Coulomb collisional processes in space plasmas; relaxation of suprathermal particle distributions

Nonequilibrium distributions of space plasmas are often characterized by extended high-energy tails. This paper provides a detailed analysis of the relaxation of such isotropic nonequilibrium plasmas. We consider an energetic charged species dilutely dispersed in a fully ionized plasma, which acts as a heat bath at equilibrium. The minor constituent is referred to as a “test particle” and collisions between test particles are not included. We study the approach to equilibrium with a finite difference method of solution of the Fokker–Planck equation appropriate for collisions between charged particles. The solution of the Fokker–Planck equation is also presented formally as an expansion in the eigenfunctions of the Fokker–Planck operator. The main objective of the paper is the calculation of the energy-dependent relaxation times of the distribution function. A strong energy dependence for these relaxation times is anticipated since, for Coulomb collisions, the Rutherford cross-section varies with relative speed g as g−4. Analogous results for neutral species are presented for comparison in the following paper.