Energy flux of Alfven waves in weakly ionized plasma

TL;DR

This paper investigates how weak ionization and plasma-neutral collisions in the solar photosphere significantly alter Alfvén wave properties, reducing their velocity and energy flux compared to ideal plasma assumptions.

Contribution

It demonstrates that in weakly ionized, collisional plasma, Alfvén waves involve neutrals, leading to reduced wave velocity and energy flux, which challenges previous idealized models.

Findings

Alfvén waves involve both plasma and neutrals in the photosphere.

Wave velocity and energy flux are significantly reduced due to collisions.

Ions and electrons are un-magnetized in the weakly ionized photosphere.

Abstract

The overshooting convective motions in the solar photosphere are frequently proposed as the source for the excitation of Alfv\'en waves. However, the photosphere is a) very weakly ionized, and, b) the dynamics of the plasma particles in this region is heavily influenced by the plasma-neutral collisions. The purpose of this work is to check the consequences of these two facts on the above scenario and their effects on the electromagnetic waves. It is shown that the ions and electrons in the photosphere are both un-magnetized; their collision frequency with neutrals is much larger than the gyro-frequency. This implies that eventual Alfv\'en-type electromagnetic perturbations must involve the neutrals as well. This has the following serious consequences: i) in the presence of perturbations, the whole fluid (plasma + neutrals) moves; ii) the Alfv\'en velocity includes the total (plasma + neutrals) density and is thus considerably smaller compared to the collision-less case; iii) the perturbed velocity of a unit volume, which now includes both plasma and neutrals, becomes much smaller compared to the ideal (collision-less) case; and iv) the corresponding wave energy flux for the given parameters becomes much smaller compared to the ideal case.