Author name code: kontar ADS astronomy entries on 2022-09-14 author:"Kontar, Eduard" ------------------------------------------------------------------------ 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. Bibcode: 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. 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 Bibcode: 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 3 - 1.63X 2 + 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 , depending on whether the type III emissions are assumed to be at the fundamental or the harmonic. 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 Bibcode: 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. 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 Bibcode: 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. 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. Bibcode: 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.
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.
Methods: We used the data products provided by the different subsystems of RPW to study Venus' induced magnetosphere.
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.
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 (RV) 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 ∼70RV in the far distant tail region. 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. Bibcode: 2021A&A...656A..34M Altcode: 2021arXiv210913713M
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.
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.
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.

ARRAY(0x18a8790) Title: The Spatial and Temporal Variations of Turbulence in a Solar Flare Authors: Stores, Morgan; Jeffrey, Natasha L. S.; Kontar, Eduard P. Bibcode: 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. 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. Bibcode: 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. 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. Bibcode: 2021ExA...tmp..135M Altcode: As a frequent and energetic particle accelerator, our Sun provides us with an excellent astrophysical laboratory for understanding the fundamental process of particle acceleration. The exploitation of radiative diagnostics from electrons has shown that acceleration operates on sub-second time scales in a complex magnetic environment, where direct electric fields, wave turbulence, and shock waves all must contribute, although precise details are severely lacking. Ions were assumed to be accelerated in a similar manner to electrons, but γ-ray imaging confirmed that emission sources are spatially separated from X-ray sources, suggesting distinctly different acceleration mechanisms. Current X-ray and γ-ray spectroscopy provides only a basic understanding of accelerated particle spectra and the total energy budgets are therefore poorly constrained. Additionally, the recent detection of relativistic ion signatures lasting many hours, without an electron counterpart, is an enigma. We propose a single platform to directly measure the physical conditions present in the energy release sites and the environment in which the particles propagate and deposit their energy. To address this fundamental issue, we set out a suite of dedicated instruments that will probe both electrons and ions simultaneously to observe; high (seconds) temporal resolution photon spectra (4 keV - 150 MeV) with simultaneous imaging (1 keV - 30 MeV), polarization measurements (5-1000 keV) and high spatial and temporal resolution imaging spectroscopy in the UV/EUV/SXR (soft X-ray) regimes. These instruments will observe the broad range of radiative signatures produced in the solar atmosphere by accelerated particles. Title: 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. Bibcode: 2021A&A...654C...2M Altcode: No abstract at ADS 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 Bibcode: 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 ~108 cm, with brightness temperature as high as 1013 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. 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. Bibcode: 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. Title: Fine structure of type III solar radio bursts from Langmuir wave motion in turbulent plasma Authors: Reid, Hamish A. S.; Kontar, Eduard P. Bibcode: 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. 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. Bibcode: 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. Title: Solar type III radio burst fine structure from Langmuir wave motion through turbulent plasma Authors: Kontar, Eduard; Reid, Hamish Bibcode: 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. 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 Bibcode: 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 Bibcode: 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 Bibcode: 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 Bibcode: 2021EGUGA..2310620Z Altcode: The solar atmosphere is fluctuated and highly refractive for low frequency waves (<300MHz), the observed features of solar radio sources have indicated the existence of complex propagation effects. The propagation effect has two major parts: refraction and scattering, these two parts have combined influence on the observed source size and position of radio imaging and temporal-frequency features in the radio spectroscopy.We present a parametric simulation for the propagation effect of the radio wave from solar radio bursts, with the method of parametric simulation, we can build connections between the solar atmosphere plasma condition and the observed radio source properties. By comparing the simulation results with the observed source size and property we estimated the scattering rate and the degree of anisotropic of the background electron, and from the simulation results we propose a possible explanation for the co-spatial phenomena of the fundamental wave and harmonic wave in single frequency. Title: Parametric Simulation Studies on the Wave Propagation of Solar Radio Emission: The Source Size, Duration, and Position Authors: Zhang, PeiJin; Wang, ChuanBing; Kontar, Eduard P. Bibcode: 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 = ɛ2/h0, where ɛ2 = ⟨δn2⟩/n2 is the density fluctuation level and h0 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 × 10-5 km-1 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 × 10-4 km-1 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 Bibcode: 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. Bibcode: 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 Bibcode: 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 fpe ≍ 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 fpe ≍ 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. Bibcode: 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.

(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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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.
Aims: This paper aims to extend the previous investigation to a 3D magnetic reconnection configuration and to study the effect on test particle orbits.
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.
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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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 Bibcode: 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. Bibcode: 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 104 (up to ∼105) km s-1, which often exceeds the velocities inferred from the drift rate (∼104 km s-1). 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 E = 0.57 ± 0.08, which is consistent with an earlier study based on the simplified approximation of the warm target model alone (q E = 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 Bibcode: 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 Bibcode: 2019AAS...23422501C Altcode: The Focusing Optics X-ray Solar Imager (FOXSI), a SMEX mission concept in Phase A, is the first-ever solar-dedicated, direct-imaging, hard X-ray telescope. FOXSI provides a revolutionary new approach to viewing explosive magnetic-energy release on the Sun by detecting signatures of accelerated electrons and hot plasma directly in and near the energy-release sites of solar eruptive events (e.g., solar flares). FOXSI's primary science objective is to understand the mystery of how impulsive energy release leads to solar eruptions, the primary drivers of space weather at Earth, and how those eruptions are energized and evolve. FOXSI addresses three important science questions: (1) How are particles accelerated at the Sun? (2) How do solar plasmas get heated to high temperatures? (3) How does magnetic energy released on the Sun lead to flares and eruptions? These fundamental physics questions are key to our understanding of phenomena throughout the Universe from planetary magnetospheres to black hole accretion disks. FOXSI measures the energy distributions and spatial structure of accelerated electrons throughout solar eruptive events for the first time by directly focusing hard X-rays from the Sun. This naturally enables high imaging dynamic range, while previous instruments have typically been blinded by bright emission. FOXSI provides 20-100 times more sensitivity as well as 20 times faster imaging spectroscopy than previously available, probing physically relevant timescales (<1 second) never before accessible. FOXSI's launch in July 2022 is aligned with the peak of the 11-year solar cycle, enabling FOXSI to observe the many large solar eruptions that are expected to take place throughout its two-year mission. Title: Warm-Target Modeling and a Solution to The Low-energy Cut-off Problem Authors: Emslie, A. Gordon; Kontar, Eduard; Jeffrey, Natasha L. S. Bibcode: 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. Bibcode: 2019ApJ...873...33B Altcode: 2019arXiv190200239B Electromagnetic wave scattering off density inhomogeneities in the solar corona is an important process that determines both the apparent source size and the time profile of radio bursts observed at 1 au. Here we model the scattering process using a Fokker-Planck equation and apply this formalism to several regimes of interest. In the first regime the density fluctuations are considered quasi-static and diffusion in wavevector space is dominated by angular diffusion on the surface of a constant energy sphere. In the small-angle (“pencil-beam”) approximation, this diffusion further occurs over a small solid angle in wavevector space. The second regime corresponds to a much later time, by which scattering has rendered the photon distribution near-isotropic, resulting in a spatial diffusion of the radiation. The third regime involves time-dependent fluctuations and, therefore, Fermi acceleration of photons. Combined, these results provide a comprehensive theoretical framework within which to understand several important features of propagation of radio burst waves in the solar corona: emitted photons are accelerated in a relatively small inner region and then diffuse outward to larger distances. En route, angular diffusion results both in source sizes that are substantially larger than the intrinsic source and in observed intensity-versus-time profiles that are asymmetric, with a sharp rise and an exponential decay. Both of these features are consistent with observations of solar radio bursts. Title: Frequency-Distance Structure of Solar Radio Sources Observed by LOFAR Authors: Gordovskyy, Mykola; Kontar, Eduard; Browning, Philippa; Kuznetsov, Alexey Bibcode: 2019ApJ...873...48G Altcode: Low-frequency radio observations make it possible to study the solar corona at distances up to 2-3 R . 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 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 Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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 km2. 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 Bibcode: 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 . 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. Bibcode: 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 ∼104 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 104 - 106 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. Bibcode: 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. Bibcode: 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. Bibcode: 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−1. 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 ∼10−3, their characteristic length scale is less than 1000 km, and the characteristic propagation speed is in the range of 400 -800 kms−1. 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. Bibcode: 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.
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.
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.
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 θBn 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. Bibcode: 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-1, 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 Bibcode: 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 Bibcode: 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. Bibcode: 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.
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.
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.
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.
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 Bibcode: 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. Bibcode: 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 Bibcode: 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 Bibcode: 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 Bibcode: 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 Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 2018ApJ...857...82K Altcode: Type III bursts are generated by fast electron beams originated from magnetic reconnection sites of solar flares. As propagation of radio waves in the interplanetary medium is strongly affected by random electron density fluctuations, type III bursts provide us with a unique diagnostic tool for solar wind remote plasma measurements. Here, we performed a statistical survey of 152 simple and isolated type III bursts observed by the twin-spacecraft Solar TErrestrial RElations Observatory mission. We investigated their time-frequency profiles in order to retrieve decay times as a function of frequency. Next, we performed Monte Carlo simulations to study the role of scattering due to random electron density fluctuations on time-frequency profiles of radio emissions generated in the interplanetary medium. For simplification, we assumed the presence of isotropic electron density fluctuations described by a power law with the Kolmogorov spectral index. Decay times obtained from observations and simulations were compared. We found that the characteristic exponential decay profile of type III bursts can be explained by the scattering of the fundamental component between the source and the observer despite restrictive assumptions included in the Monte Carlo simulation algorithm. Our results suggest that relative electron density fluctuations < δ {n}{{e}}> /{n}{{e}} 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 Bibcode: 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. Bibcode: 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.
Aims: We study the energy dependence of the scattering mean free path of energetic electrons in the solar corona.
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.
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.
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. Bibcode: 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. Bibcode: 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 }T 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}7 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 }T. 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 108 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 107 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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.
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.
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.
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.
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. Bibcode: 2017MNRAS.471...89J Altcode: 2017arXiv170604031J We study the energetics of non-thermal electrons produced in small acceleration episodes in the solar corona. We carried out an extensive survey spanning 2004-2015 and shortlisted six impulsive electron events detected at 1 au that were not associated with large solar flares (GOES soft X-ray class > C1) or with coronal mass ejections. Each of these events had weak, but detectable hard X-ray (HXR) emission near the west limb, and were associated with interplanetary type III bursts. In some respects, these events seem like weak counterparts of 'cold/tenuous' flares. The energy carried by the HXR producing electron population was ≈1023-1025 erg, while that in the corresponding population detected at 1 au was ≈1024-1025 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. Bibcode: 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. Bibcode: 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 me/mi. 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 1010K for continuum emission, and can exceed 1011K 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. Bibcode: 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.
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.
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.
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.
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 Bibcode: 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 Bibcode: 2017SPD....4810304A Altcode: Recent high spatial and temporal resolution imaging of <250 MHz solar radio emission has enabled us to observe rapid variations in Type-III solar radio burst characteristics, revealing fast growth of the Type-III source and movement of the source centroid. In this work, we use a Monte-Carlo ray tracing simulation to model the passage of low frequency (5-240 MHz) radio waves through the solar corona from a point source, considering both isotropic and dipole emission. We model the effects of random density fluctuations and an isotropic density gradient on the transport of the rays, varying the strength of the scattering to observe the effects on images of the source from an observer at 1 AU. Absorption of photons is included, and the effects on the reproduced images and flux curves are observed. The apparent source size and centroid position are tracked through the simulation, and we find a general increase in source size with time, and a variation of centroid position in both directions throughout the simulation. We find that the size of the variation is strongly dependant upon frequency, with lower frequency sources appearing to move further on the disk than higher frequency sources. We also observe the strength of the effects at different viewing angles, finding that the greatest variation occurs closer to the solar limb. Further observational work is required to limit the scattering parameters, in order to allow for comparison with current radio images. Title: A weak thermal response on a strong electron acceleration in a ‘cold’ flare Authors: Fleishman, Gregory D.; Motorina, Galina; Nita, Gelu M.; Kontar, Eduard Bibcode: 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. Bibcode: 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.
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.
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.
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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 eco. From these parameters, we calculate the total nonthermal energy Ent in electrons with two different methods: (1) using the observed cross-over energy eco as low-energy cutoff, and (2) using the low-energy cutoff ewt predicted by the warm thick-target bremsstrahlung model of Kontar et al. Based on a mean temperature of Te=8.6MK in active regions, we find low-energy cutoff energies of ewt=6.2+/-1.6keV for the warm-target model, which is significantly lower than the cross-over energies eco=21+/-6keV. Comparing with the statistics of magnetically dissipated energies Emag and thermal energies Eth from the two previous studies, we find the following mean (logarithmic) energy ratios with the warm-target model: Ent=0.41Emag, Eth=0.08Emag, and Eth=0.15Ent. 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.

(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 Bibcode: 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}{nt}), the energy of direct heating ({E}{dir}), and the energy in CMEs ({E}{CME}), which are the primary energy dissipation processes in a flare, is found to have a ratio of ({E}{nt}+{E}{dir}+{E}{CME})/{E}{mag}=0.87+/- 0.18, compared with the dissipated magnetic free energy {E}{mag}, 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}c≈ 6 {keV}, based on the mean peak temperature of the differential emission measure of T e = 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. Bibcode: 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 Bibcode: 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 Bibcode: 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 }T associated with the turbulent scattering process, we identify four main cooling scenarios. The overall temperature evolution, from an initial temperature in excess of 107 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 }T 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. Bibcode: 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. Bibcode: 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-1 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 × 1011 ± 1.76 × 1011 cm-3, 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 × 1012 ± 4.95 × 1011 g s-1. Finally, we reveal a close proximity between the model predictions of {10}5.8 K and the observed properties between {10}5.9 and {10}6.2 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. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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 co. From these parameters, we calculate the total nonthermal energy E nt in electrons with two different methods: (1) using the observed cross-over energy e co as low-energy cutoff, and (2) using the low-energy cutoff e wt predicted by the warm thick-target bremsstrahlung model of Kontar et al. Based on a mean temperature of T e = 8.6 MK in active regions, we find low-energy cutoff energies of {e}{wt}=6.2+/- 1.6 {keV} for the warm-target model, which is significantly lower than the cross-over energies {e}{co}=21+/- 6 {keV}. Comparing with the statistics of magnetically dissipated energies E mag and thermal energies E th from the two previous studies, we find the following mean (logarithmic) energy ratios with the warm-target model: {E}{nt}=0.41 {E}{mag}, {E}{th}=0.08 {E}{mag}, and {E}{th}=0.15 {E}{nt}. 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. Bibcode: 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. Bibcode: 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 Bibcode: 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 Bibcode: 2016cosp...41E1374M Altcode: Solar flares are associated with efficient particle acceleration, in particular with the production of energetic electrons which are diagnosed through the X-ray and radio emissions that they produce when interacting with the solar atmosphere. Particle transport from the acceleration sites to the radiation sites remains of the challenging topic in the field of high energy solar physics and has an important impact on the interpretation of the particle emissions in the context of acceleration models. In order to address the transport of flare associated energetic electrons in the low corona, we use imaging spectroscopic observations from RHESSI of the 2004 May 21 solar flare which presents together with the usually observed HXR footpoints a well observed coronal non-thermal X-ray source. The number of X-ray emitting energetic electrons in the coronal source is compared to the number of electrons needed to produce the hard X-ray emission in the footpoints and is found twice as large. Such an excess of the number of electrons in the coronal source cannot be explained in the context of the standard model of X-ray emissions in which the dominant electron transport is collisional. In the present flare, an additional process is needed to explain how energetic electrons can be efficiently trapped in the corona. In the hypothesis of turbulent pitch-angle scattering of hard X-ray producing energetic electrons (Kontar et al, 2014), diffusive transport can indeed lead to a confinement of energetic electrons in the coronal source. Based on this assumption, we estimated for the present event the mean-free path of energetic electrons and found a value of 10^8 - 10^9 meters, much smaller than the size of the observed flaring loop itself. This implies that a diffusive transport of energetic electrons is dominant in this flare which is in good agreement with the results of a previous study based on the gyrosynchrotron emissions from the energetic electrons (Kuznetsov & Kontar, 2015). Title: The electric field induced by high-energy solar electron beams Authors: Reid, Hamish; Kontar, Eduard Bibcode: 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 Bibcode: 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 Bibcode: 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. Bibcode: 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.
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.
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.
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-1 around 1 MK to about 200-400 km s-1 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 2015AGUFMSH21A2375A Altcode: In the past decade, analysis of near-relativistic (~27 keV - 300 keV) electron events at 1 AU have highlighted two transport effects which require explanation. Firstly, several events feature delayed electron arrival with respect to solar radio and hard x-ray emission, and secondly, the peak-flux spectrum of electrons at 1 AU does not match the predicted spectrum from hard x-ray observations. We analyse several near-relativistic electron events observed via both RHESSI hard x-ray observations at the Sun and in-situ measurements from the Wind/3DP detector at 1 AU. Numerical simulations of electron transport outwards from the Sun are made, which take the electron injection time and peak-flux spectrum from RHESSI data, and the flux subsequently passing 1 AU is calculated. We consider the effects of adiabatic focusing and pitch angle diffusion on the particle transport, and a momentum and distance dependent form of the parallel mean free path for electrons is employed. The simulated lightcurves, peak-flux spectrum, pitch angle distribution, and delay times are then compared with Wind observations. We find that, for higher energy electrons (>40 keV), the simulated flux matches well with observations, showing that stochastic pitch angle scattering is able to explain apparent delayed particle injection at the Sun. The lower energy observations, however, remain unmatched by models, which predict much more impulsive events at Earth than are observed. We also find that pitch angle scattering is too weak to vary the peak-flux spectrum sufficiently, thus requiring further exploration. Title: The Formation of Kappa-Distribution Accelerated Electron Populations in Solar Flares Authors: Bian, N. H.; Kontar, E.; Emslie, G. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 2015A&A...580A.137K Altcode: 2015arXiv150706874K
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.
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.
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.

Appendix A is available in electronic form at http://www.aanda.org Title: Stopping frequency of type III solar radio bursts in expanding magnetic flux tubes Authors: Reid, Hamish A. S.; Kontar, Eduard P. Bibcode: 2015A&A...577A.124R Altcode: 2015arXiv150303395R
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.
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.
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 Bibcode: 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. Bibcode: 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 Bibcode: 2015TESS....130705S Altcode: The presence of extended coronal hard X-ray sources, for example the 2005 Aug 23 flare, has opened up opportunities for probing the electrons accelerated by solar flares. The loop morphology of the source enables, through the use of RHESSI imaging spectroscopy, detailed analysis of changes in the electron flux spectrum along the spatial extent of the emission. We present a realistic one-dimensional model of the corona subject to coulomb collisions and localised stochastic acceleration akin to a looptop acceleration region. Both ballistic and diffusive transport are examined and the variation of the density weighted mean electron flux,<nVF> , along our numerical loop will be compared with RHESSI observations to determine the dominant processes in the corona. We will also comment on the validity of the leaky-box Fokker-Planck approximation. Title: Three-dimensional Radio and X-Ray Modeling and Data Analysis Software: Revealing Flare Complexity Authors: Nita, Gelu M.; Fleishman, Gregory D.; Kuznetsov, Alexey A.; Kontar, Eduard P.; Gary, Dale E. Bibcode: 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. Bibcode: 2015SoPh..290...79K Altcode: 2014SoPh..tmp...69K; 2014arXiv1403.5751K We investigated in detail the 21 May 2004 flare using simultaneous observations of the Nobeyama Radioheliograph, the Nobeyama Radiopolarimeters, the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI), and the Solar and Heliospheric Observatory (SOHO). The flare images in different spectral ranges reveal a well-defined single flaring loop in this event. We simulated the gyrosynchrotron microwave emission using the recently developed interactive IDL tool GX Simulator. By comparing the simulation results with the observations, we deduced the spatial and spectral properties of the non-thermal electron distribution. The microwave emission has been found to be produced by the high-energy electrons (> 100 keV) with a relatively hard spectrum (δ≃2); the electrons were strongly concentrated near the loop top. At the same time, the number of high-energy electrons near the footpoints was too low to be detected in the RHESSI images and spatially unresolved data. The SOHO Extreme-ultraviolet Imaging Telescope images and the low-frequency microwave spectra suggest the presence of an extended "envelope" of the loop with lower magnetic field. Most likely, the energetic electron distribution in the considered flare reflects the localized (near the loop top) particle acceleration (injection) process accompanied by trapping and scattering. Title: The Formation of Kappa-distribution Accelerated Electron Populations in Solar Flares Authors: Bian, Nicolas H.; Emslie, A. Gordon; Stackhouse, Duncan J.; Kontar, Eduard P. Bibcode: 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 τacc ~ E 3/2. 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 2014A&A...567A..85R Altcode: 2014arXiv1403.1839R
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.
Methods: To obtain a list of suitable events we considered the RHESSI catalogue of X-ray flares and the Phoenix 2 catalogue of type III radio bursts. From the 200 events that showed both type III and X-ray signatures, we selected 30 events which had simultaneous emission in both wavelengths, good signal to noise in the X-ray domain and >20 s duration.
Results: We find that >50% of the selected events show a good correlation between the starting frequencies of the groups of type III bursts and the hard X-ray spectral indices. A low-high-low trend for the starting frequency of type III bursts is frequently observed. Assuming a background electron density model and the thick target approximation for X-ray observations, this leads to a correlation between starting heights of the type III emission and the beam electron spectral index. Using this correlation we infer the altitude and vertical extents of the flare acceleration regions. We find heights from 183 Mm down to 25 Mm while the sizes range from 13 Mm to 2 Mm. These values agree with previous work that places an extended flare acceleration region high in the corona. We also analyse the assumptions that are required to obtain our estimates and explore possible extensions to our assumed model. We discuss these results with respect to the acceleration heights and sizes derived from X-ray observations alone.

Appendices are available in electronic form at http://www.aanda.org Title: The Formation of Accelerated Electron Distributions in Solar Flares Authors: Emslie, A. Gordon; Bian, Nicolas; Kontar, Eduard Bibcode: 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. Bibcode: 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 Bibcode: 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 Bibcode: 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. Bibcode: 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 Bibcode: 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 Bibcode: 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 ~ (108-109) 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 Bibcode: 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 Bibcode: 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. Bibcode: 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.
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.
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.
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 Bibcode: 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 Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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. 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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, ne <~ 2 × 109 cm-3. 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. Bibcode: 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. Bibcode: 2013A&A...553A..10H Altcode: 2012arXiv1212.5529H
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).
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.
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-1 in the temperature range ≥3-4 MK and between 1029-1246 km s-1 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 × 109 cm-3, 6 × 109 cm-3, 9 × 108 cm-3 in the plasmoid core, stem, and surrounding envelope of material. This gives thermal energy estimates of 5 × 1029 erg, 1 × 1029 erg, and 2 × 1030 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.
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.

A movie is available in electronic form at http://www.aanda.org Title: Stochastic Acceleration by Multi-Island Contraction during Turbulent Magnetic Reconnection Authors: Bian, Nicolas H.; Kontar, Eduard P. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 2013A&A...550A..51H Altcode: 2012arXiv1211.6015H
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.
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.
Results: We find that the interaction of the Langmuir waves with the background electron density gradient shifts the waves to a higher phase velocity where they then resonate with higher velocity electrons. The consequence is that some of the electrons are shifted to higher energies, producing more high-energy X-rays than expected if the density inhomogeneity is not considered. We find that the level of energy gain is strongly dependent on the initial electron beam density at higher energy and the magnitude of the density gradient in the background plasma. The most significant gains are for steep (soft) spectra that initially had few electrons at higher energies. If the X-ray spectrum of the simulated footpoint emission are fitted with the standard "thick-target" model (as is routinely done with RHESSI observations) some simulation scenarios produce more than an order-of-magnitude overestimate of the number of electrons >50 keV in the source coronal distribution. Title: Science enabled by high precision inertial formation flying Authors: Skinner, Gerry K.; Dennis, Brian R.; Krizmanic, John F.; Kontar, Eduard P. Bibcode: 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. Bibcode: 2012AGUFMSH51C..07H Altcode: The current wealth of solar observations presents a unique opportunity to study energy release in solar flares, particularly particle acceleration and plasma heating. The spatial and temporal resolution of SDO/AIA EUV data give an unprecedented view of dynamical heating in solar flares yet to fully exploit this resource the underlying thermal properties of the emitting plasma needs to be recovered. This is difficult as it is an ill-posed inverse problem and there is copious data. Our recently implemented regularized inversion method (Hannah & Kontar A&A 2012a,b) can quickly and robustly find the Differential Emission Measure (DEM) solution (and its uncertainties), with the resulting EM maps allowing the temperature and density evolution to be studied both spatially and temporally. Combing this with the hard X-ray imaging and spectroscopy of RHESSI, we present a study of the non-thermal energy input and thermal response in some flares of the rising phase of cycle 24. We also look at the relationship between the energetics of flares and the underlying magnetic field configurations. Title: The Effects of Langmuir Wave Evolution on Type III Radio Emission Authors: Ratcliffe, H.; Kontar, E. P.; Bian, N. Bibcode: 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. Bibcode: 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 1017 cm-3 near the photosphere to 1010 cm-3 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 D is of the order of the quasilinear timescale τql, while when τ D Lt τql, 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. Bibcode: 2012A&A...544A.148K Altcode: 2012arXiv1207.6248K
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.
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.
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.
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. Bibcode: 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. Bibcode: 2012A&A...543A..53G Altcode: 2012arXiv1206.0477G
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.
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.
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.
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 Bibcode: 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 Bibcode: 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. Bibcode: 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 (>1011 cm-3) 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. Bibcode: 2012AAS...22020403H Altcode: The unprecedented RHESSI observations of solar flare hard X-rays (HXR) has forced us to consider mechanisms in addition to the traditional collisional view of coronal electron transport. The self-consistent generation of Langmuir waves by the electron beam is one such process, thought to be the source of the reverse drift decimetric radio emission seen in some flares. We have previously shown that the inclusion of Langmuir waves flattens the electron spectrum (Hannah et al. ApJ 2009) and produces a spectral index difference between the coronal and footpoint sources closer to observations (Hannah and Kontar A&A 2011). However the wave growth also results in fainter HXR emission requiring a higher flux of electrons to be accelerated, compounding the “number” problem. In this work we show that the addition of the interaction of the Langmuir waves with turbulent density fluctuations in the background plasma greatly alleviates this problem. We demonstrate the consequences of these self-consistent numerical simulations in the context of the observable HXR spectrum for a variety of forms of the density fluctuations. Title: Dynamical Heating In Flares Observed With SDO/AIA & RHESSI Authors: Hannah, Iain; Fletcher, L.; Kontar, E. P. Bibcode: 2012AAS...22032202H Altcode: The spatial and temporal resolution of SDO/AIA data presents an unprecedented view of the dynamics of heating during solar flares. This combined with the non-thermal energetics from RHESSI hard X-ray imaging and spectroscopy provides constraints on the flaring energy release. The recently implemented regularized inversion method (Hannah & Kontar A&A 2012) robustly recovers the underlying thermal distribution (the Differential Emission Measure, DEM) of the coronal plasma from SDO/AIA images. Crucially it is not limited to the isothermal or Gaussian-model approximation that some other approaches depend upon. Our method provides the uncertainties in the DEM and is computationally quick, producing DEMs per pixel for a series of SDO/AIA images allowing temperature maps and movies to be created. We use the regularized inversion method to study the temporal and spatial evolution of the plasma heating in flares and show how the non-thermal energy relates to this. We also investigate how the calibration errors/uncertainties affect the inferred DEMs and errors. Title: Effects Of Langmuir Waves On Flare-accelerated Electrons In The Inhomogeneous Coronal Plasma Authors: Ratcliffe, Heather; Kontar, E. P. Bibcode: 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. Bibcode: 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.

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. Bibcode: 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 Bibcode: 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 LaBr3 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. Bibcode: 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.
Aims: We aim to investigate the role of the spectrally evolving Langmuir turbulence on the population of energetic electrons in the solar corona.
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.
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.
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. Bibcode: 2012A&A...539A.146H Altcode: 2012arXiv1201.2642H
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.
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.
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.
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. Bibcode: 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. Bibcode: 2011A&A...536A..93J Altcode: 2011arXiv1110.4993J
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.
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.
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.
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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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.
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.
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.
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. Bibcode: 2011ApJ...739...96K Altcode: We report on high-resolution optical and hard X-ray observations of solar flare ribbons seen during the GOES X6.5 class white-light flare of 2006 December 6. The data consist of imaging observations at 430 nm (the Fraunhofer G band) taken by the Hinode Solar Optical Telescope with the hard X-rays observed by the Reuven Ramaty High Energy Solar Spectroscopic Imager. The two sets of data show closely similar ribbon structures, strongly suggesting that the flare emissions in white light and in hard X-rays have physically linked emission mechanisms. While the source structure along the ribbons is resolved at both wavelengths (length ~ 30''), only the G-band observations resolve the width of the ribbon, with values between ~0farcs5 and ~1farcs8. The unresolved hard X-ray observations reveal an even narrower ribbon in hard X-rays (the main footpoint has a width perpendicular to the ribbon of <1farcs1 compared to the G-band width of ~1farcs8) suggesting that the hard X-ray emission comes from the sharp leading edge of the G-band ribbon. Applying the thick-target beam model, the derived energy deposition rate is >5 × 1012 erg s-1 cm-2 provided by an electron flux of 1 × 1020 electrons s-1 cm-2 above 18 keV. This requires that the beam density of electrons above 18 keV be at least 1 × 1010 cm-3. 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. Bibcode: 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.
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.
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.
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.
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.

Two movies are available in electronic form at http://www.aanda.org 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 2011A&A...530A..47I Altcode: 2011arXiv1102.5349I
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.
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.
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.
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. Bibcode: 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 (1035 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.

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. Bibcode: 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. Bibcode: 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.

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. Bibcode: 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.

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.

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. Bibcode: 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.

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. Bibcode: 2011A&A...529A.109H Altcode: 2011arXiv1103.2257H
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.
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.
Results: We find that the addition of wave-particle interactions to collisional transport for a transient initially injected electron beam flattens the spectrum of the footpoint source. The coronal source is unchanged and so the difference in the spectral indices between the coronal and footpoint sources is Δγ > 2, which is larger than expected from purely collisional transport. A steady-state beam shows little difference between the two cases, as has been previously found, as a transiently injected electron beam is required to produce significant wave growth, especially at higher velocities. With this transiently injected beam the wave-particle interactions dominate in the corona whereas the collisional losses dominate in the chromosphere. The shape of the spectrum is different with increasing electron beam density in the wave-particle interaction case whereas with purely collisional transport only the normalisation is changed. We also find that the starting height of the source electron beam above the photosphere affects the spectral index of the footpoint when Langmuir wave growth is included. This may account for the differing spectral indices found between double footpoints if asymmetrical injection has occurred in the flaring loop. Title: Acceleration, Magnetic Fluctuations, and Cross-field Transport of Energetic Electrons in a Solar Flare Loop Authors: Kontar, E. P.; Hannah, I. G.; Bian, N. H. Bibcode: 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 × 109 cm and width ~7 × 108 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. Bibcode: 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 (1035 electrons s-1 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. Bibcode: 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.
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.
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.
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. Bibcode: 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).
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.
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.
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.
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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 2010A&A...519A.114B Altcode: 2010arXiv1006.2662B
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.
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 kρ_s≪1 down to the small “kinetic” scales with kρs ≫1, ρ_s being the ion sound gyroradius.
Results: Scaling relations are obtained for the magnitude of the turbulent electromagnetic fluctuations, as a function of k and k, showing that the electric field develops a component parallel to the magnetic field at large MHD scales.
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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 2010PhPl...17f2308B Altcode: 2010arXiv1006.2659B Anisotropic Alfvénic fluctuations with k/k<<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 and k. 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. Bibcode: 2010AAS...21640405D Altcode: 2010BAAS...41..900D RHESSI has detected compact hard (25 - 100 keV) X-ray sources that are ɜ arcseconds (FWHM) in extent for certain flares (Dennis and Pernak (2009). These sources are believed to be at magnetic loop footpoints that are known from observations at other wavelengths to be very small. Flare ribbons seen in the UV with TRACE, for example, are 1 arcsecond in width, and white light flares show structure at the 1 arcsecond level. However, Kontar and Jeffrey (2010) have shown that the measured extent should be >6 arcseconds, even if the X-ray emitting thick-target source is point-like. This is because of the strong albedo contribution in the measured energy range for a source located at the expected altitude of 1 Mm near the top of the chromosphere. This discrepancy between observations and model predictions may indicate that the source altitude is significantly lower than assumed or that the RHESSI image reconstruction procedures are not sensitive to the more diffuse albedo patch in the presence of a strong compact source. Results will be presented exploring the latter possibility using the Pixon image reconstruction procedure and other methods based on visibilities.

Dennis, B. R. and Pernak, R. L., 2009, ApJ, 698, 2131-2143.

Kontar, E. P. and Jeffrey, N. L. S., 2010, A&A, in press. Title: Hard X-Ray Structure of Loop Footpoints in a Solar Limb Flare Authors: Kontar, Eduard; Jeffrey, N.; Hannah, I.; Battaglia, M. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 2010A&A...513L...2K Altcode: 2010arXiv1003.0884K
Aims: The positions and source sizes of X-ray sources taking into account Compton backscattering (albedo) are investigated.
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.
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.
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. Bibcode: 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 sun is measured with a high time cadence of <=2.5 minutes. For deriving CME velocity and acceleration, we apply and test a new algorithm based on regularization methods. The CME maximum acceleration is achieved at heights h <= 0.4 R sun, and the peak velocity at h <= 2.1 R sun (in one case, as small as 0.5 R sun). 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. Bibcode: 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 Bibcode: 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 Bibcode: 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 Bibcode: 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 Bibcode: 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 Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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.
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.
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.
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.
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 Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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.
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.
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)-1 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. Bibcode: 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. Bibcode: 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. Bibcode: 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.

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 Bibcode: 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.

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-1 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. Bibcode: 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 Bibcode: 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 Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 2007ApJ...663L.109K Altcode: We compare hard X-ray (HXR) photon spectra observed by the RHESSI with the spectra of the electrons in the associated solar impulsive particle events observed near 1 AU by the WIND 3D Plasma and Energetic Particle (3DP) instrument. For prompt events, where the inferred injection time at the Sun coincides with the HXR burst, the HXR photon power-law spectral index γ and the in situ observed electron spectral index δ measured above 50 keV show a good linear fit, δ=γ+0.1(+/-0.1), with correlation coefficient of 0.83, while for delayed events (inferred injection >10 minutes after the HXR burst) only a weak correlation with a coefficient of 0.43 is seen. The observed relationship for prompt events is inconsistent, however, with both the thin target case, where the escaping electrons come from the X-ray-producing electron population, and the thick target case where some of the accelerated source population escapes to 1 AU and the rest produce the HXRs while losing all their energy to collisions. Furthermore, the derived total number of escaping electrons correlates with the number of electrons required to produce observed X-ray flux but is only about ~0.2% of the number of HXR-producing electrons. Title: Hard X-ray spectra and positions of solar flares observed by RHESSI: photospheric albedo, directivity and electron spectra Authors: Kašparová, J.; Kontar, E. P.; Brown, J. C. Bibcode: 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.
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.
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. Bibcode: 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. Bibcode: 2007LNP...725...65B Altcode: 2006astro.ph..7440B Hard X-rays and γ-rays are the most direct signatures of energetic electrons and ions in the sun’s atmosphere which is optically thin at these energies and their radiation involves no coherent processes. Being collisional they are complementary to gyro-radiation in probing atmospheric density as opposed to magnetic field and the electrons are primarily 10 100~keV in energy, complementing the (>100 keV) electrons likely responsible for microwave bursts. The pioneering results of the Ramaty High Energy Solar Spectroscopic Imager (RHESSI) are raising the first new major questions concerning solar energetic particles in many years. Some highlights of these results are discussed primarily around RHESSI topics on which the authors have had direct research involvement particularly when they are raising the need for re-thinking of entrenched ideas. Results and issues are broadly divided into discoveries in the spatial, temporal and spectral domains, with the main emphasis on flare hard X-rays/fast electrons but touching also on γ-rays/ions, non-flare emissions, and the relationship to radio bursts. Title: Stereoscopic Electron Spectroscopy of Solar Hard X-Ray Flares with a Single Spacecraft Authors: Kontar, Eduard P.; Brown, John C. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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'(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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 (γ1 = 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. Bibcode: 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

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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 F0(E0) (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. Bibcode: 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.

RHESSI hard X-ray source spatial structure in relation to theoretical models and loop density structure.

Energy budget of flare electrons and the Neupert effect.

Spectral deconvolution methods including blind target testing and results for RHESSI HXR spectra, including the reality and implications of dips inferred in electron spectra.

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. Bibcode: 2004AGUFMSH13A1129K Altcode: The Sun frequently accelerates electrons in solar flares and type III radio bursts. Some of the accelerated electrons lose their energy by collisions in the denser, lower solar atmosphere producing hard X-ray (HXR) emissions and heat the corona, while others escape into interplanetary space. Whether the HXR producing and the escaping electrons are accelerated by the same mechanism is not known. Combining RHESSI X-ray observations with in-situ observations of energetic electrons from the WIND spacecraft allows for the first time a detailed temporal, spatial, and spectral study. Statistical results of 16 events with a close temporal agreement between the HXR and the in-situ detected electrons (taking the time of flight of the escaping electrons into account) show a correlation between the HXR photon spectral index and the electron spectral index observed in-situ thus indicating a common acceleration mechanism. Furthermore, the solar X-ray source structure of these events look similar showing hot loops with HXR footpoints plus an additional HXR source separated from the loop by typically ~15". This source structure can be explained by a simple magnetic reconnection model with newly emerging flux tubes that reconnect with previously open field lines, so-called interchange reconnection. Events with a delayed timing between the HXRs and the solar release of escaping electrons (Krucker et al. 1999, Haggerty & Roelof 2002) are presently investigated. If these delayed events are indeed accelerated later in the event by shocks, no correlation between the HXR photon spectrum and the in-situ observed electron spectrum is expect to be found. Title: Generalized Regularization Techniques with Constraints for the Analysis of Solar Bremsstrahlung X-ray Spectra Authors: Kontar, Eduard P.; Piana, Michele; Massone, Anna Maria; Emslie, A. Gordon; Brown, John C. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 3He-rich solar energetic particle events Authors: Krucker, S.; Lin, R. P.; Kontar, E. P.; Mason, G. M.; Wiedenbeck, M. E. Bibcode: 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 3He-rich solar energetic particles observed at 1 AU. A series of very large 3He-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-1. X-ray emission is seen from the main flaring loops as well; however, the most prominent source in >30 keV hard X-rays appears to be displaced from the flaring loops, at the footpoint of the field lines along which the jet is moving outward. We investigate the possibility that the HXR footpoint is produced by the downward moving part of the same electron population that escapes to 1 AU, by comparing the derived electron spectrum from the HXR footpoint source with the in situ observed electron spectrum. Title: Regularized Electron Flux Spectra in the 2002 July 23 Solar Flare Authors: Piana, Michele; Massone, Anna Maria; Kontar, Eduard P.; Emslie, A. Gordon; Brown, John C.; Schwartz, Richard A. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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 vBPS≈0.35 c at fundamental and harmonic frequencies are found to be equal to TF=1013 K, TH=1016 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. Bibcode: 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. Bibcode: 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.

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. Bibcode: 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. Bibcode: 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 F0(E0) is much improved when ionisation structure is included. The expression involves the column depth N* 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. Bibcode: 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)-1.3 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 ne(z) = 1.25×1013 (z/1 Mm)-2.5 cm-3. 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. Bibcode: 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. Bibcode: 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)−1.3 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 ne(z)=1.25×1013(z/1 Mm)−2.5 cm−3. 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. Bibcode: 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)=n0 (z/z0)−b which yields numerical results for z(ε) well fit by z(ε)∼ε−α, 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 n0,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. Bibcode: 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 F0(E0)) is disturbingly non-unique. However, we show that it is relatively simple to allow for the effect in forward fitting of parametric models of F0(E0)) and provide an expression to evaluate it for the usual single power-law form of F0(E0)).The expression involves the column depth N* of the transition region in the flare loop as one of the parameters so data fitting can enable derivation of N* (and its evaporative evolution) as part of the fitting procedure. The fit to RHESSI data on four flares for a single power law F0(E0)) 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 F0(E0)) 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Title: The spread of the hot electron cloud in the solar corona Authors: Mel'nik, V. N.; Kontar, E. P. Bibcode: 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. Title: Propagation of a Monoenergetic Electron Beam in the Solar Corona Authors: Mel'nik, V. N.; Lapshin, V.; Kontar, E. Bibcode: 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. Title: Beam-Plasma Structures at Propagation of Electron Beams in Plasma Authors: Mel'nik, V. N.; Kontar, E. P. Bibcode: 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. Title: Plasma emission of electron beams in the low corona Authors: Kontar, E. P.; Melnik, V. N. Bibcode: 1997jena.confE..43K Altcode: Radioemission recorded at the high resolution spectrograms in the range 6.3-8.7 GHz is characterized by the fine structure i.e. time profiles consist of great number of elements [1]. Each element is supposed to be generated by an electron beam. The brightness temperature and radio flux density is calculated for a single beam with exp-like distribution with duration 25ms [2]. It is considered that Langmuir waves initially one-dimensional convert into radioemission through either ion off scattering or fusion of two Langmuir waves. Radioemission at the harmonic (2 omega_p) is calculated when izotropic part of Langmuir waves is much smaller than one-dimensional one. It is shown that for beam velocities v > 5 times 10^9 cm/s transformation of one-dimensional spectrum into the izotropic spectrum rise sharply with velocity that leads to the stop of electron beam with higher velocities. The maximum radio flux density at 2 omega_p is obtained equal to 110 s.f.u.. Radioemission at fundamental frequency omega_p give higher brightness temperatures and radio flux density than for the harmonic frequency. Title: A Fly Off of the Fast Electron Flows Generating Type III Bursts Authors: Mel'nik, V. N.; Kontar, E. P. Bibcode: 1997pre4.conf..421M Altcode: Recent observation data show the existence of a fine structure in solar type III bursts. A solar burst consists of several impulses of radioemission with a short duration - much shorter than burst duration. These impulses are thought to be generated by the different electron beams propagating in the solar corona. It is important to find out the variation electron distribution function taking into account that electrons emit and absorb plasma waves. An initial electron distribution function is taken as a set of mono-energetic beams. Then the spread of electrons is described by the propogation of mono-energetic beams. Recently it has been shown that a mono-energetic beam propagates with constant velocity in the form of a beam-plasma structure that consists of electrons and plasmons. The resultant electron distribution function in every point is determined as a result of the beam-plasma structure interaction. Such an approach allows us to obtain the electron distribution function and the spectral energy density of Langmuir waves. The electron distribution function looks like a common plateau and a staircase. The spectral energy density of plasma waves turns to be equal to zero at v = u_i and the respective stair (u_i < v < u_{i+1}) is separated from the common plateau. Thus the maximum velocity of the common plateau is decreased jumping down from v = u_{i+1} to v = u_i but its height is increased. For velocities v, which are greater than the maximum velocity u_i, the electron distribution function has a staircase-form with stair heights decreasing with velocity. The solutions of the problem obtained in the paper are compared with numerical simulations of spreading of electron flows which generate type III bursts.