Abstract. This paper, a review of the present status of existing models for particle acceleration during impulsive solar flares, was inspired by a week-long workshop held in the Fall of 1993 at NASA Goddard Space Flight Center. Recent observations from Yohkoh and the Compton Gamma Ray Observatory, and a reanalysis of older observations from the Solar Maximum Mission, have led to important new results concerning the location, timing, and eificiency of particle acceleration in flares. These are summarized in the first part of the review. Particle acceleration processes are then discussed, with particular emphasis on new developments in stochastic acceleration by magnetohydrodynamic waves and direct electric field acceleration by both sub-and super-Dreicer electric fields. Finally, issues that arise when these mechanisms are incorporated into the large-scale flare structure are considered. Stochastic and super-Dreicer acceleration may occur either in a single large coronal reconnection site or at multiple "fragmented" energy release sites. Sub-Dreicer acceleration requires a highly filamented coronal current pattern. A particular issue that needs to be confronted by all theories is the apparent need for large magnetic field strengths in the flare energy release region.

We present and analyze the first high-resolution hard X-ray spectra from a solar flare observed in both X-ray/ g-ray continuum and g-ray lines. Spatially integrated photon flux spectra obtained by the Ramaty High Energy Solar Spectroscopic Imager (RHESSI) are well fitted between 10 and 300 keV by the combination of an isothermal component and a double power law. The flare plasma temperature peaks at 40 MK around the time of peak hard X-ray emission and remains above 20 MK 37 minutes later. We derive the nonthermal mean electron flux distribution in one time interval by directly fitting the RHESSI X-ray spectrum with the thin-target bremsstrahlung from a double-power-law electron distribution with a low-energy cutoff. We find that relativistic effects significantly impact the bremsstrahlung spectrum above 100 keV and, therefore, the deduced mean electron flux distribution. We derive the evolution of the injected electron flux distribution on the assumption that the emission is thick-target bremsstrahlung. The injected nonthermal electrons are well described throughout the flare by a double-power-law distribution with a low-energy cutoff that is typically between 20 and 40 keV. We find that the power in nonthermal electrons peaks before the impulsive rise of the hard X-ray and g-ray emissions. We compare the energy contained in the nonthermal electrons with the energy content of the thermal flare plasma observed by RHESSI and GOES. The minimum total energy deposited into the flare plasma by nonthermal electrons, ergs, is on the order of the energy in the thermal plasma. 31 2.6 # 10 Subject headings: Sun: flares -Sun: X-rays, gamma raysThe time history of the flare emission in three energy bands is shown in Figure 1a. The Ramaty High Energy Solar Spectroscopic Imager (RHESSI) uses two sets of aluminum attenuators, known as thin shutters and thick shutters, to avoid saturating the detectors during large flares. The July 23 flare was observed in two attenuator states. The instrument was primarily in the A3 state, with both sets of attenuators in place. Early in the flare, before 00:26:08 UT, and late in the flare, after 00:59:21 UT, the instrument was in the A1 state, with only the thin shutters in place. There were also four brief periods during which the instrument switched from A3 to A1 and back to A3. These transitions in attenuator state are apparent in the time history of the lowest energy band in Figure 1a. The flux calibration is currently uncertain during these four brief periods, so these time periods appear as gaps in subsequent results derived from the data.We corrected the observed counts for pulse pileup and decimation (see Smith et al. 2002). Pulse pileup occurs at high count rates, with multiple photons recorded as a single photon with an energy equal to the sum of the energies of the individual photons. Decimation conserves onboard memory by recording only a fraction of the incident photons. Background counts were determined from the data by linearly interpolating between the background level...

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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.

We present X-ray evidence for the formation of a large-scale current sheet in a flare observed by the Ramaty High-Energy Solar Spectroscopic Imager on 2002 April 15. The flare occurred on the northwest limb, showing a cusp-shaped flare loop in the rise phase. When the impulsive rise in hard X-rays (125 keV) began, the cusp part of the coronal source separated from the underlying flare loop and remained stationary for about 2 minutes. During this time, the underlying flare loops shrank at ∼9 km s . The temperature of the underlying loops Ϫ1 increased toward higher altitudes, while the temperature of the coronal source increased toward lower altitudes. These results indicate that a current sheet formed between the top of the flare loops and the coronal source during the early impulsive phase. After the hard X-ray peak, the flare loops grew outward at ∼8 km s , and the coronal Ϫ1Spectrometric Coronagraph C2 and C3 detectors, also propagated outward at ∼300 km s . These observations Ϫ1 are all consistent with the continued expansion of the current sheet.