Decimeter Burst Emission and Particle Acceleration

Abstract

The radio emission of solar flares at decimeter wavelengths includes a variety of emission processes of a plasma thought to have a high beta. Very intense coherent emissions are observed at frequencies smaller than about 9 GHz. They are caused by plasma instabilities driving various wave modes that in turn may emit observable radio waves. Particularly important are type III bursts, caused by electron beams exciting Langmuir waves. Their sources may be used to trace the path of the electrons back in the corona to the acceleration region. Less known are radio emissions from trapped electrons driving loss-cone unstable waves, suspected for type IV bursts. These types of coherent radio emission give clues on the geometry and plasma parameters near the acceleration region.

More speculative are emissions that are directly produced by the acceleration process. A possible group of such phenomena are narrowband, short peaks of emission. Narrowband spikes are seen sometimes at frequencies above the start of metric type III events. There is mounting evidence for the hypothesis that these spikes coincide with the energy release region. Much less clear is the situation for decimetric spikes, which are associated with hard X-ray flares. More frequently than spikes, however, there is fluctuating broadband decimetric emission during the hard X-ray phase of flares. The use of these coherent radio emissions as a diagnostic tool for the primary energy release requires a solid understanding of the emission process. At the moment we are still far away from an accepted theory. Complementary observations of energetic electrons and the thermal coronal background in EUV lines and soft X-rays can put coherent emissions into context and test the different emission scenarios. In combination with other wavelengths, spectroscopic imaging by FASR would provide exciting possibilities for the diagnostics of the acceleration process.