Behavior of the flare-produced coronal MHD wavefront and the occurrence of type II radio bursts

Abstract

The propagation of the weak MHD fast-mode shock emitted into the corona by flares at their explosive phase is computer-simulated. It is shown as the result that the shock wave is refracted towards the low Alfven velocity regions pre-existing in the corona, and the strength of the shock, which is otherwise weak, is drastically enhanced on encountering low- VA regions due to the focussing effect by refraction and also due to the lowered propagation velocity of the shock in such regions. It is expected that electron acceleration takes place in such a drastic strengthening of the shock, leading to the local excitation of plasma waves and eventually to the occurrence of radio bursts at such locations. Such locations of shock strength enhancement, when computed by using HAO realistic models of coronal density and magnetic field of the day of certain type II burst events, actually coincide roughly with the observed positions of type II bursts. Peculiar configurations of type II burst sources as well as their occurrence even beyond the horizon of the responsible flare are explained consistently by the large scale refraction and the local enhancement of the shock due to the global and local distribution of Alfven velocity in the corona. A unified interpretation is given for the occurrence of type II bursts and Moreton's wave phenomena, and also the relation of our MHD fast-mode disturbance with other flare-associated dynamical phenomena is discussed.