Critical issues for understanding particle acceleration in impulsive solar flares

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 efficiency 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.