Analytic solution to charge-exchange source terms between Maxwellian and kappa-distributed velocity distributions in the heliosphere

Abstract

Many space and astrophysical environments are dominated by magnetohydrodynamic plasma interactions that are virtually collisionless. However, pseudo-elastic collisions are present through mechanisms of ion-neutral charge-exchange. In this paper, we investigate how charge-exchange collisions affect the balance of momentum and energy between ions and neutrals within the plasma. Our primary application is the heliosphere, where the transfer of momentum and energy between protons and interstellar hydrogen ultimately modifies its shape and structure. We derive analytical formulas for charge-exchange coupling between protons with suprathermal tails, modeled as a kappa distribution, and a relatively cool population of thermal hydrogen that is moving with respect to the plasma. We discuss differences in the energy and momentum transfer rates depending on whether the plasma is modeled by a Maxwellian or kappa distribution.