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 impulsive phase, showing fast positively drifting bursts superimposed on a slowly negatively drifting burst, is presented. Analyzing this radio spectrum it was found that the flare started somewhere near the transition 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, which bombarded dense layers of the solar atmosphere, were accelerated. It seems that, simultaneously with the increase of beam bombardment intensity, the intensity of MHD waves was increasing and thus the MHD shock wave generation and the electron beam acceleration and bombardment formed a self-consistently amplifying flare process. At higher coronal 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 of the shock-wave generation and propagation in space from a flare site near the transition region up to 3 solar radii was made. Comparing the 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.