Implication of Kappa models in realistic parameterization of the kinetic anisotropy and the resulting instabilities in space plasmas

Abstract:

Direct in-situ measurements of the velocity distributions of plasma particles in the solar wind reveal two distinct components: a Maxwellian (thermal) core, and a less dense but hotter halo in the high-energy (suprathermal) tails of the distribution, which are well-described by Kappa power-laws. Despite these evidences, the present attempts to parameterize the observed anisotropy and the resulting plasma wave instabilities are limited to idealized models of the distributions. These are, for instance, simplified models which ignore the suprathermal populations, or minimize the role of the core, assuming this component is cold, and model only the suprathermal tails with a Kappa distribution function. It is worthwhile to asses to which extent these models are relevant in realistic situations. Here, we present a comparative analysis with more realistic approaches, which combine a Maxwellian core, and one or two Kappa distributed components (the halo and the field-aligned strahl in the fast wind). A comparison is provided for the particular case of the cyclotron instabilities, which enables us to emphasize the effects produced by the thermal spread of plasma particles from the core, and extend approaching complex situations frequently observed in the solar wind, when both the core and halo populations are anisotropic. Correlated with the radial profile of Kappa components in the heliosphere, these effects help us to build a realistic picture on the role played by these instabilities in major processes like heating and energy dissipation in the solar wind.