On the influence of the shape of kappa distributions of ions and electrons on the ion-cyclotron instability

Abstract

The dispersion relation for ion-cyclotron waves propagating along the direction of the ambient magnetic field is investigated numerically by considering different forms of kappa functions as velocity distributions of ions and electrons. General forms of kappa distributions, isotropic and anisotropic, are defined and used to obtain the dispersion relations for ion-cyclotron waves. With suitable choice of parameters, the general forms reduce to anisotropic versions of the kappa distributions most frequently employed in the literature. The analysis is focused in cases with a small value of the kappa index, for which the non thermal character of the kappa distributions is enhanced. The results show the effects of the superthermal tails of the velocity distributions of both particle species (ions and electrons) on the growth rate of the ion-cyclotron instability. It is seen that different forms of anisotropic kappa distributions, which are used in the current literature, can have a significantly different effect on the growth rates of the instability.