Plasma Diagnostics of the Solar Corona Using Decimetric Radio Waves (review) 1. Solar and Stellar Coronae

The solar corona is a very dynamic plasma on time scales down to a few tens of microseconds. The various emissions at decimetric wavelength are brieey reviewed. Several of them seem to be caused by energetic (non-thermal) electrons accelerated in ares or are-like processes releasing free magnetic energy. The use of decimetric radiation as diagnostics of the acceleration process, the ambient plasma and the non-thermal particles requires a solid understanding of the emission process. Although we are still far from this in most cases, some information can already be derived from the observations. The solar corona has become a scientiic problem of rst importance when W. Grotrian and B. Edl en suggested in 1939 that it consists of a thin plasma of some million degrees temperature. This unexpectedly high temperature is still an enigma today. It is one of the reasons why the solar corona will be a primary target of solar research in the coming years. Soft X-ray motion pictures show ceaseless variability in active regions. Loops brighten up and disappear within a few hours. About 1500 X-ray bright points light up per day in quiet regions and coronal holes. They consist of several parallel loops and have a mean lifetime of eight hours.