Chapter 14 - Importance of Kappa Distributions to Solar Radio Bursts

Abstract

Non-Maxwellian electron distributions are observed widely throughout the solar system and are often well described as kappa distributions. This chapter addresses the importance of kappa distributions to the growth and damping of plasma waves (specifically Langmuir waves), the reflection of electrons from shocks, the evolution of electron beams, and the generation of type II and III solar radio bursts. Kappa distributions extend to much higher speeds and have many more fast particles than Maxwellians. Accordingly, kappa distributions lead to a much larger spontaneous emission of plasma waves and radio emissions produced by incoherent processes. Similarly, shocks that accelerate the background electrons have much higher numbers of fast electrons upstream for kappa-distributed electrons than for Maxwellian electrons, leading to stronger electron beams and stronger Langmuir waves and radio emissions. This is demonstrated theoretically for type II solar radio bursts. The extension of kappa distributions to larger speeds than for Maxwellian distributions leads to larger damping rates for electron beam-generated Langmuir waves. It also limits the relaxation of the beam to higher speeds and reduces energy transfers to the Langmuir waves and any radio emissions generated therefrom. These effects are demonstrated theoretically for type III solar radio bursts, allowing us to understand why kappa distributions for the background electrons lead naturally to faster beams with speeds of ≳0.5c and to the type III bursts being weaker, having faster frequency drift rates, and starting at lower frequencies for smaller values of the kappa index, κ.