Resistive processes in the preflare phase of eruptive flares

Abstract

It is now widely accepted that fast magnetic reconnection based on locally enhanced resistivity plays an important role in such violent phenomena as solar flares. Here we study how such localization of resistivity occurs in flare evolution. We start with a 2.5-dimensional force-free current sheet under a uniformly distributed resistivity, which is subject to a very small random velocity perturbation. Then the evolution enters the linear stage of the tearing instability and later a sufficient amount of thermal energy is produced in the nonlinear stage, which is considered to have a relation with the preflare heating. As the nonlinear evolution proceeds, the magnetic fields perpendicular to current sheet (perpendicular magnetic fields) flow away from the X-points formed in the current sheet and eventually the current-sheet collapse occurs at these points. This collapse greatly reduced the thickness of current sheet into the range of microscopic values if the magnetic Reynolds number is quite large and the plasma beta is quite low. Since the formation of thin current sheet leads to the occurrence of a locally enhanced resistivity (anomalous resistivity), the transition from the gradual energy-release phase under a uniformly distributed resistivity to the rapid phase with a locally enhanced anomalous resistivity can be accomplished. This transition is responsible for various explosive phenomena in the sun.