On the role of stochastic Fermi acceleration in setting the dissipation scale of turbulence in the interstellar medium

TL;DR

This paper investigates how stochastic Fermi acceleration influences the dissipation scale of turbulence in the interstellar medium, suggesting it affects radio scintillation models and provides a heating mechanism for the Reynolds Layer.

Contribution

It demonstrates that STFA sets a dissipation scale larger than observed scintillation scales and supports alternative scintillation models involving ionized boundary surfaces.

Findings

Dissipation scale via STFA is ≥ 10^{12} cm for certain turbulence spectra.

STFA dissipation aligns with non-cascading scintillation models involving HII region boundaries.

STFA provides a viable heating mechanism for the Reynolds Layer plasma.

Abstract

We consider the dissipation by Fermi acceleration of magnetosonic turbulence in the Reynolds Layer of the interstellar medium. The scale in the cascade at which electron acceleration via stochastic Fermi acceleration (STFA) becomes comparable to further cascade of the turbulence defines the inner scale. For any magnetic turbulent spectra equal to or shallower than Goldreich-Sridhar this turns out to be $\ge 10^{12}$cm, which is much larger than the shortest length scales observed in radio scintillation measurements. While STFA for such spectra then contradict models of scintillation which appeal directly to an extended, continuous turbulent cascade, such a separation of scales is consistent with the recent work of \citet{Boldyrev2} and \citet{Boldyrev3} suggesting that interstellar scintillation may result from the passage of radio waves through the galactic distribution of thin ionized boundary surfaces of HII regions, rather than density variations from cascading turbulence. The presence of STFA dissipation also provides a mechanism for the non-ionizing heat source observed in the Reynolds Layer of the interstellar medium \citep{Reynolds}. STFA accommodates the proper heating power, and the input energy is rapidly thermalized within the low density Reynolds layer plasma.