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.

---------------------------------------------------------
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 (&lt;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
  (&lt;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.

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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 &lt; δ {n}<SUB>{{e</SUB>}}&gt;
  /{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 &gt; 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 &lt;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 &amp; 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 (&gt;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,&lt;nVF&gt; , 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 (&gt; 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 &gt;20 s duration. <BR /> Results:
  We find that &gt;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> &lt;~ 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 &gt;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 &amp; Kontar A&amp;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 (&gt;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&amp;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 &amp; 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 &amp; Kontar
  A&amp;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 &lt;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 &gt;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 &gt; 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>&lt;&lt;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 &gt;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&amp;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 &lt;=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 &lt;= 0.4 R <SUB>sun</SUB>,
  and the peak velocity at h &lt;= 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 &gt;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 (&gt;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 &amp; 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
  &gt;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.

---------------------------------------------------------
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.

---------------------------------------------------------
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.

---------------------------------------------------------
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.

---------------------------------------------------------
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.

---------------------------------------------------------
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.

---------------------------------------------------------
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 &gt; 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 &lt; v &lt; 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.