Coronal Hard X-ray Source Research Articles

Temporal and Spatial Characteristics of Hard X-Ray Sources in a Flare Model with a Vertical Current Sheet

Abstract We analyzed changes in the height of the coronal hard X-ray (HXR) source for flares SOL2013-05-13T01:50 and SOL2013-05-13T15:51. Analysis of the Reuven Ramaty High Energy Solar Spectroscopic Imager data revealed the downward motion of the HXR source and the separation of the sources by energy and height. In the early stages of the flares, a negative correlation was found between the HXR source area in the corona and HXR flux. For the SOL2013-05-13T15:51 event, an increasing trend in the time delay spectra at the footpoints was obtained. For both events, the spectra of the time delays in the coronal HXR source showed a decreasing trend with energy in certain flare phases. To interpret the observed phenomena, we considered a flare model of collapsing traps and calculated the distribution functions of accelerated electrons along the magnetic loop using a nonstationary relativistic kinetic equation. This approach considers betatron and Fermi first-order acceleration mechanisms. The increasing trend of the time delay spectra at the footpoints was explained by the high mirror ratio in the magnetic loop and betatron acceleration mechanism. The observed features in the spatial and temporal behavior of the HXR sources, such as the negative correlation between the HXR source area and HXR flux, can be interpreted by the collapsing trap model.

A Model of Double Coronal Hard X-Ray Sources in Solar Flares

A number of double coronal X-ray sources have been observed during solar flares by RHESSI, where the two sources reside at different sides of the inferred reconnection site. However, where and how these X-ray-emitting electrons are accelerated remains unclear. Here we present the first model of the double coronal hard X-ray (HXR) sources, where electrons are accelerated by a pair of termination shocks driven by bidirectional fast reconnection outflows. We model the acceleration and transport of electrons in the flare region by numerically solving the Parker transport equation using velocity and magnetic fields from the macroscopic magnetohydrodynamic simulation of a flux rope eruption. We show that electrons can be efficiently accelerated by the termination shocks and high-energy electrons mainly concentrate around the two shocks. The synthetic HXR emission images display two distinct sources extending to >100 keV below and above the reconnection region, with the upper source much fainter than the lower one. The HXR energy spectra of the two coronal sources show similar spectral slopes, consistent with the observations. Our simulation results suggest that the flare termination shock can be a promising particle acceleration mechanism in explaining the double-source nonthermal emissions in solar flares.

Early-stage Coronal Hard X-Ray Source in Solar Flares in the Collapsing Trap Model

A bright hard X-ray coronal source observed at the early stage of solar flares is considered. The plasma density in a quiet corona is not enough to explain the hard X-ray bremsstrahlung radiation. The generally accepted concept of increasing plasma density in the looptop is associated with the effect of evaporation of hot chromosphere plasma. We discuss the increase in plasma density at the looptop at the early stage of a flare, due to magnetic loop contraction during the relaxation of the magnetic field (the so-called collapsing trap model). In this case, the increase in the plasma density at the looptop occurs on a timescale of seconds–tens of seconds, while the process of plasma evaporation increases the plasma density for much longer. The Fokker–Planck kinetic equation for accelerated electrons with a betatron and Fermi terms is solved numerically. We calculate increases in the energy of the accelerated electrons, the energy spectrum, and the pitch-angle anisotropy due to betatron and Fermi first-order acceleration. For a collapse time of 8 s, the total energy of the accelerated electrons increases by ∼20%–200%, depending on the model parameters. The ratio of the looptop/total hard X-ray flux at 29–58 keV increases by 15%–30% in the collapsing trap model. It is shown that this model can explain the appearance of bright coronal hard X-ray sources in the first seconds–tens of seconds after the hard X-ray flux starts growing.

Моделирование коронального источника жесткого рентгеновского излучения в турбулентной плазме солнечных вспышек

The kinetics of electron beams accelerated in the collisional plasma of solar (stellar) flares is considered, taking into account the stationary ion-acoustic mode localized at the magnetic looptop and magnetic fluctuations. The astrophysical aspect of the propagation process is related to the interpretation of hard X-rays in the plasma of flare loops. It is shown that when the plasma density in the coronal part of the solar flare loops does not exceed 1010 cm-3, taking into account the additional scattering on the ion-acoustic mode with the ratio of the turbulence energy density to the thermal energy of the plasma ~5 * 10-5-10-3 and magnetic fluctuations with a level of 5·10-2 does not lead to the appearance of a bright hard X-ray source in the coronal part of the loop in the model with the isotropic pitch-angle distribution of accelerated electrons. In the anisotropic case with a hard electron energy spectrum, the coronal hard X-ray source, in the presence of ion-acoustic turbulence, can exist for a short time after the beginning of turbulence generation. And only in the case of a soft energy spectrum of accelerated electrons (power spectrum index >5) and a relatively high plasma density at the magnetic looptop >1010 cm-3, a bright coronal hard X-ray source is generated at energies of 25-50 keV, regardless of the pitch-angular distribution of accelerated electrons at the moment of injection. A significant effect of turbulence on the distribution of the linear degree of hard X-ray polarization along the loop is shown, leading to a decrease in the extreme values in the coronal part by 5-35%. The integral values of the hard X-ray polarization do not exceed 10%.

A Remarkably Narrow RHESSI X-Ray Flare on 2011 September 25

Abstract The unusually narrow X-ray source imaged with the Ramaty High Energy Solar Spectroscopic Imager (RHESSI) during an impulsive spike lasting for ∼10 s during the Geostationary Operational Environmental Satellite C7.9 flare on 2011 September 25 (SOL2011-09-25T03:32) was only ∼2″ wide and ∼10″ long. Comparison with Helioseismsic and Magnetic Imager magnetograms and Atmospheric Imaging Assembly images at 1700 Å shows that the X-ray emission was primarily from a long ribbon in the region of positive polarity with little if any emission from the negative polarity ribbon. However, a thermal plasma source density of ∼1012 cm−3 estimated from the RHESSI-derived emission measure and source area showed that this could best be interpreted as a coronal hard X-ray source in which the accelerated electrons with energies less than ∼50 keV were stopped by Coulomb collisions in the corona, thus explaining the lack of the more usual bright X-ray footpoints. Analysis of RHESSI spectra shows greater consistency with a multi-temperature distribution and a low-energy cutoff to the accelerated electron spectrum of 22 keV compared to 12 keV if a single-temperature distribution is assumed. This leads to a change in the lower limit on the total energy in electrons by an order of magnitude, given the steepness of the best-fit electron spectrum with a power-law index of ∼6.

Multiwavelength Stereoscopic Observation of the 2013 May 1 Solar Flare and CME

Abstract A M-class behind-the-limb solar flare on 2013 May 1 (SOL2013-05-01T02:32), accompanied by a (∼400 km s−1) coronal mass ejection (CME), was observed by several space-based observatories with different viewing angles. We investigated the RHESSI-observed occulted hard X-ray (HXR) emissions that originated at least 0.1 above the flare site. Emissions below ∼10 keV revealed a hot, extended (11 MK, >60″) thermal source from the escaping CME core, with densities around 109 cm−3. In such a tenuous hot plasma, ionization times scales are several minutes, consistent with the nondetection of the hot CME core in SDO/AIA’s 131 Å filter. The nonthermal RHESSI source originated from an even larger area (∼100″) at lower densities (108 cm−3) located above the hot core, but still behind the CME front. This indicates that the observed part of the nonthermal electrons are not responsible for heating the CME core. Possibly the hot core was heated by nonthermal electrons before it became visible from Earth, meaning that the unocculted part of the nonthermal emission likely originates from a more tenuous part of the CME core, where nonthermal electrons survive long enough to became visible from Earth. Simultaneous HXR spectra from the Mars Odyssey mission, which viewed the flare on disk, indicated that the number of nonthermal electrons >20 keV within the high coronal source is ∼0.1%–0.5% compared with the number within the chromospheric flare ribbons. The detection of high coronal HXR sources in this moderate size event suggests that such sources are likely a common feature within solar eruptive events.

Joint hard X-ray observations with ASO-S/HXI and SO/STIX

This paper discusses the potential of future joint hard X-ray solar flare observations between the Hard X-ray Imager (HXI) onboard the Advanced Space-based Solar Observatory (ASO-S) mission and the Spectrometer/Telescope for Imaging X-rays (STIX) on Solar Orbiter. The different viewing perspectives of the two telescopes relative to the Sun will allow us for the first time to systematically study non-thermal hard X-ray emissions stereoscopically. During the 4-years of the nominal mission of ASO-S, we expect to jointly observe about 160 flares above GOES M1 class to systematically study hard X-ray directivity. For about 16 partially limb-occulted STIX flares, we will have observations of the entire flare by HXI. Such observations will enable us to simultaneously study the all-important coronal hard X-ray sources, which are generally lost in the instrument’s individual imaging dynamic range, in combination with the chromospheric footpoint emissions. The two different detector systems used in the two telescopes make the relative calibration between the two instruments a key task that needs to be addressed before creditable science results can be published. If an accurate inter-calibration can be achieved using jointly observed flares on the disk, observations with HXI and STIX will provide new and essential key diagnostics for solar flare physics.

Dynamic Spectral Imaging of Decimetric Fiber Bursts in an Eruptive Solar Flare

Abstract Fiber bursts are a type of fine structure that is often superposed on type IV radio continuum emission during solar flares. Although studied for many decades, its physical exciter, emission mechanism, and association with the flare energy release remain unclear, partly due to the lack of simultaneous imaging observations. We report the first dynamic spectroscopic imaging observations of decimetric fiber bursts, which occurred during the rise phase of a long-duration eruptive flare on 2012 March 3, as obtained by the Karl G. Jansky Very Large Array in 1–2 GHz. Our results show that the fiber sources are located near and above one footpoint of the flare loops. The fiber source and the background continuum source are found to be co-spatial and share the same morphology. It is likely that they are associated with nonthermal electrons trapped in the converging magnetic fields near the footpoint, as supported by a persistent coronal hard X-ray source present during the flare rise phase. We analyze three groups of fiber bursts in detail with dynamic imaging spectroscopy and obtain their mean frequency-dependent centroid trajectories in projection. By using a barometric density model and magnetic field based on a potential field extrapolation, we further reconstruct the 3D source trajectories of fiber bursts, for comparison with expectations from the whistler wave model and two MHD-based models. We conclude that the observed fiber burst properties are consistent with an exciter moving at the propagation velocity expected for whistler waves, or models that posit similar exciter velocities.

Effect of binary collisions on electron acceleration in magnetic reconnection

Context. The presence of energetic X-ray sources in the solar corona indicates there are additional transport effects in the acceleration region. A prime method of investigation is to add collisions into models of particle behaviour at the reconnection region.Aims. We investigate electron test particle acceleration in a simple model of an X-type reconnection region. In particular, we explore the possibility that collisions will cause electrons to re-enter the acceleration more frequently, in turn causing particles to be accelerated to high energies.Methods. The deterministic (Lorentz) description of particle gyration and acceleration has been coupled to a model for the effects of collisions. The resulting equations are solved numerically using Honeycutt’s extension of the RK4 method to stochastic differential equations. This approach ensures a correct description of collisional energy loss and pitch-angle scattering combined with a sufficiently precise description of gyro-motion and acceleration.Results. Even with initially mono-energetic electrons, the competition between collisions and acceleration results in a distribution of electron energies. When realistic model parameters are used, electrons achieve X-ray energies. A possible model for coronal hard X-ray sources is indicated.Conclusions. Even in competition with energy losses, pitch-angle scattering results in a small proportion of electrons reaching higher energies than they would in a collisionless situation.

CORONAL THICK TARGET HARD X-RAY EMISSIONS AND RADIO EMISSIONS

Recently a distinctive class of hard X ray (HXR) sources located in the corona was found, which implies that the collisionally thick target model (CTTM) applies even to the corona. We investigated whether this idea can independently be verified by microwave radiations that have been known as the best companion to HXRs. The study is made for the GOES M2.3 class flare occurred on 2002 September 9 that were observed by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and the Owens Valley Solar Array (OVSA). Interpreting the observed energy dependent variation of HXR source size under the CTTM the coronal density should be as high as $5\times 10^{11}$ cm$^{-3}$ over the distance up to 12$"$. To explain the cut-off feature of microwave spectrum at 3 GHz, we however, need density no higher than $1\times 10^{11}$ cm$^{-3}$. Additional constraints need to be placed on temperature and magnetic field of the coronal source in order to reproduce the microwave spectrum as a whole. Firstly, a spectral feature called the Razin suppression requires the magnetic field in a range of 250--350 gauss along with high viewing angles around 75$^{\rm o}$. Secondly, to avoid excess fluxes at high frequencies due to the free-free emission that were not observed, we need a high temperature $\geq2\times 10^7$ K. These two microwave spectral features, Razzin suppression and free-free emissions, become more significant at regions of high thermal plasma density and are essential for validating and for determining additional parameters for the coronal HXR sources.

Spatially resolved observations of a split-band coronal type II radio burst

Context. The origin of coronal type-II radio bursts and of their band-splitting are still not fully understood. Aims. To make progress in solving this problem on the basis of one extremely well observed solar eruptive event. Methods. The relative dynamics of multi-thermal eruptive plasmas, observed in detail by the SDO/AIA and of the harmonic type-II burst sources, observed by the NRH at ten frequencies from 445 to 151 MHz, is studied for the partially behind the limb event on 3 November 2010. Special attention is given to the band-splitting of the burst. Analysis is supplemented by investigation of coronal hard X-ray (HXR) sources observed by the RHESSI. Results. It is found that the flare impulsive phase was accompanied by the formation of a double coronal HXR source, whose upper part coincided with the hot (T~10 MK) eruptive plasma blob. The leading edge (LE) of the eruptive plasmas (T~1-2 MK) moved upward from the flare region with the speed of v=900-1400 km/s. The type II burst source initially appeared just above the LE apex and moved with the same speed and in the same direction. After about 20 s it started to move about twice faster, but still in the same direction. At any given moment the low frequency component (LFC) source of the splitted type-II burst was situated above the high frequency component (HFC) source, which in turn was situated above the LE. It is also found that at a given frequency the HFC source was located slightly closer to the photosphere than the LFC source. Conclusions. The shock wave, which could be responsible for the observed type-II radio burst, was initially driven by the multi-temperature eruptive plasmas, but later transformed to a freely propagating blast shock wave. The most preferable interpretation of the type-II burst splitting is that its LFC was emitted from the upstream region of the shock, whereas the HFC - from the downstream region.

HARD X-RAY OBSERVATIONS OF A JET AND ACCELERATED ELECTRONS IN THE CORONA

We report the first hard X-ray observation of a solar jet on the limb with flare footpoints occulted, so that faint emission from accelerated electrons in the corona can be studied in detail. In this event on 2003 August 21, RHESSI observed a double coronal hard X-ray source in the pre-impulsive phase at both thermal and nonthermal energies. In the impulsive phase, the first of two hard X-ray bursts consists of a single thermal/nonthermal source coinciding with the lower of the two earlier sources, and the second burst shows an additional nonthermal, elongated source, spatially and temporally coincident with the coronal jet. Analysis of the jet hard X-ray source shows that collisional losses by accelerated electrons can deposit enough energy to generate the jet. The hard X-ray time profile above 20 keV matches that of the accompanying Type III and broadband gyrosynchrotron radio emission, indicating both accelerated electrons escaping outward along the jet path and electrons trapped in the flare loop. The double coronal hard X-ray source, the open field lines indicated by Type III bursts, and the presence of a small post-flare loop are consistent with significant electron acceleration in an interchange reconnection geometry.

THE ROLE OF INVERSE COMPTON SCATTERING IN SOLAR CORONAL HARD X-RAY AND γ-RAY SOURCES

Coronal hard X-ray (HXR) and continuum gamma-ray sources associated with the impulsive phase of solar flares have been the subject of renewed interest in recent years. They have been interpreted in terms of thin-target, nonthermal bremsstrahlung emission. This interpretation has led to rather extreme physical requirements in some cases. For example, in one case, essentially all of the electrons in the source must be accelerated to nonthermal energies to account for the coronal HXR source. In other cases, the extremely hard photon spectra of the coronal continuum gamma-ray emission suggest that the low energy cutoff of the electron energy distribution lies in the MeV energy range. Here we consider the role of inverse Compton scattering (ICS) as an alternate emission mechanism in both the ultra- and mildly relativistic regimes. It is known that relativistic electrons are produced during powerful flares; these are capable of up-scattering soft photospheric photons to HXR and gamma-ray energies. Previously overlooked is the fact that mildly relativistic electrons, generally produced in much greater numbers in flares of all sizes, can up-scatter EUV/SXR photons to HXR energies. We also explore ICS on anisotropic electron distributions and show that the resulting emission can be significantly enhanced over an isotropic electron distribution for favorable viewing geometries. We briefly review results from bremsstrahlung emission and reconsider circumstances under which nonthermal bremsstrahlung or ICS would be favored. Finally, we consider a selection of coronal HXR and gamma-ray events and find that in some cases the ICS is a viable alternative emission mechanism.

IMPULSIVE PHASE CORONAL HARD X-RAY SOURCES IN AN X3.9 CLASS SOLAR FLARE

[Abridged]We present analysis of a pair of unusually energetic coronal hard X-ray (HXR) sources detected by RHESSI during the impulsive phase of an X3.9 class solar flare on 2003 November 3, which simultaneously shows two intense footpoint (FP) sources. A distinct loop top (LT) coronal source is detected up to ~150 keV and a second (upper) coronal source up to ~80 keV. These photon energies are much higher than commonly observed in coronal sources and pose grave modeling challenges. The LT source in general appears higher in altitude with increasing energy and exhibits a more limited motion compared to the expansion of the thermal loop. The high energy LT source shows an impulsive time profile and its nonthermal power law spectrum exhibits soft-hard-soft evolution during the impulsive phase, similar to the FP sources. The upper coronal source exhibits an opposite spatial gradient and a similar spectral slope compared to the LT source. These properties are consistent with the model of stochastic acceleration of electrons by plasma waves or turbulence. However, the LT and FP spectral index difference (varying from ~0-1) is much smaller than commonly measured and than that expected from a simple stochastic acceleration model. Additional confinement or trapping mechanisms of high energy electrons in the corona are required. Comprehensive modeling including both kinetic effects and the macroscopic flare structure may shed light on this behavior. These results highlight the importance of imaging spectroscopic observations of the LT and FP sources up to high energies in understanding electron acceleration in solar flares. Finally, we show that the electrons producing the upper coronal HXR source may very likely be responsible for the type III radio bursts at the decimetric/metric wavelength observed during the impulsive phase of this flare.

Energy Release and Particle Acceleration in Flares: Summary and Future Prospects

RHESSI measurements relevant to the fundamental processes of energy release and particle acceleration in flares are summarized. RHESSI’s precise measurements of hard X-ray continuum spectra enable model-independent deconvolution to obtain the parent electron spectrum. Taking into account the effects of albedo, these show that the low energy cut-off to the electron power-law spectrum is typically ≲tens of keV, confirming that the accelerated electrons contain a large fraction of the energy released in flares. RHESSI has detected a high coronal hard X-ray source that is filled with accelerated electrons whose energy density is comparable to the magnetic-field energy density. This suggests an efficient conversion of energy, previously stored in the magnetic field, into the bulk acceleration of electrons. A new, collisionless (Hall) magnetic reconnection process has been identified through theory and simulations, and directly observed in space and in the laboratory; it should occur in the solar corona as well, with a reconnection rate fast enough for the energy release in flares. The reconnection process could result in the formation of multiple elongated magnetic islands, that then collapse to bulk-accelerate the electrons, rapidly enough to produce the observed hard X-ray emissions. RHESSI’s pioneering γ-ray line imaging of energetic ions, revealing footpoints straddling a flare loop arcade, has provided strong evidence that ion acceleration is also related to magnetic reconnection. Flare particle acceleration is shown to have a close relationship to impulsive Solar Energetic Particle (SEP) events observed in the interplanetary medium, and also to both fast coronal mass ejections and gradual SEP events. New instrumentation to provide the high sensitivity and wide dynamic range hard X-ray and γ-ray measurements, plus energetic neutral atom (ENA) imaging of SEPs above ∼2 R⊙, will enable the next great leap forward in understanding particle acceleration and energy release is large solar eruptions—solar flares and associated fast coronal mass ejections (CMEs).

Location of Decimetric Pulsations in Solar Flares

This work investigates the spatial relation between coronal X-ray sources and coherent radio emissions, both generally thought to be signatures of particle acceleration. Two limb events were selected during which the radio emission was well correlated in time with hard X-rays. The radio emissions were of the type of decimetric pulsations as determined from the spectrogram observed by Phoenix-2 of ETH Zurich. The radio positions were measured from observations with the Nancay Radioheliograph between 236 and 432 MHz and compared to the position of the coronal X-ray source imaged with RHESSI. The radio pulsations originated at least 30 - 240 Mm above the coronal hard X-ray source. The altitude of the radio emission increases generally with lower frequency. The average positions at different frequencies are on a line pointing approximately to the coronal hard X-ray source. Thus, the pulsations cannot be caused by electrons trapped in the flare loops, but are consistent with emission from a current sheet above the coronal source.

AN ALTERNATIVE VIEW OF THE “MASUDA” FLARE

The limb flare on 1992 January 13, the so-called Masuda flare, has stimulated scientists to refine theory of solar flares based on two-dimensional magnetic reconnection. This is primarily because of the hard X-ray (HXR) source seen above the clearly defined flare loop, and the outward motions in soft X-rays (SXRs) interpreted as ejections. We have revisited Yohkoh HXR and SXR data for this and other limb flares and found that the Masuda flare is still unique in terms of the location and spectral properties of the coronal HXR source. However, the outward motions in SXR outside the flare loop may not be as simply characterized as plasmoid ejections as in other flares, nor are they particularly fast. The motions appear complex partly because we also see trans-equatorial loops in motion, one of whose legs anchors close to the main flare loop. It is possible that these large-scale loops represent post-flare loops, and that the flare may also be explained in terms of three-dimensional quadrupolar reconnection, similar to those flares where a pair of two loops exchange their footpoints through magnetic reconnection. It appears that expansion and brightening of large-scale loops offset from the main flare loop are not common, possibly providing a reason for the unusual coronal HXR source in the Masuda flare.

Inverse Compton X-rays from relativistic flare electrons and positrons

In solar flares, inverse Compton scattering (ICS) of photospheric photons might give rise to detectable hard X-ray photon fluxes from the corona where ambient densities are too low for significant bremsstrahlung or recombination. Gamma-ray lines and continuum in some large flares imply the presence of the necessary ~100 MeV electrons and positrons, the latter as by-products of GeV energy ions. Recent observations of coronal hard X-ray sources in particular prompt us to reconsider here the possible contribution of ICS. We aim to evaluate the ICS X-ray fluxes to be expected from prescribed populations of relativistic electrons and positrons in the solar corona. The ultimate aim is to determine if ICS coronal X-ray sources might offer a new diagnostic window on relativistic electrons and ions in flares. We use the complete formalism of ICS to calculate X-ray fluxes from possible populations of flare primary electrons and secondary positrons, paying attention to the incident photon angular distribution near the solar surface and thus improving on the assumption of isotropy made in previous solar discussions. Both primary electrons and secondary positrons produce very hard ICS X-ray spectra. The anisotropic primary radiation field results in pronounced centre-to-limb variation in predicted fluxes and spectra, with the most intense spectra, extending to the highest photon energies, expected from limb flares. Acceptable numbers of electrons or positrons could account for RHESSI coronal X/gamma-ray sources. Some coronal X-ray sources at least might be interpreted in terms of ICS by relativistic electrons or positrons, particularly when sources appear at such low ambient densities that bremsstrahlung appears implausible.

FLARE HARD X-RAY SOURCES DOMINATED BY NONTHERMAL RECOMBINATION

It was recently shown that, in the hottest regions of flare plasma, nonthermal hard X-ray (HXR) emission in the few deka-keV range from nonthermal electrons by recombination (NTR) onto heavy ions (especially Fe) exceeds bremsstrahlung (NTB), contrary to earlier assumptions. Here we discuss what types of HXR events are so dominated. Though significant even at temperatures T down to 106 K, the dominance of such NTR radiation over NTB needs T > 10 MK in order for Fe22+ ions and above to be plentiful. Furthermore, even for an accelerated fraction of only 0.01, the total hot plasma thermal emission begins to exceed NTR only for T > 25 MK. The relative NTR contribution is greatest when the electron flux spectrum is steep and extends to low energies. Thus, in proper modeling of hot HXR sources, inclusion of NTR as well as NTB is essential and reduces the HXR electron number and power requirements by over an order of magnitude in some cases. This alleviates problems of electron acceleration efficiency, especially in coronal HXR sources. Even some chromospheric footpoint HXR sources may be NTR-dominated if the hot soft X-ray (SXR) footpoint plasma there contains fast electrons. Only a small fraction of the plasma emission measure observed in SXR footpoints need be in the form of nonthermals to provide the necessary HXR emission measure. Compared with the standard cold thick target (bremsstrahlung) model (CTTM), such a scenario would give fast electrons a lesser role in the flare energy budget and help solve various problems with the CTTM.

Parallel Motions of Coronal Hard X-Ray Source and Hα Ribbons

During solar flares Hα ribbons form and often move away from the local magnetic polarity inversion line (PIL). While the motion perpendicular to the PIL has been taken as evidence for coronal magnetic reconnection in the so-called CSHKP standard model, the other velocity component parallel to the PIL is much less adopted as a property of the magnetic reconnection process. In this Letter we report an event in which the motion parallel to the PIL is found in both Hα ribbons and a thermal hard X-ray source. Such commonality would indicate a link between the coronal magnetic reconnection and footpoint emissions as in the standard solar flare model. However, its direction implies a reconnection region that is increasing in length, a feature missing from the standard two-dimensional model. We present a modified framework in which the variation along the third dimension is allowed, in order to assess the effect of such a proper motion on estimation of the magnetic reconnection rate. Data used are hard X-ray maps from the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI), Hα filtergrams of Big Bear Solar Observatory (BBSO), and the SOHO Michelson Doppler Imager (MDI) magnetogram obtained for the 2004 March 30 flare.