Critical issues for understanding particle acceleration in impulsive solar flares

TLDR

Particle acceleration in impulsive solar flares is studied through models of stochastic and super‑Dreicer processes that may occur in single or fragmented coronal reconnection sites, with sub‑Dreicer acceleration requiring highly filamented currents and theories challenged by the need for large magnetic field strengths. The review aims to discuss particle acceleration processes, focusing on recent developments in stochastic acceleration by magnetohydrodynamic waves and direct electric field acceleration, and to examine issues arising when these mechanisms are incorporated into large‑scale flare structures. The authors review existing models inspired by a 1993 workshop, analyze stochastic acceleration by MHD waves and direct electric fields, and evaluate how these mechanisms fit within the overall flare structure. Recent Yohkoh, CGRO, and reanalyzed Solar Maximum Mission observations reveal new insights into the location, timing, and efficiency of particle acceleration in flares, which are summarized in the review's first part.

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.