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.
Magnetohydrodynamic simulations of the evolution of a flux tube accelerated through a stationary magnetized plasma are presented. As the flux tube moves through the external plasma, its shape becomes distorted and reconnection can take place between the flux tube and external fields. The coupling between the moving flux tube and the external plasma is generally efficient, with simulated flux tube velocities many times smaller than those expected from frictionless motion. The reconnection between the flux tube and external field takes place when there is a unidirectional external field component in the direction of flux tube propagation. The reconnection is intrinsically nonsymmetric around the flux tube boundary. The principal reconnection site is at the rear of the flux tube, where strong vortices convect the external field toward the flux tube. Drag coefficients (C'D) that parameterize this interaction have been determined. When the flux tube is continually accelerated, CD > 1 is appropriate, consistent with previously used ad hoc values. Examples of when the flux tube is accelerated for a short time but allowed to continue interacting with the external plasma are presented. It is shown that in the absence of reconnection, the coupling time is several Alfv•n wave transit times across the flux tube. However, when reconnection takes place, this coupling can cease to occur, and the flux tube may move frictiordessly (C'D m 0). The results are discussed in terms of interplanetary magnetic clouds, and it is suggested that the observations of cornoving coronal mass ejection and solar wind plasma can be accounted for by drag between the two. 1. Introduction Magnetic flux tubes are important elemental plasma structures in a wide range of astrophysical environments. Their structure and dynamics have been investigated extensively in space and solar plasmas. An extensive compendium of papers on this subject are given by Russell et al. [1990]. The structure of flux tubes has been studied in the solar convection zone, in the solar corona, in interplanetary space, and in the Earth's magnetosphere. The dynamical properties, such as eruption and subsequent propagation of flux tubes, are particularly important for understanding solar wind structures of solar origin that can influence geomagnetic conditions. The flux structures arriving at 1 AU are the product of forces responsible for their propagation and interaction with the ambient plasma medium. Thus the nature of the interaction between a moving flux tube and the ambient plasma is important in understanding This paper is not subject to U.S. copyright. Published in 1996 by the American Geophysical Union. Paper number 95JA03769. how to relate sparse in situ measurements at 1 AU to the properties of the large-scale structures. A qualitative picture of the interaction of a flux tube with the external plasma can be obtained by using knowledge obtained from fluid flows around a rigid body. It is well known [e.g., Batckelor, 1967] that at high Reynolds number, vortices form on the...
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