Self-initialised Fermi-1 acceleration by pitch-angle re-scattering of solar wind ions reflected from the parallel termination shock

Abstract

In a recent kinetic calculation a rather high amount of reflected ions was shown to occur at the solar wind termination shock under a parallel magnetic field orientation. The fate of these reflected ions is analysed within the scope of this paper. In the inner heliosphere their further transport is expected to be mainly influenced by pitch-angle scattering similar to the process governing pick-up ions (PUIs). This diffusive scattering isotropises the ions on a shell-like distribution in the comoving upstream solar wind reference frame. The resulting distribution is calculated and the result is applied on our earlier kinetic model. The isotropised ions, when again convected into the shock, tend to attenuate the shock a little and lead to an additional energetic tail population in the downstream distribution function. A consistent incorporation of the reflected ions into the model is iteratively possible. It turns out that a part of the reflected and isotropised ions offer properties which allow for the acceleration to the anomalous cosmic ray (ACR) regime as an additional contribution to the injection by pick-up ions (i.e. we propose a self-injection mechanism to the Fermi-1 acceleration process). © Author(s) 2008.