CRITICAL ISSUES FOR UNDERSTANDING PARTICLE-ACCELERATION IN IMPULSIVE SOLAR-FLARES

This paper, a review of the present status of existing models for part icle acceleration during impulsive solar flares, was inspired by a wee k-long workshop held in the Fall of 1993 at NASA Goddard Space Flight Center. Recent observations from Yohkoh and the Compton Gamma Ray Obse rvatory, and a reanalysis of older observations from the Solar Maximum Mission, have led to important new results concerning the location, t iming, and efficiency of particle acceleration in flares. These are su mmarized in the first part of the review. Particle acceleration proces ses are then discussed, with;particular emphasis on new developments i n stochastic acceleration by magnetohydrodynamic waves and direct elec tric field acceleration by both sub- and super-Dreicer electric fields . Finally, issues that arise when these mechanisms are incorporated in to the large-scale flare structure are considered. Stochastic and supe r-Dreicer acceleration may occur either in a single large coronal reco nnection site or at multiple ''fragmented'' energy release sites. Sub- Dreicer acceleration requires a highly filamented coronal current patt ern. A particular issue that needs to be confronted by all theories is the apparent need for large magnetic field strengths in the flare ene rgy release region.