explanation blue bibcodes open ADS page with paths to full text
Author name code: kontar
ADS astronomy entries on 2022-09-14
author:"Kontar, Eduard"
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Title: Sizes and Shapes of Sources in Solar Metric Radio Bursts
Authors: Gordovskyy, Mykola; Kontar, Eduard P.; Clarkson, Daniel L.;
Chrysaphi, Nicolina; Browning, Philippa K.
2022ApJ...925..140G Altcode: 2021arXiv211107777G
Metric and decametric radio emissions from the Sun are the only direct
source of information about the dynamics of nonthermal electrons
in the upper corona. In addition, the combination of spectral and
imaging (sizes, shapes, and positions) observations of low-frequency
radio sources can be used as a unique diagnostic tool to probe plasma
turbulence in the solar corona and inner heliosphere. The geometry of
the low-frequency sources and its variation with frequency are still
not understood, primarily due to the relatively low spatial resolution
available for solar observations. Here we report the first detailed
multifrequency analysis of the sizes of solar radio sources observed
by the Low Frequency Array. Furthermore, we investigate the source
shapes by approximating the derived intensity distributions using
2D Gaussian profiles with elliptical half-maximum contours. These
measurements have been made possible by a novel empirical method for
evaluating the instrumental and ionospheric effects on radio maps
based on known source observations. The obtained deconvolved sizes
of the sources are found to be smaller than previous estimations,
and often show higher ellipticity. The sizes and ellipticities of the
sources inferred using 2D Gaussian approximation, and their variation
with frequency are consistent with models of anisotropic radio-wave
scattering in the solar corona.
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Title: Spectral Analysis of Solar Radio Type III Bursts from 20 kHz
to 410 MHz
Authors: Sasikumar Raja, K.; Maksimovic, Milan; Kontar, Eduard P.;
Bonnin, Xavier; Zarka, Philippe; Lamy, Laurent; Reid, Hamish; Vilmer,
Nicole; Lecacheux, Alain; Krupar, Vratislav; Cecconi, Baptiste; Nora,
Lahmiti; Denis, Laurent
2022ApJ...924...58S Altcode: 2021arXiv211010935S
We present the statistical analysis of the spectral response of solar
radio type III bursts over the wide frequency range between 20 kHz and
410 MHz. For this purpose, we have used observations that were carried
out using both spaced-based (Wind/Waves) and ground-based (Nançay
Decameter Array and Nançay Radioheliograph) facilities. In order to
compare the flux densities observed by the different instruments, we
have carefully calibrated the data and displayed them in solar flux
units. The main result of our study is that type III bursts, in the
metric to hectometric wavelength range, statistically exhibit a clear
maximum of their median radio flux density around 2 MHz. Although
this result was already reported by inspecting the spectral profiles
of type III bursts in the frequency range 20 kHz-20 MHz, our study
extends such analysis for the first time to metric radio frequencies
(i.e., from 20 kHz to 410 MHz) and confirms the maximum spectral
response around 2 MHz. In addition, using a simple empirical model
we show that the median radio flux S of the studied data set obeys
the polynomial form Y = 0.04X <SUP>3</SUP> - 1.63X <SUP>2</SUP>
+ 16.30X - 41.24, with $X=\mathrm{ln}({F}_{\mathrm{MHz}})$ and
with $Y=\mathrm{ln}({S}_{\mathrm{SFU}})$ . Using the Sittler and
Guhathakurtha model for coronal streamers, we have found that
the maximum of radio power therefore falls in the range 4 to 10
R <SUB>⊙</SUB>, depending on whether the type III emissions are
assumed to be at the fundamental or the harmonic.
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Title: Thermal and Nonthermal Electron Energy Gain in Macroscale
Magnetic Reconnection
Authors: Arnold, Harry; Drake, James; Swisdak, Marc; Guo, Fan; Dahlin,
Joel; Zhang, Qile; Chen, Bin; Fleishman, Gregory; Glesener, Lindsay;
Kontar, Eduard; Phan, Tai; Shen, Chengcai
2021AGUFMSH22B..01A Altcode:
The first self-consistent simulations of electron acceleration
duringmagnetic reconnection in a macroscale system are
presented. Consistentwith solar flare observations the spectra of
energetic electrons takethe form of power-laws that extend more than
two decades inenergy. The drive mechanism for these nonthermal electrons
is Fermireflection in growing and merging magnetic flux ropes. A strong
guidefield suppresses the production of nonthermal electrons byweakening
the Fermi drive mechanism. For a weak guide field the totalenergy
content of nonthermal electrons dominates that of the hotthermal
electrons even though their number density remains small. Ourresults
are benchmarked with the hard x-ray, radio and extremeultra-violet
(EUV) observations of the X8.2-class solar flare onSeptember 10,
2017. Additionally, recent simulations suggest that the parallel
electric field resulting from large scale gradients in the electron
pressure and bound the reconnection exhausts is sufficient to produce
the energy gain of hot thermal electrons during flare energy release.
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Title: Simulations of radio-wave anisotropic scattering to interpret
type III radio burst measurements by Solar Orbiter, Parker Solar
Probe, STEREO and Wind
Authors: Musset, Sophie; Maksimovic, Milan; Kontar, Eduard; Krupar,
Vratislav; Chrysaphi, Nicolina; Bonnin, Xavier; Vecchio, Antonio;
Cecconi, Baptiste; Issautier, Karine; Bale, Stuart; Pulupa, Marc
2021AGUFMSH21A..11M Altcode:
The launch of Solar Orbiter and Parker Solar Probe provide for the first
time the opportunity to study type III radio burst emission measured
at 4 different spacecraft in the heliosphere, with measurements at 1
a.u. by Wind and STEREO-A. These measurements are used to characterize
the directivity of the type III radio burst emission, as well as the
variation of the radio time profiles from different angles. These
observations are compared to the predictions of radio propagation
simulations with an anisotropic scattering of the radio-wave on the
turbulent density fluctuations of the ambient plasma. We will present
here the first measurements of type III radio burst emission by 4
widely-spaced spacecraft and how these measurements provide new insights
on the radio sources properties, and how they provide a remote-sensing
tool to estimate the level of anisotropy of the density fluctuations
of ambient plasma in the inner heliosphere.
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Title: Solar Orbiter's first Venus flyby: Observations from the
Radio and Plasma Wave instrument
Authors: Hadid, L. Z.; Edberg, N. J. T.; Chust, T.; Píša, D.;
Dimmock, A. P.; Morooka, M. W.; Maksimovic, M.; Khotyaintsev, Yu. V.;
Souček, J.; Kretzschmar, M.; Vecchio, A.; Le Contel, O.; Retino, A.;
Allen, R. C.; Volwerk, M.; Fowler, C. M.; Sorriso-Valvo, L.; Karlsson,
T.; Santolík, O.; Kolmašová, I.; Sahraoui, F.; Stergiopoulou, K.;
Moussas, X.; Issautier, K.; Dewey, R. M.; Klein Wolt, M.; Malandraki,
O. E.; Kontar, E. P.; Howes, G. G.; Bale, S. D.; Horbury, T. S.;
Martinović, M.; Vaivads, A.; Krasnoselskikh, V.; Lorfèvre, E.;
Plettemeier, D.; Steller, M.; Štverák, Š.; Trávníček, P.;
O'Brien, H.; Evans, V.; Angelini, V.; Velli, M. C.; Zouganelis, I.
2021A&A...656A..18H Altcode:
Context. On December 27, 2020, Solar Orbiter completed its first
gravity assist manoeuvre of Venus (VGAM1). While this flyby was
performed to provide the spacecraft with sufficient velocity to get
closer to the Sun and observe its poles from progressively higher
inclinations, the Radio and Plasma Wave (RPW) consortium, along
with other operational in situ instruments, had the opportunity to
perform high cadence measurements and study the plasma properties in
the induced magnetosphere of Venus. <BR /> Aims: In this paper, we
review the main observations of the RPW instrument during VGAM1. They
include the identification of a number of magnetospheric plasma wave
modes, measurements of the electron number densities computed using
the quasi-thermal noise spectroscopy technique and inferred from
the probe-to-spacecraft potential, the observation of dust impact
signatures, kinetic solitary structures, and localized structures at the
bow shock, in addition to the validation of the wave normal analysis
on-board from the Low Frequency Receiver. <BR /> Methods: We used the
data products provided by the different subsystems of RPW to study
Venus' induced magnetosphere. <BR /> Results: The results include the
observations of various electromagnetic and electrostatic wave modes
in the induced magnetosphere of Venus: strong emissions of ∼100 Hz
whistler waves are observed in addition to electrostatic ion acoustic
waves, solitary structures and Langmuir waves in the magnetosheath of
Venus. Moreover, based on the different levels of the wave amplitudes
and the large-scale variations of the electron number densities, we
could identify different regions and boundary layers at Venus. <BR />
Conclusions: The RPW instrument provided unprecedented AC magnetic
and electric field measurements in Venus' induced magnetosphere for
continuous frequency ranges and with high time resolution. These
data allow for the conclusive identification of various plasma
waves at higher frequencies than previously observed and a detailed
investigation regarding the structure of the induced magnetosphere
of Venus. Furthermore, noting that prior studies were mainly focused
on the magnetosheath region and could only reach 10-12 Venus radii
(R<SUB>V</SUB>) down the tail, the particular orbit geometry of Solar
Orbiter's VGAM1, allowed the first investigation of the nature of the
plasma waves continuously from the bow shock to the magnetosheath,
extending to ∼70R<SUB>V</SUB> in the far distant tail region.
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Title: Simulations of radio-wave anisotropic scattering to interpret
type III radio burst data from Solar Orbiter, Parker Solar Probe,
STEREO, and Wind
Authors: Musset, S.; Maksimovic, M.; Kontar, E.; Krupar, V.; Chrysaphi,
N.; Bonnin, X.; Vecchio, A.; Cecconi, B.; Zaslavsky, A.; Issautier,
K.; Bale, S. D.; Pulupa, M.
2021A&A...656A..34M Altcode: 2021arXiv210913713M
<BR /> Aims: We use multi-spacecraft observations of individual
type III radio bursts to calculate the directivity of the radio
emission. We compare these data to the results of ray-tracing
simulations of the radio-wave propagation and probe the plasma
properties of the inner heliosphere. <BR /> Methods: We used
ray-tracing simulations of radio-wave propagation with anisotropic
scattering on density inhomogeneities to study the directivity of radio
emissions. Simultaneous observations of type III radio bursts by four
widely separated spacecraft were used to calculate the directivity
and position of the radio sources. The shape of the directivity
pattern deduced for individual events is compared to the directivity
pattern resulting from the ray-tracing simulations. <BR /> Results: We
show that simultaneous observations of type radio III bursts by four
different probes provide an opportunity to estimate the radio source
positions and the directivity of the radio emission. The shape of the
directivity varies from one event to another and it is consistent with
anisotropic scattering of the radio waves. <P />ARRAY(0x18a8790)
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Title: The Spatial and Temporal Variations of Turbulence in a
Solar Flare
Authors: Stores, Morgan; Jeffrey, Natasha L. S.; Kontar, Eduard P.
2021ApJ...923...40S Altcode: 2021arXiv211001542S
Magnetohydrodynamic plasma turbulence is believed to play a vital
role in the production of energetic electrons during solar flares,
and the nonthermal broadening of spectral lines is a key sign of this
turbulence. Here, we determine how flare turbulence evolves in time and
space using spectral profiles of Fe XXIV, Fe XXIII, and Fe XVI, observed
by the Hinode/EUV Imaging Spectrometer. Maps of nonthermal velocity are
created for times covering the X-ray rise, peak, and decay. For the
first time, the creation of kinetic energy density maps reveal where
energy is available for energization, suggesting that similar levels
of energy may be available to heat and/or accelerate electrons in large
regions of the flare. We find that turbulence is distributed throughout
the entire flare, often greatest in the coronal loop tops, and decaying
at different rates at different locations. For hotter ions (Fe XXIV and
Fe XXIII), the nonthermal velocity decreases as the flare evolves and
during/after the X-ray peak shows a clear spatial variation decreasing
linearly from the loop apex toward the ribbon. For the cooler ion (Fe
XVI), the nonthermal velocity remains relativity constant throughout
the flare, but steeply increases in one region corresponding to the
southern ribbon, peaking just prior to the peak in hard X-rays before
declining. The results suggest turbulence has a more complex temporal
and spatial structure than previously assumed, while newly introduced
turbulent kinetic energy maps show the availability of the energy and
identify important spatial inhomogeneities in the macroscopic plasma
motions leading to turbulence.
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Title: First observations and performance of the RPW instrument on
board the Solar Orbiter mission
Authors: Maksimovic, M.; Souček, J.; Chust, T.; Khotyaintsev, Y.;
Kretzschmar, M.; Bonnin, X.; Vecchio, A.; Alexandrova, O.; Bale, S. D.;
Bérard, D.; Brochot, J. -Y.; Edberg, N. J. T.; Eriksson, A.; Hadid,
L. Z.; Johansson, E. P. G.; Karlsson, T.; Katra, B.; Krasnoselskikh,
V.; Krupař, V.; Lion, S.; Lorfèvre, E.; Matteini, L.; Nguyen, Q. N.;
Píša, D.; Piberne, R.; Plettemeier, D.; Rucker, H. O.; Santolík,
O.; Steinvall, K.; Steller, M.; Štverák, Š.; Trávníček, P.;
Vaivads, A.; Zaslavsky, A.; Chaintreuil, S.; Dekkali, M.; Astier,
P. -A.; Barbary, G.; Boughedada, K.; Cecconi, B.; Chapron, F.; Collin,
C.; Dias, D.; Guéguen, L.; Lamy, L.; Leray, V.; Malac-Allain, L. R.;
Pantellini, F.; Parisot, J.; Plasson, P.; Thijs, S.; Fratter, I.;
Bellouard, E.; Danto, P.; Julien, S.; Guilhem, E.; Fiachetti, C.;
Sanisidro, J.; Laffaye, C.; Gonzalez, F.; Pontet, B.; Quéruel, N.;
Jannet, G.; Fergeau, P.; Dudok de Wit, T.; Vincent, T.; Agrapart,
C.; Pragout, J.; Bergerard-Timofeeva, M.; Delory, G. T.; Turin, P.;
Jeandet, A.; Leroy, P.; Pellion, J. -C.; Bouzid, V.; Recart, W.;
Kolmašová, I.; Krupařová, O.; Uhlíř, L.; Lán, R.; Baše, J.;
André, M.; Bylander, L.; Cripps, V.; Cully, C.; Jansson, S. -E.;
Puccio, W.; Břínek, J.; Ottacher, H.; Angelini, V.; Berthomier,
M.; Evans, V.; Goetz, K.; Hellinger, P.; Horbury, T. S.; Issautier,
K.; Kontar, E.; Le Contel, O.; Louarn, P.; Martinović, M.; Müller,
D.; O'Brien, H.; Owen, C. J.; Retino, A.; Rodríguez-Pacheco, J.;
Sahraoui, F.; Sanchez, L.; Walsh, A. P.; Wimmer-Schweingruber, R. F.;
Zouganelis, I.
2021A&A...656A..41M Altcode:
The Radio and Plasma Waves (RPW) instrument on the ESA Solar Orbiter
mission is designed to measure in situ magnetic and electric fields
and waves from the continuum up to several hundred kHz. The RPW also
observes solar and heliospheric radio emissions up to 16 MHz. It was
switched on and its antennae were successfully deployed two days after
the launch of Solar Orbiter on February 10, 2020. Since then, the
instrument has acquired enough data to make it possible to assess its
performance and the electromagnetic disturbances it experiences. In this
article, we assess its scientific performance and present the first RPW
observations. In particular, we focus on a statistical analysis of the
first observations of interplanetary dust by the instrument's Thermal
Noise Receiver. We also review the electro-magnetic disturbances that
RPW suffers, especially those which potential users of the instrument
data should be aware of before starting their research work.
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Title: The high-energy Sun - probing the origins of particle
acceleration on our nearest star
Authors: Matthews, S. A.; Reid, H. A. S.; Baker, D.; Bloomfield, D. S.;
Browning, P. K.; Calcines, A.; Del Zanna, G.; Erdelyi, R.; Fletcher,
L.; Hannah, I. G.; Jeffrey, N.; Klein, L.; Krucker, S.; Kontar, E.;
Long, D. M.; MacKinnon, A.; Mann, G.; Mathioudakis, M.; Milligan,
R.; Nakariakov, V. M.; Pesce-Rollins, M.; Shih, A. Y.; Smith, D.;
Veronig, A.; Vilmer, N.
2021ExA...tmp..135M Altcode:
As a frequent and energetic particle accelerator, our Sun provides
us with an excellent astrophysical laboratory for understanding
the fundamental process of particle acceleration. The exploitation
of radiative diagnostics from electrons has shown that acceleration
operates on sub-second time scales in a complex magnetic environment,
where direct electric fields, wave turbulence, and shock waves all
must contribute, although precise details are severely lacking. Ions
were assumed to be accelerated in a similar manner to electrons, but
γ-ray imaging confirmed that emission sources are spatially separated
from X-ray sources, suggesting distinctly different acceleration
mechanisms. Current X-ray and γ-ray spectroscopy provides only a basic
understanding of accelerated particle spectra and the total energy
budgets are therefore poorly constrained. Additionally, the recent
detection of relativistic ion signatures lasting many hours, without
an electron counterpart, is an enigma. We propose a single platform
to directly measure the physical conditions present in the energy
release sites and the environment in which the particles propagate and
deposit their energy. To address this fundamental issue, we set out
a suite of dedicated instruments that will probe both electrons and
ions simultaneously to observe; high (seconds) temporal resolution
photon spectra (4 keV - 150 MeV) with simultaneous imaging (1 keV -
30 MeV), polarization measurements (5-1000 keV) and high spatial and
temporal resolution imaging spectroscopy in the UV/EUV/SXR (soft X-ray)
regimes. These instruments will observe the broad range of radiative
signatures produced in the solar atmosphere by accelerated particles.
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Title: The Solar Orbiter Radio and Plasma Waves (RPW) instrument
(Corrigendum)
Authors: Maksimovic, M.; Bale, S. D.; Chust, T.; Khotyaintsev, Y.;
Krasnoselskikh, V.; Kretzschmar, M.; Plettemeier, D.; Rucker, H. O.;
Souček, J.; Steller, M.; Štverák, Š.; Trávníček, P.; Vaivads,
A.; Chaintreuil, S.; Dekkali, M.; Alexandrova, O.; Astier, P. -A.;
Barbary, G.; Bérard, D.; Bonnin, X.; Boughedada, K.; Cecconi,
B.; Chapron, F.; Chariet, M.; Collin, C.; de Conchy, Y.; Dias, D.;
Guéguen, L.; Lamy, L.; Leray, V.; Lion, S.; Malac-Allain, L. R.;
Matteini, L.; Nguyen, Q. N.; Pantellini, F.; Parisot, J.; Plasson,
P.; Thijs, S.; Vecchio, A.; Fratter, I.; Bellouard, E.; Lorfèvre,
E.; Danto, P.; Julien, S.; Guilhem, E.; Fiachetti, C.; Sanisidro,
J.; Laffaye, C.; Gonzalez, F.; Pontet, B.; Quéruel, N.; Jannet,
G.; Fergeau, P.; Brochot, J. -Y.; Cassam-Chenai, G.; Dudok de Wit,
T.; Timofeeva, M.; Vincent, T.; Agrapart, C.; Delory, G. T.; Turin,
P.; Jeandet, A.; Leroy, P.; Pellion, J. -C.; Bouzid, V.; Katra, B.;
Piberne, R.; Recart, W.; Santolík, O.; Kolmašová, I.; Krupař,
V.; Krupařová, O.; Píša, D.; Uhlíř, L.; Lán, R.; Baše, J.;
Ahlèn, L.; André, M.; Bylander, L.; Cripps, V.; Cully, C.; Eriksson,
A.; Jansson, S. -E.; Johansson, E. P. G.; Karlsson, T.; Puccio, W.;
Břínek, J.; Öttacher, H.; Panchenko, M.; Berthomier, M.; Goetz,
K.; Hellinger, P.; Horbury, T. S.; Issautier, K.; Kontar, E.; Krucker,
S.; Le Contel, O.; Louarn, P.; Martinović, M.; Owen, C. J.; Retino,
A.; Rodríguez-Pacheco, J.; Sahraoui, F.; Wimmer-Schweingruber, R. F.;
Zaslavsky, A.; Zouganelis, I.
2021A&A...654C...2M Altcode:
No abstract at ADS
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Title: First Frequency-time-resolved Imaging Spectroscopy Observations
of Solar Radio Spikes
Authors: Clarkson, Daniel L.; Kontar, Eduard P.; Gordovskyy, Mykola;
Chrysaphi, Nicolina; Vilmer, Nicole
2021ApJ...917L..32C Altcode: 2021arXiv210806191C
Solar radio spikes are short duration and narrow bandwidth fine
structures in dynamic spectra observed from the GHz to tens of MHz
range. Their very short duration and narrow frequency bandwidth
are indicative of subsecond small-scale energy release in the
solar corona, yet their origin is not understood. Using the LOw
Frequency ARray, we present spatially, frequency, and time resolved
observations of individual radio spikes associated with a coronal mass
ejection. Individual radio spike imaging demonstrates that the observed
area is increasing in time and the centroid positions of the individual
spikes move superluminally parallel to the solar limb. Comparison of
spike characteristics with that of individual Type IIIb striae observed
in the same event show similarities in duration, bandwidth, drift rate,
polarization, and observed area, as well the spike and striae motion in
the image plane suggesting fundamental plasma emission with the spike
emission region on the order of ~10<SUP>8</SUP> cm, with brightness
temperature as high as 10<SUP>13</SUP> K. The observed spatial,
spectral, and temporal properties of the individual spike bursts are
also suggestive of the radiation responsible for spikes escaping through
anisotropic density turbulence in closed loop structures with scattering
preferentially along the guiding magnetic field oriented parallel to
the limb in the scattering region. The dominance of scattering on the
observed time profile suggests the energy release time is likely to be
shorter than what is often assumed. The observations also imply that
the density turbulence anisotropy along closed magnetic field lines
is higher than along open field lines.
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Title: Energy Budget of Plasma Motions, Heating, and Electron
Acceleration in a Three-loop Solar Flare
Authors: Fleishman, Gregory D.; Kleint, Lucia; Motorina, Galina G.;
Nita, Gelu M.; Kontar, Eduard P.
2021ApJ...913...97F Altcode: 2021arXiv210400811F
Nonpotential magnetic energy promptly released in solar flares is
converted to other forms of energy. This may include nonthermal energy
of flare-accelerated particles, thermal energy of heated flaring
plasma, and kinetic energy of eruptions, jets, upflows/downflows,
and stochastic (turbulent) plasma motions. The processes or parameters
governing partitioning of the released energy between these components
are an open question. How these components are distributed between
distinct flaring loops and what controls these spatial distributions
are also unclear. Here, based on multiwavelength data and 3D modeling,
we quantify the energy partitioning and spatial distribution
in the well-observed SOL2014-02-16T064620 solar flare of class
C1.5. Nonthermal emission of this flare displayed a simple impulsive
single-spike light curve lasting about 20 s. In contrast, the thermal
emission demonstrated at least three distinct heating episodes, only
one of which was associated with the nonthermal component. The flare
was accompanied by upflows and downflows and substantial turbulent
velocities. The results of our analysis suggest that (i) the flare
occurs in a multiloop system that included at least three distinct flux
tubes; (ii) the released magnetic energy is divided unevenly between
the thermal and nonthermal components in these loops; (iii) only
one of these three flaring loops contains an energetically important
amount of nonthermal electrons, while two other loops remain thermal;
(iv) the amounts of direct plasma heating and that due to nonthermal
electron loss are comparable; and (v) the kinetic energy in the flare
footpoints constitutes only a minor fraction compared with the thermal
and nonthermal energies.
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Title: Fine structure of type III solar radio bursts from Langmuir
wave motion in turbulent plasma
Authors: Reid, Hamish A. S.; Kontar, Eduard P.
2021NatAs...5..796R Altcode: 2021arXiv210308424R; 2021NatAs.tmp...96R
The Sun frequently accelerates near-relativistic electron beams
that travel out through the solar corona and interplanetary
space. Interacting with their plasma environment, these beams produce
type III radio bursts—the brightest astrophysical radio sources
seen from Earth. The formation and motion of type III fine frequency
structures is a puzzle, but is commonly believed to be related to plasma
turbulence in the solar corona and solar wind. Combining a theoretical
framework with kinetic simulations and high-resolution radio type III
observations using the Low-Frequency Array, we quantitatively show
that the fine structures are caused by the moving intense clumps
of Langmuir waves in a turbulent medium. Our results show how type
III fine structure can be used to remotely analyse the intensity and
spectrum of compressive density fluctuations, and can infer ambient
temperatures in astrophysical plasma, substantially expanding the
current diagnostic potential of solar radio emission.
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Title: Electron Acceleration during Macroscale Magnetic Reconnection
Authors: Arnold, H.; Drake, J. F.; Swisdak, M.; Guo, F.; Dahlin, J. T.;
Chen, B.; Fleishman, G.; Glesener, L.; Kontar, E.; Phan, T.; Shen, C.
2021PhRvL.126m5101A Altcode: 2020arXiv201101147A
The first self-consistent simulations of electron acceleration during
magnetic reconnection in a macroscale system are presented. Consistent
with solar flare observations, the spectra of energetic electrons
take the form of power laws that extend more than two decades in
energy. The drive mechanism for these nonthermal electrons is Fermi
reflection in growing and merging magnetic flux ropes. A strong guide
field suppresses the production of nonthermal electrons by weakening
the Fermi drive mechanism. For a weak guide field the total energy
content of nonthermal electrons dominates that of the hot thermal
electrons even though their number density remains small. Our results
are benchmarked with the hard x-ray, radio, and extreme ultraviolet
observations of the X8.2-class solar flare on September 10, 2017.
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Title: Solar type III radio burst fine structure from Langmuir wave
motion through turbulent plasma
Authors: Kontar, Eduard; Reid, Hamish
2021EGUGA..2312013K Altcode:
The Sun frequently accelerates near-relativistic electron beams
that travel out through the solar corona and interplanetary
space. Interacting with their plasma environment, these beams produce
type III radio bursts, the brightest astrophysical radio sources
detected by humans. The formation and motion of type III fine frequency
structures is a puzzle but is commonly believed to be related to plasma
turbulence in the solar corona and solar wind. Combining a theoretical
framework with kinetic simulations and high-resolution radio type
III observations, we quantitatively show that the fine structures are
caused by the moving intense clumps of Langmuir waves in a turbulent
medium. Our results show how type III fine structure can be used to
remotely analyse the intensity and spectrum of compressive density
fluctuations, and can infer ambient temperatures in astrophysical
plasma, both significantly expanding the current diagnostic potential
of solar radio emission.
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Title: Transport of energetic particles from reconnecting current
sheets in flaring corona to the heliosphere
Authors: Browning, Philippa; Gordovskyy, Mykola; Inoue, Satashi;
Kontar, Eduard; Kusano, Kanya; Vekstein, Gregory
2021EGUGA..2315163B Altcode:
In this study, we inverstigate the acceleration of electrons and ions
at current sheets in the flaring solar corona, and their transport
into the heliosphere. We consider both generic solar flare models and
specific flaring events with a data-driven approach. The aim is to
answer two questions: (a) what fraction of particles accelerated in
different flares can escape into the heliosphere?; and (b) what are
the characteristics of the particle populations propagating towards
the chromosphere and into the heliosphere?We use a combination of
data-driven 3D magnetohydrodynamics simulations with drift-kinetic
particle simulations to model the evolution of the magnetic field
and both thermal and non-thermal plasma and to forward-model
observable characteristics. Particles are accelerated in current
sheets associated with flaring reconnection. When applied to a
specific flare, the model successfully predicts observed features
such as the location and relative intensity of hard X-ray sources
and helioseismic source locations. This confirms the viability of
the approach.Using these MHD-particle models, we will show how the
magnetic field evolution and particle transport processes affect the
characteristics of both energetic electrons and ions in the the inner
corona and the heliosphere. The implications for interpretation of in
situ measurements of energetic particles by Solar Orbiter and Parker
Solar Probe will be discussed.
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Title: Observing solar wind turbulence in the corona with ground-based
radio telescopes
Authors: Toit Strauss, Du; Botha, Gert; Chibueze, James; Kontar,
Eduard; Engelbrecht, Eugene; Lotz, Stefan; Wicks, Robert; Krupar,
Vratislav; Bale, Stuart; Maharaj, Shimul; Jeffrey, Natasha; Nel,
Amore; Steyn, Ruhann; van den Berg, Jabus
2021EGUGA..2314180T Altcode:
When point-like galactic and extragalactic radio sources are observed
through the solar corona by ground-based radio telescopes, plasma
density fluctuations in the turbulent solar wind scatter these photons,
leading to an observed broadening and/or elongation of such sources. By
observing this broadening for several sources, over several days, we
can get information about e.g. the wavenumber and radial dependence
of solar wind density fluctuations at very small scales (~30m - 8km)
inside the Alfven radius, thereby capturing details of the turbulence
dissipation range. Here, we present very initial results of such a
study with the MeerKAT radio telescope in South Africa (being, of
course, a precursor to the much larger Square Kilometer Array, SKA),
discuss the preliminary results, and compare these with theoretical
estimates and previous observations.
---------------------------------------------------------
Title: Sizes of solar radio sources observed by LOFAR
Authors: Gordovskyy, Mykola; Kontar, Eduard; Clarkson, Daniel;
Browning, Philippa
2021EGUGA..2315852G Altcode:
Decametric radio emission provides a unique insight into the physics
of solar and heliospheric plasmas. Along with dynamic spectra, the
spatial characteristics of the emission sources observed in solar
radio bursts yield important information about the behaviour of
high-energy non-thermal electrons, and the state of thermal plasma in
the upper solar corona. Recently, it has been shown that sizes and
locations of radio sources in the 10-100 MHz range can be used as a
diagnostic tool for plasma turbulence in the upper corona and inner
heliosphere. However, observations in this spectral range can be
strongly affected by limited spatial resolution of the instrument,
as well as by the effect of the Earth's ionosphere on radio wave
propagation.We describe a new method for correcting radio intensity
maps for instrumental and ionospheric effects using observations of
a known radio source at an arbitrary location in the sky. Based on
this method, we derive sizes and areas of the emission sources in the
solar radio bursts observed by the Low-Frequency Array (LOFAR) in 30-45
MHz range. It is shown that the sizes of sources are of the order of
ten arcminutes and decrease with increasing frequency. Overall, we
find that the sizes and their variation, as well as the shapes of the
sources in the considered events are consistent with the theoretical
models of turbulent radio-wave scattering in the solar corona developed
by Kontar et al. 2019 (Astrophys.J., 884, 122).
---------------------------------------------------------
Title: Parametric simulation studies on the wave propagation of
solar radio emission: the source size, duration, and position
Authors: Zhang, Peijin; Wang, Chuanbing; Kontar, Eduard
2021EGUGA..2310620Z Altcode:
The solar atmosphere is fluctuated and highly refractive for low
frequency waves (<300MHz), the observed features of solar radio
sources have indicated the existence of complex propagation effects. The
propagation effect has two major parts: refraction and scattering,
these two parts have combined influence on the observed source size
and position of radio imaging and temporal-frequency features in
the radio spectroscopy.We present a parametric simulation for the
propagation effect of the radio wave from solar radio bursts, with
the method of parametric simulation, we can build connections between
the solar atmosphere plasma condition and the observed radio source
properties. By comparing the simulation results with the observed source
size and property we estimated the scattering rate and the degree of
anisotropic of the background electron, and from the simulation results
we propose a possible explanation for the co-spatial phenomena of the
fundamental wave and harmonic wave in single frequency.
---------------------------------------------------------
Title: Parametric Simulation Studies on the Wave Propagation of
Solar Radio Emission: The Source Size, Duration, and Position
Authors: Zhang, PeiJin; Wang, ChuanBing; Kontar, Eduard P.
2021ApJ...909..195Z Altcode: 2021arXiv210100911Z
The observed features of the radio source indicate that the waves of
solar radio bursts are convoluted with complex propagation effects. In
this work, we perform ray-tracing simulations on radio wave transport in
the corona and interplanetary region with anisotropic electron density
fluctuations. For the first time, the variation of the apparent source
size, burst duration, and source position for the fundamental emission
and harmonic emission at the frequency of 35 MHz are simulated as a
function of the anisotropic parameter α and the angular scattering rate
coefficient η = ɛ<SUP>2</SUP>/h<SUB>0</SUB>, where ɛ<SUP>2</SUP> =
⟨δn<SUP>2</SUP>⟩/n<SUP>2</SUP> is the density fluctuation level
and h<SUB>0</SUB> is its correlation length near the wave excitation
site. It is found that isotropic fluctuations produce a much larger
decay time than a highly anisotropic fluctuation for fundamental
emission. By comparing the observed duration and source size with
the simulation results in the parameter space, we can estimate the
scattering coefficient and the anisotropic parameter η = 8.9 ×
10<SUP>-5</SUP> km<SUP>-1</SUP> and α = 0.719 with a point pulse source
assumption. Position offsets due to wave scattering and refraction can
produce the co-spatial of the fundamental and harmonic waves in the
observation of some type III radio bursts. The visual speed due to the
wave propagation effect can reach 1.5c for η = 2.4 × 10<SUP>-4</SUP>
km<SUP>-1</SUP> and α = 0.2 for the fundamental emission in the sky
plane, accompanied with large expansion rate of the source size. The
direction of the visual speed is mostly identical to the direction of
the offset, thus, for the observation aimed at obtaining the source
position, the source centroid at the starting time is closer to the
wave excitation site.
---------------------------------------------------------
Title: Constraints on the acceleration region of type III radio
bursts from decimetric radio spikes and faint X-ray bursts
Authors: Musset, Sophie; Kontar, Eduard; Glesener, Lindsay; Vilmer,
Nicole; Hamini, Abdallah
2021arXiv210107543M Altcode:
We study the release of energy during the gradual phase of a
flare, characterized by faint bursts of non-thermal hard X-ray
(HXR) emission associated with decimetric radio spikes and type
III radio bursts starting at high frequencies and extending to the
heliosphere. We characterize the site of electron acceleration in the
corona and study the radial evolution of radio source sizes in the
high corona. Imaging and spectroscopy of the HXR emission with Fermi
and RHESSI provide a diagnostic of the accelerated electrons in the
corona as well as a lower limit on the height of the acceleration
region. Radio observations in the decimetric range with the ORFEES
spectrograph provide radio diagnostics close to the acceleration
region. Radio spectro-imaging with LOFAR in the meter range provide the
evolution of the radio source sizes with their distance from the Sun,
in the high corona. Non-thermal HXR bursts and radio spikes are well
correlated on short timescales. The spectral index of non-thermal HXR
emitting electrons is -4 and their number is about $2\times 10^{33}$
electrons/s. The density of the acceleration region is constrained
between $1-5 \times 10^9$ cm$^{-3}$. Electrons accelerated upward
rapidly become unstable to Langmuir wave production, leading to high
starting frequencies of the type III radio bursts, and the elongation
of the radio beam at its source is between 0.5 and 11.4 Mm. The radio
source sizes and their gradient observed with LOFAR are larger than the
expected size and gradient of the size of the electron beam, assuming
it follows the expansion of the magnetic flux tubes. These observations
support the idea that the fragmentation of the radio emission into
spikes is linked to the fragmentation of the acceleration process
itself. The combination of HXR and radio diagnostics in the corona
provides strong constrains on the site of electron acceleration.
---------------------------------------------------------
Title: The Micro Solar Flare Apparatus (MiSolFA)
Authors: Hayes, L. A.; Christe, S.; Ryan, D.; Krucker, S.; Martinez
Oliveros, J. C.; Kontar, E.; Jeffrey, N.; Caspi, A.; Saint-Hilaire,
P.; Limousin, O.; Meuris, A.; Battaglia, M.; Casadei, D.
2020AGUFMSH056..08H Altcode:
During a solar flare the rapid release of magnetic energy drives
extremely efficient particle acceleration through a mechanism which
remains largely unknown. Hard X-ray observations are one of the
most direct signatures of flare accelerated energetic electrons
at and near the acceleration site, and X-ray spectra can provide
key diagnostics to the physical processes occurring in flares. In
particular, a measure of the electron angular distribution (the hard
X-ray directivity) is a prime diagnostic of the unknown acceleration
mechanism. However, to-date, observational constraints of directivity
have not been clear. Looking towards the next solar cycle, stereoscopic
X-ray observations of solar flares will make significant advances at
measuring the hard X-ray directivity. The Micro Solar Flare Apparatus
(MiSolFA) is a CubeSat Observatory that will observe solar flares at
the same time as Solar Orbiter/STIX using cross-calibrated flight-spare
detectors. During the rise of the next solar maximum, STIX on board
Solar Orbiter will perform X-ray observations of solar flares from
0.28 AU (at perihelion) and up to inclinations of ∼25 degrees at
heliospheric angles significantly different than the Earth. These two
instruments working together will be the first pair of cross-calibrated
X-ray spectrometers to observe solar flares from very different
points of view allowing us to measure the anisotropy of the flare
hard x-rays and finally confidently constrain the flare-accelerated
electron directivity in individual flares for the first time.
---------------------------------------------------------
Title: Subsecond Time Evolution of Type III Solar Radio Burst Sources
at Fundamental and Harmonic Frequencies
Authors: Chen, Xingyao; Kontar, Eduard P.; Chrysaphi, Nicolina;
Jeffrey, Natasha L. S.; Gordovskyy, Mykola; Yan, Yihua; Tan, Baolin
2020ApJ...905...43C Altcode: 2020arXiv201008782C
Recent developments in astronomical radio telescopes opened new
opportunities in imaging and spectroscopy of solar radio bursts at
subsecond timescales. Imaging in narrow frequency bands has revealed
temporal variations in the positions and source sizes that do not fit
into the standard picture of type III solar radio bursts, and require a
better understanding of radio-wave transport. In this paper, we utilize
3D Monte Carlo ray-tracing simulations that account for the anisotropic
density turbulence in the inhomogeneous solar corona to quantitatively
explain the image dynamics at the fundamental (near plasma frequency)
and harmonic (double) plasma emissions observed at ∼32 MHz. Comparing
the simulations with observations, we find that anisotropic scattering
from an instantaneous emission point source can account for the observed
time profiles, centroid locations, and source sizes of the fundamental
component of type III radio bursts (generated where f<SUB>pe</SUB>
≍ 32 MHz). The best agreement with observations is achieved when the
ratio of the perpendicular to the parallel component of the wavevector
of anisotropic density turbulence is around 0.25. Harmonic emission
sources observed at the same frequency (∼32 MHz, but generated where
f<SUB>pe</SUB> ≍ 16 MHz) have apparent sizes comparable to those
produced by the fundamental emission, but demonstrate a much slower
temporal evolution. The simulations of radio-wave propagation make it
possible to quantitatively explain the variations of apparent source
sizes and positions at subsecond timescales both for the fundamental
and harmonic emissions, and can be used as a diagnostic tool for the
plasma turbulence in the upper corona.
---------------------------------------------------------
Title: VizieR Online Data Catalog: Global energetics of solar
flares. VIII. (Aschwanden+, 2019)
Authors: Aschwanden+; Aschwanden, M. J.; Kontar, E. P.; Jeffrey,
N. L. S.
2020yCat..18810001A Altcode:
We use the same Ramaty High Energy Solar Spectroscopic Imager (RHESSI)
data of 191 flare events as previously analyzed in Paper III, using
the Object Spectral Executive (OSPEX) software. <P />(1 data file).
---------------------------------------------------------
Title: Probing solar flare accelerated electron distributions with
prospective X-ray polarimetry missions
Authors: Jeffrey, Natasha L. S.; Saint-Hilaire, Pascal; Kontar,
Eduard P.
2020A&A...642A..79J Altcode: 2020arXiv200807849J
Solar flare electron acceleration is an extremely efficient process,
but the method of acceleration is not well constrained. Two of the
essential diagnostics, electron anisotropy (velocity angle to the
guiding magnetic field) and the high energy cutoff (highest energy
electrons produced by the acceleration conditions: mechanism, spatial
extent, and time), are important quantities that can help to constrain
electron acceleration at the Sun but both are poorly determined. Here,
by using electron and X-ray transport simulations that account for both
collisional and non-collisional transport processes, such as turbulent
scattering and X-ray albedo, we show that X-ray polarization can be used
to constrain the anisotropy of the accelerated electron distribution
and the most energetic accelerated electrons together. Moreover, we
show that prospective missions, for example CubeSat missions without
imaging information, can be used alongside such simulations to determine
these parameters. We conclude that a fuller understanding of flare
acceleration processes will come from missions capable of both X-ray
flux and polarization spectral measurements together. Although imaging
polarimetry is highly desired, we demonstrate that spectro-polarimeters
without imaging can also provide strong constraints on electron
anisotropy and the high energy cutoff.
---------------------------------------------------------
Title: The Solar Orbiter Radio and Plasma Waves (RPW) instrument
Authors: Maksimovic, M.; Bale, S. D.; Chust, T.; Khotyaintsev, Y.;
Krasnoselskikh, V.; Kretzschmar, M.; Plettemeier, D.; Rucker, H. O.;
Souček, J.; Steller, M.; Štverák, Š.; Trávníček, P.; Vaivads,
A.; Chaintreuil, S.; Dekkali, M.; Alexandrova, O.; Astier, P. -A.;
Barbary, G.; Bérard, D.; Bonnin, X.; Boughedada, K.; Cecconi,
B.; Chapron, F.; Chariet, M.; Collin, C.; de Conchy, Y.; Dias, D.;
Guéguen, L.; Lamy, L.; Leray, V.; Lion, S.; Malac-Allain, L. R.;
Matteini, L.; Nguyen, Q. N.; Pantellini, F.; Parisot, J.; Plasson,
P.; Thijs, S.; Vecchio, A.; Fratter, I.; Bellouard, E.; Lorfèvre,
E.; Danto, P.; Julien, S.; Guilhem, E.; Fiachetti, C.; Sanisidro,
J.; Laffaye, C.; Gonzalez, F.; Pontet, B.; Quéruel, N.; Jannet,
G.; Fergeau, P.; Brochot, J. -Y.; Cassam-Chenai, G.; Dudok de Wit,
T.; Timofeeva, M.; Vincent, T.; Agrapart, C.; Delory, G. T.; Turin,
P.; Jeandet, A.; Leroy, P.; Pellion, J. -C.; Bouzid, V.; Katra, B.;
Piberne, R.; Recart, W.; Santolík, O.; Kolmašová, I.; Krupař,
V.; Krupařová, O.; Píša, D.; Uhlíř, L.; Lán, R.; Baše, J.;
Ahlèn, L.; André, M.; Bylander, L.; Cripps, V.; Cully, C.; Eriksson,
A.; Jansson, S. -E.; Johansson, E. P. G.; Karlsson, T.; Puccio, W.;
Břínek, J.; Öttacher, H.; Panchenko, M.; Berthomier, M.; Goetz,
K.; Hellinger, P.; Horbury, T. S.; Issautier, K.; Kontar, E.; Krucker,
S.; Le Contel, O.; Louarn, P.; Martinović, M.; Owen, C. J.; Retino,
A.; Rodríguez-Pacheco, J.; Sahraoui, F.; Wimmer-Schweingruber, R. F.;
Zaslavsky, A.; Zouganelis, I.
2020A&A...642A..12M Altcode:
The Radio and Plasma Waves (RPW) instrument on the ESA Solar Orbiter
mission is described in this paper. This instrument is designed
to measure in-situ magnetic and electric fields and waves from the
continuous to a few hundreds of kHz. RPW will also observe solar radio
emissions up to 16 MHz. The RPW instrument is of primary importance
to the Solar Orbiter mission and science requirements since it is
essential to answer three of the four mission overarching science
objectives. In addition RPW will exchange on-board data with the other
in-situ instruments in order to process algorithms for interplanetary
shocks and type III langmuir waves detections.
---------------------------------------------------------
Title: The Energetic Particle Detector. Energetic particle instrument
suite for the Solar Orbiter mission
Authors: Rodríguez-Pacheco, J.; Wimmer-Schweingruber, R. F.; Mason,
G. M.; Ho, G. C.; Sánchez-Prieto, S.; Prieto, M.; Martín, C.;
Seifert, H.; Andrews, G. B.; Kulkarni, S. R.; Panitzsch, L.; Boden,
S.; Böttcher, S. I.; Cernuda, I.; Elftmann, R.; Espinosa Lara, F.;
Gómez-Herrero, R.; Terasa, C.; Almena, J.; Begley, S.; Böhm, E.;
Blanco, J. J.; Boogaerts, W.; Carrasco, A.; Castillo, R.; da Silva
Fariña, A.; de Manuel González, V.; Drews, C.; Dupont, A. R.;
Eldrum, S.; Gordillo, C.; Gutiérrez, O.; Haggerty, D. K.; Hayes,
J. R.; Heber, B.; Hill, M. E.; Jüngling, M.; Kerem, S.; Knierim,
V.; Köhler, J.; Kolbe, S.; Kulemzin, A.; Lario, D.; Lees, W. J.;
Liang, S.; Martínez Hellín, A.; Meziat, D.; Montalvo, A.; Nelson,
K. S.; Parra, P.; Paspirgilis, R.; Ravanbakhsh, A.; Richards, M.;
Rodríguez-Polo, O.; Russu, A.; Sánchez, I.; Schlemm, C. E.; Schuster,
B.; Seimetz, L.; Steinhagen, J.; Tammen, J.; Tyagi, K.; Varela, T.;
Yedla, M.; Yu, J.; Agueda, N.; Aran, A.; Horbury, T. S.; Klecker, B.;
Klein, K. -L.; Kontar, E.; Krucker, S.; Maksimovic, M.; Malandraki,
O.; Owen, C. J.; Pacheco, D.; Sanahuja, B.; Vainio, R.; Connell,
J. J.; Dalla, S.; Dröge, W.; Gevin, O.; Gopalswamy, N.; Kartavykh,
Y. Y.; Kudela, K.; Limousin, O.; Makela, P.; Mann, G.; Önel, H.;
Posner, A.; Ryan, J. M.; Soucek, J.; Hofmeister, S.; Vilmer, N.;
Walsh, A. P.; Wang, L.; Wiedenbeck, M. E.; Wirth, K.; Zong, Q.
2020A&A...642A...7R Altcode:
After decades of observations of solar energetic particles from
space-based observatories, relevant questions on particle injection,
transport, and acceleration remain open. To address these scientific
topics, accurate measurements of the particle properties in the inner
heliosphere are needed. In this paper we describe the Energetic Particle
Detector (EPD), an instrument suite that is part of the scientific
payload aboard the Solar Orbiter mission. Solar Orbiter will approach
the Sun as close as 0.28 au and will provide extra-ecliptic measurements
beyond ∼30° heliographic latitude during the later stages of the
mission. The EPD will measure electrons, protons, and heavy ions with
high temporal resolution over a wide energy range, from suprathermal
energies up to several hundreds of megaelectronvolts/nucleons. For
this purpose, EPD is composed of four units: the SupraThermal
Electrons and Protons (STEP), the Electron Proton Telescope (EPT),
the Suprathermal Ion Spectrograph (SIS), and the High-Energy Telescope
(HET) plus the Instrument Control Unit that serves as power and data
interface with the spacecraft. The low-energy population of electrons
and ions will be covered by STEP and EPT, while the high-energy
range will be measured by HET. Elemental and isotopic ion composition
measurements will be performed by SIS and HET, allowing full particle
identification from a few kiloelectronvolts up to several hundreds
of megaelectronvolts/nucleons. Angular information will be provided
by the separate look directions from different sensor heads, on the
ecliptic plane along the Parker spiral magnetic field both forward
and backwards, and out of the ecliptic plane observing both northern
and southern hemispheres. The unparalleled observations of EPD will
provide key insights into long-open and crucial questions about the
processes that govern energetic particles in the inner heliosphere.
---------------------------------------------------------
Title: Forward Modeling of Particle Acceleration and Transport in
an Individual Solar Flare
Authors: Gordovskyy, Mykola; Browning, Philippa K.; Inoue, Satoshi;
Kontar, Eduard P.; Kusano, Kanya; Vekstein, Grigory E.
2020ApJ...902..147G Altcode: 2020arXiv200910130G
The aim of this study is to generate maps of the hard X-ray emission
produced by energetic electrons in a solar flare and compare them
with observations. The ultimate goal is to test the viability of the
combined MHD/test-particle approach for data-driven modeling of active
events in the solar corona and their impact on the heliosphere. Based
on an MHD model of X-class solar flare observed on 2017 September 8,
we calculate trajectories of a large number of electrons and protons
using the relativistic guiding-center approach. Using the obtained
particle trajectories, we deduce the spatial and energy distributions of
energetic electrons and protons, and calculate bremsstrahlung hard X-ray
emission using the "thin-target" approximation. Our approach predicts
some key characteristics of energetic particles in the considered
flare, including the size and location of the acceleration region,
energetic particle trajectories and energy spectra. Most importantly,
the hard X-ray bremsstrahlung intensity maps predicted by the model
are in good agreement with those observed by RHESSI. Furthermore, the
locations of proton and electron precipitation appear to be close to
the sources of helioseismic response detected in this flare. Therefore,
the adopted approach can be used for observationally driven modeling
of individual solar flares, including manifestations of energetic
particles in the corona, as well as the inner heliosphere.
---------------------------------------------------------
Title: The Solar Orbiter Science Activity Plan. Translating solar
and heliospheric physics questions into action
Authors: Zouganelis, I.; De Groof, A.; Walsh, A. P.; Williams, D. R.;
Müller, D.; St Cyr, O. C.; Auchère, F.; Berghmans, D.; Fludra,
A.; Horbury, T. S.; Howard, R. A.; Krucker, S.; Maksimovic, M.;
Owen, C. J.; Rodríguez-Pacheco, J.; Romoli, M.; Solanki, S. K.;
Watson, C.; Sanchez, L.; Lefort, J.; Osuna, P.; Gilbert, H. R.;
Nieves-Chinchilla, T.; Abbo, L.; Alexandrova, O.; Anastasiadis, A.;
Andretta, V.; Antonucci, E.; Appourchaux, T.; Aran, A.; Arge, C. N.;
Aulanier, G.; Baker, D.; Bale, S. D.; Battaglia, M.; Bellot Rubio,
L.; Bemporad, A.; Berthomier, M.; Bocchialini, K.; Bonnin, X.; Brun,
A. S.; Bruno, R.; Buchlin, E.; Büchner, J.; Bucik, R.; Carcaboso,
F.; Carr, R.; Carrasco-Blázquez, I.; Cecconi, B.; Cernuda Cangas, I.;
Chen, C. H. K.; Chitta, L. P.; Chust, T.; Dalmasse, K.; D'Amicis, R.;
Da Deppo, V.; De Marco, R.; Dolei, S.; Dolla, L.; Dudok de Wit, T.;
van Driel-Gesztelyi, L.; Eastwood, J. P.; Espinosa Lara, F.; Etesi,
L.; Fedorov, A.; Félix-Redondo, F.; Fineschi, S.; Fleck, B.; Fontaine,
D.; Fox, N. J.; Gandorfer, A.; Génot, V.; Georgoulis, M. K.; Gissot,
S.; Giunta, A.; Gizon, L.; Gómez-Herrero, R.; Gontikakis, C.; Graham,
G.; Green, L.; Grundy, T.; Haberreiter, M.; Harra, L. K.; Hassler,
D. M.; Hirzberger, J.; Ho, G. C.; Hurford, G.; Innes, D.; Issautier,
K.; James, A. W.; Janitzek, N.; Janvier, M.; Jeffrey, N.; Jenkins,
J.; Khotyaintsev, Y.; Klein, K. -L.; Kontar, E. P.; Kontogiannis,
I.; Krafft, C.; Krasnoselskikh, V.; Kretzschmar, M.; Labrosse, N.;
Lagg, A.; Landini, F.; Lavraud, B.; Leon, I.; Lepri, S. T.; Lewis,
G. R.; Liewer, P.; Linker, J.; Livi, S.; Long, D. M.; Louarn, P.;
Malandraki, O.; Maloney, S.; Martinez-Pillet, V.; Martinovic, M.;
Masson, A.; Matthews, S.; Matteini, L.; Meyer-Vernet, N.; Moraitis,
K.; Morton, R. J.; Musset, S.; Nicolaou, G.; Nindos, A.; O'Brien,
H.; Orozco Suarez, D.; Owens, M.; Pancrazzi, M.; Papaioannou, A.;
Parenti, S.; Pariat, E.; Patsourakos, S.; Perrone, D.; Peter, H.;
Pinto, R. F.; Plainaki, C.; Plettemeier, D.; Plunkett, S. P.; Raines,
J. M.; Raouafi, N.; Reid, H.; Retino, A.; Rezeau, L.; Rochus, P.;
Rodriguez, L.; Rodriguez-Garcia, L.; Roth, M.; Rouillard, A. P.;
Sahraoui, F.; Sasso, C.; Schou, J.; Schühle, U.; Sorriso-Valvo, L.;
Soucek, J.; Spadaro, D.; Stangalini, M.; Stansby, D.; Steller, M.;
Strugarek, A.; Štverák, Š.; Susino, R.; Telloni, D.; Terasa, C.;
Teriaca, L.; Toledo-Redondo, S.; del Toro Iniesta, J. C.; Tsiropoula,
G.; Tsounis, A.; Tziotziou, K.; Valentini, F.; Vaivads, A.; Vecchio,
A.; Velli, M.; Verbeeck, C.; Verdini, A.; Verscharen, D.; Vilmer, N.;
Vourlidas, A.; Wicks, R.; Wimmer-Schweingruber, R. F.; Wiegelmann,
T.; Young, P. R.; Zhukov, A. N.
2020A&A...642A...3Z Altcode: 2020arXiv200910772Z
Solar Orbiter is the first space mission observing the solar plasma
both in situ and remotely, from a close distance, in and out of the
ecliptic. The ultimate goal is to understand how the Sun produces
and controls the heliosphere, filling the Solar System and driving
the planetary environments. With six remote-sensing and four in-situ
instrument suites, the coordination and planning of the operations are
essential to address the following four top-level science questions:
(1) What drives the solar wind and where does the coronal magnetic field
originate?; (2) How do solar transients drive heliospheric variability?;
(3) How do solar eruptions produce energetic particle radiation that
fills the heliosphere?; (4) How does the solar dynamo work and drive
connections between the Sun and the heliosphere? Maximising the
mission's science return requires considering the characteristics
of each orbit, including the relative position of the spacecraft
to Earth (affecting downlink rates), trajectory events (such
as gravitational assist manoeuvres), and the phase of the solar
activity cycle. Furthermore, since each orbit's science telemetry
will be downloaded over the course of the following orbit, science
operations must be planned at mission level, rather than at the level
of individual orbits. It is important to explore the way in which those
science questions are translated into an actual plan of observations
that fits into the mission, thus ensuring that no opportunities are
missed. First, the overarching goals are broken down into specific,
answerable questions along with the required observations and the
so-called Science Activity Plan (SAP) is developed to achieve this. The
SAP groups objectives that require similar observations into Solar
Orbiter Observing Plans, resulting in a strategic, top-level view of
the optimal opportunities for science observations during the mission
lifetime. This allows for all four mission goals to be addressed. In
this paper, we introduce Solar Orbiter's SAP through a series of
examples and the strategy being followed.
---------------------------------------------------------
Title: Radio Echo in the Turbulent Corona and Simulations of Solar
Drift-pair Radio Bursts
Authors: Kuznetsov, Alexey A.; Chrysaphi, Nicolina; Kontar, Eduard P.;
Motorina, Galina
2020ApJ...898...94K Altcode: 2020arXiv200714648K
Drift-pair bursts are an unusual type of solar low-frequency
radio emission, which appear in the dynamic spectra as two parallel
drifting bright stripes separated in time. Recent imaging spectroscopy
observations allowed for the quantitative characterization of the
drifting pairs in terms of source size, position, and evolution. Here,
the drift-pair parameters are qualitatively analyzed and compared
with the newly developed Monte Carlo ray-tracing technique simulating
radio-wave propagation in the inhomogeneous anisotropic turbulent
solar corona. The results suggest that drift-pair bursts can be formed
due to a combination of refraction and scattering processes, with the
trailing component being the result of turbulent reflection (turbulent
radio echo). The formation of drift-pair bursts requires an anisotropic
scattering with the level of plasma density fluctuations comparable to
that in type III bursts, but with a stronger anisotropy at the inner
turbulence scale. The anisotropic radio-wave scattering model can
quantitatively reproduce the key properties of drift-pair bursts: the
apparent source size and its increase with time at a given frequency,
the parallel motion of the source centroid positions, and the delay
between the burst components. The trailing component is found to be
virtually cospatial and following the main component. The simulations
suggest that drift-pair bursts are likely to be observed closer to
the disk center and below 100 MHz due to the effects of free-free
absorption and scattering. The exciter of drift pairs is consistent
with propagating packets of whistlers, allowing for a fascinating way
to diagnose the plasma turbulence and the radio emission mechanism.
---------------------------------------------------------
Title: Spectral Analysis of Solar Radio Type III Bursts from 10 kHz
to 80 MHz
Authors: Sasikumar Raja, Kantepalli; Maksimovic, Milan; Bonnin, Xavier;
Zarka, Philippe; Lamy, Laurent; Kontar, Eduard P.; Lecacheux, Alain;
Krupar, Vratislav; Cecconi, Baptiste; Lahmiti, Nora; Denis, Laurent
2020EGUGA..22.1252S Altcode:
Solar radio type III bursts are produced by electron beams that are
propagating along the open magnetic field lines in the corona and
interplanetary medium (IPM). They are the intense, fast drifting, and
frequently observed bursts. Recently, it was reported that observations
of type III bursts show a maximum spectral response at around 1 MHz. But
this behavior of type III bursts is not sufficiently discussed in the
literature. In order to understand this behavior we have revisited
this problem and studied 2279 isolated type III bursts that are
observed with Wind/Waves instrument (from space during 1995-2009) in
the frequency range 10 kHz-14 MHz and found that all of them show a
maximum spectral response at around 1 MHz. Since type III bursts are
somewhat directive, we have studied separately, another 115 type III
bursts that are simultaneously observed (in 2013-2014) using Wind/Waves
and ground-based facility Nancay Decameter Array (10-80 MHz) and
compared the spectral profiles. In this presentation, we will discuss
the observations, applied calibration techniques and the possible
theoretical explanation of why type III bursts show such behavior.
---------------------------------------------------------
Title: First Observation of a Type II Solar Radio Burst Transitioning
between a Stationary and Drifting State
Authors: Chrysaphi, Nicolina; Reid, Hamish A. S.; Kontar, Eduard P.
2020ApJ...893..115C Altcode: 2020arXiv200311101C
Standing shocks are believed to be responsible for stationary
Type II solar radio bursts, whereas drifting Type II bursts are
excited by moving shocks often related to coronal mass ejections
(CMEs). Observations of either stationary or drifting Type II bursts
are common, but a transition between the two states has not yet been
reported. Here, we present a Type II burst which shows a clear,
continuous transition from a stationary to a drifting state, the
first observation of its kind. Moreover, band splitting is observed
in the stationary parts of the burst, as well as intriguing negative
and positive frequency-drift fine structures within the stationary
emissions. The relation of the radio emissions to an observed jet
and a narrow CME were investigated across multiple wavelengths,
and the mechanisms leading to the transitioning Type II burst were
determined. We find that a jet eruption generates a streamer-puff CME
and that the interplay between the CME-driven shock and the streamer
is likely to be responsible for the observed radio emissions.
---------------------------------------------------------
Title: Particle acceleration with anomalous pitch angle scattering
in 3D separator reconnection
Authors: Borissov, A.; Neukirch, T.; Kontar, E. P.; Threlfall, J.;
Parnell, C. E.
2020A&A...635A..63B Altcode: 2020arXiv200107548B
Context. Understanding how the release of stored magnetic energy
contributes to the generation of non-thermal high energy particles
during solar flares is an important open problem in solar physics. There
is a general consensus that magnetic reconnection plays a fundamental
role in the energy release and conversion processes taking place during
flares. A common approach for investigating how reconnection contributes
to particle acceleration is to use test particle calculations in
electromagnetic fields derived from numerical magnetohydrodynamic (MHD)
simulations of reconnecting magnetic fields. These MHD simulations use
anomalous resistivities that are orders of magnitude larger than the
Spitzer resistivity that is based on Coulomb collisions. The processes
leading to such an enhanced resistivity should also affect the test
particles, for example, through pitch angle scattering. This study
explores the effect of such a link between the level of resistivity
and its impact on particle orbits and builds on a previous study using
a 2D MHD simulation of magnetic reconnection. <BR /> Aims: This paper
aims to extend the previous investigation to a 3D magnetic reconnection
configuration and to study the effect on test particle orbits. <BR />
Methods: We carried out orbit calculations using a 3D MHD simulation of
reconnection in a magnetic field with a magnetic separator. The orbit
calculations use the relativistic guiding centre approximation but,
crucially, they also include pitch angle scattering using stochastic
differential equations. The effects of varying the resistivity and
the models for pitch angle scattering on particle orbit trajectories,
final positions, energy spectra, final pitch angle distribution,
and orbit duration are all studied in detail. <BR /> Results: Pitch
angle scattering widens highly collimated beams of unscattered orbit
trajectories, allowing orbits to access previously unaccessible field
lines; this causes final positions to spread along other topological
structures which could not be accessed without scattering. Scattered
orbit energy spectra are found to be predominantly affected by the
level of anomalous resistivity, with the pitch angle scattering model
only playing a role in specific, isolated cases. This is in contrast
to the study involving a 2D MHD simulation of magnetic reconnection,
where pitch angle scattering had a more noticeable effect on the energy
spectra. Pitch scattering effects are found to play a crucial role in
determining the pitch angle and orbit duration distributions.
---------------------------------------------------------
Title: Spatiotemporal Energy Partitioning in a Nonthermally Dominated
Two-loop Solar Flare
Authors: Motorina, Galina G.; Fleishman, Gregory D.; Kontar, Eduard P.
2020ApJ...890...75M Altcode: 2020arXiv200102009M
Solar flares show remarkable variety in the energy partitioning
between thermal and nonthermal components. Those with a prominent
nonthermal component but only a modest thermal one are particularly
well suited for study of the direct effect of the nonthermal electrons
on plasma heating. Here, we analyze such a well-observed, impulsive
single-spike nonthermal event, a solar flare SOL2013-11-05T035054,
where the plasma heating can be entirely attributed to the energy
losses of these impulsively accelerated electrons. Evolution of
the energy budget of thermal and nonthermal components during the
flare is analyzed using X-ray, microwave, and EUV observations and
three-dimensional modeling. The results suggest that (I) the flare
geometry is consistent with a two-loop morphology and the magnetic
energy is likely released due to interaction between these two loops;
(II) the released magnetic energy is converted to the nonthermal
energy of accelerated electrons only, which is subsequently converted
to the thermal energy of the plasma; (III) the energy is partitioned
in these two flaring loops in comparable amounts; (IV) one of these
flaring loops remained relatively tenuous but rather hot, while the
other remained relatively cool but denser than the first. Therefore,
this solar flare demonstrates an extreme efficiency of conversion of the
free magnetic energy to the nonthermal energy of particle acceleration
and the flow of energy into two loops from the nonthermal component
to the thermal one with negligible direct heating.
---------------------------------------------------------
Title: Density Fluctuations in the Solar Wind Based on Type III
Radio Bursts Observed by Parker Solar Probe
Authors: Krupar, Vratislav; Szabo, Adam; Maksimovic, Milan; Kruparova,
Oksana; Kontar, Eduard P.; Balmaceda, Laura A.; Bonnin, Xavier; Bale,
Stuart D.; Pulupa, Marc; Malaspina, David M.; Bonnell, John W.;
Harvey, Peter R.; Goetz, Keith; Dudok de Wit, Thierry; MacDowall,
Robert J.; Kasper, Justin C.; Case, Anthony W.; Korreck, Kelly E.;
Larson, Davin E.; Livi, Roberto; Stevens, Michael L.; Whittlesey,
Phyllis L.; Hegedus, Alexander M.
2020ApJS..246...57K Altcode: 2020arXiv200103476K
Radio waves are strongly scattered in the solar wind, so that their
apparent sources seem to be considerably larger and shifted than the
actual ones. Since the scattering depends on the spectrum of density
turbulence, a better understanding of the radio wave propagation
provides indirect information on the relative density fluctuations,
$\epsilon =\langle \delta n\rangle /\langle n\rangle $ , at the
effective turbulence scale length. Here, we analyzed 30 type III
bursts detected by Parker Solar Probe (PSP). For the first time,
we retrieved type III burst decay times, ${\tau }_{{\rm{d}}}$ ,
between 1 and 10 MHz thanks to an unparalleled temporal resolution
of PSP. We observed a significant deviation in a power-law slope for
frequencies above 1 MHz when compared to previous measurements below
1 MHz by the twin-spacecraft Solar TErrestrial RElations Observatory
(STEREO) mission. We note that altitudes of radio bursts generated
at 1 MHz roughly coincide with an expected location of the Alfvén
point, where the solar wind becomes super-Alfvénic. By comparing
PSP observations and Monte Carlo simulations, we predict relative
density fluctuations, ɛ, at the effective turbulence scale length
at radial distances between 2.5 and 14 ${R}_{\odot }$ to range from
0.22 to 0.09. Finally, we calculated relative density fluctuations,
ɛ, measured in situ by PSP at a radial distance from the Sun of 35.7
${R}_{\odot }$ during perihelion #1, and perihelion #2 to be 0.07
and 0.06, respectively. It is in a very good agreement with previous
STEREO predictions ( $\epsilon =0.06\mbox{--}0.07$ ) obtained by remote
measurements of radio sources generated at this radial distance.
---------------------------------------------------------
Title: Density Fluctuations in the Solar Wind Deduced from Radio
Measurements by Parker Solar Probe
Authors: Krupar, V.; Szabo, A.; Maksimovic, M.; Kruparova, O.; Kontar,
E.; Bale, S. D.; Pulupa, M.; Malaspina, D.; Bonnell, J. W.; Harvey,
P.; Goetz, K.; Dudok de Wit, T.; MacDowall, R. J.; Kasper, J. C.;
Case, A. W.; Korreck, K. E.; Larson, D. E.; Livi, R.; Stevens, M. L.;
Whittlesey, P. L.; Hegedus, A. M.
2019AGUFMSH21C3322K Altcode:
Radio waves are strongly scattered in the solar wind, so that their
apparent sources seem to be considerably larger and shifted compared
to the actual sources. Since the effect of radio wave scattering
depends on the spectrum of density turbulence, better understanding
of the radio wave propagation provides indirect information on the
density fluctuations. Here, we have analyzed 30 type III bursts
detected by Parker Solar Probe to retrieve decay times as a function
of frequency. We observed a significant deviation for frequencies
above 1 MHz when compared to previous observations by the STEREO
spacecraft. Next, we performed Monte Carlo simulations to study the
role of scattering on time-frequency profiles of radio emissions. By
comparing Parker Solar Probe observations and Monte Carlo simulations
we predicted relative density fluctuations between 2.5 and 14 solar
radii. Finally, we calculated relative density fluctuations measured
in situ by Parker Solar Probe at characteristic scale times of Monte
Carlo simulations during the perihelion #1 and perihelion #2, and
compared them with STEREO predictions.
---------------------------------------------------------
Title: Global Energetics of Solar Flares and Coronal Mass Ejections
Authors: Aschwanden, Markus J.; Caspi, Amir; Cohen, Christina M. S.;
Holman, Gordon; Jing, Ju; Kretzschmar, Matthieu; Kontar, Eduard
P.; McTiernan, James M.; Mewaldt, Richard A.; O'Flannagain, Aidan;
Richardson, Ian G.; Ryan, Daniel; Warren, Harry P.; Xu, Yan
2019JPhCS1332a2002A Altcode:
We investigate the global energetics and energy closure of various
physical processes that are energetically important in solar flares
and coronal mass ejections (CMEs), which includes: magnetic energies,
thermal energies, nonthermal energies (particle acceleration),
direct and indirect plasma heating processes, kinetic CME energies,
gravitational CME energies, aerodynamic drag of CMEs, solar
energetic particle events, EUV and soft X-ray radiation, white-light,
and bolometric energies. Statistics on these forms of energies is
obtained from 400 GOES M- and X-class events during the first 3.5
years of the Solar Dynamics Observatory (SDO) mission. A primary
test addressed in this study is the closure of the various energies,
such as the equivalence of the dissipated magnetic energies and the
primary dissipated are energies (accelerated particles, direct heating,
CME acceleration), which faciliate the energy of secondary processes
(plasma heating, shock acceleration) and interactions with the solar
wind (aerodynamic drag). Our study demonstrates energy closure in the
statistical average, while individual events may have considerable
uncertainties, requiring improved nonlinear force-free field models,
and particle acceleration models with observationally constrained
low-energy cutoffs.
---------------------------------------------------------
Title: On the Source Position and Duration of a Solar Type III Radio
Burst Observed by LOFAR
Authors: Zhang, PeiJin; Yu, SiJie; Kontar, Eduard P.; Wang, ChuanBing
2019ApJ...885..140Z Altcode: 2019arXiv190908773Z
The flux of solar type III radio bursts have a time profile of rising
and decay phases at a given frequency, which has been actively studied
since the 1970s. Several factors that may influence the duration of a
type III radio burst have been proposed. In this work, to study the
dominant cause of the duration, we investigate the source positions
of the front edge, the peak, and the tail edge in the dynamic spectrum
of a single and clear type III radio burst. The duration of this type
III burst at a given frequency is about 3 s for decameter wave. The
beam-formed observations by the LOw-Frequency ARray are used, which can
provide the radio source positions and the dynamic spectra at the same
time. We find that, for this burst, the source positions of the front
edge, the peak, and the tail edge split with each other spatially. The
radial speed of the electrons exciting the front edge, the peak, and
the tail edge is 0.42c, 0.25c, and 0.16c, respectively. We estimate the
influences of the corona density fluctuation and the electron velocity
dispersion on the duration, and the scattering effect by comparison
with a few short-duration bursts from the same region. The analysis
yields that, in the frequency range of 30-41 MHz, the electron velocity
dispersion is the dominant factor that determines the time duration
of type III radio bursts with long duration, while scattering may play
an important role in the duration of short bursts.
---------------------------------------------------------
Title: First imaging spectroscopy observations of solar drift
pair bursts
Authors: Kuznetsov, A. A.; Kontar, E. P.
2019A&A...631L...7K Altcode: 2019arXiv191009864K
Drift pairs are an unusual and puzzling type of fine structure sometimes
observed in dynamic spectra of solar radio emission. They appear as
two identical short narrowband drifting stripes separated in time; both
positive and negative frequency drifts are observed. Currently, due to
the lack of imaging observations, there is no satisfactory explanation
for this phenomenon. Using the Low Frequency Array (LOFAR), we report
unique observations of a cluster of drift pair bursts in the frequency
range of 30-70 MHz made on 12 July 2017. Spectral imaging capabilities
of the instrument have allowed us for the first time to resolve the
temporal and frequency evolution of the source locations and sizes at
a fixed frequency and along the drifting pair components. Sources of
two components of a drift pair have been imaged and found to propagate
in the same direction along nearly the same trajectories. Motion of
the second component source is seen to be delayed in time with respect
to that of the first one. The source trajectories can be complicated
and non-radial; positive and negative frequency drifts correspond to
opposite propagation directions. The drift pair bursts with positive
and negative frequency drifts, as well as the associated broadband
type-III-like bursts, are produced in the same regions. The visible
source velocities are variable from zero to a few 10<SUP>4</SUP> (up to
∼10<SUP>5</SUP>) km s<SUP>-1</SUP>, which often exceeds the velocities
inferred from the drift rate (∼10<SUP>4</SUP> km s<SUP>-1</SUP>). The
visible source sizes are of about 10'-18'; they are more compact than
typical type III sources at the same frequencies. The existing models of
drift pair bursts cannot adequately explain the observed features. We
discuss the key issues that need to be addressed, and in particular
the anisotropic scattering of the radio waves. The broadband bursts
observed simultaneously with the drift pairs differ in some aspects from
common type III bursts and may represent a separate type of emission.
---------------------------------------------------------
Title: Anisotropic Radio-wave Scattering and the Interpretation of
Solar Radio Emission Observations
Authors: Kontar, Eduard P.; Chen, Xingyao; Chrysaphi, Nicolina;
Jeffrey, Natasha L. S.; Emslie, A. Gordon; Krupar, Vratislav;
Maksimovic, Milan; Gordovskyy, Mykola; Browning, Philippa K.
2019ApJ...884..122K Altcode: 2019arXiv190900340K
The observed properties (i.e., source size, source position, time
duration, and decay time) of solar radio emission produced through
plasma processes near the local plasma frequency, and hence the
interpretation of solar radio bursts, are strongly influenced by
propagation effects in the inhomogeneous turbulent solar corona. In
this work, a 3D stochastic description of the propagation process
is presented, based on the Fokker-Planck and Langevin equations
of radio-wave transport in a medium containing anisotropic electron
density fluctuations. Using a numerical treatment based on this model,
we investigate the characteristic source sizes and burst decay times
for Type III solar radio bursts. Comparison of the simulations with
the observations of solar radio bursts shows that predominantly
perpendicular density fluctuations in the solar corona are required,
with an anisotropy factor of ∼0.3 for sources observed at around 30
MHz. The simulations also demonstrate that the photons are isotropized
near the region of primary emission, but the waves are then focused by
large-scale refraction, leading to plasma radio emission directivity
that is characterized by a half width at half maximum of about 40°
near 30 MHz. The results are applicable to various solar radio bursts
produced via plasma emission.
---------------------------------------------------------
Title: Coronal Loop Scaling Laws for Various Forms of Parallel
Heat Conduction
Authors: Bradshaw, Stephen J.; Emslie, A. Gordon; Bian, N. H.; Kontar,
Eduard P.
2019ApJ...880...80B Altcode: 2019arXiv190603332B
The solar atmosphere is dominated by loops of magnetic fluxes
that connect the multi-million degree corona to the much cooler
chromosphere. The temperature and density structure of quasi-static
loops are determined by the continuous flow of energy from the
hot corona to the lower solar atmosphere. Loop scaling laws provide
relationships between global properties of the loop (such as the peak
temperature, pressure, and length); they follow from the physical
variable dependencies of various terms in the energy equation, and,
hence, the form of the loop scaling law provides insight into the
key physics that control the loop structure. Traditionally, scaling
laws have been derived under the assumption of collision-dominated
thermal conduction. Here, we examine the impact of different regimes
of thermal conduction—collision-dominated, turbulence-dominated,
and free-streaming—on the form of the scaling laws relating the loop
temperature and heating rate to its pressure and half-length. We
show that the scaling laws for turbulence-dominated conduction
are fundamentally different than those for collision-dominated and
free-streaming conduction, inasmuch as the form of the scaling laws
now depend primarily on conditions at the low-temperature, rather than
high-temperature, part of the loop. We also establish regimes in the
temperature and density space in which each of the applicable scaling
laws prevail.
---------------------------------------------------------
Title: Global Energetics of Solar Flares. VIII. The Low-energy Cutoff
Authors: Aschwanden, Markus J.; Kontar, Eduard P.; Jeffrey, Natasha
L. S.
2019ApJ...881....1A Altcode: 2019arXiv190605835A
One of the key problems in solar flare physics is the determination
of the low-energy cut-off: the value that determines the energy of
nonthermal electrons and hence flare energetics. We discuss different
approaches to determine the low-energy cut-off in the spectrum of
accelerated electrons: (i) the total electron number model, (ii) the
time-of-flight model (based on the equivalence of the time-of-flight
and the collisional deflection time), (iii) the warm target model of
Kontar et al., and (iv) the model of the spectral cross-over between
thermal and nonthermal components. We find that the first three models
are consistent with a low-energy cutoff with a mean value of ≈10 keV,
while the cross-over model provides an upper limit for the low-energy
cutoff with a mean value of ≈21 keV. Combining the first three models
we find that the ratio of the nonthermal energy to the dissipated
magnetic energy in solar flares has a mean value of q <SUB> E </SUB>
= 0.57 ± 0.08, which is consistent with an earlier study based on
the simplified approximation of the warm target model alone (q <SUB>
E </SUB> = 0.51 ± 0.17). This study corroborates the self-consistency
between three different low-energy cutoff models in the calculation
of nonthermal flare energies.
---------------------------------------------------------
Title: The Role of Energy Diffusion in the Deposition of Energetic
Electron Energy in Solar and Stellar Flares
Authors: Jeffrey, Natasha L. S.; Kontar, Eduard P.; Fletcher, Lyndsay
2019ApJ...880..136J Altcode: 2019arXiv190601887J
During solar flares, a large fraction of the released magnetic energy
is carried by energetic electrons that transfer and deposit energy
in the Sun’s atmosphere. Electron transport is often approximated
by a cold thick-target model, assuming that electron energy is much
larger than the temperature of the ambient plasma, and electron energy
evolution is modeled as a systematic loss. Using kinetic modeling
of electrons, we reevaluate the transport and deposition of flare
energy. Using a full collisional warm-target model (WTM), we account
for electron thermalization and for the properties of the ambient
coronal plasma such as its number density, temperature and spatial
extent. We show that the deposition of nonthermal electron energy in the
lower atmosphere is highly dependent on the properties of the flaring
coronal plasma. In general, thermalization and a reduced WTM energy
loss rate leads to an increase of nonthermal energy transferred to
the chromosphere, and the deposition of nonthermal energy at greater
depths. The simulations show that energy is deposited in the lower
atmosphere initially by high-energy nonthermal electrons, and later by
lower energy nonthermal electrons that partially or fully thermalize
in the corona, over timescales of seconds, unaccounted for in previous
studies. This delayed heating may act as a diagnostic of both the
injected nonthermal electron distribution and the coronal plasma,
vital for constraining flare energetics.
---------------------------------------------------------
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: Warm-Target Modeling and a Solution to The Low-energy Cut-off
Problem
Authors: Emslie, A. Gordon; Kontar, Eduard; Jeffrey, Natasha L. S.
2019AAS...23422506E Altcode:
Solar flare hard X-ray (HXR) spectroscopy serves as a key diagnostic
of the accelerated electron spectrum. However, the standard approach
using the collisional cold thick-target model poorly constrains the
lower-energy part of the accelerated electron spectrum, hence the
overall energetics of the accelerated electrons and consequently the
flare energetics are typically constrained only to within one or two
orders of magnitude. I will discuss the development and application of
a physically self-consistent, warm-target approach that involves the
use of both HXR spectroscopy and imaging data. The approach allows
an accurate determination of the electron distribution low-energy
cutoff, and hence the electron acceleration rate and the contribution
of accelerated electrons to the total energy released, by constraining
the coronal plasma parameters. Using a solar flare observed in X-rays by
RHESSI, we demonstrate that using the standard cold-target methodology,
the low-energy cutoff (hence the energy content in electrons) is
essentially undetermined. However, the warm-target methodology can
determine the low-energy electron cutoff with ∼7% uncertainty at
the 3σ level, hence it permits an accurate quantitative study of the
importance of accelerated electrons in solar flare energetics.
---------------------------------------------------------
Title: A Fokker-Planck Framework for Studying the Diffusion of Radio
Burst Waves in the Solar Corona
Authors: Bian, N. H.; Emslie, A. G.; Kontar, E. P.
2019ApJ...873...33B Altcode: 2019arXiv190200239B
Electromagnetic wave scattering off density inhomogeneities in the solar
corona is an important process that determines both the apparent source
size and the time profile of radio bursts observed at 1 au. Here we
model the scattering process using a Fokker-Planck equation and apply
this formalism to several regimes of interest. In the first regime
the density fluctuations are considered quasi-static and diffusion
in wavevector space is dominated by angular diffusion on the surface
of a constant energy sphere. In the small-angle (“pencil-beam”)
approximation, this diffusion further occurs over a small solid angle
in wavevector space. The second regime corresponds to a much later time,
by which scattering has rendered the photon distribution near-isotropic,
resulting in a spatial diffusion of the radiation. The third regime
involves time-dependent fluctuations and, therefore, Fermi acceleration
of photons. Combined, these results provide a comprehensive theoretical
framework within which to understand several important features of
propagation of radio burst waves in the solar corona: emitted photons
are accelerated in a relatively small inner region and then diffuse
outward to larger distances. En route, angular diffusion results both
in source sizes that are substantially larger than the intrinsic source
and in observed intensity-versus-time profiles that are asymmetric,
with a sharp rise and an exponential decay. Both of these features
are consistent with observations of solar radio bursts.
---------------------------------------------------------
Title: Frequency-Distance Structure of Solar Radio Sources Observed
by LOFAR
Authors: Gordovskyy, Mykola; Kontar, Eduard; Browning, Philippa;
Kuznetsov, Alexey
2019ApJ...873...48G Altcode:
Low-frequency radio observations make it possible to study the
solar corona at distances up to 2-3 R <SUB>⊙</SUB>. Frequency of
plasma emission is a proxy for electron density of the emitting
plasma and, therefore, observations of solar radio bursts can be
used to probe the density structure of the outer corona. In this
study, positions of solar radio sources are investigated using the
Low-Frequency Array (LOFAR) spectral imaging in the frequency range
30-50 MHz. We show that there are events where apparent positions
of the radio sources cannot be explained using the standard coronal
density models. Namely, the apparent heliocentric positions of the
sources are 0.1-0.7 R <SUB>⊙</SUB> further from the Sun compared
with the positions predicted by the Newkirk model, and these shifts
are frequency-dependent. We discuss several possible explanations for
this effect, including enhanced plasma density in the flaring corona,
as well as scattering and refraction of the radio waves.
---------------------------------------------------------
Title: Electron Distribution and Energy Release in Magnetic
Reconnection Outflow Regions during the Pre-impulsive Phase of a
Solar Flare
Authors: Battaglia, Marina; Kontar, Eduard P.; Motorina, Galina
2019ApJ...872..204B Altcode: 2019arXiv190107767B
We present observations of electron energization in magnetic
reconnection outflows during the pre-impulsive phase of solar flare
SOL2012-07-19T05:58. During a time-interval of about 20 minutes,
starting 40 minutes before the onset of the impulsive phase, two
X-ray sources were observed in the corona, one above the presumed
reconnection region and one below. For both of these sources, the mean
electron distribution function as a function of time is determined over
an energy range from 0.1 keV up to several tens of keV, for the first
time. This is done by simultaneous forward fitting of X-ray and extreme
ultraviolet (EUV) data. Imaging spectroscopy with RHESSI provides
information on the high-energy tail of the electron distribution in
these sources while EUV images from SDO/Atmospheric Imaging Assembly
are used to constrain the low specific electron energies. The measured
electron distribution spectrum in the magnetic reconnection outflows
is consistent with a time-evolving kappa-distribution with κ =
3.5-5.5. The spectral evolution suggests that electrons are accelerated
to progressively higher energies in the source above the reconnection
region, while in the source below, the spectral shape does not change
but an overall increase of the emission measure is observed, suggesting
density increase due to evaporation. The main mechanisms by which
energy is transported away from the source regions are conduction and
free-streaming electrons. The latter dominates by more than one order
of magnitude and is comparable to typical nonthermal energies during
the hard X-ray peak of solar flares, suggesting efficient acceleration
even during this early phase of the event.
---------------------------------------------------------
Title: Preface: Solar physics advances from the interior to the
heliosphere
Authors: Georgoulis, Manolis K.; Kontar, Eduard P.
2019AdSpR..63.1387G Altcode:
Solar Physics has been experiencing a golden era of unprecedented
observations and voluminous data for nearly three decades. While
much of the progress in the 1990s and 2000s was spurred by flagship
space missions, the latest decade has also seen the culmination
of game-changing ground-based facilities, long sought after by the
community. Observing the Sun from space has indisputable benefits;
however, space missions have a relatively limited lifespan, mainly
because of the unforgiving deep space or orbital conditions and our
limited ability to maintain them after launch. This is not the case
for ground-based facilities that can, in principle, serve and train
entire generations of researchers.
---------------------------------------------------------
Title: Determination of the Total Accelerated Electron Rate and
Power Using Solar Flare Hard X-Ray Spectra
Authors: Kontar, Eduard P.; Jeffrey, Natasha L. S.; Emslie, A. Gordon
2019ApJ...871..225K Altcode: 2018arXiv181209474K
Solar flare hard X-ray (HXR) spectroscopy serves as a key diagnostic
of the accelerated electron spectrum. However, the standard approach
using the collisional cold thick-target model poorly constrains
the lower-energy part of the accelerated electron spectrum, hence
the overall energetics of the accelerated electrons are typically
constrained only to within one or two orders of magnitude. Here, we
develop and apply a physically self-consistent, warm-target approach
that involves the use of both HXR spectroscopy and imaging data. This
approach allows an accurate determination of the electron distribution
low-energy cutoff, and hence the electron acceleration rate and the
contribution of accelerated electrons to the total energy released,
by constraining the coronal plasma parameters. Using a solar flare
observed in X-rays by RHESSI, we demonstrate that using the standard
cold-target methodology, the low-energy cutoff (hence the energy
content in electrons) is essentially undetermined. However, the
warm-target methodology can determine the low-energy electron cutoff
with ∼7% uncertainty at the 3σ level, hence it permits an accurate
quantitative study of the importance of accelerated electrons in solar
flare energetics.
---------------------------------------------------------
Title: Solar physics with the Square Kilometre Array
Authors: Nindos, A.; Kontar, E. P.; Oberoi, D.
2019AdSpR..63.1404N Altcode: 2018arXiv181004951N
The Square Kilometre Array (SKA) will be the largest radio
telescope ever built, aiming to provide collecting area larger than 1
km<SUP>2</SUP>. The SKA will have two independent instruments, SKA-LOW
comprising of dipoles organized as aperture arrays in Australia and
SKA-MID comprising of dishes in South Africa. Currently the phase-1 of
SKA, referred to as SKA1, is in its late design stage and construction
is expected to start in 2020. Both SKA1-LOW (frequency range of 50-350
MHz) and SKA1-MID Bands 1, 2, and 5 (frequency ranges of 350-1050,
950-1760, and 4600-15,300 MHz, respectively) are important for solar
observations. In this paper we present SKA's unique capabilities
in terms of spatial, spectral, and temporal resolution, as well as
sensitivity and show that they have the potential to provide major new
insights in solar physics topics of capital importance including (i) the
structure and evolution of the solar corona, (ii) coronal heating, (iii)
solar flare dynamics including particle acceleration and transport,
(iv) the dynamics and structure of coronal mass ejections, and (v)
the solar aspects of space weather. Observations of the Sun jointly
with the new generation of ground-based and space-borne instruments
promise unprecedented discoveries.
---------------------------------------------------------
Title: CME-driven Shock and Type II Solar Radio Burst Band Splitting
Authors: Chrysaphi, Nicolina; Kontar, Eduard P.; Holman, Gordon D.;
Temmer, Manuela
2018ApJ...868...79C Altcode: 2018arXiv181008026C
Coronal mass ejections (CMEs) are believed to be effective in
producing shocks in the solar corona and interplanetary space. One of
the important signatures of shocks and shock acceleration are Type II
solar radio bursts that drift with the shock speed and produce bands of
fundamental and higher harmonic plasma radio emission. An intriguing
aspect of Type II radio bursts is the occasional split of a harmonic
band into thinner lanes, known as band splitting. Here we report a
detailed imaging and spectroscopic observation of a CME-driven shock
producing band splitting in a Type II burst. Using LOFAR, we examine the
spatial and temporal relation of the Type II burst to the associated
CME event, use source imaging to calculate the apparent coronal
density, and demonstrate how source imaging can be used to estimate
projection effects. We consider two widely accepted band-splitting
models that make opposing predictions regarding the locations of the
true emission sources with respect to the shock front. Our observations
suggest that the locations of the upper and lower subband sources
are spatially separated by ∼0.2 ± 0.05 R <SUB>⊙</SUB>. However,
we quantitatively show, for the first time, that such separation is
consistent with radio-wave scattering of plasma radio emission from
a single region, implying that the split-band Type II sources could
originate from nearly cospatial locations. Considering the effects
of scattering, the observations provide supporting evidence for the
model that interprets the band splitting as emission originating in
the upstream and downstream regions of the shock front, two virtually
cospatial areas.
---------------------------------------------------------
Title: Frequency rising sub-THz emission from solar flare ribbons
Authors: Kontar, E. P.; Motorina, G. G.; Jeffrey, N. L. S.; Tsap,
Y. T.; Fleishman, G. D.; Stepanov, A. V.
2018A&A...620A..95K Altcode: 2018arXiv181003922K
Observations of solar flares at sub-THz frequencies (millimetre
and sub-millimetre wavelengths) over the last two decades often
show a spectral component rising with frequency. Unlike a typical
gyrosynchrotron spectrum decreasing with frequency or a weak thermal
component from hot coronal plasma, the observations can demonstrate a
high flux level (up to ∼10<SUP>4</SUP> solar flux units at 0.4 THz)
and fast variability on sub-second timescales. Although, many models
have been put forward to explain the puzzling observations, none of them
has clear observational support. Here we propose a scenario to explain
the intriguing sub-THz observations. We show that the model, based on
free-free emission from the plasma of flare ribbons at temperatures
10<SUP>4</SUP> - 10<SUP>6</SUP> K, is consistent with all existing
observations of frequency-rising sub-THz flare emission. The model
provides a temperature diagnostic of the flaring chromosphere and
suggests fast heating and cooling of the dense transition region plasma.
---------------------------------------------------------
Title: Spatial Expansion and Speeds of Type III Electron Beam Sources
in the Solar Corona
Authors: Reid, Hamish A. S.; Kontar, Eduard P.
2018ApJ...867..158R Altcode: 2018arXiv180900887R
A component of space weather, electron beams are routinely accelerated
in the solar atmosphere and propagate through interplanetary
space. Electron beams interact with Langmuir waves resulting in type
III radio bursts. They expand along the trajectory and, using kinetic
simulations, we explore the expansion as the electrons propagate
away from the Sun. Specifically, we investigate the front, peak,
and back of the electron beam in space from derived radio brightness
temperatures of fundamental type III emission. The front of the
electron beam travels at speeds from 0.2c to 0.7c, significantly
faster than the back of the beam, which travels at speeds between
0.12c and 0.35c. The difference in speed between the front and the
back elongates the electron beam in time. The rate of beam elongation
has a 0.98 correlation coefficient with the peak velocity, in line
with predictions from type III observations. The inferred speeds of
electron beams initially increase close to the acceleration region
and then decrease through the solar corona. Larger starting densities
and harder initial spectral indices result in longer and faster type
III sources. Faster electron beams have higher beam energy densities,
and produce type IIIs with higher peak brightness temperatures and
shorter FWHM durations. Higher background plasma temperatures also
increase speed, particularly at the back of the beam. We show how our
predictions of electron beam evolution influences type III bandwidth and
drift rates. Our radial predictions of electron beam speed and expansion
can be tested by the upcoming in situ electron beam measurements made
by Solar Orbiter and Parker Solar Probe.
---------------------------------------------------------
Title: Energy Deposition by Energetic Electrons in a Diffusive
Collisional Transport Model
Authors: Emslie, A. Gordon; Bian, Nicolas H.; Kontar, Eduard P.
2018ApJ...862..158E Altcode: 2018arXiv180608158E
A considerable fraction of the energy in a solar flare is released
as suprathermal electrons; such electrons play a major role in energy
deposition in the ambient atmosphere, and hence the atmospheric response
to flare heating. Historically, the transport of these particles has
been approximated through a deterministic approach in which first-order
secular energy loss to electrons in the ambient target is treated
as the dominant effect, with second-order diffusive terms (in both
energy and angle) being generally either treated as a small correction
or neglected. However, it has recently been pointed out that while
neglect of diffusion in energy may indeed be negligible, diffusion
in angle is of the same order as deterministic scattering and hence
must be included. Here we therefore investigate the effect of angular
scattering on the energy deposition profile in the flaring atmosphere. A
relatively simple compact expression for the spatial distribution of
energy deposition into the ambient plasma is presented and compared with
the corresponding deterministic result. For unidirectional injection
there is a significant shift in heating from the lower corona to the
upper corona; this shift is much smaller for isotropic injection. We
also compare the heating profiles due to return current ohmic heating
in the diffusional and deterministic models.
---------------------------------------------------------
Title: LOFAR Observations of Fine Spectral Structure Dynamics in
Type IIIb Radio Bursts
Authors: Sharykin, I. N.; Kontar, E. P.; Kuznetsov, A. A.
2018SoPh..293..115S Altcode: 2018arXiv180601046S
Solar radio emission features a large number of fine structures
demonstrating great variability in frequency and time. We present
spatially resolved spectral radio observations of type IIIb bursts in
the 30 - 80 MHz range made by the Low Frequency Array (LOFAR). The
bursts show well-defined fine frequency structuring called "stria"
bursts. The spatial characteristics of the stria sources are
determined by the propagation effects of radio waves; their movement
and expansion speeds are in the range of (0.1 -0.6 )c . Analysis
of the dynamic spectra reveals that both the spectral bandwidth and
the frequency drift rate of the striae increase with an increase of
their central frequency. The striae bandwidths are in the range of
≈(20 -100 ) kHz and the striae drift rates vary from zero to ≈0.3
MHzs−<SUP>1</SUP>. The observed spectral characteristics of the
stria bursts are consistent with the model involving modulation of
the type III burst emission mechanism by small-amplitude fluctuations
of the plasma density along the electron beam path. We estimate that
the relative amplitude of the density fluctuations is of Δ n /n
∼10<SUP>−3</SUP>, their characteristic length scale is less than
1000 km, and the characteristic propagation speed is in the range
of 400 -800 kms−<SUP>1</SUP>. These parameters indicate that the
observed fine spectral structures could be produced by propagating
magnetohydrodynamic waves.
---------------------------------------------------------
Title: Shock location and CME 3D reconstruction of a solar type II
radio burst with LOFAR
Authors: Zucca, P.; Morosan, D. E.; Rouillard, A. P.; Fallows, R.;
Gallagher, P. T.; Magdalenic, J.; Klein, K. -L.; Mann, G.; Vocks, C.;
Carley, E. P.; Bisi, M. M.; Kontar, E. P.; Rothkaehl, H.; Dabrowski,
B.; Krankowski, A.; Anderson, J.; Asgekar, A.; Bell, M. E.; Bentum,
M. J.; Best, P.; Blaauw, R.; Breitling, F.; Broderick, J. W.; Brouw,
W. N.; Brüggen, M.; Butcher, H. R.; Ciardi, B.; de Geus, E.; Deller,
A.; Duscha, S.; Eislöffel, J.; Garrett, M. A.; Grießmeier, J. M.;
Gunst, A. W.; Heald, G.; Hoeft, M.; Hörandel, J.; Iacobelli, M.;
Juette, E.; Karastergiou, A.; van Leeuwen, J.; McKay-Bukowski, D.;
Mulder, H.; Munk, H.; Nelles, A.; Orru, E.; Paas, H.; Pandey, V. N.;
Pekal, R.; Pizzo, R.; Polatidis, A. G.; Reich, W.; Rowlinson, A.;
Schwarz, D. J.; Shulevski, A.; Sluman, J.; Smirnov, O.; Sobey, C.;
Soida, M.; Thoudam, S.; Toribio, M. C.; Vermeulen, R.; van Weeren,
R. J.; Wucknitz, O.; Zarka, P.
2018A&A...615A..89Z Altcode: 2018arXiv180401025Z
Context. Type II radio bursts are evidence of shocks in the solar
atmosphere and inner heliosphere that emit radio waves ranging from
sub-meter to kilometer lengths. These shocks may be associated with
coronal mass ejections (CMEs) and reach speeds higher than the
local magnetosonic speed. Radio imaging of decameter wavelengths
(20-90 MHz) is now possible with the Low Frequency Array (LOFAR),
opening a new radio window in which to study coronal shocks that
leave the inner solar corona and enter the interplanetary medium and
to understand their association with CMEs. <BR /> Aims: To this end,
we study a coronal shock associated with a CME and type II radio burst
to determine the locations at which the radio emission is generated,
and we investigate the origin of the band-splitting phenomenon. <BR />
Methods: Thetype II shock source-positions and spectra were obtained
using 91 simultaneous tied-array beams of LOFAR, and the CME was
observed by the Large Angle and Spectrometric Coronagraph (LASCO) on
board the Solar and Heliospheric Observatory (SOHO) and by the COR2A
coronagraph of the SECCHI instruments on board the Solar Terrestrial
Relation Observatory(STEREO). The 3D structure was inferred using
triangulation of the coronographic observations. Coronal magnetic
fields were obtained from a 3D magnetohydrodynamics (MHD) polytropic
model using the photospheric fields measured by the Heliospheric
Imager (HMI) on board the Solar Dynamic Observatory (SDO) as lower
boundary. <BR /> Results: The type II radio source of the coronal shock
observed between 50 and 70 MHz was found to be located at the expanding
flank of the CME, where the shock geometry is quasi-perpendicular with
θ<SUB>Bn</SUB> 70°. The type II radio burst showed first and second
harmonic emission; the second harmonic source was cospatial with the
first harmonic source to within the observational uncertainty. This
suggests that radio wave propagation does not alter the apparent
location of the harmonic source. The sources of the two split bands
were also found to be cospatial within the observational uncertainty,
in agreement with the interpretation that split bands are simultaneous
radio emission from upstream and downstream of the shock front. The
fast magnetosonic Mach number derived from this interpretation was
found to lie in the range 1.3-1.5. The fast magnetosonic Mach numbers
derived from modelling the CME and the coronal magnetic field around
the type II source were found to lie in the range 1.4-1.6.
---------------------------------------------------------
Title: Origin of the Modulation of the Radio Emission from the Solar
Corona by a Fast Magnetoacoustic Wave
Authors: Kolotkov, Dmitrii Y.; Nakariakov, Valery M.; Kontar, Eduard P.
2018ApJ...861...33K Altcode: 2018arXiv180508282K
Observational detection of quasi-periodic drifting fine structures in a
type III radio burst associated with a solar flare SOL2015-04-16T11:22,
with the LOw Frequency ARray (LOFAR), is presented. Although similar
modulations of the type III emission have been observed before and
were associated with the plasma density fluctuations, the origin
of those fluctuations was unknown. Analysis of the striae of the
intensity variation in the dynamic spectrum allowed us to reveal two
quasi-oscillatory components. The shorter component has an apparent
wavelength of ∼2 Mm, phase speed of ∼657 km s<SUP>-1</SUP>, which
gives an oscillation period of ∼3 s, and a relative amplitude of
∼0.35%. The longer component has a wavelength of ∼12 Mm and relative
amplitude of ∼5.1%. The short frequency range of the detection does
not allow us to estimate its phase speed. However, the properties of
the shorter oscillatory component allowed us to interpret it as a fast
magnetoacoustic wave guided by a plasma nonuniformity along the magnetic
field outwards from the Sun. The assumption that the intensity of the
radio emission is proportional to the amount of plasma in the emitting
volume allowed us to show that the superposition of the plasma density
modulation by a fast wave and a longer-wavelength oscillation of an
unspecified nature could readily reproduce the fine structure of the
observed dynamic spectrum. The observed parameters of the fast wave
give an absolute value for the magnetic field in the emitting plasma
of ∼1.1 G, which is consistent with the radial magnetic field model.
---------------------------------------------------------
Title: Radio wave propagation in the solar corona:
high-time-resolution with LOFAR.
Authors: Bian, Nicolas; Kontar, Eduard; Kuznetsov, Alexey; Subramanian,
Prasad; Jeffrey, Natasha; A. Gordon Emslie, ., , Dr; Alcock, Benjamin;
Yu, Sijie; Melnik, Valentin
2018cosp...42E.341B Altcode:
Using the Low Frequency Array (LOFAR), we analyse the source sizes
and locations of the fine frequency structuresin a solar radio
burst. The high time resolution allows us to determine the location
and the size of the radioemission source, and its evolution with time,
following the radio emission propagation through the solar corona.It
is found that intrinsically very small radio sources have an apparent
size that is a thousand times larger thanthe actual region where the
radio waves originate [1]. The observations suggest that it is radio
wave propagationeffects, rather than the intrinsic properties of the
emission source, that determine the observed spatial characteristicsof
the plasma emission radio bursts. In addition, the observations provide
a new opportunity for diagnostics ofsmall-scale plasma fluctuations by
imaging the radio source halos as the radio waves move in the solar
corona.[1] Kontar et al.: Imaging Spectroscopy of Solar Radio Burst
Fine Structures, Nature Communications 8, Articlenumber: 1515 (2017)
DOI: 10.1038/s41467-017-01307-8
---------------------------------------------------------
Title: Particle acceleration and transport in the solar atmosphere
Authors: Kontar, Eduard
2018cosp...42E1811K Altcode:
During periods of sporadic are activity, the Sun releases energy stored
in the magnetic field into the plasma of the solar atmosphere. This
is an extremely efficient process, with a large fraction of the
magnetic energy going into plasma particles. The solar ares are
accompanied by prompt electromagnetic emission virtually over the entire
electromagnetic spectrum from gamma-raysdown to radio frequencies. The
Sun, through its activity, also plays a driving role in the Sun-Earth
system that substantially influences geophysical space. Solar are
energetic particles from the Sun are detected in interplanetary
space by in-situ measurements making them a vitalcomponent of
the single Sun-Earth system. Although a qualitative picture is
generally agreed upon, many processes solar are processes are poorly
understood. Specifically, the processes of acceleration and propagation
of energetic particles interacting on various physical scales remain
major challenges in solar physics and basic plasma physics. In the
talk, I will review the current understanding of solar flare energetic
particles focusing on recent observational progress in the view of
Solar Orbiter and Praker Solar Probe.
---------------------------------------------------------
Title: Solar type III radio burst time characteristics at LOFAR
frequencies and the implications for electron beam transport
Authors: Reid, Hamish A. S.; Kontar, Eduard P.
2018A&A...614A..69R Altcode: 2018arXiv180201507R
Context. Solar type III radio bursts contain a wealth of information
about the dynamics of electron beams in the solar corona and the inner
heliosphere; this information is currently unobtainable through other
means. However, the motion of different regions of an electron beam
(front, middle, and back) have never been systematically analysed
before. <BR /> Aims: We characterise the type III burst frequency-time
evolution using the enhanced resolution of LOFAR (LOw Frequency ARray)
in the frequency range 30-70 MHz and use this to probe electron beam
dynamics. <BR /> Methods: The rise, peak, and decay times with a 0.2
MHz spectral resolution were defined for a collection of 31 type III
bursts. The frequency evolution was used to ascertain the apparent
velocities of the front, middle, and back of the type III sources,
and the trends were interpreted using theoretical and numerical
treatments. <BR /> Results: The type III time profile was better
approximated by an asymmetric Gaussian profile and not an exponential,
as was used previously. Rise and decay times increased with decreasing
frequency and showed a strong correlation. Durations were shorter
than previously observed. Drift rates from the rise times were
faster than from the decay times, corresponding to inferred mean
electron beam speeds for the front, middle, and back of 0.2, 0.17,
0.15 c, respectively. Faster beam speeds correlate with shorter type
III durations. We also find that the type III frequency bandwidth
decreases as frequency decreases. <BR /> Conclusions: The different
speeds naturally explain the elongation of an electron beam in space
as it propagates through the heliosphere. The expansion rate is
proportional to the mean speed of the exciter; faster beams expand
faster. Beam speeds are attributed to varying ensembles of electron
energies at the front, middle, and back of the beam.
---------------------------------------------------------
Title: Combined Radio and Space-Based Solar Observations: From
Techniques to New Results - Preface
Authors: Kontar, Eduard P.; Nindos, Alexander
2018SoPh..293...90K Altcode: 2018arXiv180605919K
The phenomena observed at the Sun have a variety of unique radio
signatures that can be used to diagnose the processes in the solar
atmosphere. The insights provided by radio observations are further
enhanced when they are combined with observations from space-based
telescopes. This Topical collection demonstrates the power of
combination methodology at work and provides new results on i) type I
solar radio bursts and thermal emission to study active regions; ii)
type II and IV bursts to better understand the structure of coronal
mass ejections; and iii) non-thermal gyro-synchrotron and/or type
III bursts to improve the characterisation of particle acceleration
in solar flares. The ongoing improvements in time, frequency, and
spatial resolutions of ground-based telescopes reveal new levels in
the complexity of solar phenomena and pose new questions.
---------------------------------------------------------
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: Non-Local Heat Conduction Effects in Active Region, Flaring
and Post-Flare Loops
Authors: Emslie, Gordon; Bian, Nicolas Horace; Kontar, Eduard
2018tess.conf11401E Altcode:
It is, of course, a given that gradients of electron temperature lead
to the transport of energy by heat conduction. However, solution of
the pertinent electron-transport Fokker-Planck diffusion equation
via a Legendre polynomial expansion, or equivalently via a continuous
time random walk analysis, shows that in general the heat flux at a
particular point in space is determined not just by the temperature
gradient at that point, but rather by a convolution of the temperature
gradient over a finite region with a kernel that has the form of
a bi-exponential function. Somewhat surprisingly, the kernel has a
characteristic width equal to several (approximately 7) mean free paths,
so that nonlocal effects can be important even in situations where the
temperature scale length is quite long compared to the collisional
mean free path. We explore the consequences of non-local effects in
determining the temperature profiles of static coronal loops and in
modelling the transport of thermal energy in flaring and post-flare
loops.
---------------------------------------------------------
Title: Fokker-Planck modelling of coronal scattering of radiation
from solar radio sources
Authors: Bian, Nicolas Horace; Emslie, Gordon; Kontar, Eduard
2018tess.conf11406B Altcode:
It has long been known that owing to turbulent scattering by density
irregularities a radio point source in the solar corona is observed
to have a finite angular extent. Recent high-time-resolution imaging
spectroscopy of type III bursts with LOFAR show that the role
wave transport effects in determining the observed spatiotemporal
characteristics of solar radio bursts is as important as, or even more
important than, the properties of the intrinsic source. In this work,
we model turbulent scattering of a radio wave. Quasilinear theory
is applied to obtain the diffusion tensor in wavevector space in
the presence of a Kolmogorov spectrum of density fluctuations. The
diffusion tensor is shown to take a Lorentz form familiar from plasma
kinetic theory. Accordingly, the photon distribution function, in both
space and momentum space, evolves in time according to a Fokker-Planck
equation. This Fokker-Planck equation is solved analytically in the
forward scattering regime from the source to the observer, assuming
negligible back-scattering of rays along their paths, and the results
used to determine forms for the angular broadening and arrival time
profiles of the scattered radiation. The arrival time profile form,
which has in the past been computed via pure probabilistic arguments, is
instead derived in an analytic form amenable to universal scaling. The
method is finally compared with the standard Gaussian or "two-moments"
closure of the Fokker-Planck equation and to numerical Monte-Carlo
simulations of this equation in the forward scattering regime.
---------------------------------------------------------
Title: Ion acceleration and the development of turbulence during 3D
magnetic reconnection in impulsive flares
Authors: Drake, James Frederick; Dahlin, Joel; Kontar, Eduard;
Swisdak, Marc
2018tess.conf41306D Altcode:
Ion heating, including protons and alpha particles, is being explored
---------------------------------------------------------
Title: Radio wave propagation in the solar corona:
high-time-resolution observations with LOFAR
Authors: Kontar, Eduard; Yu, Sijie; Kuznetsov, Alexey; Emslie,
Gordon; Alcock, Ben; Jeffrey, Natasha; Melnik, Valentin; Bian, Nic;
Subramanian, Prasad
2018EGUGA..20.3278K Altcode:
Using the Low Frequency Array (LOFAR), we analyse the source sizes and
locations of the fine frequency structures in a solar radio burst. The
high time resolution allows us to determine the location and the size
of the radio emission source, and its evolution with time, following
the radio emission propagation through the solar corona. It is found
that intrinsically very small radio sources have an apparent size
that is a thousand times larger than the actual region where the
radio waves originate [1]. The observations suggest that it is radio
wave propagation effects, rather than the intrinsic properties of the
emission source, that determine the observed spatial characteristics of
the plasma emission radio bursts. In addition, the observations provide
a new opportunity for diagnostics of small-scale plasma fluctuations
by imaging the radio source halos as the radio waves move in the solar
corona. [1] Kontar et al.: Imaging Spectroscopy of Solar Radio Burst
Fine Structures, Nature Communications 8, Article number: 1515 (2017)
DOI: 10.1038/s41467-017-01307-8
---------------------------------------------------------
Title: Spatial and frequency structure of solar LOFAR radio sources
Authors: Gordovskyy, Mykola; Browning, Philippa; Kontar, Eduard;
Kuznetsov, Alexey
2018EGUGA..2013823G Altcode:
We investigate frequency-position structure of radio sources in solar
type III and type IV bursts in the frequency range 30-50 MHz observed
by LOFAR. These sources are produced by fundamental and harmonic plasma
emission induced by propagating suprathermal electrons. Therefore, the
frequency is a proxy for the electron density in the emitting plasma,
and these observations can be used to estimate the plasma density in the
outer corona. Our analysis indicates that coronal plasma, which produces
the emission, is denser and has larger hydrodynamic scale height (i.e.,
it is less stratified or more uniform) compared to Newkirk's density
model. We interpret this as the result of local plasma gradients
induced by plasma motion in the corona above solar active regions.
---------------------------------------------------------
Title: Spatially inhomogeneous acceleration of electrons in solar
flares
Authors: Stackhouse, Duncan J.; Kontar, Eduard P.
2018A&A...612A..64S Altcode: 2018arXiv180104743S
The imaging spectroscopy capabilities of the Reuven Ramaty high energy
solar spectroscopic imager (RHESSI) enable the examination of the
accelerated electron distribution throughout a solar flare region. In
particular, it has been revealed that the energisation of these
particles takes place over a region of finite size, sometimes resolved
by RHESSI observations. In this paper, we present, for the first time,
a spatially distributed acceleration model and investigate the role of
inhomogeneous acceleration on the observed X-ray emission properties. We
have modelled transport explicitly examining scatter-free and diffusive
transport within the acceleration region and compare with the analytic
leaky-box solution. The results show the importance of including
this spatial variation when modelling electron acceleration in solar
flares. The presence of an inhomogeneous, extended acceleration region
produces a spectral index that is, in most cases, different from the
simple leaky-box prediction. In particular, it results in a generally
softer spectral index than predicted by the leaky-box solution, for
both scatter-free and diffusive transport, and thus should be taken
into account when modelling stochastic acceleration in solar flares.
---------------------------------------------------------
Title: Interplanetary Type III Bursts and Electron Density
Fluctuations in the Solar Wind
Authors: Krupar, V.; Maksimovic, M.; Kontar, E. P.; Zaslavsky, A.;
Santolik, O.; Soucek, J.; Kruparova, O.; Eastwood, J. P.; Szabo, A.
2018ApJ...857...82K Altcode:
Type III bursts are generated by fast electron beams originated from
magnetic reconnection sites of solar flares. As propagation of radio
waves in the interplanetary medium is strongly affected by random
electron density fluctuations, type III bursts provide us with a unique
diagnostic tool for solar wind remote plasma measurements. Here, we
performed a statistical survey of 152 simple and isolated type III
bursts observed by the twin-spacecraft Solar TErrestrial RElations
Observatory mission. We investigated their time-frequency profiles
in order to retrieve decay times as a function of frequency. Next, we
performed Monte Carlo simulations to study the role of scattering due to
random electron density fluctuations on time-frequency profiles of radio
emissions generated in the interplanetary medium. For simplification,
we assumed the presence of isotropic electron density fluctuations
described by a power law with the Kolmogorov spectral index. Decay times
obtained from observations and simulations were compared. We found that
the characteristic exponential decay profile of type III bursts can
be explained by the scattering of the fundamental component between
the source and the observer despite restrictive assumptions included
in the Monte Carlo simulation algorithm. Our results suggest that
relative electron density fluctuations < δ {n}<SUB>{{e</SUB>}}>
/{n}<SUB>{{e</SUB>}} in the solar wind are 0.06-0.07 over wide range
of heliospheric distances.
---------------------------------------------------------
Title: Fine Structures of Solar Radio Type III Bursts and Their
Possible Relationship with Coronal Density Turbulence
Authors: Chen, Xingyao; Kontar, Eduard P.; Yu, Sijie; Yan, Yihua;
Huang, Jing; Tan, Baolin
2018ApJ...856...73C Altcode: 2018arXiv180107545C
Solar radio type III bursts are believed to be the most sensitive
signatures of near-relativistic electron beam propagation in the
corona. A solar radio type IIIb-III pair burst with fine frequency
structures, observed by the Low Frequency Array (LOFAR) with high
temporal (∼10 ms) and spectral (12.5 kHz) resolutions at 30-80 MHz,
is presented. The observations show that the type III burst consists
of many striae, which have a frequency scale of about 0.1 MHz in both
the fundamental (plasma) and the harmonic (double plasma) emission. We
investigate the effects of background density fluctuations based on the
observation of striae structure to estimate the density perturbation in
the solar corona. It is found that the spectral index of the density
fluctuation spectrum is about -1.7, and the characteristic spatial
scale of the density perturbation is around 700 km. This spectral
index is very close to a Kolmogorov turbulence spectral index of
-5/3, consistent with a turbulent cascade. This fact indicates that
the coronal turbulence may play the important role of modulating the
time structures of solar radio type III bursts, and the fine structure
of radio type III bursts could provide a useful and unique tool to
diagnose the turbulence in the solar corona.
---------------------------------------------------------
Title: Diffusive transport of energetic electrons in the solar corona:
X-ray and radio diagnostics
Authors: Musset, S.; Kontar, E. P.; Vilmer, N.
2018A&A...610A...6M Altcode: 2017arXiv171000765M
Context. Imaging spectroscopy in X-rays with RHESSI provides the
possibility to investigate the spatial evolution of X-ray emitting
electron distribution and therefore, to study transport effects on
energetic electrons during solar flares. <BR /> Aims: We study the
energy dependence of the scattering mean free path of energetic
electrons in the solar corona. <BR /> Methods: We used imaging
spectroscopy with RHESSI to study the evolution of energetic electrons
distribution in various parts of the magnetic loop during the 2004 May
21 flare. We compared these observations with the radio observations of
the gyrosynchrotron radiation of the same flare and with the predictions
of a diffusive transport model. <BR /> Results: X-ray analysis shows a
trapping of energetic electrons in the corona and a spectral hardening
of the energetic electron distribution between the top of the loop
and the footpoints. Coronal trapping of electrons is stronger for
radio-emitting electrons than for X-ray-emitting electrons. These
observations can be explained by a diffusive transport model. <BR />
Conclusions: We show that the combination of X-ray and radio diagnostics
is a powerful tool to study electron transport in the solar corona
in different energy domains. We show that the diffusive transport
model can explain our observations, and in the range 25-500 keV,
the scattering mean free path of electrons decreases with electron
energy. We can estimate for the first time the scattering mean free
path dependence on energy in the corona.
---------------------------------------------------------
Title: Radio Imaging Spectroscopy of Physical Processes in the
Inner Heliosphere
Authors: Kontar, E. P.; Emslie, A. G.
2018LPICo2063.3185K Altcode:
Radio observations below 100 MHz made using an array of small
radio antennae on the lunar surface can provide unique insight into
non-thermal processes in the corona and heliosphere. Such an array
fits within reasonable weight, power, telemetry, and cost constraints.
---------------------------------------------------------
Title: Heating and Cooling of Coronal Loops with Turbulent Suppression
of Parallel Heat Conduction
Authors: Bian, Nicolas; Emslie, A. Gordon; Horne, Duncan; Kontar,
Eduard P.
2018ApJ...852..127B Altcode: 2017arXiv171111388B
Using the “enthalpy-based thermal evolution of loops” (EBTEL) model,
we investigate the hydrodynamics of the plasma in a flaring coronal
loop in which heat conduction is limited by turbulent scattering of the
electrons that transport the thermal heat flux. The EBTEL equations
are solved analytically in each of the two (conduction-dominated and
radiation-dominated) cooling phases. Comparison of the results with
typical observed cooling times in solar flares shows that the turbulent
mean free path {λ }<SUB>T</SUB> lies in a range corresponding to a
regime in which classical (collision-dominated) conduction plays at
most a limited role. We also consider the magnitude and duration of
the heat input that is necessary to account for the enhanced values of
temperature and density at the beginning of the cooling phase and for
the observed cooling times. We find through numerical modeling that in
order to produce a peak temperature ≃ 1.5× {10}<SUP>7</SUP> K and
a 200 s cooling time consistent with observations, the flare-heating
profile must extend over a significant period of time; in particular,
its lingering role must be taken into consideration in any description
of the cooling phase. Comparison with observationally inferred values
of post-flare loop temperatures, densities, and cooling times thus
leads to useful constraints on both the magnitude and duration of the
magnetic energy release in the loop, as well as on the value of the
turbulent mean free path {λ }<SUB>T</SUB>.
---------------------------------------------------------
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: 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: Diffusive transport of energetic electrons in the solar corona:
X-ray and radio diagnostics
Authors: Musset, S.; Kontar, E.; Vilmer, N.
2017AGUFMSH43C..04M Altcode:
Solar flares are associated with efficient particule
acceleration. Energetic electrons are diagnosed through X-ray and radio
emissions produced as they interact in the solar atmosphere. Particle
transport from the acceleration region to the emission sites has a
crucial impact on the interpretation of particle emissions in the
context of acceleration models, and remains one of the challenging
topics in the field of high energy solar physics. In order to address
the transport of flare accelerated electrons in the low corona, we
used the imaging spectroscopy capabilities of the RHESSI spacecraft
to analyze X-ray emissions during the 2004 May 21 flare. We show that
non-thermal energetic electrons are trapped in the coronal part of the
flaring loop. In the hypothesis of turbulent pitch-angle scattering
of energetic electrons (Kontar et al, 2014), diffusive transport of
energetic electrons can lead to a confinement of accelerated electrons
in the coronal part of the loop. We show that this model can explain
the X-ray observations with a scattering mean free path of the order of
10<SUP>8</SUP> cm, much smaller than the length of the whole loop. Such
results are compared to the observation of the gyrosynchrotron emission
of the same flare (Kuznetsov et al, 2015). The diffusive transport
model can explain the radio observations with a scattering mean free
path of the order or 10<SUP>7</SUP> cm. The presented combination of
X-ray and radio diagnostics during a flare leads to the first estimate
of the energy dependence of the scattering mean free path of energetic
electrons in the low corona. This result is comparable with studies
of the energy dependence of the scattering mean free path of electrons
in the interplanetary medium.
---------------------------------------------------------
Title: Effects of Anomalous Resistivity on Particle Acceleration
due to Pitch Angle Scattering.
Authors: Borissov, A.; Kontar, E.; Neukirch, T.; Threlfall, J. W.;
Stevenson, J.; Parnell, C. E.
2017AGUFMSH41B2765B Altcode:
The mechanisms for generation of non-thermal accelerated particles in
flares is one of the outstanding problems in solar physics. The energy
for powering solar flares fundamentally comes from the coronal magnetic
field and its release involves magnetic reconnection. One direct way
of accelerating charged particles is due to the parallel electric field
generated during magnetic reconnection. To achieve a sufficiently rapid
release of energy an anomalous resistivity, several orders of magnitude
larger than the Spitzer resistivity in the corona, is often invoked,
particularly when performing mganetohydrodynamic (MHD) simulations
of solar flares. Since resistivity is fundamentally connected to
particle scattering, an enhanced anomalous resistivity relative to the
Spitzer resistivity should result in an enhanced scattering frequency
relative to the Coulomb scattering rate. We present results of test
particle simulations that attempt to account for these phenomena by
introducing pitch angle scattering at a rate dependent on the ratio of
the anomalous to Spitzer resistivity in the context of MHD simulations
of magnetic reconnection. We find that test particle trajectories and
durations are significantly modified by the presence of resistivity
dependent pitch angle scattering, with particle energy spectra also
being affected in some cases
---------------------------------------------------------
Title: Non-Local Diffusion of Energetic Electrons during Solar Flares
Authors: Bian, N. H.; Emslie, G.; Kontar, E.
2017AGUFMSH41B2764B Altcode:
The transport of the energy contained in suprathermal electrons in
solar flares plays a key role in our understanding of many aspects
of flare physics, from the spatial distributions of hard X-ray
emission and energy deposition in the ambient atmosphere to global
energetics. Historically the transport of these particles has been
largely treated through a deterministic approach, in which first-order
secular energy loss to electrons in the ambient target is treated as
the dominant effect, with second-order diffusive terms (in both energy
and angle) generally being either treated as a small correction or even
neglected. Here, we critically analyze this approach, and we show that
spatial diffusion through pitch-angle scattering necessarily plays
a very significant role in the transport of electrons. We further
show that a satisfactory treatment of the diffusion process requires
consideration of non-local effects, so that the electron flux depends
not just on the local gradient of the electron distribution function but
on the value of this gradient within an extended region encompassing a
significant fraction of a mean free path. Our analysis applies generally
to pitch-angle scattering by a variety of mechanisms, from Coulomb
collisions to turbulent scattering. We further show that the spatial
transport of electrons along the magnetic field of a flaring loop can
be modeled as a Continuous Time Random Walk with velocity-dependent
probability distribution functions of jump sizes and occurrences, both
of which can be expressed in terms of the scattering mean free path.
---------------------------------------------------------
Title: Imaging spectroscopy of solar radio burst fine structures
Authors: Kontar, E. P.; Yu, S.; Kuznetsov, A. A.; Emslie, A. G.;
Alcock, B.; Jeffrey, N. L. S.; Melnik, V. N.; Bian, N. H.; Subramanian,
P.
2017NatCo...8.1515K Altcode: 2017arXiv170806505K
Solar radio observations provide a unique diagnostic of the outer
solar atmosphere. However, the inhomogeneous turbulent corona
strongly affects the propagation of the emitted radio waves, so
decoupling the intrinsic properties of the emitting source from the
effects of radio wave propagation has long been a major challenge
in solar physics. Here we report quantitative spatial and frequency
characterization of solar radio burst fine structures observed with
the Low Frequency Array, an instrument with high-time resolution that
also permits imaging at scales much shorter than those corresponding to
radio wave propagation in the corona. The observations demonstrate that
radio wave propagation effects, and not the properties of the intrinsic
emission source, dominate the observed spatial characteristics of radio
burst images. These results permit more accurate estimates of source
brightness temperatures, and open opportunities for quantitative study
of the mechanisms that create the turbulent coronal medium through
which the emitted radiation propagates.
---------------------------------------------------------
Title: Imaging spectroscopy of type U and J solar radio bursts
with LOFAR
Authors: Reid, Hamish A. S.; Kontar, Eduard P.
2017A&A...606A.141R Altcode: 2017arXiv170607410R
Context. Radio U-bursts and J-bursts are signatures of electron beams
propagating along magnetic loops confined to the corona. The more
commonly observed type III radio bursts are signatures of electron
beams propagating along magnetic loops that extend into interplanetary
space. Given the prevalence of solar magnetic flux to be closed in
the corona, why type III bursts are more frequently observed than
U-bursts or J-bursts is an outstanding question. <BR /> Aims: We
use Low-Frequency Array (LOFAR) imaging spectroscopy between 30-80
MHz of low-frequency U-bursts and J-bursts, for the first time,
to understand why electron beams travelling along coronal loops
produce radio emission less often. Radio burst observations provide
information not only about the exciting electron beams but also about
the structure of large coronal loops with densities that are too
low for standard extreme ultraviolet (EUV) or X-ray analysis. <BR />
Methods: We analysed LOFAR images of a sequence of two J-bursts and
one U-burst. The different radio source positions were used to model
the spatial structure of the guiding magnetic flux tube and then deduce
the energy range of the exciting electron beams without the assumption
of a standard density model. We also estimated the electron density
along the magnetic flux rope and compared it to coronal models. <BR />
Results: The radio sources infer a magnetic loop that is 1 solar radius
in altitude with the highest frequency sources starting around 0.6
solar radii. Electron velocities were found between 0.13 c and 0.24 c
with the front of the electron beam travelling faster than the back of
the electron beam. The velocities correspond to energy ranges within
the beam from 0.7-11 keV to 0.7-43 keV. The density along the loop is
higher than typical coronal density models and the density gradient is
smaller. <BR /> Conclusions: We found that a more restrictive range
of accelerated beam and background plasma parameters can result in
U-bursts or J-bursts, causing type III bursts to be more frequently
observed. The large instability distances required before Langmuir
waves are produced by some electron beams, and the small magnitude of
the background density gradients makes closed loops less facilitative
for radio emission than loops that extend into interplanetary space.
---------------------------------------------------------
Title: Small electron acceleration episodes in the solar corona
Authors: James, Tomin; Subramanian, Prasad; Kontar, Eduard P.
2017MNRAS.471...89J Altcode: 2017arXiv170604031J
We study the energetics of non-thermal electrons produced in small
acceleration episodes in the solar corona. We carried out an extensive
survey spanning 2004-2015 and shortlisted six impulsive electron events
detected at 1 au that were not associated with large solar flares (GOES
soft X-ray class > C1) or with coronal mass ejections. Each of these
events had weak, but detectable hard X-ray (HXR) emission near the west
limb, and were associated with interplanetary type III bursts. In some
respects, these events seem like weak counterparts of 'cold/tenuous'
flares. The energy carried by the HXR producing electron population was
≈10<SUP>23</SUP>-10<SUP>25</SUP> erg, while that in the corresponding
population detected at 1 au was ≈10<SUP>24</SUP>-10<SUP>25</SUP>
erg. The number of electrons that escape the coronal acceleration
site and reach 1 au constitute 6 per cent to 148 per cent of those
that precipitate downwards to produce thick target HXR emission.
---------------------------------------------------------
Title: Particle acceleration with anomalous pitch angle scattering
in 2D magnetohydrodynamic reconnection simulations
Authors: Borissov, A.; Kontar, E. P.; Threlfall, J.; Neukirch, T.
2017A&A...605A..73B Altcode: 2017arXiv170900305B
The conversion of magnetic energy into other forms (such as plasma
heating, bulk plasma flows, and non-thermal particles) during solar
flares is one of the outstanding open problems in solar physics. It
is generally accepted that magnetic reconnection plays a crucial
role in these conversion processes. In order to achieve the rapid
energy release required in solar flares, an anomalous resistivity,
which is orders of magnitude higher than the Spitzer resistivity, is
often used in magnetohydrodynamic (MHD) simulations of reconnection
in the corona. The origin of Spitzer resistivity is based on Coulomb
scattering, which becomes negligible at the high energies achieved
by accelerated particles. As a result, simulations of particle
acceleration in reconnection events are often performed in the absence
of any interaction between accelerated particles and any background
plasma. This need not be the case for scattering associated with
anomalous resistivity caused by turbulence within solar flares,
as the higher resistivity implies an elevated scattering rate. We
present results of test particle calculations, with and without pitch
angle scattering, subject to fields derived from MHD simulations
of two-dimensional (2D) X-point reconnection. Scattering rates
proportional to the ratio of the anomalous resistivity to the local
Spitzer resistivity, as well as at fixed values, are considered. Pitch
angle scattering, which is independent of the anomalous resistivity,
causes higher maximum energies in comparison to those obtained without
scattering. Scattering rates which are dependent on the local anomalous
resistivity tend to produce fewer highly energised particles due to
weaker scattering in the separatrices, even though scattering in the
current sheet may be stronger when compared to resistivity-independent
scattering. Strong scattering also causes an increase in the number of
particles exiting the computational box in the reconnection outflow
region, as opposed to along the separatrices as is the case in the
absence of scattering.
---------------------------------------------------------
Title: Solar Plasma Radio Emission in the Presence of Imbalanced
Turbulence of Kinetic-Scale Alfvén Waves
Authors: Lyubchyk, O.; Kontar, E. P.; Voitenko, Y. M.; Bian, N. H.;
Melrose, D. B.
2017SoPh..292..117L Altcode: 2017arXiv170702295L
We study the influence of kinetic-scale Alfvénic turbulence on the
generation of plasma radio emission in the solar coronal regions
where the ratio β of plasma to magnetic pressure is lower than the
electron-to-ion mass ratio m<SUB>e</SUB>/m<SUB>i</SUB>. The present
study is motivated by the phenomenon of solar type I radio storms that
are associated with the strong magnetic field of active regions. The
measured brightness temperature of the type I storms can be up to
10<SUP>10</SUP>K for continuum emission, and can exceed 10<SUP>11</SUP>K
for type I bursts. At present, there is no generally accepted theory
explaining such high brightness temperatures and some other properties
of the type I storms. We propose a model with an imbalanced turbulence
of kinetic-scale Alfvén waves that produce an asymmetric quasi-linear
plateau on the upper half of the electron velocity distribution. The
Landau damping of resonant Langmuir waves is suppressed and their
amplitudes grow spontaneously above the thermal level. The estimated
saturation level of Langmuir waves is high enough to generate observed
type I radio emission at the fundamental plasma frequency. Harmonic
emission does not appear in our model because the backward-propagating
Langmuir waves undergo strong Landau damping. Our model predicts 100 %
polarization in the sense of the ordinary (o-) mode of type I emission.
---------------------------------------------------------
Title: Measuring X-ray anisotropy in solar flares. Prospective
stereoscopic capabilities of STIX and MiSolFA
Authors: Casadei, Diego; Jeffrey, Natasha L. S.; Kontar, Eduard P.
2017A&A...606A...2C Altcode: 2017arXiv170208795C
Context. During a solar flare, a large percentage of the magnetic
energy released goes into the kinetic energy of non-thermal
particles, with X-ray observations providing a direct connection to keV
flare-accelerated electrons. However, the electron angular distribution,
a prime diagnostic tool of the acceleration mechanism and transport,
is poorly known. <BR /> Aims: During the next solar maximum, two
upcoming space-borne X-ray missions, STIX on board Solar Orbiter and
MiSolFA, will perform stereoscopic X-ray observations of solar flares
at two different locations: STIX at 0.28 AU (at perihelion) and up to
inclinations of 25°, and MiSolFA in a low-Earth orbit. The combined
observations from these cross-calibrated detectors will allow us to
infer the electron anisotropy of individual flares confidently for
the first time. <BR /> Methods: We simulated both instrumental and
physical effects for STIX and MiSolFA including thermal shielding,
background and X-ray Compton backscattering (albedo effect) in the
solar photosphere. We predict the expected number of observable
flares available for stereoscopic measurements during the next
solar maximum. We also discuss the range of useful spacecraft
observation angles for the challenging case of close-to-isotropic
flare anisotropy. <BR /> Results: The simulated results show that
STIX and MiSolFA will be capable of detecting low levels of flare
anisotropy, for M1-class or stronger flares, even with a relatively
small spacecraft angular separation of 20-30°. Both instruments
will directly measure the flare X-ray anisotropy of about 40 M- and
X-class solar flares during the next solar maximum. <BR /> Conclusions:
Near-future stereoscopic observations with Solar Orbiter/STIX and
MiSolFA will help distinguishing between competing flare-acceleration
mechanisms, and provide essential constraints regarding collisional and
non-collisional transport processes occurring in the flaring atmosphere
for individual solar flares.
---------------------------------------------------------
Title: Diffusive transport of energetic electrons in the solar corona:
X-ray and radio diagnostics
Authors: Musset, Sophie; Kontar, Eduard; Vilmer, Nicole
2017SPD....4810203M Altcode:
Solar flares are associated with efficient particle acceleration. In
particular, energetic electrons are diagnosed through X-ray and radio
emissions produced as they interact with the solar atmosphere. Particle
transport from the acceleration region to the emission sites remains
one of the challenging topics in the field of high energy solar physics
and has a crucial impact on the interpretation of particles emissions
in the context of acceleration models.In order to address the transport
of flare associated energetic electrons in the low corona, we used the
imaging spectroscopy capabilities of the RHESSI spacecraft to analyze
the X-ray emission during the 2004 May 21 solar flare. We show that
non-thermal X-ray emitting energetic electrons are trapped in the
coronal part of the flaring loop. In the hypothesis of turbulent
pitch-angle scattering of energetic electrons (Kontar et al. 2014),
diffusive transport can lead to a confinement of energetic electrons
in the coronal part of the loop. We show that this model can explain
the X-ray observations with a scattering mean free path of the order
of 10^8 cm, much smaller than the length of the loop itself.Such
results are compared with the study by Kuznetsov and Kontar (2015)
of the gyrosynchrotron emission of the same flare. The diffusive
transport model can explain the radio observations with a scattering
mean free path of the order of 10^7 cm. This combination of X-ray
and radio observations during a flare leads to the first estimate of
the energy dependence of the scattering mean free path of energetic
electrons in the low corona. This result is comparable with studies
of the energy dependence of the scattering mean free path of electrons
in the interplanetary medium.
---------------------------------------------------------
Title: Modelling The Effects of Density Gradients and Fluctuations on
the Apparent Sizes and Positions of Low Frequency Solar Radio Sources
Authors: Alcock, Benjamin Thomas; Kontar, Eduard; Jeffrey, Natasha
2017SPD....4810304A Altcode:
Recent high spatial and temporal resolution imaging of <250 MHz
solar radio emission has enabled us to observe rapid variations in
Type-III solar radio burst characteristics, revealing fast growth of
the Type-III source and movement of the source centroid. In this work,
we use a Monte-Carlo ray tracing simulation to model the passage of
low frequency (5-240 MHz) radio waves through the solar corona from
a point source, considering both isotropic and dipole emission. We
model the effects of random density fluctuations and an isotropic
density gradient on the transport of the rays, varying the strength
of the scattering to observe the effects on images of the source
from an observer at 1 AU. Absorption of photons is included, and the
effects on the reproduced images and flux curves are observed. The
apparent source size and centroid position are tracked through the
simulation, and we find a general increase in source size with time,
and a variation of centroid position in both directions throughout the
simulation. We find that the size of the variation is strongly dependant
upon frequency, with lower frequency sources appearing to move further
on the disk than higher frequency sources. We also observe the strength
of the effects at different viewing angles, finding that the greatest
variation occurs closer to the solar limb. Further observational work
is required to limit the scattering parameters, in order to allow for
comparison with current radio images.
---------------------------------------------------------
Title: A weak thermal response on a strong electron acceleration in
a ‘cold’ flare
Authors: Fleishman, Gregory D.; Motorina, Galina; Nita, Gelu M.;
Kontar, Eduard
2017SPD....4810302F Altcode:
Solar flares are sudden explosive processes in the solar atmosphere,
which demonstrate remarkable variety of the partitions between
various energy components. Understanding the flare acceleration
site requires knowledge of exactly how flare energization works
and what is the partition between nonthermal, thermal and kinetic
energies. These partitions are known to vary broadly resulting in both
‘entirely thermal’ and primarily nonthermal, so-called ‘cold’
flares. These ‘cold flares’ are characterized by domination of
nonthermal component, but very weak thermal emission and almost no soft
X-ray enhancement; thus GOES often does not recognize such events as
flares. Here we attempt to quantify the thermal and nonthermal energies
and their evolving relationship in a 2013-Nov-05 cold flare. For
nonthermal diagnostics we use the RHESSI data, while the AIA data are
employed for the thermal diagnostics. We applied RHESSI spectral fits,
with both ‘cold’ and ‘warm’ target to bracketing the low-energy
cutoff, to quantify the rate of the nonthermal energy deposition
in this flare as well to characterize a (tiny) hot component. We
then computed evolving differential emission measure maps using
the regularized inversion method and derived from them the emission
measure and temperature maps. These inputs allowed us to accurately
calculate the evolving thermal energy in the flare. This thermal energy
was compared with the mentioned above rate of the nonthermal energy
deposition. This comparison suggests that the observed plasma heating
is entirely supplied by the loss of the nonthermal energy released
in the impulsive phase of the flare. Using vector magnetic data from
SDO/HMI we created a nonlinear force-free field reconstruction of the
region of interest, and, using the available X-ray and EUV data set as
a constraint, we developed a 3D model of the flare capable of correctly
reproducing the data set. To validate the model, we used microwave
data from Nobeyama and BBMS/SSRT instruments. Finally, this validated
model has been used to quantify the nonthermal and thermal energies
directly from the model 3D volume. We discuss physical implications
of the obtained results.
---------------------------------------------------------
Title: Polarisation of microwave emission from reconnecting twisted
coronal loops
Authors: Gordovskyy, M.; Browning, P. K.; Kontar, E. P.
2017A&A...604A.116G Altcode: 2016arXiv161102237G
Context. Magnetic reconnection and particle acceleration due to the
kink instability in twisted coronal loops can be a viable scenario
for confined solar flares. Detailed investigation of this phenomenon
requires reliable methods for observational detection of magnetic twist
in solar flares, which may not be possible solely through extreme UV
and soft X-ray thermal emission. Polarisation of microwave emission in
flaring loops can be used as one of the detection criteria. <BR /> Aims:
The aim of this study is to investigate the effect of magnetic twist in
flaring coronal loops on the polarisation of gyro-synchrotron microwave
(GSMW) emission, and determine whether it could provide a means for
magnetic twist detection. <BR /> Methods: We consider time-dependent
magnetohydrodynamic and test-particle models developed using the LARE3D
and GCA codes to investigate twisted coronal loops that relax after kink
instability. Synthetic GSMW emission maps (I and V Stokes components)
are calculated using GX simulator. <BR /> Results: It is found that
flaring twisted coronal loops produce GSMW radiation with a gradient
of circular polarisation across the loop. However, these patterns
may be visible only for a relatively short period of time owing to
fast magnetic reconfiguration after the instability. Their visibility
also depends on the orientation and position of the loop on the solar
disk. Typically, it would be difficult to see these characteristic
polarisation patterns in a twisted loop seen from the top (I.e. close
to the centre of the solar disk), but easier in a twisted loop seen
from the side (I.e. observed very close to the limb).
---------------------------------------------------------
Title: Understanding CMEs using plasma diagnostics of the related
dimmings
Authors: Vanninathan, Kamalam; Veronig, Astrid; Gomory, Peter;
Dissauer, Karin; Temmer, Manuela; Hannah, Iain; Kontar, Eduard
2017EGUGA..19.1571V Altcode:
Coronal Mass Ejections (CMEs) are often associated with dimmings that
are well observed in Extreme Ultra-violet (EUV) wavelengths. Such
dimmings are suggested to represent the evacuation of mass that is
carried out by CMEs and are a unique and indirect means to study CME
properties. While Earth-directed CMEs (on-disk CMEs) are difficult to
observe due to the bright background solar disk and projection effects,
their corresponding dimmings are clearly discernible and ideally suited
for analysis. Using data from the 6 EUV channels of Solar Dynamics
Observatory/Atmospheric Imaging Assembly for Differential Emission
Measure (DEM) diagnostics, we determine the plasma characteristics of
the dimming region. These data are well suited for this kind of study
due to the good temperature ranges covered by the multiple passbands
of the instrument. We analyse 7 on-disk and 5 off-limb events and
derive the weighted density and temperature as a function of time,
from the DEMs. From such an analysis we differentiate 2 types of dimming
regions: core and secondary dimmings. Core dimmings often occur in pairs
lying on either sides of the active region and in opposite polarity
regions while the secondary dimming is more extended. In both the
regions the derived plasma parameters reach a minimum within 30-60
min after the flare. For each event the core dimming region shows
a higher decrease in density and temperature than the corresponding
secondary dimming regions. The values of these parameters remains low
within the core dimming region for the entire duration of this study
( 10 hrs after the flare) while the secondary dimming region starts to
show a gradual increase after 1-2 hrs. We also use spectroscopic data
from Hinode/Extreme-Ultraviolet Imaging Spectrometer to differentiate
core and secondary dimming regions. We find that the Fe XIII 202 Å
line shows double component profiles within the core dimming region
with strong blueshifts of 100 km/s while the secondary dimming region
has weak upflows of 10 km/s. We conclude that the core dimming region
corresponds to footpoints of the erupting flux rope from where there
is continuous strong upflowing plasma for at least 10 hrs after the
flare, while the secondary dimming region begins to refill within 1-2
hrs. These measurements can be used to deduce information about the
mass of on-disk CMEs where white light measurements can fail. We also
confirm that the dimmings are mainly caused by density decrease and
not temperature changes. DEM analysis is a strong tool to decipher
CME properties from dimming regions.
---------------------------------------------------------
Title: Turbulent Kinetic Energy in the Energy Balance of a Solar Flare
Authors: Kontar, E. P.; Perez, J. E.; Harra, L. K.; Kuznetsov, A. A.;
Emslie, A. G.; Jeffrey, N. L. S.; Bian, N. H.; Dennis, B. R.
2017PhRvL.118o5101K Altcode: 2017arXiv170302392K
The energy released in solar flares derives from a reconfiguration of
magnetic fields to a lower energy state, and is manifested in several
forms, including bulk kinetic energy of the coronal mass ejection,
acceleration of electrons and ions, and enhanced thermal energy that
is ultimately radiated away across the electromagnetic spectrum
from optical to x rays. Using an unprecedented set of coordinated
observations, from a suite of instruments, we here report on a
hitherto largely overlooked energy component—the kinetic energy
associated with small-scale turbulent mass motions. We show that the
spatial location of, and timing of the peak in, turbulent kinetic
energy together provide persuasive evidence that turbulent energy may
play a key role in the transfer of energy in solar flares. Although
the kinetic energy of turbulent motions accounts, at any given time,
for only ∼(0.5 - 1 )% of the energy released, its relatively rapid
(∼1 - 10 s ) energization and dissipation causes the associated
throughput of energy (i.e., power) to rival that of major components
of the released energy in solar flares, and thus presumably in other
astrophysical acceleration sites.
---------------------------------------------------------
Title: Langmuir wave electric fields induced by electron beams in
the heliosphere
Authors: Reid, Hamish A. S.; Kontar, Eduard P.
2017A&A...598A..44R Altcode: 2016arXiv161107901R
Solar electron beams responsible for type III radio emission generate
Langmuir waves as they propagate out from the Sun. The Langmuir waves
are observed via in situ electric field measurements. These Langmuir
waves are not smoothly distributed but occur in discrete clumps,
commonly attributed to the turbulent nature of the solar wind electron
density. Exactly how the density turbulence modulates the Langmuir wave
electric fields is understood only qualitatively. Using weak turbulence
simulations, we investigate how solar wind density turbulence changes
the probability distribution functions, mean value and variance
of the beam-driven electric field distributions. Simulations show
rather complicated forms of the distribution that are dependent upon
how the electric fields are sampled. Generally the higher magnitude
of density fluctuations reduce the mean and increase the variance
of the distribution in a consistent manor to the predictions from
resonance broadening by density fluctuations. We also demonstrate how
the properties of the electric field distribution should vary radially
from the Sun to the Earth and provide a numerical prediction for the
in situ measurements of the upcoming Solar Orbiter and Solar Probe
Plus spacecraft.
---------------------------------------------------------
Title: VizieR Online Data Catalog: Global energetics of solar
flares. III. (Aschwanden+, 2016)
Authors: Aschwanden, M. J.; Holman, G.; O'Flannagain, A.; Caspi, A.;
McTiernan, J. M.; Kontar, E. P.
2017yCat..18320027A Altcode:
This study entails the third part of a global flare energetics project,
in which Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) data
of 191 M and X-class flare events from the first 3.5yrs of the Solar
Dynamics Observatory mission are analyzed. We fit a thermal and a
nonthermal component to RHESSI spectra, yielding the temperature
of the differential emission measure (DEM) tail, the nonthermal
power-law slope and flux, and the thermal/nonthermal cross-over energy
e<SUB>co</SUB>. From these parameters, we calculate the total nonthermal
energy E<SUB>nt</SUB> in electrons with two different methods: (1)
using the observed cross-over energy e<SUB>co</SUB> as low-energy
cutoff, and (2) using the low-energy cutoff e<SUB>wt</SUB> predicted
by the warm thick-target bremsstrahlung model of Kontar et al. Based
on a mean temperature of T<SUB>e</SUB>=8.6MK in active regions, we
find low-energy cutoff energies of e<SUB>wt</SUB>=6.2+/-1.6keV for the
warm-target model, which is significantly lower than the cross-over
energies e<SUB>co</SUB>=21+/-6keV. Comparing with the statistics
of magnetically dissipated energies E<SUB>mag</SUB> and thermal
energies E<SUB>th</SUB> from the two previous studies, we find the
following mean (logarithmic) energy ratios with the warm-target model:
E<SUB>nt</SUB>=0.41E<SUB>mag</SUB>, E<SUB>th</SUB>=0.08E<SUB>mag</SUB>,
and E<SUB>th</SUB>=0.15E<SUB>nt</SUB>. The total dissipated magnetic
energy exceeds the thermal energy in 95% and the nonthermal energy in
71% of the flare events, which confirms that magnetic reconnection
processes are sufficient to explain flare energies. The nonthermal
energy exceeds the thermal energy in 85% of the events, which largely
confirms the warm thick-target model. <P />(1 data file).
---------------------------------------------------------
Title: Global Energetics of Solar Flares. V. Energy Closure in Flares
and Coronal Mass Ejections
Authors: Aschwanden, Markus J.; Caspi, Amir; Cohen, Christina M. S.;
Holman, Gordon; Jing, Ju; Kretzschmar, Matthieu; Kontar, Eduard
P.; McTiernan, James M.; Mewaldt, Richard A.; O'Flannagain, Aidan;
Richardson, Ian G.; Ryan, Daniel; Warren, Harry P.; Xu, Yan
2017ApJ...836...17A Altcode: 2017arXiv170101176A
In this study we synthesize the results of four previous studies
on the global energetics of solar flares and associated coronal
mass ejections (CMEs), which include magnetic, thermal, nonthermal,
and CME energies in 399 solar M- and X-class flare events observed
during the first 3.5 yr of the Solar Dynamics Observatory (SDO)
mission. Our findings are as follows. (1) The sum of the mean
nonthermal energy of flare-accelerated particles ({E}<SUB>{nt</SUB>}),
the energy of direct heating ({E}<SUB>{dir</SUB>}), and the
energy in CMEs ({E}<SUB>{CME</SUB>}), which are the primary
energy dissipation processes in a flare, is found to have a ratio of
({E}<SUB>{nt</SUB>}+{E}<SUB>{dir</SUB>}+{E}<SUB>{CME</SUB>})/{E}<SUB>{mag</SUB>}=0.87+/-
0.18, compared with the dissipated magnetic free energy
{E}<SUB>{mag</SUB>}, which confirms energy closure within the
measurement uncertainties and corroborates the magnetic origin of
flares and CMEs. (2) The energy partition of the dissipated magnetic
free energy is: 0.51 ± 0.17 in nonthermal energy of ≥slant 6 {keV}
electrons, 0.17 ± 0.17 in nonthermal ≥slant 1 {MeV} ions, 0.07 ±
0.14 in CMEs, and 0.07 ± 0.17 in direct heating. (3) The thermal
energy is almost always less than the nonthermal energy, which is
consistent with the thick-target model. (4) The bolometric luminosity
in white-light flares is comparable to the thermal energy in soft
X-rays (SXR). (5) Solar energetic particle events carry a fraction
≈ 0.03 of the CME energy, which is consistent with CME-driven shock
acceleration. (6) The warm-target model predicts a lower limit of the
low-energy cutoff at {e}<SUB>c</SUB>≈ 6 {keV}, based on the mean peak
temperature of the differential emission measure of T <SUB>e</SUB> =
8.6 MK during flares. This work represents the first statistical study
that establishes energy closure in solar flare/CME events.
---------------------------------------------------------
Title: The Role of Diffusion in the Transport of Energetic Electrons
during Solar Flares
Authors: Bian, Nicolas H.; Emslie, A. Gordon; Kontar, Eduard P.
2017ApJ...835..262B Altcode: 2016arXiv161209456B
The transport of the energy contained in suprathermal electrons in
solar flares plays a key role in our understanding of many aspects
of flare physics, from the spatial distributions of hard X-ray
emission and energy deposition in the ambient atmosphere to global
energetics. Historically the transport of these particles has been
largely treated through a deterministic approach, in which first-order
secular energy loss to electrons in the ambient target is treated as
the dominant effect, with second-order diffusive terms (in both energy
and angle) generally being either treated as a small correction or even
neglected. Here, we critically analyze this approach, and we show that
spatial diffusion through pitch-angle scattering necessarily plays
a very significant role in the transport of electrons. We further
show that a satisfactory treatment of the diffusion process requires
consideration of non-local effects, so that the electron flux depends
not just on the local gradient of the electron distribution function but
on the value of this gradient within an extended region encompassing
a significant fraction of a mean free path. Our analysis applies
generally to pitch-angle scattering by a variety of mechanisms, from
Coulomb collisions to turbulent scattering. We further show that the
spatial transport of electrons along the magnetic field of a flaring
loop can be modeled rather effectively as a Continuous Time Random
Walk with velocity-dependent probability distribution functions of
jump sizes and occurrences, both of which can be expressed in terms
of the scattering mean free path.
---------------------------------------------------------
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: Anomalous Cooling of Coronal Loops with Turbulent Suppression
of Thermal Conduction
Authors: Bian, Nicolas H.; Watters, Jonathan M.; Kontar, Eduard P.;
Emslie, A. Gordon
2016ApJ...833...76B Altcode: 2016arXiv161004732B
We investigate the impact of turbulent suppression of parallel heat
conduction on the cooling of post-flare coronal loops. Depending on
the value of the mean free path {λ }<SUB>T</SUB> associated with
the turbulent scattering process, we identify four main cooling
scenarios. The overall temperature evolution, from an initial
temperature in excess of 10<SUP>7</SUP> K, is modeled in each case,
highlighting the evolution of the dominant cooling mechanism throughout
the cooling process. Comparison with observed cooling times allows the
value of {λ }<SUB>T</SUB> to be constrained, and interestingly this
range corresponds to situations where collision-dominated conduction
plays a very limited role, or even no role at all, in the cooling of
post-flare coronal loops.
---------------------------------------------------------
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: Observing the Formation of Flare-driven Coronal Rain
Authors: Scullion, E.; Rouppe van der Voort, L.; Antolin, P.;
Wedemeyer, S.; Vissers, G.; Kontar, E. P.; Gallagher, P. T.
2016ApJ...833..184S Altcode: 2016arXiv161009255S
Flare-driven coronal rain can manifest from rapidly cooled plasma
condensations near coronal loop tops in thermally unstable postflare
arcades. We detect five phases that characterize the postflare decay:
heating, evaporation, conductive cooling dominance for ∼120 s,
radiative/enthalpy cooling dominance for ∼4700 s, and finally
catastrophic cooling occurring within 35-124 s, leading to rain
strands with a periodicity of 55-70 s. We find an excellent agreement
between the observations and model predictions of the dominant
cooling timescales and the onset of catastrophic cooling. At the
rain-formation site, we detect comoving, multithermal rain clumps
that undergo catastrophic cooling from ∼1 MK to ∼22,000 K. During
catastrophic cooling, the plasma cools at a maximum rate of 22,700
K s<SUP>-1</SUP> in multiple loop-top sources. We calculated the
density of the extreme-ultraviolet (EUV) plasma from the differential
emission measure of the multithermal source employing regularized
inversion. Assuming a pressure balance, we estimate the density of
the chromospheric component of rain to be 9.21 × 10<SUP>11</SUP>
± 1.76 × 10<SUP>11</SUP> cm<SUP>-3</SUP>, which is comparable with
quiescent coronal rain densities. With up to eight parallel strands
in the EUV loop cross section, we calculate the mass loss rate from
the postflare arcade to be as much as 1.98 × 10<SUP>12</SUP> ±
4.95 × 10<SUP>11</SUP> g s<SUP>-1</SUP>. Finally, we reveal a close
proximity between the model predictions of {10}<SUP>5.8</SUP> K and the
observed properties between {10}<SUP>5.9</SUP> and {10}<SUP>6.2</SUP>
K, which defines the temperature onset of catastrophic cooling. The
close correspondence between the observations and numerical models
suggests that indeed acoustic waves (with a sound travel time of 68 s)
could play an important role in redistributing energy and sustaining
the enthalpy-based radiative cooling.
---------------------------------------------------------
Title: On the relaxation toward Kappa-Distribution Accelerated
Electron Populations in Solar Flares
Authors: Bian, N. H.; Kontar, E.; Emslie, G.
2016AGUFMSH13D..04B Altcode:
Driven by recent RHESSI observations of confined loop-top hard
X-ray sources in solar flares, we consider stochastic acceleration
of electrons in the presence of Coulomb collisions. It is shown
that when the diffusive acceleration time scales as the collisonal
friction time then the electron distribution function relaxes toward a
kappa distribution. We also show that the evolution toward this kappa
distribution involves a "wave front" propagating forwards in velocity
space so that electrons of higher energy are accelerated later. While
the relaxation toward a Maxwellian distribution is well known to imply
spatially diffusive transport, it is shown that relaxation toward
this kappa distribution is likely to involve anomalous (fractional)
diffusion of the particles in the acceleration region.
---------------------------------------------------------
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: Quasi-periodic Acceleration of Electrons in the Flare on 2012
July 19
Authors: Huang, Jing; Kontar, Eduard P.; Nakariakov, Valery M.;
Gao, Guannan
2016ApJ...831..119H Altcode:
Quasi-periodic pulsations (QPPs) of nonthermal emission in an M7.7
class flare on 2012 July 19 are investigated with spatially resolved
observations at microwave and HXR bands and with spectral observations
at decimetric, metric waves. Microwave emission at 17 GHz of two
footpoints, HXR emission at 20-50 keV of the north footpoint and
loop top, and type III bursts at 0.7-3 GHz show prominent in-phase
oscillations at 270 s. The microwave emission of the loop leg has less
pulsation but stronger emission. Through the estimation of plasma
density around the loop top from EUV observations, we find that the
local plasma frequency would be 1.5 GHz or even higher. Thus, type
III bursts at 700 MHz originate above the loop top. Quasi-periodic
acceleration or injection of energetic electrons is proposed to
dominate these in-phase QPPs of nonthermal emission from footpoints,
loop top, and above. In the overlying region, drifting pulsations
(DPS) at 200-600 MHz oscillate at a distinct period (200 s). Its global
structure drifts toward lower frequency, which is closely related to
upward plasmoids observed simultaneously from EUV emission. Hence,
nonthermal emission from overlying plasmoids and underlying flaring
loops show different oscillating periods. Two individual systems of
quasi-periodic acceleration of electrons are proposed to coincide in
the bi-direction outflows from the reconnection region.
---------------------------------------------------------
Title: Global Energetics of Solar Flares. III. Nonthermal Energies
Authors: Aschwanden, Markus J.; Holman, Gordon; O'Flannagain, Aidan;
Caspi, Amir; McTiernan, James M.; Kontar, Eduard P.
2016ApJ...832...27A Altcode: 2016arXiv160706488A
This study entails the third part of a global flare energetics project,
in which Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) data
of 191 M and X-class flare events from the first 3.5 years of the
Solar Dynamics Observatory mission are analyzed. We fit a thermal and
a nonthermal component to RHESSI spectra, yielding the temperature
of the differential emission measure (DEM) tail, the nonthermal
power-law slope and flux, and the thermal/nonthermal cross-over energy e
<SUB>co</SUB>. From these parameters, we calculate the total nonthermal
energy E <SUB>nt</SUB> in electrons with two different methods: (1)
using the observed cross-over energy e <SUB>co</SUB> as low-energy
cutoff, and (2) using the low-energy cutoff e <SUB>wt</SUB> predicted by
the warm thick-target bremsstrahlung model of Kontar et al. Based on a
mean temperature of T <SUB> e </SUB> = 8.6 MK in active regions, we find
low-energy cutoff energies of {e}<SUB>{wt</SUB>}=6.2+/- 1.6 {keV} for
the warm-target model, which is significantly lower than the cross-over
energies {e}<SUB>{co</SUB>}=21+/- 6 {keV}. Comparing with the statistics
of magnetically dissipated energies E <SUB>mag</SUB> and thermal
energies E <SUB>th</SUB> from the two previous studies, we find the
following mean (logarithmic) energy ratios with the warm-target model:
{E}<SUB>{nt</SUB>}=0.41 {E}<SUB>{mag</SUB>}, {E}<SUB>{th</SUB>}=0.08
{E}<SUB>{mag</SUB>}, and {E}<SUB>{th</SUB>}=0.15 {E}<SUB>{nt</SUB>}. The
total dissipated magnetic energy exceeds the thermal energy in 95%
and the nonthermal energy in 71% of the flare events, which confirms
that magnetic reconnection processes are sufficient to explain flare
energies. The nonthermal energy exceeds the thermal energy in 85%
of the events, which largely confirms the warm thick-target model.
---------------------------------------------------------
Title: Diagnosing the Source Region of a Solar Burst on 26 September
2011 by Using Microwave Type-III Pairs
Authors: Tan, B. L.; Karlický, M.; Mészárosová, H.; Kashapova,
L.; Huang, J.; Yan, Y.; Kontar, E. P.
2016SoPh..291.2407T Altcode: 2016SoPh..tmp..143T; 2016arXiv160605410T
We report a peculiar and interesting train of microwave Type-III pair
bursts in the impulsive rising phase of a solar flare on 26 September
2011. The observations include radio spectrometers at frequencies
of 0.80 - 2.00 GHz from the Ondřejov radiospectrograph in the
Czech Republic (ORSC), hard X-ray from the Ramaty High-Energy Solar
Spectroscopic Imager (RHESSI) and Gamma-Ray Burst Monitor onboard
the Fermi Space Telescope (Fermi/GRB), EUV images from the Sun
Watcher using APS detectors and image Processing instrument onboard
the Project for Onboard Autonomy 2 (SWAP/PROBA2), and magnetograms
from the Helioseismic and Magnetic Imager onboard the Solar Dynamic
Observatory (SDO/HMI). By using a recently developed method (Tan et al.,
Res. Astron. Astrophys.16, 82, 2016a), we diagnosed the plasma density,
temperature, plasma-β , magnetic field near the source region, the
energy of energetic electrons, and the distance between the acceleration
region and the emission start sites of Type-III bursts. From the
diagnostics, we find that i) The plasma density, temperature, magnetic
field, and the distance between the acceleration region and the emission
start sites have almost no obvious variations during the period of
Type-III pair trains, while the energy of electrons has an obvious
peak value that is consistent with the hard X-ray emission. ii) The
plasma-β is much higher than unity, showing a highly dynamic process
near the emission start site of Type-III bursts. iii) Although the
reversed-slope Type-III branches drift more slowly by one order of
magnitude than that of the normal Type-IIIs, the related descending
and ascending electrons still could have energy of the same order of
magnitude. These facts indicate that both the ascending and descending
electrons are possibly accelerated by a similar mechanism and in a
small source region. These diagnostics can help us to understand the
physics in the source region of solar bursts.
---------------------------------------------------------
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: Particle acceleration and transport in the solar atmosphere
Authors: Kontar, Eduard
2016cosp...41E1038K Altcode:
During periods of sporadic flare activity, the Sun releases
energy stored in the magnetic field into the plasma of the solar
atmosphere. This is an extremely efficient process, with a large
fraction of the magnetic energy going into plasma particles. The
solar flares are accompanied by prompt electromagnetic emission
virtually over the entire electromagnetic spectrum from gamma-rays
down to radio frequencies. The Sun, through its activity, also plays
a driving role in the Sun-Earth system that substantially influences
geophysical space. Solar flare energetic particles from the Sun are
detected in interplanetary space by in-situ measurements making them a
vital component of the single Sun-Earth system. Although a qualitative
picture is generally agreed upon, many processes solar flare processes
are poorly understood. Specifically, the processes of acceleration and
propagation of energetic particles interacting on various physical
scales remain major challenges in solar physics and basic plasma
physics. In the talk, I will review the current understanding of solar
flare energetic particles focusing on recent observational progress,
which became possible due to the numerous spacecraft and ground-based
observations.
---------------------------------------------------------
Title: Diffusive transport of energetic electrons in the 2004,
May 21 solar flare
Authors: Musset, Sophie; Kontar, Eduard; Vilmer, Nicole
2016cosp...41E1374M Altcode:
Solar flares are associated with efficient particle acceleration,
in particular with the production of energetic electrons which are
diagnosed through the X-ray and radio emissions that they produce when
interacting with the solar atmosphere. Particle transport from the
acceleration sites to the radiation sites remains of the challenging
topic in the field of high energy solar physics and has an important
impact on the interpretation of the particle emissions in the context
of acceleration models. In order to address the transport of flare
associated energetic electrons in the low corona, we use imaging
spectroscopic observations from RHESSI of the 2004 May 21 solar flare
which presents together with the usually observed HXR footpoints a
well observed coronal non-thermal X-ray source. The number of X-ray
emitting energetic electrons in the coronal source is compared to the
number of electrons needed to produce the hard X-ray emission in the
footpoints and is found twice as large. Such an excess of the number of
electrons in the coronal source cannot be explained in the context of
the standard model of X-ray emissions in which the dominant electron
transport is collisional. In the present flare, an additional process
is needed to explain how energetic electrons can be efficiently trapped
in the corona. In the hypothesis of turbulent pitch-angle scattering of
hard X-ray producing energetic electrons (Kontar et al, 2014), diffusive
transport can indeed lead to a confinement of energetic electrons in the
coronal source. Based on this assumption, we estimated for the present
event the mean-free path of energetic electrons and found a value of
10^8 - 10^9 meters, much smaller than the size of the observed flaring
loop itself. This implies that a diffusive transport of energetic
electrons is dominant in this flare which is in good agreement with
the results of a previous study based on the gyrosynchrotron emissions
from the energetic electrons (Kuznetsov & Kontar, 2015).
---------------------------------------------------------
Title: The electric field induced by high-energy solar electron beams
Authors: Reid, Hamish; Kontar, Eduard
2016cosp...41E1633R Altcode:
Solar electron beam responsible for type III emission generate Langmuir
waves as they propagate out from the Sun. The Langmuir waves are
observed through in-situ electric field measurements. The increase in
the electric field is not observed to be smoothly distributed as the
electron beam passes spacecraft but is spikey, with the waves occurring
in discrete clumps. The clumpy behaviour is commonly attributed to
the turbulent nature of the solar wind electron density modulating the
effective growth rate of Langmuir waves from the propagating electron
beam. Exactly how the intensity of the density turbulence modulates
the induced electric field distribution is known quantitatively. Using
quasilinear simulations we investigate how increasing the level of
density turbulence in the solar wind plasma changes the distribution
of the beam-driven electric field distribution. For plasma conditions
indicative of 1 AU we demonstrate how the electric field distribution
that is peaked at the maximum electric field for unperturbed plasma,
spreads out more uniformly in magnitude as density turbulence increases,
and is also able to reach higher electric fields. We show how the
electric field distribution changes as an electron beam travels through
plasma from the Sun to the Earth through the inner heliosphere. Our
simulations provide predictions of the radial behaviour that the
upcoming Solar Orbiter and Solar Probe Plus spacecraft will detect as
they travel towards the Sun.
---------------------------------------------------------
Title: Suppression of Parallel Transport in Turbulent Magnetized
Plasmas and Its Impact on the Non-thermal and Thermal Aspects of
Solar Flares
Authors: Bian, Nicolas H.; Kontar, Eduard P.; Emslie, A. Gordon
2016ApJ...824...78B Altcode: 2016arXiv160308672B
The transport of the energy contained in electrons, both thermal and
suprathermal, in solar flares plays a key role in our understanding of
many aspects of the flare phenomenon, from the spatial distribution
of hard X-ray emission to global energetics. Motivated by recent
RHESSI observations that point to the existence of a mechanism that
confines electrons to the coronal parts of flare loops more effectively
than Coulomb collisions, we here consider the impact of pitch-angle
scattering off turbulent magnetic fluctuations on the parallel transport
of electrons in flaring coronal loops. It is shown that the presence
of such a scattering mechanism in addition to Coulomb collisional
scattering can significantly reduce the parallel thermal and electrical
conductivities relative to their collisional values. We provide
illustrative expressions for the resulting thermoelectric coefficients
that relate the thermal flux and electrical current density to the
temperature gradient and the applied electric field. We then evaluate
the effect of these modified transport coefficients on the flare coronal
temperature that can be attained, on the post-impulsive-phase cooling of
heated coronal plasma, and on the importance of the beam-neutralizing
return current on both ambient heating and the energy loss rate of
accelerated electrons. We also discuss the possible ways in which
anomalous transport processes have an impact on the required overall
energy associated with accelerated electrons in solar flares.
---------------------------------------------------------
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: Plasma motions and non-thermal line broadening in flaring
twisted coronal loops
Authors: Gordovskyy, M.; Kontar, E. P.; Browning, P. K.
2016A&A...589A.104G Altcode: 2015arXiv150806412G
Context. Observation of coronal extreme ultra-violet (EUV) spectral
lines sensitive to different temperatures offers an opportunity to
evaluate the thermal structure and flows in flaring atmospheres. This,
in turn, can be used to estimate the partitioning between the thermal
and kinetic energies released in flares. <BR /> Aims: Our aim is to
forward-model large-scale (50-10 000 km) velocity distributions to
interpret non-thermal broadening of different spectral EUV lines
observed in flares. The developed models allow us to understand
the origin of the observed spectral line shifts and broadening,
and link these features to particular physical phenomena in flaring
atmospheres. <BR /> Methods: We use ideal magnetohydrodynamics
(MHD) to derive unstable twisted magnetic fluxtube configurations
in a gravitationally stratified atmosphere. The evolution of these
twisted fluxtubes is followed using resistive MHD with anomalous
resistivity depending on the local density and temperature. The model
also takes thermal conduction and radiative losses in the continuum
into account. The model allows us to evaluate average velocities
and velocity dispersions, which would be interpreted as non-thermal
velocities in observations, at different temperatures for different
parts of the models. <BR /> Results: Our models show qualitative and
quantitative agreement with observations. Thus, the line-of-sight
(LOS) velocity dispersions demonstrate substantial correlation
with the temperature, increasing from about 20-30 km s<SUP>-1</SUP>
around 1 MK to about 200-400 km s<SUP>-1</SUP> near 10-20 MK. The
average LOS velocities also correlate with velocity dispersions,
although they demonstrate a very strong scattering compared to the
observations. We also note that near footpoints the velocity dispersions
across the magnetic field are systematically lower than those along the
field. We conclude that the correlation between the flow velocities,
velocity dispersions, and temperatures are likely to indicate that
the same heating mechanism is responsible for heating the plasma,
its turbulisation, and expansion/evaporation.
---------------------------------------------------------
Title: Suppression of Parallel Transport in Turbulent Magnetized
Plasmas and Its Impact on Non-Thermal and Thermal Aspects of Solar
Flares
Authors: Emslie, A. Gordon; Bian, Nicolas H.; Kontar, Eduard
2016SPD....47.0603E Altcode:
Motivated by recent RHESSI observations that point to the existence
of a mechanism that confines electrons to the coronal parts of flare
loops more effectively than Coulomb collisions, we consider the impact
of pitch-angle scattering off turbulent magnetic fluctuations on the
parallel transport of electrons in flaring coronal loops. It is shown
that the presence of such a scattering mechanism in addition to Coulomb
collisional scattering can significantly reduce the parallel thermal
and electrical conductivities relative to their collisional values. We
provide illustrative expressions for the resulting thermoelectric
coefficients that relate the thermal flux and electrical current density
to the temperature gradient and the applied electric field. We then
evaluate the effect of these modified transport coefficients on several
items of interest to the modeling of flares, including: the peak flare
coronal temperature that can be attained, the post-impulsive-phase
cooling time of heated coronal plasma, and the importance of the
beam-neutralizing return current on both ambient heating and the energy
loss rate of accelerated electrons. We also discuss the ways in which
anomalous transport processes have an impact on the required overall
energy content of accelerated electrons in solar flares.
---------------------------------------------------------
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: On the origin of 140 GHz emission from the 4 July 2012
solar flare
Authors: Tsap, Yuriy T.; Smirnova, Victoria V.; Morgachev, Alexander
S.; Motorina, Galina G.; Kontar, Eduard P.; Nagnibeda, Valery G.;
Strekalova, Polina V.
2016AdSpR..57.1449T Altcode: 2016arXiv160401530T
The sub-THz event observed on the 4 July 2012 with the Bauman Moscow
State Technical University Radio Telescope RT-7.5 at 93 and 140 GHz
as well as Kislovodsk and Metsähovi radio telescopes, Radio Solar
Telescope Network (RSTN), GOES, RHESSI, and SDO orbital stations is
analyzed. The spectral flux between 93 and 140 GHz has been observed
increasing with frequency. On the basis of the SDO/AIA data the
differential emission measure has been calculated. It is shown that
the thermal coronal plasma with the temperature above 0.5 MK cannot be
responsible for the observed sub-THz flare emission. The non-thermal
gyrosynchrotron mechanism can be responsible for the microwave emission
near 10 GHz but the observed millimeter spectral characteristics are
likely to be produced by the thermal bremsstrahlung emission from
plasma with a temperature of about 0.1 MK.
---------------------------------------------------------
Title: Division E Commission 49: Interplanetary Plasma and Heliosphere
Authors: Mann, Ingrid; Manoharan, P. K.; Gopalswamy, Natchimuthuk;
Briand, Carine; Chashei, Igor V.; Gibson, Sarah E.; Lario, David;
Hanaoka, Yoichiro; Malandraki, Olga; Kontar, Eduard; Richardson,
John D.
2016IAUTA..29..300M Altcode:
After a little more than forty years of work related to the
interplanetary plasma and the heliosphere the IAU's Commission 49 was
formally discontinued in 2015. The commission started its work when
the first spacecraft were launched to measure the solar wind in-situ
away from Earth orbit, both inward and outward from 1 AU. It now
hands over its activities to a new commission during an era of space
research when Voyager 1 measures in-situ the parameters of the local
interstellar medium at the edge of the heliosphere. The commission will
be succeeded by C.E3 with a similar area of responsibility but with more
focused specific tasks that the community intends to address during the
coming several years. This report includes a short description of the
motivation for this commission and of the historical context. It then
describes work from 2012 to 2015 during the present solar cycle 24 that
has been the weakest in the space era so far. It gave rise to a large
number of studies on solar energetic particles and cosmic rays. Other
studies addressed e.g. the variation of the solar wind structure
and energetic particle fluxes on long time scales, the detection of
dust in the solar wind and the Voyager measurements at the edge of the
heliosphere. The research is based on measurements from spacecraft that
are at present operational and motivated by the upcoming Solar Probe
+ and Solar Orbiter missions to explore the vicinity of the Sun. We
also report here the progress on new and planned radio instruments
and their importance for heliospheric studies. Contributors to this
report are Carine Briand, Yoichiro Hanaoka, Eduard Kontar, David Lario,
Ingrid Mann, John D. Richardson.
---------------------------------------------------------
Title: Two-dimensional time evolution of beam-plasma instability in
the presence of binary collisions
Authors: Tigik, S. F.; Ziebell, L. F.; Yoon, P. H.; Kontar, E. P.
2016A&A...586A..19T Altcode:
Energetic electrons produced during solar flares are known to be
responsible for generating solar type III radio bursts. The radio
emission is a byproduct of Langmuir wave generation via beam-plasma
interaction and nonlinear wave-wave and wave-particle interaction
processes. In addition to type III radio bursts, electrons traveling
downwards toward the chromosphere lead to the hard X-ray emission
via electron-ion collisions. Recently, the role of Langmuir waves
on the X-ray-producing electrons has been identified as important,
because Langmuir waves may alter the electron distribution, thereby
affecting the X-ray profile. Both Coulomb collisions and wave-particle
interactions lead electrons to scattering and energy exchange that
necessitates considering the two-dimensional (2D) problem in velocity
space. The present paper investigates the influence of binary collisions
on the beam-plasma instability development in 2D in order to elucidate
the nonlinear dynamics of Langmuir waves and binary collisions. The
significance of the present findings in the context of solar physics
is discussed.
---------------------------------------------------------
Title: SSALMON - The Solar Simulations for the Atacama Large
Millimeter Observatory Network
Authors: Wedemeyer, S.; Bastian, T.; Brajša, R.; Barta, M.; Hudson,
H.; Fleishman, G.; Loukitcheva, M.; Fleck, B.; Kontar, E.; De Pontieu,
B.; Tiwari, S.; Kato, Y.; Soler, R.; Yagoubov, P.; Black, J. H.;
Antolin, P.; Gunár, S.; Labrosse, N.; Benz, A. O.; Nindos, A.;
Steffen, M.; Scullion, E.; Doyle, J. G.; Zaqarashvili, T.; Hanslmeier,
A.; Nakariakov, V. M.; Heinzel, P.; Ayres, T.; Karlicky, M.
2015AdSpR..56.2679W Altcode: 2015arXiv150205601W
The Solar Simulations for the Atacama Large Millimeter Observatory
Network (SSALMON) was initiated in 2014 in connection with two ALMA
development studies. The Atacama Large Millimeter/submillimeter Array
(ALMA) is a powerful new tool, which can also observe the Sun at
high spatial, temporal, and spectral resolution. The international
SSALMONetwork aims at co-ordinating the further development of solar
observing modes for ALMA and at promoting scientific opportunities
for solar physics with particular focus on numerical simulations,
which can provide important constraints for the observing modes and
can aid the interpretation of future observations. The radiation
detected by ALMA originates mostly in the solar chromosphere - a
complex and dynamic layer between the photosphere and corona, which
plays an important role in the transport of energy and matter and the
heating of the outer layers of the solar atmosphere. Potential targets
include active regions, prominences, quiet Sun regions, flares. Here,
we give a brief overview over the network and potential science cases
for future solar observations with ALMA.
---------------------------------------------------------
Title: Multithermal Representation of the Kappa-distribution of
Solar Flare Electrons and Application to Simultaneous X-Ray and
EUV Observations
Authors: Battaglia, Marina; Motorina, Galina; Kontar, Eduard P.
2015ApJ...815...73B Altcode: 2015arXiv151101328B
Acceleration of particles and plasma heating is one of the fundamental
problems in solar flare physics. An accurate determination of the
spectrum of flare-energized electrons over a broad energy range is
crucial for our understanding of aspects such as the acceleration
mechanism and the total flare energy. Recent years have seen a growing
interest in the kappa-distribution as a representation of the total
spectrum of flare-accelerated electrons. In this work we present the
kappa-distribution as a differential emission measure. This allows
for inferring the electron distribution from X-ray observations and
EUV observations by simultaneously fitting the proposed function to
RHESSI and SDO/AIA data. This yields the spatially integrated electron
spectra of a coronal source from less than 0.1 keV up to several tens
of keV. The method is applied to a single-loop GOES C4.1 flare. The
results show that the total energy can only be determined accurately
by combining RHESSI and AIA observations. Simultaneously fitting the
proposed representation of the kappa-distribution reduces the electron
number density in the analyzed flare by a factor of ∼30 and the
total flare energy by a factor of ∼5 compared with the commonly used
fitting of RHESSI spectra. The spatially integrated electron spectrum
of the investigated flare between 0.043 and 24 keV is consistent with
the combination of a low-temperature (∼2 MK) component and a hot
(∼11 MK) kappa-like component with spectral index 4, reminiscent of
solar wind distributions.
---------------------------------------------------------
Title: Capabilities of a FOXSI Small Explorer
Authors: Inglis, A. R.; Christe, S.; Glesener, L.; Krucker, S.; Dennis,
B. R.; Shih, A.; Wilson-Hodge, C.; Gubarev, M.; Hudson, H. S.; Kontar,
E.; Buitrago Casas, J. C.; Drake, J. F.; Caspi, A.; Holman, G.; Allred,
J. C.; Ryan, D.; Alaoui, M.; White, S. M.; Saint-Hilaire, P.; Klimchuk,
J. A.; Hannah, I. G.; Antiochos, S. K.; Grefenstette, B.; Ramsey,
B.; Jeffrey, N. L. S.; Reep, J. W.; Schwartz, R. A.; Ireland, J.
2015AGUFMSH43B2456I Altcode:
We present the FOXSI (Focusing Optics X-ray Solar Imager) small explorer
(SMEX) concept, a mission dedicated to studying particle acceleration
and energy release on the Sun. FOXSI is designed as a 3-axis stabilized
spacecraft in low-Earth orbit making use of state-of-the-art grazing
incidence focusing optics, allowing for direct imaging of solar
X-rays. The current design being studied features three telescope
modules deployed in a low-inclination low-earth orbit (LEO). With a 15
meter focal length enabled by a deployable boom, FOXSI will observe
the Sun in the 3-50 keV energe range. The FOXSI imaging concept has
already been tested on two sounding rocket flights, in 2012 and 2014
and on the HEROES balloon payload flight in 2013. FOXSI will image
the Sun with an angular resolution of 5”, a spectral resolution of
0.5 keV, and sub-second temporal resolution using CdTe detectors. In
this presentation we investigate the science objectives and targets
which can be accessed from this mission. Because of the defining
characteristic of FOXSI is true imaging spectroscopy with high dynamic
range and sensitivity, a brand-new perspective on energy release on the
Sun is possible. Some of the science targets discussed here include;
flare particle acceleration processes, electron beams, return currents,
sources of solar energetic particles (SEPs), as well as understanding
X-ray emission from active region structures and the quiescent corona.
---------------------------------------------------------
Title: Pitch Angle Scattering of Solar Flare Electrons in the
Interplanetary Medium: Observations and Modelling
Authors: Alcock, B.; Kontar, E.; Agueda, N.
2015AGUFMSH21A2375A Altcode:
In the past decade, analysis of near-relativistic (~27 keV - 300
keV) electron events at 1 AU have highlighted two transport effects
which require explanation. Firstly, several events feature delayed
electron arrival with respect to solar radio and hard x-ray emission,
and secondly, the peak-flux spectrum of electrons at 1 AU does not
match the predicted spectrum from hard x-ray observations. We analyse
several near-relativistic electron events observed via both RHESSI
hard x-ray observations at the Sun and in-situ measurements from the
Wind/3DP detector at 1 AU. Numerical simulations of electron transport
outwards from the Sun are made, which take the electron injection time
and peak-flux spectrum from RHESSI data, and the flux subsequently
passing 1 AU is calculated. We consider the effects of adiabatic
focusing and pitch angle diffusion on the particle transport, and a
momentum and distance dependent form of the parallel mean free path for
electrons is employed. The simulated lightcurves, peak-flux spectrum,
pitch angle distribution, and delay times are then compared with
Wind observations. We find that, for higher energy electrons (>40
keV), the simulated flux matches well with observations, showing that
stochastic pitch angle scattering is able to explain apparent delayed
particle injection at the Sun. The lower energy observations, however,
remain unmatched by models, which predict much more impulsive events
at Earth than are observed. We also find that pitch angle scattering
is too weak to vary the peak-flux spectrum sufficiently, thus requiring
further exploration.
---------------------------------------------------------
Title: The Formation of Kappa-Distribution Accelerated Electron
Populations in Solar Flares
Authors: Bian, N. H.; Kontar, E.; Emslie, G.
2015AGUFMSH33C..07B Altcode:
Driven by recent RHESSI observations of confined loop-top hard
X-ray sources in solar flares, we consider stochastic acceleration
of electrons in the presence of Coulomb collisions. If electron
escape from the acceleration region can be neglected, the electron
distribution function is determined by a balance between diffusive
acceleration and collisions. Such a scenario admits a stationary
solution for the electron distribution function that takes the form
of a kappa distribution. We show that the evolution toward this kappa
distribution involves a "wave front" propagating forwards in velocity
space, so that electrons of higher energy are accelerated later; the
acceleration time scales as the power three-half of the energy. At
sufficiently high energies escape from the finite-length acceleration
region will eventually dominate. For such energies, the electron
velocity distribution function is obtained by solving a time-dependent
Fokker-Planck equation in the "leaky-box" approximation. Solutions
are obtained in the limit of a small escape rate from an acceleration
region that can effectively be considered a thick target.
---------------------------------------------------------
Title: High-temperature differential emission measure and altitude
variations in the temperature and density of solar flare coronal
X-ray sources
Authors: Jeffrey, Natasha L. S.; Kontar, Eduard P.; Dennis, Brian R.
2015A&A...584A..89J Altcode: 2015arXiv151004095J
The detailed knowledge of plasma heating and acceleration region
properties presents a major observational challenge in solar flare
physics. Using the Ramaty High Energy Solar Spectroscopic Imager
(RHESSI), the high temperature differential emission measure, DEM(T),
and the energy-dependent spatial structure of solar flare coronal
sources were studied quantitatively. The altitude of the coronal
X-ray source was observed to increase with energy by ~+0.2 arcsec/keV
between 10 and 25 keV. Although an isothermal model can fit the thermal
X-ray spectrum observed by RHESSI, such a model cannot account for the
changes in altitude, and multi-thermal coronal sources are required
where the temperature increases with altitude. For the first time,
we show how RHESSI imaging information can be used to constrain the
DEM(T) of a flaring plasma. We developed a thermal bremsstrahlung X-ray
emission model with inhomogeneous temperature and density distributions
to simultaneously reproduce i) DEM(T); ii) altitude as a function of
energy; and iii) vertical extent of the flaring coronal source versus
energy. We find that the temperature-altitude gradient in the region
is ~+0.08 keV/arcsec (~1.3 MK/Mm). Similar altitude-energy trends
in other flares suggest that the majority of coronal X-ray sources
are multi-thermal and have strong vertical temperature and density
gradients with a broad DEM(T).
---------------------------------------------------------
Title: Differential emission measure and electron distribution
function reconstructed from RHESSI and SDO observations
Authors: Motorina, G. G.; Kontar, E. P.
2015Ge&Ae..55..995M Altcode: 2015arXiv151006755M
To solve a number of problems in solar physics related to mechanisms
of energy release in solar corona parameters of hot coronal plasma are
required, such as energy distribution, emission measure, differential
emission measure, and their evolution with time. Of special interest is
the distribution of solar plasma by energies, which can evolve from a
nearly Maxwellian distribution to a distribution with a more complex
structure during a solar flare. The exact form of this distribution
for low-energy particles, which receive the bulk of flare energy, is
still poorly known; therefore, detailed investigations are required. We
present a developed method of simultaneous fitting of data from two
spacecrafts Solar Dynamics Observatory/Atmospheric Imaging Assembly
(SDO/AIA) and Reuven Ramaty High Energy Solar Spectroscopic Imager
(RHESSI), using a differential emission measure and a thin target
model for the August 14, 2010 flare event.
---------------------------------------------------------
Title: Coronal Response to an EUV Wave from DEM Analysis
Authors: Vanninathan, K.; Veronig, A. M.; Dissauer, K.; Madjarska,
M. S.; Hannah, I. G.; Kontar, E. P.
2015ApJ...812..173V Altcode: 2015arXiv150905269V
Extreme-Ultraviolet (EUV) waves are globally propagating disturbances
that have been observed since the era of the Solar and Heliospheric
Observatory/Exteme-ultraviolet Imaging Telescope instrument. Although
the kinematics of the wave front and secondary wave components have been
widely studied, there is not much known about the generation and plasma
properties of the wave. In this paper we discuss the effect of an EUV
wave on the local plasma as it passes through the corona. We studied the
EUV wave, generated during the 2011 February 15 X-class flare/coronal
mass ejection event, using Differential Emission Measure diagnostics. We
analyzed regions on the path of the EUV wave and investigated the local
density and temperature changes. From our study we have quantitatively
confirmed previous results that during wave passage the plasma visible
in the Atmospheric Imaging Assembly (AIA) 171 Å channel is getting
heated to higher temperatures corresponding to AIA 193 and 211 Å
channels. We have calculated an increase of 6%-9% in density and 5%-6%
in temperature during the passage of the EUV wave. We have compared
the variation in temperature with the adiabatic relationship and
have quantitatively demonstrated the phenomenon of heating due to
adiabatic compression at the wave front. However, the cooling phase
does not follow adiabatic relaxation but shows slow decay indicating
slow energy release being triggered by the wave passage. We have also
identified that heating is taking place at the front of the wave pulse
rather than at the rear. Our results provide support for the case that
the event under study here is a compressive fast-mode wave or a shock.
---------------------------------------------------------
Title: The collisional relaxation of electrons in hot flaring plasma
and inferring the properties of solar flare accelerated electrons
from X-ray observations.
Authors: Jeffrey, N. L. S.; Kontar, E. P.; Emslie, A. G.; Bian, N. H.
2015JPhCS.642a2013J Altcode: 2015arXiv150706785J
X-ray observations are a direct diagnostic of fast electrons produced in
solar flares, energized during the energy release process and directed
towards the Sun. Since the properties of accelerated electrons can
be substantially changed during their transport and interaction with
the background plasma, a model must ultimately be applied to X-ray
observations in order to understand the mechanism responsible for
their acceleration. A cold thick target model is ubiquitously used for
this task, since it provides a simple analytic relationship between
the accelerated electron spectrum and the emitting electron spectrum
in the X-ray source, with the latter quantity readily obtained from
X-ray observations. However, such a model is inappropriate for the
majority of solar flares in which the electrons propagate in a hot
megaKelvin plasma, because it does not take into account the physics of
thermalization of fast electrons. The use of a more realistic model,
properly accounting for the properties of the background plasma,
and the collisional diffusion and thermalization of electrons, can
alleviate or even remove many of the traditional problems associated
with the cold thick target model and the deduction of the accelerated
electron spectrum from X-ray spectroscopy, such as the number problem
and the need to impose an ad hoc low energy cut-off.
---------------------------------------------------------
Title: Collisional Relaxation of Electrons in a Warm Plasma and
Accelerated Nonthermal Electron Spectra in Solar Flares
Authors: Kontar, Eduard P.; Jeffrey, Natasha L. S.; Emslie, A. Gordon;
Bian, N. H.
2015ApJ...809...35K Altcode: 2015arXiv150503733K
Extending previous studies of nonthermal electron transport in solar
flares, which include the effects of collisional energy diffusion
and thermalization of fast electrons, we present an analytic method
to infer more accurate estimates of the accelerated electron spectrum
in solar flares from observations of the hard X-ray spectrum. Unlike
for the standard cold-target model, the spatial characteristics of
the flaring region, especially the necessity to consider a finite
volume of hot plasma in the source, need to be taken into account
in order to correctly obtain the injected electron spectrum from the
source-integrated electron flux spectrum (a quantity straightforwardly
obtained from hard X-ray observations). We show that the effect
of electron thermalization can be significant enough to nullify the
need to introduce an ad hoc low-energy cutoff to the injected electron
spectrum in order to keep the injected power in non-thermal electrons at
a reasonable value. Rather, the suppression of the inferred low-energy
end of the injected spectrum compared to that deduced from a cold-target
analysis allows the inference from hard X-ray observations of a more
realistic energy in injected non-thermal electrons in solar flares.
---------------------------------------------------------
Title: On the speed and acceleration of electron beams triggering
interplanetary type III radio bursts
Authors: Krupar, V.; Kontar, E. P.; Soucek, J.; Santolik, O.;
Maksimovic, M.; Kruparova, O.
2015A&A...580A.137K Altcode: 2015arXiv150706874K
<BR /> Aims: Type III radio bursts are intense radio emissions
triggered by beams of energetic electrons often associated with solar
flares. These exciter beams propagate outwards from the Sun along
an open magnetic field line in the corona and in the interplanetary
(IP) medium. <BR /> Methods: We performed a statistical survey of
29 simple and isolated IP type III bursts observed by STEREO/Waves
instruments between January 2013 and September 2014. We investigated
their time-frequency profiles in order to derive the speed and
acceleration of exciter electron beams. <BR /> Results: We show
these beams noticeably decelerate in the IP medium. Obtained speeds
range from ~0.02c up to ~0.35c depending on initial assumptions. It
corresponds to electron energies between tens of eV and hundreds of
keV, and in order to explain the characteristic energies or speeds
of type III electrons (~0.1c) observed simultaneously with Langmuir
waves at 1 au, the emission of type III bursts near the peak should
be predominately at double plasma frequency. Derived properties
of electron beams can be used as input parameters for computer
simulations of interactions between the beam and the plasma in the
IP medium. <P />Appendix A is available in electronic form at <A
href="http://www.aanda.org/10.1051/0004-6361/201425308/olm">http://www.aanda.org</A>
---------------------------------------------------------
Title: Stopping frequency of type III solar radio bursts in expanding
magnetic flux tubes
Authors: Reid, Hamish A. S.; Kontar, Eduard P.
2015A&A...577A.124R Altcode: 2015arXiv150303395R
<BR /> Aims: Understanding the properties of type III radio bursts
in the solar corona and interplanetary space is one of the best ways
to remotely deduce the characteristics of solar accelerated electron
beams and the solar wind plasma. One feature of all type III bursts is
the lowest frequency they reach (or stopping frequency). This feature
reflects the distance from the Sun that an electron beam can drive
the observable plasma emission mechanism. The stopping frequency
has never been systematically studied before from a theoretical
perspective. <BR /> Methods: Using numerical kinetic simulations, we
explore the different parameters that dictate how far an electron beam
can travel before it stops inducing a significant level of Langmuir
waves, responsible for plasma radio emission. We use the quasilinear
approach to model the resonant interaction between electrons and
Langmuir waves self-consistently in inhomogeneous plasma, and take
into consideration the expansion of the guiding magnetic flux tube and
the turbulent density of the interplanetary medium. <BR /> Results:
We find that the rate of radial expansion has a significant effect
on the distance an electron beam travels before enhanced levels of
Langmuir waves, hence radio waves, cease. Radial expansion of the
guiding magnetic flux tube rarefies the electron stream to the extent
that the density of non-thermal electrons is too low to drive Langmuir
wave production. The initial conditions of the electron beam have a
significant effect, where decreasing the beam density or increasing
the spectral index of injected electrons would cause higher type
III stopping frequencies. We also demonstrate how the intensity of
large-scale density fluctuations increases the highest frequency to
which Langmuir waves can be driven by the beam and how the magnetic
field geometry can be the cause of type III bursts that are only
observed at high coronal frequencies.
---------------------------------------------------------
Title: Collisional Diffusion and Thick-Target Energy Losses in Solar
Flares -- Death to the "Low-Energy Cufoff"
Authors: Emslie, Gordon; Bian, Nicolas; Jeffrey, Natasha; Kontar,
Eduard
2015TESS....130204E Altcode:
We extend previous studies of nonthermal electron transport in solar
flares by including the effects of collisional diffusion on the energy
loss rate of the electron distribution as a whole. We conclude that
previous estimates of electron energy loss, particularly at energies
E ~ 10kT or less, have been greatly overestimated. Consequently the
required number of electrons at the low-energy end of the accelerated
electron spectrum, and concomitantly the overall energy content
in the accelerated electrons, are significantly reduced. Use of an
artificially-imposed “low-energy cutoff” in the accelerated spectrum
is therefore not only unwarranted, but also unnecessary.
---------------------------------------------------------
Title: Solar and Heliospheric Physics with the Square Kilometre Array
Authors: Nakariakov, V.; Bisi, M. M.; Browning, P. K.; Maia,
D.; Kontar, E. P.; Oberoi, D.; Gallagher, P. T.; Cairns, I. H.;
Ratcliffe, H.
2015aska.confE.169N Altcode: 2015PoS...215E.169N; 2015arXiv150700516N
The fields of solar radiophysics and solar system radio physics,
or radio heliophysics, will benefit immensely from an instrument
with the capabilities projected for SKA. Potential applications
include interplanetary scintillation (IPS), radio-burst tracking,
and solar spectral radio imaging with a superior sensitivity. These
will provide breakthrough new insights and results in topics of
fundamental importance, such as the physics of impulsive energy
releases, magnetohydrodynamic oscillations and turbulence, the
dynamics of post-eruptive processes, energetic particle acceleration,
the structure of the solar wind and the development and evolution of
solar wind transients at distances up to and beyond the orbit of the
Earth. The combination of the high spectral, time and spatial resolution
and the unprecedented sensitivity of the SKA will radically advance
our understanding of basic physical processes operating in solar and
heliospheric plasmas and provide a solid foundation for the forecasting
of space weather events.
---------------------------------------------------------
Title: Numerical and Observational Examination of the Spectral
Variation of Extended Coronal Hard X-Ray Sources
Authors: Stackhouse, Duncan James; Kontar, Eduard
2015TESS....130705S Altcode:
The presence of extended coronal hard X-ray sources, for example the
2005 Aug 23 flare, has opened up opportunities for probing the electrons
accelerated by solar flares. The loop morphology of the source enables,
through the use of RHESSI imaging spectroscopy, detailed analysis of
changes in the electron flux spectrum along the spatial extent of the
emission. We present a realistic one-dimensional model of the corona
subject to coulomb collisions and localised stochastic acceleration
akin to a looptop acceleration region. Both ballistic and diffusive
transport are examined and the variation of the density weighted
mean electron flux,<nVF> , along our numerical loop will be
compared with RHESSI observations to determine the dominant processes
in the corona. We will also comment on the validity of the leaky-box
Fokker-Planck approximation.
---------------------------------------------------------
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: Spatially Resolved Energetic Electron Properties for the 21
May 2004 Flare from Radio Observations and 3D Simulations
Authors: Kuznetsov, A. A.; Kontar, E. P.
2015SoPh..290...79K Altcode: 2014SoPh..tmp...69K; 2014arXiv1403.5751K
We investigated in detail the 21 May 2004 flare using simultaneous
observations of the Nobeyama Radioheliograph, the Nobeyama
Radiopolarimeters, the Reuven Ramaty High Energy Solar Spectroscopic
Imager (RHESSI), and the Solar and Heliospheric Observatory (SOHO). The
flare images in different spectral ranges reveal a well-defined
single flaring loop in this event. We simulated the gyrosynchrotron
microwave emission using the recently developed interactive IDL tool GX
Simulator. By comparing the simulation results with the observations, we
deduced the spatial and spectral properties of the non-thermal electron
distribution. The microwave emission has been found to be produced by
the high-energy electrons (> 100 keV) with a relatively hard spectrum
(δ≃2); the electrons were strongly concentrated near the loop top. At
the same time, the number of high-energy electrons near the footpoints
was too low to be detected in the RHESSI images and spatially unresolved
data. The SOHO Extreme-ultraviolet Imaging Telescope images and the
low-frequency microwave spectra suggest the presence of an extended
"envelope" of the loop with lower magnetic field. Most likely, the
energetic electron distribution in the considered flare reflects
the localized (near the loop top) particle acceleration (injection)
process accompanied by trapping and scattering.
---------------------------------------------------------
Title: The Formation of Kappa-distribution Accelerated Electron
Populations in Solar Flares
Authors: Bian, Nicolas H.; Emslie, A. Gordon; Stackhouse, Duncan J.;
Kontar, Eduard P.
2014ApJ...796..142B Altcode: 2014arXiv1410.0819B
Driven by recent RHESSI observations of confined loop-top hard
X-ray sources in solar flares, we consider stochastic acceleration
of electrons in the presence of Coulomb collisions. If electron
escape from the acceleration region can be neglected, the electron
distribution function is determined by a balance between diffusive
acceleration and collisions. Such a scenario admits a stationary
solution for the electron distribution function that takes the form
of a kappa distribution. We show that the evolution toward this kappa
distribution involves a "wave front" propagating forward in velocity
space, so that electrons of higher energy are accelerated later; the
acceleration timescales with energy according to τ<SUB>acc</SUB>
~ E <SUP>3/2</SUP>. At sufficiently high energies escape from the
finite-length acceleration region will eventually dominate. For such
energies, the electron velocity distribution function is obtained by
solving a time-dependent Fokker-Planck equation in the "leaky-box"
approximation. Solutions are obtained in the limit of a small escape
rate from an acceleration region that can effectively be considered
a thick target.
---------------------------------------------------------
Title: Large-scale simulations of solar type III radio bursts:
flux density, drift rate, duration, and bandwidth
Authors: Ratcliffe, H.; Kontar, E. P.; Reid, H. A. S.
2014A&A...572A.111R Altcode: 2014arXiv1410.2410R
Non-thermal electrons accelerated in the solar corona can produce
intense coherent radio emission, known as solar type III radio
bursts. This intense radio emission is often observed from hundreds
of MHz in the corona down to the tens of kHz range in interplanetary
space. It involves a chain of physical processes from the generation of
Langmuir waves to non-linear processes of wave-wave interaction. We
develop a self-consistent model to calculate radio emission from
a non-thermal electron population over a large frequency range,
including the effects of electron transport, Langmuir wave-electron
interaction, the evolution of Langmuir waves due to non-linear wave-wave
interactions, Langmuir wave conversion into electromagnetic emission,
and finally escape of the electromagnetic waves. For the first time
we simulate escaping radio emission over a broad frequency range
from 500 MHz down to a few MHz and infer key properties of the radio
emission observed: the onset (starting) frequency, identification as
fundamental or harmonic emission, peak flux density, instantaneous
frequency bandwidth, and timescales for rise and decay. By comparing
these large-scale simulations with the observations, we can identify
the processes governing the major type III solar radio burst
characteristics.
---------------------------------------------------------
Title: On Speeds of Exciter Beams of Interplanetary Type III Radio
Bursts
Authors: Krupar, V.; Kontar, E.; Soucek, J.; Santolik, O.; Maksimovic,
M.; Kruparova, O.
2014AGUFMSH13B4113K Altcode:
Type III radio bursts are intense radio emissions triggered by beams of
energetic electrons often associated with solar flares. These exciter
beams propagate outward the Sun along an open magnetic field line in
the corona and the interplanetary medium at large distances beyond 1
AU, where energetic electrons can be detected in situ by spacecraft. We
performed a statistical survey of 20 simple and isolated interplanetary
Type III radio bursts observed by STEREO/Waves between January 2013
and June 2014. We investigated time - frequency profiles to derive
speeds of exciter electron beams. We present evidence that these beams
decelerate in the solar wind. Obtained beam speeds range from 0.05c up
to 0.55c depending on initial assumptions. It corresponds to electron
energies between tens of eV and hundreds of keV.
---------------------------------------------------------
Title: Statistical Survey of Type III Radio Bursts at Long Wavelengths
Observed by the Solar TErrestrial RElations Observatory (STEREO)/
Waves Instruments: Radio Flux Density Variations with Frequency
Authors: Krupar, V.; Maksimovic, M.; Santolik, O.; Kontar, E. P.;
Cecconi, B.; Hoang, S.; Kruparova, O.; Soucek, J.; Reid, H.;
Zaslavsky, A.
2014SoPh..289.3121K Altcode: 2014SoPh..tmp...61K; 2014arXiv1410.2053K
We have performed a statistical study of 152 Type III radio bursts
observed by Solar TErrestrial RElations Observatory (STEREO)/Waves
between May 2007 and February 2013. We investigated the flux density
between 125 kHz and 16 MHz. Both high- and low-frequency cutoffs
were observed in 60 % of events, suggesting an important role of
propagation. As already reported by previous authors, we observed
that the highest flux density occurs at 1 MHz on both spacecraft. We
developed a simplified analytical model of the flux density as a
function of radial distance and compared it with the STEREO/Waves data.
---------------------------------------------------------
Title: The low-high-low trend of type III radio burst starting
frequencies and solar flare hard X-rays
Authors: Reid, Hamish A. S.; Vilmer, Nicole; Kontar, Eduard P.
2014A&A...567A..85R Altcode: 2014arXiv1403.1839R
<BR /> Aims: Using simultaneous X-ray and radio observations from
solar flares, we investigate the link between the type III radio burst
starting frequency and hard X-ray spectral index. For a proportion of
events the relation derived between the starting height (frequency)
of type III radio bursts and the electron beam velocity spectral index
(deduced from X-rays) is used to infer the spatial properties (height
and size) of the electron beam acceleration region. Both quantities
can be related to the distance travelled before an electron beam
becomes unstable to Langmuir waves. <BR /> Methods: To obtain a list
of suitable events we considered the RHESSI catalogue of X-ray flares
and the Phoenix 2 catalogue of type III radio bursts. From the 200
events that showed both type III and X-ray signatures, we selected 30
events which had simultaneous emission in both wavelengths, good signal
to noise in the X-ray domain and >20 s duration. <BR /> Results:
We find that >50% of the selected events show a good correlation
between the starting frequencies of the groups of type III bursts
and the hard X-ray spectral indices. A low-high-low trend for the
starting frequency of type III bursts is frequently observed. Assuming
a background electron density model and the thick target approximation
for X-ray observations, this leads to a correlation between starting
heights of the type III emission and the beam electron spectral
index. Using this correlation we infer the altitude and vertical
extents of the flare acceleration regions. We find heights from 183 Mm
down to 25 Mm while the sizes range from 13 Mm to 2 Mm. These values
agree with previous work that places an extended flare acceleration
region high in the corona. We also analyse the assumptions that are
required to obtain our estimates and explore possible extensions
to our assumed model. We discuss these results with respect to the
acceleration heights and sizes derived from X-ray observations
alone. <P />Appendices are available in electronic form at <A
href="http://www.aanda.org/10.1051/0004-6361/201321973/olm">http://www.aanda.org</A>
---------------------------------------------------------
Title: The Formation of Accelerated Electron Distributions in
Solar Flares
Authors: Emslie, A. Gordon; Bian, Nicolas; Kontar, Eduard
2014AAS...22412322E Altcode:
Driven by RHESSI observations of dense compact coronal hard X-ray
sources in solar flares, we study electron acceleration in such
regions. We consider the acceleration of electrons by a stochastic
process that is characterized by a diffusion coefficient D_turb ~
1/v in a collisional medium of finite length. If electron escape can
be neglected, the electron distribution function is determined by a
balance between stochastic acceleration and collisional friction. Such
a scenario admits a stationary solution for the electron distribution
function that takes the form of a kappa-distribution. We show how the
growth toward this stationary distribution can be described as a "wave"'
propagating forwards in velocity space, so that electrons of higher
energy E are accelerated later than lower-energy ones; quantitatively,
the acceleration time τ scales with E according to τ ~ E^{3/2}. Since
such an approach towards a stationary kappa distribution becomes
progressively slower at high energies, escape from the acceleration
region (of finite length L) will, at sufficiently high energies,
eventually dominate over collisions, and a different stationary
solution, corresponding to a balance between diffusive acceleration and
particle escape, is applicable in this energy range. Using a numerical
treatment, we derive the time evolution toward the stationary solution
for a range of parameters appropriate to the solar flare situation.
---------------------------------------------------------
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: Resonance broadening due to particle scattering and mode
coupling in the quasi-linear relaxation of electron beams
Authors: Bian, Nicolas H.; Kontar, Eduard P.; Ratcliffe, Heather
2014JGRA..119.4239B Altcode: 2015arXiv150301710B
Of particular interest for radio and hard X-ray diagnostics of
accelerated electrons during solar flares is the understanding of
the basic nonlinear mechanisms regulating the relaxation of electron
beams propagating in turbulent plasmas. In this work, it is shown
that in addition to scattering of beam electrons, scattering of the
beam-generated Langmuir waves via for instance mode coupling can
also result in broadening of the wave-particle resonance. We obtain
a resonance-broadened version of weak turbulence theory with mode
coupling to ion sound modes. Resonance broadening is presented here as
a unified framework which can quantitatively account for the reduction
and possible suppression of the beam instability due to background
scattering of the beam electrons themselves or due to scattering of
the beam-generated Langmuir waves in fluctuating plasmas. Resonance
broadening being essentially equivalent to smoothing of the electron
phase space distribution is used to construct an intuitive physical
picture for the stability of inverted populations of fast electrons
that are commonly observed in situ to propagate in the solar wind.
---------------------------------------------------------
Title: On the Variation of Solar Flare Coronal X-Ray Source Sizes
with Energy
Authors: Jeffrey, Natasha L. S.; Kontar, Eduard P.; Bian, Nicolas H.;
Emslie, A. Gordon
2014ApJ...787...86J Altcode: 2014arXiv1404.1962J
Observations with RHESSI have enabled the detailed study of the
structure of dense hard X-ray coronal sources in solar flares. The
variation of source extent with electron energy has been discussed in
the context of streaming of non-thermal particles in a one-dimensional
cold target model and the results used to constrain both the physical
extent of, and density within, the electron acceleration region. Here,
we extend this investigation to a more physically realistic model of
electron transport that takes into account the finite temperature
of the ambient plasma, the initial pitch angle distribution of the
accelerated electrons, and the effects of collisional pitch angle
scattering. The finite temperature results in the thermal diffusion
of electrons, which leads to the observationally inferred value of
the acceleration region volume being an overestimate of its true
value. The different directions of the electron trajectories, a
consequence of both the non-zero injection pitch angle and scattering
within the target, cause the projected propagation distance parallel
to the guiding magnetic field to be reduced, so that a one-dimensional
interpretation can overestimate the actual density by a factor of
up to ~6. The implications of these results for the determination of
acceleration region properties (specific acceleration rate, filling
factor, etc.) are discussed.
---------------------------------------------------------
Title: Plasma radio emission from inhomogeneous collisional plasma
of a flaring loop
Authors: Ratcliffe, H.; Kontar, E. P.
2014A&A...562A..57R Altcode: 2013arXiv1312.2792R
The evolution of a solar flare accelerated non-thermal electron
population and associated plasma emission is considered in collisional
inhomogeneous plasma. Non-thermal electrons collisionally evolve
to become unstable and generate Langmuir waves, which may lead
to intense radio emission. We self-consistently simulated the
collisional relaxation of electrons, wave-particle interactions,
and non-linear Langmuir wave evolution in plasma with density
fluctuations. Additionally, we simulated the scattering, decay,
and coalescence of the Langmuir waves which produce radio emission
at the fundamental or the harmonic of the plasma frequency, using an
angle-averaged emission model. Long-wavelength density fluctuations,
such as are observed in the corona, are seen to strongly suppress
the levels of radio emission, meaning that a high level of Langmuir
waves can be present without visible radio emission. Additionally, in
homogeneous plasma, the emission shows time and frequency variations
that could be smoothed out by density inhomogeneities.
---------------------------------------------------------
Title: Accelerated particles and their observational signatures from
confined solar flares in twisted coronal loops
Authors: Browning, Philippa; Kontar, Eduard; Vilmer, Nicole;
Gordovskyy, Mykola; Pinto, Rui; Bian, Nicolas
2014cosp...40E.416B Altcode:
Twisted magnetic fields provide a reservoir of free magnetic energy,
and are ubiquitous in the solar corona. Recent theoretical studies
suggest that the onset of the kink instability in twisted coronal loops
may generate fragmented current sheets throughout the loop, leading
to fast magnetic reconnection which dissipates magnetic energy. This
provides a viable model for small self-contained flares. Using a
combination of 3D MHD and guiding-centre test-particle simulations,
incorporating collisions with the background plasma, we study
the kinetics of non-thermal particles accelerated during magnetic
reconnection in a flaring twisted coronal loop. It is shown that this
model can provide the number of high-energy electrons and acceleration
efficiency comparable with those obtained from observations of small
flares. We consider various geometries: including idealised cylindrical
loop models, as well as, more realistically, curved loops. The effects
of gravitational stratification, which has very significant effects
on the non-thermal particles through collisions, are included. The
calculated loop temperatures and densities, and the energy spectra and
pitch-angles of the accelerated particles, are used to forward-model
the emission in both Soft X-rays and Hard X-rays, predicting spatial
distributions and temporal evolution, as well as radio emission arising
from cyclotron/synchrotron radiation. These properties may be compared
with observations.
---------------------------------------------------------
Title: Turbulent Pitch-angle Scattering and Diffusive Transport of
Hard X-Ray-producing Electrons in Flaring Coronal Loops
Authors: Kontar, Eduard P.; Bian, Nicolas H.; Emslie, A. Gordon;
Vilmer, Nicole
2014ApJ...780..176K Altcode: 2013arXiv1312.0266K
Recent observations from RHESSI have revealed that the number of
non-thermal electrons in the coronal part of a flaring loop can exceed
the number of electrons required to explain the hard X-ray-emitting
footpoints of the same flaring loop. Such sources cannot, therefore, be
interpreted on the basis of the standard collisional transport model, in
which electrons stream along the loop while losing their energy through
collisions with the ambient plasma; additional physical processes, to
either trap or scatter the energetic electrons, are required. Motivated
by this and other observations that suggest that high-energy electrons
are confined to the coronal region of the source, we consider turbulent
pitch-angle scattering of fast electrons off low-frequency magnetic
fluctuations as a confinement mechanism, modeled as a spatial diffusion
parallel to the mean magnetic field. In general, turbulent scattering
leads to a reduction of the collisional stopping distance of non-thermal
electrons along the loop, and hence to an enhancement of the coronal
hard X-ray source relative to the footpoints. The variation of source
size L with electron energy E becomes weaker than the quadratic behavior
pertinent to collisional transport, with the slope of L(E) depending
directly on the mean free path λ associated with the non-collisional
scattering mechanism. Comparing the predictions of the model with
observations, we find that λ ~ (10<SUP>8</SUP>-10<SUP>9</SUP>) cm for
~30 keV, less than the length of a typical flaring loop and smaller
than, or comparable to, the size of the electron acceleration region.
---------------------------------------------------------
Title: The variation of solar flare coronal X-ray source lengths
with energy
Authors: Jeffrey, Natasha; Kontar, Eduard; Emslie, A. Gordon; Bian,
Nicolas
2014cosp...40E1328J Altcode:
Observations with the Ramaty High Energy Solar Spectroscopic Imager
(RHESSI) have enabled the detailed study of the structure of dense
hard X-ray coronal sources in solar flares. The variation of coronal
source extent with electron energy has been discussed in the context
of streaming non-thermal particles in a one-dimensional cold target
model, and the results used to constrain both the physical extent of,
and density within, the electron acceleration region. I will discuss how
this investigation was extended to a more physically realistic model
of electron transport that takes into account the finite temperature
of the ambient plasma, the initial pitch angle distribution of the
accelerated electrons, and the effects of collisional pitch-angle
scattering. The finite temperature results in thermal diffusion of
electrons, which leads to the observationally-inferred value of the
acceleration region volume being an overestimate of its true value. The
different directions of the electron trajectories, a consequence
of both the non-zero injection pitch angle and scattering within
the target, cause the projected propagation distance parallel to
the guiding magnetic field to be reduced, so that a one-dimensional
interpretation can give an overestimate of the actual density by a
factor of 3-6. The implications of these results for the determination
of acceleration region properties such as specific acceleration rate
and filling factor will also be discussed.
---------------------------------------------------------
Title: The detailed height, temperature and density structure of
solar flare acceleration regions
Authors: Jeffrey, Natasha; Kontar, Eduard; Dennis, Brian
2014cosp...40E1329J Altcode:
Using Ramaty High Energy Solar Spectroscopic Imager (RHESSI) imaging
and spectroscopy observations, the height structure of flaring plasma
for two well-observed near-limb events with coronal X-ray emission is
investigated. For each event, the centroid height and X-ray energy
between 10-20 keV is found to correlate and can be fitted with a
straight line. We find that the average gradient for both events is 0.20
arcsec/keV. By using a simple non-isothermal X-ray source and assuming
only thermal bremsstrahlung emission, an equation relating the X-ray
energy at each height with the temperature and number density structure
of the region is derived. Multi-thermal functions are also fitted to the
lower energy spectrum of each event and the results are compared with
the predictions of different vertical temperature and number density
models input into our derived equation and via simple one-dimensional
modelling. One such model assumes that the temperature structure of the
region is controlled entirely by thermal conduction along the guiding
field of the loop, reproducing many of the results observed by RHESSI.
---------------------------------------------------------
Title: Particle acceleration and transport in reconnecting twisted
loops in a stratified atmosphere
Authors: Gordovskyy, M.; Browning, P. K.; Kontar, E. P.; Bian, N. H.
2014A&A...561A..72G Altcode: 2015arXiv150106418G
Context. Twisted coronal loops should be ubiquitous in the solar
corona. Twisted magnetic fields contain excess magnetic energy, which
can be released during magnetic reconnection, causing solar flares. <BR
/> Aims: The aim of this work is to investigate magnetic reconnection,
and particle acceleration and transport in kink-unstable twisted coronal
loops, with a focus on the effects of resistivity, loop geometry and
atmospheric stratification. Another aim is to perform forward-modelling
of bremsstrahlung emission and determine the structure of hard X-ray
sources. <BR /> Methods: We use a combination of magnetohydrodynamic
(MHD) and test-particle methods. First, the evolution of the kinking
coronal loop is considered using resistive MHD model, incorporating
atmospheric stratification and loop curvature. Then, the obtained
electric and magnetic fields and density distributions are used to
calculate electron and proton trajectories using a guiding-centre
approximation, taking into account Coulomb collisions. <BR />
Results: It is shown that electric fields in twisted coronal loops
can effectively accelerate protons and electrons to energies up to
10 MeV. High-energy particles have hard, nearly power-law energy
spectra. The volume occupied by high-energy particles demonstrates
radial expansion, which results in the expansion of the visible
hard X-ray loop and a gradual increase in hard X-ray footpoint
area. Synthesised hard X-ray emission reveals strong footpoint sources
and the extended coronal source, whose intensity strongly depends on
the coronal loop density.
---------------------------------------------------------
Title: a Statistic Study of Loop-Structured Solar
FLARES—IMPLICATIONS for Electron Acceleration Models
Authors: Guo, Jingnan; Piana, Michele; Kontar, Eduard; Emslie,
A. Gordon
2014cosp...40E1108G Altcode:
We analyze electron flux maps based on RHESSI hard X-ray imaging
spectroscopy data for a number of extended coronal loop flare
events. For each event, we determine the variation of the characteristic
loop length with electron energy, and we fit this observed behavior with
models that incorporate an extended acceleration region and an exterior
propagation region, and which may include collisional modification of
the accelerated electron spectrum inside the acceleration region. The
models are characterized by two parameters: the plasma density in,
and the longitudinal extent of, the acceleration region. Determination
of the best-fit values of these parameters permits inference of the
volume that encompasses the acceleration region and of the total number
of particles within it. It is then straightforward to compute values
for the emission filling factor and for the specific acceleration rate
(electrons per second per ambient electron above a chosen reference
energy). For the events studied, the range of inferred filling factors
is consistent with a value of unity. The inferred mean value of the
specific acceleration rate above 20 keV is about 0.01. We compare these
values with the predictions of several models, including acceleration
by large-scale, weak (sub-Dreicer) fields, by strong (super-Dreicer)
electric fields in a reconnecting current sheet, and by stochastic
acceleration processes.
---------------------------------------------------------
Title: Radio and X-ray Diagnostics of Electron Beams in Solar Flares
Authors: Vilmer, Nicole; Kontar, Eduard; Hamish; Reid, A. S.;
Maksimovic, Milan
2014cosp...40E3526V Altcode:
Solar flares are associated with efficient production of energetic
particles at all energies. While energetic electrons and ions
interacting with the solar atmosphere produce high energy X-rays
and gamma-rays, the energetic electrons escaping to the corona and
interplanetary medium produce coherent radio emissions (in particular
type III bursts) and may be directly detected by experiments aboard
spacecraft. We shall present the results of two statistical studies
combining X-ray observations from RHESSI and of type III bursts observed
in the decimeter/meter range and imaged by the Nançay Radioheliograph
We shall show how the combination of X-ray and radio observations
allows for some events to deduce the characteristics of the electron
beam acceleration sites (height and size). We shall also present the
results of a recent study on the percentage of decimetric/metric
type III bursts observed with Nançay which have a counterpart at
lower frequencies (namely in the range 14 to 1 MHz ) observed with
Wind/Waves. This study is based on a list of events for which X-ray
emission (by RHESSI) is also observed in connection with the type III
bursts. We shall discuss the different reasons which could explain
the extent or not of the metric type III burst to the hectometric range.
---------------------------------------------------------
Title: Electron spectra in the plasma of solar flares on the basis
of RHESSI, SDO/AIA observations
Authors: Motorina, Galina; Kontar, Eduard
2014cosp...40E2188M Altcode:
The temperature distribution of emitting plasma can be described by
the differential emission measure (DEM), which allows to get detailed
information about the mean temperature, the total emission measure,
mean electron flux spectrum. Hard X-rays resulting of bremsstrahlung
give direct information on the electron energy distribution of
electrons heated/accelerated during solar flares. The analyze of one
well-observed flare was made in an attempt to improve the electron
diagnostics from SDO/AIA and RHESSI. Combining these observations
allows the mean electron flux spectrum to be deduced from 0.1 keV
up to tens of keV. Mean electron flux spectrum was obtained from DEM
results derived from AIA images in different EUV wavelength channels
and RHESSI images. In this way we reconstructed the energy distribution
of electrons from different sources and made an examination of the
total spectrum.
---------------------------------------------------------
Title: Electron Distribution Functions in Solar Flares from Combined
X-Ray and Extreme-ultraviolet Observations
Authors: Battaglia, M.; Kontar, E. P.
2013ApJ...779..107B Altcode: 2013arXiv1310.3930B
Simultaneous solar flare observations with SDO and RHESSI provide
spatially resolved information about hot plasma and energetic particles
in flares. RHESSI allows the properties of both hot (gsim8 MK)
thermal plasma and non-thermal electron distributions to be inferred,
while SDO/AIA is more sensitive to lower temperatures. We present and
implement a new method to reconstruct electron distribution functions
from SDO/AIA data. The combined analysis of RHESSI and AIA data allows
the electron distribution function to be inferred over the broad
energy range from 0.1 keV up to a few tens of keV. The analysis of two
well-observed flares suggests that the distributions in general agree
to within a factor of three when the RHESSI values are extrapolated
into the intermediate range 1-3 keV, with AIA systematically predicting
lower electron fluxes. Possible instrumental and numerical effects,
as well as potential physical origins for this discrepancy, are
discussed. The inferred electron distribution functions in general
show one or two nearly Maxwellian components at energies below ~15
keV and a non-thermal tail above.
---------------------------------------------------------
Title: Solar Eruptive Events (SEE) 2020 Mission Concept
Authors: Lin, R. P.; Caspi, A.; Krucker, S.; Hudson, H.; Hurford,
G.; Bandler, S.; Christe, S.; Davila, J.; Dennis, B.; Holman, G.;
Milligan, R.; Shih, A. Y.; Kahler, S.; Kontar, E.; Wiedenbeck, M.;
Cirtain, J.; Doschek, G.; Share, G. H.; Vourlidas, A.; Raymond, J.;
Smith, D. M.; McConnell, M.; Emslie, G.
2013arXiv1311.5243L Altcode:
Major solar eruptive events (SEEs), consisting of both a large flare and
a near simultaneous large fast coronal mass ejection (CME), are the most
powerful explosions and also the most powerful and energetic particle
accelerators in the solar system, producing solar energetic particles
(SEPs) up to tens of GeV for ions and hundreds of MeV for electrons. The
intense fluxes of escaping SEPs are a major hazard for humans in space
and for spacecraft. Furthermore, the solar plasma ejected at high speed
in the fast CME completely restructures the interplanetary medium
(IPM) - major SEEs therefore produce the most extreme space weather
in geospace, the interplanetary medium, and at other planets. Thus,
understanding the flare/CME energy release process(es) and the related
particle acceleration processes are major goals in Heliophysics. To
make the next major breakthroughs, we propose a new mission concept,
SEE 2020, a single spacecraft with a complement of advanced new
instruments that focus directly on the coronal energy release and
particle acceleration sites, and provide the detailed diagnostics of
the magnetic fields, plasmas, mass motions, and energetic particles
required to understand the fundamental physical processes involved.
---------------------------------------------------------
Title: The Location of Non-thermal Velocity in the Early Phases of
Large Flares—Revealing Pre-eruption Flux Ropes
Authors: Harra, Louise K.; Matthews, Sarah; Culhane, J. L.; Cheung,
Mark C. M.; Kontar, Eduard P.; Hara, Hirohisa
2013ApJ...774..122H Altcode:
Non-thermal velocity measurements of the solar atmosphere, particularly
from UV and X-ray emission lines have demonstrated over the decades
that this parameter is important in understanding the triggering of
solar flares. Enhancements have often been observed before intensity
enhancements are seen. However, until the launch of Hinode, it has
been difficult to determine the spatial location of the enhancements to
better understand the source region. The Hinode EUV Imaging Spectrometer
has the spectral and spatial resolution to allow us to probe the early
stages of flares in detail. We analyze four events, all of which
are GOES M- or X-classification flares, and all are located toward
the limb for ease of flare geometry interpretation. Three of the
flares were eruptive and one was confined. In all events, pre-flare
enhancement in non-thermal velocity at the base of the active region
and its surroundings has been found. These enhancements seem to be
consistent with the footpoints of the dimming regions, and hence may
be highlighting the activation of a coronal flux rope for the three
eruptive events. In addition, pre-flare enhancements in non-thermal
velocity were found above the looptops for the three eruptive events.
---------------------------------------------------------
Title: Evolution of the Solar Flare Energetic Electrons in the
Inhomogeneous Inner Heliosphere
Authors: Reid, Hamish A. S.; Kontar, Eduard P.
2013SoPh..285..217R Altcode: 2012SoPh..tmp..109R; 2012arXiv1209.5347R
Solar flare accelerated electrons escaping into the interplanetary
space and seen as type III solar radio bursts are often detected
near the Earth. Using numerical simulations we consider the evolution
of energetic electron spectrum in the inner heliosphere and near the
Earth. The role of Langmuir wave generation, heliospheric plasma density
fluctuations, and expansion of magnetic field lines on the electron peak
flux and fluence spectra is studied to predict the electron properties
as could be observed by Solar Orbiter and Solar Probe Plus. Considering
various energy loss mechanisms we show that the substantial part of the
initial energetic electron energy is lost via wave-plasma processes due
to plasma inhomogeneity. For the parameters adopted, the results show
that the electron spectrum changes mostly at the distances before ∼
20 R<SUB>⊙</SUB>. Further into the heliosphere, the electron flux
spectrum of electrons forms a broken power law relatively similar to
what is observed at 1 AU.
---------------------------------------------------------
Title: Stochastic Simulations of the Pitch-angle Scattering of High
Energy Electrons
Authors: Dickson, Ewan C.; Kontar, E.; Fletcher, L.
2013SPD....44...56D Altcode:
The angular variation of high energy electrons during a solar flare is
key to understanding the acceleration mechanism. Regularised inversion
of RHESSI X-ray spectra, using the effect of photospheric albedo, allows
us to estimate the angular distributions of the emitting electrons. The
results for all flares studied are consistent with an isotropic
pitch-angle distribution, and inconsistent with a ratio of downward to
upward going electron flux greater than 3:1. To attempt to understand
these results, I have performed stochastic simulations of electron
pitch-angle scattering by Coulomb collisions, including the effects
of collisional energy loss, and of magnetic field convergence. This
allows us to estimate what constraints these observations put on the
parameters of the electron beam, such as initial directionality, and of
the characteristics of the loop itself. These simulations suggest that
Coulomb collisions cannot sufficiently isotropise the distribution to
be consistent with the observations, even for an initially isotropic
injected distribution.
---------------------------------------------------------
Title: Temporal Variations of X-Ray Solar Flare Loops: Length,
Corpulence, Position, Temperature, Plasma Pressure, and Spectra
Authors: Jeffrey, Natasha; Kontar, E.
2013SPD....4420101J Altcode:
The spatial and spectral properties of three solar flare coronal
X-ray loops are studied before, during, and after the peak X-ray
emission. Using observations from the Ramaty High Energy Solar
Spectroscopic Imager (RHESSI), we deduce the temporal changes in
emitting X-ray length, corpulence, volume, position, number density,
and thermal pressure. We observe a decrease in the loop length, width,
and volume before the X-ray peak, and an increasing number density
and thermal pressure. After the X-ray peak, volume increases and loop
corpulence grows due to increasing width. The volume variations are more
pronounced than the position variations, often known as magnetic field
line contraction. We believe this is the first dedicated study examining
the temporal evolution of X-ray loop lengths and widths. Collectively,
the observations also show for the first time three temporal phases
given by peaks in temperature, X-ray emission, and thermal pressure,
with the minimum volume coinciding with the X-ray peak. Although
the volume of the flaring plasma decreases before the peak in X-ray
emission, the relationship between temperature and volume does not
support simple compressive heating in a collapsing magnetic trap
model. Within a low β plasma, shrinking loop widths perpendicular
to the guiding field can be explained by squeezing the magnetic field
threading the region. Plasma heating leads to chromospheric evaporation
and growing number density. This produces increasing thermal pressure
and decreasing loop lengths as electrons interact at shorter distances
and we believe after the X-ray peak, the increasing loop corpulence.
---------------------------------------------------------
Title: Effect of Collisions and Magnetic Convergence on Electron
Acceleration and Transport in Reconnecting Twisted Solar Flare Loops
Authors: Gordovskyy, M.; Browning, P. K.; Kontar, E. P.; Bian, N. H.
2013SoPh..284..489G Altcode: 2012SoPh..tmp..225G; 2015arXiv150106436G
We study a model of particle acceleration coupled with an MHD model
of magnetic reconnection in unstable twisted coronal loops. The kink
instability leads to the formation of helical currents with strong
parallel electric fields resulting in electron acceleration. The motion
of electrons in the electric and magnetic fields of the reconnecting
loop is investigated using a test-particle approach taking into account
collisional scattering. We discuss the effects of Coulomb collisions and
magnetic convergence near loop footpoints on the spatial distribution
and energy spectra of high-energy electron populations and possible
implications on the hard X-ray emission in solar flares.
---------------------------------------------------------
Title: Measurements of Electron Anisotropy in Solar Flares Using
Albedo with RHESSI X-Ray Data
Authors: Dickson, E. C. M.; Kontar, E. P.
2013SoPh..284..405D Altcode: 2012arXiv1210.4757M; 2012SoPh..tmp..301D
The angular distribution of electrons accelerated in solar flares is a
key parameter in the understanding of the acceleration and propagation
mechanisms that occur there. However, the anisotropy of energetic
electrons is still a poorly known quantity, with observational studies
producing evidence for an isotropic distribution and theoretical
models mainly considering the strongly beamed case. We use the effect
of photospheric albedo to infer the pitch-angle distribution of X-ray
emitting electrons using Hard X-ray data from RHESSI. A bi-directional
approximation is applied and a regularised inversion is performed for
eight large flare events to deduce the electron spectra in both downward
(towards the photosphere) and upward (away from the photosphere)
directions. The electron spectra and the electron anisotropy ratios are
calculated for a broad energy range, from about ten up to ∼ 300 keV,
near the peak of the flares. The variation of electron anisotropy over
short periods of time lasting 4, 8 and 16 seconds near the impulsive
peak has been examined. The results show little evidence for strong
anisotropy and the mean electron flux spectra are consistent with
the isotropic electron distribution. The 3σ level uncertainties,
although energy and event dependent, are found to suggest that
anisotropic distribution with anisotropy larger than ∼ three are not
consistent with the hard X-ray data. At energies above 150 - 200 keV,
the uncertainties are larger and thus the possible electron anisotropies
could be larger.
---------------------------------------------------------
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: Transport of solar flare-accelerated energetic electron from
the Sun to the Earth
Authors: Kontar, E. P.; Reid, H.
2013AGUSMSH51C..03K Altcode:
During the periods of solar activity, the Sun produces accelerated
electron populations which escape often into the interplanetary
space. These escaping energetic electrons often manifest themselves
via observed type III solar radio bursts and can be directly detected
in-situ near the Earth. The transport of energetic electrons is
affected by number of processes including scattering, generation
and re-absorption of Langmuir waves. In addition, Langmuir waves
are affected by density inhomogeneities that, in turn, affect the
electron transport. I will present the recent numerical simulations of
the evolution of energetic electron spectrum in the inner heliosphere
and near the Earth. The role of Langmuir wave generation, heliospheric
plasma density fluctuations, and expansion of magnetic field lines on
the electron peak flux and fluence spectra will be discussed. Various
energy loss mechanisms are compared and we show that the substantial
part of the initial energetic electron energy is lost via wave-plasma
processes due to plasma inhomogeneity. It is also shown how the
initially power-law spectrum evolves to a broken power law, which is
similar to what is observed at 1 AU.
---------------------------------------------------------
Title: Multi-thermal dynamics and energetics of a coronal mass
ejection in the low solar atmosphere
Authors: Hannah, I. G.; Kontar, E. P.
2013A&A...553A..10H Altcode: 2012arXiv1212.5529H
<BR /> Aims: The aim of this work is to determine the multi-thermal
characteristics and plasma energetics of an eruptive plasmoid and
occulted flare observed by the Solar Dynamics Observatory's Atmospheric
Imaging Assembly (SDO/AIA). <BR /> Methods: We study a 2010 Nov. 3
event (peaking at 12:20 UT in GOES soft X-rays) of a coronal mass
ejection and occulted flare that demonstrates the morphology of a
classic erupting flux rope. The high spatial and time resolution and
six coronal channels of the SDO/AIA images allows the dynamics of the
multi-thermal emission during the initial phases of eruption to be
studied in detail. The differential emission measure is calculated,
using an optimized version of a regularized inversion method, for each
pixel across the six channels at different times, resulting in emission
measure maps and movies in a variety of temperature ranges. <BR />
Results: We find that the core of the erupting plasmoid is hot (8-11,
11-14 MK) with a similarly hot filamentary "stem" structure connecting
it to the lower atmosphere, which could be interpreted as the current
sheet in the flux rope model, though is wider than these models
suggest. The velocity of the leading edge of the eruption is 597-664
km s<SUP>-1</SUP> in the temperature range ≥3-4 MK and between
1029-1246 km s<SUP>-1</SUP> for ≤2-3 MK. We estimate the density
(in 11-14 MK) of the erupting core and stem during the impulsive phase
to be about 3 × 10<SUP>9</SUP> cm<SUP>-3</SUP>, 6 × 10<SUP>9</SUP>
cm<SUP>-3</SUP>, 9 × 10<SUP>8</SUP> cm<SUP>-3</SUP> in the plasmoid
core, stem, and surrounding envelope of material. This gives thermal
energy estimates of 5 × 10<SUP>29</SUP> erg, 1 × 10<SUP>29</SUP>
erg, and 2 × 10<SUP>30</SUP> erg. The kinetic energy for the core
and envelope is slightly lower. The thermal energy of the core
and current sheet grows during the eruption, suggesting continuous
influx of energy presumably via reconnection. <BR /> Conclusions:
The combination of the optimized regularized inversion method and
SDO/AIA data allows the multi-thermal characteristics (i.e. velocity,
density, and thermal energies) of the plasmoid eruption to be
determined. <P />A movie is available in electronic form at <A
href="http://www.aanda.org">http://www.aanda.org</A>
---------------------------------------------------------
Title: Stochastic Acceleration by Multi-Island Contraction during
Turbulent Magnetic Reconnection
Authors: Bian, Nicolas H.; Kontar, Eduard P.
2013PhRvL.110o1101B Altcode: 2013arXiv1302.6090B
The acceleration of charged particles in magnetized plasmas is
considered during turbulent multi-island magnetic reconnection. The
particle acceleration model is constructed for an ensemble of islands
which produce adiabatic compression of the particles. The model
takes into account the statistical fluctuations in the compression
rate experienced by the particles during their transport in the
acceleration region. The evolution of the particle distribution
function is described as a simultaneous first- and second-order Fermi
acceleration process. While the efficiency of the first-order process
is controlled by the average rate of compression, the second-order
process involves the variance in the compression rate. Moreover,
the acceleration efficiency associated with the second-order process
involves both the Eulerian properties of the compression field and the
Lagrangian properties of the particles. The stochastic contribution
to the acceleration is nonresonant and can dominate the systematic
part in the case of a large variance in the compression rate. The
model addresses the role of the second-order process, how the latter
can be related to the large-scale turbulent transport of particles,
and explains some features of the numerical simulations of particle
acceleration by multi-island contraction during magnetic reconnection.
---------------------------------------------------------
Title: Temporal Variations of X-Ray Solar Flare Loops: Length,
Corpulence, Position, Temperature, Plasma Pressure, and Spectra
Authors: Jeffrey, Natasha L. S.; Kontar, Eduard P.
2013ApJ...766...75J Altcode: 2013arXiv1302.2860J
The spatial and spectral properties of three solar flare coronal
X-ray loops are studied before, during, and after the peak X-ray
emission. Using observations from the Ramaty High Energy Solar
Spectroscopic Imager (RHESSI), we deduce the temporal changes in
emitting X-ray length, corpulence, volume, position, number density,
and thermal pressure. We observe a decrease in the loop length, width,
and volume before the X-ray peak, and an increasing number density
and thermal pressure. After the X-ray peak, volume increases and loop
corpulence grows due to increasing width. The volume variations are more
pronounced than the position variations, often known as magnetic field
line contraction. We believe this is the first dedicated study examining
the temporal evolution of X-ray loop lengths and widths. Collectively,
the observations also show for the first time three temporal phases
given by peaks in temperature, X-ray emission, and thermal pressure,
with the minimum volume coinciding with the X-ray peak. Although
the volume of the flaring plasma decreases before the peak in X-ray
emission, the relationship between temperature and volume does not
support simple compressive heating in a collapsing magnetic trap
model. Within a low β plasma, shrinking loop widths perpendicular
to the guiding field can be explained by squeezing the magnetic field
threading the region. Plasma heating leads to chromospheric evaporation
and growing number density. This produces increasing thermal pressure
and decreasing loop lengths as electrons interact at shorter distances
and we believe after the X-ray peak, the increasing loop corpulence.
---------------------------------------------------------
Title: Implications for electron acceleration and transport from
non-thermal electron rates at looptop and footpoint sources in
solar flares
Authors: Simões, P. J. A.; Kontar, E. P.
2013A&A...551A.135S Altcode: 2013arXiv1301.7591S
The interrelation of hard X-ray (HXR) emitting sources and the
underlying physics of electron acceleration and transport presents one
of the major questions in high-energy solar flare physics. Spatially
resolved observations of solar flares often demonstrate the presence of
well-separated sources of bremsstrahlung emission, so-called coronal
and footpoint sources. Using spatially resolved X-ray observations
by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI)
and recently improved imaging techniques, we investigate in detail
the spatially resolved electron distributions in a few well-observed
solar flares. The selected flares can be interpreted as having a
standard geometry with chromospheric HXR footpoint sources related to
thick-target X-ray emission and the coronal sources characterised by a
combination of thermal and thin-target bremsstrahlung. Using imaging
spectroscopy techniques, we deduce the characteristic electron rates
and spectral indices required to explain the coronal and footpoint
X-ray sources. We found that, during the impulsive phase, the electron
rate at the looptop is several times (a factor of 1.7-8) higher than
at the footpoints. The results suggest that a sufficient number of
electrons accelerated in the looptop explain the precipitation into
the footpoints and imply that electrons accumulate in the looptop. We
discuss these results in terms of magnetic trapping, pitch-angle
scattering, and injection properties. Our conclusion is that the
accelerated electrons must be subject to magnetic trapping and/or
pitch-angle scattering, keeping a fraction of the population trapped
inside the coronal loops. These findings put strong constraints on
the particle transport in the coronal source and provide quantitative
limits on deka-keV electron trapping/scattering in the coronal source.
---------------------------------------------------------
Title: Effect of turbulent density-fluctuations on wave-particle
interactions and solar flare X-ray spectra
Authors: Hannah, I. G.; Kontar, E. P.; Reid, H. A. S.
2013A&A...550A..51H Altcode: 2012arXiv1211.6015H
<BR /> Aims: The aim of this paper is to demonstrate the effect of
turbulent background density-fluctuations on flare-accelerated electron
transport in the solar corona. <BR /> Methods: Using the quasi-linear
approximation, we numerically simulated the propagation of a beam
of accelerated electrons from the solar corona to the chromosphere,
including the self-consistent response of the inhomogeneous background
plasma in the form of Langmuir waves. We calculated the X-ray spectrum
from these simulations using the bremsstrahlung cross-section and fitted
the footpoint spectrum using the collisional "thick-target" model,
a standard approach adopted in observational studies. <BR /> Results:
We find that the interaction of the Langmuir waves with the background
electron density gradient shifts the waves to a higher phase velocity
where they then resonate with higher velocity electrons. The consequence
is that some of the electrons are shifted to higher energies, producing
more high-energy X-rays than expected if the density inhomogeneity
is not considered. We find that the level of energy gain is strongly
dependent on the initial electron beam density at higher energy and
the magnitude of the density gradient in the background plasma. The
most significant gains are for steep (soft) spectra that initially
had few electrons at higher energies. If the X-ray spectrum of the
simulated footpoint emission are fitted with the standard "thick-target"
model (as is routinely done with RHESSI observations) some simulation
scenarios produce more than an order-of-magnitude overestimate of the
number of electrons >50 keV in the source coronal distribution.
---------------------------------------------------------
Title: Science enabled by high precision inertial formation flying
Authors: Skinner, Gerry K.; Dennis, Brian R.; Krizmanic, John F.;
Kontar, Eduard P.
2013IJSSE...1..331S Altcode: 2013arXiv1311.6955S
The capability of maintaining two satellites in precise relative
position, stable in a celestial coordinate system, would enable major
advances in a number of scientific disciplines and with a variety
of types of instrumentation. The common requirement is for formation
flying of two spacecraft with the direction of their vector separation
in inertial coordinates precisely controlled and accurately determined
as a function of time. We consider here the scientific goals that could
be achieved with such technology and review some of the proposals that
have been made for specific missions. Types of instrumentation that will
benefit from the development of this type of formation flying include 1)
imaging systems, in which an optical element on one spacecraft forms
a distant image recorded by a detector array on the other spacecraft,
including telescopes capable of very high angular resolution; 2) systems
in which the front spacecraft of a pair carries an occulting disk,
allowing very high dynamic range observations of the solar corona and
exoplanets; 3) interferometers, another class of instrument that aims
at very high angular resolution and which, though usually requiring
more than two spacecraft, demands very much the same developments.
---------------------------------------------------------
Title: Particle acceleration and dynamical heating in Cycle 24 flares
Authors: Hannah, I. G.; Fletcher, L.; Kontar, E. P.
2012AGUFMSH51C..07H Altcode:
The current wealth of solar observations presents a unique opportunity
to study energy release in solar flares, particularly particle
acceleration and plasma heating. The spatial and temporal resolution
of SDO/AIA EUV data give an unprecedented view of dynamical heating
in solar flares yet to fully exploit this resource the underlying
thermal properties of the emitting plasma needs to be recovered. This
is difficult as it is an ill-posed inverse problem and there is
copious data. Our recently implemented regularized inversion method
(Hannah & Kontar A&A 2012a,b) can quickly and robustly find the
Differential Emission Measure (DEM) solution (and its uncertainties),
with the resulting EM maps allowing the temperature and density
evolution to be studied both spatially and temporally. Combing this with
the hard X-ray imaging and spectroscopy of RHESSI, we present a study of
the non-thermal energy input and thermal response in some flares of the
rising phase of cycle 24. We also look at the relationship between the
energetics of flares and the underlying magnetic field configurations.
---------------------------------------------------------
Title: The Effects of Langmuir Wave Evolution on Type III Radio
Emission
Authors: Ratcliffe, H.; Kontar, E. P.; Bian, N.
2012AGUFMSH43A2145R Altcode:
High energy electron beams are produced during solar flares and
propagate through the inhomogeneous plasma of the solar corona,
generating high levels of Langmuir waves which can produce Type III
radio bursts. We consider the effects of long length-scale density
fluctuations on these Langmuir waves using a diffusive approximation,
and calculate the diffusion coefficients. We use 1-D simulations to
follow the time-evolution of the Langmuir waves, and the subsequent
effects on the electron beam. In addition, we simulate the Type III
radio emission from the Langmuir waves, and consider the modification
of brightness and spectrum due to inhomogeneities.
---------------------------------------------------------
Title: X-ray and EUV Observations of the Chromospheric and Coronal
Plasma Parameters during a Solar Flare
Authors: Battaglia, M.; Kontar, E. P.
2012AGUFMSH43B2161B Altcode:
X-ray and EUV observations are an important diagnostic tool of various
plasma parameters of the solar atmosphere during solar flares. Soft
X-ray and EUV observations often show coronal sources near the top
of flaring loops, while hard X-ray emission is mostly observed from
the chromospheric footpoints. Combining X-ray and EUV observations
allows for the first time to determine the thermal properties and the
density of the whole flaring loop as a function of height above the
photosphere. We present observations of a limb flare where we find
the emission measure, temperature, and density combining RHESSI with
simultaneous SDO/AIA observations. Soft X-ray images and spectra give
the plasma parameters of the coronal source while the chromospheric
density is found using RHESSI visibility analysis of the hard X-ray
footpoints. A regularized inversion technique is applied to AIA images
of the flare to find the differential emission measure DEM(T). Using
DEM maps, we determine the density and temperature structure along the
loop and compare it with RHESSI results. The combined EUV and X-ray
observations suggest that the density in the loop legs decreases
with increasing height from around 10<SUP>17</SUP> cm<SUP>-3</SUP>
near the photosphere to 10<SUP>10</SUP> cm<SUP>-3</SUP> in the coronal
source. The hottest plasma is found near the coronal loop top source
while the temperature along the legs of the loop is constant within
uncertainties.
---------------------------------------------------------
Title: RHESSI and SDO/AIA Observations of the Chromospheric and
Coronal Plasma Parameters during a Solar Flare
Authors: Battaglia, M.; Kontar, E. P.
2012ApJ...760..142B Altcode: 2012arXiv1210.3367B
X-ray and extreme ultraviolet (EUV) observations are an important
diagnostic of various plasma parameters of the solar atmosphere during
solar flares. Soft X-ray and EUV observations often show coronal
sources near the top of flaring loops, while hard X-ray emission is
mostly observed from chromospheric footpoints. Combining RHESSI with
simultaneous Solar Dynamics Observatory/Atmospheric Imaging Assembly
(AIA) observations, it is possible for the first time to determine the
density, temperature, and emission profile of the solar atmosphere
over a wide range of heights during a flare, using two independent
methods. Here we analyze a near limb event during the first of three
hard X-ray peaks. The emission measure, temperature, and density of
the coronal source is found using soft X-ray RHESSI images while the
chromospheric density is determined using RHESSI visibility analysis of
the hard X-ray footpoints. A regularized inversion technique is applied
to AIA images of the flare to find the differential emission measure
(DEM). Using DEM maps, we determine the emission and temperature
structure of the loop, as well as the density, and compare it with
RHESSI results. The soft X-ray and hard X-ray sources are spatially
coincident with the top and bottom of the EUV loop, but the bulk of
the EUV emission originates from a region without cospatial RHESSI
emission. The temperature analysis along the loop indicates that
the hottest plasma is found near the coronal loop-top source. The
EUV observations suggest that the density in the loop legs increases
with increasing height while the temperature remains constant within
uncertainties.
---------------------------------------------------------
Title: Multi-flare Study on Acceleration Regions using Correlated
X-ray and Radio Observations
Authors: Reid, H.; Vilmer, N.; Kontar, E. P.
2012AGUFMSH43A2151R Altcode:
X-ray and radio observations are found to be both temporally and
spatially correlated in a large number of solar flares. We use both
wavelength ranges to estimate properties about the spatial region in
solar flares which accelerate electrons. A list of events were studied
using the RHESSI X-ray flare catalogue and the PHOENIX 2 type III burst
catalogue. We find that some events show a very good anti-correlation
between the hard X-ray spectral index and the starting frequency of
type III radio bursts. Through an analytical relation we then constrain
the distance an upwardly travelling electron beam can propagate before
it becomes unstable to the generation of Langmuir waves. By assuming a
background density model we then infer the height and vertical extent
of a variety of different solar flare acceleration regions. Spatially
resolved images of each flare in both X-ray and radio wavelengths via
RHESSI and the Nançay Radioheliograph respectively are then used to
check the validity of the predicted heights.
---------------------------------------------------------
Title: Second-order Fermi acceleration by multi-island contraction
during turbulent magnetic reconnection
Authors: Bian, N.; Kontar, E. P.
2012AGUFMSH51B2239B Altcode:
Turbulence plays an important role in the energization of particles
throughout the heliosphere. The acceleration of particles in
magnetized plasmas is considered during turbulent multi-island magnetic
reconnection. The stochastic acceleration model is constructed for
an ensemble of islands, which undergo adiabatical compression or
expansion. The evolution of the particle distribution function is
described as simultaneous first and second order Fermi acceleration
processes. The second order process becomes efficient for strong
particle diffusion and dominates the first order one in the case of
a large spread in the relative magnitude of the reconnecting magnetic
fields. The model emphasizes the role of turbulence and explains some
features of the numerical PIC simulations of multi-island contraction
during turbulent magnetic reconnection.
---------------------------------------------------------
Title: The time evolving spatial and spectral properties of coronal
X-ray sources from solar flares
Authors: Jeffrey, N. L.; Kontar, E. P.
2012AGUFMSH43B2159J Altcode:
X-rays from solar flares serve as an important and direct observational
tool for determining how and why electrons are accelerated. Using
observations from the Ramaty High Energy Solar Spectroscopic Imager
(RHESSI) and the imaging algorithm of visibility forward fitting
(VIS FWDFIT) we studied the dynamics of source spatial properties:
lengths parallel to the guiding field, widths perpendicular to the
guiding field and the centroid positions of three coronal X-ray loop
top sources. We observed how these parameters changed in time during
the evolution of each flare; before, during and after the peak X-ray
emission at energies between 10-25 keV. For the first time, it was
observed that the lengths and widths, and hence the volumes, of each
source decreased while the X-ray emission was increasing. After the
peak in X-ray emission, the volume of each source increased, mainly
due to a growth in source width. For one event situated at the limb,
we also observed a decrease in altitude before the peak in X-ray
emission and an increase after the peak, consistent with the results
of other coronal loop source observations. However, we note that
the changes in loop volume were the dominant changes for each event,
over any position changes. Using our imaging parameters in combination
with spectral parameters (emission measure and plasma temperature),
we also inferred how the number density, thermal pressure and energy
density evolved during the observational time for each event. This
allowed us to build a fuller picture of how the coronal region changed
during the evolution of each flare. From our results, energy release
occurs during the entire observational time for each event; before,
during and after the peaks in X-ray emission and during both the
compression and expansion phases of the loop volumes.
---------------------------------------------------------
Title: Electron acceleration during beam relaxation and the
interpretation of hard X-ray spectra in solar flares
Authors: Kontar, E. P.; Ratcliffe, H.; Bian, N.
2012AGUFMSH43B2155K Altcode:
Non-thermal electrons accelerated during solar flares are abundant in
the solar corona and in interplanetary space. Commonly, the number and
energy of non-thermal electrons at the Sun is estimated through hard
X-ray (HXR) spectral observations (e.g. RHESSI) and a collisional
approximation. We investigate the role of the spectrally evolving
Langmuir turbulence on the population of energetic electrons in the
solar corona. The relaxation of a power-law non-thermal electron
population is simulated in a collisional inhomogeneous plasma,
including wave-particle and wave-wave interactions. Effects of
constant density gradient, density fluctuations and non-linear
interactions are considered. The simulations show that the long-time
evolution of electron population above 20 keV deviates substantially
from the collisional approximation when wave-particle interactions
in non-uniform plasma are taken into account. The evolution of
the Langmuir wave spectrum towards smaller wavenumbers, caused
by large-scale density fluctuations and wave-wave interactions,
leads to an effective acceleration of electrons. Furthermore, the
time-integrated spectrum of non-thermal electrons, which is normally
observed with HXR above around 20 keV, is noticeably increased because
of acceleration of non-thermal electrons through Langmuir waves. The
results suggest that the observed HXR spectrum, when interpreted in
terms of collisional relaxation, can lead to an overestimated number
and energy of energetic electrons accelerated in the corona.
---------------------------------------------------------
Title: Non-thermal Electron Rate at Loop-top and Foot-point Sources
of Solar Flares: Implications for Electron Acceleration
Authors: Simões, P. J.; Kontar, E. P.
2012AGUFMSH43B2157S Altcode:
Spatially resolved observations of solar flares often demonstrate
the presence of well separated sources of bremsstrahlung emission,
so-called coronal and footpoint sources. The interrelation of hard X-ray
emitting sources and the underlying physics of electron acceleration and
transport presents one of the major questions in the high energy solar
flare physics. Using spatially resolved X-ray observations by the Reuven
Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and recently
improved imaging techniques, we investigate in detail the electron
distributions in a few well observed solar flares. The selected flares
can be interpreted as having a standard geometry with chromospheric
sources (Hard X-ray (HXR) foot-points) related to thick-target X-ray
emission, while the coronal sources are normally characterized by a
combination of thermal and thin-target bremsstrahlung. Using imaging
spectroscopy technique, we deduce the characteristic electron rates and
spectral indices required to explain the coronal and footpoints X-ray
sources. We found that, during the impulsive phase, the electron rate is
a few times higher at the loop-top than at the foot-points. The results
are consistent with the electron acceleration source approximately
co-spatial with the thermal source and subsequent electron transport
towards the chromosphere. These findings put strong constraints on the
particles transport for the models of particle acceleration requiring
faster acceleration rate than scattering.
---------------------------------------------------------
Title: Density Fluctuations and the Acceleration of Electrons by
Beam-generated Langmuir Waves in the Solar Corona
Authors: Ratcliffe, H.; Bian, N. H.; Kontar, E. P.
2012ApJ...761..176R Altcode: 2012arXiv1211.2587R
Non-thermal electron populations are observed throughout the
heliosphere. The relaxation of an electron beam is known to produce
Langmuir waves which, in turn, may substantially modify the electron
distribution function. As the Langmuir waves are refracted by
background density gradients and as the solar and heliospheric plasma
density is naturally perturbed with various levels of inhomogeneity,
the interaction of Langmuir waves with non-thermal electrons in
inhomogeneous plasmas is an important topic. We investigate the role
played by ambient density fluctuations on the beam-plasma relaxation,
focusing on the effect of acceleration of beam electrons. The
scattering of Langmuir waves off turbulent density fluctuations is
modeled as a wavenumber diffusion process which is implemented in
numerical simulations of the one-dimensional quasilinear kinetic
equations describing the beam relaxation. The results show that
a substantial number of beam electrons are accelerated when the
diffusive timescale in wavenumber space τ<SUB> D </SUB> is of
the order of the quasilinear timescale τ<SUB>ql</SUB>, while
when τ<SUB> D </SUB> Lt τ<SUB>ql</SUB>, the beam relaxation is
suppressed. Plasma inhomogeneities are therefore an important means
of energy redistribution for waves and hence electrons, and so must
be taken into account when interpreting, for example, hard X-ray or
Type III emission from flare-accelerated electrons.
---------------------------------------------------------
Title: Electron acceleration during three-dimensional relaxation of
an electron beam-return current plasma system in a magnetic field
Authors: Karlický, M.; Kontar, E. P.
2012A&A...544A.148K Altcode: 2012arXiv1207.6248K
<BR /> Aims: We investigate the effects of acceleration during
non-linear electron-beam relaxation in magnetized plasma in the case of
electron transport in solar flares. <BR /> Methods: The evolution of
electron distribution functions is computed using a three-dimensional
particle-in-cell electromagnetic code. Analytical estimations under
simplified assumptions are made to provide comparisons. <BR /> Results:
We show that, during the non-linear evolution of the beam-plasma system,
the accelerated electron population appears. We found that, although
the electron beam loses its energy efficiently to the thermal plasma,
a noticeable part of the electron population is accelerated. For
model cases with initially monoenergetic beams in uniform plasma, we
found that the amount of energy in the accelerated electrons above the
injected beam-electron energy varies depending the plasma conditions and
could be around 10-30% of the initial beam energy. <BR /> Conclusions:
This type of acceleration could be important for the interpretation
of non-thermal electron populations in solar flares. Its neglect could
lead to the over-estimation of accelerated electron numbers. The results
emphasize that collective plasma effects should not be treated simply
as an additional energy-loss mechanism, when hard X-ray emission in
solar flares is interpreted, notably in the case of RHESSI data.
---------------------------------------------------------
Title: A Classification Scheme for Turbulent Acceleration Processes
in Solar Flares
Authors: Bian, Nicolas; Emslie, A. Gordon; Kontar, Eduard P.
2012ApJ...754..103B Altcode: 2012arXiv1206.0472B
We establish a classification scheme for stochastic acceleration
models involving low-frequency plasma turbulence in a strongly
magnetized plasma. This classification takes into account both the
properties of the accelerating electromagnetic field, and the nature
of the transport of charged particles in the acceleration region. We
group the acceleration processes as either resonant, non-resonant,
or resonant-broadened, depending on whether the particle motion is
free-streaming along the magnetic field, diffusive, or a combination
of the two. Stochastic acceleration by moving magnetic mirrors and
adiabatic compressions are addressed as illustrative examples. We obtain
expressions for the momentum-dependent diffusion coefficient D(p), both
for general forms of the accelerating force and for the situation when
the electromagnetic force is wave-like, with a specified dispersion
relation ω = ω(k). Finally, for models considered, we calculate the
energy-dependent acceleration time, a quantity that can be directly
compared with observations of the time profile of the radiation field
produced by the accelerated particles, such as those occuring during
solar flares.
---------------------------------------------------------
Title: Determination of the acceleration region size in a
loop-structured solar flare
Authors: Guo, J.; Emslie, A. G.; Kontar, E. P.; Benvenuto, F.; Massone,
A. M.; Piana, M.
2012A&A...543A..53G Altcode: 2012arXiv1206.0477G
<BR /> Aims: To study the acceleration and propagation of
bremsstrahlung-producing electrons in solar flares, we analyze the
evolution of the flare loop size with respect to energy at a variety
of times. A GOES M3.7 loop-structured flare starting around 23:55
on 2002 April 14 is studied in detail using Ramaty High Energy Solar
Spectroscopic Imager (RHESSI) observations. <BR /> Methods: We construct
photon and mean-electron-flux maps in 2-keV energy bins by processing
observationally-deduced photon and electron visibilities, respectively,
through several image-processing methods: a visibility-based forward-fit
(FWD) algorithm, a maximum entropy (MEM) procedure and the uv-smooth
(UVS) approach. We estimate the sizes of elongated flares (i.e.,
the length and width of flaring loops) by calculating the second
normalized moments of the intensity in any given map. Employing a
collisional model with an extended acceleration region, we fit the
loop lengths as a function of energy in both the photon and electron
domains. <BR /> Results: The resulting fitting parameters allow us
to estimate the extent of the acceleration region which is between ~
13 arcsec and ~19 arcsec. Both forward-fit and uv-smooth algorithms
provide substantially similar results with a systematically better
fit in the electron domain. <BR /> Conclusions: The consistency of
the estimates from these methods provides strong support that the
model can reliably determine geometric parameters of the acceleration
region. The acceleration region is estimated to be a substantial
fraction (~1/2) of the loop extent, indicating that this dense flaring
loop incorporates both acceleration and transport of electrons, with
concurrent thick-target bremsstrahlung emission.
---------------------------------------------------------
Title: Multi-flare study of acceleration region characteristics
using combined X-ray and Radio Observations
Authors: Reid, Hamish; Kontar, Eduard; Vilmer, Nicole
2012cosp...39.1605R Altcode: 2012cosp.meet.1605R
Using emission in X-ray and radio wavelengths, we infer properties
of accelerated electrons to indirectly obtain estimates about flare
acceleration regions. We have selected a list of events using the
RHESSI flare catalogue and the PHOENIX 2 radio burst list that show
temporally correlated X-ray and radio emission. We find some events
show a very good anti-correlation between the hard X-ray spectral index
and the starting frequency of type III bursts. We use this information
to constrain the distance an outwardly propagating electron beam can
travel before it undergoes the bump-in-tail instability. By assuming
the height dependence of the background electron density we are able to
observationally estimate the height and vertical extent of a variety of
different solar flare acceleration regions. We verify the feasibility
of these predictions by using kinetic simulations to check the Langmuir
wave-particle instability distance for electron beam.
---------------------------------------------------------
Title: RHESSI and SDO observations of X-ray, EUV, and white-light
footpoint emissions in a gamma-ray solar flare
Authors: Kontar, Eduard; Battaglia, Marina
2012cosp...39..968K Altcode: 2012cosp.meet..968K
Using simultaneous RHESSI and SDO data, we study chromospheric heights
of hard X-ray (HXR), EUV and white-light continuum sources in a well
observed gamma-ray solar flare. HXr visibility approach allows us to
determine the height of the HXR sources as a function of energy above
the photosphere. Co-aligning AIA/SDO and HMI/SDO images with 35-100 keV
HXR RHESSI data, we infer the heights and characteristic densities of
HXR, EUV and continuum sources in a flaring footpoint. The maximum of
white-light emission appears between the HXR and EUV emission presumably
in the transition between ionized and neutral atmospheres. We note
that the energy deposited by low energy electrons is sufficient to
explain the energetics of optical and UV emissions.
---------------------------------------------------------
Title: Numerical Simulations of Chromospheric Hard X-Ray Source
Sizes in Solar Flares
Authors: Battaglia, M.; Kontar, E. P.; Fletcher, L.; MacKinnon, A. L.
2012ApJ...752....4B Altcode: 2012arXiv1204.1151B
X-ray observations are a powerful diagnostic tool for transport,
acceleration, and heating of electrons in solar flares. Height and
size measurements of X-ray footpoint sources can be used to determine
the chromospheric density and constrain the parameters of magnetic
field convergence and electron pitch-angle evolution. We investigate
the influence of the chromospheric density, magnetic mirroring, and
collisional pitch-angle scattering on the size of X-ray sources. The
time-independent Fokker-Planck equation for electron transport is
solved numerically and analytically to find the electron distribution
as a function of height above the photosphere. From this distribution,
the expected X-ray flux as a function of height, its peak height, and
full width at half-maximum are calculated and compared with RHESSI
observations. A purely instrumental explanation for the observed
source size was ruled out by using simulated RHESSI images. We find
that magnetic mirroring and collisional pitch-angle scattering tend
to change the electron flux such that electrons are stopped higher in
the atmosphere compared with the simple case with collisional energy
loss only. However, the resulting X-ray flux is dominated by the
density structure in the chromosphere and only marginal increases in
source width are found. Very high loop densities (>10<SUP>11</SUP>
cm<SUP>-3</SUP>) could explain the observed sizes at higher energies,
but are unrealistic and would result in no footpoint emission below
about 40 keV, contrary to observations. We conclude that within
a monolithic density model the vertical sizes are given mostly by
the density scale height and are predicted smaller than the RHESSI
results show.
---------------------------------------------------------
Title: Simulating The Effects Of Turbulent Density Fluctuations On
Solar Flare X-ray Spectrum
Authors: Hannah, Iain; Kontar, E. P.; Reid, H. A. S.
2012AAS...22020403H Altcode:
The unprecedented RHESSI observations of solar flare hard X-rays (HXR)
has forced us to consider mechanisms in addition to the traditional
collisional view of coronal electron transport. The self-consistent
generation of Langmuir waves by the electron beam is one such process,
thought to be the source of the reverse drift decimetric radio emission
seen in some flares. We have previously shown that the inclusion of
Langmuir waves flattens the electron spectrum (Hannah et al. ApJ 2009)
and produces a spectral index difference between the coronal and
footpoint sources closer to observations (Hannah and Kontar A&A
2011). However the wave growth also results in fainter HXR emission
requiring a higher flux of electrons to be accelerated, compounding
the “number” problem. In this work we show that the addition of the
interaction of the Langmuir waves with turbulent density fluctuations in
the background plasma greatly alleviates this problem. We demonstrate
the consequences of these self-consistent numerical simulations in
the context of the observable HXR spectrum for a variety of forms of
the density fluctuations.
---------------------------------------------------------
Title: Dynamical Heating In Flares Observed With SDO/AIA & RHESSI
Authors: Hannah, Iain; Fletcher, L.; Kontar, E. P.
2012AAS...22032202H Altcode:
The spatial and temporal resolution of SDO/AIA data presents an
unprecedented view of the dynamics of heating during solar flares. This
combined with the non-thermal energetics from RHESSI hard X-ray imaging
and spectroscopy provides constraints on the flaring energy release. The
recently implemented regularized inversion method (Hannah & Kontar
A&A 2012) robustly recovers the underlying thermal distribution (the
Differential Emission Measure, DEM) of the coronal plasma from SDO/AIA
images. Crucially it is not limited to the isothermal or Gaussian-model
approximation that some other approaches depend upon. Our method
provides the uncertainties in the DEM and is computationally quick,
producing DEMs per pixel for a series of SDO/AIA images allowing
temperature maps and movies to be created. We use the regularized
inversion method to study the temporal and spatial evolution of the
plasma heating in flares and show how the non-thermal energy relates
to this. We also investigate how the calibration errors/uncertainties
affect the inferred DEMs and errors.
---------------------------------------------------------
Title: Effects Of Langmuir Waves On Flare-accelerated Electrons In
The Inhomogeneous Coronal Plasma
Authors: Ratcliffe, Heather; Kontar, E. P.
2012AAS...22052101R Altcode:
Solar flares accelerate beams of electrons, the number and energy
of which may be estimated from the hard X-rays (HXR) they emit. The
standard collisional thick target model assumes the emitting electrons
are modified only by collisions. However, the electron beam produces
Langmuir waves as it propagates through the inhomogeneous solar
corona. The interaction between these Langmuir waves and density
fluctuations, in the limit of fluctuations with spatial scale much
larger than the Langmuir wavelength, can be described by a diffusion
equation. We evaluate the diffusion coefficient for various spectra of
density fluctuations, considering both those due to waves with a defined
dispersion relation, and those with arbitrary frequency and wavenumber
spectra. Simulations for the case of a 1-dimensional electron beam
generating Langmuir waves which then diffuse, showed an acceleration
effect on the electrons, increasing the number at high energies in the
time-integrated distribution. The magnitude and form of the diffusion
coefficient may therefore be important in the interpretation of HXR
observations of non-thermal electrons, as the increased number of
electrons at high energies could lead to an overestimate of the total
number and energy of the originally accelerated electrons.
---------------------------------------------------------
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: The Energy-Dependent Growth Of RHESSI HXR Loops As A Possible
Signature Of Turbulent Acceleration
Authors: Hannah, Iain; Kontar, E. P.; Jeffrey, N. L. S.
2012AAS...22020404H Altcode:
Plasma turbulence is thought to be associated with various physical
processes involved in solar flares, including magnetic reconnection,
particle acceleration, and transport, though there is no direct
observational basis. Using RHESSI observations and the hard X-ray (HXR)
visibility analysis, we determine the spatial and spectral distributions
of energetic electrons for a number of flares with HXR loops. The loop
length and width is energy-dependent, its growth a signature of the
transport of tens of keV electrons both along and across the guiding
magnetic field of the loop. We show that the cross-field transport
is consistent with the presence of magnetic turbulence in the loop,
where electrons are accelerated, and estimate the magnitude of the
field line diffusion coefficient for different phases of the flares. The
relative energy density of magnetic fluctuations is calculated for given
magnetic field correlation lengths and the level of the fluctuations is
investigated as a function of the number of accelerated electrons for
different phases of the flares. These HXR observations are consistent
with the scenario that magnetic turbulence governs the evolution of
energetic electrons in dense flaring loops and is suggestive of their
turbulent acceleration.
---------------------------------------------------------
Title: Solar Particle Acceleration Radiation and Kinetics (SPARK). A
mission to understand the nature of particle acceleration
Authors: Matthews, Sarah A.; Williams, David R.; Klein, Karl-Ludwig;
Kontar, Eduard P.; Smith, David M.; Lagg, Andreas; Krucker, Sam;
Hurford, Gordon J.; Vilmer, Nicole; MacKinnon, Alexander L.; Zharkova,
Valentina V.; Fletcher, Lyndsay; Hannah, Iain G.; Browning, Philippa
K.; Innes, Davina E.; Trottet, Gerard; Foullon, Clare; Nakariakov,
Valery M.; Green, Lucie M.; Lamoureux, Herve; Forsyth, Colin; Walton,
David M.; Mathioudakis, Mihalis; Gandorfer, Achim; Martinez-Pillet,
Valentin; Limousin, Olivier; Verwichte, Erwin; Dalla, Silvia; Mann,
Gottfried; Aurass, Henri; Neukirch, Thomas
2012ExA....33..237M Altcode: 2011ExA...tmp..124M
Energetic particles are critical components of plasma populations
found throughout the universe. In many cases particles are accelerated
to relativistic energies and represent a substantial fraction of
the total energy of the system, thus requiring extremely efficient
acceleration processes. The production of accelerated particles
also appears coupled to magnetic field evolution in astrophysical
plasmas through the turbulent magnetic fields produced by diffusive
shock acceleration. Particle acceleration is thus a key component
in helping to understand the origin and evolution of magnetic
structures in, e.g. galaxies. The proximity of the Sun and the range
of high-resolution diagnostics available within the solar atmosphere
offers unique opportunities to study the processes involved in particle
acceleration through the use of a combination of remote sensing
observations of the radiative signatures of accelerated particles, and
of their plasma and magnetic environment. The SPARK concept targets the
broad range of energy, spatial and temporal scales over which particle
acceleration occurs in the solar atmosphere, in order to determine how
and where energetic particles are accelerated. SPARK combines highly
complementary imaging and spectroscopic observations of radiation from
energetic electrons, protons and ions set in their plasma and magnetic
context. The payload comprises focusing-optics X-ray imaging covering
the range from 1 to 60 keV; indirect HXR imaging and spectroscopy
from 5 to 200 keV, γ-ray spectroscopic imaging with high-resolution
LaBr<SUB>3</SUB> scintillators, and photometry and source localisation
at far-infrared wavelengths. The plasma environment of the regions
of acceleration and interaction will be probed using soft X-ray
imaging of the corona and vector magnetography of the photosphere
and chromosphere. SPARK is designed for solar research. However,
in addition it will be able to provide exciting new insights into the
origin of particle acceleration in other regimes, including terrestrial
gamma-ray flashes (TGF), the origin of γ-ray bursts, and the possible
existence of axions.
---------------------------------------------------------
Title: Wave-particle interactions in non-uniform plasma and the
interpretation of hard X-ray spectra in solar flares
Authors: Kontar, E. P.; Ratcliffe, H.; Bian, N. H.
2012A&A...539A..43K Altcode: 2011arXiv1112.4448K
Context. High-energy electrons accelerated during solar flares are
abundant in the solar corona and in interplanetary space. Commonly,
the number and energy of non-thermal electrons at the Sun is estimated
through hard X-ray (HXR) spectral observations (e.g. RHESSI) and a
single-particle collisional approximation. <BR /> Aims: We aim to
investigate the role of the spectrally evolving Langmuir turbulence
on the population of energetic electrons in the solar corona. <BR
/> Methods: We numerically simulated the relaxation of a power-law
non-thermal electron population in a collisional inhomogeneous plasma,
including wave-particle and wave-wave interactions. <BR /> Results:
The numerical simulations show that the long-time evolution of electron
population above 20 keV deviates substantially from the collisional
approximation when wave-particle interactions in non-uniform plasma
are taken into account. The evolution of the Langmuir wave spectrum
towards smaller wavenumbers, caused by large-scale density fluctuations
and wave-wave interactions, leads to an effective acceleration of
electrons. Furthermore, the time-integrated spectrum of non-thermal
electrons, which is normally observed with HXR above 20 keV, is
noticeably increased because of acceleration of non-thermal electrons
through Langmuir waves. <BR /> Conclusions: The results show that
the observed HXR spectrum, when interpreted in terms of collisional
relaxation, can lead to an overestimated number and energy of energetic
electrons accelerated in the corona.
---------------------------------------------------------
Title: Differential emission measures from the regularized inversion
of Hinode and SDO data
Authors: Hannah, I. G.; Kontar, E. P.
2012A&A...539A.146H Altcode: 2012arXiv1201.2642H
<BR /> Aims: To demonstrate the capabilities of regularized inversion
to recover differential emission measures (DEMs) from multi-wavelength
observations provided by telescopes such as Hinode and SDO. <BR />
Methods: We develop and apply an enhanced regularization algorithm,
used in RHESSI X-ray spectral analysis, to constrain the ill-posed
inverse problem that is determining the DEM from solar observations. We
demonstrate this computationally fast technique applied to a range of
DEM models simulating broadband imaging data from SDO/AIA and high
resolution line spectra from Hinode/EIS, as well as actual active
region observations with Hinode/EIS and XRT. As this regularization
method naturally provides both vertical and horizontal (temperature
resolution) error bars we are able to test the role of uncertainties
in the data and response functions. <BR /> Results: The regularization
method is able to successfully recover the DEM from simulated data
of a variety of model DEMs (single Gaussian, multiple Gaussians and
CHIANTI DEM models). It is able to do this, at best, to over four
orders of magnitude in DEM space but typically over two orders of
magnitude from peak emission. The combination of horizontal and
vertical error bars and the regularized solution matrix allows us
to easily determine the accuracy and robustness of the regularized
DEM. We find that the typical range for the horizontal errors is Δlog
T ≈ 0.1-0.5 and this is dependent on the observed signal to noise,
uncertainty in the response functions as well as the source model and
temperature. With Hinode/EIS an uncertainty of 20% greatly broadens
the regularized DEMs for both Gaussian and CHIANTI models although
information about the underlying DEMs is still recoverable. When
applied to real active region observations with Hinode/EIS and XRT the
regularization method is able to recover a DEM similar to that found
via a MCMC method but in considerably less computational time. <BR />
Conclusions: Regularized inversion quickly determines the DEM from solar
observations and provides reliable error estimates (both horizontal
and vertical) which allows the temperature spread of coronal plasma
to be robustly quantified.
---------------------------------------------------------
Title: Electron Acceleration Associated with Solar Jets
Authors: Krucker, Säm; Kontar, E. P.; Christe, S.; Glesener, L.;
Lin, R. P.
2011ApJ...742...82K Altcode:
This paper investigates the solar source region of supra-thermal
(few keV up to the MeV range) electron beams observed near Earth
by combining in situ measurements of the three-dimensional Plasma
and Energetic Particles experiment on the WIND spacecraft with
remote-sensing hard X-ray observations by the Reuven Ramaty High
Energy Solar Spectroscopic Imager. The in situ observations are used
to identify events, and the hard X-ray observations are then searched
for signatures of supra-thermal electrons radiating bremsstrahlung
emission in the solar atmosphere. Only prompt events detected above
50 keV with a close temporal correlation between the flare hard
X-ray emission and the electrons seen near Earth are selected,
limiting the number of events to 16. We show that for 7 of these
16 events, hard X-ray imaging shows three chromospheric sources:
two at the footpoints of the post-flare loop and one related to an
apparently open field line. The remaining events show two footpoints
(seven events, four of which show elongated sources possibly hiding
a third source) or are spatially unresolved (two events). Out of the
16 events, 6 have a solar source region within the field of view of
the Transition Region and Corona Explorer (TRACE). All events with
TRACE data show EUV jets that have the same onset as the hard X-ray
emission (within the cadence of tens of seconds). After the hard X-ray
burst ends, the jets decay. These results suggest that escaping prompt
supra-thermal electron events observed near Earth are accelerated in
flares associated with reconnection between open and closed magnetic
field lines, the so-called interchange reconnection scenario.
---------------------------------------------------------
Title: Spatially resolved hard X-ray polarization in solar flares:
effects of Compton scattering and bremsstrahlung
Authors: Jeffrey, N. L. S.; Kontar, E. P.
2011A&A...536A..93J Altcode: 2011arXiv1110.4993J
<BR /> Aims: We study the polarization of hard X-ray (HXR) sources
in the solar atmosphere, including Compton backscattering of photons
in the photosphere (the albedo effect) and the spatial distribution
of polarization across the source. <BR /> Methods: HXR photon
polarization and spectra produced via electron-ion bremsstrahlung
emission are calculated from various electron distributions typical
for solar flares. Compton scattering and photoelectric absorption
are then modelled using Monte Carlo simulations of photon transport
in the photosphere to study the observed (primary and albedo)
sources. Polarization maps across HXR sources (primary and albedo
components) for each of the modelled electron distributions are
calculated at various source locations from the solar centre to
the limb. <BR /> Results: We show that Compton scattering produces a
distinct polarization variation across the albedo patch at peak albedo
energies of 20-50 keV for all anisotropies modelled. The results
show that there are distinct spatial polarization changes in both
the radial and perpendicular to radial directions across the extent
of the HXR source at a given disk location. In the radial direction,
the polarization magnitude and direction at specific positions along
the HXR source will either increase or decrease with increased photon
distribution directivity towards the photosphere. We also show how high
electron cutoff energies influence the direction of polarization at
above ~100 keV. <BR /> Conclusions: Spatially resolved HXR polarization
measurements can provide important information about the directivity
and energetics of the electron distribution. Our results indicate the
preferred angular resolution of polarization measurements required
to distinguish between the scattered and primary components. We also
show how spatially resolved polarization measurements could be used to
probe the emission pattern of an HXR source, using both the magnitude
and the direction of the polarization.
---------------------------------------------------------
Title: High Energy Solar Physics Data in Europe (HESPE): a European
project for the exploitation of hard X-ray data in solar flare physics
Authors: Piana, M.; Csillaghy, A.; Kontar, E. P.; Fletcher, L.;
Veronig, A. M.; Vilmer, N.; Hurford, G. J.; Dennis, B. R.; Schwartz,
R. A.; Massone, A.; Krucker, S.; Benvenuto, F.; Etesi, L. I.; Guo,
J.; Hochmuth, N.; Reid, H.
2011AGUFMSH33B2068P Altcode:
It has been recognized since the early days of the space program that
high-energy observations play a crucial role in understanding the basic
mechanisms of solar eruptions. Unfortunately, the peculiar nature of
this radiation makes it so difficult to extract useful information
from it that non-conventional observational techniques together with
complex data analysis procedures must be adopted. HESPE is a European
project funded within the seventh Framework Program, with the aim of
realizing computational methods for solar high-energy data analysis and
technological tools for the intelligent exploitation of science-ready
products. Such products and methods are put at disposal of the solar,
heliospheric and space weather communities, who will exploit them in
order to build flare prediction models and to integrate the information
extracted from hard X-rays and gamma rays data, with the one extracted
from other wavelengths data.
---------------------------------------------------------
Title: 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: RHESSI and SDO observations of X-ray, EUV, and white-light
footpoint emission in a gamma-ray solar flare
Authors: Kontar, E. P.; Battaglia, M.
2011AGUFMSH33A2038K Altcode:
Using simultaneous RHESSI and SDO data, we study chromospheric heights
of hard X-ray (HXR), EUV and white-light continuum sources in a well
observed gamma-ray solar flare. HXr visibility approach allows us to
determine the height of the HXR sources as a function of energy above
the photosphere. Co-aligning AIA/SDO and HMI/SDO images with 35-100 keV
HXR RHESSI data, we infer the heights and characteristic densities of
HXR, EUV and continuum sources in a flaring footpoint. The maximum of
white-light emission appears between the HXR and EUV emission presumably
in the transition between ionized and neutral atmospheres. We note
that the energy deposited by low energy electrons is sufficient to
explain the energetics of optical and UV emissions.
---------------------------------------------------------
Title: Turbulent cross-field transport of non-thermal electrons in
coronal loops: theory and observations
Authors: Bian, N. H.; Kontar, E. P.; MacKinnon, A. L.
2011A&A...535A..18B Altcode: 2011arXiv1110.0935B
Context. A fundamental problem in astrophysics is the interaction
between magnetic turbulence and charged particles. It is now possible to
use Ramaty High Energy Solar Spectroscopic Imager (RHESSI) observations
of hard X-rays (HXR) emitted by electrons to identify the presence of
turbulence and to estimate the magnitude of the magnetic field line
diffusion coefficient at least in dense coronal flaring loops. <BR />
Aims: We discuss the various possible regimes of cross-field transport
of non-thermal electrons resulting from broadband magnetic turbulence
in coronal loops. The importance of the Kubo number K as a governing
parameter is emphasized and results applicable in both the large and
small Kubo number limits are collected. <BR /> Methods: Generic models,
based on concepts and insights developed in the statistical theory of
transport, are applied to the coronal loops and to the interpretation of
hard X-ray imaging data in solar flares. The role of trapping effects,
which become important in the non-linear regime of transport, is
taken into account in the interpretation of the data. <BR /> Results:
For this flaring solar loop, we constrain the ranges of parallel and
perpendicular correlation lengths of turbulent magnetic fields and
possible Kubo numbers. We show that a substantial amount of magnetic
fluctuations with energy ~1% (or more) of the background field can be
inferred from the measurements of the magnetic diffusion coefficient
inside thick-target coronal loops.
---------------------------------------------------------
Title: High-resolution Imaging of Solar Flare Ribbons and Its
Implication on the Thick-target Beam Model
Authors: Krucker, Säm; Hudson, H. S.; Jeffrey, N. L. S.; Battaglia,
M.; Kontar, E. P.; Benz, A. O.; Csillaghy, A.; Lin, R. P.
2011ApJ...739...96K Altcode:
We report on high-resolution optical and hard X-ray observations of
solar flare ribbons seen during the GOES X6.5 class white-light flare
of 2006 December 6. The data consist of imaging observations at 430 nm
(the Fraunhofer G band) taken by the Hinode Solar Optical Telescope
with the hard X-rays observed by the Reuven Ramaty High Energy Solar
Spectroscopic Imager. The two sets of data show closely similar ribbon
structures, strongly suggesting that the flare emissions in white light
and in hard X-rays have physically linked emission mechanisms. While
the source structure along the ribbons is resolved at both wavelengths
(length ~ 30”), only the G-band observations resolve the width of the
ribbon, with values between ~0farcs5 and ~1farcs8. The unresolved
hard X-ray observations reveal an even narrower ribbon in hard
X-rays (the main footpoint has a width perpendicular to the ribbon
of <1farcs1 compared to the G-band width of ~1farcs8) suggesting
that the hard X-ray emission comes from the sharp leading edge of
the G-band ribbon. Applying the thick-target beam model, the derived
energy deposition rate is >5 × 10<SUP>12</SUP> erg s<SUP>-1</SUP>
cm<SUP>-2</SUP> provided by an electron flux of 1 × 10<SUP>20</SUP>
electrons s<SUP>-1</SUP> cm<SUP>-2</SUP> above 18 keV. This requires
that the beam density of electrons above 18 keV be at least 1 ×
10<SUP>10</SUP> cm<SUP>-3</SUP>. Even if field lines converge toward
the chromospheric footpoints, the required beam in the corona has too
high a density to be described as a dilute tail population on top of
a Maxwellian core. We discuss this issue and others associated with
this extreme event, which poses serious questions to the standard
thick target beam interpretation of solar flares.
---------------------------------------------------------
Title: Height structure of X-ray, EUV, and white-light emission in
a solar flare
Authors: Battaglia, M.; Kontar, E. P.
2011A&A...533L...2B Altcode: 2011arXiv1107.3808B
Context. The bulk of solar flare emission originates from very
compact sources located in the lower solar atmosphere and observable
at a broad range of wavelengths such as near optical, UV, EUV, soft
and hard X-rays, and gamma-rays. Nevertheless, very few spatially
resolved imaging observations have been performed to determine the
structure of these compact regions. <BR /> Aims: We investigate the
above-the-photosphere heights of hard X-ray (HXR), EUV, and white-light
(6173 Å) continuum sources in the low atmosphere and the corresponding
densities at these heights. By considering the collisional transport
of solar energetic electrons, we also determine where and how much
energy is deposited and compare these values with the emissions
observed in HXR, EUV, and the continuum. <BR /> Methods: Simultaneous
EUV/continuum images from AIA/HMI on-board SDO and HXR RHESSI images
are compared to study a well-observed gamma-ray limb flare. Using
RHESSI X-ray visibilities, we determine the height of the HXR sources
as a function of energy above the photosphere. Co-aligning AIA/SDO and
HMI/SDO images with RHESSI, we infer, for the first time, the heights
and characteristic densities of HXR, EUV, and continuum (white-light)
sources in the flaring footpoint of the magnetic loop. <BR /> Results:
We find 35-100 keV HXR sources at heights of between 1.7 and 0.8 Mm
above the photosphere, below the 6173 Å continuum emission that appears
at heights 1.5-3 Mm and the peak of EUV emission originating near 3
Mm. <BR /> Conclusions: The EUV emission locations are consistent with
energy deposition from low energy electrons of ~12 keV occurring in
the top layers of the fully ionized chromosphere/low corona and not by
≳ 20 keV electrons that produce HXR footpoints in the lower neutral
chromosphere. The maximum of white-light continuum emission appears
between the HXR and EUV emission, presumably in the transition between
ionized and neutral atmospheres, implying that it consists of free-bound
and free-free continuum emission. We note that the energy deposited by
low energy electrons is sufficient to explain the energetics of both the
optical and UV emissions. <P />Two movies are available in electronic
form at <A href="http://www.aanda.org">http://www.aanda.org</A>
---------------------------------------------------------
Title: Implications of X-ray Observations for Electron Acceleration
and Propagation in Solar Flares
Authors: Holman, G. D.; Aschwanden, M. J.; Aurass, H.; Battaglia, M.;
Grigis, P. C.; Kontar, E. P.; Liu, W.; Saint-Hilaire, P.; Zharkova,
V. V.
2011SSRv..159..107H Altcode: 2011SSRv..tmp..162H; 2011SSRv..tmp..242H; 2011SSRv..tmp..260H;
2011SSRv..tmp...86H; 2011arXiv1109.6496H
High-energy X-rays and γ-rays from solar flares were discovered
just over fifty years ago. Since that time, the standard for
the interpretation of spatially integrated flare X-ray spectra
at energies above several tens of keV has been the collisional
thick-target model. After the launch of the Reuven Ramaty High
Energy Solar Spectroscopic Imager ( RHESSI) in early 2002, X-ray
spectra and images have been of sufficient quality to allow a greater
focus on the energetic electrons responsible for the X-ray emission,
including their origin and their interactions with the flare plasma
and magnetic field. The result has been new insights into the flaring
process, as well as more quantitative models for both electron
acceleration and propagation, and for the flare environment with
which the electrons interact. In this article we review our current
understanding of electron acceleration, energy loss, and propagation
in flares. Implications of these new results for the collisional
thick-target model, for general flare models, and for future flare
studies are discussed.
---------------------------------------------------------
Title: Deducing Electron Properties from Hard X-ray Observations
Authors: Kontar, E. P.; Brown, J. C.; Emslie, A. G.; Hajdas, W.;
Holman, G. D.; Hurford, G. J.; Kašparová, J.; Mallik, P. C. V.;
Massone, A. M.; McConnell, M. L.; Piana, M.; Prato, M.; Schmahl,
E. J.; Suarez-Garcia, E.
2011SSRv..159..301K Altcode: 2011arXiv1110.1755K; 2011SSRv..tmp..279K
X-radiation from energetic electrons is the prime diagnostic of
flare-accelerated electrons. The observed X-ray flux (and polarization
state) is fundamentally a convolution of the cross-section for the hard
X-ray emission process(es) in question with the electron distribution
function, which is in turn a function of energy, direction, spatial
location and time. To address the problems of particle propagation
and acceleration one needs to infer as much information as possible on
this electron distribution function, through a deconvolution of this
fundamental relationship. This review presents recent progress toward
this goal using spectroscopic, imaging and polarization measurements,
primarily from the Reuven Ramaty High Energy Solar Spectroscopic
Imager ( RHESSI). Previous conclusions regarding the energy, angular
(pitch angle) and spatial distributions of energetic electrons in
solar flares are critically reviewed. We discuss the role and the
observational evidence of several radiation processes: free-free
electron-ion, free-free electron-electron, free-bound electron-ion,
photoelectric absorption and Compton backscatter (albedo), using both
spectroscopic and imaging techniques. This unprecedented quality of
data allows for the first time inference of the angular distributions
of the X-ray-emitting electrons and improved model-independent
inference of electron energy spectra and emission measures of
thermal plasma. Moreover, imaging spectroscopy has revealed hitherto
unknown details of solar flare morphology and detailed spectroscopy of
coronal, footpoint and extended sources in flaring regions. Additional
attempts to measure hard X-ray polarization were not sufficient to put
constraints on the degree of anisotropy of electrons, but point to the
importance of obtaining good quality polarization data in the future.
---------------------------------------------------------
Title: Recent Advances in Understanding Particle Acceleration
Processes in Solar Flares
Authors: Zharkova, V. V.; Arzner, K.; Benz, A. O.; Browning, P.;
Dauphin, C.; Emslie, A. G.; Fletcher, L.; Kontar, E. P.; Mann, G.;
Onofri, M.; Petrosian, V.; Turkmani, R.; Vilmer, N.; Vlahos, L.
2011SSRv..159..357Z Altcode: 2011SSRv..tmp..156Z; 2011SSRv..tmp..249Z; 2011SSRv..tmp..232Z;
2011arXiv1110.2359Z; 2011SSRv..tmp..278Z
We review basic theoretical concepts in particle acceleration,
with particular emphasis on processes likely to occur in regions of
magnetic reconnection. Several new developments are discussed, including
detailed studies of reconnection in three-dimensional magnetic field
configurations (e.g., current sheets, collapsing traps, separatrix
regions) and stochastic acceleration in a turbulent environment. Fluid,
test-particle, and particle-in-cell approaches are used and results
compared. While these studies show considerable promise in accounting
for the various observational manifestations of solar flares, they
are limited by a number of factors, mostly relating to available
computational power. Not the least of these issues is the need to
explicitly incorporate the electrodynamic feedback of the accelerated
particles themselves on the environment in which they are accelerated. A
brief prognosis for future advancement is offered.
---------------------------------------------------------
Title: Hard X-Ray Footpoint Sizes and Positions as Diagnostics of
Flare Accelerated Energetic Electrons in the Low Solar Atmosphere
Authors: Battaglia, M.; Kontar, E. P.
2011ApJ...735...42B Altcode: 2011arXiv1104.2997B
The hard X-ray (HXR) emission in solar flares comes almost exclusively
from a very small part of the flaring region, the footpoints of
magnetic loops. Using RHESSI observations of solar flare footpoints, we
determine the radial positions and sizes of footpoints as a function of
energy in six near-limb events to investigate the transport of flare
accelerated electrons and the properties of the chromosphere. HXR
visibility forward fitting allows us to find the positions/heights and
the sizes of HXR footpoints along and perpendicular to the magnetic
field of the flaring loop at different energies in the HXR range. We
show that in half of the analyzed events, a clear trend of decreasing
height of the sources with energy is found. Assuming collisional
thick-target transport, HXR sources are located between 600 and 1200
km above the photosphere for photon energies between 120 and 25 keV,
respectively. In the other events, the position as a function of energy
is constant within the uncertainties. The vertical sizes (along the
path of electron propagation) range from 1.3 to 8 arcsec which is up to
a factor four larger than predicted by the thick-target model even in
events where the positions/heights of HXR sources are consistent with
the collisional thick-target model. Magnetic mirroring, collisional
pitch-angle scattering, and X-ray albedo are discussed as potential
explanations of the findings.
---------------------------------------------------------
Title: Instrumental oscillations in RHESSI count rates during
solar flares
Authors: Inglis, A. R.; Zimovets, I. V.; Dennis, B. R.; Kontar, E. P.;
Nakariakov, V. M.; Struminsky, A. B.; Tolbert, A. K.
2011A&A...530A..47I Altcode: 2011arXiv1102.5349I
<BR /> Aims: We seek to illustrate the analysis problems posed
by RHESSI spacecraft motion by studying persistent instrumental
oscillations found in the lightcurves measured by RHESSI's X-ray
detectors in the 6-12 keV and 12-25 keV energy range during the
decay phase of the flares of 2004 November 4 and 6. <BR /> Methods:
The various motions of the RHESSI spacecraft which may contribute
to the manifestation of oscillations are studied. The response of
each detector in turn is also investigated. <BR /> Results: We find
that on 2004 November 6 the observed oscillations correspond to the
nutation period of the RHESSI instrument. These oscillations are of
greatest amplitude for detector 5, while in the lightcurves of many
other detectors the oscillations are small or undetectable. We also
find that the variation in detector pointing is much larger during this
flare than the counterexample of 2004 November 4. <BR /> Conclusions:
Sufficiently large nutation motions of the RHESSI spacecraft lead to
clearly observable oscillations in count rates, posing a significant
hazard for data analysis. This issue is particularly problematic for
detector 5 due to its design characteristics. Dynamic correction of
the RHESSI counts, accounting for the livetime, data gaps, and the
transmission of the bi-grid collimator of each detector, is required
to overcome this issue. These corrections should be applied to all
future oscillation studies.
---------------------------------------------------------
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: Characteristics of the flare acceleration region derived from
simultaneous hard X-ray and radio observations
Authors: Reid, H. A. S.; Vilmer, N.; Kontar, E. P.
2011A&A...529A..66R Altcode: 2011arXiv1102.2342R
We investigate the type III radio bursts and X-ray signatures of
accelerated electrons in a well-observed solar flare in order to
find the spatial properties of the acceleration region. Combining
simultaneous RHESSI hard X-ray flare data and radio data from
Phoenix-2 and the Nançay radioheliograph, the outward transport
of flare accelerated electrons is analysed. The observations show
that the starting frequencies of type III bursts are anti-correlated
with the HXR spectral index of solar flare accelerated electrons. We
demonstrate both analytically and numerically that the type III burst
starting location is dependent upon the accelerated electron spectral
index and the spatial acceleration region size, but weakly dependent
on the density of energetic electrons for relatively intense electron
beams. Using this relationship and the observed anti-correlation, we
estimate the height and vertical extent of the acceleration region,
giving values of around 50 Mm and 10 Mm, respectively. The inferred
acceleration height and size suggest that electrons are accelerated
well above the soft X-ray loop-top, which could be consistent with the
electron acceleration between 40 Mm and 60 Mm above the flaring loop.
---------------------------------------------------------
Title: Measurement of Anisotropy in Solar Flares
Authors: Dickson, Ewan; Kontar, E.
2011SPD....42.2212D Altcode: 2011BAAS..43S.2212D
The angular variation of high energy electrons during a solar flare is
key to understanding the acceleration mechanism. High resolution X-ray
spectra observed by RHESSI can be used to estimate this anisotropy. The
effect of photospheric albedo, Compton scattering of X-ray photons
from the photosphere, should greatly influence the observed spectrum
if the X-ray emitting electrons are highly beamed. The observed
spectra will thus contain signatures of the anisotropy. The technique
of regularised inversion is used to determine the proportion of the
electron flux directed downwards towards the photosphere compared to
the electron flux directed towards the observer. The RHESSI flare
database has been searched and analysis performed on all flares
found to have statistically significant counts above 300 keV. In
total 9 flares suitable for analysis were found. The anisotropy of
these flares both over the entire impulsive phase and for shorter
time intervals was measured and the flares have all been found to
exhibit angular distributions which are close to isotropic. <P />EMD
gratefully acknowledges the support of an SPD and STFC studentship. EK
gratefully acknowledges financial support from an STFC rolling grant
and STFC Advanced Fellowship.
---------------------------------------------------------
Title: Accounting for the Albedo Flux in RHESSI Image Reconstructions
Authors: Schwartz, Richard A.; Kontar, E.; Jeffrrey, N.; Massone, A.
2011SPD....42.1504S Altcode: 2011BAAS..43S.1504S
Solar flare hard x-ray emission over the disk of the Sun must be
accompanied by a high percentage of back-scattered x-rays from the
photosphere. This albedo flux is an inevitable result of the high
ratio of Compton scattering to photoelectric absorption. While we
know that a substantial fraction of the emission should be albedo
we have been unable to separate this flux in images made with RHESSI
because the albedo may be more diffuse or very closely aligned with the
direct flux. <P />In this study we will take a fresh approach where we
include the contribution of the albedo flux for a point source where
we have assumed the height of the source above the photosphere as
well as its up/down directivity. For this we use a Green's function
approach to modify the expected count rates of the point source or
alternatively determine the true visibilities from the measured x-ray
visibilities. We will examine how this affects images obtained for
two solar flares, one on 20 Feb. 2002 at 11:06 UT and the other on 20
Aug. 2002 at 8:24 UT. Both flares are observed in energies above 100
keV with footpoints above the visible solar disk. We will make images
assuming several different combinations of the height and directivity
to see how the spectrum and image have changed. <P />This activity has
been supported by NASA and the European Community FrameworkProgramme 7,
'High Energy Solar Physics Data in Europe (HESPE).'
---------------------------------------------------------
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: The spectral difference between solar flare HXR coronal and
footpoint sources due to wave-particle interactions
Authors: Hannah, I. G.; Kontar, E. P.
2011A&A...529A.109H Altcode: 2011arXiv1103.2257H
<BR /> Aims: We investigate the spatial and spectral evolution of hard
X-ray (HXR) emission from flare accelerated electron beams subject
to collisional transport and wave-particle interactions in the solar
atmosphere. <BR /> Methods: We numerically follow the propagation
of a power-law of accelerated electrons in 1D space and time with
the response of the background plasma in the form of Langmuir waves
using the quasilinear approximation. <BR /> Results: We find that the
addition of wave-particle interactions to collisional transport for
a transient initially injected electron beam flattens the spectrum
of the footpoint source. The coronal source is unchanged and so the
difference in the spectral indices between the coronal and footpoint
sources is Δγ > 2, which is larger than expected from purely
collisional transport. A steady-state beam shows little difference
between the two cases, as has been previously found, as a transiently
injected electron beam is required to produce significant wave growth,
especially at higher velocities. With this transiently injected beam
the wave-particle interactions dominate in the corona whereas the
collisional losses dominate in the chromosphere. The shape of the
spectrum is different with increasing electron beam density in the
wave-particle interaction case whereas with purely collisional transport
only the normalisation is changed. We also find that the starting height
of the source electron beam above the photosphere affects the spectral
index of the footpoint when Langmuir wave growth is included. This
may account for the differing spectral indices found between double
footpoints if asymmetrical injection has occurred in the flaring loop.
---------------------------------------------------------
Title: Acceleration, Magnetic Fluctuations, and Cross-field Transport
of Energetic Electrons in a Solar Flare Loop
Authors: Kontar, E. P.; Hannah, I. G.; Bian, N. H.
2011ApJ...730L..22K Altcode: 2011arXiv1102.3664K
Plasma turbulence is thought to be associated with various physical
processes involved in solar flares, including magnetic reconnection,
particle acceleration, and transport. Using RHESSI observations and
the X-ray visibility analysis, we determine the spatial and spectral
distributions of energetic electrons for a flare (GOES M3.7 class,
2002 April 14, 23:55 UT), which was previously found to be consistent
with a reconnection scenario. It is demonstrated that because of the
high density plasma in the loop, electrons have to be continuously
accelerated about the loop apex of length ~2 × 10<SUP>9</SUP> cm and
width ~7 × 10<SUP>8</SUP> cm. Energy-dependent transport of tens of
keV electrons is observed to occur both along and across the guiding
magnetic field of the loop. We show that the cross-field transport
is consistent with the presence of magnetic turbulence in the loop,
where electrons are accelerated, and estimate the magnitude of the
field line diffusion coefficient for different phases of the flare. The
energy density of magnetic fluctuations is calculated for given magnetic
field correlation lengths and is larger than the energy density of the
non-thermal electrons. The level of magnetic fluctuations peaks when the
largest number of electrons is accelerated and is below detectability
or absent at the decay phase. These hard X-ray observations provide
the first observational evidence that magnetic turbulence governs
the evolution of energetic electrons in a dense flaring loop and is
suggestive of their turbulent acceleration.
---------------------------------------------------------
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: Parallel electric field amplification by phase mixing of
Alfven waves
Authors: Bian, N. H.; Kontar, E. P.
2011A&A...527A.130B Altcode: 2010arXiv1006.2729B
Context. Several numerical studies have identified phase mixing of
low-frequency Alfven waves as a means of parallel electric field
amplification and acceleration of electrons in a collisionless
plasma. <BR /> Aims: Theoretical explanations are given of how phase
mixing amplifies the parallel electric field and, as a consequence,
also leads to enhanced collisionless damping of the wave by energy
transfer to the electrons. <BR /> Methods: Our results are based on
the properties of the Alfven waves in a warm plasma. These results
are obtained within the framework of drift-kinetic theory. <BR />
Results: Phase mixing in a collisionless low-β plasma proceeds in a
manner very similar to the resistive case, except that electron Landau
damping is the primary energy dissipation channel. The time and length
scales involved are evaluated. We also focus on the evolution of the
parallel electric field and calculate its maximum value in the course
of its amplification
---------------------------------------------------------
Title: The influence of albedo on the size of hard X-ray flare sources
Authors: Battaglia, M.; Kontar, E. P.; Hannah, I. G.
2011A&A...526A...3B Altcode: 2010arXiv1010.5387B
Context. Hard X-rays from solar flares are an important diagnostic of
particle acceleration and transport in the solar atmosphere. However,
any observed X-ray flux from on-disc sources is composed of direct
emission plus Compton backscattered photons (albedo). This affects both
the observed spectra and images and the physical quantities derived
from them, such as the spatial and spectral distributions of accelerated
electrons or characteristics of the solar atmosphere (e.g. density). <BR
/> Aims: We propose a new indirect method to measure albedo and to infer
the directivity of X-rays in imaging using RHESSI data. We describe this
method and demonstrate its application to a compact disc event observed
with RHESSI. <BR /> Methods: Visibility forward fitting is used to
determine the size (second moment) of a disc event observed by RHESSI
as a function of energy. Using a Monte Carlo simulation code of photon
transport in the chromosphere, maps for different degrees of downward
directivity and true source sizes are computed. The resulting sizes from
the simulated maps are compared with the sizes from the observations
to find limits on the true source size and the directivity. <BR />
Results: The observed full width half maximum of the source varies in
size between 7.4 arcsec and 9.1 arcsec with the maximum between 30 and
40 keV. Such behaviour is expected in the presence of albedo and is
found in the simulations. The uncertainties in the data are not small
enough to make unambiguous statements about the true source size and
the directivity simultaneously. However, a source size smaller than
6 arcsec is improbable for modest directivities, and the true source
size is likely to be around 7 arcsec for small directivities. <BR />
Conclusions: While it is difficult to image the albedo patch directly,
the effect of backscattered photons on the observed source size can
be estimated. This is demonstrated here on observations for the first
time. The increase in source size caused by albedo has to be accounted
for when computing physical quantities that include the size as a
parameter, such as flare energetics. At the same time, studying the
albedo signature provides vital information about the directivity of
X-rays and related electrons.
---------------------------------------------------------
Title: Relationship Between Hard and Soft X-ray Emission Components
of a Solar Flare
Authors: Guo, Jingnan; Liu, Siming; Fletcher, Lyndsay; Kontar,
Eduard P.
2011ApJ...728....4G Altcode: 2010arXiv1012.4346G
X-ray observations of solar flares routinely reveal an impulsive
high-energy and a gradual low-energy emission component, whose
relationship is one of the key issues of solar flare study. The gradual
and impulsive emission components are believed to be associated with,
respectively, the thermal and nonthermal components identified in
spectral fitting. In this paper, a prominent ~50 s hard X-ray (HXR)
pulse of a simple GOES class C7.5 flare on 2002 February 20 is used to
study the association between high-energy, non-thermal, and impulsive
evolution, and low-energy, thermal, and gradual evolution. We use
regularized methods to obtain time derivatives of photon fluxes to
quantify the time evolution as a function of photon energy, obtaining
a break energy between impulsive and gradual behavior. These break
energies are consistent with a constant value of ~11 keV in agreement
with those found spectroscopically between thermal and non-thermal
components, but the relative errors of the former are greater than
15% and much greater than the few percent errors found from the
spectral fitting. These errors only weakly depend on assuming an
underlying spectral model for the photons, pointing to the current
data being inadequate to reduce the uncertainties rather than there
being a problem associated with an assumed model. The time derivative
method is used to test for the presence of a "pivot energy" in this
flare. Although these pivot energies are marginally consistent with
a constant value of ~9 keV, its values in the HXR rise phase appear
to be lower than those in the decay phase. Assuming that electrons
producing the high-energy component have a power-law distribution and
are accelerated from relatively hot regions of a background plasma
responsible for the observed thermal component, a low limit is obtained
for the low-energy cutoff. This limit is always lower than the break and
pivot energies and is located in the tail of the Maxwellian distribution
of the thermal component.
---------------------------------------------------------
Title: Combined Radio and X-ray Diagnostics of Electron Acceleration
Region in the Solar Corona
Authors: Reid, H.; Vilmer, N.; Kontar, E. P.
2010AGUFMSH42B..08R Altcode:
Solar flares are believed to accelerate both upward and downward
propagating electron beams which can radiate emission at radio and X-ray
wavelengths correspondingly. The correlation between X-ray and radio
emissions in a well observed solar flare allowed us detailed study
of the electron acceleration region properties. We used the Nancey
Radioheliograph, Phoenix-2 and RHESSI to infer the type III position,
type III starting frequency and spectral index of the HXR emission
respectively. Using these datasets, we were able to infer not only
the location (the height in the corona), but to estimate the spatial
size of the electron acceleration site. Using numerical simulations
of the electron transport of the electron beam in the corona plasma
to relate X-ray and radio data, we find that the spatial size of 10
Mm at an altitude of 50 Mm above the photosphere are consistent with
the observations.
---------------------------------------------------------
Title: Parallel electric field fluctuations produced by Alfvenic
turbulence
Authors: Bian, N.; Kontar, E. P.
2010AGUFMSH43D..01B Altcode:
We discuss the spectral structure of parallel electric field
fluctuations produced by Alfvenic turbulence in relation to
particle acceleration and cross-field transport in solar wind plasma
conditions. Based on the simplest truncation of the electromagnetic
gyrofluid equations in a homogeneous plasma, a model for the
energy cascade produced by strong anisotropic Alfvenic turbulence
is constructed. It smoothly connects the large magnetohydrodynamics
(MHD) scales and the small dispersive scales. Scaling relations are
obtained for the parallel electric field fluctuations, as a function
of perpendicular and parallel wavenumbers. The reason is the potential
implication of this parallel electric field in turbulent acceleration
and transport, an issue raised some time ago in [A. Hasegawa, K. Mima,
J. Geophys. Res. 83, 1117 (1978)]. The possibility of measurement of
this electric field component in the solar wind will be discussed.
---------------------------------------------------------
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: Parallel electric field generation by Alfvén wave turbulence
Authors: Bian, N. H.; Kontar, E. P.; Brown, J. C.
2010A&A...519A.114B Altcode: 2010arXiv1006.2662B
<BR /> Aims: This work aims to investigate the spectral structure
of the parallel electric field generated by strong anisotropic
and balanced Alfvénic turbulence in relation with the problem of
electron acceleration from the thermal population in solar flare
plasma conditions. <BR /> Methods: We consider anisotropic Alfvénic
fluctuations in the presence of a strong background magnetic
field. Exploiting this anisotropy, a set of reduced equations
governing non-linear, two-fluid plasma dynamics is derived. The
low-β limit of this model is used to follow the turbulent cascade
of the energy resulting from the non-linear interaction between
kinetic Alfvén waves, from the large magnetohydrodynamics (MHD)
scales with k<SUB>⊥</SUB>ρ_s≪1 down to the small “kinetic”
scales with k<SUB>⊥</SUB>ρ<SUB>s</SUB> ≫1, ρ_s being the ion
sound gyroradius. <BR /> Results: Scaling relations are obtained
for the magnitude of the turbulent electromagnetic fluctuations, as
a function of k<SUB>⊥</SUB> and k<SUB>∥</SUB>, showing that the
electric field develops a component parallel to the magnetic field at
large MHD scales. <BR /> Conclusions: The spectrum we derive for the
parallel electric field fluctuations can be effectively used to model
stochastic resonant acceleration and heating of electrons by Alfvén
waves in solar flare plasma conditions
---------------------------------------------------------
Title: Solar Wind Density Turbulence and Solar Flare Electron
Transport from the Sun to the Earth
Authors: Reid, Hamish A. S.; Kontar, Eduard P.
2010ApJ...721..864R Altcode: 2010arXiv1007.5310R
Solar flare accelerated electron beams propagating away from the
Sun can interact with the turbulent interplanetary media, producing
plasma waves and Type III radio emission. These electron beams are
detected near the Earth with a double power-law energy spectrum. We
simulate electron beam propagation from the Sun to the Earth in the weak
turbulent regime taking into account the self-consistent generation of
plasma waves and subsequent wave interaction with density fluctuations
from low-frequency MHD turbulence. The rate at which plasma waves are
induced by an unstable electron beam is reduced by background density
fluctuations, most acutely when fluctuations have large amplitudes or
small wavelengths. This suppression of plasma waves alters the wave
distribution which changes the electron beam transport. Assuming a 5/3
Kolmogorov-type power-density spectrum of fluctuations often observed
near the Earth, we investigate the corresponding energy spectrum
of the electron beam after it has propagated 1 AU. We find a direct
correlation between the spectrum of the double power-law below the break
energy and the turbulent intensity of the background plasma. For an
initial spectral index of 3.5, we find a range of spectra below the
break energy between 1.6 and 2.1, with higher levels of turbulence
corresponding to higher spectral indices.
---------------------------------------------------------
Title: The Sub-arcsecond Hard X-ray Structure of Loop Footpoints in
a Solar Flare
Authors: Kontar, E. P.; Hannah, I. G.; Jeffrey, N. L. S.; Battaglia, M.
2010ApJ...717..250K Altcode: 2010arXiv1005.0599K
The newly developed X-ray visibility forward fitting technique is
applied to the RHESSI data of a limb flare to investigate the energy and
height dependence on sizes, shapes, and position of hard X-ray (HXR)
chromospheric footpoint sources. This provides information about the
electron transport and chromospheric density structure. The spatial
distribution of two footpoint X-ray sources is analyzed using PIXON,
Maximum Entropy Method, CLEAN, and visibility forward fit algorithms at
nonthermal energies from ~20 to ~200 keV. We report, for the first time,
the vertical extents and widths of HXR chromospheric sources measured
as a function of energy for a limb event. Our observations suggest that
both the vertical and horizontal sizes of footpoints are decreasing
with energy. Higher energy emission originates progressively deeper in
the chromosphere, consistent with downward flare accelerated streaming
electrons. The ellipticity of the footpoints grows with energy from
~0.5 at ~20 keV to ~0.9 at ~150 keV. The positions of X-ray emission
are in agreement with an exponential density profile of scale height
~150 km. The characteristic size of the HXR footpoint source along the
limb decreases with energy, suggesting a converging magnetic field in
the footpoint. The vertical sizes of X-ray sources are inconsistent with
simple collisional transport in a single density scale height but can be
explained using a multi-threaded density structure in the chromosphere.
---------------------------------------------------------
Title: Solar Physics with LOFAR
Authors: Kontar, E.
2010sks3.conf....4K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: A gyrofluid description of Alfvénic turbulence and its
parallel electric field
Authors: Bian, N. H.; Kontar, E. P.
2010PhPl...17f2308B Altcode: 2010arXiv1006.2659B
Anisotropic Alfvénic fluctuations with
k<SUB>∥</SUB>/k<SUB>⊥</SUB><<1 remain at frequencies much
smaller than the ion cyclotron frequency in the presence of a strong
background magnetic field. Based on the simplest truncation of the
electromagnetic gyrofluid equations in a homogeneous plasma, a model
for the energy cascade produced by Alfvénic turbulence is constructed,
which smoothly connects the large magnetohydrodynamics scales and
the small “kinetic” scales. Scaling relations are obtained for
the electromagnetic fluctuations, as a function of k<SUB>⊥</SUB>
and k<SUB>∥</SUB>. Moreover, a particular attention is paid to the
spectral structure of the parallel electric field which is produced
by Alfvénic turbulence. The reason is the potential implication of
this parallel electric field in turbulent acceleration and transport
of particles. For electromagnetic turbulence, this issue was raised
some time ago in Hasegawa and Mima [J. Geophys. Res. 83, 1117 (1978)].
---------------------------------------------------------
Title: Hard X-ray Footpoint Source Sizes
Authors: Dennis, Brian R.; Kontar, E. P.; Gopie, A. A.; Tolbert,
A. K.; Schwartz, R. A.
2010AAS...21640405D Altcode: 2010BAAS...41..900D
RHESSI has detected compact hard (25 - 100 keV) X-ray sources that
are ɜ arcseconds (FWHM) in extent for certain flares (Dennis and
Pernak (2009). These sources are believed to be at magnetic loop
footpoints that are known from observations at other wavelengths to
be very small. Flare ribbons seen in the UV with TRACE, for example,
are 1 arcsecond in width, and white light flares show structure at
the 1 arcsecond level. However, Kontar and Jeffrey (2010) have shown
that the measured extent should be >6 arcseconds, even if the
X-ray emitting thick-target source is point-like. This is because
of the strong albedo contribution in the measured energy range for
a source located at the expected altitude of 1 Mm near the top of
the chromosphere. This discrepancy between observations and model
predictions may indicate that the source altitude is significantly
lower than assumed or that the RHESSI image reconstruction procedures
are not sensitive to the more diffuse albedo patch in the presence of a
strong compact source. Results will be presented exploring the latter
possibility using the Pixon image reconstruction procedure and other
methods based on visibilities. <P />Dennis, B. R. and Pernak, R. L.,
2009, ApJ, 698, 2131-2143. <P />Kontar, E. P. and Jeffrey, N. L. S.,
2010, A&A, in press.
---------------------------------------------------------
Title: Hard X-Ray Structure of Loop Footpoints in a Solar Limb Flare
Authors: Kontar, Eduard; Jeffrey, N.; Hannah, I.; Battaglia, M.
2010AAS...21640430K Altcode: 2010BAAS...41..904K
We apply newly-developed X-ray visibility forward fitting technique to
RHESSI data of a well-observed limb flare to investigate the energy and
height dependence on sizes, shapes, and position of hard X-ray footpoint
sources. The positions, the vertical extents and the widths of hard
X-ray sources are measured as a function of energy. Our observations
suggest that the vertical and horizontal sizes of footpoints are
decreasing with energy while higher energy emission originates
progressively deeper in the chromosphere. The characteristic widths of
the hard X-ray footpoint source along the limb decrease with height and
are consistent with a converging magnetic field in the footpoint. The
vertical sizes of X-ray sources are larger than predicted by collisional
thick-target transport in a single density scale height chromosphere but
can be explained using a multi-threaded density structure of the loop.
---------------------------------------------------------
Title: Solar wind density turbulence and solar flare electron
transport from the Sun to the Earth
Authors: Reid, Hamish; Kontar, Eduard
2010EGUGA..12.1042R Altcode:
Electron beams accelerated during solar flare events and associated
with Type III solar radio bursts can be observed near the Earth with a
double power-law energy spectra. We simulate self-consistently energetic
electron propagation from the Sun to the Earth in the weak turbulent
regime, taking into account the turbulent solar wind plasma. The density
fluctuations from the solar wind turbulence influence the spectrum of
high frequency beam-driven electron plasma waves and hence can alter the
beam energy spectra. Taking a Kolmogorov-type power spectrum of density
fluctuations often observed at 1AU, we investigate the formation of
energetic electron spectrum near the Earth. We show that an initial
power-law electron spectra changes as a result of transport to a
double power-law with a break in the deka-keV range flattening at low
energies. With an initial power-law index of 3.5, the simulated spectrum
below the break at the Earth was found between 1.7-2.1 dependent upon
the level of density fluctuations present in the background solar wind.
---------------------------------------------------------
Title: The Interaction of Solar Flare Accelerated Electron Beams
with the Turbulent Solar Wind
Authors: Reid, Hamish; Kontar, E.
2010AAS...21632107R Altcode: 2010BAAS...41..912R
Solar flare accelerated electron beams propagating away from the
Sun can interact with the turbulent interplanetary media, producing
plasma waves and type III radio emission. These electron beams are
detected near the Earth with a double power-law energy spectra. We
simulate electron beam propagation from the Sun to the Earth in the weak
turbulent regime taking into account the self-consistent generation of
plasma waves and subsequent wave interaction with density fluctuations
from low frequency MHD turbulence. The rate which plasma waves are
induced by an unstable electron beam is reduced by background density
fluctuations, most acutely when fluctuations have large amplitudes or
small wavelengths. This suppression of plasma waves alters the wave
distribution which changes the electron beam transport. Assuming a 5/3
Kolmogorov-type power spectra of density fluctuations often observed
near the Earth, we investigate the corresponding energy spectra of the
electron beam after it has propagated 1AU. We find a direct correlation
between the spectra of the double power-law below the break energy
and the turbulent intensity of the background plasma. By varying the
radial dependency of density fluctuations, injected electron spectra
with spectral index 3-4 produce double power-laws near the Earth with
spectra below the break energy within the range 1.6-2.4, agreeing with
observational values.
---------------------------------------------------------
Title: Positions and sizes of X-ray solar flare sources
Authors: Kontar, E. P.; Jeffrey, N. L. S.
2010A&A...513L...2K Altcode: 2010arXiv1003.0884K
<BR /> Aims: The positions and source sizes of X-ray sources taking
into account Compton backscattering (albedo) are investigated. <BR
/> Methods: Using a Monte Carlo simulation of X-ray photon transport
including photo-electric absorption and Compton scattering, we calculate
the apparent source sizes and positions of X-ray sources at the solar
disk for various source sizes, spectral indices and directivities of
the primary source. <BR /> Results: We show that the albedo effect
can alter the true source positions and substantially increase the
measured source sizes. The source positions are shifted by up to ~0.5”
radially towards the disk centre and 5 arcsec source sizes can be two
times larger even for an isotropic source (minimum albedo effect) at
1 Mm above the photosphere. The X-ray sources therefore should have
minimum observed sizes, and thus their FWHM source size (2.35 times
second-moment) will be as large as ~7” in the 20-50 keV range for
a disk-centered point source at a height of 1 Mm (~1.4”) above the
photosphere. The source size and position change is greater for flatter
primary X-ray spectra, a stronger downward anisotropy, for sources
closer to the solar disk centre, and between the energies of 30 and 50
keV. <BR /> Conclusions: Albedo should be taken into account when X-ray
footpoint positions, footpoint motions or source sizes from e.g. RHESSI
or Yohkoh data are interpreted, and we suggest that footpoint sources
should be larger in X-rays than in either optical or EUV ranges.
---------------------------------------------------------
Title: Combined STEREO/RHESSI Study of Coronal Mass Ejection
Acceleration and Particle Acceleration in Solar Flares
Authors: Temmer, M.; Veronig, A. M.; Kontar, E. P.; Krucker, S.;
Vršnak, B.
2010ApJ...712.1410T Altcode: 2010arXiv1002.3080T
Using the potential of two unprecedented missions, Solar Terrestrial
Relations Observatory (STEREO) and Reuven Ramaty High-Energy Solar
Spectroscopic Imager (RHESSI), we study three well-observed fast coronal
mass ejections (CMEs) that occurred close to the limb together with
their associated high-energy flare emissions in terms of RHESSI hard
X-ray (HXR) spectra and flux evolution. From STEREO/EUVI and STEREO/COR1
data, the full CME kinematics of the impulsive acceleration phase up to
~4 R <SUB>sun</SUB> is measured with a high time cadence of <=2.5
minutes. For deriving CME velocity and acceleration, we apply and
test a new algorithm based on regularization methods. The CME maximum
acceleration is achieved at heights h <= 0.4 R <SUB>sun</SUB>,
and the peak velocity at h <= 2.1 R <SUB>sun</SUB> (in one case,
as small as 0.5 R <SUB>sun</SUB>). We find that the CME acceleration
profile and the flare energy release as evidenced in the RHESSI HXR flux
evolve in a synchronized manner. These results support the "standard"
flare/CME model which is characterized by a feedback relationship
between the large-scale CME acceleration process and the energy release
in the associated flare.
---------------------------------------------------------
Title: Sub-Thz Radiation Mechanisms in Solar Flares
Authors: Fleishman, Gregory D.; Kontar, Eduard P.
2010ApJ...709L.127F Altcode: 2009arXiv0911.5335F
Observations in the sub-THz range of large solar flares have revealed
a mysterious spectral component increasing with frequency and hence
distinct from the microwave component commonly accepted to be produced
by gyrosynchrotron (GS) emission from accelerated electrons. Evidently,
having a distinct sub-THz component requires either a distinct
emission mechanism (compared to the GS one), or different properties
of electrons and location, or both. We find, however, that the list
of possible emission mechanisms is incomplete. This Letter proposes
a more complete list of emission mechanisms, capable of producing
a sub-THz component, both well known and new in this context, and
calculates a representative set of their spectra produced by (1)
free-free emission, (2) GS emission, (3) synchrotron emission from
relativistic positrons/electrons, (4) diffusive radiation, and (5)
Cherenkov emission. We discuss the possible role of the mechanisms in
forming the sub-THz emission and emphasize their diagnostics potential
for flares.
---------------------------------------------------------
Title: Plasma turbulence in inhomogeneous plasma and the number of
accelerated electrons in solar flares
Authors: Kontar, Eduard
2010cosp...38.2992K Altcode: 2010cosp.meet.2992K
he number of accelerated electrons in solar flares is normally
estimated using purely collisional models and often presents a
substantial challenge for electron acceleration models. Here, the
relaxation of energetic electrons is considered taking into account
the self-consistent description of Langmuir waves in the inhomogeneous
plasma. Full numerical solutions of the set of kinetic equations
for electrons and Langmuir waves are obtained for an inhomogeneous
plasma. The results show that the presence of inhomogeneity
significantly changes the overall evolution of the system. The
inhomogeneity is effective in shifting the wave numbers of the Langmuir
waves, and can thus re-distribute the energy accelerating the tail of
electron distribution. It is shown that X-ray spectra calculated from
such distributions and interpreted using purely collisional models
will overestimate the actual number of energetic electrons.
---------------------------------------------------------
Title: Sizes and position of on-disk X-ray solar flares sources:
effect of X-ray albedo
Authors: Kontar, Eduard; Jeffrey, Natasha
2010cosp...38.3027K Altcode: 2010cosp.meet.3027K
Using Monte Carlo simulation of X-ray photon transport, we calculate
the apparent source sizes and positions of X-ray sources at the solar
disk for various source sizes, spectral indices and directivities
of the primary X-ray emitting sources. It is shown the albedo effect
will alter the true source positions and substantially increase the
measured source sizes. The source positions are shifted up to about
0.5 arcsecond radially towards the disk centre and the source sizes
can be two times larger even for an isotropic source (minimum albedo
effect) at 1 Mm above the photosphere. The source size and position
change is the largest for flatter primary X-ray spectra, stronger
downward anisotropy, for sources closer to the solar disk centre,
and between the energies of 30 and 50 keV.
---------------------------------------------------------
Title: Footpoint sizes and positions of limb flares observed with
RHESSI
Authors: Battaglia, Marina; Kontar, Eduard
2010cosp...38.3026B Altcode: 2010cosp.meet.3026B
X-ray observations of solar flare footpoints are an important diagnostic
tool of the physics of electron transport in the solar chromosphere
and can be used to infer the chromospheric density structure. With
the newly developed technique of X-ray visibility forward fitting, the
moments (position, size) of X-ray emission in flare sources observed
with RHESSI can be determined with sub-arcsecond resolution. For the
first time, we use this technique to systematically study the position
(first moment) and size (second moment) of footpoints as a function
of energy in 7 limb events observed with RHESSI. In the collisional
thick target model, the source positions at different energy ranges
correspond to different heights in the chromosphere. The vertical
sizes of the footpoints present additional constraints on electron
transport along the field lines of the magnetic loop, while the sizes
in the perpendicular direction give us crucial information about the
width of this loop along which the electrons propagate.
---------------------------------------------------------
Title: Imaging observations of X-ray albedo in a compact disc flare
Authors: Battaglia, Marina; Kontar, Eduard
2010cosp...38.3031B Altcode: 2010cosp.meet.3031B
X-rays from solar flare sources are an important diagnostic tool for
particle acceleration and transport in the solar atmosphere. However,
the observed flux at Earth is composed of direct emission and
photons which are Compton backscattered from the photosphere. This
contribu-tion can account for up to 40 We present imaging observations
of a compact flare on the solar disc. The source full-width-half
maximum was determined at different energies using X-ray visibility
forward fitting. The observed source size increases and decreases with
energy with a maximum size at about 40 keV, contrary to observations
made in limb events. The behavior is consistent with predictions from
Monte Carlo simulations of X-ray photon transport in which X-ray
visibilities were computed from simulated maps and fitted using
visibility forward fit.
---------------------------------------------------------
Title: Solar flare accelerated electron transport through the
turbulent solar wind
Authors: Reid, Hamish; Kontar, Eduard
2010cosp...38.2989R Altcode: 2010cosp.meet.2989R
Solar flare accelerated electron beams can become unstable during
transport from the Sun to the Earth, producing plasma waves in
the turbulent inner heliosphere. We simulate solar electron beam
propagation to the Earth in the weak turbulent regime taking into
account the self-consistent generation of plasma waves. Induced plasma
waves interact with the density fluctuations from low frequency MHD
turbulence present in the background plasma. These fluctuations act to
suppress the generation of waves, most acutely when fluctuations have
large amplitudes or small wavelengths. The reduction of plasma wave
generation alters the wave distribution which changes electron beam
transport. Assuming an observed 5/3 Kolmogorov-type power density
spectra of fluctuations, we investigate the energy spectra of the
electron beam near the Earth. We find the presence of turbulence in the
background plasma alters the spectral index below the break energy of
the double power-law formed at 1AU. From an initial single power-law
electron distribution, we find a range of spectra below the break
energy, with higher levels of turbulence corresponding to a higher
spectral index.
---------------------------------------------------------
Title: The effect of turbulent density perturbations on solar flare
electron transport and X-ray spectrum
Authors: Hannah, Iain; Kontar, Eduard
2010cosp...38.2977H Altcode: 2010cosp.meet.2977H
RHESSI solar flare hard X-ray observations sometimes cannot be
adequately interpreted in terms of purely collisional electron
transport. We instead present numerical simulations where we consider
the energetic electron-beam interactions in the presence of low
frequency den-sity perturbations. We demonstrate how the turbulent
density perturbations affect the high frequency Langmuir waves and in
turn, the flare accelerated electron distribution. The conse-quences
of this self-consistent treatment are discussed for the observable
X-ray spectrum.
---------------------------------------------------------
Title: Discussion of a high-energy mission for solar eruptions in
ESA's Cosmic Vision Programme
Authors: Kontar, Eduard; MacKinnon, Alexander; Klein, Karl-Ludwig;
Vilmer, Nicole; Green, Lucie M.; Matthews, Sarah A.
2010cosp...38.2983K Altcode: 2010cosp.meet.2983K
In this paper we emphasize the effect of a self-induced electric field
on the distributions of electron beams during their precipitation
into flaring atmospheres and their hard X-ray (HXR) and microwave (MW)
emission. For the beam precipitation the time-dependent Fokker-Planck
approach is applied by taking into account collisional and Ohmic
losses in a converging magnetic field with different level of
convergence. The energy range of beam electrons covers from 12 keV to
1.2 MeV, for HXR emission angle-dependent relativistic cross-sections
are considered, for MW the effects of radiative transfer of ordinary
and extra-ordinary waves are also taken into account. We compare the
effects of self-induced electric field on the HXR and MW emission and
polarization in flares. We also produce some recommendation for future
interpretation of the simultaneous HXR and MW observations.
---------------------------------------------------------
Title: The Effect of Wave-Particle Interactions on Low-Energy Cutoffs
in Solar Flare Electron Spectra
Authors: Hannah, I. G.; Kontar, E. P.; Sirenko, O. K.
2009ApJ...707L..45H Altcode: 2009arXiv0911.0314H
Solar flare hard X-ray (HXR) spectra from Reuven Ramaty High Energy
Solar Spectrometer (RHESSI) are normally interpreted in terms of purely
collisional electron beam propagation, ignoring spatial evolution
and collective effects. In this Letter, we present self-consistent
numerical simulations of the spatial and temporal evolution of
an electron beam subject to collisional transport and beam-driven
Langmuir wave turbulence. These wave-particle interactions represent
the background plasma's response to the electron beam propagating
from the corona to chromosphere and occur on a far faster timescale
than Coulomb collisions. From these simulations, we derive the mean
electron flux spectrum, comparable to such spectra recovered from
high-resolution HXRs observations of solar flares with RHESSI. We
find that a negative spectral index (i.e., a spectrum that increases
with energy), or local minima when including the expected thermal
spectral component at low energies, occurs in the standard thick-target
model, when Coulomb collisions are only considered. The inclusion of
wave-particle interactions does not produce a local minimum, maintaining
a positive spectral index. These simulations are a step toward a more
complete treatment of electron transport in solar flares and suggest
that a flat spectrum (spectral index of 0-1) down to thermal energies
maybe a better approximation instead of a sharp cutoff in the injected
electron spectrum.
---------------------------------------------------------
Title: Formation of a Broken Power-Law Electron Spectrum in Impulsive
Solar Flare Energetic Electron Events
Authors: Reid, H.; Kontar, E.
2009AGUFMSH22A..06R Altcode:
Impulsive solar energetic particle events have an attractive diagnostic
potential for poorly understood particle acceleration processes in
solar flares. Some solar energetic particle events are believed to
propagate almost scatter-free from the Sun, and hence their spectrum
should be that of accelerated particles. We simulate propagation of
energetic electrons from the Sun to the Earth including wave-particle
interactions in the inhomogeneous heliospheric plasma. We show the
that an injected single power-law spectrum will be detected at 1AU as a
broken power-law due to wave-particle interaction in the inhomogeneous
plasma. It is demonstrated that the large scale density inhomogeneity
as well as small scale density fluctuations affecting Langmuir waves
are indirectly responsible for flattening the electron spectra below
the break energy.
---------------------------------------------------------
Title: Local re-acceleration and a modified thick target model of
solar flare electrons
Authors: Brown, J. C.; Turkmani, R.; Kontar, E. P.; MacKinnon, A. L.;
Vlahos, L.
2009A&A...508..993B Altcode: 2009arXiv0909.4243B
Context: The collisional thick target model (CTTM) of solar hard
X-ray (HXR) bursts has become an almost “standard model” of
flare impulsive phase energy transport and radiation. However, it
faces various problems in the light of recent data, particularly the
high electron beam density and anisotropy it involves.<BR /> Aims: We
consider how photon yield per electron can be increased, and hence fast
electron beam intensity requirements reduced, by local re-acceleration
of fast electrons throughout the HXR source itself, after injection.<BR
/> Methods: We show parametrically that, if net re-acceleration rates
due to e.g. waves or local current sheet electric (E) fields are a
significant fraction of collisional loss rates, electron lifetimes, and
hence the net radiative HXR output per electron can be substantially
increased over the CTTM values. In this local re-acceleration thick
target model (LRTTM) fast electron number requirements and anisotropy
are thus reduced. One specific possible scenario involving such
re-acceleration is discussed, viz, a current sheet cascade (CSC) in a
randomly stressed magnetic loop.<BR /> Results: Combined MHD and test
particle simulations show that local E fields in CSCs can efficiently
accelerate electrons in the corona and and re-accelerate them after
injection into the chromosphere. In this HXR source scenario, rapid
synchronisation and variability of impulsive footpoint emissions can
still occur since primary electron acceleration is in the high Alfvén
speed corona with fast re-acceleration in chromospheric CSCs. It is
also consistent with the energy-dependent time-of-flight delays in
HXR features.<BR /> Conclusions: Including electron re-acceleration in
the HXR source allows an LRTTM modification of the CTTM in which beam
density and anisotropy are much reduced, and alleviates theoretical
problems with the CTTM, while making it more compatible with radio and
interplanetary electron numbers. The LRTTM is, however, different in
some respects such as spatial distribution of atmospheric heating by
fast electrons.
---------------------------------------------------------
Title: The Location of Centroids in Photon and Electron Maps of
Solar Flares
Authors: Prato, Marco; Emslie, A. Gordon; Kontar, Eduard P.; Massone,
Anna Maria; Piana, Michele
2009ApJ...706..917P Altcode:
We explore the use of centroid coordinates as a means to identify the
"locations" of electron-proton bremsstrahlung hard X-ray sources in
solar flares. Differences between the coordinates of the electron and
photon centroids are derived and explained. For electron propagation in
a collision-dominated target, with either a uniform or an exponential
density profile, the position of the electron centroid can be calculated
analytically. We compare these analytic forms to data from a flare
event on 2002 February 20. We first spectrally invert the native photon
visibility data to obtain "electron visibilities," which are in turn
used to construct electron flux images at various electron energies
E. Centroids of these maps are then obtained by straightforward
numerical integration over the electron maps. This comparison allows
us to infer the density structure in the two compact sources visible,
and we discuss the (somewhat unexpected) results thus obtained.
---------------------------------------------------------
Title: Hard X-ray Imaging of Solar Flares Using Interpolated
Visibilities
Authors: Massone, Anna Maria; Emslie, A. Gordon; Hurford, G. J.;
Prato, Marco; Kontar, Eduard P.; Piana, Michele
2009ApJ...703.2004M Altcode:
RHESSI produces solar flare images with the finest angular and
spectral resolutions ever achieved at hard X-ray energies. Because
this instrument uses indirect, collimator-based imaging techniques,
the "native" output of which is in the form of "visibilities"
(two-dimensional spatial Fourier components of the image), the
development and application of robust, accurate, visibility-based image
reconstruction techniques is required. Recognizing that the density
of spatial-frequency (u, v) coverage by RHESSI is much sparser than
that normally encountered in radio astronomy, we therefore introduce a
method for image reconstruction from a relatively sparse distribution
of sampled visibilities. The method involves spline interpolation
at spatial frequencies less than the largest sampled frequency and
the imposition of a positivity constraint on the image to reduce the
ringing effects resulting from an unconstrained Fourier transform
inversion procedure. Using simulated images consisting both of assumed
mathematical forms and of the type of structure typically associated
with solar flares, we validate the fidelity, accuracy, and robustness
with which the new procedure recovers input images. The method
faithfully recovers both single and multiple sources, both compact
and extended, over a dynamic range of ~10:1. The performance of the
method, which we term as uv_smooth, is compared with other RHESSI
image reconstruction algorithms currently in use and its advantages
summarized. We also illustrate the application of the method using
RHESSI observations of four solar flares.
---------------------------------------------------------
Title: Solar energetic particles: radio and X-ray signatures
Authors: Kontar, E.
2009sksp.conf....8K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Onsets and Spectra of Impulsive Solar Energetic Electron
Events Observed Near the Earth
Authors: Kontar, Eduard P.; Reid, Hamish A. S.
2009ApJ...695L.140K Altcode: 2009arXiv0903.2576K
Impulsive solar energetic electrons are often observed in the
interplanetary space near the Earth and have an attractive diagnostic
potential for poorly understood solar flare acceleration processes. We
investigate the transport of solar flare energetic electrons in
the heliospheric plasma to understand the role of transport to the
observed onset and spectral properties of the impulsive solar electron
events. The propagation of energetic electrons in solar wind plasma
is simulated from the acceleration region at the Sun to the Earth,
taking into account self-consistent generation and absorption of
electrostatic electron plasma (Langmuir) waves, effects of nonuniform
plasma, collisions, and Landau damping. The simulations suggest that
the beam-driven plasma turbulence and the effects of solar wind density
inhomogeneity play a crucial role and lead to the appearance of (1)
a spectral break for a single power-law injected electron spectrum,
with the spectrum flatter below the break, (2) apparent early onset
of low-energy electron injection, and (3) the apparent late maximum
of low-energy electron injection. We show that the observed onsets,
spectral flattening at low energies, and formation of a break energy at
tens of keV is the direct manifestation of wave-particle interactions
in nonuniform plasma of a single accelerated electron population with
an initial power-law spectrum.
---------------------------------------------------------
Title: Modelling Langmuir Wave Interaction with Plasma Inhomogeneities
in the Flaring Solar Corona
Authors: Hannah, I. G.; Kontar, E. P.
2008AGUFMSH21A1577H Altcode:
We present simulations of the time and spatial evolution of electron
beam driven Langmuir wave turbulence interacting with plasma
inhomogeneities. This code can be used to investigate a variety of
particle acceleration and propagation scenarios in the flaring corona
and interplanetary space. Here we present one simple model: simulations
of a beam of accelerated electrons propagating from an acceleration
region at the top of a coronal loop down to the chromosphere. RHESSI
imaging spectroscopy shows a flatter X-ray spectrum at the footpoints
compared to coronal sources, so we investigate whether plasma waves
and inhomogeneities can change the electron distribution in such a way.
---------------------------------------------------------
Title: A visibility-based approach using regularization for
imaging-spectroscopy in solar X-ray astronomy
Authors: Prato, M.; Massone, A. M.; Piana, M.; Emslie, A. G.; Hurford,
G. J.; Kontar, E. P.; Schwartz, R. A.
2008JPhCS.135a2084P Altcode:
The Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) is
a nine-collimators satellite detecting X-rays and γ-rays emitted by
the Sun during flares. As the spacecraft rotates, imaging information
is encoded as rapid time-variations of the detected flux. We recently
proposed a method for the construction of electron flux maps at
different electron energies from sets of count visibilities (i.e.,
direct, calibrated measurements of specific Fourier components of the
source spatial structure) measured by RHESSI. The method requires the
application of regularized inversion for the synthesis of electron
visibility spectra and of imaging techniques for the reconstruction
of two-dimensional electron flux maps. The method, already tested on
real events registered by RHESSI, is validated in this paper by means
of simulated realistic data.
---------------------------------------------------------
Title: Chromospheric magnetic field and density structure measurements
using hard X-rays in a flaring coronal loop
Authors: Kontar, E. P.; Hannah, I. G.; MacKinnon, A. L.
2008A&A...489L..57K Altcode: 2008arXiv0808.3334K
Aims: A novel method of using hard X-rays as a diagnostic for
chromospheric density and magnetic structures is developed to infer
sub-arcsecond vertical variation of magnetic flux tube size and neutral
gas density. <BR />Methods: Using Reuven Ramaty High Energy Solar
Spectroscopic Imager (RHESSI) X-ray data and the newly developed
X-ray visibilities forward fitting technique we find the FWHM and
centroid positions of hard X-ray sources with sub-arcsecond resolution
(~0.2”) for a solar limb flare. We show that the height variations
of the chromospheric density and the magnetic flux densities can be
found with an unprecedented vertical resolution of ~150 km by mapping
18-250 keV X-ray emission of energetic electrons propagating in the
loop at chromospheric heights of 400-1500 km. <BR />Results: Our
observations suggest that the density of the neutral gas is in good
agreement with hydrostatic models with a scale height of around 140 ±
30 km. FWHM sizes of the X-ray sources decrease with energy suggesting
the expansion (fanning out) of magnetic flux tubes in the chromosphere
with height. The magnetic scale height B(z)(dB/dz)<SUP>-1</SUP> is found
to be of the order of 300 km and a strong horizontal magnetic field is
associated with noticeable flux tube expansion at a height of ~900 km.
---------------------------------------------------------
Title: Low-Energy Cutoffs in Electron Spectra of Solar Flares:
Statistical Survey
Authors: Kontar, E. P.; Dickson, E.; Kašparová, J.
2008SoPh..252..139K Altcode: 2008arXiv0805.1470K; 2008SoPh..tmp..149K
The Reuven Ramaty High Energy Spectroscopic Imager (RHESSI) X-ray
data base (February 2002 - May 2006) has been searched to find solar
flares with weak thermal components and flat photon spectra. Using a
regularized inversion technique, we determine the mean electron flux
distribution from count spectra for a selection of events with flat
photon spectra in the 15 - 20 keV energy range. Such spectral behavior
is expected for photon spectra either affected by photospheric albedo
or produced by electron spectra with an absence of electrons in a
given energy range (e.g., a low-energy cutoff in the mean electron
spectra of nonthemal particles). We have found 18 cases that exhibit a
statistically significant local minimum (a dip) in the range of 13 - 19
keV. The positions and spectral indices of events with low-energy cutoff
indicate that such features are likely to be the result of photospheric
albedo. It is shown that if the isotropic albedo correction is applied,
all low-energy cutoffs in the mean electron spectrum are removed,
and hence the low-energy cutoffs in the mean electron spectrum of
solar flares above ∼ 12 keV cannot be viewed as real features. If
low-energy cutoffs exist in the mean electron spectra, their energies
should be less than ∼ 12 keV.
---------------------------------------------------------
Title: X-ray Measurements of Magnetic Field and Density Structure
in Flaring Solar Coronal Loops
Authors: Kontar, E.; MacKinnon, A.
2008ESPM...122.108K Altcode:
Hard X-ray emission of solar flares is analysed to infer the magnetic
loop sizes and density structures in the solar chromosphere. Using
Ramaty Solar Spectroscopic Imager (RHESSI) X-ray data and newly
developed X-ray visibilities technique we find the FWHM and centroid
positions of hard X-ray sources with subarcsecond resolution 0.2”
for a limb solar flare. We show that the height variations of
the chromospheric density and the magnetic loop size can be found
with vertical resolution of around 150 km by mapping 20-200 keV
X-ray emission of energetic electrons propagating in the loop. Our
observations suggest that the density of the neutral gas is in a
good agreement with hydrostatic models with scale height around 147±
28km. FWHM sizes of the X-ray sources decrease with energy suggesting
expansion of magnetic flux tube in the cromosphere with height. The
magnetic scale height is found to be of the order of 300km.
---------------------------------------------------------
Title: Particle Acceleration and Energy Release in RHESSI Era
Authors: Kontar, E.
2008ESPM...12.2.82K Altcode:
Since high energy emission (X-rays and gamma-rays) represents
optically-thin radiation from energetic particles, it is a relatively
straightforward, and hence extremely valuable, tool in the diagnostic
study of flare-accelerated electrons and ions at the Sun. The
observed X-ray/gamma-ray flux is fundamentally a convolution of
the cross-section for the emission process(es) in question with the
distribution function(s) of accelerated particles, which are in turn
functions of energy, direction, spatial location and time. To address
the key problems of particle acceleration, propagation as well as
energy release one needs to infer as much information as possible
on the particle distribution function, through a de-convolution
of this fundamental relationship. <P />This review presents recent
observational progress toward the understanding of energy release and
particle acceleration using spectroscopic, imaging and polarization
measurements, primarily from the Ramaty High Energy Solar Spectroscopic
Imager (RHESSI). Previous conclusions regarding the energy, angular
(pitch angle) and spatial distributions of energetic electrons and
ions in solar flares are critically reviewed. The diagnostics of
radiation processes, particle transport, and acceleration, using
both spectroscopic and imaging techniques will be discussed. The
unprecedented quality of the RHESSI data in combination with novel
data analysis techniques have revealed previously unknown details of
energetic particle distributions and imposed new challenging constraints
on the particle acceleration.
---------------------------------------------------------
Title: Determining the Spatial Variation of Accelerated Electron
Spectra in Solar Flares
Authors: Emslie, A. Gordon; Hurford, G. J.; Kontar, Eduard P.; Massone,
Anna Maria; Piana, Michele; Prato, Marco; Xu, Yan
2008AIPC.1039....3E Altcode:
The RHESSI spacecraft images hard X-ray emission from solar flares
with an angular resolution down to ~2” and an energy resolution of
1 keV. For such a Rotating Modulation Collimator (RMC) instrument,
imaging information is gathered not as a set of spatial images, but
rather as a set of (energy-dependent) spatial Fourier components (termed
visibilities). We report here on a novel technique which uses these
spatial Fourier components in count space to derive, via a regularized
spectral inversion process, the corresponding spatial Fourier components
for the electron distribution, in such a way that the resulting
electron visibilities, and so the images that are constructed from
them, vary smoothly with electron energy E. “Stacking” such images
then results in smooth, physically plausible, electron spectra for
prominent features in the flare. <P />Application of visibility-based
analysis techniques has also permitted an assessment of the density
and volume of the electron acceleration region, and so the number of
particles it contains. This, plus information on the rate of particle
acceleration to hard-X-ray-producing energies [obtained directly
from the hard X-ray spectrum I(ɛ)] allows us to deduce the specific
acceleration rate (particles s<SUP>-1</SUP> per particle). The values
of this key quantity are compared with the predictions of various
electron acceleration scenarios.
---------------------------------------------------------
Title: The way forward for coronal heating
Authors: De Moortel, Ineke; Browning, Philippa; Bradshaw, Stephen J.;
Pintér, Balázs; Kontar, Eduard P.
2008A&G....49c..21D Altcode:
Ineke De Moortel, Philippa K Browning, Stephen J Bradshaw, Balázs
Pintér and Eduard P Kontar consider approaches to the longstanding
and enigmatic problem of coronal heating, as presented at the RAS
discussion meeting on 11 January 2008.
---------------------------------------------------------
Title: Nonthermal particles at the Sun and beyond: RHESSI results
Authors: Kontar, Eduard
2008cosp...37.1576K Altcode: 2008cosp.meet.1576K
Hard X-rays and gamma-rays are considered to be the most direct
signatures of solar-flare accelerated particles. High quality X-ray
data from RHESSI (Ramaty High Energy Solar Spectroscopic Imager)
has substantially advanced our understanding of solar flare physics
as well as has set new challenging questions. In this talk, I will
highlight some RHESSI results that enhanced our understanding of
nonthermal processes at the Sun and beyond with the focus on the new
observational results on the non-thermal energetic electrons in solar
flares inferred from RHESSI data.
---------------------------------------------------------
Title: Implications of X-Ray Spectra for Accelerated Electrons in
Solar Eruptions
Authors: Kontar, Eduard
2008cosp...37.1575K Altcode: 2008cosp.meet.1575K
X-rays represent prompt, optically-thin, radiation from energetic
electrons, they are a relatively straightforward, and hence the key
diagnostic tool of flare-accelerated electrons. The observed X-ray
flux at the Earth is simply a linear convolution of the cross-section
for the hard X-ray emission processes with the electron distribution
function at the Sun. The successful operation of Ramaty High Energy
Solar Spectroscopic Imager (RHESSI) for the last 6 years has opened
new horizons in diagnostics of solar flare energetic electrons and has
substantially advanced our understanding of solar flare physics through
a deconvolution of this relationship. In this talk I will review the
recent progress in inference as much information as possible on the
electron distribution function to address the fundamental problems of
electron propagation and acceleration.
---------------------------------------------------------
Title: Electron Flux Maps of Solar Flares: A Regularization Approach
to Rhessi Imaging Spectroscopy
Authors: Massone, A. M.; Piana, M.; Prato, M.; Emslie, A. G.; Hurford,
G. J.; Kontar, E. P.; Scwartz, R. A.
2008mss..conf...48M Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Low-Energy Cut-Offs In Electron Spectra Of Solar Flares:
Statistical Survey
Authors: Kontar, Eduard; Dickson, Ewan; Kasparova, Jana
2008cosp...37.1577K Altcode: 2008cosp.meet.1577K
Ramaty High Energy Solar Spectroscopic Imager (RHESSI) X-ray data base
(February 2002 - May 2006) has been searched to find solar flares with
weak thermal component and flat photon spectra. Using a regularised
inversion technique, we determine the mean electron flux distribution
from count spectra of the events which had flat photon spectra in
the 15-20 keV energy range. Such spectral behaviour is expected for
photon spectra either affected by photospheric albedo or produced by
electron spectra with an absence of electrons in some energy range,
e.g. low-energy cutoff in electron spectra. We have found a number of
cases which exhibit a statistically significant dip in the range of
10-20 keV. The positions and spectral indices of events with low-energy
cutoffs indicate that such feature could be a result of photospheric
albedo. It is shown that if the isotropic albedo correction was applied,
all low-energy cutoffs in mean electron spectrum were removed.
---------------------------------------------------------
Title: Imaging spectroscopy from visibilities in the RHESSI era
Authors: Massone, Anna Maria; Piana, Michele; Prato, Marco; Emslie,
A. Gordon; Hurford, Gordon J.; Kontar, Eduard; Schwartz, Richard A.
2008cosp...37.1943M Altcode: 2008cosp.meet.1943M
The Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) is a
nine-collimators satellite detecting X—rays and γ--rays emitted by
the Sun during flares. As the spacecraft rotates, imaging information
is encoded as rapid time-variations of the detected flux. We propose
a method for the construction of electron flux maps at different
electron energies from sets of count visibilities (i.e., calibrated
measurements of specific Fourier components of the source spatial
structure) measured by RHESSI. The method requires the application of
regularized inversion for the synthesis of electron visibility spectra
and of imaging techniques for the reconstruction of two-dimensional
electron flux maps. The method, is tested on both real RHESSI and
simulated realistic data.
---------------------------------------------------------
Title: Electron-Electron Bremsstrahlung Emission and the Inference
of Electron Flux Spectra in Solar Flares
Authors: Kontar, Eduard P.; Emslie, A. Gordon; Massone, Anna Maria;
Piana, Michele; Brown, John C.; Prato, Marco
2007ApJ...670..857K Altcode: 2007arXiv0707.4225K
Although both electron-ion and electron-electron bremsstrahlung
contribute to the hard X-ray emission from solar flares, the latter
is normally ignored. Such an omission is not justified at electron
(and photon) energies above ~300 keV, and inclusion of the additional
electron-electron bremsstrahlung in general makes the electron
spectrum required to produce a given hard X-ray spectrum steeper at
high energies. Unlike electron-ion bremsstrahlung, electron-electron
bremsstrahlung cannot produce photons of all energies up to the electron
energy involved. The maximum possible photon energy depends on the angle
between the direction of the emitting electron and the emitted photon,
and this suggests a diagnostic for an upper cutoff energy and/or for
the degree of beaming of the accelerated electrons. We analyze the
large event of 2005 January 17 and show that the upward break around
400 keV in the observed hard X-ray spectrum is naturally accounted for
by the inclusion of electron-electron bremsstrahlung. Indeed, the mean
source electron spectrum recovered through a regularized inversion
of the hard X-ray spectrum, using a cross section that includes both
electron-ion and electron-electron terms, has a relatively constant
spectral index δ over the range from electron kinetic energy E=200 keV
to E=1 MeV. Such a spectrum is indicative of an acceleration mechanism
without a characteristic energy or corresponding scale.
---------------------------------------------------------
Title: Electron Flux Spectral Imaging of Solar Flares through
Regularized Analysis of Hard X-Ray Source Visibilities
Authors: Piana, Michele; Massone, Anna Maria; Hurford, G. J.; Prato,
Marco; Emslie, A. Gordon; Kontar, Eduard P.; Schwartz, Richard A.
2007ApJ...665..846P Altcode:
We introduce a new method for imaging spectroscopy analysis of
hard X-ray emission during solar flares. The method avoids the
“traditional” noise-sensitive step of stacking independent images
made in different count-based energy intervals. Rather, it involves
regularized inversion of the count visibility spectra (i.e., the
two-dimensional spatial Fourier transforms of the spectral image)
to obtain smoothed (regularized) forms of the corresponding electron
visibility spectra. Application of conventional visibility-based imaging
algorithms then yields images of the electron flux that vary smoothly
with energy. We apply the method to a solar flare observed on 2002
February 20 by the RHESSI instrument. The event is characterized by two
bright footpoints with a more diffuse emission between them. Analysis
of the regularized electron flux images reveals that the electron
flux spectra at the footpoints are systematically harder than those
in the region between the footpoints and that the observed degree of
hardening is consistent with that produced by Coulomb collisions between
an acceleration site high in the corona and the dense chromospheric
footpoint regions.
---------------------------------------------------------
Title: Solar Flare Electron Spectra at the Sun and near the Earth
Authors: Krucker, Säm; Kontar, E. P.; Christe, S.; Lin, R. P.
2007ApJ...663L.109K Altcode:
We compare hard X-ray (HXR) photon spectra observed by the RHESSI with
the spectra of the electrons in the associated solar impulsive particle
events observed near 1 AU by the WIND 3D Plasma and Energetic Particle
(3DP) instrument. For prompt events, where the inferred injection
time at the Sun coincides with the HXR burst, the HXR photon power-law
spectral index γ and the in situ observed electron spectral index δ
measured above 50 keV show a good linear fit, δ=γ+0.1(+/-0.1), with
correlation coefficient of 0.83, while for delayed events (inferred
injection >10 minutes after the HXR burst) only a weak correlation
with a coefficient of 0.43 is seen. The observed relationship for
prompt events is inconsistent, however, with both the thin target case,
where the escaping electrons come from the X-ray-producing electron
population, and the thick target case where some of the accelerated
source population escapes to 1 AU and the rest produce the HXRs while
losing all their energy to collisions. Furthermore, the derived total
number of escaping electrons correlates with the number of electrons
required to produce observed X-ray flux but is only about ~0.2% of
the number of HXR-producing electrons.
---------------------------------------------------------
Title: Hard X-ray spectra and positions of solar flares observed by
RHESSI: photospheric albedo, directivity and electron spectra
Authors: Kašparová, J.; Kontar, E. P.; Brown, J. C.
2007A&A...466..705K Altcode: 2007astro.ph..1871K
Aims:We investigate the signature of the photospheric albedo
contribution in solar flare hard X-ray spectra, the effect of low
energy cutoffs in electron spectra, and the directivity of hard X-ray
emission. <BR />Methods: Using Ramaty High Energy Solar Spectroscopic
Imager (RHESSI) flare data we perform a statistical analysis of
spatially integrated spectra and positions of solar flares. <BR
/>Results: We demonstrate clear centre-to-limb variation of photon
spectral indices in the 15-20 keV energy range and a weaker dependency
in the 20-50 keV range which is consistent with photospheric albedo as
the cause. The results also suggest that low-energy cutoffs sometimes
inferred in mean electron spectra are an artefact of albedo. We also
derive the anisotropy (ratio of downward/observer directed photons)
of hard X-ray emission in the 15-20 keV range for various heliocentric
angles.
---------------------------------------------------------
Title: Determination of Electron Flux Spectrum Images in Solar Flares
using Regularized Analysis of Hard X-Ray Source Visibilities
Authors: Emslie, A. G.; Piana, M.; Massone, A. M.; Hurford, G. J.;
Prato, M.; Kontar, E. P.; Schwartz, R. A.
2007AAS...210.3704E Altcode: 2007BAAS...39..151E
We introduce a new method for imaging spectroscopy analysis of
hard X-ray emission during solar flares. The new method allows the
construction of images of both count and electron flux spectra that
are smoothed with respect to energy, and so more suitable for further
analysis. The procedure involves regularized inversion of the count
visibility spectra (i.e., the two-dimensional spatial Fourier transforms
of the spectral image) to obtain smoothed forms of the corresponding
electron visibility spectra. We apply the method to a solar flare
observed on February 20, 2002 by the RHESSI instrument. The event is
characterized by two bright footpoints with a "strand" of more diffuse
emission between them. We find that the electron flux spectra at the
footpoints are systematically harder than those in the region between
the footpoints, and that the observed degree of hardening is consistent
with that produced by Coulomb collisions between an acceleration site
high in the corona and the dense chromospheric footpoint regions.
---------------------------------------------------------
Title: RHESSI Results Time for a Rethink?
Authors: Brown, J. C.; Kontar, E. P.; Veronig, A. M.
2007LNP...725...65B Altcode: 2006astro.ph..7440B
Hard X-rays and γ-rays are the most direct signatures of energetic
electrons and ions in the sun’s atmosphere which is optically thin at
these energies and their radiation involves no coherent processes. Being
collisional they are complementary to gyro-radiation in probing
atmospheric density as opposed to magnetic field and the electrons
are primarily 10 100~keV in energy, complementing the (>100 keV)
electrons likely responsible for microwave bursts. The pioneering
results of the Ramaty High Energy Solar Spectroscopic Imager (RHESSI)
are raising the first new major questions concerning solar energetic
particles in many years. Some highlights of these results are discussed
primarily around RHESSI topics on which the authors have had direct
research involvement particularly when they are raising the need for
re-thinking of entrenched ideas. Results and issues are broadly divided
into discoveries in the spatial, temporal and spectral domains, with the
main emphasis on flare hard X-rays/fast electrons but touching also on
γ-rays/ions, non-flare emissions, and the relationship to radio bursts.
---------------------------------------------------------
Title: Stereoscopic Electron Spectroscopy of Solar Hard X-Ray Flares
with a Single Spacecraft
Authors: Kontar, Eduard P.; Brown, John C.
2006ApJ...653L.149K Altcode: 2006astro.ph.11170K
Hard X-ray (HXR) spectroscopy is the most direct method of diagnosing
energetic electrons in solar flares. Here we present a technique that
allows us to use a single HXR spectrum to determine an effectively
stereoscopic electron energy distribution. Considering the Sun's
surface to act as a “Compton mirror” allows us to look at emitting
electrons also from behind the source, providing vital information
on downward-propagating particles. Using this technique we determine
simultaneously the electron spectra of downward- and upward-directed
electrons for two solar flares observed by the Ramaty High Energy
Solar Spectroscopic Imager (RHESSI). The results reveal surprisingly
near-isotropic electron distributions, which contrast strongly with
the expectations from the standard model that invokes strong downward
beaming, including a collisional thick-target model.
---------------------------------------------------------
Title: RHESSI survey of photospheric albedo and directivity of solar
flare hard X-ray spectra
Authors: Kasparova, J.; Kontar, E. P.
2006IAUJD...1E..48K Altcode:
Hard X-ray spectra of solar flares are generated as bremsstrahlung
of accelerated electrons propagating in solar corona. The photons
emitted towards the photosphere have a high probability to undergo
Compton backscattering into observers direction. They form so called
photospheric albedo component and modify the spatially integrated
photon spectra of solar flares. Analysing 409 solar flares observed
by RHESSI, we show significant centre-to-limb variation of observed
photon spectra in energies ~ 20 keV, which is consistent with the
photospheric albedo. Moreover, we also show that the low-energy cutoff
in the mean electron spectrum can be viewed as an artifact of the
albedo component. Noting that the amount of backscattered photons
strongly depends on the downward directed photon flux, we determine
for the first time the directivity of the downward photon flux. The
results favour near-isotropic photon distribution and represent a
problem for the models with downward propagating electron beam.
---------------------------------------------------------
Title: Regularized Reconstruction of the Differential Emission
Measure from Solar Flare Hard X-Ray Spectra
Authors: Prato, M.; Piana, M.; Brown, J. C.; Emslie, A. G.; Kontar,
E. P.; Massone, A. M.
2006SoPh..237...61P Altcode: 2006SoPh..tmp....6P
We address the problem of how to test whether an observed solar hard
X-ray bremsstrahlung spectrum (I(∊)) is consistent with a purely
thermal (locally Maxwellian) distribution of source electrons, and,
if so, how to reconstruct the corresponding differential emission
measure (ξ(T)). Unlike previous analysis based on the Kramers and
Bethe-Heitler approximations to the bremsstrahlung cross-section,
here we use an exact (solid-angle-averaged) cross-section. We show
that the problem of determining ξ(T) from measurements of I(∊)
invOlves two successive inverse problems: the first, to recover the
mean source-electron flux spectrum ($\overline{F}$(E)) from I(∊)
and the second, to recover ξ(T) from $\overline{F}$(E). We discuss
the highly pathological numerical properties of this second problem
within the framework of the regularization theory for linear inverse
problems. In particular, we show that an iterative scheme with
a positivity constraint is effective in recovering δ-like forms
of ξ(T) while first-order Tikhonov regularization with boundary
conditions works well in the case of power-law-like forms. Therefore,
we introduce a restoration approach whereby the low-energy part of
$\overline{F}$ (E), dominated by the thermal component, is inverted by
using the iterative algorithm with positivity, while the high-energy
part, dominated by the power-law component, is inverted by using
first-order regularization. This approach is first tested by using
simulated $\overline{F}$(E) derived from a priori known forms of ξ(T)
and then applied to hard X-ray spectral data from the Reuven Ramaty
High Energy Solar Spectroscopic Imager (RHESSI).
---------------------------------------------------------
Title: Evaluation of Algorithms for Reconstructing Electron Spectra
from Their Bremsstrahlung Hard X-Ray Spectra
Authors: Brown, John C.; Emslie, A. Gordon; Holman, Gordon D.;
Johns-Krull, Christopher M.; Kontar, Eduard P.; Lin, Robert P.;
Massone, Anna Maria; Piana, Michele
2006ApJ...643..523B Altcode:
The Ramaty High Energy Solar Spectroscopic Imager (RHESSI) has
yielded solar flare hard X-ray spectra with unprecedented resolution,
enabling reconstruction of mean source electron energy spectra F(E) by
deconvolution of photon energy spectra I(ɛ). While various algorithms
have been proposed, the strengths and weaknesses of each have yet to
be explored in a systematic fashion. For real data F(E) is unknown,
so these various algorithms must instead be tested on simulated data
for which the “true” F(E) is known. Accordingly, we devised several
forms of F(E) with “interesting” features, generated the corresponding
(noise-added) I(ɛ), and recovered F(E) using a variety of algorithms,
including zero- and first-order Tikhonov regularizations, triangular
matrix row elimination, and forward fitting using a parametric
form consisting of a double power law with low/high cutoffs plus an
isothermal component. All inversion methods reconstructed the general
magnitude and form of F(E) well, suffering only from (1) blurring of
sharp features and (2) poor recovery at low electron energies E in
cases in which F<SUP>'</SUP>(E) was positive and large. Addition of a
steep thermal component at low E did not prevent recovery of features
at higher values of E. Forward fitting did recover large-scale forms
and features well but, inevitably, failed to recover local features not
expressible within the parametric used. This confirms that inversions
are the most dependable way to discover such features. However,
examination of the pattern of I(ɛ) residuals can suggest feature
locations and so help refine the parametric form used. Since quite
smooth F(E) forms do reproduce the observed I(ɛ) form with relatively
small residuals, it appears that sharp features may be uncommon in
actual flares.
---------------------------------------------------------
Title: Solar HXR Emission and Energetic Electron events seen at 1~AU:
A temporal and spectral comparison
Authors: Krucker, S.; Christe, S. D.; Kontar, E. P.; Lin, R. P.
2006AGUSMSH41A..04K Altcode:
Onset time analysis of impulsive electron events at 1 AU suggest that
there are two classes of events: (1) events with a solar release time
in close temporal agreement with solar hard X-ray (HXR) flares, and (2)
events with a delayed solar release time relative to the HXR emission
that seem to be accelerated later possibly at coronal shocks. Simple
onset time analysis, however, are criticized for not taking propagation
effect properly into account producing falsely delayed release
times. Here we present a further test of the existence of two classes
of events by comparing the in-situ observed electron spectra with the
solar HXR photon spectra remotely observed by RHESSI. For events with
a good temporal agreement, a clear correlation between the spectra has
been reported indicating a common acceleration mechanism. Delayed events
have not yet been analyzed. However, if indeed a second acceleration
mechanism independent of the HXR flare exists that release energetic
electron into interplanetary space, no correlation between the in-situ
observed electron spectra and the HXR photon spectra is expected to
be found.
---------------------------------------------------------
Title: Compton backscattered and primary X-rays from solar flares:
angle dependent Green's function correction for photospheric albedo
Authors: Kontar, E. P.; MacKinnon, A. L.; Schwartz, R. A.; Brown, J. C.
2006A&A...446.1157K Altcode: 2005astro.ph.10167K
The observed hard X-ray (HXR) flux spectrum I(ɛ) from solar flares is a
combination of primary bremsstrahlung photons I_P(ɛ) with a spectrally
modified component from photospheric Compton backscatter of downward
primary emission. The latter can be significant, distorting or hiding
the true features of the primary spectrum which are key diagnostics
for acceleration and propagation of high energy electrons and of their
energy budget. For the first time in solar physics, we use a Green's
function approach to the backscatter spectral deconvolution problem,
constructing a Green's matrix including photoelectric absorption. This
approach allows spectrum-independent extraction of the primary spectrum
for several HXR flares observed by the Ramaty High Energy Solar
Spectroscopic Imager (RHESSI). We show that the observed and primary
spectra differ very substantially for flares with hard spectra close
to the disk centre. We show in particular that the energy dependent
photon spectral index γ (ɛ)=-d log I/d log ɛ is very different for
I_P(ɛ) and for I(ɛ) and that inferred mean source electron spectra
F(E) differ greatly. Even for a forward fitting of a parametric F(E) to
the data, a clear low-energy cutoff required to fit I(ɛ) essentially
disappears when the fit is to I_P(ɛ) - i.e. when albedo correction
is included. The self-consistent correction for backscattered photons
is thus shown to be crucial in determining the energy spectra of flare
accelerated electrons, and hence their total number and energy.
---------------------------------------------------------
Title: Solar flare hard X-ray spectra possibly inconsistent with
the collisional thick target model
Authors: Kontar, Eduard P.; Brown, John C.
2006AdSpR..38..945K Altcode: 2005astro.ph..8418K
Recent progress in solar hard X-ray (HXR) observations with RHESSI data
and methods for spectral inversion allow us to study model-independent
mean electron flux spectra in solar flares. We report several hard
X-ray events observed by RHESSI in which the photon spectra I(γ) are
such that the inferred source mean electron spectra are not consistent
with the standard model of collisional transport in solar flares. The
observed photon spectra are so flat locally that the recovered mean
electron flux spectra show a dip around 17 31 keV. While we note that
alternative explanations, unrelated to electron transport, have not
been ruled out, we focus on the physical implications of this tentative
result for the collisional thick target model.
---------------------------------------------------------
Title: Angular and energy distributions of energetic electrons in
solar flares: results with RHESSI
Authors: Kontar, E. P.; Brown, J. C.
2006cosp...36.1013K Altcode: 2006cosp.meet.1013K
X-ray spectroscopy of solar flares is the key tool in diagnostics of
high-energy particles in the low solar atmosphere However an observed
X-ray spectrum is the convolution of radiation propagation effects as
well as the instrumental response Specifically Compton back-scattering
from the photosphere can account for 30-90 of the observed flux at
energies 30-50 keV Starting with RHESSI Hard X-ray spectra and using
recently developed inversion tools we compute the spectra of energetic
electrons The angular and energy distribution of energetic electrons
can be viewed as acceleration and propagation model fingerprints The
electron spectra inferred from RHESSI solar flare spectra put new
tighter observational constrains on possible acceleration propagation
models
---------------------------------------------------------
Title: Multi-Wavelength Analysis of High-Energy Electrons in Solar
Flares: A Case Study of the August 20, 2002 Flare
Authors: Kašparová, Jana; Karlický, Marian; Kontar, Eduard P.;
A. Schwartz, Richard; Dennis, Brian R.
2005SoPh..232...63K Altcode: 2005astro.ph..8636K
A multi-wavelength spatial and temporal analysis of solar high-energy
electrons is conducted using the August 20, 2002 flare of an unusually
flat (γ<SUB>1</SUB> = 1.8) hard X-ray spectrum. The flare is studied
using RHESSI, Hα, radio, TRACE, and MDI observations with advanced
methods and techniques never previously applied in the solar flare
context. A new method to account for X-ray Compton backscattering in the
photosphere (photospheric albedo) has been used to deduce the primary
X-ray flare spectra. The mean electron flux distribution has been
analysed using both forward fitting and model-independent inversion
methods of spectral analysis. We show that the contribution of the
photospheric albedo to the photon spectrum modifies the calculated mean
electron flux distribution, mainly at energies below ∼100 keV. The
positions of the Hα emission and hard X-ray sources with respect to
the current-free extrapolation of the MDI photospheric magnetic field
and the characteristics of the radio emission provide evidence of the
closed geometry of the magnetic field structure and the flare process in
low altitude magnetic loops. In agreement with the predictions of some
solar flare models, the hard X-ray sources are located on the external
edges of the Hα emission and show chromospheric plasma heated by the
non-thermal electrons. The fast changes of Hα intensities are located
not only inside the hard X-ray sources, as expected if they are the
signatures of the chromospheric response to the electron bombardment,
but also away from them.
---------------------------------------------------------
Title: Fast electron slowing-down and diffusion in a high temperature
coronal X-ray source
Authors: Galloway, R. K.; MacKinnon, A. L.; Kontar, E. P.; Helander, P.
2005A&A...438.1107G Altcode: 2005astro.ph..5197G
Finite thermal velocity modifications to electron slowing-down rates
may be important for the deduction of solar flare total electron
energy. Here we treat both slowing-down and velocity diffusion of
electrons in the corona at flare temperatures, for the case of a simple,
spatially homogeneous source. Including velocity diffusion yields
a consistent treatment of both “accelerated” and “thermal”
electrons. It also emphasises that one may not invoke finite thermal
velocity target effects on electron lifetimes without simultaneously
treating the contribution to the observed X-ray spectrum from thermal
electrons. We present model calculations of the X-ray spectra resulting
from injection of a power-law energy distribution of electrons into a
source with finite temperature. Reducing the power-law distribution
low-energy cutoff to <P />lower and lower energies only increases
the relative magnitude of the thermal component of the spectrum,
because the lowest energy electrons simply join the background thermal
distribution. Acceptable fits to RHESSI flare data are obtained using
this model. These also demonstrate, however, that observed spectra
may in consequence be acceptably consistent with rather a wide range
of injected electron parameters.
---------------------------------------------------------
Title: Comparison of Algorithms for Reconstructing Electron Spectra
from Solar Flare Hard X-Ray Spectra
Authors: Emslie, G.; Brown, J. C.; Holman, G. D.; Johns-Krull, C.;
Kontar, E. P.; Massone, A. M.; Piana, M.
2005AGUSMSP21A..05E Altcode:
The Ramaty High Energy Solar Spectroscopic Imager (RHESSI) is yielding
solar flare hard X-ray (HXR) spectra with unprecedented resolution and
precision. Such spectra enable the reconstruction of the effective
mean source electron spectrum F?(E) by deconvolution of the photon
spectrum I(ɛ) through the bremsstrahlung cross-section Q(ɛ,E). In
this paper we report on an evaluation of three distinct "inverting"
reconstruction techniques and one forward fitting procedure. We
synthesized a variety of hypothetical F?(E) forms, with a variety
of empirical features designed to represent diagnostics of electron
acceleration and transport processes, generated the corresponding I(ɛ)
with realistic random noise added, and performed "blind" (i.e. without
knowledge of F?[E] in advance) recoveries of F?(E) for comparison with
the originally assumed forms. In most cases the inversion methods
gave very good reconstructions of F?(E). The forward fitting method
did well in recovering large-scale features but, somewhat inevitably,
failed to recover features outwith the parametric forms of F?(E),
such as dips, bumps and positive slopes. However, examination of the
distribution of photon spectrum residuals over ɛ should in principle
permit refinement of the parametric form used.
---------------------------------------------------------
Title: Regularized Energy-Dependent Solar Flare Hard X-Ray Spectral
Index
Authors: Kontar, Eduard P.; Mackinnon, Alexander L.
2005SoPh..227..299K Altcode: 2005astro.ph..6097K
The deduction from solar flare X-ray photon spectroscopic data of the
energy-dependent model-independent spectral index is considered as an
inverse problem. Using the well-developed regularization approach we
analyze the energy dependency of spectral index for a high-resolution
energy spectrum provided by Ramaty High Energy Solar Spectroscopic
Imager (RHESSI). The regularization technique produces much smoother
derivatives while avoiding additional errors typical of finite
differences. It is shown that observations imply a spectral index
varying significantly with energy, in a way that also varies with
time as the flare progresses. The implications of these findings are
discussed in the solar flare context.
---------------------------------------------------------
Title: Nonlinear wave interactions as a model for naturally enhanced
ion acoustic lines in the ionosphere
Authors: Kontar, E. P.; Pécseli, H. L.
2005GeoRL..32.5110K Altcode: 2005GeoRL..3205110K; 2019arXiv190503115K
Incoherent radar scatter from the ionosphere will, for equilibrium
conditions, show two symmetric ion-acoustic lines, one for each
direction of wave propagation. Many observations, from the EISCAT
Svalbard Radar (ESR) for instance, demonstrate that the symmetry
of this ion line can be broken. An enhanced, nonthermal, level of
fluctuations, i.e., Naturally Enhanced Ion-Acoustic Lines (NEIALs)
might be observed. Several models have been proposed for explaining
these naturally enhanced lines. Here, we consider one of these,
suggesting that decay of electron beam excited Langmuir waves gives
rise to enhanced asymmetric ion lines in the ionosphere. We use a
weak-turbulence approximation, and identify crucial parameters for
Langmuir decay processes to be effective in generating the observed
signals.
---------------------------------------------------------
Title: Determination of Electron Flux Spectra in a Solar Flare with
an Augmented Regularization Method: Application to Rhessi Data
Authors: Kontar, Eduard P.; Emslie, A. Gordon; Piana, Michele; Massone,
Anna Maria; Brown, John C.
2005SoPh..226..317K Altcode: 2004astro.ph..9691K
Kontar et al. (2004) have shown how to recover mean source electron
spectra $\bar F(E)$ in solar flares through a physical constraint
regularization analysis of the bremsstrahlung photon spectra I(ε) that
they produce. They emphasize the use of non-square inversion techniques,
and preconditioning combined with physical properties of the spectra
to achieve the most meaningful solution to the problem. Higher-order
regularization techniques may be used to generate $\bar F(E)$ forms with
certain desirable properties (e.g., higher-order derivatives). They
further note that such analysis may be used to infer properties of
the electron energy spectra at energies well above the maximum photon
energy observed. In this paper we apply these techniques to data from
a solar flare observed by RHESSI on 26 February, 2002. Results using
different orders of regularization are presented and compared for
various time intervals. Clear evidence is presented for a change in the
value of the high-energy cutoff in the mean source electron spectrum
with time. We also show how the construction of the injected electron
spectrum F<SUB>0</SUB>(E<SUB>0</SUB>) (assuming that Coulomb collisions
in a cold target dominate the electron transport) is facilitated by
the use of higher-order regularization methods.
---------------------------------------------------------
Title: Problems and progress in flare fast particle diagnostics
Authors: Brown, John C.; Kontar, Eduard P.
2005AdSpR..35.1675B Altcode: 2005astro.ph..8417B
Recent progress in the diagnosis of flare fast particles is
critically discussed with the main emphasis on high resolution
hard X-ray (HXR) data from RHESSI and coordinated data from other
instruments. Spectacular new photon data findings are highlighted
as are advances in theoretical aspects of their use as fast particle
diagnostics, and some important comparisons made with interplanetary
particle data. More specifically the following topics are addressed:
RHESSI data on HXR (electron) versus gamma-ray line (ion) source
locations. <P />RHESSI hard X-ray source spatial structure in relation
to theoretical models and loop density structure. <P />Energy budget
of flare electrons and the Neupert effect. <P />Spectral deconvolution
methods including blind target testing and results for RHESSI HXR
spectra, including the reality and implications of dips inferred
in electron spectra. <P />The relation between flare in situ and
interplanetary particle data.
---------------------------------------------------------
Title: Solar X-rays and Energetic Electrons Escaping from the Sun
Authors: Krucker, S.; Kontar, E. P.; Lin, R. P.
2004AGUFMSH13A1129K Altcode:
The Sun frequently accelerates electrons in solar flares and type III
radio bursts. Some of the accelerated electrons lose their energy
by collisions in the denser, lower solar atmosphere producing hard
X-ray (HXR) emissions and heat the corona, while others escape into
interplanetary space. Whether the HXR producing and the escaping
electrons are accelerated by the same mechanism is not known. Combining
RHESSI X-ray observations with in-situ observations of energetic
electrons from the WIND spacecraft allows for the first time a detailed
temporal, spatial, and spectral study. Statistical results of 16 events
with a close temporal agreement between the HXR and the in-situ detected
electrons (taking the time of flight of the escaping electrons into
account) show a correlation between the HXR photon spectral index and
the electron spectral index observed in-situ thus indicating a common
acceleration mechanism. Furthermore, the solar X-ray source structure of
these events look similar showing hot loops with HXR footpoints plus an
additional HXR source separated from the loop by typically ~15". This
source structure can be explained by a simple magnetic reconnection
model with newly emerging flux tubes that reconnect with previously open
field lines, so-called interchange reconnection. Events with a delayed
timing between the HXRs and the solar release of escaping electrons
(Krucker et al. 1999, Haggerty & Roelof 2002) are presently
investigated. If these delayed events are indeed accelerated later in
the event by shocks, no correlation between the HXR photon spectrum
and the in-situ observed electron spectrum is expect to be found.
---------------------------------------------------------
Title: Generalized Regularization Techniques with Constraints for
the Analysis of Solar Bremsstrahlung X-ray Spectra
Authors: Kontar, Eduard P.; Piana, Michele; Massone, Anna Maria;
Emslie, A. Gordon; Brown, John C.
2004SoPh..225..293K Altcode: 2004astro.ph..9688K
Hard X-ray spectra in solar flares provide knowledge of the electron
spectrum that results from acceleration and propagation in the solar
atmosphere. However, the inference of the electron spectra from solar
X-ray spectra is an ill-posed inverse problem. Here, we develop and
apply an enhanced regularization algorithm for this process making
use of physical constraints on the form of the electron spectrum. The
algorithm incorporates various features not heretofore employed in
the solar flare context: Generalized Singular Value Decomposition
(GSVD) to deal with different orders of constraints; rectangular form
of the cross-section matrix to extend the solution energy range;
regularization with various forms of the smoothing operator; and
"preconditioning" of the problem. We show by simulations that this
technique yields electron spectra with considerably more information
and higher quality than previous algorithms.
---------------------------------------------------------
Title: Spectra of Solar Energetic Electrons in Flares and near Earth
Authors: Kontar, E. P.; Krucker, S.; Lin, R. P.
2004AGUFMSH13A1130K Altcode:
Successful operation of Reuven Ramaty High Energy Solar Spectroscopic
Imager (RHESSI) allows us to observe hard X-ray spectrum of many solar
flares with unprecendent energy resolution. X-ray spectra provide us
with vital information about the spectral properties of highly energetic
electrons at the Sun. The mean electron spectrum of the solar flare
can be recovered through a newly developed constraint regularization
analysis of the bremsstrahlung photon spectra that energetic electrons
produce. We emphasize the use of non-square inversion techniques
combined with the correction of the observed spectrum for the effect
of Compton photon back-scatter to achieve the most meaningful solution
to the problem. For 16 solar flares the electron spectrum of temporally
related solar energetic electron events has been measured using WIND/3DP
allowing us simultaneous analysis of electron flux spectral properties
at the Sun and near the Earth. Electron spectra from solar flares show
strong correlations with the spectrum of solar energetic electrons,
though vary with energy. Weaker correlation at lower energies can be
viewed as a propogation and/or escape effect. Results suggest that
the commonly used model of a collisional transport (thick-target)
for flare electrons plus free streaming for interplanetary particles
cannot explain the observed spectra.
---------------------------------------------------------
Title: Anisotropic Bremsstrahlung Emission and the Form of Regularized
Electron Flux Spectra in Solar Flares
Authors: Massone, Anna Maria; Emslie, A. Gordon; Kontar, Eduard P.;
Piana, Michele; Prato, Marco; Brown, John C.
2004ApJ...613.1233M Altcode:
The cross section for bremsstrahlung photon emission in solar flares is,
in general, a function of the angle θ between the incoming electron
and the outgoing photon directions. Thus the electron spectrum required
to produce a given photon spectrum is a function of this angle, which
is related to the position of the flare on the solar disk and the
direction(s) of the precollision electrons relative to the local solar
vertical. We compare mean electron flux spectra for the flare of 2002
August 21 using cross sections for parameterized ranges of the angle
θ. Implications for the shape of the mean source electron spectrum
and for the injected power in nonthermal electrons are discussed.
---------------------------------------------------------
Title: Comparing Solar Hard X-ray Emissions and Impulsive Electron
Events seen at 1AU
Authors: Krucker, S.; Lin, R. P.; Kontar, E. P.
2004AGUSMSH22A..02K Altcode:
The Sun frequently accelerates electrons in solar flares and type III
radio bursts. Some of the accelerated electrons lose their energy by
collisions in the denser, lower solar atmosphere producing hard X-ray
(HXR) emissions, while others escape into interplanetary space. Whether
the HXR producing and the escaping electrons are accelerated by the
same mechanism is not known. We present a combined study of RHESSI
X-ray observations and WIND/3dp in situ electron observations taken
near 1~AU. Electron events with a solar release time in close temporal
agreement with the HXR peak time are selected. For these events,
the electron spectrum measured at 1~AU is compared with the electron
spectrum derived from the HXR observations. We find the derived and the
observed electron spectrum do not agree with a simple model of electron
acceleration high in the corona with downward moving electrons producing
HXRs in the lower, denser corona (thick target model) and upwards moving
electrons escaping into interplanetary space without energy changes;
the observed electron spectrum at 1 AU would predict a much harder
HXR spectrum than what is observed. More complicated models including
the effects of how particle escape from the acceleration cite are
need. That a high coronal acceleration can be excluded makes it hard
to explain how the low energy electrons, down to a few hundred eV,
can escape to ~1~AU. This suggests that two different mechanisms may
be accelerating electrons and that the HXR emission is not related to
the electrons seen at 1~AU despite the close temporal correlation.
---------------------------------------------------------
Title: Regularized mean and accelerated electron flux spectra in
solar flares
Authors: Kontar, E. P.; Emslie, A. G.; Piana, M.; Massone, A. M.;
Brown, J. C.
2004cosp...35.3941K Altcode: 2004cosp.meet.3941K
Hard X-ray spectra in solar flares permit, through knowledge of
the bremsstrahlung cross-section, inference of the mean source
electron spectrum that results from acceleration and propagation
of electrons in the solar atmosphere. Here we develop and apply an
enhanced regularization algorithm for this process which makes use of
a variety of physical constraints on the possible form of the electron
spectrum. The algorithm incorporates various features not heretofore
employed in the solar flare context, such as the use of Generalized
Singular Value Decomposition (GSVD), a rectangular representation of the
discretized problem (so that the electron and photon energy ranges used
are not necessarily the same), regularization using various smoothing
operators. The use of non-square inversion techniques, with physical
properties of the spectra to achieve the most meaningful solution
to the problem. We apply these techniques to data from a few solar
flares observed by RHESSI. Results using different regularization are
presented and compared for various time intervals. We further note
that such analyses may be used to infer properties of the electron
energy spectrum that lie at energies well above the maximum photon
energy observed. We also show how the construction of the accelerated
(injected) electron spectrum (assuming that Coulomb collisions in a
cold target dominate the electron energetics) is facilitated by the
use of higher-order regularization methods. Clear evidence is presented
for a change in the value of the high-energy cutoff in the mean source
electron spectrum with time.
---------------------------------------------------------
Title: The X-ray source region of <SUP>3</SUP>He-rich solar energetic
particle events
Authors: Krucker, S.; Lin, R. P.; Kontar, E. P.; Mason, G. M.;
Wiedenbeck, M. E.
2003AGUFMSH11D1130K Altcode:
The Reuven Ramaty High Energy Spectroscopic Imager RHESSI allows
for the first time to study simultaneously the spatial and spectral
characteristics of solar hard X-ray emission in detail. In this paper,
RHESSI X-ray imaging spectroscopy is used to investigate the source
regions of <SUP>3</SUP>He-rich solar energetic particles observed
at 1 AU. A series of very large <SUP>3</SUP>He-rich events that
occurred between August 19-21, 2002 are investigated. The timing
of the simultaneously observed electron events suggests that these
particle events are related to a series of GOES M class flares. All
events originated from the same active region AR0069 and show a
similar behavior: Next to the main flaring loops, TRACE observations
additionally show an EUV jet that appears to escape from the Sun with
a speed of up to 500 km s<SUP>-1</SUP>. X-ray emission is seen from
the main flaring loops as well; however, the most prominent source in
>30 keV hard X-rays appears to be displaced from the flaring loops,
at the footpoint of the field lines along which the jet is moving
outward. We investigate the possibility that the HXR footpoint is
produced by the downward moving part of the same electron population
that escapes to 1 AU, by comparing the derived electron spectrum from
the HXR footpoint source with the in situ observed electron spectrum.
---------------------------------------------------------
Title: Regularized Electron Flux Spectra in the 2002 July 23 Solar
Flare
Authors: Piana, Michele; Massone, Anna Maria; Kontar, Eduard P.;
Emslie, A. Gordon; Brown, John C.; Schwartz, Richard A.
2003ApJ...595L.127P Altcode:
By inverting the Reuven Ramaty High Energy Solar Spectroscopic Imager
(RHESSI) hard X-ray photon spectrum with the Tikhonov regularization
algorithm, we infer the effective mean electron source spectrum for
a time interval near the peak of the 2002 July 23 event. This inverse
approach yields the smoothest electron flux spectrum consistent with the
data while retaining real features, such as local minima, that cannot
be found with forward model-fitting methods that involve only a few
parameters. A significant dip in the recovered mean source electron
spectrum near E=55 keV is noted, and its significance briefly discussed.
---------------------------------------------------------
Title: RHESSI Hard X-Ray Imaging Spectroscopy of the Large Gamma-Ray
Flare of 2002 July 23
Authors: Emslie, A. Gordon; Kontar, Eduard P.; Krucker, Säm; Lin,
Robert P.
2003ApJ...595L.107E Altcode:
We present Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI)
hard X-ray images in different energy bands for the large X-class flare
of 2002 July 23; these images are used to construct spatially resolved
hard X-ray spectra for each of four prominent features: a bright,
soft source high in the corona, two localized, hard footpoints in
opposite polarity magnetic regions that show highly correlated flux
and spectral variations in time, and a third footpoint bounded by
the other three sources. The power-law spectral indices of the two
correlated footpoints differ by ~0.3-0.4, which may be the result of
differing column densities from the electron source.
---------------------------------------------------------
Title: An Explanation for Non-Power-Law Behavior in the Hard X-Ray
Spectrum of the 2002 July 23 Solar Flare
Authors: Kontar, Eduard P.; Brown, John C.; Emslie, A. Gordon;
Schwartz, Richard A.; Smith, David M.; Alexander, R. Calum
2003ApJ...595L.123K Altcode:
High-resolution Reuven Ramaty High Energy Solar Spectroscopic Imager
(RHESSI) data reveal that solar flare hard X-ray spectra show systematic
deviations from power-law behavior. Even for injection of a power-law
electron spectrum, such deviations are expected because of a number
of effects, including nonuniform target ionization and solar albedo
backscattering of the primary hard X-ray flux. In this Letter, we
examine 1 keV resolution hard X-ray spectra for the intense 2002 July
23 event, corrected for the effects of decimation, pulse pileup,
and background. We find that the observed spectra indeed deviate
from a power-law behavior in a manner consistent with the effects
of nonuniform target ionization. Further, this interpretation of
the observed deviations requires that the amount of coronal material
increases during the initial phase of the flare. The implications of
this discovery for models of atmospheric response to flare heating
are discussed.
---------------------------------------------------------
Title: The Determination and Use of Mean Electron Flux Spectra in
Solar Flares
Authors: Brown, John C.; Emslie, A. Gordon; Kontar, Eduard P.
2003ApJ...595L.115B Altcode:
Hard X-ray spectra in solar flares provide information on electron
acceleration and propagation processes. We here point out that
the inference of these processes involves two distinct steps: (1)
the model-independent deconvolution of the hard X-ray spectrum to
obtain the effective mean electron spectrum F(E) in the source and (2)
the model-dependent interpretation of this mean spectrum in terms of
physical processes operating in that source. Thus, the mean electron
spectrum is a natural “middle ground” on which to compare the
predictions of models with observations, and we urge the presentation
of results, both from analysis of photon spectra and from modeling
of candidate physical processes, in the form of F(E) spectra. We
consider the constraints that various source models impose on F(E),
and we present explicit forms for an illustrative F(E) corresponding
to the injection of a power-law spectrum of electrons into a thick
target with a nonuniform ionization level.
---------------------------------------------------------
Title: Plasma Radio Emission of Beam-Plasma Structures in the
Solar Corona
Authors: Mel'Nik, V. N.; Kontar, E. P.
2003SoPh..215..335M Altcode:
Recent progress in the description of electron beam propagation
in a plasma allows us to obtain explicit expressions for Langmuir
turbulence generated by the beam. Radio emission of an electron beam
accompanied by Langmuir turbulence (beam-plasma structure - BPS),
propagating in the solar corona is considered within the plasma emission
mechanism. The maximum brightness temperatures of a BPS radio emission
with velocity v<SUB>BPS</SUB>≈0.35 c at fundamental and harmonic
frequencies are found to be equal to T<SUB>F</SUB>=10<SUP>13</SUP> K,
T<SUB>H</SUB>=10<SUP>16</SUP> K, respectively. It is shown that the
temperature of radio emission sharply declines with the decrease of BPS
velocity. The dominant drift velocity of Type III sources (≈0.3 c)
and broad range of observable brightness temperatures are naturally
explained by the latter fact.
---------------------------------------------------------
Title: Implications of solar flare hard X-ray “knee” spectra
observed by RHESSI
Authors: Conway, A. J.; Brown, J. C.; Eves, B. A. C.; Kontar, E.
2003A&A...407..725C Altcode:
We analyse the RHESSI photon spectra of four flares that exhibit
significant deviations from power laws - i.e. changes in the “local”
Hard X-ray spectral index. These spectra are characterised by two
regions of constant power law index connected by a region of changing
spectral index - the “knee”. We develop theoretical and numerical
methods of describing such knees in terms of variable photon spectral
indices and we study the results of their inversions for source mean
thin target and collisional thick target injection electron spectra. We
show that a particularly sharp knee can produce unphysical negative
values in the electron spectra, and we derive inequalities that
can be used to test for this without the need for an inversion to
be performed. Such unphysical features would indicate that source
model assumptions were being violated, particularly strongly for
the collisional thick target model which assumes a specific form for
electron energy loss. For all four flares considered here we find that
the knees do not correspond to unphysical electron spectra. In the
three flares that have downward knees we conclude that the knee can be
explained in terms of transport effects through a region of non-uniform
ionisation. In the other flare, which has an upward knee, we conclude
that it is most likely a feature of the accelerated spectrum.
---------------------------------------------------------
Title: RHESSI Hard X-Ray Imaging Spectroscopy of the July 23, 2002
Solar Flare
Authors: Emslie, A. G.; Kontar, E. P.; Krucker, S.; Lin, R. P.
2003SPD....34.2208E Altcode: 2003BAAS...35..851E
We present hard X-ray images in different energy bands, as obtained
by the unique combination of rotating modulation collimators and
high-resolution germanium spectrometers on the RHESSI satellite,
for the large X-class flare of July 23, 2002. These data are then
used to construct spatially-resolved hard X-ray spectra for each
of four prominent features evident in these images. These four main
features are a bright, low-energy (soft spectrum) source high in the
corona, two localized high-energy (hard spectrum) footpoints and a
bright feature bounded by the other three, which could either be an
additional footpoint or a source near the top of a magnetic structure
connecting the other two. The temporal evolution of the spectrum of
each feature is described, with allowances for the dynamic range of
the RHESSI instrument, which obscures weak sources at a given photon
energy when one or more much brighter sources at this photon energy
are also present. A comparison with the temporal evolution of the
spatially-integrated hard X-ray spectrum is also provided. <P />This
work was supported by NASA's Office of Space Science and by a PPARC
Award.
---------------------------------------------------------
Title: Flare electron energy budgets - what is RHESSI telling us?
Authors: Brown, J. C.; Kontar, E.; MacKinnon, A. L.; Aschwanden, M. J.
2002ESASP.506..253B Altcode: 2002ESPM...10..253B; 2002svco.conf..253B
We address the idea that energetic particles may play a key role in the
dissipation and transport of energy in flares. After three decades of
predictions of spatial, spectral and temporal distributions of hard X-
and γ-rays, the various models can now be quantitatively tested against
RHESSI high resolution spectral imaging data. It is shown that RHESSI
results for a number of HXR flares are in very good agreement with
predictions of the basic thick target model (Brown 1971) regarding
source height as a function of energy and of global HXR spectrum. A
single power-law injection spectrum and purely collisional transport
(no wave generation) fit well the decrease of source peak height
with increasing energy for very plausible chromospheric density
structures. When the target ionisation drop across the transition one
is included, the global HXR spectrum agrees well with observed "knee"
spectra without any feature added to a scale-less power-law electron
injection spectrum. This result favours statistically distributed, as
opposed to single large scale, E-field acceleration. Whether energetic
electron beams actually dominate flare energy transport still depends
on accurate inference of the low energy thermal/nonthermal spectral
transition though RHESSI results to date support the idea. The ion
energy budget is also briefly mentioned.
---------------------------------------------------------
Title: X-ray observations with RHESSI and collisional thick target
model with nonuniform target ionisation
Authors: Kontar, Eduard P.; Brown, John C.; McArthur, Guillian K.
2002ESASP.506..311K Altcode: 2002ESPM...10..311K; 2002svco.conf..311K
Past analysis of the flare Hard X-Ray (HXR) spectra have largely ignored
the effect of nonuniform ionisation along the electron paths in the
thick target model, though it is very significant for well-resolved
spectra. The fit to RHESSI data on four flares for a single powerlaw
F<SUB>0</SUB>(E<SUB>0</SUB>) is much improved when ionisation structure
is included. The expression involves the column depth N<SUB>*</SUB>
of the transition region in the flare loop as one of the parameters.
---------------------------------------------------------
Title: Chromospheric density and height measurements of the
2002-Feb-20 flare observed with RHESSI
Authors: Aschwanden, Markus J.; Brown, John C.; Kontar, Eduard P.
2002ESASP.506..275A Altcode: 2002ESPM...10..275A; 2002svco.conf..275A
We present the first chromospheric density and height measurements
made with the Ramaty High Energy Solar Spectroscopic Imager (RHESSI)
spacecraft during the flare of 2002-Feb-22, 11:06 UT. Thanks to the
high energy resolution of the germanium-cooled hard X-ray detectors on
RHESSI we can measure the flare source positions with a high accuracy
as a function of energy. Using a forward-fitting algorithm for image
reconstruction, we find a systematic decrease in the altitudes of the
source centroids z(ɛ) as a function of increasing hard X-ray energy
ɛ, as expected in the thick-target bremsstrahlung model of Brown. The
altitude of hard X-ray emission as a function of photon energy ɛ can
be characterized by a powerlaw function in the ɛ = 15-50 keV energy
range, viz. z(ɛ) ≍ 2.3 (ɛ/20 keV)<SUP>-1.3</SUP> Mm. Based on a
purely collisional 1-D thick-target model, this height dependence can
be inverted into a chromospheric density model n(z), which follows
the powerlaw function n<SUB>e</SUB>(z) = 1.25×10<SUP>13</SUP> (z/1
Mm)<SUP>-2.5</SUP> cm<SUP>-3</SUP>. This density is comparable with
models based on optical/UV spectrometry in the chromospheric height
range, while at a height of h≍1000-2500 km, it is more consistent
with the "spicular extended-chromosphere model" inferred from radio
sub-mm observations. In coronal heights of the flare loop, the RHESSI
inferred desities are comparable with soft X-ray and radio observations.
---------------------------------------------------------
Title: Nonlinear development of Langmuir turbulence in inhomogeneous
solar coronal plasma
Authors: Kontar, Eduard P.; Pécseli, Hans L.
2002ESASP.506..315K Altcode: 2002svco.conf..315K; 2002ESPM...10..315K
The Langmuir wave turbulence is a key issue in the theory of solar
plasma emission. The self-consistent description of Langmuir wave
and ion-sound wave turbulence in the presence of an electron beam is
presented for inhomogeneous non-isothermal plasma. The results show
that the presence of inhomogeneity significantly changes the overall
evolution in the system. The inhomogeneity is very effective in shifting
the wave numbers of the Langmuir waves, and thus can switch between
different nonlinear processes.
---------------------------------------------------------
Title: Chromospheric Height and Density Measurements in a Solar
Flare Observed with RHESSI II. Data Analysis
Authors: Aschwanden, Markus J.; Brown, John C.; Kontar, Eduard P.
2002SoPh..210..383A Altcode:
We present an analysis of hard X-ray imaging observations from one
of the first solar flares observed with the Reuven Ramaty High-Energy
Solar Spectroscopic Imager (RHESSI) spacecraft, launched on 5 February
2002. The data were obtained from the 22 February 2002, 11:06 UT flare,
which occurred close to the northwest limb. Thanks to the high energy
resolution of the germanium-cooled hard X-ray detectors on RHESSI
we can measure the flare source positions with a high accuracy as
a function of energy. Using a forward-fitting algorithm for image
reconstruction, we find a systematic decrease in the altitudes of
the source centroids z(ε) as a function of increasing hard X-ray
energy ε, as expected in the thick-target bremsstrahlung model of
Brown. The altitude of hard X-ray emission as a function of photon
energy ε can be characterized by a power-law function in the ε=15-50
keV energy range, viz., z(ε)≈2.3(ε/20 keV)<SUP>−1.3</SUP>
Mm. Based on a purely collisional 1-D thick-target model, this
height dependence can be inverted into a chromospheric density model
n(z), as derived in Paper I, which follows the power-law function
n<SUB>e</SUB>(z)=1.25×10<SUP>13</SUP>(z/1 Mm)<SUP>−2.5</SUP>
cm<SUP>−3</SUP>. This density is comparable with models based on
optical/UV spectrometry in the chromospheric height range of h≲1000
km, suggesting that the collisional thick-target model is a reasonable
first approximation to hard X-ray footpoint sources. At h≈1000-2500
km, the hard X-ray based density model, however, is more consistent
with the `spicular extended-chromosphere model' inferred from radio
sub-mm observations, than with standard models based on hydrostatic
equilibrium. At coronal heights, h≈2.5-12.4 Mm, the average flare
loop density inferred from RHESSI is comparable with values from
hydrodynamic simulations of flare chromospheric evaporation, soft
X-ray, and radio-based measurements, but below the upper limits set
by filling-factor insensitive iron line pairs.
---------------------------------------------------------
Title: Chromospheric Height and Density Measurements in a Solar
Flare Observed with RHESSI I. Theory
Authors: Brown, John C.; Aschwanden, Markus J.; Kontar, Eduard P.
2002SoPh..210..373B Altcode:
We obtain a theoretical description of the height (z) distribution
of flare hard X-rays in the collisional thick-target model as a
function of photon energy ε. This depends on the target atmosphere
density structure n(z) and on the beam spectral index δ. We
show that by representing the data in terms of the 1-D function
z(ε) defining where the emission peaks as a function of ε it is
possible to derive n(z) from data on z(ε). This is done first on
the basis of a simple stopping depth argument then refined to allow
for the dependence on spectral index δ. The latter is worked out
in detail for the case of a parameterization n(z)=n<SUB>0</SUB>
(z/z<SUB>0</SUB>)<SUP>−b</SUP> which yields numerical results for
z(ε) well fit by z(ε)∼ε<SUP>−α</SUP>, with α dependent on δ,
which is also found to fit well to actual observations. This enables
derivation of flare loop n(z) in terms of n<SUB>0</SUB>,b from RHESSI
data in an entirely novel way, independent of other density diagnostic
methods, and also of how n(z) varies with time in flares such as by
evaporation, as detailed in companion Paper II.
---------------------------------------------------------
Title: Nonuniform Target Ionization and Fitting Thick Target Electron
Injection Spectra to RHESSI Data
Authors: Kontar, Eduard P.; Brown, John C.; McArthur, Guillian K.
2002SoPh..210..419K Altcode:
Past analyses of flare hard X-ray (HXR) spectra have largely
ignored the effect of nonuniform ionization along the electron
paths in the thick-target model, though it is very significant
for well-resolved spectra. The inverse problem (photon spectrum
to electron injection spectrum F<SUB>0</SUB>(E<SUB>0</SUB>)) is
disturbingly non-unique. However, we show that it is relatively simple
to allow for the effect in forward fitting of parametric models of
F<SUB>0</SUB>(E<SUB>0</SUB>)) and provide an expression to evaluate it
for the usual single power-law form of F<SUB>0</SUB>(E<SUB>0</SUB>)).The
expression involves the column depth N<SUB>*</SUB> of the transition
region in the flare loop as one of the parameters so data fitting can
enable derivation of N<SUB>*</SUB> (and its evaporative evolution)
as part of the fitting procedure. The fit to RHESSI data on four
flares for a single power law F<SUB>0</SUB>(E<SUB>0</SUB>)) is much
improved when ionization structure is included compared to when the
usual fully ionized approximation is used. This removes the need, in
these events at least, to invoke broken power laws, or other forms,
of the acceleration spectrum F<SUB>0</SUB>(E<SUB>0</SUB>)) to explain
the observed photon spectrum
---------------------------------------------------------
Title: Nonlinear development of electron-beam-driven weak turbulence
in an inhomogeneous plasma
Authors: Kontar, E. P.; Pécseli, H. L.
2002PhRvE..65f6408K Altcode: 2019arXiv190308368K
The self-consistent description of Langmuir wave and ion-sound wave
turbulence in the presence of an electron beam is presented for
inhomogeneous nonisothermal plasmas. Full numerical solutions of
the complete set of kinetic equations for electrons, Langmuir waves,
and ion-sound waves are obtained for an inhomogeneous unmagnetized
plasma. The results show that the presence of inhomogeneity
significantly changes the overall evolution of the system. The
inhomogeneity is effective in shifting the wave numbers of the Langmuir
waves, and can thus switch between different processes governing the
weakly turbulent state. The results can be applied to a variety of
plasma conditions, where we choose solar coronal parameters as an
illustration, when performing the numerical analysis.
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Title: Dynamics of electron beams in the solar corona plasma with
density fluctuations
Authors: Kontar, E. P.
2001A&A...375..629K Altcode: 2019arXiv190405650K
The problem of beam propagation in a plasma with small scale and
low intensity inhomogeneities is investigated. It is shown that the
electron beam propagates in a plasma as a beam-plasma structure and is a
source of Langmuir waves. The plasma inhomogeneity changes the spatial
distribution of the waves. The spatial distribution of the waves is
fully determined by the distribution of plasma inhomogeneities. The
possible applications to the theory of radio emission associated with
electron beams are discussed.
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Title: Dynamics of electron beams in the inhomogeneous solar corona
plasma
Authors: Kontar, Eduard P.
2001SoPh..202..131K Altcode: 2019arXiv190308867K
Dynamics of a spatially-limited electron beam in the inhomogeneous
solar corona plasma is considered in the framework of weak turbulence
theory when the temperature of the beam significantly exceeds that
of surrounding plasma. The numerical solution of kinetic equations
manifests that generally the beam accompanied by Langmuir waves
propagates as a beam-plasma structure with a decreasing velocity. Unlike
the uniform plasma case the structure propagates with the energy
losses in the form of Langmuir waves. The results obtained are
compared with the results of observations of type III bursts. It
is shown that the deceleration of type III sources can be explained
by corona inhomogeneity. The frequency drift rates of the type III
sources are found to be in good agreement with the numerical results
of beam dynamics.
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Title: Numerical consideration of quasilinear electron cloud dynamics
in plasma
Authors: Kontar, Eduard P.
2001CoPhC.138..222K Altcode: 2019arXiv190308651K
The dynamics of a hot electron cloud in the solar corona-like plasma
based on the numerical solution of kinetic equations of weak turbulence
theory is considered. Different finite difference schemes are examined
to fit the exact analytical solutions of quasilinear equations in
hydrodynamic limit (gas-dynamic solution). It is shown that the scheme
suggested demonstrates correct asymptotic behavior and can be employed
to solve initial value problems for an arbitrary initial electron
distribution function.
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Title: Plasma Emission of Beam-Plasma Structure in the Solar Corona
Authors: Mel'nik, V. N.; Kontar, E. P.
2001pre5.conf..479M Altcode:
The plasma emission mechanism is usually used for an explanation of
Solar radio bursts at decimetric and longer wavelengths with high
brightness temperature. This mechanism needs a high level of Langmuir
turbulence generated, for example by fast electron beams or shocks. In
recent times it was shown that fast electrons propagate through the
plasma in the form of a beam-plasma structure, a new nonlinear object,
analogous to a soliton. This structure consists of electrons and
Langmuir waves and propagates large distances without energy losses. The
energy in Langmuir waves is compared with that in fast electrons, so
the beam-plasma structure can be a powerful source of radioemission
due to plasma mechanism. In the paper the properties of plasma emission
of beam-plasma structure are presented. The application of the results
to the explanation of type III burst characteristics is discussed.
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Title: Propagation of a Maxwellian Electron Cloud in a Plasma
Authors: Mel'nik, V. N.; Kontar, E. P.; Lapshin, V. I.
2000SoPh..196..199M Altcode:
We consider the dynamics of an electron cloud with an initially
Maxwellian electron distribution and a temperature significantly
exceeding that of the surrounding plasma. It is demonstrated that
only the fastest electrons propagate into the plasma as a beam-plasma
structure, whereas the main part of the cloud of electrons is locked
by the Langmuir turbulence generated by the electrons remaining.
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Title: To gasdynamic description of a hot electron cloud in a
cold plasma
Authors: Mel'nik, V. N.; Kontar, E. P.
2000NewA....5...35M Altcode:
The quasi-gasdynamic equations are reassessed and the nonconsistency of
these equations is shown. The correct gasdynamic system of equations
is found. The solution of these equations are obtained in special
cases. The solution presents a new nonlinear object, beam-plasma
structure, that consists of electrons and Langmuir waves, and moves
with a constant speed. Numerical simulations corroborate the gasdynamic
theory very well.
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Title: The spread of the hot electron cloud in the solar corona
Authors: Mel'nik, V. N.; Kontar, E. P.
1999NewA....4...41M Altcode:
The spread of the electron cloud generating solar type III bursts
is discussed. It is shown that the maximum velocity of the electron
distribution function, plateau, cannot exceed the velocity of free
propagation u= x/ t. It appeared that despite the plateau formation
at the electron distribution function the asymptotic self-similar
solution u( x, t)=2 x/ t never occurred.
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Title: Propagation of a Monoenergetic Electron Beam in the Solar
Corona
Authors: Mel'nik, V. N.; Lapshin, V.; Kontar, E.
1999SoPh..184..353M Altcode:
The dynamics of an electron beam is considered when the initial electron
distribution is localized in a space region. Analysis is conducted for
the parameters of the beam and plasma that give radio emission. We
demonstrate both numerically and analytically that beam electrons
propagate as a beam-plasma structure at large distances. The speed
of the beam-plasma structure is equal to half of the maximum velocity
of the electrons involved in this structure. The structure conserves
the shape of the initial spatial distribution of electrons. A plateau
with a constant maximum velocity is formed at the electron distribution
function in each spatial point.
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Title: Beam-Plasma Structures at Propagation of Electron Beams
in Plasma
Authors: Mel'nik, V. N.; Kontar, E. P.
1998PhyS...58..510M Altcode:
In the frame of the theory of weak turbulence the propagation of
electron beams is considered. Using the smallness of quasilinear
time transition from kinetic equations to gas-dynamic ones has been
done. The obtained gas-dynamic equations are solved for one, two and N
monoenergetic beams. It is shown that each beam generally propagates
as a beam-plasma structure consisting of electrons and Langmuir
waves. Interaction of the structures at their propagation leads to
electron exchange and spatial shape changing. Different situations
are discussed in dependence on initial beam velocities and densities.
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Title: Plasma emission of electron beams in the low corona
Authors: Kontar, E. P.; Melnik, V. N.
1997jena.confE..43K Altcode:
Radioemission recorded at the high resolution spectrograms in the range
6.3-8.7 GHz is characterized by the fine structure i.e. time profiles
consist of great number of elements [1]. Each element is supposed to
be generated by an electron beam. The brightness temperature and radio
flux density is calculated for a single beam with exp-like distribution
with duration 25ms [2]. It is considered that Langmuir waves initially
one-dimensional convert into radioemission through either ion off
scattering or fusion of two Langmuir waves. Radioemission at the
harmonic (2 omega_p) is calculated when izotropic part of Langmuir
waves is much smaller than one-dimensional one. It is shown that
for beam velocities v > 5 times 10^9 cm/s transformation of
one-dimensional spectrum into the izotropic spectrum rise sharply
with velocity that leads to the stop of electron beam with higher
velocities. The maximum radio flux density at 2 omega_p is obtained
equal to 110 s.f.u.. Radioemission at fundamental frequency omega_p
give higher brightness temperatures and radio flux density than for
the harmonic frequency.
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Title: A Fly Off of the Fast Electron Flows Generating Type III Bursts
Authors: Mel'nik, V. N.; Kontar, E. P.
1997pre4.conf..421M Altcode:
Recent observation data show the existence of a fine structure in
solar type III bursts. A solar burst consists of several impulses
of radioemission with a short duration - much shorter than burst
duration. These impulses are thought to be generated by the different
electron beams propagating in the solar corona. It is important to
find out the variation electron distribution function taking into
account that electrons emit and absorb plasma waves. An initial
electron distribution function is taken as a set of mono-energetic
beams. Then the spread of electrons is described by the propogation of
mono-energetic beams. Recently it has been shown that a mono-energetic
beam propagates with constant velocity in the form of a beam-plasma
structure that consists of electrons and plasmons. The resultant
electron distribution function in every point is determined as a result
of the beam-plasma structure interaction. Such an approach allows us
to obtain the electron distribution function and the spectral energy
density of Langmuir waves. The electron distribution function looks like
a common plateau and a staircase. The spectral energy density of plasma
waves turns to be equal to zero at v = u_i and the respective stair
(u_i < v < u_{i+1}) is separated from the common plateau. Thus
the maximum velocity of the common plateau is decreased jumping
down from v = u_{i+1} to v = u_i but its height is increased. For
velocities v, which are greater than the maximum velocity u_i, the
electron distribution function has a staircase-form with stair heights
decreasing with velocity. The solutions of the problem obtained in the
paper are compared with numerical simulations of spreading of electron
flows which generate type III bursts.