Angular Scattering in Charge Exchange: Issues and Implications for Secondary Interstellar Hydrogen

TL;DR

This study investigates how angular scattering during charge exchange collisions affects the properties of interstellar hydrogen atoms, revealing significant impacts on their velocities, temperatures, and distributions relevant for heliospheric observations.

Contribution

It introduces a numerical analysis of angular scattering effects in charge exchange, highlighting their importance for accurate modeling of interstellar hydrogen atoms.

Findings

Secondary hydrogen atoms have higher bulk velocities when scattering is included.

Temperatures of secondary atoms increase by approximately 1200-2400 K due to scattering.

High relative velocities lead to non-Maxwellian distributions of secondary atoms.

Abstract

Interstellar neutral atoms provide a remote diagnostic of plasma in the outer heliosheath and the very local interstellar medium via charge exchange collisions that convert ions into atoms and vice versa. So far, most studies of interstellar atoms assumed that daughter hydrogen atoms directly inherit the kinetic properties of parent protons. This assumption neglects angular scattering of the interacting particles. However, for low relative velocities, as expected for charge exchanges in the outer heliosheath, this scattering is significant. In this study, we present how the parameters of daughter populations depend on the relative velocity and temperatures of parent populations. For this purpose, we numerically compute collision terms with and without this scattering. We find that the secondary population of interstellar hydrogen atoms, for the parent populations with the relative bulk velocity of 20 km s$^{-1}$ and equal temperatures of 7500 K, has ~2 km s$^{-1}$ higher bulk velocity if the scattering is taken into account. Additionally, temperatures are higher by ~2400 K and ~1200 K in parallel and perpendicular direction to the relative motion of parent populations, respectively. Moreover, a significant departure of secondary atoms from the Maxwell-Boltzmann distribution is expected for high relative velocities of parent populations. This process affects the distribution and density of interstellar atoms in the heliosphere and production of pickup ions. Thus, we show that angular scattering in charge exchange collisions is important to include in analyses of interstellar neutral atoms and pickup ions observed at 1 au and in the outer heliosphere.