Self-consistent Simulations of Plasma-Neutral in a Partially Ionized Astrophysical Turbulent Plasma

TL;DR

This paper develops a self-consistent two-dimensional fluid simulation model to study how charge-exchange interactions between plasma and neutral hydrogen affect turbulence energy transfer in the heliosheath and outer heliosphere.

Contribution

It introduces a novel turbulence model that incorporates charge-exchange processes, revealing their impact on spectral transfer and turbulence spectra in partially ionized astrophysical plasmas.

Findings

Charge-exchange interactions modify spectral transfer in turbulence.

Turbulent spectra are steeper than Kolmogorov predictions.

Charge-exchange prolongs the inertial range energy cascade.

Abstract

A local turbulence model is developed to study energy cascades in the heliosheath and outer heliosphere (OH) based on self-consistent two-dimensional fluid simulations. The model describes a partially ionized magnetofluid OH that couples a neutral hydrogen fluid with a plasma primarily through charge-exchange interactions. Charge-exchange interactions are ubiquitous in warm heliospheric 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 turbulent cascade rate prolongs spectral transfer among inertial range turbulent modes. Turbulent spectra associated with the neutral and plasma fluids are therefore steeper than those predicted by Kolmogorov's phenomenology. Our work is important in the context of the global heliospheric interaction, the energization and transport of cosmic rays, gamma-ray bursts, interstellar density spectra, etc. Furthermore, the plasma-neutral coupling is crucial in understanding the energy dissipation mechanism in molecular clouds and star formation processes.