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 ter- mination 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 trans- port is expected to be mainly influenced by pitch-angle scat- tering 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 re- sult is applied on our earlier kinetic model. The isotropised ions, when again convected into the shock, tend to attenu- ate the shock a little and lead to an additional energetic tail population in the downstream distribution function. A con- sistent 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 ac- celeration 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 accel- eration process).