Obliquely Propagating Electromagnetic Waves in Magnetized Kappa Plasmas

Abstract:

The effects of velocity distribution functions (VDFs) that exhibit a power-law dependence on the high-energy tail have been the subject
of intense research by the space plasma community. Such functions, known as kappa or superthermal distributions, have been
found to provide a better fitting to the VDF measured by spacecraft in the solar wind. One of the problems that is being addressed on this new light is the temperature anisotropy of solar wind protons and electrons. An anisotropic kappa VDF contains a large amount of free energy that can excite waves in the solar wind. Conversely, the wave-particle interaction is important to determine the shape of the
observed particle distributions.
In the literature, the general treatment for waves excited by (bi-)Maxwellian plasmas is well-established. However, for kappa distributions, either isotropic or anisotropic, the wave characteristics have been studied mostly for the limiting cases of purely parallel or perpendicular propagation. Contributions for the general case of obliquely-propagating electromagnetic waves have been scarcely reported so far. The absence of a general treatment prevents a complete analysis of the wave-particle interaction in kappa plasmas, since some instabilities, such as the firehose, can operate simultaneously both in the parallel and oblique directions.
In a recent work [1], we have obtained expressions for the dielectric tensor and dispersion relations for the low-frequency, quasi-perpendicular dispersive Alfvén waves resulting from a kappa VDF. In the present work, we generalize the formalism introduced by [1] for the
general case of electrostatic and/or electromagnetic waves propagating in a kappa plasma in any frequency range and for arbitrary angles.
We employ an isotropic distribution, but the methods used here can be easily applied to more general anisotropic distributions,
such as the bi-kappa or product-bi-kappa.

[1] R. Gaelzer and L. F. Ziebell, Journal of Geophysical Research 119, 9334 (2014).