Importance of Kappa Background Electron Distributions to Solar Radio Bursts

Abstract:

The Sun produces intense radio emissions by collective processes and by incoherent single-particle processes, both of which depend sensitively on the electron distribution function present. Examples of the former include metric type II and III bursts produced by the so-called plasma emission processes, while examples of the latter include decametric and metric continua produced by the bremstrahlung and gyrosynchrotron processes. Kappa electron distributions, which appear power-law at high energies, qualitatively alter the spectral shape for the incoherent processes and increase the total flux (due to the increased number of high energy electrons). Kappa distributions are also vital for collective processes, due to the increased number of fast electrons (and so energy available) again and also due to the greatly ncreased level of the nonthermal background distribution onto which is superposed the distribution function of fast particles responsible for the collectively-produced waves and radio emissions. Two examples are presented, one for each reason. For type II bursts the level and frequency-time structures (above background) of predicted radio emission change qualitatively when kappa rather than Maxwellian background electrons are assumed, due to the increased numbers of fast electrons reflected by the type II shock. While this effect is also important for type III bursts produced by fast electron beams, our simulations show that the crucial qualitative change is that beam speeds above 0.3 c only develop in the simulations when kappa background electrons are present, due to the high level of the kappa tail preventing quasilinear relaxation of the beam electrons to smaller speeds.