Energy release processes in active regions.

A standard model of impulsive energy release has emerged during the recent years: Magnetic energy is dumped into coronal electrons (and possibly ions) accelerating them to some tens of keV. These particles mostly precipitate into the chromosphere, radiate hard X-rays and heat it to millions of degrees. The hot chromospheric material is ejected into the corona and produces the soft X-ray flare. The theory behind the energy release is reconnection, proposed for various geometries. The standard model is here confronted with observations showing a large variety of energy releases on different time scales from the smallest noise storms and high coronal flares to the large coronal mass ejections. In view of the differences of these phenomena it is unlikely that a simple uniform model can explain them all. Hard X-ray and Hα emissions suggest a total duration of a flare of ten minutes to one hour with individual episodes of contiguous acceleration of one minute. Elementary hard X-ray peaks have 5-10 s duration, corresponding to groups of beams observable as type III radio bursts. The rise time of these beams is of order 0.1 s. The smallest time scale is indicated in narrowband radio spikes of a few 0.01 s duration. They suggest spatial scales between less than 12 and 300 km. Yohkoh observations have allowed us to measure and locate the thermal energy of the ejected material. It can readily be related to the non-thermal energy input observed in other wavelengths. The geometry of the energy release has been studied in hard X-ray and radio emissions. New observations of the coherent radiation of microflares in the 1-3 GHz region show bursts that are neither visible in synchrotron nor thermal emissions. On the other hand, there seem to be also incoherent radio events with no coherent signatures. Significant differences exist between electron acceleration at high altitudes producing interplanetary electron beams and low-altitude activity in the network of the quiet Sun. [ABSTRACT FROM AUTHOR]