THE GENERATION OF MHD SHOCK-WAVES DURING THE IMPULSIVE PHASE OF THE FEBRUARY 27, 1992 FLARE

In this paper a unique 2.3-4.2 GHz radio spectrum of the flare impulsi ve phase, showing fast positively drifting bursts superimposed on a sl owly negatively drifting burst, is presented. Analyzing this radio spe ctrum it was found that the flare started somewhere near the transitio n region, where upward propagating MHD waves were generated during the whole impulsive phase. Moreover, it was found that behind a front of these ascending MHD waves the downward propagating electron beams, whi ch bombarded dense layers of the solar atmosphere, were accelerated. I t seems that, simultaneously with the increase of beam bombardment int ensity, the intensity of MI-ID waves was increasing and thus the MHD s hock wave generation and the electron beam acceleration and bombardmen t formed a self-consistently amplifying flare process. At higher coron al heights this process was followed by a type II radio burst, i.e. by the MHD flare shock. To verify this concept, the numerical modeling o f the shock-wave generation and propagation in space from a flare site near the transition region up to 3 solar radii was made. Comparing th e thermal and magnetic field disturbances, it was found that those of magnetic origin are more relevant in this case. Combining the results of interpretation and numerical simulation, a model of the February 27 , 1992 flare is suggested and new aspects of this model are discussed.