NUMERICAL-SIMULATION OF MAGNETIC RECONNECTION IN ERUPTIVE FLARES

Prompted by the Yohkoh observations of solar flares, which have establ ished the essential role of magnetic reconnection in the release of en ergy, we have studied the evolution of eruptive flares in some detail based on the reconnection model by means of the two-dimensional magnet ohydrodynamic (MHD) simulations. We are interested in what factor affe cts the time evolution of solar flares and how the related phenomena, particularly observed loop-top source and plasmoid eruption, can be ex plained by this model. We have studied the dependence of the structure and evolution of the system on plasma beta (ratio of gas pressure to magnetic pressure), adiabatic index, gamma, and rho(c) (initial densit y in the current sheet). If the timescale and velocity are normalized by Alfven time and Alfven speed, respectively, we find that the main r esults (e.g., reconnection rate, plasmoid velocity, etc.) are rather i nsensitive to the plasma beta. The rho(c) value, on the other hand, cr ucially affects the motion of a plasmoid: the ejection velocity of pla smoid grows in proportion to rho(c)(-1/2) in the early phase, which su ggests that the observed plasmoid velocity can be reproduced when we a ssign rho(c) similar or equal to 40 rho(0) (initial density outside th e current sheet). When adiabatic index gamma is small, corresponding t o the case of efficient thermal conduction, plasma heating will be gen erally suppressed, but the compression effect can be rather enhanced, which plays an important role in forming the high-density loop-top sou rce. We discuss loop-top sources, plasmoid eruption, and the rise moti on of a loop in comparison with the observations. Our simulations can well account for the existence of the loop-top, hard X-ray sources dis covered in the impulsive flares. We concluded that both the impulsive flares and the LDE (long duration event) flares can be generally under stood by the reconnection model for the cusp-type flares.