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

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.

Filtration of Interstellar Neutral Helium by Elastic and Charge Exchange Collisions in Heliospheric Boundaries

“…We seek common distribution functions describing these populations at 100 au to describe the higher moments of the distribution function, which require higher statistics of Monte Carlo integrated trajectories. Due to the asymmetry of these distributions (Swaczyna et al 2019b; Paper I), we need to rotate the obtained state vector to a coordinate system where the local bulk flow defines one of the axes. Consequently, in the first step, we rotate the obtained velocities at 100 au, v, according to the local bulk flow velocity of the respective population, v pri sec , obtained from the analytic formulae (Appendix B) at the point at which the atom entered the inner boundary:…”

Future Exploration of the Outer Heliosphere and Very Local Interstellar Medium by Interstellar Probe

“… a From the acceptable range given in the paper b At the HP, with a slightly increased speed in the pristine VLISM (25.9 km/s) due to elastic collisions c Uncertainty translated from acceptable range of speeds of ISNs d At the HP e Due to removing the heating by elastic collisions given in Swaczyna et al. ( 2019 ) f Directional shift in 2005 originally reported by Krüger et al. ( 2007 ) and further confirmed by Krüger et al.…”

“…New models of the velocity distribution of plasma in the outer heliosphere are beginning to include nonthermal components through the use of kappa functions (Vasyliunas 1968 ) that differ from Maxwell–Boltzmann velocity distributions by including high-velocity tails (Swaczyna et al. 2019 ). Nonthermal ions in the VLISM (Gloeckler et al.…”

Slowdown and Heating of Interstellar Neutral Helium by Elastic Collisions beyond the Heliopause

“…𝒗 9,MNA = 𝑚 I@JK 𝑚 I@JK + 𝑚 0 𝒓G𝒗 9,M − 𝒗 O' , 𝜃, 𝜙I + 𝒗 O' , The rotation function 𝒓(𝒗, 𝜃, 𝜙) returns scattered velocity in the center-of-mass frame (moving with velocity 𝒗 O' ) and is defined in Appendix A in Swaczyna et al (2019b). Each atom is tracked until its position along the z-axis is less than 110 au (i.e., the heliopause distance in this direction).…”

“…The resonant character of this process is often used to claim that the momentum of the newly created atom is the same as the momentum of the parent ion. Swaczyna et al (2019b) demonstrated that this simplification is not justified for typical collision speeds in the outer heliosheath, and angular scattering of colliding particles impacts the distribution function of the created neutral population. However, this angular scattering can be neglected in studies of the global heliosphere because the amount of momentum exchange is not significantly changed (Heerikhuisen et al 2009;Izmodenov et al 2000).…”