Fast-to-Alfv\'en Mode Conversion Mediated by Hall Current. I. Cold Plasma Model

TL;DR

This paper investigates how Hall currents can facilitate fast Alfvén mode conversion in weakly ionized plasmas, with implications for solar, stellar, and star-forming regions, especially at higher frequencies.

Contribution

It introduces a model analyzing Hall-mediated Alfvén mode conversion in cold plasma, highlighting conditions where significant wave oscillations can occur in weakly ionized environments.

Findings

Hall coupling occurs mainly when wave vector is nearly field-aligned.

At low frequencies, Hall effects are minimal due to small Hall parameter.

Higher frequency waves may experience significant mode oscillations in star-forming regions.

Abstract

The photospheric temperature minimum in the Sun and solar-like stars is very weakly ionized, with ionization fraction $f$ as low as $10^{-4}$. In galactic star forming regions, $f$ can be $10^{-10}$ or lower. Under these circumstances, the Hall current can couple low frequency Alfv\'en and magneto\-acoustic waves via the dimensionless Hall parameter $\epsilon=\omega/\Omega_\text{i}f$, where $\omega$ is the wave frequency and $\Omega_\text{i}$ is the mean ion gyrofrequency. This is analysed in the context of a cold (zero-$\beta$) plasma, and in less detail for a warm plasma. It is found that Hall coupling preferentially occurs where the wave vector is nearly field-aligned. In these circumstances, Hall coupling in theory produces a continual oscillation between fast and Alfv\'en modes as the wave passes through the weakly ionized region. At low frequencies (mHz), characteristic of solar and stellar normal modes, $\epsilon$ is probably too small for more than a fraction of one oscillation to occur. On the other hand, the effect may be significant at the far higher frequencies (Hz) associated with magnetic reconnection events. In another context, characteristic parameters for star forming gas clouds suggest that $\mathcal{O}(1)$ or more full oscillations may occur in one cloud crossing. This mechanism is not expected to be effective in sunspots, due to their high ion gyrofrequencies and Alfv\'en speeds, since the net effect depends inversely on both and therefore inverse quadratically on field strength.