On the radial evolution of κ distributions of pickup protons in the supersonic solar wind

It is well known that (pickup) ions in the inner heliosphere do not maintain Maxwellian distributions but tend to nonequilibrium distributions with extended suprathermal tails. Such states have been classified as quasi‐equilibria which in many cases can well be described by so‐called κ distributions. With the present study we start out from a phase space transport equation for pickup ions in the inner heliosphere that adequately describes the most important processes such as injection, convection, cooling, and diffusion in velocity space. Assuming that the underlying distribution functions are κ distributions, we proceed from this transport equation to a second‐order moment, i.e., pressure equation which represents an ordinary differential equation for the κ parameter as function of heliocentric distance. This strategy allows one to describe the transition from an initial Vasyliunas‐Siscoe distribution to κ distributions with gradually more pronounced suprathermal tails. While the velocity dependence of the velocity diffusion coefficient determines the systematic reduction of the parameter κ, the latter always has the (formal) asymptotic value κ∞=3/2. This translates into values of 1.5≤κTS≤2.2 in the upstream region of the (upwind) solar wind termination shock that defines the outer validity range of the model. Key Points Evolution of kappa distributionsPickup ion transportVelocity diffusion