Heating of the Warm Ionized Medium by Low-Energy Cosmic Rays

TL;DR

This paper investigates how low-energy cosmic rays significantly heat the Warm Ionized Medium, potentially balancing radiative cooling and influencing the thermal state of the interstellar environment.

Contribution

It demonstrates that low-energy cosmic rays can provide sufficient heating to maintain the WIM, highlighting their dominant role in its thermal balance across the galaxy.

Findings

LECR effectively heat plasma more than neutrals.

Measured ionization rates imply LECR can sustain WIM temperature.

Higher LECR fluxes in the inner Galaxy suggest even greater heating contributions.

Abstract

In light of evidence for a high ionization rate due to Low-Energy Cosmic Rays (LECR), in diffuse molecular gas in the solar neighbourhood, we evaluate their heat input to the Warm Ionized Medium (WIM). LECR are much more effective at heating plasma than they are at heating neutrals. We show that the upper end of the measured ionization rates corresponds to a local LECR heating rate sufficient to maintain the WIM against radiative cooling, independent of the nature of the ionizing particles or the detailed shape of their spectrum. Elsewhere in the Galaxy the LECR heating rates may be higher than measured locally. In particular, higher fluxes of LECR have been suggested for the inner Galactic disk, based on the observed hard X-ray emission, with correspondingly larger heating rates implied for the WIM. We conclude that LECR play an important, perhaps dominant role in the thermal balance of the WIM.