Impact of Stochastic Acceleration on the Pickup Proton Distribution and ENA Fluxes in the Heliosphere

Abstract

The paper describes a kinetic model of pickup proton distribution in the heliosphere that considers the process of stochastic acceleration by the small-scale Alfvenic and large-scale magnetosonic fluctuations. Using this model, we investigated the impact of this process on the velocity distribution function of pickup protons. The model results showed that stochastic acceleration leads to diffusion in the velocity space and the formation of a high-velocity ``tail'' in the distribution function of pickup protons. We also simulated the energy spectra of energetic neutral atom (ENA) fluxes from the inner heliosheath and compared them to the observations of IBEX-Lo/Hi instruments onboard the IBEX spacecraft in Earth's orbit. The comparison showed that (a) consideration of the large-scale oscillations of the solar wind velocity allows for explaining the IBEX-Hi observations at the highest energy steps, and (b) the velocity diffusion is responsible for an ENA flux increase in the 0.1 -- 0.5 keV range, partially reconciling existing discrepancies between IBEX-Lo observations and models that do not consider this process.