Cosmic Ray Heating of the Warm Ionized Medium

TL;DR

This paper explores cosmic ray heating as a potential additional heating mechanism in the warm ionized medium, showing it can be significant and stable, possibly explaining observed line ratio discrepancies.

Contribution

It demonstrates that MHD-wave mediated cosmic ray heating can be a viable, stable supplemental heating source in the warm ionized medium, with a density dependence weaker than cooling.

Findings

Cosmic ray heating can be significant with appropriate energy densities.

The heating mechanism is stable to perturbations.

Applicable to warm ionized gas in other galaxies.

Abstract

Observations of line ratios in the Milky Way's warm ionized medium (WIM) suggest that photoionization is not the only heating mechanism present. For the additional heating to explain the discrepancy it would have to have a weaker dependence on the gas density than the cooling rate, $\Lambda n_e^2$. \cite{reynolds99} suggested turbulent dissipation or magnetic field reconnection as possible heating sources. We investigate here the viability of MHD-wave mediated cosmic ray heating as a supplemental heating source. This heating rate depends on the gas density only through its linear dependence on the Alfv\'en speed, which goes as $n_e^{-1/2}$. We show that, scaled to appropriate values of cosmic ray energy density, cosmic ray heating can be significant. Furthermore, this heating is stable to perturbations. These results should also apply to warm ionized gas in other galaxies.