Ion-Neutral Collisions in the Interstellar Medium: Wave Damping and Elimination of Collisionless Processes

TL;DR

This paper investigates how ion-neutral collisions affect wave damping and turbulence in the interstellar medium, revealing that such collisions suppress collisionless processes and alter turbulence characteristics in different interstellar environments.

Contribution

It provides the first detailed analysis of ion-neutral collision effects on wave damping and turbulence in the diffuse ionized gas and local clouds of the interstellar medium.

Findings

Ion-neutral collisions damp MHD waves within less than a parsec.

VLISM turbulence lacks features of solar wind turbulence, such as ion mass-dependent temperatures.

Ion-neutral interactions distribute energy among ions and neutrals, affecting turbulence properties.

Abstract

Most phases of the interstellar medium contain neutral atoms in addition to ions and electrons. This introduces differences in plasma physics processes in those media relative to the solar corona and the solar wind at a heliocentric distance of 1 astronomical unit. In this paper, we consider two well-diagnosed, partially-ionized interstellar plasmas. The first is the Diffuse Ionized Gas (DIG) which is probably the extensive phase in terms of volume. The second is the gas that makes up the Local Clouds of the Very Local Interstellar Medium (VLISM). Ion-neutral interactions seem to be important in both media. In the DIG, ion-neutral collisions are relatively rare, but sufficiently frequent to damp magnetohydrodynamic (MHD) waves (as well as propagating MHD eddies) within less than a parsec of the site of generation. This result raises interesting questions about the sources of turbulence in the DIG. In the case of the VLISM, the ion-neutral collision frequency is higher than that in the DIG, because the hydrogen is partially neutral rather than fully ionized. We present results showing that prominent features of coronal and solar wind turbulence seem to be absent in VLISM turbulence. For example, ion temperature does not depend on ion mass. This difference may be attributable to ion-neutral collisions, which distribute power from more effectively heated massive ions such as iron to other ion species and neutral atoms.