Energy Cascades in Astrophysical Plasma

TL;DR

This paper develops a local turbulence model for the interstellar medium, showing how charge exchange interactions between neutral and plasma components modify energy cascades and steepen turbulent spectra.

Contribution

It introduces a self-consistent two-dimensional fluid simulation model that incorporates charge exchange effects in partially ionized astrophysical plasma.

Findings

Charge exchange interactions alter spectral transfer in ISM turbulence.

Turbulent spectra become steeper than Kolmogorov predictions.

Prolonged spectral transfer due to charge exchange processes.

Abstract

A local turbulence model is developed to study energy cascades in the interstellar medium (ISM) based on self-consistent two-dimensional fluid simulations. The model describes a partially ionized magnetofluid interstellar medium (ISM) that couples a neutral hydrogen fluid with a plasma primarily through charge exchange interactions. Charge exchange interactions are ubiquitous in warm ISM plasma and the strength of the interaction depends largely on the relative speed between the plasma and the neutral fluid. Unlike small length-scale linear collisional dissipation in a single fluid, charge exchange processes introduce channels that can be effective on a variety of length-scales that depend on the neutral and plasma densities, temperature, relative velocities, charge exchange cross section and the characteristic length scales. We find, from scaling arguments and nonlinear coupled fluid simulations, that charge exchange interactions modify spectral transfer associated with large scale energy containing eddies. Consequently, the warm ISM turbulent cascade rate prolongs spectral transfer amongst inertial range turbulent modes. Turbulent spectra associated with the neutral and plasma ISM fluids are therefore steeper than those predicted by Kolmogorov's phenomenology.