Energetic pickup proton population downstream of the termination shock as revealed by the IBEX-Hi data

Abstract

The pickup protons originate as a result of the ionization of hydrogen atoms in the supersonic solar wind, forming the suprathermal component of protons in the heliosphere. While picked by the heliospheric magnetic field and convected into the heliosheath, the pickup protons may suffer the stochastic acceleration by the solar wind turbulence in the region from the Sun up to the heliospheric termination shock, where they can also experience the shock-drift acceleration or the reflection from the cross-shock potential. These processes create a high-energy "tail" in the pickup ion energy distribution. The properties of this energetic pickup proton population are still not well-defined, despite they are vital for the models to simulate energetic neutral atom fluxes. We have considered two scenarios for pickup proton velocity distribution downstream of the heliospheric termination shock (filled shell with energetic power-law "tail" and bi-Maxwellian). Based on the numerical kinetic model and observations of the energetic neutral atom fluxes from the inner heliosheath by the IBEX-Hi instrument, we have characterized the pickup proton distribution and provided estimations on the properties of the energetic pickup proton population downstream of the termination shock.