) falls as an
approximate power law with increasing flux level for the frequency bands
investigated(1-10 and 10-20 GHz). Also, the number of events with peak
flux density >10<SUP>3</SUP> SFU varies, as expected, with the solar
cycle in the bands investigated (1-2, 2-4, and 4-10 GHz). We discuss the
rate of occurrence of events (>10<SUP>3</SUP> SFU) in the context
of the noise levels in typical wireless communications systems. We
find that statistically, depending upon wireless system parameters,
several solar events per year are likely to occur that could cause
severe interference in a given cell site during solar maximum periods.
---------------------------------------------------------
Title: Motion of Flare Footpoint Emission and Inferred Electric
Field in Reconnecting Current Sheets
Authors: Qiu, Jiong; Lee, Jeongwoo; Gary, Dale E.; Wang, Haimin
2002ApJ...565.1335Q Altcode:
A systematic motion of Hα kernels during solar flares can be regarded
as the chromospheric signature of progressive magnetic reconnection in
the corona, in that the magnetic field lines swept through by the kernel
motion are those connected to the diffusion region at the reconnection
point. In this paper, we present high-cadence and high-resolution
Hα-1.3 Å observations of an impulsive flare that exhibits a systematic
kernel motion and relate them to the reconnecting current sheet (RCS)
in the corona. Through analyses of X-ray and microwave observations,
we further examine the role of the macroscopic electric field inside
the RCS in accelerating electrons. We measure the velocity of the
kernel motion to be 20-100 km s<SUP>-1</SUP>. This is used together
with the longitudinal magnetic field to infer an electric field as
high as 90 V cm<SUP>-1</SUP> at the flare maximum. This event shows a
special magnetic field configuration and motion pattern of Hα kernels,
in that a light bridge divides a flare kernel into two parts that move
in different manners: one moving into the stronger magnetic field and
the other moving along the isogauss contour of the longitudinal magnetic
field. The temporal variation of the electric field inferred from the
former type of kernel motion is found to be correlated with 20-85
keV hard X-ray light curves during the rise of the major impulsive
phase. This would support the scenario of magnetic energy release
via current dissipation inside the RCS, along with the hypothesis of
the DC electric field acceleration of X-ray-emitting electrons below
100 keV. However, there is no good temporal correlation between the
hard X-ray emission and the inferred electric field from the other
motion pattern. Furthermore, the microwave emission, which supposedly
comes from higher energy electrons, shows a time profile and electron
spectrum that differs from those of the X-ray bursts. We conclude that
either the two-dimensional magnetic reconnection theory related to the
Hα kernel motion is applicable to only some part of the flare region
due to its special magnetic geometry, or the electron acceleration is
dominated by other mechanisms depending on the electron energy.
---------------------------------------------------------
Title: Noise in wireless systems from solar radio bursts
Authors: Lanzerotti, L.; Gary, D.; Nita, G.; Thomson, D.; Maclennan, C.
2002cosp...34E.501L Altcode: 2002cosp.meetE.501L
Solar radio bursts were first discovered as a result of their
interference in early defensive radar systems during the second World
War (1942). Such bursts can still affect radar systems, as well as
new wireless technologies. We have investigated a forty year record
of solar radio burst data (1960 - 2000) as well as several individual
radio events in the 23rd solar cycle. We show statistically that for
f1 GHz there can be a burst > 103 solar flux units (SFU) every
few days during solar maximum conditions, and that such burst levels
can produce problems in contemporary wireless systems. We also discuss
individual burst events and show that the event of April 2001 had signal
levels > 103 SFU for more than three quarters of the event duration
(order 30 minutes). Such long duration, high intensity, bursts can be
especially disruptive to wireless systems.
---------------------------------------------------------
Title: First detection of the impulsive and extended phases of a
solar radio burst above 200 GHz
Authors: Trottet, G.; Raulin, J. -P.; Kaufmann, P.; Siarkowski, M.;
Klein, K. -L.; Gary, D. E.
2002A&A...381..694T Altcode:
We present a detailed analysis of radio observations obtained at
212 and 405 GHz during the 2000 March 22 Hα 2N flare that occured in
AR8910 at ~ 1834 UT. These data are compared with microwave, soft X-ray
and hard X-ray measurements of this flare. While the flare emission
is not clearly detected at 405 GHz, the time profile of the 212 GHz
emission exhibits an impulsive burst, associated in time with the
1-18 GHz impulsive microwave burst and a long-lasting thermal burst
which finishes at about the same time as the soft X-ray emission but
reaches its maximum later. The 212 GHz impulsive emission and the lack
of detection at 405 GHz are consistent with synchrotron radiation from a
population of ultrarelativistic electrons in an average magnetic field
of 400-600 G. This radiating population of electrons has a hard energy
spectrum (power law index ~2.7). The expected >1 MeV gamma-ray
continuum emission from the radio emitting electrons is comparable
to that detected for mid-size electron-dominated events and the hard
X-ray flux they would produce at 100 keV is consistently lower than
the upper limit inferred from the observations. It is shown that the
212 GHz thermal source has to be different from that radiating the
soft X-ray and microwave thermal emission. The present observations
of a solar burst provide the first evidence of the extension of the
gyrosynchrotron spectrum of an impulsive radio burst in the synchrotron
domain above 200 GHz.
---------------------------------------------------------
Title: First Detection of the Impulsive and Extended Phases of a
Solar Radio Burst above 200 GHZ
Authors: Raulin, J. P.; Kaufmann, P.; Trottet, G.; Klein, K. L.;
Gary, D.; Siarkowski, M.
2002RMxAC..14..108R Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The frequency agile solar telescope (FASR)
Authors: Bastian, T.; Gary, D.; White, S.; Hurford, G.
2002cosp...34E1870B Altcode: 2002cosp.meetE1870B
The Frequency Agile Solar Radiotelescope (FASR) will be a large-N,
solar- dedicated, Fourier synthesis array designed to perform broadband
imaging spectroscopy across a frequency range of approximately 0.1-30
GHz with an angular resolution as high as 1" (>20 GHz). This
paper reviews the FASR science drivers -- including coronal magnetic
fields, energetic phenomena, and thermal physics -- and the associated
instrument requirements and specifications. Applications for synoptic
studies and space weather are also discussed. Ongoing activities are
briefly described, including site surveys, configuration studies,
science simulations, and system design.
---------------------------------------------------------
Title: 2800 MHz Solar Radio Bursts: A Statistical Analysis of 40
years of Data
Authors: Balachandran, B.; Lanzerotti, L. J.; Gary, D. E.
2001AGUFMSH31A0692B Altcode:
The daily values of solar flux and radio bursts at 2800 MHz (10.7 cm
wavelength) are known to be closely related to various manifestations
of solar activity. The flux values, which vary slowly with time, have
long been used as indicators of solar activity. Also, the number of
radio bursts shows a variation with the phase of the solar cycle. The
close relationships between the 2800 MHz bursts, the associated flares
and geophysical phenomena such as shortwave fadeouts have been studied
extensively and were established as early as the 1960s. Therefore,
a constant monitoring of the Sun at this frequency would enable
us to forecast the terrestrial disturbances following the solar
activity. Moreover, a detailed study based on past data would help
understand solar activity phenomena as well as the origin of these burst
events. In the present analysis, we are revisiting some of these points
by carrying out an analysis of 40 year data of solar radio bursts with
special emphasis on 2800 MHz bursts. A scatter plot of the intensity vs
duration shows that the distribution is not completely random but is
double--pronged. This result is consistent with earlier works (e.g.,
Covington, 1959). The two-pronged distribution suggests the existence
of two distinct types of burst events: impulsive and gradual rise and
fall. The mechanisms that cause the emission of the two types of bursts
are also different: the former due to nonthermal processes and the
latter due to thermal processes. Our present analysis shows that the
intensity--duration plot has a significant variation with the phase
of the solar cycle. In addition to this, we present the behaviour of
risetime vs duration as well as the frequency distribution of peak flux
of these events. The analysis has also been extended to high frequency
(> 10 GHz) bursts and the behaviour is contrasted to that of 2800
MHz bursts.
---------------------------------------------------------
Title: Solar Microwave Imaging Software for the Owens Vally Solar
Array
Authors: Gary, D. E.; Lee, J.; Bong, S.
2001AGUSM..SH41B18G Altcode:
A package of solar microwave imaging software has been developed
mainly to support general users of microwave data obtained with
the Owens Valley Solar Array (OVSA). While the Astronomical Image
Processing System (AIPS) maintained by NRAO is widely used for radio
interferometry data and can be used for the OVSA data as well, we
consider it worthwhile to have new imaging software optimized for OVSA
data, especially for accommodating the large number of frequencies in
the OVSA data. Another motivation, in collaboration with the High Energy
Solar Spectroscopic Imager (HESSI--which uses Fourier-Transform imaging
at energies of hard X rays), is to make the OVSA imaging software
conform to the HESSI software. The OVSA imaging software is written in
IDL, and conforms to the Solarsoft software tree while maintaining radio
standards. With typical input parameters set by the user, the imaging
software carries out the standard Clean algorithm and then prompts for
further processing options. These include Self-Calibration, Maximum
Entropy, and Forward-Fitting algorithms. We describe the software and
demonstrate the capabilities of each algorithm when applied to solar
microwave imaging with OVSA. We also describe progress on an improved
version of the Spatial/Spectral Maximum Entropy Method and test it
against the above-mentioned, more conventional techniques.
---------------------------------------------------------
Title: Optical, EUV, and Microwave Observations of the March 22,
2000 X-class Flare.
Authors: Gallagher, P. T.; Gary, D. E.; Lee, J.
2001AGUSM..SP42A08G Altcode:
The evolution of an X-class flare which occured in NOAA 8910 on March
22, 2000 is discussed using observations from the Owens Valley Solar
Array (ovsa), Big Bear Solar Observatory (bbso), the Transition
Region and Coronal Explorer (trace), and the Michelson Doppler
Imager (mdi). The main energy release occurs in a set of coronal
loops visible in the trace 171 Å (1 x 10<SUP>6</SUP>~K) wavelength
band, which is rapidly followed by the emergence of two bright Hα
ribbons. High-cadence radio images at around 5 GHz obtained using the
ovsa appears in the middle of two Hα ribbons, which thus indicates
presence of energetic electrons flowing across the two ribbons. During
the impulsive phase, short-lived (τ <= 30 s), narrow-band (BW
<= 500 MHz), and highly polarized microwave emission is observed in
radio images in the 1-2 GHz frequency range, which is possibly due to
plasma radiation. At optically-thin, high frequencies ovsa total power
spectra also show a typical soft-hard-soft evolution indicative of
highly efficient nonthermal electron acceleration during the impulsive
phase. The microwave emission from this flare is surprisingly short
considering that active region is large (> 200 arc sec in diameter)
and the flare is energetic (GOES X class). We consider that the compact
magnetic field geometry in the flaring region and subsequent strong
chromospheric evaporation into it may have limited life of high energy
electrons trapped in the loops to such a short duration.
---------------------------------------------------------
Title: Shifting Emission Source in an Impulsive Flare
Authors: Qiu, J.; Gary, D. E.; Wang, H.
2001AGUSM..SP42A05Q Altcode:
We study high resolution observations of a compact impulsive C9.0
flare. We include in the analysis high cadence Hα -1.3~Å filtergrams
from the Big Bear Solar Observatory (BBSO), radio data from Owens
Valley Solar Array (OVSA), soft and hard X-ray data from Yohkoh, and
UV~1600~Å continuum images from Transition Region And Corona Explorer
(TRACE). Observations show two emission kernels located at the feet of
a general bipolar structure. The impulsive phase of the flare consisted
of two major emission peaks in hard X-ray, radio and Hα emission,
each lasting for about a minute, indicating the existence of two major
episodes of flare energy release. We find that these different episodes
in the flare temporal evolution were related to different spatial
locations as shown by the Hα kernels. The high cadence time profiles
at various wavelengths exhibit temporal structures on a shorter time
scales from a few seconds to less than a second, while Hα - 1.3~Å
and radio images show that the location of the footpoint emission was
shifting. This may be an indication that emission in these two episodes
was produced in different magnetic structures, which we explore through
spectral analysis of hard X-ray and microwave emission.
---------------------------------------------------------
Title: Noise in Wireless Systems Produced by Solar Radio Bursts
Authors: Balachandran, B.; Lanzerotti, L. J.; Gary, D. E.; Thomson,
D. J.
2001AGUSM..SP52B07B Altcode:
We have carried out an investigation of 40 years of solar radio burst
data in a wide range of frequencies that have been reported by observing
stations aroundthe world during 1960-1999. The data were compiled by
the NGDC of NOAA. This period covers three full and two partial solar
cycles. We find that the number of bursts per day with amplitudes
>10<SUP>3</SUP> solar flux units (SFU) falls as an approximate
power law with increasing burst flux level for the frequency bands
investigated (1-10 and 10-20 GHz). We also find that the number of
eventswith peak flux >10<SUP>3</SUP> SFU varies, as expecterd,
with the solar cycle in the bands investigated (1-2, 2-4, and 4-10
GHz). We discuss the event (>10<SUP>3</SUP> SFU) occurrence rate
in the context of the noise levels in typical wireless communications
systems. We find that there are likely to be a few solar events per year
that could cause severe interference in a given cell site during solar
maximum conditions. We also discuss other aspects of possible solar
radio influence on wireless systems, including the potential impacts
of lower flux levels of solar radio bursts on system bit error rates.
---------------------------------------------------------
Title: Radio and X ray Observations of a Limb Flare during the Max
Millennium Campaign
Authors: Lee, J.; Gallagher, P. T.; Gary, D. E.; Harra, L. K.
2001AGUSM..SP51A03L Altcode:
A powerful (GOES Class M9.8), limb flare was observed on 1999 August
20 above AR 8673 during the second Max Millennium campaign. Due to its
location on the limb as well as its strength, the flare observation
provides an ideal case of studying vertical structure of electron
acceleration and its transport. The hard X ray images from Yohkoh/HXT
along with EUV images from SoHO/EIT show an impulsive, compact double
brightening in the lower atmosphere and a hypothesis of footpoint
emission from a compact loop seems likely. In contrast, microwave
visibilities obtained using the Owens Valley Solar Array (OVSA) suggest
a region of high energy electrons high in the corona (2x 10<SUP>5</SUP>
km), requiring large loops that can act as a good trap of electrons. The
duration of radio emission is also much extended compared with the
hard X ray emission. We present this set of observations as a good
example of the trap-plus-precipitation hypothesis, and derive numbers
of electrons emitting the microwaves and hard X rays, respectively, as
a measure for the trap vs. precipitation. The derived time evolution
of electron numbers in energy space is not compatible with a simple
hypothesis of energy-independent acceleration solely under Coulomb
collisions, but instead requires an acceleration or transport process
that is highly energy-dependent.
---------------------------------------------------------
Title: 40 Years of Solar Microwave Burst Flux Measurements
Authors: Nita, G. M.; Gary, D. E.; Lanzerotti, L. J.; Thomson, D. J.
2001AGUSM..SP41A03N Altcode:
We report results from an investigation of 40 years of solar radio
burst data, from 1960-1999, compiled by the National Geophysical
Data Center at NOAA, Boulder, CO. This dataset consists of more than
a half-million entries corresponding to several hundred thousand
bursts measured at many frequencies from approximately 150 MHz to 35
GHz, and covering more than three 11-year solar activity cycles. We
discuss the ways in which limitations and biases in the dataset are
identified and, where possible, corrected. We present the corrected
number distributions for bursts greater than 10 SFU (solar flux units,
1 SFU = 10<SUP>-22</SUP> W/m<SUP>2</SUP>/Hz), and powerlaw fits to them,
as a function of frequency, time, and phase of the solar cycle. We
compare these distributions and their variation with similar measures
in other wavelengths, and discuss the implications for coronal heating
and other aspects of flare physics.
---------------------------------------------------------
Title: The Frequency-Agile Solar Radiotelescope (FASR) (invited)
Authors: Gary, D. E.; Bastian, T. S.; White, S. M.; Hurford, G. J.
2001aprs.conf..236G Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Correlation of Microwave and Hard X-Ray Spectral Parameters
Authors: Silva, Adriana V. R.; Wang, Haimin; Gary, Dale E.
2000ApJ...545.1116S Altcode:
We present the analysis of 27 solar flares with multiple peaks that
were observed at hard X-ray and microwave wavelengths. A total of
57 simultaneous peaks were observed by BATSE (hard X-rays) and Owens
Valley Radio Observatory (microwaves). Throughout the duration of a
flare, its spectra at both wavelengths are fitted independently at all
times. The hard X-ray spectra were fitted by a single power law in most
cases, whereas the microwave spectra were fitted as gyrosynchrotron
emission. For each individual peak, the parameters at both wavelengths
(peak flux, turnover frequency, spectral indices, and delays between
hard X-ray and microwave peak emission) were then compared and
correlated. We have also studied impulsive and nonimpulsive bursts
individually. The main results obtained were as follows. (1) In 75% of
the bursts, the inferred index of the electron energy distribution of
the microwave-emitting electrons, δ<SUB>r</SUB>, is harder than that
of the lower energy hard X-ray-emitting electrons, δ<SUB>X</SUB>,
on average by 0.5-2.0. This implies that there is a breakup in the
energy spectra of the electrons, as is sometimes observed in the hard
X-ray spectra of giant flares. (2) A soft-hard-harder spectral index
temporal evolution is more commonly seen in the microwave spectra
(47%) than in the hard X-ray observations (32%) and in nonimpulsive
flares than in impulsive ones. (3) Delays larger than 2 s were observed
between the radio and hard X-ray peaks in 65% of the bursts, with the
delays decreasing as the hard X-ray energy increased. (4) Nonimpulsive
flares are more microwave rich, have higher delays between their radio
emission and the hard X-ray peaks, and display harder spectral indices
than impulsive bursts.
---------------------------------------------------------
Title: Solar Microwave Bursts and Injection Pitch-Angle Distribution
of Flare Electrons
Authors: Lee, J.; Gary, Dale E.
2000ApJ...543..457L Altcode:
We calculate the time variation of the energy and pitch angle of
high-energy electrons injected into a magnetic loop and subsequently
trapped there because of magnetic mirroring. We use the evolving
distribution in the calculation of gyrosynchrotron emission, as
an aid to interpretation of a particular microwave burst observed
using the Owens Valley Solar Array (OVSA) during a GOES class C2.8
flare on 1993 June 3. The electrons are assumed to have a Gaussian
pitch-angle distribution, whose width and mean pitch angle are
calculated as they evolve in time, taking into account the electron
energy loss and a specific magnetic loop structure set as a model for
the target active region. Various temporal behaviors of the microwave
spectrum are found as a function of injection and trap conditions,
which can be used to infer some of the injection properties directly
from the observed microwave spectra. As a main result we found that
initial pitch-angle distribution plays an important role in the
microwave spectral evolution. This is largely due to the fact that
pitch-angle diffusion of electrons under Coulomb collisions markedly
differs at those electron energies to which the microwave spectrum
is sensitive. This effect cannot be reproduced by adjusting the trap
properties and therefore could be used to determine whether the initial
pitch-angle distribution is isotropic or narrowly beamed. The microwave
burst spectra observed during the 1993 June 3 flare are found to be most
consistent with the hypothesis of an initially narrow beamed injection
(<=30°) into a low-density (~4×10<SUP>9</SUP> cm<SUP>-3</SUP>)
magnetic trap. This result explains the observed asymmetric microwave
time curve consisting of a relatively short rise (~32 s) and a long
decay (>=5 minutes) in terms of a transport effect rather than
acceleration characteristics. The physical connection of the proposed
microwave model to hard X-ray models in thin/thick targets is briefly
discussed.
---------------------------------------------------------
Title: First results from the upgraded Owens Valley Solar Array.
Authors: Gary, D. E.; Hurford, G. J.; Lee, J.; Gallagher, P. T.
2000BAAS...32..818G Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Magnetic Reconnection and Energetics of the 2000 March 22
Solar Flare
Authors: Gallagher, P. T.; Gary, D. E.; Lee, J.
2000SPD....31.0260G Altcode: 2000BAAS...32..821G
The evolution and properties of the 2000 March 22 X--class flare from
the β δ active region NOAA 8910 are discussed using high resolution
radio observations from the Owen's Valley Solar Array, TRACE UV and
EUV filtergrams, GOES soft X-rays, and magnetograms from MDI on board
SOHO. The episodic flare evolution is found to be extremely complicated
at all observed wavelengths and releases a large amount of both thermal
and non--thermal energy. From the UV and EUV observations, the flare is
shown to be consistent with the model in which the release of energy
is caused by several, successive loop interactions while the radio
observations identify multiple non--thermal electron acceleration
sites. The final stage of the flare is then characterised by the
emergence of a set of post--flare loops parallel to the magnetic
neutral line as the system relaxes.
---------------------------------------------------------
Title: Electron Acceleration During the 1999 August 20 Flare
Authors: Lee, J.; Gary, D. E.; Gallagher, P. T.
2000SPD....31.0250L Altcode: 2000BAAS...32..820L
A powerful flare (GOES SX class of M9.8) occurred on 1999 August 20 at
23:06 UT near the eastern limb during a Max Millennium campaign. The
Owens Valley Solar Array (OVSA) at the time was operating in a high
time resolution (1 s) mode to observe an impulsive microwave burst
rising very rapidly (within 3 s) at all observing frequencies in the
range of 1.4 to 18 GHz. As a main characteristic of the burst, the
microwave spectrum decays in a remarkably well-defined exponential
profile with a timescale varying in the range of ~ 30 s to ~ 4 min,
in proportion to the wavelength. The accompanying hard X ray emission
from the BATSE DISCLA data is confined to a short time interval ( ~ 30
s), and its light curve is very similar to that of the high-frequency
(15--18 GHz) microwaves. Our analysis is therefore focused on whether
these observartions are consistent with theoretical predictions
for the microwave emission from trapped electrons in the corona and
thick target X-ray emission from the electrons precipitating into
the chromosphere. In addition, we infer the magnetic reconnection
geometry from EUV images obtained from SoHO/EIT which is used as
another constraint to study the property of the acceleration. The OVSA
is supported by NSF grants ATM-9796213 and AST-9796238 and NASA grant
NAG5-6381 to New Jersey Institute of Technology.
---------------------------------------------------------
Title: Observations and models of a flaring loop.
Authors: Nindos, A.; White, S. M.; Kundu, M. R.; Gary, D. E.
2000BAAS...32..818N Altcode:
No abstract at ADS
---------------------------------------------------------
Title: First Results from the Upgraded Owens Valley Solar Array
Authors: Gary, D. E.; Hurford, G. J.; Lee, Jeongwoo; Gallagher, P. T.
2000SPD....31.0244G Altcode:
The Owens Valley Solar Array (OVSA) has undergone extensive hardware
and software upgrades in preparation for the current solar maximum
and the launch of HESSI. We present an overview of the now completed
upgrade from 5 to 6 antennas, and show first results from the newly
expanded instrument. We show results from several recent flares,
as well as multi-frequency maps of active region coronal structure,
to demonstrate the improvements now possible with the instrument. The
data and analysis software are freely available on the web, and we
invite all who are interested in working with these data to contact
the associated website.
---------------------------------------------------------
Title: Observations and Models of a Flaring Loop
Authors: Nindos, A.; White, S. M.; Kundu, M. R.; Gary, D. E.
2000SPD....31.0243N Altcode:
Simultaneous images of a flaring loop at two frequencies are used to
model the magnetic structure of the loop and the energy distribution of
the radiating electrons. The imaging data were obtained with the VLA at
5 and 15 GHz. Additional spectral data were provided by the OVRO Solar
Array at several frequencies between 2 GHz and 15 GHz. At 15 GHz, the
flare emission was optically thin and came from the footpoints of the
flaring loop, while at 5 GHz the loop itself was outlined. Most of the
5 GHz emission was optically thick and its spatial maximum was close
to the loop top. A striking feature of the observations is that the 5
GHz emission does not reach down to the 15 GHz footpoints. We compare
the observations with calculations of gyrosynchrotron emission from an
inhomogeneous magnetic loop in order to determine the conditions in the
flaring loop. The best fit to the OVRO fluxes was reached with a model
flaring loop with photospheric footpoint magnetic field strength of 870
G. The thickness of the model loop was small compared to its footpoint
separation. The energy spectral index of the energetic electrons was 3.7
and their number density was 7.9 x 10<SUP>7</SUP> cm<SUP>-3</SUP>. The
low and high energy cutoffs of the nonthermal electrons were 8 and
210 keV. The 5 GHz emission in this model is at low harmonics (3--7)
and harmonic effects are responsible for the weak 5 GHz emission at
the footpoints. The absence of electrons above 210 keV is necessary in
this model to explain why no emission is observed from the loop top at
15 GHz. That model reproduced well the high frequency part of the OVRO
flux spectrum as well as the VLA spatial structure. Thus comparisons
between the spatially--resolved observations and models reveal the
three-dimensional structure of the loop geometry.
---------------------------------------------------------
Title: Observations and Models of a Flaring Loop
Authors: Nindos, A.; White, S. M.; Kundu, M. R.; Gary, D. E.
2000ApJ...533.1053N Altcode:
Simultaneous images of a flaring loop at two frequencies are used to
model the magnetic structure of the loop and the energy distribution of
the radiating electrons. The imaging data were obtained with the VLA at
5 and 15 GHz. Additional spectral data were provided by the Owens Valley
Radio Observatory (OVRO) solar array at several frequencies between
2 and 15 GHz. At 15 GHz, the flare emission was optically thin and
came from the footpoints of the flaring loop, while at 5 GHz the loop
itself was outlined. Most of the 5 GHz emission was optically thick,
and its spatial maximum was close to the loop top. A striking feature
of the observations is that the 5 GHz emission does not reach down to
the 15 GHz footpoints. We compare the observations with calculations of
gyrosynchrotron emission from an inhomogeneous magnetic loop in order
to determine the conditions in the flaring loop. The best fit to the
OVRO fluxes was reached with a model flaring loop with photospheric
footpoint magnetic field strength of 870 G. The thickness of the
model loop was small compared with its footpoint separation. The
energy spectral index of the energetic electrons was 3.7, and their
number density was 7.9×10<SUP>7</SUP> cm<SUP>-3</SUP>. The low- and
high-energy cutoffs of the nonthermal electrons were 8 and 210 keV,
respectively. The 5 GHz emission in this model is at low harmonics
(3-7), and harmonic effects are responsible for the weak 5 GHz emission
at the footpoints. The absence of electrons above 210 keV is necessary
in this model to explain why no emission is observed from the loop
top at 15 GHz. That model reproduced well the high-frequency part of
the OVRO flux spectrum as well as the VLA spatial structure. Thus,
comparisons between the spatially resolved observations and models
reveal the three-dimensional structure of the loop geometry.
---------------------------------------------------------
Title: Magnetic Trapping and Electron Injection in Two Contrasting
Solar Microwave Bursts
Authors: Lee, J.; Gary, Dale E.; Shibasaki, K.
2000ApJ...531.1109L Altcode:
We study two microwave bursts (which occurred about 12 minutes apart
in the same active region) that show contrasting characteristics
in morphology and spectral variation in microwave data from the
Owens Valley Radio Observatory (OVRO) Solar Array and the Nobeyama
Radioheliograph (NoRH). The first flare (X-ray class C2.8, 23:22 UT)
shows radio morphology that changes from a single source at a low
frequency (5 GHz) to a double source at high frequencies (10-17 GHz),
and shows a total power spectrum that hardens in the decay phase. In
contrast, the second flare (X-ray class M1.0, 23:35 UT) shows a simple,
single-source morphology at 7-17 GHz. This source is located at one
of the two footpoints of a small loop identified in Yohkoh/HXT maps,
and its spectrum evolves in a typical soft-hard-soft pattern. We infer
the trap properties of these two loops from the microwave spectral
behaviors and a coronal field extrapolation from the vector magnetogram
of the active region obtained from Mees Solar Observatory, along
with X-ray maps obtained from Yohkoh/SXT. It is shown that the radio
characteristics of the first event are consistent with the hypothesis
in which Coulomb collisions dominate in a highly anisotropic loop with
low ambient electron density ~5x10<SUP>9</SUP> cm<SUP>-3</SUP>. The
second flare involves a small loop in which a more uniform field
strength allows rapid loss of electrons by precipitation, resulting
in a spectral variation mainly controlled by the energy dependence and
time profile of the injection. In spite of the significantly differing
decay times (~8 and ~1 minutes, respectively), very similar injection
times of ~30 s are inferred from the analysis of time profiles under
the above hypotheses. We therefore present a view that the largely
different morphologies and the corresponding spectral variations are
consequences of the differing trap properties rather than dissimilar
injection properties. The low trap density inferred for the first event
(~5x10<SUP>9</SUP> cm<SUP>-3</SUP>) as compared with the second event
(~8x10<SUP>10</SUP> cm<SUP>-3</SUP>) also explains why this event
showed richer microwave output in spite of weaker GOES activity.
---------------------------------------------------------
Title: Microwave Bursts and Electron Pitch Angles
Authors: Lee, J.; Gary, D. E.
2000ASPC..206..323L Altcode: 2000hesp.conf..323L
No abstract at ADS
---------------------------------------------------------
Title: Solar Radio Burst Locator
Authors: Dougherty, B. L.; Freely, W. B.; Zirin, H.; Gary, D. E.;
Hurford, G. J.
2000ASPC..206..367D Altcode: 2000hesp.conf..367D
No abstract at ADS
---------------------------------------------------------
Title: OVRO Solar Array Analysis Software in Support of HESSI
Authors: Gallagher, P. T.; Gary, D. E.; Lee, J.
2000ASPC..206..363G Altcode: 2000hesp.conf..363G
No abstract at ADS
---------------------------------------------------------
Title: Radio View of Particle Acceleration and Complementarity
with HESSI
Authors: Gary, Dale E.
2000ASPC..206..297G Altcode: 2000hesp.conf..297G
No abstract at ADS
---------------------------------------------------------
Title: Observations and Models of a Flaring Loop
Authors: Nindos, A.; White, S. M.; Kundu, M. R.; Gary, D. E.
2000ASPC..206..359N Altcode: 2000hesp.conf..359N
No abstract at ADS
---------------------------------------------------------
Title: Microwave and Extreme Ultraviolet Observations of Solar
Polar Regions
Authors: Nindos, A.; Kundu, M. R.; White, S. M.; Gary, D. E.;
Shibasaki, K.; Dere, K. P.
1999ApJ...527..415N Altcode:
The poles of the Sun are brighter than the rest of the quiet Sun's
emission in a limited range of radio frequencies from 17 GHz to 87
GHz. We have studied microwave images of the quiet Sun made with
the Nobeyama radioheliograph at 17 GHz. They show that the so-called
polar-cap brightening consists of two components: a diffuse component
of 1500 K excess brightness and patchy compact sources with localized
excess brightness of about 3500 K. We test the reality of the compact
sources using the maximum entropy method deconvolution. The total flux
and the number of compact polar sources as well as the north-south
extent of the diffuse polar emission are larger in the pole that is
closest to the Earth. We compared the microwave polar emission with
nearly simultaneous SOHO EIT images taken in the lines of He II at 304
Å and Fe XII at 195 Å. No one-to-one correlation between the compact
radio sources and the bright EUV features was found: most of the radio
emission arises between the plumes visible to EIT. The boundaries of
the polar-cap brightenings did not match exactly the boundaries of the
coronal holes as seen in either the Fe XII 195 Å images or the He II
304 Å images. The temporal variations of the compact microwave sources
did not correspond to any significant changes in EUV emission. On the
other hand, most He II 304 Å changing features were associated with the
diffuse polar microwave emission, which was practically constant. Our
data suggest that the origin of the polar brightening is not coronal;
it seems that the bulk of the patchy radio emission comes from heights
below the 80,000 K layer.
---------------------------------------------------------
Title: Radio Counterparts to SXR Transients
Authors: Gary, D. E.
1999spro.proc..129G Altcode:
By now several studies have been done on small-scale brightenings seen
at radio, UV, EUV, and soft X-ray wavelengths. These are to be reviewed
by Kundu in these proceedings. In this talk we concentrate on the
radio counterpart of a particular type of brightening---the soft X-ray
transient brightenings of Shimizu. These brightenings are associated
with active regions, and a study of radio counterparts by White et
al. (1995) using Nobeyama data found an excellent correspondence between
the 17 GHz and SXR brightenings, both spatially and temporally. However,
this study found that both the SXR and microwave emissions could be
satisfactorily explained as purely thermal emission, and a search
of BATSE hard X-ray data showed no nonthermal counterpart. White et
al. (1995) were forced to conclude that the events may be different from
flares. A more sensitive search for nonthermal emission was needed,
in particular using lower frequency microwaves where the influence
of nonthermal electrons would be more easily detected. Gary, Hartl
and Shimizu (1997) found 34 SXR transient brightenings over a 10-day
period in May 1992, for which OVRO (1-18 GHz) total power data were
available. A comparison of the data showed a number of clear nonthermal
signatures. In addition, one of the events was seen in the lowest
energy (6-9.3 keV) channel of the BATSE SPEC detector, suggesting
a connection between the microflares discovered in hard X-rays by
Lin et al. (1984). The evidence that SXR transient brightenings are
microflares is reviewed in this talk. We also attempt to place other
small-scale brightenings in context with regard to SXR transient
brightenings and microflares.
---------------------------------------------------------
Title: Comparison of Microwave and HXR Spectra from Solar Flares
Authors: Silva, A. V. R.; Wang, H.; Gary, D. E.
1999spro.proc..255S Altcode:
We analyze simultaneous hard X-ray and microwave emission from 28
solar flares with multiple peaks. A total of 44 simultaneous peaks
were observed at both wavelengths by BATSE (hard X-ray) and Owens
Valley Radio Observatory (microwave). Spectra during each burst were
fitted throughout the duration of the flare. The hard X-ray spectra
were fitted by a single power-law in most cases, whereas the microwave
spectra were fitted as gyrosynchrotron emission. The parameters at
both wavelengths (peak flux, turnover frequency, duration, spectral
indices, and delays between hard X-ray and microwave peak emission)
are then compared and correlated for each individual peak. We find
that for 74% of the bursts, the inferred index of the electron energy
distribution of the microwave emitting electrons, δ<SUB>r</SUB>, is
harder than that of the lower energy hard X-ray emitting electrons,
δ<SUB>x</SUB>. This implies that there is a breakup in the energy
spectra of the electrons, as is sometimes observed in the hard X-ray
spectra of giant flares. Moreover, 65% of all bursts with δ<SUB>x</SUB>
> δ<SUB>r</SUB> also showed delayed radio peak emission with
respect to hard X-ray maximum. Implications on electron acceleration
mechanisms are discussed.
---------------------------------------------------------
Title: OVRO Solar Array Upgrades in Preparation for MAX 2000
Authors: Gary, D. E.; Hurford, G. J.
1999spro.proc..429G Altcode:
Several significant changes to the OVRO Solar Array are underway in
preparation for the coming solar maximum in the year 2000. The array
currently consists of 5 antennas with a maximum baseline of about
670 m. By mid-2000, two additional antennas will be operating, with a
maximum baseline of about 1250 m, nearly doubling the spatial resolution
(to 5” at 10 GHz). The number of baselines recorded typically during
the previous solar maximum was 7. A new data system, coupled with
the additional antennas, will allow us to record 21 baselines for a
factor of three improvement in imaging. The new data system, which is
already in place, offers many additional advantages in ease of use of
the data, as well as improved calibration. In concert with the HESSI
(High Energy Solar Spectroscopic Imager) team, we plan to make the
OVRO data and the IDL-based analysis software widely available via the
World Wide Web. The current state of these improvements is discussed.
---------------------------------------------------------
Title: Microwave and Extreme Ultraviolet Observations of Solar
Polar Regions
Authors: Nindos, A.; Kundu, M. R.; White, S. M.; Gary, D. E.;
Shibasaki, K.; Dere, K. P.
1999AAS...194.3207N Altcode: 1999BAAS...31..871N
The radio emission of solar poles is brighter than the rest of the
quiet Sun's emission in a limited range of frequencies from 17 GHz to
87 GHz. We have studied microwave images of the quiet Sun made with
the Nobeyama Radioheliograph at 17 GHz. They show that the so-called
polar-cap brightening consists of two components: a diffuse component
of 1500 K excess brightness, and patchy compact sources with localized
excess brightness of about 3500 K. The total flux and the number of
compact polar sources as well as the North-South extent of the diffuse
polar emission are larger in the pole which is closest to the Earth. We
compared the microwave polar emission with nearly simultaneous SoHO EIT
images taken in the lines of He ii at 304 Angstroms and Fe xii at 195
Angstroms. No one-to-one correlation between the compact radio sources
and the bright EUV features was found: most of the radio emission arises
between the plumes visible to EIT. The boundaries of the polar-cap
brightenings did not match exactly the boundaries of the coronal holes
as seen in the Fe xii 195 Angstroms images. The temporal variations of
the compact microwave sources did not correspond to any significant
changes in EUV emission. On the other hand, most He ii 304 Angstroms
changing features were associated with the diffuse polar microwave
emission which was practically constant. Our data suggest that the
origin of the polar brightening is not coronal; it seems that the bulk
of the patchy radio emission comes from heights below the 80000 K layer.
---------------------------------------------------------
Title: The OVRO Solar Array During Max Millenium
Authors: Gary, D. E.; Hurford, G. J.
1999AAS...194.8007G Altcode: 1999BAAS...31..965G
The Owens Valley Radio Observatory (OVRO) Solar Array is a
solar-dedicated radio imaging instrument that obtains flare and active
region data in the range 1-18 GHz. The array is currently undergoing
extensive software and hardware upgrades, to be completed by mid-2000
-- in time for the launch of the HESSI spacecraft. The hardware
improvements include adding two antennas to the 5 existing ones,
bringing the number of antennas to 7 and the number of baselines
to 21. This addition will improve the image quality and double the
east-west spatial resolution, to 5" at 10 GHz. Along with the hardware
improvements, the computer control and data systems are being completely
revamped to modernize the data handling and make the data available
and useful to outside users on a daily basis. Certain standard data
products (e.g. calibrated total power spectra of flares), will be
available by this summer, via web server, for open use by the solar
community. Complete interferometry data will be available by the time
of the HESSI launch, in self-contained data files that can be analyzed
using freely-available, user-friendly IDL widget-driven software. It
is planned to model the analysis software after the HESSI analysis
software, so that those who have mastered the use of the HESSI software
will be able to use the OVRO software without difficulty. This is
possible since the HESSI and OVRO data are parallel in many ways. Both
obtain spatial data in the form of time profiles of spatial fourier
components, at many spectral points. Both pertain to emission from both
high-energy and thermal electrons in the corona. The two data-sets are
highly complementary, and it is expected that the OVRO data will be of
great interest to many researchers working on HESSI data. Ultimately,
the full OVRO data will be archived at the HESSI European Data Center
(HEDC). The OVRO Solar Array is supported through NSF grants AST-9796238
and ATM-9796213, and NASA grant NAG5-6831 to New Jersey Institute
of Technology.
---------------------------------------------------------
Title: FASR - A Frequency-Agile Solar Radiotelescope
Authors: Hurford, G. J.; Gary, D. E.; Bastian, T. S.; White, S. M.
1999AAS...194.7603H Altcode: 1999BAAS...31..956H
The Frequency-Agile Solar Radiotelescope (FASR) is a multi-frequency
imaging array designed specifically for imaging spectroscopy of the
Sun. Using < 100 antennas, it will combine high-quality/high spatial
resolution imaging (1" resolution at 20 GHz) with spectroscopy (dnu
/nu 0.01-0.03) and high time resolution (<1 s) across two decades in
frequency from 0.3-30 GHz. In so doing, it will produce a continuous,
dynamic, three-dimensional picture of the solar atmosphere from the
chromosphere through the mid-corona. These capabilities represent a
quantum leap beyond existing solar radio instruments, yet are well
within reach of emerging technologies. The range of science that
can be addressed by such an instrument is as broad as solar physics
itself. Virtually every solar feature from within a few hundred km
of the visible surface of the Sun to high in the solar corona can be
studied in detail with the unique diagnostics available in the radio
regime. Particular diagnostics include measuring the properties of
both thermal and nonthermal electrons accelerated in solar flares
from the largest events to the tiniest microflares/nanoflares,
measuring coronal magnetic field strengths in active regions and
elsewhere (coronal magnetography), and mapping kinetic electron
temperatures throughout the chromosphere and corona. In addition,
FASR's far-reaching exploration of the Sun in the radio regime gives
the instrument tremendous potential for new discoveries beyond those
that we can now anticipate. FASR is expected to be one of the major
new ground-based solar instruments of the next decade, and can be
operational by 2006, well before the decade is out. It will play a
major role in supporting NASA space missions with the unique diagnostics
and perspective provided by high-resolution radio imaging/spectroscopy.
---------------------------------------------------------
Title: Magnetic Trapping of Flare Electrons and Microwave Emission
Authors: Lee, J.; Gary, D. E.
1999AAS...194.2204L Altcode: 1999BAAS...31..859L
The topic of trapping of flare-produced electrons in magnetic loops
and their evolution under Coulomb collision has received considerable
attention in relation to interpreting hard X ray observations, since
the first work by Melrose and Brown in 1976. However, application
of the idea to the study of microwave radiation has been more
limited. Petrosian in 1982 illustrated how the magnetic field affects
the trapping and beaming of electrons to predict spatial morphology of
microwave emission given magnetic structure and location of a flaring
loop. Mel'nikov in 1994 used a model for trap and precipitation to
study relative intensities and time delays between microwaves and
hard X rays. We present a detailed modeling of microwave emission from
electrons undergoing Coulomb interaction in magnetic traps, designed
for quantitative analysis of spatially-resolved, multiwavelength
microwave observations such as those of the Solar Arrays at Owens
Valley Radio Observatory (OVRO). Our main concern is to properly
relate the precipitation rate and pitch angle diffusion to magnetic
quantities of the flaring loop and injection parameters. In this
approach, we use coronal field extrapolation and overlays of soft X
ray loops to provide the magnetic quantities so that the microwave
spectrum can be used mainly as the electron diagnostic. We discuss the
model capabilities and apply the results to a flare that occurred in
AR 7515 on 1993 June 3. This flare showed spectral flattening in the
decay phase along with morphological variation suggestive of a magnetic
trap around the loop top, and the spectral flattening is interpreted
as driven by Coulomb collision in the magnetic trap. The OVRO Solar
Array is supported through NSF grants AST-9796238 and ATM-9796213,
and NASA grant NAG5-6831 to New Jersey Institute of Technology.
---------------------------------------------------------
Title: IDL-based Database of Solar Active Regions
Authors: Gary, D. E.; Grechnev, V. V.; Shabarova, L. V.; Vourlidas,
A.; Nishio, M.
1999ASPC..172..391G Altcode: 1999adass...8..391G
A database on solar active regions has been implemented in IDL
(Interactive Data Language). The observational parameters of the regions
are stored in a multi-level structure array that is distributed as a
standard IDL save file. Convenient access to the data is provided by an
application equipped with a graphical user interface (GUI). A variety
of search modes are implemented. Full-disk 17 GHz radio maps produced
by the Nobeyama Radioheliograph provide the basis for the database.
---------------------------------------------------------
Title: Broadband microwave imaging spectroscopy with a solar-dedicated
array
Authors: Bastian, Tim S.; Gary, D. E.; White, S. M.; Hurford, Gordon J.
1998SPIE.3357..609B Altcode:
For many years, ground-based radio observations of the Sun have
proceeded into two directions: (1) high resolution imaging at a few
discrete wavelengths; (2) spectroscopy with limited or no spatial
resolution at centimeter, decimeter, and meter wavelengths. Full
exploitation of the radio spectrum to measure coronal magnetic fields
in both quiescent active regions and flares, to probe the thermal
structure of the solar atmosphere, and to study energy release and
particle energization in transient events, requires a solar-dedicated,
frequency-agile solar radiotelescope, capable of high-time, - spatial,
and -spectral resolution imaging spectroscopy. In this paper we
summarize the science program and instrument requirements for such a
telescope, and present a strawman interferometric array composed of
many (greater than 40), small (2 m) antenna elements, each equipped
with a frequency- agile receiver operating over the range 1 - 26.5 GHz.
---------------------------------------------------------
Title: Radio Emission from Solar Flares
Authors: Bastian, T. S.; Benz, A. O.; Gary, D. E.
1998ARA&A..36..131B Altcode:
Radio emission from solar flares offers a number of unique diagnostic
tools to address long-standing questions about energy release,
plasma heating, particle acceleration, and particle transport
in magnetized plasmas. At millimeter and centimeter wavelengths,
incoherent gyrosynchrotron emission from electrons with energies of
tens of kilo electron volts (keV) to several mega electron volts (MeV)
plays a dominant role. These electrons carry a significant fraction
of the energy released during the impulsive phase of flares. At
decimeter and meter wavelengths, coherent plasma radiation can play a
dominant role. Particularly important are type III and type III-like
radio bursts, which are due to upward- and downward-directed beams
of nonthermal electrons, presumed to originate in the energy release
site. With the launch of Yohkoh and the Compton Gamma-Ray Observatory,
the relationship between radio emission and energetic photon emissions
has been clarified. In this review, recent progress on our understanding
of radio emission from impulsive flares and its relation to X-ray
emission is discussed, as well as energy release in flare-like phenomena
(microflares, nanoflares) and their bearing on coronal heating.
---------------------------------------------------------
Title: Toward a Frequency-Agile Solar Radiotelescope
Authors: Bastian, T. S.; Gary, D. E.; White, S. M.; Hurford, G. J.
1998ASPC..140..563B Altcode: 1998ssp..conf..563B
No abstract at ADS
---------------------------------------------------------
Title: First Images of Impulsive Millimeter Emission and Spectral
Analysis of the 1994 August 18 Solar Flare
Authors: Silva, Adriana V. R.; Gary, Dale E.; White, Stephen M.; Lin,
R. P.; de Pater, Imke
1997SoPh..175..157S Altcode:
We present here the first images of impulsive millimeter emission of
a flare. The flare on 1994 August 18 was simultaneously observed at
millimeter (86 GHz), microwave (1-18 GHz), and soft and hard X-ray
wavelengths. Images of millimeter, soft and hard X-ray emission show
the same compact ( 8”) source. Both the impulsive and the gradual
phases are studied in order to determine the emission mechanisms. During
the impulsive phase, the radio spectrum was obtained by combining the
millimeter with simultaneous microwave emission. Fitting the nonthermal
radio spectra as gyrosynchrotron radiation from a homogeneous source
model with constant magnetic field yields the physical properties
of the flaring source, that is, total number of electrons, power-law
index of the electron energy distribution, and the nonthermal source
size. These results are compared to those obtained from the hard X-ray
spectra. The energy distribution of the energetic electrons inferred
from the hard X-ray and radio spectra is found to follow a double
power-law with slope ∼6-8 below ∼50 keV and ∼3-4 above those
energies. The temporal evolution of the electron energy spectrum and its
implication for the acceleration mechanism are discussed. Comparison of
millimeter and soft X-ray emissions during the gradual phase implies
that the millimeter emission is free-free radiation from the same hot
soft X-ray emitting plasma, and further suggests that the flare source
contains multiple temperatures.
---------------------------------------------------------
Title: On the feasibility of imaging coronal mass ejections at
radio wavelengths
Authors: Bastian, T. S.; Gary, D. E.
1997JGR...10214031B Altcode:
Coronal mass ejections (CMEs) can have a profound impact on the
interplanetary medium and the near-Earth environment. We discuss the
feasibility of detecting coronal mass ejections at radio wavelengths
with a ground-based instrument. In particular, we explore the
possibility that a radio telescope employing Fourier synthesis imaging
techniques can detect thermal bremsstrahlung emission from CMEs. Using
a simulated database from such a telescope, we explore three detection
schemes: direct detection, an approximate differential detection scheme,
and an “exact” differential detection scheme. We conclude that thermal
bremsstrahlung emission from CMEs can be detected by such a telescope
provided differential techniques are employed. While the approximate
differential detection scheme may be sufficient for CMEs viewed near
the solar limb, detection of CMEs against the solar disk may require
the more sensitive exact differential scheme. The detection and imaging
of nonthermal radio emissions from CMEs is also discussed.
---------------------------------------------------------
Title: Imaging the Chromospheric Evaporation of the 1994 June 30
Solar Flare
Authors: Silva, Adriana V. R.; Wang, Haimin; Gary, Dale E.; Nitta,
Nariaki; Zirin, Harold
1997ApJ...481..978S Altcode:
We analyze simultaneous Hα images (from the Big Bear Solar
Observatory), soft and hard X-ray images and spectra (from the soft
X-ray telescope [SXT], the Bragg Crystal Spectrometer [BCS], and the
hard X-ray telescope [HXT] on Yohkoh), and radio time profiles (from
the Owens Valley Radio Observatory) during the first 3 minutes of the
1994 June 30 flare. The strong blueshifts observed in the Ca XIX soft
X-ray line are interpreted as evidence of chromospheric evaporation,
with maximum up-flow velocities occurring 2 minutes prior to the hard
X-ray emission peak. In this study, we search for moving sources in
Hα, soft and hard X-ray images that correspond to the blueshifted
component. The chromospheric evaporation in this flare is divided
into two phases: an early phase with up-flow velocities of 350-450
km s<SUP>-1</SUP>, and a later phase (during the hard X-ray peak)
characterized by velocities of 100-200 km s<SUP>-1</SUP>. During the
first chromospheric evaporation phase, the footpoints of a loop seen
in HXT low-energy maps are seen to move toward the loop-top source. No
source displacement is observed in SXT images at this time. Images
of the later phase of chromospheric evaporation show a change in
the source morphology. The early HXT loop is no longer visible, and
HXT maps during this time display the two footpoints of a new loop
visible in SXT images. Now the HXT sources are stationary, and a SXT
footpoint source is seen to move toward the loop top. We interpret
the observed displacement of footpoint sources in HXT (early phase)
and SXT (later phase) maps to be the images of the evaporating front
projected onto the solar disk, while the up-flow velocities (inferred
from the blueshifts) are due to the movement of the same evaporating
material along the line of sight. By combining the up-flow velocities
with the proper motion of the footpoint sources seen in the maps,
we constructed a three-dimensional view of the magnetic loop for each
chromospheric evaporation phase. The early loop is almost semicircular,
with a height of 1.7 × 10<SUP>9</SUP> cm, whereas the later magnetic
loop is more elongated (a height of 3.2 × 10<SUP>9</SUP> cm), with
its apex closer to the footpoint where most of the evaporation took
place. The implications of these magnetic configurations and the
distinct evaporation phases are discussed.
---------------------------------------------------------
Title: Nobeyama/SOHO/BBSO Comparison of Solar Polar Coronal Holes
Authors: Gary, D. E.; Enome, S.; Shibasaki, K.; Gurman, J. B.; Shine,
R. A.
1997SPD....28.0801G Altcode: 1997BAAS...29..912G
Although it is not widely known outside the discipline of solar
radiophysics, a long-standing puzzle exists: the poles of the Sun
appear brighter than the rest of the quiet Sun in a restricted range
of wavelengths roughly from 15 GHz to about 48 GHz (cf. Kosugi et
al. 1986). At somewhat lower radio frequencies the poles appear darker
than the quiet Sun due to a deficit of coronal material, while at
mm-wavelengths the polar and non-polar quiet Sun appear quite uniform
due to the similarity of the atmospheric structure at lower heights
in the chromosphere. The excess brightness at the poles has also been
reported in coronal holes on the disk, and so is apparently related to
the phenomenon of coronal holes. The brightening likely corresponds to
an elevated temperature in the upper chromosphere in coronal holes
relative to normal quiet Sun. The phenomenon is especially well
suited to study via radio emission due to the unique sensitivity of
radio waves to this height range in the chromosphere. The possibility
exists that the different chromospheric structure for coronal holes
implied by the radio brightening may offer some clue to the origin of
the fast solar wind, which is now well established to arise in coronal
holes. Radio brightening of coronal holes is a difficult observational
problem because an instrument is needed that can image large areas of
the Sun at relatively high resolution. The Nobeyama Radioheliograph
has the required capability and operates at 17 and 34 GHz, nicely
within the frequency range where the brightening occurs. We compare
Nobeyama radio synthesis images on several days in 1996 with images
from the EIT, CDS, and MDI experiments on the Solar and Heliospheric
Observatory (SOHO) spacecraft, and with high resolution images from
the Big Bear Solar Observatory, with the aim of determining the spatial
and temporal characteristics of the brightening. We compare the extent
of the radio brightening with the boundaries of the coronal holes seen
from the SOHO data, to establish the previously suggested identity of
the polar brightening with coronal holes. We investigate whether the
brightening is primarily associated with network features, faculae,
or perhaps bipolar magnetic elements, or whether it is instead a
diffuse brightening more-or-less uniformly covering the coronal hole
area. We look for temporal variations, and their correlation with
changing features seen from SOHO. We conclude with some ideas for how
the chromospheric structure may be different in coronal holes than in
normal quiet Sun, and speculate on the implications for acceleration
of the fast solar wind.
---------------------------------------------------------
Title: Sunspot Gyroresonance Emission at 17 GHz: A Statistical Study
Authors: Vourlidas, A.; Gary, D. E.; Shibasaki, K.
1997SPD....28.0134V Altcode: 1997BAAS...29..885V
Our statistical study is based on a compilation of the daily Nobeyama
Radioheliograph full-disk maps obtained at 17 GHz between July 1992 --
December 1994. It includes 533 active regions, 20% of which appear to
have gyroresonance cores during some part of their disk crossing. At
least one of the regions reaches coronal temperatures (T>10(6)
K) while several others show a polarization reversal at the extreme
limb. Our study indicates that the gyroresonance cores are polarized
in the sense of the x-mode and are due to s=3 (B=2024 G) and/or s=4
(B=1518 G) gyroresonance absorption in the transition region and/or
corona. We also investigate the dependence between various physical
quantities such as brightness temperature, degree of polarization,
spot area, photospheric magnetic field and heliographic longitude and
comment on the polarization reversals at the limb. A more detailed
study of individual active regions, that have been also observed with
the Owens Valley Solar Array, will follow.
---------------------------------------------------------
Title: A spatial and spectral maximum entropy method as applied to
OVRO solar data
Authors: Komm, R. W.; Hurford, G. J.; Gary, D. E.
1997A&AS..122..181K Altcode:
We present first results of applying a Maximum Entropy Method (MEM)
algorithm that acts in both the spatial and spectral domains to
data obtained with the frequency-agile solar interferometer at Owens
Valley Radio Observatory (OVRO) taken at 45 frequencies in the range
1-18 GHz. The traditional MEM algorithm does not exploit the spatial
information available at adjacent frequencies in the OVRO data, but
rather applies separately to each frequency. We seek an algorithm that
obtains a global solution to the visibilities in both the spatial and
spectral domains. To simplify the development process, the algorithm
is at present limited to the one-dimensional spatial case. We apply
our 1-d algorithm to observations taken with the OVRO frequency-agile
interferometer of active region AR 5417 near the solar limb on March
20, 1989 (vernal equinox). The interferometer's two 27 m antennas
and 40 m antenna were arranged in a linear east-west array, which
at the vernal equinox gives a good match to the 1-d algorithm. Our
results show that including the spectral MEM term greatly improves the
dynamic range of the reconstructed image compared with a reconstruction
without using this information. The derived brightness temperature
spectra show that for AR 5417 the dominant radio emission mechanism
is thermal gyroresonance and we use this information to deduce the
spatial variation of electron temperature and magnetic field strength
in the corona above the active region.
---------------------------------------------------------
Title: Nonthermal Radio Emission from Solar Soft X-Ray Transient
Brightenings
Authors: Gary, Dale E.; Hartl, Michael D.; Shimizu, Toshifumi
1997ApJ...477..958G Altcode:
We compare microwave total power spectral data from the Owens Valley
Radio Observatory Solar Array with soft X-ray transient brightenings
observed with the Yohkoh soft X-ray telescope. We find that the
transient brightenings are clearly detected in microwaves in 12
of 34 events (35%), possibly detected in another 17 of 34 events
(50%), and only five of 34 events (15%) had no apparent microwave
counterpart. Comparing the radio and soft X-ray characteristics, we
find that (1) the soft X-ray peak is delayed relative to the microwave
peak in 16 of 20 events, (2) the microwave flux is correlated with
the flux seen in soft X-rays, (3) when radio fluence is used instead
of radio flux (24 events) the correlation increases substantially,
(4) the microwave spectra in the range 1-18 GHz vary greatly from
event to event, (5) the microwave spectra often peak in the range 5-10
GHz (13 of 16 events), and (6) the microwave spectra of some events
show narrowband spectra with a steep low-frequency slope. <P />We
conclude that the emission from at least some events is the result of
a nonthermal population of electrons, and that transient brightenings
as a whole can therefore be identified as microflares, the low-energy
extension of the general flare energy distribution. Soft X-ray transient
brightenings, and therefore microflares, cannot heat the corona.
---------------------------------------------------------
Title: Prospects for the Solar Radio Telescope
Authors: Bastian, Timothy S.; Gary, Dale E.
1997LNP...483..218B Altcode: 1997cprs.conf..218B
The Solar Radio Telescope (SRT) is an instrument concept for a powerful
solar-dedicated radio telescope. As presently conceived. it would
combine a high-resolution imaging capability (2″ at 20 GHz) with a
broadband spectroscopic capability (0.3-26 GHz). In other words, the SRT
would perform broadband imaging spectroscopy on a wide range of quiet-
and active-Sun phenomena. On 17-20 April, 1995, a workshop was held in
San Juan Capistrano, California. The purpose of the workshop, which was
attended by more than 40 scientists from the US and around the world,
was to discuss the science that could be done with a solar-dedicated
radio synthesis telescope, and to discuss the design constraints
imposed by the science envisioned. Special attention was also given
to nighttime uses for the instrument. We summarize the "strawman"
concept for the instrument here.
---------------------------------------------------------
Title: Comprehensive Multiwavelength Observations of the 1992 January
7 Solar Flare
Authors: Silva, Adriana V. R.; White, Stephen M.; Lin, Robert P.;
de Pater, Imke; Gary, Dale E.; McTiernan, James M.; Hudson, Hugh S.;
Doyle, J. Gerry; Hagyard, Mona J.; Kundu, Mukul R.
1996ApJS..106..621S Altcode:
Observations of a solar flare that occurred at 2022 UT on 1992
January 7, during the 1991 December/1992 January Max `91 campaign,
are presented. This flare was observed simultaneously in Hα, radio
(at microwave and millimeter wavelengths), and soft and hard X-rays
(by the Yohkoh spacecraft) with high spatial and moderate spectral
resolution. A comparison of magneto grams before and after the flare
shows evidence of the emergence of new magnetic flux of opposite
polarity at the flare site. Although this flare was only of moderate
size (GOES classification C8.9 and Hα importance SF), it exhibited
several distinct bursts and at least 10 spatially distinct hard/soft
X-ray sources. Cospatial Hα brightenings suggest that most of the
X-ray sources are located at footpoints of magnetic loops. Two of the
hard X-ray sources have no Hα counterparts and are therefore believed
to be located at loop tops. The flare consisted of three bursts
of particle acceleration followed by a purely thermal phase. High
spectral resolution Ca XIX line profiles indicate upflows shortly
after the second acceleration phase. Analysis of the microwave/hard
X-ray/soft X-ray emission from individual sources provides information
on the radio emission mechanisms, the energetic electron population,
the magnetic field strength, and the plasma density. These parameters
were estimated for the two microwave sources observed during the
third acceleration burst; these sources were simultaneously detected
in soft X-rays, and one of the sources is also seen in hard X-ray
maps. Although the microwave emission is consistent with the gyro
synchrotron mechanism, the millimeter emission, which peaks during the
thermal phase when all nonthermal activity has ceased, is likely due
to thermal bremsstrahlung from the soft X-ray emitting hot plasma. The
energy lost to collisions by the energetic (>15 keV) electrons
and the energy contained in the thermal plasma are calculated for
each source. The energy injected by the nonthermal electrons from all
sources is estimated to be 10<SUP>30</SUP> ergs. Only the soft X-ray
sources with gradual time profiles seem to show the Neupert effect.
---------------------------------------------------------
Title: Flare energetics: analysis of a large flare on YZ Canis
Minoris observed simultaneously in the ultraviolet, optical and radio.
Authors: van den Oord, G. H. J.; Doyle, J. G.; Rodono, M.; Gary,
D. E.; Henry, G. W.; Byrne, P. B.; Linsky, J. L.; Haisch, B. M.;
Pagano, I.; Leto, G.
1996A&A...310..908V Altcode:
The results of coordinated observations of the dMe star YZ CMi
at optical, UV and radio wavelengths during 3-7 February 1983 are
presented. YZ CMi showed repeated optical flaring with the largest
flare having a magnitude of 3.8 in the U-band. This flare coincided
with an IUE exposure which permits a comparison of the emission measure
curves of YZ CMi in its flaring and quiescent state. During the flare a
downward shift of the transition zone is observed while the radiative
losses in the range 10^4^-10^7^K strongly increase. The optical flare
is accompanied with a radio flare at 6cm, while at 20cm no emission
is detected. The flare is interpreted in terms of optically thick
synchrotron emission. We present a combined interpretation of the
optical/radio flare and show that the flare can be interpreted within
the context of solar two-ribbon/white-light flares. Special attention
is paid to the bombardment of dMe atmospheres by particle beams. We
show that the characteristic temperature of the heated atmosphere is
almost independent of the beam flux and lies within the range of solar
white-light flare temperatures. We also show that it is unlikely that
stellar flares emit black-body spectra. The fraction of accelerated
particles, as follows from our combined optical/radio interpretation
is in good agreement with the fraction determined by two-ribbon flare
reconnection models.
---------------------------------------------------------
Title: Microwave and Soft X-Ray Emission from a Flare-activated
Coronal Loop
Authors: Gary, D. E.; Wang, H.; Nitta, N.; Kosugi, T.
1996ApJ...464..965G Altcode:
We study the microwave and soft X-ray emissions produced by a
flare-activated coronal loop, using total power spectral data from
the Owens Valley Radio Observatory Solar Array, as well as soft X-ray
data from the Yohkoh Soft X-ray Telescope, and the GOES soft X-ray
monitors. We show that both the microwave and soft X-ray emissions,
late in the loop development, can be well understood as thermal
bremsstrahlung (free-free emission) from a hot (∼8 MK) plasma. From
an investigation of the microwave spectrum we suggest that an overlying
or cospatial, cooler (∼4 MK) plasma may also be present. <P />We
also study the time development of the loop, as observed with the
three instruments. The loop appears to grow in size from a lower
lying, initially invisible loop. New loops appear at greater heights,
or else the initial loop rises at a top speed of more than 150 km
s<SUP>-1</SUP>, but the growth of the loop or loops slows after about 3
minutes and becomes stable at its inflated size. We study the subsequent
brightening of the loop, using the SXT, GOES, and microwave data to
investigate the changing physical parameters of the loop as it evolves.
---------------------------------------------------------
Title: Imaging the Chromospheric Evaporation of the 1994 June 30
Solar Flare
Authors: Silva, Adriana V. R.; Wang, H.; Gary, D. E.; Zirin, H.;
Nitta, N.
1996AAS...188.3310S Altcode: 1996BAAS...28R.869S
We analyze simultaneous H _alpha images (from Big Bear Solar
Observatory), soft and hard X-ray images and spectra (from Yohkoh
during the first three minutes of the 1994 June 30 flare. The strong
blueshifts observed in the Ca XIX soft X-ray line are interpreted as
evidence of chromospheric evaporation, with maximum up--flow velocities
occurring two minutes prior to the hard X-ray emission peak. In this
paper, we search for moving sources in H_alpha , soft and hard X-ray
images that correspond to the blueshifted component. The chromospheric
evaporation in this flare is divided into two phases: an early phase
with up-flow velocities of 300-450 km s(-1) , and a later phase (during
the hard X-ray peak) characterized by velocities of 100-200 km s(-1)
. During the first chromospheric evaporation phase, the footpoints
of a loop seen in HXT maps are seen to move towards the loop top
source. No source displacement is observed in SXT images. The hard
X-ray spectra of individual sources, obtained from HXT maps, display a
very steep slope (gamma ~ 10-12). Thermal fitting of the spectra yield
temperatures of 20-50 MK. Images of the later phase of chromospheric
evaporation show the magnetic configuration to have changed. The early
HXT loop is no longer visible and HXT maps during this time display
the two footpoints of a new loop also visible in SXT images. Now
the HXT sources are stationary and a SXT footpoint source is seen to
move toward the loop top. We interpret the observed displacement of
footpoint sources in HXT (early phase) and SXT (later phase) maps to
be the images of the evaporating front projected onto the solar disk,
while the up--flow velocities (inferred from the blueshifts) are due
to the movement of the same evaporating material along the line of
sight. By combining the up--flow velocities with the proper motion
of the footpoint sources seen in the maps, we constructed a 3-D view
of the magnetic loop for each chromospheric evaporation phase. The
early loop is almost semi--circular with a height of 1.7x 10(9) cm,
whereas the later magnetic loop is more elongated (height of 2.3x 10(9)
cm) and asymmetric with its apex closer to the footpoint where most
of the evaporation took place. The implications of these magnetic
configurations and the distinct evaporation phases are discussed.
---------------------------------------------------------
Title: Large Scale Features of the Radio Sun
Authors: White, S. M.; Gary, D. E.; Kundu, M. R.
1996AAS...188.7907W Altcode: 1996BAAS...28Q.956W
We present full-disk images of the Sun at 5 GHz made by observing 26
different fields with the Very Large Array and combining them using
mosaicking techniques. The resulting image combines sensitivity to
large-scale structures with good resolution. Full-disk images at 0.33,
1.4 and 17 GHz, a high-resolution magnetogram and a soft X-ray image
are compared with the 5 GHz image to investigate the physical properties
of large-scale features.
---------------------------------------------------------
Title: Nonthermal Microwave Emission from Soft X-ray Transient
Brightenings.
Authors: Gary, D. E.; Hartl, M.; Shimizu, T.
1996AAS...188.2609G Altcode: 1996BAAS...28..858G
Soft X-ray transient brightenings (TBs) are small enhancements,
generally associated with active region loops, that last for 5-10
min. Shimizu et al. (1994) has shown that the rate of energy release in
TBs has the same form as that for flares, and if they are interpreted
as tiny flares they would extend the flare energy release rate to lower
energy by two orders of magnitude. However, in initial investigation of
radio counterparts of TBs near 15 GHz (Gopalswamy et al 1995; White et
al. 1995) showed no conclusive evidence for nonthermal electrons, which
brings into question whether TBs are flare-like energy releases. The
presence of nonthermal electrons is most easily seen at somewhat lower
radio frequencies, which led us to search for such emission in radio
data from the OVRO Solar Array in the range 1-18 GHz. Using soft X-ray
observations from Yohkoh, we identified 34 transient brightenings in
solar active region AR7172 observed from 20-29 May 1992. A comparison
with radio data from OVRO yielded the following new results: 1) enhanced
radio emission can be positively associated with TBs in 12 events (35%),
with another 17 (50%) showing a possible association; 2) a number of
the positive associations show radio peaks that occur near the onset
of the soft X-ray enhancement, with a Neupert-Effect-like relationship
(mean delay of the soft X-ray peak relative to the microwave peak is
2.5 min); 3) the radio power spectra of those events intense enough
to give good spectra indicate the presence of a nonthermal electron
population; 4) the correlation between peak radio flux and peak soft
X-ray flux is weak. For the 7 events with the clearest Neupert-Effect
relationship, there is a much higher correlation between integrated
radio flux and peak soft X-ray flux. Together these results suggest
that TBs are the analog of solar flares extended below the subflare
energy range. We may now state that the low-energy extension of flare
events is not sufficient to heat the corona, but the study of TBs can
shed light on flare processes that may be masked in larger flares.
---------------------------------------------------------
Title: Spatial/Spectral MEM Applied to OVRO Flare Data
Authors: Komm, R. W.; Gary, D. E.; Hurford, G. J.
1996AAS...188.8502K Altcode: 1996BAAS...28..961K
We present observations of a flare that occured on 1995 Oct 11, in AR
7912 obtained with the frequency-agile solar interferometer at Owens
Valley Radio Observatory (OVRO) made with both high spatial and spectral
resolution. We analyzed data at 32 frequencies in the range 1.2-12.4 GHz
for both left-handed and right-handed circular polarizations applying
the recently completed Spatial/Spectral Maximum Entropy Method (MEM). In
contrast to the traditional MEM algorithm, which does not exploit the
spatial information available at adjacent frequencies in the OVRO data,
the new algorithm obtains a global solution to the visibilities in
both the spatial and spectral domains and leads to spectra which are
greatly improved in smoothness and dynamic range. A comparison with
BBSO data shows that the optical flare is associated with the trailing
sunspot of the active region, while the leading sunspot shows no flare
related brightning. The reconstructed radio images show two sources;
one of them, the primary source, is associated with the optical flare
and has its peak emission at about 7.0 GHz, while the other one, the
secondary source, is associated with the leading sunspot with a peak
emission at about 2.8 GHz. The slopes of the brightness temperature
spectra imply that in both cases non-thermal gyrosynchrotron emission
is the process responsible for the microwave radiation. A preliminary
model fit of the spectra shows that the emission of both sources comes
from low-order harmonics and that the estimated magnetic field strength
and the estimated density of nonthermal electrons of the primary
source are much larger than the corresponding values of the secondary
source. The results suggest that the two sources are connected by a
large secondary loop and that the emission of the secondary source is
caused by nonthermal electrons escaping from the primary source. The
time evolution of the two sources within about +/- 3 min of the flux
peak time can be described by an increase in the number of nonthermal
electrons up to the peak time and by a subsequent decrease.
---------------------------------------------------------
Title: Ground-based Gamma-Ray Burst Follow-up Efforts: Results of
the First Two Years of the BATSE/COMPTEL/NMSU Rapid Response Network
Authors: McNamara, Bernard J.; Harrison, Thomas E.; Ryan, J.;
Kippen, R. M.; McConnell, M.; Macri, J.; Kouveliotou, C.; Fishman,
G. J.; Meegan, C. A.; Green, D. A.; Koranyi, D. M.; Warner, P. J.;
Waldram, E. M.; Hanlon, L.; Bennett, K.; Spoelstra, T. A. Th.; Metlov,
V. G.; Metlova, N. V.; Feigelson, E.; Beasley, A. J.; Palmer, D. M.;
Barthelmy, S. D.; Gary, Dale E.; Olsen, E. T.; Levin, S.; Wannier,
P. G.; Janssen, M. A.; MACHO Collaboration; Borovicka, J.; Pravec,
P.; Hudec, R.; Coe, M. J.
1996ApJS..103..173M Altcode:
In this paper we describe the capabilities of the BATSE/COMPTEL/NMSU
Rapid Response Network and report on results obtained during its first
2 years of operation. This network is a worldwide association of 22
radio and optical observatories that perform follow-up searches of
newly discovered gamma-ray burst error boxes by the Compton Gamma Ray
Observatory. During the last 2 years, it has deeply imaged 10 gamma-ray
error boxes over time frames from a few hours to a month after burst
detection, and it finds no sources that can be associated unambiguously
with a gamma-ray burst. We suggest that significant optical or radio
emission is not produced by gamma-ray bursts more than a day after
the burst. This result is consistent with recent theoretical models by
Katz, Meszaros, Rees, & Papathanassiou, and Paczynski & Rhoads;
however, our hours to days optical response time and radio sensitivity
limits allow only a weak constraint to be placed on these models. Based
upon this study and other published works, we surest that future work
should concentrate on acquiring deep optical images (m >= 12) of
small gamma-ray error boxes well within a day of the burst. Ideally,
radio observations should begin as soon after the burst as possible,
reach a sensitivity of <= 1 mJy, and be continued with occasional
images being acquired for at least a month following burst detection.
---------------------------------------------------------
Title: Imaging Spectroscopy of the Non-Flaring Sun
Authors: Gary, D. E.
1996ASPC...93..387G Altcode: 1996ress.conf..387G
No abstract at ADS
---------------------------------------------------------
Title: Broadband Imaging Spectroscopy with the Solar Radio Telescope
Authors: Bastian, T. S.; Gary, D. E.; Hurford, G. J.; Hudson, H. S.;
Klimchuk, J. A.; Petrosian, V.; White, S. M.
1996ASPC...93..430B Altcode: 1996ress.conf..430B
No abstract at ADS
---------------------------------------------------------
Title: OVRO, BBSO, BATSE, and YOHKOH Observations of a Twin Solar
Flare
Authors: Wang, H.; Gary, D. E.; Zirin, H.; Nitta, N.; Schwartz, R. A.;
Kosugi, T.
1996ApJ...456..403W Altcode:
We present the results of studies on two solar flares that occurred
on 1993 February 11: an M1.1 flare at 18:07 UT and an M2.7 flare at
18:31 UT. Our study was based on comprehensive observations by the
following observatories: Owens Valley Radio Observatory, which obtains
1-18 GHz microwave images; Big Bear Solar Observatory, which obtains
magnetograms, Hα and He D3 filtergrams; BATSE on board Compton Gamma
Ray Observatory, which obtains high-resolution hard X-ray spectra;
and the Japanese satellite Yohkoh, which obtains high-resolution soft
and hard X-ray images. We find the following: (1) While the optical
and hard X-ray emissions are confined to a small loop near the leading
spot of the active region for both flares, a large-scale soft X-ray
loop connects from the leading to the following spot 160" away. In
low-frequency micro-waves (<4 GHz), sources appear at each end
of the big loop, and the source near the following spot (away from
the Hα flare site) dominates at frequencies <2.8 GHz. For both
flares, as frequency increases, the source near the leading spot
becomes dominant, and the source near the following spot vanishes
gradually. (2) As frequency increases, the centroid of the leading
microwave source moves progressively downward until it reaches the
footpoint at high frequencies. (3) For the M2.7 event, in the compact
loop near the leading spot, two footpoints are seen in both soft and
hard X-rays. The dominant hard X-ray source has a softer spectrum than
the weaker one, suggesting that the weaker one may become dominant
at the higher energies (>100 keV) responsible for the microwave
emission. The high-frequency microwave emission is better associated
with this latter footpoint. (4) The large soft X-ray loop in the M2.7
flare is the postflare loop of the M1.1 flare. This flare is associated
with a different compact loop which is 40" away from the main flare. (5)
For the M2.7 flare, the microwave brightness temperature spectra in
the sources at the two ends of the big loop require very different
source parameters. The primary source near the leading spot can be
explained by nonthermal gyrosynchrotron emission from electrons with
a power-law energy index (δ) of 5.3. The same group of electrons can
explain the observed BATSE hard X-ray spectra. The low-frequency radio
source near the following spot is due to either a thermal component, or
a nonthermal component with a steep energy index (δ= 9.4). Based on the
available information, we cannot distinguish these two possibilities.
---------------------------------------------------------
Title: Recent Results from the OVRO Solar Array
Authors: Gary, D. E.
1996ASPC...93..409G Altcode: 1996ress.conf..409G
No abstract at ADS
---------------------------------------------------------
Title: Coordinated OVRO, BATSE, Yohkoh, and BBSO Observations of
the 1992 June 25 M1.4 Flare
Authors: Wang, H.; Gary, D. E.; Zirin, H.; Schwartz, R. A.; Sakao,
T.; Kosugi, T.; Shibata, K.
1995ApJ...453..505W Altcode:
We compare 1-14 GHz microwave images observed at the Owens Valley Radio
Observatory (OVRO), 16- and 256-channel hard X-ray spectra obtained by
the Burst and Transient Source Experiment (BATSE) onboard the Compton
Gamma Ray Observatory (CGRO), soft and and hard X-ray images obtained
by Yohkoh, and Hα images and magnetograms observed at the Big Bear
Solar Observatory (BBSO) for the 1992 June 25 M1.4 flare. We find the
following unique properties for this flare: (1) Soft X-ray emissions
connect two foot- points, the primary microwave source is located at
one footpoint, and hard X-ray emissions are concentrated in the other
footpoint The radio footpoint is associated with an umbra and may
have stronger magnetic field. (2) During the period that 256-channel
BATSE data are available, the hard X-ray photon spectrum consists
of two components: a superhot component with a temperature of 8.4
× 10<SUP>7</SUP> K and emission measure of 2.5 × 10<SUP>46</SUP>
cm<SUP>-3</SUP> and a power-law component with a photon index of
4.2. This is the first time that such a high temperature is reported
for the hard X-ray thermal components. It is even more interesting
that such a superhot component is identified before the peak of the
flare. The microwave brightness temperature spectra during the same
period also demonstrate two components: a thermal component near the
loop top and a nonthermal component at the footpoint of the loop. The
microwave thermal component has the similar temperature as that of
the hard X-ray superhot component. These measurements are consistent
with the theory that the microwaves and hard X-rays are due to the
same group of electrons, despite the fact that they are separated by
35,000 km. (3) The soft X-ray emissions brighten the existing loops and
co-align with Hα emissions throughout the entire duration of the flare.
---------------------------------------------------------
Title: Solar Rotation Stereoscopy in Microwaves
Authors: Aschwanden, Markus J.; Lim, Jeremy; Gary, Dale E.; Klimchuk,
James A.
1995ApJ...454..512A Altcode:
We present here the first stereoscopic altitude measurements of active
region sources observed at microwave frequencies (10-14 GHz The active
region NOAA 7128 was observed with the Owens Valley Radio Observatory
(OVRO) on 1992 April 13, 14, 15, and 16 as it passed through the central
meridian. From white-light data of the underlying sunspot we determined
the rotation rate of the active region, which was found to have a
relative motion of dL/dt = +0°.240 day<SUP>-1</SUP> with respect to the
standard photospheric differential rotation rate. Based on this rotation
rate we determine for the microwave sources stereoscopic altitudes of
3.3-11.0 Mm above the photosphere. The altitude spectrum h(v) of the
right circular polarization (RCP) main source shows a discontinuity
at 12 GHz and can be satisfactorily fitted with a dipole model with a
transition from the second to the third harmonic level at 12 GHz. The
dominance of the third harmonic for frequencies above 12 GHz occurs
because the second harmonic level drops below the transition region, at
a height of 2.6±0.6 Mm according to the microwave data. The altitude
spectrum h(v) serves also to invert the temperature profile T(h)
from the optically thick parts of the radio brightness temperature
spectrum T<SUB>B</SUB>(ν[h]). The microwave emission in both circular
polarizations can be modeled with gyroresonance emission, with x-mode
for RCP and o-mode in LCP, with the same harmonics at each frequency,
but different emission angles in both modes. The contributions from
free-free emission are negligible in both polarizations, based on the
peak emission measure of EM ≍ 6 × 10<SUP>28</SUP> cm<SUP>-5</SUP>
observed in soft X-rays by Yohkoh/SXT. <P />This study demonstrates
that the height dependence of the coronal magnetic field B(h) and
the plasma temperature T(h) in an active region can be inverted from
the stereoscopic altitude spectra h(v) and the observed brightness
temperature spectra T<SUB>B</SUB>(ν).
---------------------------------------------------------
Title: A Comparison of Active Region Temperatures and Emission
Measures Observed in Soft X-Rays and Microwaves and Implications
for Coronal Heating
Authors: Klimchuk, J. A.; Gary, D. E.
1995ApJ...448..925K Altcode:
We have determined active region temperatures and emission measures
using both broad-band soft X-ray images from the Yohkoh satellite and
spatially and spectrally resolved microwave data from the Owens Valley
Radio Observatory (OVRO). This work differs from previous work in that
the microwave temperatures and emission measures are directly measured
from the microwave spectrum, and are not model-dependent. The soft
X-ray temperatures and emission measures are ≍2.5 times greater
than the corresponding microwave values, on average. Detailed
error analysis indicates that the temperature differences are real,
but that the emission measure differences may not be. <P />We have
simulated Yohkoh and OVRO observations of idealized plane-parallel and
nested-loop coronal models. The plane-parallel model reproduces the
observed temperature differences if the coronal temperature decreases
exponentially with height from a maximum value of 4 × 10<SUP>6</SUP>
K at the base to an asymptotic value of ≍10<SUP>6</SUP> K. The
nested-loop model, which assumes quasi-static loop equilibrium, also
reproduces the observed temperature differences and indicates that the
volumetric coronal heating rate varies inversely with loop length to a
power greater than 2. Both models predict microwave emission measures
that are larger than observed. We suggest that a more complex model
is required to explain the observed emission measures and that more
than one coronal heating mechanism may be operative in solar active
regions. <P />We present derivations of the temperature and emission
measure uncertainties that result from random and systematic errors in
the Yohkoh observations. The expression for the random error emission
measure uncertainty is different from that used previously and is
especially important for observations of nonflaring plasmas.
---------------------------------------------------------
Title: The Microwave and H alpha Sources of the 1992 January 13 Flare
Authors: Wang, H.; Gary, D. E.; Zirin, H.; Kosugi, T.; Schwartz,
R. A.; Linford, G.
1995ApJ...444L.115W Altcode:
We compare X-ray, microwave and H-alpha observations for the 1992
January 13 limb flare. The soft and hard X-ray images of the flare
have been studied thoroughly by Masuda et al. (1994) with Yohkoh
SXT and HXT images. We find that during the hard X-ray emission peak
there is no H-alpha brightening on the disk nor at the limb, so the
main ribbons of this flare must be beyond the limb. The microwave
source maintains a fixed distance about 10 arcsecs from the optical
limb in the frequency range 2.8-14.0 GHz. We interpret this limit
in source position as due to the presence of a microwave limb that
extends higher than the white-light limb -- to a height of 7300 +/-
1500 km. We believe that the high-frequency microwave emissions are
occulted by this extended limb, while the soft and hard X-ray emissions
are able to pass through largely unaffected. We also believe, however,
that the hard X-ray footpoints are also partially occulted by the
photospheric limb, despite the appearance of 'footpoint sources'
in HXT data shown by Masuda et al. The smooth X-ray and microwave
time profiles, microwave-rich emission relative to hard X-rays, and
progressive hard X-ray spectral hardening through the flare peak are
all characteristics that we interpret as being a direct result of the
occultation of footpoint emission.
---------------------------------------------------------
Title: Solar Microwave and Soft X-ray Observations of Thermal
Bremsstrahlung from a Post-Flare Loop
Authors: Gary, D. E.; Wang, H.; Nitta, N.; Kosugi, T.
1995SPD....26.1214G Altcode: 1995BAAS...27..986G
No abstract at ADS
---------------------------------------------------------
Title: The Flare of 1992 July 16: Gyrosynchrotron Radiation and
Razin Suppression
Authors: Belkora, L.; Kiplinger, A.; Gary, D. E.
1995SPD....26..803B Altcode: 1995BAAS...27..972B
No abstract at ADS
---------------------------------------------------------
Title: A Solar Radio Telescope for the Future: Science Summary from
the SRT Workshop
Authors: Gary, D. E.; Bastian, T. S.; Hudson, H. S.; Hurford, G. J.;
Klimchuk, J. A.; Petrosian, V.; White, S. M.
1995SPD....26..801G Altcode: 1995BAAS...27..971G
No abstract at ADS
---------------------------------------------------------
Title: The Microwave and Hα Sources of the 1992 January 13 Flare
Authors: Wang, H.; Gary, D. E.; Zirin, H.; Kosugi, T.; Schwartz,
R. A.; Linford, G.
1995SPD....26..805W Altcode: 1995BAAS...27..972W
No abstract at ADS
---------------------------------------------------------
Title: The Application of Spatial and Spectral MEM to OVRO Solar Data
Authors: Komm, R. W.; Hurford, G. J.; Gary, D. E.
1995SPD....26.1301K Altcode: 1995BAAS...27..986K
No abstract at ADS
---------------------------------------------------------
Title: Initial Operation of the Solar Radio Burst Locator
Authors: Hurford, G. J.; Freely, W. F.; Gary, D. E.
1995SPD....26.1311H Altcode: 1995BAAS...27..989H
No abstract at ADS
---------------------------------------------------------
Title: A Solar Radio Telescope for the Future: Strawman Concept from
the SRT Workshop
Authors: Hurford, G. J.; Bastian, T. S.; Gary, D. E.; Hudson, H. S.;
Klimchuk, J. A.; Petrosian, V.; White, S. M.
1995SPD....26..802H Altcode: 1995BAAS...27..971H
No abstract at ADS
---------------------------------------------------------
Title: A Multisource Limb Flare Observed at Multiple Radio Wavelengths
Authors: Kucera, T. A.; Dulk, G. A.; Gary, D. E.; Bastian, T. S.
1994ApJ...433..875K Altcode:
A flare with several radio sources occurred on the solar limb at 2155
UT on 1989 June 20. It was observed by the Very Large Array (VLA) and
the Owens Valley Radio Observatory (OVRO). The VLA data consisted of
images at 1.4 GHz, while OVRO provided spectral and spatial information
over the range 1-15 GHz. We develop a new gyrosynchrotron model to
analyze the sources observed at flare peak. This model differs from
many previous ones in that it contains spatial variations of both
the magnetic field and accelerated particle density. It uses a new
gyrosynchrotron approximation which is valid at very low harmonics of
the gyrofrequency. For the first time we find that the cause of the
change of microwave source size with frequency in the event studied
was due primarily to the spatial variation of the accelerated electron
density. This is contrary to the common assumption that the variation
in size is due to a nonuniform magnetic field. We also investigate a
polarized source which brightened later in the flare, finding that it
could have been due to plasma radiation or gyrosynchrotron emission.
---------------------------------------------------------
Title: Microwave Spectral Imaging, H alpha , and Hard X-Ray
Observations of a Solar Limb Flare
Authors: Wang, H.; Gary, D. E.; Lim, J.; Schwartz, R. A.
1994ApJ...433..379W Altcode:
We compare the microwave, H-alpha, and hard X-ray observations for a
west limb C7.3 flare that occurred at 17:10 UT, 1992 June 26. H-alpha
movies were obtained at Big Bear Solar Observatory. Before the onset
of the flare, overexposed H-alpha images show the complicated flux
loop structure above the limb. Material was observed to descend along
the loops toward the site where the flare occurred hours later. Using
the five-antenna solar array at Owens Valley Radio Observatory, we
obtain two-dimensional maps of flare emission from 1.4 to 14 GHz. In
all three temporal peaks of the microwave bursts, the maps show the
same characteristics. The peak low-frequency emission comes from
the top of one bundle of the H-alpha loops and gradually shifts to
the foot-point of the loops (the location of H-alpha flare) as the
frequency increases. The location of the emission peak shifts 88 sec
between 1 and 14 GHz. Seventy percent of the shift occurs between 1
and 5 GHz. The locus of the shift of the emission peak follows the
shape of an H-alpha surge that occurred after the flare. For each
point along the locus, we create the microwave brightness temperature
spectrum and compare the radio-derived electron distribution with that
derived from the high-resolution hard X-ray spectra measured with
Burst and Transient Source Experiment (BATSE) on board the Compton
Gamma Ray Observatory (CGRO). We find that the peak frequency changes
from approximately 3 GHz at the loop top to approximately 7 GHz at the
footprint, presumably due to the increase of the magnetic field from
approximately 160 GHz at the loop top to approximately 300 G at the
footpoint. The high-frequency slope of the microwave power-law spectrum
decreases from approximately 10 at the loop top to approximately 5
at the footprint due to a change in the energy distribution of the
dominant electrons. The microwave brightness temperature spectral index
predicted by the BATSE power-law hard X-ray spectra agrees with the
measured value only at the footpoint. At the loop top, the emission may
be thermal gyrosynchrotron with a temperature of 3.5 x 10<SUP>7</SUP>
K, which is likely to correspond to the superhot component seen in
the hard X-ray emission.
---------------------------------------------------------
Title: Spectral evolution of microwaves and hard X-rays in the 1989
March 18 flare and its interpretation
Authors: Lee, Jeongwoo W.; Gary, Dale E.
1994SoPh..153..347L Altcode:
We analyze the time variation of microwave spectra and hard X-ray
spectra of 1989 March 18, which are obtained from the Solar Array
at the Owens Valley Radio Observatory (OVRO) and the Hard X-Ray
Burst Spectrometer (HXRBS) on the Solar Maximum Mission (SMM),
respectively. From this observation, it is noted that the hard X-ray
spectra gradually soften over 50-200 keV on-and-after the maximum
phase while the microwaves at 1-15 GHz show neither a change in
spectral shape nor as rapid a decay as hard X-rays. This leads to
decoupling of hard X-rays from the microwaves in the decay phase
away from their good correlation seen in the initial rise phase. To
interpret this observation, we adopt a view that microwave-emitting
particles and hard X-ray particles are physically separated in an
inhomogeneous magnetic loop, but linked via interactions with the
Whistler waves generated during flares. From this viewpoint, it is
argued that the observed decoupling of microwaves from hard X-rays may
be due to the different ability of each source region to maintain high
energy electrons in response to the Whistler waves passing through the
entire loop. To demonstrate this possibility, we solve a Fokker-Planck
equation that describes evolution of electrons interacting with the
Whistler waves, taking into account the variation of Fokker-Planck
coefficients with physical quantities of the background medium. The
numerical Fokker-Planck solutions are then used to calculate microwave
spectra and hard X-ray spectra for agreement with observations. Our
model results are as follows: in a stronger field region, the energy
loss by electron escape due to scattering by the waves is greatly
enhanced resulting in steep particle distributions that reproduce the
observed hard X-ray spectra. In a region with weaker fields and lower
density, this loss term is reduced allowing high energy electrons to
survive longer so that microwaves can be emitted there in excess of
hard X-rays during the decay phase of the flare. Our results based
on spectral fitting of a flare event are discussed in comparison with
previous studies of microwaves and hard X-rays based on either temporal
or spatial information.
---------------------------------------------------------
Title: Flat Microwave Spectra Seen at X-Class Flares
Authors: Lee, Jeongwoo W.; Gary, Dale E.; Zirin, H.
1994SoPh..152..409L Altcode:
We report peculiar spectral activity of four large microwave bursts as
obtained from the Solar Arrays at the Owens Valley Radio Observatory
during observations of X-class flares on 1990 May 24 and 1991 March
7, 8, and 22. Main observational points that we newly uncovered are:
(1) flat flux spectra over 1-18 GHz in large amounts of flux ranging
from 10<SUP>2</SUP> to 10<SUP>4</SUP> s.f.u. at the maximum phase, (2)
a common evolutionary pattern in which the spectral region of dominant
flux shifts from high frequencies at the initial rise to low frequencies
at the decaying phase, and (3) unusual time profiles that are impulsive
at high frequencies but more extended at lower frequencies.
---------------------------------------------------------
Title: OVRO and NRO Observations of the Solar Flare on 1993 June 3
Authors: Gary, D. E.; Enome, S.; Bruner, M.
1994kofu.symp..165G Altcode:
A flare that began on 1993 June 3 was observed jointly by the Owens
Valley Radio Observatory (OVRO) Solar Array and the Nobeyama Radio
Observatory (NRO) Radioheliograph. We present preliminary results
from these observations, along with soft X-ray data from the Yohkoh
SXT and GOES. The burst had a gradual time profile in the 17GHz NRO
data, except for a pair of unusual spikes in the decay phase. However,
the OVRO data show that the burst was impulsive, comprising several
peaks at progressively higher frequencies. We suggest that the gradual
17GHz emission is thermal, and discuss the relationship of the thermal
emission to the impulsive emission.
---------------------------------------------------------
Title: Multiwavelength Observations of a Solar Flare
Authors: White, S. M.; Silva, A.; de Pater, I.; Lin, R. P.; Gary,
D. E.; Hudson, H. S.; Doyle, J. G.; Hagyard, M. J.; Kundu, M. R.
1994kofu.symp..203W Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Imaging Spectroscopy of Solar Microwave Radiation. I. Flaring
Emission
Authors: Lim, Jeremy; Gary, Dale E.; Hurford, Gordon J.; Lemen,
James R.
1994ApJ...430..425L Altcode:
We present observations of an impulsive microwave burst on the
Sun with both high spatial and spectral resolution, made with the
Solar Array at the Owens Valley Radio Observatory (OVRO). We used
the measured brightness temperature spectrum to infer the emission
process responsible for each microwave source, and to derive physical
conditions in the source region. We confimed our predictions using
soft X-ray measurements from Geostationary Operational Environmental
Satellite (GOES), soft X-ray images from Yohkoh, and H-alpha flare
images together with sunspots and magnetogram images from the Big Bear
Solar Observatory.
---------------------------------------------------------
Title: Coronal Temperature, Density, and Magnetic Field Maps of a
Solar Active Region Using the Owens Valley Solar Array
Authors: Gary, Dale E.; Hurford, G. J.
1994ApJ...420..903G Altcode:
We present the first results of solar active region observations with
the recently completed five-element Owens Valley Solar Array. On
1991 October 24, maps of Active Region AR 6891 were obtained at
22 frequencies from 1.2-7.0 GHz to provide brightness temperature
spectra at each point. This is the first time that both high spatial
and frequency-resolution brightness temperature spectra have been
available over such a broad radio-frequency range. We find that over
most of the region the spectra fall into one of the two well-defined
categories: thermal free-free or thermal gyroresonance. In these
cases, we use the spectra to deduce the spatial variation of
physical parameters-electron temperature, column emission measure
(intergral n<SUP>2</SUP><SUB>e</SUB> dl), and the coronal magnetic
field strength-in and around the active region. Over a limited area
of the region, the spectra resemble neither of the simple types, and
alternative interpretations are required. The possibilties include the
presence of fine structure that is unresolved at low frequencies; the
presence of a small number of nonthermal electrons; or the presence of
overlying, cooler 10<SUP>6</SUP> K material which at low frequencies
absorbs the hot (3 x 10<SUP>6</SUP> K) thermal emission generated below.
---------------------------------------------------------
Title: Particle Acceleration and Flare Triggering in Large-Scale
Magnetic Loops Joining Widely Spaced Active Regions
Authors: Willson, Robert F.; Lang, Kenneth R.; Gary, Dale E.
1993ApJ...418..490W Altcode:
We provide new observations that contradict the canonical model of
solar flare emission in the coronal loops of a single active region,
and indicate instead particle acceleration and flare triggering
in otherwise invisible, large-scale magnetic loops joining widely
separated active regions. Very Large Array (VLA) snapshot maps and
NSO-Kitt Peak photospheric magnetograms indicate that bursts at 20 cm
wavelength occurred in two widely separated active regions, and that
91 cm radiation originated in large-scale magnetic loops joining
them. Images taken by the Yohkoh satellite showed no substantial
enhanced soft X-ray emission from coronal loops joining these active
regions either before or after the flare. Observations from the
Owens Valley Solar Array (OVRO) confirm the VLA 20 cm results while
also showing decimetric emission that is relatively narrow-band and
superposed on a broader background. Comparisons of the time profiles
of the radio radiation from the two active regions with simultaneous
hard X-ray time profiles, observed with the Burst and Transient Source
Spectrometer Experiment (BATSE) on the Compton Gamma Ray Observatory
(CGRO), suggest that energetic electrons were first accelerated in one
active region, and then injected into the largescale magnetic loops
at velocities ≥ 0.4c to trigger 20 cm burst emission in the active
region at the other footpoint of these loops, while also leading to 91
cm radiation from the entire loop system. These observations provide
another example of the diversified mechanisms for particle acceleration
and flare triggering on the Sun.
---------------------------------------------------------
Title: High spectral resolution, high sensitivity microwave and
associated hard X-ray bursts
Authors: Sawant, H. S.; Cecatto, J. R.; Dennis, B. R.; Gary, D. E.;
Hurford, G. J.
1993AdSpR..13i.191S Altcode: 1993AdSpR..13..191S
We have carried out mm-wavelength (18 - 23) GHz observations of solar
bursts in June 1989, in Brazil. Nine of the bursts were observed
simultaneously with the Hard X-ray Burst Spectrometer (HXRBS) on
the Solar Maximum Mission (SMM). The Owens Valley Radio Observatory
(OVRO) observed five of these events from 1 to 18 GHz. To date, we have
analyzed six of these events and the preliminary results are: (i) The
turnover frequency of all of these bursts were in the frequency range
of 7 - 11 GHz and the radio spectral index ranged from -0.3 to -5.3
(ii) In three bursts, dominant features of high energy (>= 200 keV)
X-rays coincided in time with mm-wavelength peaks, suggesting that the
high energy electrons are responsible for the mm-wavelength emission.
---------------------------------------------------------
Title: The Magnetic Field in the Corona above Sunspots at the Eclipse
of 1991 July 11
Authors: Gary, D. E.; Leblanc, Y.; Dulk, G. A.; Golub, L.
1993ApJ...412..421G Altcode:
A partial solar eclipse and an X-ray image are used to study the
magnetic field as a function of height in the corona above an active
region during the solar eclipse of July 11, 1991. The dominant features
of AR 6718 are two leading spots of positive polarity followed by two
spots of negative polarity about 3 arcmin to the east. Bright radio
emission coincides with the positions of the sunspots, attributable to
a gyroresonance radiation from ambient electrons above the spots. A
simplified model of the source as a function of frequency based on
the interferometer fringe amplitudes is used to obtain brightness
temperature spectra for the emission associated with the sunspots. It
is deduced that the magnetic field strength at the base of the corona
above the leading spots was 1200 G, and about 1100 G above the following
spots. The soft X-ray brightness above the sunspots was very low, about
30 times lower than that of the adjacent plage-associated emission.
---------------------------------------------------------
Title: Imaging Spectroscopy of a Solar Microwave Flare
Authors: Lim, J.; Gary, D. E.; Hurford, G. J.; Lemen, J. R.
1993BAAS...25.1198L Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Flare of July 16, 1992 Observed with OVRO and YOHKOH
Authors: Belkora, L.; Gary, D. E.
1993BAAS...25.1186B Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Comparison of Coronal Temperatures and Emission Measures
Determined from X-Ray and Microwave Observations
Authors: Klimchuk, J. A.; Gary, D. E.
1993BAAS...25.1179K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Microwave, Hα and Hard X-ray Observations of the 1992 June
26 C7.3 Solar Flare
Authors: Wang, H.; Gary, D. E.
1993BAAS...25.1198W Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Millimeter, Microwave and X-Ray Morphology and Spectra of
the 07Jan92 Flare
Authors: Silva, A. V.; Lin, R. P.; de Pater, I.; White, S. M.; Kundu,
M. R.; Gary, D. E.; Hudson, H. S.
1993BAAS...25Q1223S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: OVRO Microwave and Yohkoh HXT and SXT Observations of a
Solar Flare
Authors: Gary, D. E.; Hurford, G. J.; Sakao, T.
1993BAAS...25.1180G Altcode:
No abstract at ADS
---------------------------------------------------------
Title: OVRO Microwave Observations During the Max'91/CoMStOC'92
Campaign
Authors: Gary, D. E.; Hurford, G. J.; Bastian, T.
1993BAAS...25.1224G Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Model of Gyrosynchrotron Emission Sources Observed in the
Microwaves
Authors: Kucera, T. A.; Dulk, G. A.; Gary, D. E.
1993BAAS...25.1214K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Microwave Emission from a Sunspot - Part Two
Authors: Lee, Jeongwoo W.; Gary, Dale E.; Hurford, G. J.
1993SoPh..144..349L Altcode:
A series of microwave observations of a sunspot in the active region
NOAA 4741 was made with the Owens Valley Solar Array for the purpose
of investigating the center-to-limb variation of both the spectral
and spatial brightness distribution. In this investigation, several
properties of the sunspot microwave radiation are found. First,
sunspot microwave emission appears in two typical profiles depending
on the heliocentric position of the spot: either the ring structure
near disk center or single-peak structure near the limb. Second, the
brightness temperature at high, optically thin frequencies (>6
GHz) increases slightly as the spot approaches the limb, which we
interpret as being due to the increase of the gyroresonance opacity
of the field lines near the spot center as they gain greater viewing
angles. Third, the center-to-limb variation of the gyroresonance
spectrum seems to be mostly characterized by a change of effective
harmonic, which accompanies a discontinuous change of the degree
of polarization. Fourth, a change of spectrum from gyroresonance to
free-free emission is found in the passage of the spot over the solar
limb, which gives a determination of the height of the gyroresonance
layer to confirm its location low in the corona of the active region.
---------------------------------------------------------
Title: Microwave Emission from a Sunspot - Part One
Authors: Lee, Jeongwoo W.; Hurford, G. J.; Gary, Dale E.
1993SoPh..144...45L Altcode:
From the gyroresonance brightness temperature spectrum of a sunspot,
one can determine the magnetic field strength by using the property
that microwave brightness is limited above a frequency given by an
integer-multiple of the gyrofrequency. In this paper, we use this idea
to find the radial distribution of magnetic field at the coronal base of
a sunspot in the active region, NOAA 4741. The gyroresonance brightness
temperature spectra of this sunspot are obtained from multi-frequency
interferometric observations made at the Owens Valley Radio Observatory
at 24 frequencies in the range of 4.0-12.4 GHz with spatial resolution
2.2″-6.8″. The main results of present study are summarized as
follows: first, by comparison of the coronal magnetic flux deduced
from our microwave observation with the photospheric magnetic flux
measured by KPNO magnetograms, we show that theo-mode emission must
arise predominantly from the second harmonic of the gyrofrequency,
while thex-mode arises from the third harmonic. Second, the radial
distribution of magnetic fieldsB(r) at the coronal base of this spot
(say, 2000-4000 km above the photosphere) can be adequately fitted by
B(r) = 1420(1±0.008)exp[-(r/{11.05"(1±0.014)})<SUP>2</SUP>]G, where
r is the radial distance from the spot center at coronal base. Third,
it is found that coronal magnetic fields originate mostly from the
photospheric umbral region. Fourth, although the derived vertical
variation of magnetic fields can be approximated roughly by a dipole
model with dipole moment 1.6 × 10<SUP>30</SUP> erg G<SUP>−1</SUP>
buried at 11000 km below the photosphere, the radial field distribution
at coronal heights is found to be more confined than predicted by the
dipole model.
---------------------------------------------------------
Title: Microwave Emission From a Sunspot III. Implications for the
Force Balance in a Static Sunspot
Authors: Lee, J. W.; Gary, D. E.; Hurford, G. J.; Zirin, H.
1993ASPC...46..287L Altcode: 1993IAUCo.141..287L; 1993mvfs.conf..287L
No abstract at ADS
---------------------------------------------------------
Title: Measurement of the Solar Limb Brightness Profile at 3
Millimeters during the Total Eclipse of 1991 July 11
Authors: Belkora, L.; Hurford, G. J.; Gary, Dale E.; Woody, D. P.
1992ApJ...400..692B Altcode:
Observations of the solar limb at the point of first contact during the
eclipse of 1991 July 11, with a spatial resolution of about 1.6 arcsec,
are reported. The visibility amplitude and phase were modeled to yield
the height of the 3-mm limb above the visible photosphere, and the
data were differentiated to yield the brightness profile of the limb
in strips about 1.6 arcsec wide. The 3-mm limb was found to extend
7.5 +/- 0.8 arcsec above the visible photosphere, with no evidence
of a limb spike. The 3-mm limb, at a temperature of about 6500 K,
extends to altitudes far beyond the expected location of the transition
region in the model of Vernazza et al. (1981). A comparison of the 3-mm
profile and an off-band H-alpha photograph of the limb reveals a close
correspondence between the 3-mm limb and the height of H-alpha spicules.
---------------------------------------------------------
Title: The High-Frequency Characteristics of Solar Radio Bursts
Authors: Lim, J.; White, S. M.; Kundu, M. R.; Gary, D. E.
1992SoPh..140..343L Altcode:
We compare the millimeter, microwave, and soft X-ray emission from
a number of solar flares in order to determine the properties of the
high-frequency radio emission of flares. The millimeter observations
use a sensitive interferometer at 86 GHz which offers much better
sensitivity and spatial resolution than most previous high-frequency
observations. We find a number of important results for these
flares: (i) the 86 GHz emission onset appears often to be delayed
with respect to the microwave onset; (ii) even in large flares the
millimeter-wavelength emission can arise in sources of only a few
arc sec dimension; (iii) the millimeter emission in the impulsive
phase does not correlate with the soft X-ray emission, and thus is
unlikely to contain any significant thermal bremsstrahlung component;
and (iv) the electron energy distributions implied by the millimeter
observations are much flatter (spectral indices of 2.5 to 3.6) than
is usual for microwave or hard X-ray observations.
---------------------------------------------------------
Title: Radio observations of the M8.1 solar flare of 23 June, 1988:
Evidence for energy transport by thermal processes
Authors: Bastian, T. S.; Gary, D. E.
1992SoPh..139..357B Altcode:
The Very Large Array (VLA) and the frequency agile interferometer at
the Owens Valley Radio Observatory (OVRO) were used to observe the
M8.1 flare of 23 June, 1988. The VLA obtained images prior to and
during the flare at 333 MHz, and at 1.5 and 4.7 GHz. The frequency
agile interferometer at Owens Valley obtained interferometer amplitude
and total power spectra of the flare at 45 frequencies between 1 and
18 GHz. The observations were supplemented by radiometer measurements
made by the USAF RSTN network site at Palehua, HI, by GOES soft X-ray
observations, by USAF SOON Hα filtergrams, and by a KPNO photospheric
magnetogram.
---------------------------------------------------------
Title: Measurement of the Solar Limb Brightness Profile at 3 MM
During the Total Eclipse of 1991 July 11
Authors: Belkora, L.; Hurford, G. J.; Gary, D. E.; Woody, D. P.
1992AAS...180.4202B Altcode: 1992BAAS...24..796B
We observed the solar limb at the point of first contact during
the eclipse of 1991 July 11, with a spatial resolution ~ 1.6”. The
observations were carried out at the Owens Valley Radio Observatory
millimeter interferometer, operating at 3 millimeters. The visibility
amplitude and phase were modeled to yield the height of the 3 mm limb
above the visible photosphere, and the data were differentiated to yield
the brightness profile of the limb in strips ~ 1.6” wide. We found
that the 3 mm limb extends 7.5+/-0.8” or 5500 km above the visible
photosphere, with no evidence of a limb spike. This result, and the
overall shape of the limb profile, are similar to the interferometric
results of Wannier, Hurford, & Seielstad (1983) obtained with the
same intrument but without the benefit of an eclipse. The 3 mm limb,
at a temperature of ~ 7200 K, extends to altitudes far beyond the
expected location of the transition region in the model of Vernazza,
Avrett, & Loeser (1981). A comparison of the 3 mm profile and
an offband Hα photograph of the limb reveals a close correspondence
between the 3 mm limb and the height of Hα spicules.
---------------------------------------------------------
Title: First Results from 5-Element Observations with the OVRO
Solar Array
Authors: Gary, Dale E.; Hurford, G. J.
1992AAS...180.4201G Altcode: 1992BAAS...24R.795G
We report the first synthesis imaging results using the 5 antennas of
the newly expanded solar array at Owens Valley Radio Observatory. The
observations were made on 1991 Oct 24 and give the spatial and spectral
structure of the complex active region NOAA 6891 as well as the spatial,
spectral, and temporal structure of three flares that occurred during
our observations. We show aperture synthesis maps of the active region
from 1.2 to 7 GHz, with spatial resolution from 60” to 9” over the
same range, and plot brightness temperature spectra for various points
in the maps to show the region of dominance of free-free emission
versus the region of dominance of gyroresonance emission. From the
free-free dominated spectra we can deduce the coronal column density
and temperature as a function of position over the active region. Where
the source is dominated by gyroresonance emission, the spectra can be
used to determine the magnetic field strength at the base of the corona
(see the paper by Lee, Gary, and Hurford also being presented at this
meeting). We also investigate the three flares that occurred during our
observations, using frequency synthesis to map the spatial structure
over relatively broad frequency ranges. We find several interesting
results, including a radio emission source with no accompanying
Hα emission and motion of the radio source along the spreading Hα
ribbon. These results represent the first, but not necessarily the
best-suited, observations of active regions and flares with the new
OVRO Solar Array. They serve to show the tremendous capability of the
instrument to resolve the ambiguities that for so long have plagued
radio studies because simultaneous spatial and spectral observations
were not available.
---------------------------------------------------------
Title: OVRO Solar Array Observations During HIREGS Campaign
Authors: Gary, Dale E.; Hurford, G. J.; Belkora, L.
1992AAS...180.4509G Altcode: 1992BAAS...24..803G
The newly expanded solar interferometer array at Owens Valley Radio
Observatory (OVRO Solar Array) was in full operation with 5 antennas
before and during the Max '91 Campaign that was associated with the
HIREGS hard X-ray balloon experiment. For the two weeks prior to
1992 Jan 6, we obtained at least 21 flares within our UT time range
of ~1600-2400 UT. During this pre-campaign period we were using
an observing mode that measured 45 frequencies in both right- and
left-hand circular polarization every 12 s. For the campaign period
itself, from 1992 Jan 6-24, we obtained at least 10 large events along
with many smaller ones. For the time that the HIREGS balloon was up,
we were observing in a mode where we obtained only stokes I (total
intensity), but at 20 frequencies with a 2 s cycle time to maximize
our time resolution. In addition to the flare observations, OVRO is now
capable of synthesis imaging of the active regions each day, which will
be of interest for active region studies. We present early results from
one or two of the best observed events by OVRO and other observatories
participating in the campaign, including the event of 1941 UT on 1992
Jan 21. We emphasize multifrequency imaging of the flare as a function
of time, a capability that is unique to the expanded OVRO Solar Array.
---------------------------------------------------------
Title: Center-to-Limb Variation of Sunspot Microwave Radiation
Authors: Lee, Jeongwoo W.; Gary, Dale E.; Hurford, G. J.
1992AAS...180.4203L Altcode: 1992BAAS...24..796L
A series of microwave observations of a sunspot in the active region
NOAA 6761 was made with the Owens Valley Solar Array for the purpose
of investigating the center-to-limb variation of both spectral and
spatial brightness distribution. In this investigation, several
properties of the sunspot microwave radiation are found. First,
sunspot microwave emission appears in two typical profiles depending
on the heliocentric position of the spot: either the ring structure
near disk center or single-peak structure near the limb. Second, the
brightness temperature at high, optically thin frequencies (>6 GHz)
increases slightly as the spot approaches the limb, which we interpret
as being due to the increase of the gyroresonance opacity of the field
lines near the spot center as they gain greater viewing angles at the
position. Third, the center-to-limb variation of the gyroresonance
spectrum seems to be mostly characterized by a change of effective
harmonic, which accompanies a discontinuous change of the degree
of polarization. Fourth, a change of spectrum from gyroresonance to
free-free emission feature is found in the passage of the spot over
the solar limb, which gives a determination of the height of the
gyroresonance layer to confirm its location low in the corona of the
active region.
---------------------------------------------------------
Title: Hard X ray/microwave spectroscopy of solar flares
Authors: Gary, Dale E.
1992cait.rept.....G Altcode:
The joint study of hard x ray and microwave observations of solar
flares is extremely important because the two complementary ways of
viewing the accelerated electrons yield information that cannot be
obtained using hard x rays or microwaves alone. The microwaves can
provide spatial information lacking in the hard x rays, and the x ray
data can give information on the energy distribution of electrons that
remove ambiguities in the radio data. A prerequisite for combining
the two data-sets, however, is to first understand which range of
microwave frequencies correlate best with the hard x rays. This SMM
Guest Investigator grant enabled us to combine multi-frequency OVRO
data with calibrated hard x ray data to shed light on the relationship
between the two emissions. In particular, the questions of which
microwave frequencies correspond to which hard x ray energies, and
what is the corresponding energy of the electrons that produce both
types of emission are investigated.
---------------------------------------------------------
Title: Solar Radio Pulsation Event Observed by the VLA and OVRO
Authors: Aschwanden, M. J.; Bastian, T. S.; Gary, D. E.
1992AAS...180.4505A Altcode: 1992BAAS...24..802A
We investigate interpretational aspects of the radio pulsation
event which has been observed by the Very Large Array (VLA) and
the Owens Valley Radio Observatory (OVRO) on December 21, 1990,
1930 UT, during the second MAX'91 observing campaign. The VLA was
observing with a time resolution of 0.4 s at 0.33, 1.4 and 4.9 GHz,
while OVRO used a time resolution of 0.2 s at 1.2, 1.4, 1.6, 1.8
and 2.0 GHz. The radio pulsation event was triggered by a C3.3 class
flare in soft X-rays in active region 6412 (NOAA). The time profile
of the quasi-periodic radio emission exhibits a period of 8.8 s and
shows indications of secondary periodicities with faster periods. We
investigate whether these periodicities can be explained in terms of
(i) MHD eigen-modes in cylindrical fluxtubes (MHD surface modes and
harmonic modes), or (ii) relaxational oscillations of a nonlinear
dissipative system. We test these two options by means of Fourier
spectra and by reconstruction of the “strange attractor dimension”
of nonlinear systems. We attempt a three-dimensional reconstruction of
the pulsating source by means of magnetic field extrapolation from the
photospheric magnetogram and by using the constraints of the radio
maps at multiple frequencies. Preliminary analysis indicates that
plasma emission as well as gyrosynchrotron emission is coherently
modulated in spatially diverging magnetic structures. We investigate
whether the pulsating radio emission originates from plasma confined
in a pulsating loop structure or whether the radio emission is driven
by electron beams which are produced in a pulsating acceleration
mechanism. The latter possibility would support a model where the
primary energy release itself is governed by a nonlinear dissipative
system, e.g. by oscillatory magnetic reconnection.
---------------------------------------------------------
Title: Radio synthesis imaging during the GRO solar campaign.
Authors: Gary, Dale E.
1992NASCP3137..514G Altcode: 1992como.work..514G
The Owens Valley (OVRO) Solar Array was recently expanded to 5
antennas. Using frequency synthesis, the 5-element OVRO Solar Array
has up to 450 effective baselines, which can be employed as necessary
to make maps at frequencies in the range 1 to 18 GHz. Fortuitously,
the last of the 5 antennas was completed and brought into operation
on 7 Jun., just in time for the Gamma Ray Observatory (GRO)/Max 1991
observing campaign. Many events were observed jointly with OVRO and
the BATSE experiment on GRO, including the six larger events that
are presented in tabular form. Unfortunately, the X flares that
occurred during the campaign all occurred outside the OVRO time
range. The UV coverage of the newly expanded solar array, combined
with frequency synthesis, should give a more complete view of solar
flares in the microwave range by providing simultaneous spatial and
spectral resolution. A promising application of MEM (maximum entropy)
is also being pursued that will use smoothness criteria in both the
spatial and spectral domains to give brightness temperature maps at each
observed frequency (up to 45 frequencies every 10 s). Such maps can be
compared directly with the theory of microwave emission to yield plasma
parameters in the source - notably the number and energy distribution of
electrons, for comparison with the x ray and gamma ray results from GRO.
---------------------------------------------------------
Title: Multifrequency Observations of a Remarkable Solar Radio Burst
Authors: White, S. M.; Kundu, M. R.; Bastian, T. S.; Gary, D. E.;
Hurford, G. J.; Kucera, T.; Bieging, J. H.
1992ApJ...384..656W Altcode:
Observations of an impulsive solar-radio burst from three observatories
are presented. The striking observational aspects of this flare are
that the time profile was identical throughout at 8.6, 15, and 86
GHz, that the spectrum was apparently flat from 15 to 86 GHz, and
that there was a sharp cutoff in the spectrum between 5.0 and 8.6
GHz. The simplest interpretation of the cutoff, namely as a plasma
frequency effect, leads to the conclusion that there was exceptionally
high-density material in the solar corona (of about 5 x 10 exp 11/cu
cm). Very Large Array images at 15 GHz show a single-loop structure
which brightened uniformly and showed little change in size during the
whole impulsive phase. The flat spectrum is consistent with optically
thin thermal bremsstrahlung emission, but the lack of observed soft
X-ray emission and other properties of the flare cannot easily be
accommodated by this mechanism. The possibility is explored that the
emission is optically thick due to thermal absorption of nonthermal
gyrosynchrotron emission, or optically thin gyrosynchrotron emission
absorbed by high-density material intervening along the line of
sight. Both of these explanations also face difficulties.
---------------------------------------------------------
Title: Rotationalmodulation and flares on RS Canum Venaticorum and BY
Draconis stars. XVI. IUE spectroscopy and VLA observations of GL 182
(=V 1005 Orionis) in October 1983.
Authors: Mathioudakis, M.; Doyle, J. G.; Rodono, M.; Gibson, D. M.;
Byrne, P. B.; Avgoloupis, S.; Linsky, J. L.; Gary, D.; Mavridis,
L. N.; Varvoglis, P.
1991A&A...244..155M Altcode:
A large flare was detected simultaneously with IUE and VLA on Gl 182 on
October 5, 1983, this event showing the largest C IV flare enhancement
yet observed by IUE. A smaller flare was also detected on October 4,
although only with the IUE satellite. Line ratio and emission measure
techniques are used to derive various physical parameters of the
flares. The radiative losses in the temperature region log T(e) =
4.3-5.4 in the two flares are 2.9 x 10 to the 33rd and 8.4 x 10 to
the 32nd ergs, respectively. Total radiative losses over the whole
temperature range log T(e) = 4.0-8.0 are estimated to be of the order
of 6.4 x 10 to the 34th and 1.1 x 10 to the 34th ergs, respectively. In
the October 5, flare, a very strong ultraviolet continuum is present
with a total energy of 1.9 x 10 to the 33rd ergs over the wavelength
range 1250-1950 A.
---------------------------------------------------------
Title: Observations of Two Large Off Limb Solar Flares in the Radio,
X-Rays, and Hα
Authors: Kucera, T. A.; Dulk, G. A.; Winglee, R. M.; Kiplinger, A. L.;
Bastian, T. S.; Gary, D. E.
1991BAAS...23R1065K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Joint Microwave/Hard X-ray Observations of Solar Bursts During
March 1989
Authors: Gary, D. E.; Hurford, G. J.
1991BAAS...23.1066G Altcode:
No abstract at ADS
---------------------------------------------------------
Title: First Interferometric Observations of Solar Microwave
Millisecond Spike Bursts
Authors: Gary, Dale E.; Hurford, G. J.; Flees, D. J.
1991ApJ...369..255G Altcode:
Observations, with one-dimensional spatial resolution, of solar
microwave millisecond spikes at 2.8 GHz are reported. The observations
were made with the Owens Valley frequency-agile interferometer with 20
ms time resolution. It is found that the spikes occur at a position
different from that of the underlying gyrosynchrotron radio burst
source. All of the spikes during the rising part of the burst come
from the same one-dimensional location to within + or - 1 arcsec,
despite the fact that rapid evolution in the sense and degree of
circular polarization was observed. The one-dimensional position of the
spikes is consistent with a location over a large sunspot, suggesting
that the emission occurs in a region of strong, converging magnetic
field. The observations support the suggestion that the spikes are
due to the electron-cyclotron maser mechanism.
---------------------------------------------------------
Title: Multifrequency Observations of a Remarkable Solar Radio Burst
Authors: White, S. M.; Kundu, M. R.; Bastian, T. S.; Gary, D. E.;
Hurford, G. J.; Kucera, T.; Bieging, J. H.
1991BAAS...23.1043W Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Effects of Source Inhomogeneity on Solar Microwave Bursts
Authors: Hurford, G. J.; Gary, D. E.
1991BAAS...23.1066H Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Temporally, Spectrally, and Spatially Resolved Observations
of a Pulsating Solar Radio Burst
Authors: Bastian, T. S.; Aschwanden, M. J.; Gary, D. E.
1991BAAS...23Q1072B Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Summary of Observations with the Owens Valley Solar Array
During Max '91 Campaign 2
Authors: Gary, D. E.; Hurford, G. J.
1991BAAS...23.1072G Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Observations of Two Large Limb Flares on 20 June 1989
Authors: Kucera, T. A.; Winglee, R. M.; Dulk, G. A.; Bastian, T. S.;
Gary, D. E.
1991max..conf..172K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Multiwavelength Investigations of the 1989 June 30 Solar
Flares Including Interferometric Observations of Their Microwave
Emitting Sources
Authors: Crannell, C. J.; Gary, D. E.; Hurford, G. J.; Starr, R.;
Kucera, T.
1991max..conf..192C Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Radio Observations During Max'91 Campaign 1
Authors: Gary, D. E.
1991max..conf....1G Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Multifrequency Observations of a Solar Microwave Burst with
Two-dimensional Spatial Resolution
Authors: Gary, Dale E.; Hurford, G. J.
1990ApJ...361..290G Altcode:
Frequency-agile interferometry observations using three baselines and
the technique of frequency synthesis were used to obtain two-dimensional
positions of multiple microwave sources at several frequency ranges
in a solar flare. Source size and brightness temperature spectra were
obtained near the peak of the burst. The size spectrum shows that
the source size decreases rapidly with increasing frequency, but the
brightness temperature spectrum can be well-fitted by gyrosynchrotron
emission from a nonthermal distribution of electrons with power-law
index of 4.8. The spatial structure of the burst showed several
characteristics in common with primary/secondary bursts discussed
by Nakajima et al. (1985). A source of coherent plasma emission at
low frequencies is found near the secondary gyrosynchrotron source,
associated with the leader spots of the active region.
---------------------------------------------------------
Title: Microwave and Hard X-Ray Diagnostics of Nonthermal Electrons
in a Solar Burst
Authors: Hurford, G. J.; Gary, D. E.; Bromley, J. W.
1990BAAS...22.1196H Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Secondary Spectral Component of Solar Microwave Bursts
Authors: Stahli, M.; Gary, D. E.; Hurford, G. J.
1990SoPh..125..343S Altcode:
Microwave observations in the range 1 to 18 GHz with high spectral
resolution (40 frequencies) have shown that many events display a
complex microwave spectrum. From a set of 14 events with two or
more spectral components, we find that two different classes of
complex events can be distinguished. The first group (4 events)
is characterized by a different temporal evolution of the spectral
components, resulting in a change of the spectral shape. These events
probably can be explained by gyrosynchrotron emission from two or
more individual sources. The second class (10 events) has a constant
spectral shape, so that the two spectral components vary together
in intensity. For all ten events in this second class, the ratio of
primary to secondary peak frequencies is remarkably similar, exhibiting
an average value of 3.4, and both components show a common circular
polarization. These properties suggest either a common source for the
different spectral components or several sources which are closely
coupled. An additional example of this class of burst was observed
interferometrically to provide spatial resolution. This event suggests
that the primary and secondary components have a similar location,
but that the surface area of the secondary component is larger.
---------------------------------------------------------
Title: Microwave Structure of the Quiet Sun at 8.5 GHz
Authors: Gary, Dale E.; Zirin, Harold; Wang, Haimin
1990ApJ...355..321G Altcode:
Multifrequency VLA observations of the quiet sun near 8.5 GHz are
presented. Two regions of the sun were observed, one dominated by an
enhanced network corresponding to a decayed active region, and the
other corresponding to an enhanced network with no active features. The
full-day synthesis maps for both show nearly perfect correspondence
to H-alpha images, and to longitudinal magnetograms. The coronal loops
were observed to appear as regions of radio emission with no underlying
longitudinal magnetic fields, being aligned with H-alpha fibrils in the
photosphere, and connecting regions of opposite magnetic polarity. The
emission can be modeled as optically thin free-free emission from a
coronal loop with a peak axial density of approximately 2.4-2.8 x 10
to the 9th/cu cm, for an assumed coronal temperature of 1-2 x 10 to the
6th K. The quiet chromosphere sources are measured, and the significance
of these measurements for existing chromospheric models is discussed.
---------------------------------------------------------
Title: Solar Microwave Spectroscopy with the Owens Valley Solar Array
Authors: Hurford, G. J.; Gary, D. E.
1990BAAS...22..808H Altcode:
No abstract at ADS
---------------------------------------------------------
Title: First Observations of Millisecond Microwave Spike Bursts with
Spatial Resolution
Authors: Gary, D. E.; Hurford, G. J.; Flees, D. J.
1990BAAS...22..823G Altcode:
No abstract at ADS
---------------------------------------------------------
Title: High Resolution Microwave Spectras of Solar Bursts
Authors: Stahli, M.; Gary, D. E.; Hurford, G. J.
1989SoPh..120..351S Altcode:
Microwave observations with exceptionally high spectral resolution
are described for a set of 49 solar flares observed between May and
October 1981. Total power data were obtained at 40 frequencies between
1 and 18 GHz by the Owens Valley frequency-agile interferometer with
10 s time resolution. Statistical analysis of this sample of microwave
bursts established the following significant characteristics of their
microwave spectra: (i) Most (≈ 80%) of the microwave events displayed
complex spectra consisting of more than one component during some or
all of their lifetime. Single spectral component bursts are rare. It
is shown that the presence of more than one component can lead to
significant errors when data with low spectral resolution are used
to determine the low-side spectral index. (ii) The high-resolution
data show that many bursts have a low-side spectral index that is
larger than the maximum value of about 3 that might be expected from
theory. Possible explanations include the effect of the underlying
active region on the perceived burst spectrum and/or the necessity for
more accurate calculations for bursts with low effective temperatures,
(iii) the peak frequencies of the bursts are remarkably constant during
their lifetimes. This is contrary to expectations based on simple
models in which the source size and ambient field remain constant
during the evolution of a burst.
---------------------------------------------------------
Title: A Simple Solar Microwave Burst Observed with High Spectral
Resolution
Authors: Gary, Dale E.; Hurford, G. J.
1989ApJ...339.1115G Altcode:
A small flare that occurred on February 3, 1986 which was dominated
by a single homogeneous source is analyzed. The theory of microwave
emission from homogeneous sources was used to trace the evolution of
the burst. The spectral profile indicates a thermal origin. It was
found that the source expanded with time during the burst.
---------------------------------------------------------
Title: The Secondary Spectral Component of Solar Microwave Bursts
Authors: Hurford, G. J.; Stahli, M.; Gary, D. E.
1989BAAS...21..836H Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Owens Valley Solar Array
Authors: Hurford, G. J.; Gary, D. E.
1989BAAS...21..861H Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Solar radio burst spectral observations, particle acceleration,
and wave-particle interactions.
Authors: Gary, D. E.; Hurford, G. J.
1989GMS....54..237G Altcode: 1989sspp.conf..237G; 1989opss.conf..237G
The authors consider solar radio bursts throughout the radio spectrum
from 30 kHz to 30 GHz. The subject can be conceptualized by considering
three characteristic frequencies of the plasma: the plasma frequency
f<SUB>p</SUB>, the gyrofrequency f<SUB>B</SUB>, and the frequency
f(τ<SUB>ff</SUB> = 1) at which the plasma becomes optically thick
due to bremsstrahlung. The authors present an overview in terms of
these characteristic frequencies to show why each dominates under
various physical conditions in the Sun and solar wind. In the broad
regime where plasma emission dominates, they discuss the progress
that has been made in explaining the observed burst characteristics
theoretically, concentrating on bursts of type II (shock wave related)
and type III (excited by electron beams).
---------------------------------------------------------
Title: Coronal mass ejections and coronal structures.
Authors: Hildner, E.; Bassi, J.; Bougeret, J. L.; Duncan, R. A.;
Gary, D. E.; Gergely, T. E.; Harrison, R. A.; Howard, R. A.; Illing,
R. M. E.; Jackson, B. V.; Kahler, S. W.; Kopp, K.; Low, B. C.; Lantos,
P.; Phillips, K. J. H.; Poletto, G.; Sheeley, N. R., Jr.; Stewart,
R. T.; Svestka, Z.; Waggett, P. W.; Wu, S. T.
1989epos.conf..493H Altcode:
The work of this team was concerned with modelling of post-flare arches,
the reconnection theory of flares, the slow variation of coronal
structure, and the coronal and interplanetary detection, evolution,
and consequences of mass ejections.
---------------------------------------------------------
Title: The Owens Valley solar array
Authors: Hurford, G. J.; Gary, D. E.
1989dots.work..328H Altcode:
Solar microwave emission contains essential information for the study
of the coronal magnetic structure of active regions and of thermal and
nonthermal flare electrons. To exploit this potential requires BOTH
imaging and spectroscopy with sufficient resolution to resolve spatial
and spectral features. The VLA provides excellent solar imaging (when
in the C and D configurations) but inadequate spectral coverage. The
existing Owens Valley system has excellent spectral coverage but imaging
that is adequate only for very simple sources. The Owens Valley system
is currently undergoing an expansion, which when completed in October
1990 will provide a SOLAR-DEDICATED 5 antenna array (10 baselines). By
using frequency-synthesis, this will provide a significant imaging
capability in addition to its current spectral coverage.
---------------------------------------------------------
Title: Simultaneous multi-wavelength observations of an intense
flare on AD Leonis.
Authors: Rodono', M.; Houdebine, E. R.; Catalano, S.; Foing, B.;
Butler, C. J.; Scaltriti, F.; Cutispoto, G.; Gary, D. E.; Gibson,
D. M.; Haisch, B. M.
1989sasf.confP..53R Altcode: 1989IAUCo.104P..53R; 1988sasf.conf...53R
The authors report on the first successful coordinated observations
of stellar flares carried out on March 28, 1984 simultaneously over
a wide range of wavelengths, from UV to microwaves, using the IUE
satellite, three ESO telescopes at La Silla (Chile) and the VLA at
Socorro (NM, USA).
---------------------------------------------------------
Title: A flare on AD Leo observed in optical, UV and microwaves.
Authors: Byrne, P. B.; Gary, D. E.
1989sasf.confP..63B Altcode: 1988sasf.conf...63B; 1989IAUCo.104P..63B
The authors report a simultaneous IUE, VLA and ground-based photometric
observation of a flare on the dMe star, AD Leo, on 2nd February
1983. The optical flare was extremely impulsive, lasting in total only
about 3 mins. A relatively longlived 6 cm flare was observed with the
VLA which was initially 100% polarized. An IUE spectrum, taken ≍8
min after the onset of the optical U band flare, shows more than a
factor of 2 increase in the He II λ1640 Å emission line. The other
mid-transition region lines such as C IV λλ1548/52 Å show almost
no response.
---------------------------------------------------------
Title: Planned improvements to the Owens Valley frequency-agile
interferometer
Authors: Hurford, Gordon J.; Gary, D. E.
1988fnsm.work...98H Altcode:
Three small antennas will be added to the OVRO interferometer to
form a five-element solar-dedicated array. This would provide up to
7 or 10 baselines (compared to the present 1 or 3). This would be
sufficient to apply microwave diagnostics to most active region and
burst sources. By using frequency-synthesis it would also provide an
imaging capability comparable to that of an approximately 100 baseline
interferometer. Expansion of the array is discussed.
---------------------------------------------------------
Title: Microwave Structure of the Quiet Sun
Authors: Gary, Dale E.; Zirin, Harold
1988ApJ...329..991G Altcode:
The authors present two-frequency VLA observations of the quiet Sun
obtained on 1985 July 13, when the Sun was particularly quiet. The
4.9 GHz full-day synthesis maps correspond closely to images made
in Hα, Ca II K, and magnetograms, while the correspondence is much
reduced at 1.45 GHz. The 4.9 GHz maps have a meshlike appearance
reminiscent of chromospheric network structure. Simultaneous
magnetograms from Big Bear Solar Observatory are used to compare the
structure of the magnetic field with the radio source structure at
the two frequencies. Magnetograms of each pointing field were taken
at least once every half-hour for monitoring the changing structure
of the longitudinal component of the magnetic fields. The authors
identify changes such as the appearance of an ephemeral region and
the cancellation of opposite-polarity magnetic flux, and discuss
corresponding changes in radio sources.
---------------------------------------------------------
Title: Planned Improvements to the Owens Valley Frequency-agile
Interferometer for MAX 91
Authors: Hurford, G. J.; Gary, D. E.
1988BAAS...20..746H Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Spatially-Integrated, High Frequency Resolution, Solar Active
Region Microwave Spectra
Authors: Hurford, G. J.; Gary, D. E.
1988BAAS...20R.713H Altcode:
No abstract at ADS
---------------------------------------------------------
Title: A Simple Solar Microwave Burst Observed with High Spectral
Resolution
Authors: Gary, D. E.; Hurford, G. J.
1988BAAS...20..713G Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Solar Microwave Spectroscopy
Authors: Gary, D. E.
1988BAAS...20..723G Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Microwave Spectrum of Solar Bursts
Authors: Stähli, M.; Gary, D. E.; Hurford, G. J.
1988BAAS...20..713S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Rotational modulation and flares on RS CVn and BY DRA
stars. IV. The spatially resolved chromosphere of AR Lacertae.
Authors: Walter, F. M.; Neff, J. E.; Gibson, D. M.; Linsky, J. L.;
Rodono, M.; Gary, D. E.; Butler, C. J.
1987A&A...186..241W Altcode:
The authors observed the RS CVn system AR Lacertae systematically
over an orbital period with the International Ultraviolet Explorer in
October 1983. Contemporaneous radio observations were obtained at the
Very Large Array. The spectra of the Mg II k emission line were analyzed
using a Doppler imaging technique. In this way, the authors identified
three discrete regions of emission in the outer atmosphere of the K
star - two "plages" and a chromospheric brightening that was related
to a radio flare. The widths of the plage profiles indicate that the
two plages together cover about 2% of the visible stellar hemisphere,
and their v sin i values indicate that they lie close to the equator
of the K star. The Mg II k surface flux in the plages is about five
times the mean Mg II k surface flux of the K star. The authors then
used the far-ultraviolet spectra obtained at the eclipse phases to
separate the individual contributions of the two stars and the plage
and flare regions in order to estimate their line surface fluxes.
---------------------------------------------------------
Title: Multifrequency Observations of a Solar Active Region during
a Partial Eclipse
Authors: Gary, Dale E.; Hurford, G. J.
1987ApJ...317..522G Altcode:
Spatially-resolved microwave observations of a complex active region at
16 frequencies in the range 1.4-8 GHz are presented. One-dimensional
profiles of the active region at each frequency were obtained with
the Owens Valley frequency-agile interferometer, with 2.6-arcsec
spatial resolution provided by the lunar limb during a partial
eclipse. Simultaneous two-dimensional maps were obtained with the VLA
at 1.45 and 4.9 GHz. The combination of spatial and spectral resolution
allows the direct observation of the change in emission mechanism from
predominantly free-free emission from dense active region loops at low
frequencies to gyroresonance emission from regions of intense magnetic
fields at high frequencies. The change occurs, for this active region,
at about 3 GHz. The implications of the brightness temperature spectrum
for the coronal structure of the active region are discussed.
---------------------------------------------------------
Title: Microwave spectroscopy as a diagnostic of solar flares:
The effect of source inhomogeneity
Authors: Hurford, G. J.; Gary, D. E.
1987SoPh..113..183H Altcode: 1982SoPh..113..183H
No abstract at ADS
---------------------------------------------------------
Title: IUE/Optical/VLA Flares on AD Leo
Authors: Gary, Dale E.; Byrne, P. B.; Butler, C. J.
1987LNP...291..106G Altcode: 1987LNP87.291..106G; 1987csss....5..106G
We discuss joint observations of flares on the dMe star AD Leo made
with IUE, the VLA, and the Tinsley photometer at the University of
Hawaii's 24-inch reflector on Mauna Kea. We find that optical emission
is correlated with C IV while radio emission is better correlated with
He II. We reach some tentative conclusions concerning timescales and
source characteristics.
---------------------------------------------------------
Title: Interplanetary Effects of Coronal Mass Ejections
Authors: Hildner, E.; Bassi, J.; Bougeret, J. L.; Duncan, R. A.;
Gary, D. E.; Gergely, T. E.; Harrison, R. A.; Howard, R. A.; Illing,
R. M. E.; Jackson, B. V.; Kahler, S. W.; Kopp, K.; Low, B. C.; Lantos,
P.; Phillips, K. J. H.; Poletto, G.; Sheeley, N. R., Jr.; Steward,
R. T.; Svestka, Z.; Waggett, P. W.; Wu, S. T.
1986epos.conf.6.52H Altcode: 1986epos.confF..52H
No abstract at ADS
---------------------------------------------------------
Title: The microwave structure of quiescent solar filaments at
high resolution.
Authors: Gary, D. E.
1986NASCP2442..121G Altcode:
The author presents high resolution VLA maps of a quiescent filament
at three frequencies. At each frequency, the filament appears as a
depression in the quiet Sun background. The depression is measurably
wider and longer in extent than the corresponding Hα filament at 1.45
GHz and 4.9 GHz, indicating that the depression is due in large part to
a deficit in coronal density associated with the filament channel. In
contrast, the shape of the radio depression at 15 GHz closely
matches that of the Hα filament. In addition, the 15 GHz map shows
enhanced emission along both sides of the radio depression. A similar
enhancement is seen in an observation of a second filament obtained 4
days later, which suggests that the enhancement is a general feature
of filaments. Possible causes of the enhanced emission are explored.
---------------------------------------------------------
Title: Initiations of Coronal Mass Ejections
Authors: Hildner, E.; Bassi, J.; Bougeret, J. L.; Duncan, R. A.;
Gary, D. E.; Gergely, T. E.; Harrison, R. A.; Howard, R. A.; Illing,
R. M. E.; Jackson, B. V.; Kahler, S. W.; Kopp, K.; Low, B. C.; Lantos,
P.; Phillips, K. J. H.; Poletto, G.; Sheeley, N. R., Jr.; Steward,
R. T.; Svestka, Z.; Waggett, P. W.; Wu, S. T.
1986epos.conf.6.27H Altcode: 1986epos.confF..27H
No abstract at ADS
---------------------------------------------------------
Title: Measurement of coronal fields using spatially resolved
microwave spectroscopy.
Authors: Hurford, G. J.; Gary, D. E.
1986NASCP2442..319H Altcode: 1986copp.nasa..319H
The authors consider the potential implications of observations
which combine both high spatial and high spectral resolution. In
particular, they are interested in the ability to measure the magnetic
field at the base of the corona on a point by point basis, as in a
true magnetograph. They present model calculations of the microwave
brightness temperature spectrum along specific lines of sight near a
sunspot. They believe that spatially resolved microwave spectroscopy
provides a promising new coronal diagnostic. At present, however,
a key limitation in its application is the lack of sufficient number
of antennas with which to map complex active regions.
---------------------------------------------------------
Title: The Slowly Varying Corona Near Solar Activity Maximum
Authors: Hildner, E.; Bassi, J.; Bougeret, J. L.; Duncan, R. A.;
Gary, D. E.; Gergely, T. E.; Harrison, R. A.; Howard, R. A.; Illing,
R. M. E.; Jackson, B. V.; Kahler, S. W.; Kopp, K.; Low, B. C.; Lantos,
P.; Phillips, K. J. H.; Poletto, G.; Sheeley, N. R., Jr.; Steward,
R. T.; Svestka, Z.; Waggett, P. W.; Wu, S. T.
1986epos.conf.6.57H Altcode: 1986epos.confF..57H
No abstract at ADS
---------------------------------------------------------
Title: Modelling of Coronal Mass Ejections and POST Flare Arches
Authors: Hildner, E.; Bassi, J.; Bougeret, J. L.; Duncan, R. A.;
Gary, D. E.; Gergely, T. E.; Harrison, R. A.; Howard, R. A.; Illing,
R. M. E.; Jackson, B. V.; Kahler, S. W.; Kopp, K.; Low, B. C.; Lantos,
P.; Phillips, K. J. H.; Poletto, G.; Sheeley, N. R., Jr.; Steward,
R. T.; Svestka, Z.; Waggett, P. W.; Wu, S. T.
1986epos.conf6.366H Altcode: 1986epos.confF.366H
No abstract at ADS
---------------------------------------------------------
Title: Coronal mass ejections and coronal structures
Authors: Hildner, E.; Bassi, J.; Bougeret, J. L.; Duncan, R. A.;
Gary, D. E.; Gergely, T. E.; Harrison, R. A.; Howard, R. A.; Illing,
R. M. E.; Jackson, B. V.
1986epos.conf..6.1H Altcode: 1986epos.confF...1H
Research on coronal mass ejections (CMF) took a variety of forms, both
observational and theoretical. On the observational side there were:
case studies of individual events, in which it was attempted to provide
the most complete descriptions possible, using correlative observations
in diverse wavelengths; statistical studies of the properties CMEs and
their associated activity; observations which may tell us about the
initiation of mass ejections; interplanetary observations of associated
shocks and energetic particles even observations of CMEs traversing
interplanetary space; and the beautiful synoptic charts which show to
what degree mass ejections affect the background corona and how rapidly
(if at all) the corona recovers its pre-disturbance form. These efforts
are described in capsule form with an emphasis on presenting pictures,
graphs, and tables so that the reader can form a personal appreciation
of the work and its results.
---------------------------------------------------------
Title: Spectral Structure of Solar Microwave Bursts
Authors: Gary, D. E.; Hurford, G. J.
1986BAAS...18Q.900G Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Frequency-Synthesis Imaging of Solar Microwave Bursts
Authors: Hurford, G. J.; Gary, D. E.
1986BAAS...18..899H Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Coordinated IUE and ground-based observations of stellar
flares: YZ CMi, Proxima Cen and AD Leo.
Authors: Foing, B. H.; Rodono, M.; Cutispoto, G.; Catalano, S.; Linsky,
J. L.; Gibson, D. M.; Brown, A.; Haisch, B. M.; Butler, C. J.; Byrne,
P. B.; Andrews, A. D.; Doyle, J. G.; Gary, D. E.; Henry, G. W.; Russo,
G.; Vittone, A.; Scaltriti, F.
1986RMxAA..12..213F Altcode:
Coordinated observations of stellar flares were obtained with lUE
and several ground-based facilities in March 1984.The simultaneous
observations allowed it to cover a wide range of wavelengths from ii5nm
to 6cm.We intend to study the effect ofthe observed flares at different
atmospheric heights in order to estimate the energy budget,the time
scales and the cooling processes.Our observations includetime-resolved
IUE spectroscopy at SWP(115-195nm) and LWP(190-320nm),optical
spectroscopy at the ESO 3.6m+IDS (355-440nm),high resolution
spectroscopy at the ESO 1.4m CAT+CES(653-659nm),narrow band H alpha
and wide band optical photometry, infrared photometry at 2.2microns
and microwave observations at 2,6 and 20 cm We present for some
flare events,among the results,the first detection of infrared flux
decrease -or "negative flare"-in coincidence with the flux increase
at the other wavelengths:the broadening and changes of the Balmer H
lines,He and high excitation lines;the appearance of higher members
of the Balmer serie;the enhancements of Mg II doublet and Fe II blend
(260nm);and the flare detection at 2cm and 6cm
---------------------------------------------------------
Title: Models of Quiescent Stellar Microwave Emission
Authors: Gary, D. E.
1986LNP...254..235G Altcode: 1986csss....4..235G
Models for gyrosynchrotron emission from thermal electrons in dMe
stellar coronae are developed using the formula for electron emissivity
given by Robinson and Melrose (1984). The most physically plausible
model also closely fits VIA observations. A comparison of this model
to the observations is presented.
---------------------------------------------------------
Title: Shock waves and coronal transients: The event of 1980 April 17
Authors: Gary, D. E.; Gergely, T. E.; Kundu, M. R.
1986AdSpR...6f.311G Altcode: 1986AdSpR...6..311G
The coronal mass ejection (CME) event of 1980 April 17 was observed
with the SMM coronagraph, and an accompanying Type II (shock wave
related) burst was observed simultaneously with the Culgoora and Clark
Lake radioheliographs. The method of analysis of the combined radio
observations is presented. Although the radio positions were affected
by severe ionospheric refraction, the combined radio observations allow
a useful reduction in the range of possible positions of the 80 MHz
source when the 43 MHz source is required to lie within the range of
position angles of the CME. The positions of the 80 MHz radio sources
under this assumption are compared with the positions of the CME loop.
---------------------------------------------------------
Title: Coronal mass ejections and coronal structures.
Authors: Hildner, E.; Bassi, J.; Bougeret, J. L.; Duncan, R. A.;
Gary, D. E.; Gergely, T. E.; Harrison, R. A.; Howard, R. A.; Illing,
R. M. E.; Jackson, B. V.; Kahler, S. W.; Kopp, K.; Low, B. C.; Lantos,
P.; Phillips, K. J. H.; Poletto, G.; Sheeley, N. R., Jr.; Stewart,
R. T.; Svestka, Z.; Waggett, P. W.; Wu, S. T.
1986NASCP2439....6H Altcode:
Contents: 1. Introduction. 2. Observations. 3. Initiation of
coronal mass ejections - observations. 4. Modelling of coronal mass
ejections and post-flare arches. 5. Interplanetary effects of coronal
mass ejections. 6. The slowly varying corona near solar activity
maximum. 7. Summary.
---------------------------------------------------------
Title: Optical, UV and Radio Observations of RS Canum Venaticorum
Authors: Catalano, S.; Rodono, M.; Linsky, J. F.; Carpenter, K.;
Gibson, D.; Gary, D.; Butler, J.
1986LNP...254..253C Altcode: 1986csss....4..253C
No abstract at ADS
---------------------------------------------------------
Title: The type IV burst of 1980 June 29, 0233 UT - Harmonic plasma
emission?
Authors: Gary, D. E.; Dulk, G. A.; House, L. L.; Illing, R.; Wagner,
W. J.; Mclean, D. J.
1985A&A...152...42G Altcode:
The coronal transient event of 1980 June 29, 0233 UT, was well observed
by the HAO Coronagraph/Polarimeter aboard SMM and at meter radio
wavelengths by the Culgoora Radioheliograph. The radio event consisted
of a strong Type II (shock wave related) burst followed by weak Type
IV (storm) continuum. The authors discuss the details of the Type IV
portion of the event in terms of two possible emission mechanisms-plasma
emission (at the second harmonic) and gyro-synchrotron emission. They
find that gyro-synchrotron emission is a possible mechanism only if more
stringent requirements are met, viz., that the density of electrons
of energy greater than 10 keV is about 10% of the ambient density,
that the average energy is about 40 keV, and that the magnetic field
strength at 2.5 solar radii is about 2.8 gauss. The authors conclude
that this Type IV event is likely due to plasma emission at the 2nd
harmonic of the plasma frequency.
---------------------------------------------------------
Title: The numbers of fast electrons in solar flares as deduced from
hard X-ray and microwave spectral data
Authors: Gary, D. E.
1985ApJ...297..799G Altcode:
The previously reported discrepancy between the number of fast
solar flare electrons deduced from hard X-ray burst observations and
microwave observations of the same events is reexamined. The thin and
thick-target models for hard X-ray production and their consequences
for gyrosynchrotron emission of microwaves are discussed. Previous
work that led to the perceived discrepancy is reviewed, and it is
suggested that the discrepancy can be reduced when thick-target hard
X-ray emission is assumed. The number of electrons in microwaves
and hard X-rays are calculated assuming a homogeneous source for a
sample of flares given by Wiehl et al. (1983). The numbers are given
separately under the two assumptions of thin and thick-target hard
X-ray emission. It is found that when the latter emission is assumed,
the number of electrons deduced from the microwaves is very similar
to the number deduced from the hard X-rays.
---------------------------------------------------------
Title: The Cyg X3 Radio Outburst of 1985
Authors: Johnston, K. J.; Spencer, J. H.; Waltman, E. B.; Pooley,
G. G.; Spencer, R. E.; Angerhofer, P. E.; Florkowski, D. R.; Josties,
F. J.; McCarthy, D. D.; Matsakis, D. N.; Herford, G.; Gary, D.;
Hjellming, R. M.; Schalinski, C.; Molnar, L.; Reid, M.
1985BAAS...17..855J Altcode:
No abstract at ADS
---------------------------------------------------------
Title: A Technique for Removing Confusion Sources from VLA Data
Authors: Gary, D. E.
1985ASSL..116..385G Altcode: 1985rst..conf..385G
A procedure for removing confusing sources from VLA visibility data
is described and illustrated using the field near the flare star AD
Leo. A map containing the confusing sources is made using the entire
time range of the observation to maximize signal to noise. The map is
cleaned of all sources except the one of interest, and positions and
fluxes of the cleaned sources are saved for use in the next step. The
'dirty' visibilities are then 'cleaned' by subtracting the contribution
of each cleaned component obtained in the previous step. The resulting
visibilities contain no contribution from the confusing sources,
and maps made with the 'clean' databases contain only the source
of interest.
---------------------------------------------------------
Title: Quiescent Stellar Microwave Emission (Invited Paper)
Authors: Gary, D. E.
1985ASSL..116..185G Altcode: 1985rst..conf..185G
The quiescent microwave flux expected from hot stellar coronae, as
predicted from analogy with the Sun and from X-ray derived temperatures
and densities, is about 10 times smaller than the observed fluxes
for many dMe stars observed with the VLA. The implications of these
high observed fluxes to the coronal parameters of magnetic field
strength, temperature, and source size are discussed, and observations
are suggested that potentially allow a choice among the several
possibilities.
---------------------------------------------------------
Title: Deduction of Coronal Magnetic Fields Using Microwave
Spectroscopy
Authors: Hurford, G. J.; Gary, D. E.; Garrett, H. B.
1985ASSL..116..379H Altcode: 1985rst..conf..379H
Gyroresonance opacity renders the solar corona optically thick
at frequencies which are low integral multiples of the local
gyrofrequency. This causes the microwave spectrum of sunspots to be
sensitive to the strength of coronal magnetic fields. The concept is
illustrated by high spectral resolution observations of a sunspot
acquired with the Owens Valley frequency-agile interferometer. The
observed spectrum is compared to the results of three-dimensional
atmospheric model calculations in which the sunspot field is represented
by the potential field of a dipole located beneath the photosphere. The
comparison enables the depth, orientation and magnetic moment of the
dipole that best fits the observations to be determined. Since such
observations require that the microwave emission be resolved spectrally,
not spatially, the technique may be applicable to the study of stellar
coronal fields.
---------------------------------------------------------
Title: Coordinated Multiband Observations of Stellar Flares
Authors: Rodono, M.; Foing, B. H.; Linsky, J. L.; Butler, J. C.;
Haisch, B. M.; Gary, D. E.; Gibson, D. M.
1985Msngr..39....9R Altcode:
The March 28, 1984 flare of AD Leo is characterized on the basis of
observations obtained over the spectral range from 200 nm to 20 cm using
the IUE, the VLA, and four ESO telescopes as part of a coordinated
multiband international campaign. The data are presented graphically
and discussed, with consideration of faint negative K-band events
observed simultaneously with the optical flare; an H-alpha precursor
with longer energy-release relaxation than in the U continuum; and
remarkably enhanced UV continuum, Mg II doublet, and 260-nm Fe II
blend during the last secondary optical peak.
---------------------------------------------------------
Title: An impulsive solar burst observed in H-alpha, microwaves,
and hard X-rays
Authors: Gary, D. E.; Tang, F.
1985ApJ...288..385G Altcode:
The authors present Hα, 10.6 GHz microwave, and greater than 100 keV
X-ray observations of a single impulsive spike flare that occurred
on 1980 May 28, 1947 UT. The isolated high flux spike provides an
excellent opportunity to infer the time behavior of the electron
acceleration that is assumed to yield the microwave and hard X-ray
flux profiles. It is found that, although the time profile consists of
a simple, single peak, the spike is actually due to two acceleration
episodes in separate sources. The authors adopt a source model for
microwave and hard X-ray sources, in which the microwave source is
due to electrons trapped at the top of a magnetic loop, and the hard
X-ray source is due to thick-target emission at the footpoints.
---------------------------------------------------------
Title: High Resolution VLA Observations of an Active Region During
a Partial Solar Eclipse
Authors: Gary, D. E.; Hurford, G. J.
1984BAAS...16.1003G Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Coordinated IUE and ground-based observations of active stars:
flare events on YZ CMi, V1005 Ori, AD Leo and AR Lac.
Authors: Rodonò, M.; Cutispoto, G.; Catalano, S.; Linsky, J. L.;
Gibson, D. M.; Brown, A.; Haisch, B. M.; Butler, C. J.; Byrne, P. B.;
Andrews, A. D.; Doyle, J. G.; Gary, D. E.; Henry, G. W.; Russo, G.;
Vittone, A.; Scaltriti, F.; Foing, B.
1984ESASP.218..247R Altcode: 1984iue..conf..247R
Observations of stellar flares were obtained with IUE and ground-based
facilities simultaneously over a wide range of wavelengths in order
to study the effect of the flare radiation at different atmospheric
levels. Observations include time-resolved IUE and optical spectroscopy,
narrow and wide-band optical photometry, IR photometry, and microwave
observations. Results include detection of IR flux decrease, or negative
flare, in coincidence with flux increase at all other wavelengths.
---------------------------------------------------------
Title: Type II bursts, shock waves, and coronal transients - The
event of 1980 June 29, 0233 UT
Authors: Gary, D. E.; Dulk, G. A.; House, L.; Illing, R.; Sawyer,
C.; Wagner, W. J.; McLean, D. J.; Hildner, E.
1984A&A...134..222G Altcode:
The metric Type-II solar burst event of June 29, 1980, is characterized
on the basis of spatially resolved radioheliograph observations
obtained at Culgoora, Australia, and visible-light observations
obtained with the coronograph/polarimeter of the SMM satellite. The
data are presented in images, diagrams, and graphs and discussed in
detail. The Type-II emission is found to arise in the dense moving
material behind the transient loops, which have sky-plane width 0.5
solar radius and line-of-sight depth 0.1-0.4 solar radius. A faint arc
observed moving ahead of the transient loops at about 900 km/sec and
not associated with the Type-II burst is attributed to a shock front,
and the compression ratio and Alfven Mach number of the enhanced-density
region are estimated as n2/n1 = 1.3-3 and M(A) = 1.2-3. The ambient
material at 3 solar radii is determined to have Alfven speed 250-625
km/sec and magnetic-field strength 50-120 mG. The total mass of the
event is calculated as 700 Tg; the total magnetic energy of the loops is
(1.5-15) x 10 to the 29th ergs.
---------------------------------------------------------
Title: The Numbers of Fast Electrons in Solar Flares as Deduced From
Hard X-Ray and Microwave Spectral Data
Authors: Gary, D. E.
1984BAAS...16..523G Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Microwave emission from the coronae of late-type dwarf stars.
Authors: Linsky, J. L.; Gary, D. E.
1983ApJ...274..776L Altcode:
VLA microwave observations of 14 late-type dwarf and subgiant stars
and binary systems are examined. In this extensive set of observations,
four sources at 6 cm (Chi-1 Ori, UV Cet, YY Gem, and Wolf 630AB) were
detected and low upper limits for the remaining stars were found. The
microwave luminosities of the nondetected F-K dwarfs are as small as
0.01 those of the dMe stars. The detected emission is slowly variable
in all cases and is consistent with gyroresonant emission from thermal
electrons spiraling in magnetic fields of about 300 gauss if the source
sizes are as large as R/R(asterisk) = 3-4. This would correspond to
magnetic fields that are probably in the range 0.001-0.0001 gauss at the
photospheric level. An alternative mechanism is gyrosynchrotron emission
from a relatively small number of electrons with effective temperature.
---------------------------------------------------------
Title: Gamma-ray, Radio, and Hα Observations of a Single Spike
Solar Flare
Authors: Gary, D. E.; Tang, F.
1983BAAS...15R.919G Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The sun at 1.4 GHz: intensity and polarization.
Authors: Dulk, G. A.; Gary, D. E.
1983A&A...124..103D Altcode:
The authors present 1.4 GHz radio pictures of the sun made on 1981
September 26 using the VLA with a resolution of 40arcsec. Features
observed include active regions, limb brightening, coronal holes
and filament channels; all correspond well with features seen in
Hα or He λ10830 Å, and can be explained as relative enhancements
or depressions of free-free bremsstrahlung from the corona and upper
transition region. The degree of circular polarization of the radiation
from active regions ranges up to 0.2 and the sense corresponds, with a
few exceptions, with the polarity of the photospheric magnetograms. The
polarization is in the sense of the x-mode and is highest near the edges
of active regions, whereas the brightness is highest near the line of
zero polarization. The polarization can be explained by the different
effects of the magnetic field on the bremsstrahlung opacities of the
two modes; the observed values imply field strengths between about 20
and 70 Gauss near the boundaries of the active region corona.
---------------------------------------------------------
Title: Coordinated Ultraviolet, Microwave, and Optical Observations
of Flares on YZ CMi and AD Leo
Authors: Linsky, J. L.; Bornmann, P.; Brown, A.; Gary, D. E.; Rodono,
M.; Pazzani, V.; Andrews, A. D.; Butler, C. J.; Byrne, P. B.
1983BAAS...15..650L Altcode:
No abstract at ADS
---------------------------------------------------------
Title: VLA observations of quiescent and flare microwave emission
from late-type stars - A unique probe of coronal magnetic fields
Authors: Gary, D. E.; Linsky, J. L.; Dulk, G. A.
1983IAUS..102..387G Altcode:
No abstract at ADS
---------------------------------------------------------
Title: An unusual microwave flare with 56 second oscillations on
the M dwarf L726-8 A.
Authors: Gary, D. E.; Linsky, J. L.; Dulk, G. A.
1982ApJ...263L..79G Altcode:
Using the VLA, an unusual flare event has been observed on L726-8
A (dM5.5e), the primary star in the M dwarf system containing the
prototype flare star UV Cet. This flare had a peak flux of 8 mJy
at 6 cm and a corresponding brightness temperature greater than 10
to the 10th K, was almost entirely right-hand circularly polarized,
showed large flux variations on the 10 s time resolution of the VLA,
and exhibited quasi-periodic oscillations with a period of about 56 +
or - 5 s. While periodic flux variations have been detected during
solar flares and RS CVn type stellar flares, this is apparently the
first detection of periodicity in microwaves from M dwarf stars. It is
proposed that the observed radiation was due to maser action, probably
an electron maser, and that the energy release mechanism was modulated.
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Title: Radio Emission from Solar and Stellar Coronae.
Authors: Gary, D. E.
1982PhDT.........5G Altcode:
Several problems in radiophysics are explored utilizing new and
unique observations. Polarization measurements of two types of solar
bursts--Reverse Drift Pairs and Type V bursts--allowed identification
of the radiation as due to fundamental plasma emission for RDPs and
harmonic plasma emission for Type Vs. Two other types of solar radio
burst--Type II (shock related) and Type IV--observed at Culgoora,
Australia, are studied with the aid of Coronagraph/Polarimeter (C/P)
observations. Together, the radio and C/P data indicate the presence
of two separate shocks in the coronal transient event of 1980 June
29. One shock, without radio emission, led the transient and was driven
by it. Estimates of the density enhancement caused by the shock give
Mach number l.2 < M(,A) < 3 and, hence, magnetic field strength
in the ambient corona 0.05 < B < 0.12 gauss. The second shock,
which gave rise to the radio emission, was associated with the
loops. The Type II sources appeared to lie behind the leading edge
of the loops, and a measurement of the density from the C/P images
showed that only behind the loops was the density high enough to
account for the plasma emission. The second shock is interpreted as
a blast-wave, initiated by the impulsive flare, which traversed the
transient material and weakened when it encountered the faster moving,
upper part of the transient. The Type IV burst observed at 80 MHz in
the same event showed slight outward movement in association with a
rising of the relevant plasma level as transient material entered the
corona. The characteristics of the sources observed at 80 and 43 MHz
suggest that plasma radiation at the second harmonic was the probable
emission mechanism. The possibility of gyro-synchrotron emission is
explored, and it is found that the sources could have been due to this
mechanism only if the magnetic field strength at 2.5R(,0) was >
2.8 gauss. Finally, the discovery of quiescent microwave emission from
solar-type stars is discussed. Two sources, (chi)('1) Ori and UV Cet,
were detected. The emission is interpreted as gyroresonance emission
from hot extended coronae; free-free emission is shown to be unlikely.
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Title: Radio emission from solar and stellar coronae
Authors: Gary, Dale Everett
1982PhDT.......115G Altcode:
No abstract at ADS
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Title: Evidence for a shock wave in visible light and radio
observations of the 1980 June 29 event
Authors: Gary, D. E.; Dulk, G. A.; House, L. L.; Wagner, W. J.;
Illing, R. I.; Sawyer, C.; McLean, D. J.
1982AdSpR...2k.253G Altcode: 1982AdSpR...2..253G
Shock waves, as evidenced by type II radio bursts, often accompany
flares and coronal mass ejection transients. At present, the
density enhancements observed by coronagraphs are believed by some
to be ejected matter from the low corona, and by others to be the
compressed material behind a shock front. If the former is correct,
one would expect in some cases to see a density enhancement, associated
with the compression region of the shock, some distance ahead of the
transient ejecta. Such a density enhancement has not been previously
reported. <P />The coronal transient of 1980 June 29 (0233 UT) was
observed with the High Altitude Observatory's Coronagraph/Polarimeter
aboard SMM. This flare-associated coronal transient event was well
observed with the Culgoora Radioheliograph, including a well-developed
type II burst. Visible on the coronagraph images is a faint circular
arc moving out well ahead of the transient loops. This arc is moving
at more than 900 km s<SUP>-1</SUP> while the transient itself is
moving at a speed of about 600 km s<SUP>-1</SUP>. Both the arc and
transient appear to have originated either prior to the X-ray flare
or at some height above the flare at the time of the flare. The type
II burst observed at Culgoora is associated with the transient loops,
and no type II emission is identified with the faint arc. <P />Due to
its great speed, we interpret the faint arc as a manifestation of a
shock wave, but also envision a separate shock wave associated with
the transient loops as evidenced by the type II emission. Preliminary
density measurements are consistent with this interpretation, and show
the outer shock wave associated with the faint arc to have a Mach number
M<SUB>A</SUB> <= 1.7. At present we have no convincing explanation
for the lack of a type II burst in association with the arc. <P />This
work was supported in part by NASA through grants NSG-7287 and NAGW-91
to the University of Colorado, Boulder, and S-55989 to the High Altitude
Observatory, National Center for Atmospheric Research. The National
Center for Atmospheric Research, NCAR, is sponsored by the National
Science Foundation.
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Title: First detection of nonflare microwave emission from the
coronae of single late-type dwarf stars.
Authors: Gary, D. E.; Linsky, J. L.
1981ApJ...250..284G Altcode:
Results are presented of a search for nonflare microwave radiation
from the coronae of nearby late-type dwarf stars comparable to the sun:
single stars without evidence for either a large wind or circumstellar
envelope. The observing program consisted of flux measurements of
six stars over a 24-h period with the VLA in the C configuration
at a wavelength of 6 cm with 50 MHz bandwidth. Positive detections
at 6 cm were made for Chi 1 Ori (0.6 mJy) and the flare star UV Cet
(1.55 mJy), and upper limits were obtained for the stars Pi 1 UMa,
Xi Boo A, 70 Oph A and Epsilon Eri. It is suggested that Chi 1 Ori,
and possibly UV Cet, represent the first detected members of a new
class of radio sources which are driven by gyroresonance emission,
i.e. cyclotron emission from nonrelativistic Maxwellian electrons.
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Title: The Sun at 1.4 GHz as Observed with the VLA
Authors: Dulk, G. A.; Gary, D. E.
1981BAAS...13..878D Altcode:
No abstract at ADS
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Title: First Detection of Steady 6 cm Emission from Coronae of Single
Dwarf Stars of Spectral Type G-M
Authors: Gary, D.; Linsky, J. L.
1980BAAS...12..898G Altcode:
No abstract at ADS
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Title: Visible Light and Radio Observations of the First Coronal
Transient Event of 1980 June 29
Authors: Gary, D. E.; Dulk, G. A.; Wagner, W.; Sawyer, C.; House,
L.; Stewart, R. T.; McLean, D.
1980BAAS...12..904G Altcode:
No abstract at ADS
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Title: The position and polarization of Type V solar bursts
Authors: Dulk, G. A.; Gary, D. E.; Suzuki, S.
1980A&A....88..218D Altcode:
Observations of the position and polarization of Type V solar
radio bursts and their preceding Type III bursts are presented. The
polarization, frequency range, source position, source movement,
source size and brightness temperature of the bursts were measured
using a 24-220 MHz spectropolarimeter, an 8-8000 MHz spectrograph
and a three-frequency radioheliograph. Type V radiation is frequently
found to have the opposite sense of circular polarization from that
of the preceding Type III burst, with a degree of polarization
similar to that of harmonic Type III radiation. A reversal of
polarization is not observed when the accompanying Type III burst has no
fundamental-harmonic structure, or when the Type V radiation is poorly
developed. Possible mechanisms for the reversal are examined, including
opposite magnetic field directions in Type III and V bursts, changes
in mode coupling and a change in the mode of emission from o-mode
for Type III to x-mode for Type V, and conditions needed for the mode
change which is considered the most likely mechanism, are determined.
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Title: The Polarization of Type-V Bursts
Authors: Gary, D. E.; Suzuki, S.; Dulk, G. A.
1980IAUS...86..333G Altcode:
No abstract at ADS
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Title: Corrected formula for the polarization of second harmonic
plasma emission
Authors: Melrose, D. B.; Dulk, G. A.; Gary, D. E.
1980PASA....4...50M Altcode: 1980PASAu...4...50M
Corrections for the theory of polarization of second harmonic plasma
emission are proposed. The nontransversality of the magnetoionic
waves was not taken into account correctly and is here corrected. The
corrected and uncorrected results are compared for two simple cases of
parallel and isotropic distributions of Langmuir waves. It is found
that whereas with the uncorrected formula plausible values of the
coronal magnetic fields were obtained from the observed polarization
of the second harmonic, the present results imply fields which are
stronger by a factor of three to four.
---------------------------------------------------------
Title: Erratum: "Position and polarization of solar drift pair bursts"
[Proc. Astron. Soc. Aust., Vol. 3, p. 379 - 383 (1979)].
Authors: Suzuki, S.; Gary, D. E.
1980PASA....4..129S Altcode: 1980PASAu...4..129S
No abstract at ADS
---------------------------------------------------------
Title: Position and polarization of solar drift pair bursts
Authors: Suzuki, S.; Gary, D. E.
1979PASA....3..379S Altcode: 1979PASAu...3..379S
The paper presents an analysis of the observations of Type I bursts,
Type III bursts, and an underlying continuum made with the Culgoora
spectropolarimeter, spectrograph, and radioheliograph during a
noise storm of February 17/18, 1979. Several hundred RDP bursts, and
about fifty FDPs were observed. The results on the polarization of
drift pair bursts confirm the results of Sastry (1972) that the two
components of drift pairs are polarized in the same sense. However,
the observed significant difference in degree of polarization between
the two components of a pair has not been previously reported. Data
on RDP positional and frequency characteristics are presented, and
existing theories concerning RDPs are reviewed.
