The non-ideal finite Larmor radius effect in the solar atmosphere

TL;DR

This paper investigates the finite Larmor radius effects in the solar atmosphere, revealing how gyroviscous forces influence wave propagation and instabilities, especially in the transition region between the chromosphere and corona.

Contribution

It introduces the impact of gyroviscous effects on plasma dynamics and wave instabilities in the partially ionized solar atmosphere, highlighting their significance over macroscopic scales.

Findings

Gyroviscous effects operate over large scales in the solar atmosphere.

Gyro waves become unstable with growth rates similar to Hall instability.

Transition region may experience fast-growing gyroviscous instabilities.

Abstract

The dynamics of the partially ionized solar atmosphere is controlled by the frequent collision and charge exchange between the predominant neutral Hydrogen atoms and charged ions. At signal frequencies below or of the order of either of the collision or charge exchange frequencies the magnetic stress is {\it felt} by both the charged and neutral particles simultaneously. The resulting neutral-mass loading of the ions leads to the rescaling of the effective ion-cyclotron frequency-it becomes the Hall frequency, and the resultant effective Larmor radius becomes of the order of few kms. Thus the finite Larmor radius (FLR) effect which manifests as the ion and neutral pressure stress tensors operates over macroscopic scales. Whereas parallel and perpendicular (with respect to the magnetic field) viscous momentum transport competes with the Ohm and Hall diffusion of the magnetic field in the photosphere-chromosphre, the gyroviscous effect becomes important only in the transition region between the chromosphere and corona, where it competes with the ambipolar diffusion. The wave propagation in the gyroviscous effect dominated medium depends on the plasma $\beta$ (a ratio of the thermal and magnetic energies). The abundance of free energy makes gyro waves unstable with the onset condition exactly opposite of the Hall instability. However, the maximum growth rate is identical to the Hall instability. For a flow gradient $\sim 0.1 \,\mbox{s}^{-1}$ the instability growth time is one minute. Thus, the transition region may become subject to this fast growing, gyroviscous instability.