Radio Emission of Solar Flare Particle Acceleration

The solar corona is a very dynamic plasma on time scales of decades to a few milliseconds. Radio missions provide diagnostic tools particularly suited for the analysis of non-thermal electron distributions, enhanced levels of various kinds of plasma waves and plasma phenomena related to electron acceleration in flares. Very intense coherent emissions are observed at frequencies below about 3GHz, weaker ones up to 9GHz. They are caused by plasma instabilities driving various wave modes that in turn may emit observable radio waves. The focus here is on Type III and stationary type IV bursts from about 0.2 to 4GHz. Type III bursts can be traced back in the corona to the acceleration region of electron beams. Less known are radio emissions from magnetically trapped electrons driving loss-cone unstable waves. This is the interpretation usually given to type IV emission. It is a very powerful radiation probably also observed in stars and possibly related to acceleration after the main flare energy release phase. The comparison of the radio emissions with hard X–rays reveals surprisingly that the two emissions often do not correlate in time and thus must originate from different electron acceleration processes. In combination with other wavelengths and their recent imaging capabilities, exciting new possibilities may soon open for radio diagnostics.