Collisionless shocks in a partially ionized medium: I. Neutral return flux and its effects on acceleration of test particles

TL;DR

This paper investigates how neutral atoms influence collisionless shocks in partially ionized media, revealing the neutral return flux's role in modifying shock structure and steepening particle acceleration spectra, especially in supernova remnants.

Contribution

It introduces the concept of neutral return flux and analyzes its impact on shock compression and particle acceleration in partially ionized environments.

Findings

Neutral return flux reduces shock compression ratio.

Steepens the energy spectrum of accelerated particles.

Effects are significant for shocks below 3000 km/s.

Abstract

A collisionless shock may be strongly modified by the presence of neutral atoms through the processes of charge exchange between ions and neutrals and ionization of the latter. These two processes lead to exchange of energy and momentum between charged and neutral particles both upstream and downstream of the shock. In particular, neutrals that suffer a charge exchange downstream with shock-heated ions generate high velocity neutrals that have a finite probability of returning upstream. These neutrals might then deposit heat in the upstream plasma through ionization and charge exchange, thereby reducing the fluid Mach number. A consequence of this phenomenon, that we refer to as the "neutral return flux", is a reduction of the shock compression factor and the formation of a shock precursor upstream. The scale length of the precursor is determined by the ionization and charge exchange interaction lengths of fast neutrals moving towards upstream infinity. In the case of a shock propagating in the interstellar medium, the effects of ion-neutral interactions are especially important for shock velocities < 3000 km/s. Such propagation velocities are common among shocks associated with supernova remnants, the primary candidate sources for the acceleration of Galactic cosmic rays. We then investigate the effects of the return flux of neutrals on the spectrum of test-particles accelerated at the shock. We find that, for shocks slower than ~3000 km/s, the particle energy spectrum steepens appreciably with respect to the naive expectation for a strong shock, namely E^-2.