Plasma physics of explosive phenomena

Abstract

Some physical aspects of explosive type phenomena in the solar atmosphere are reviewed. Since the time of the Skylab experiments, we know that at least a considerable fraction of solar flares take the form of heated magnetic loops. In order to explain these observations two types of models with current-carrying magnetic loops are discussed. In the first type of model there are a limited number of loops with regular magnetic field structure and a neutral current sheet as a key element (e.g. isolated flaring loop model, loop coalescence model, emerging flux model, the central helmet streamer configuration, et al.). The main problem with these models is the high magnetic Reynolds number in the solar corona plasma and the resulting long time for neutral sheet evolution. In the second type of model there is an ensemble of the twisted flux tubes, isolated from each other, which are driven by turbulent photospheric motion. In this case the “flare” is a statistical flare and it is an ensemble of microflares that are triggered almost simultaneously either by emerging flux or large scale flows. The possibility of “statistical flare” is supported by the observations of decimetric radio spikes which indicate that there are multiple particle accelerations during the flare. Mm-wave and hard X-ray emission of solar flares sometimes show that the energy content in the fast electrons accelerated in the coronal part of flaring loop is insufficient for chromospheric plasma evaporation. It cannot be excluded that in this case the main energy release process occurs in the chromosphere. In this connection we discuss the “circuit model” of solar flares which can provide the main energy release at the footpoints of the flaring loop.