Solar wind proton reflection and injection to the ACR regime at the parallel termination shock

Abstract

Our recent kinetic model for the parallel solar wind termination shock describes the ion shock transit under the influence of a decelerating electrostatic potential and the turbulent wave-particle interaction with electrostatic plasma waves. Due to this diffusive interaction, a certain number of ions are reflected at the shock. These ions propagate with a bulk velocity comparable to the bulk velocity of the incoming solar wind backwards into the inner heliosphere. They undergo strong pitch-angle diffusion as they interact with the interplanetary Alfvénic turbulence similar to freshly ionized pick-up ions (PUIs). Their distribution function is hence isotropized to a shell in velocity space. Since their velocity magnitude in the wind frame is higher (about two times) than the estimated PUI velocity, part of the reflected and isotropized ions can directly enter an acceleration process (diffusive or stochastic, respectively) without the necessity of a pre-acceleration. This means that an additional self-initialized seed population for the injection into the anomalous cosmic ray (ACR) acceleration is shown to appear due to shock reflected solar wind ions.