Generation of Alfv\'enic Waves and Turbulence in Reconnection Jets

TL;DR

This paper investigates the linear stability of magnetohydrodynamic flows with the Hall effect and guide magnetic field, identifying unstable modes that could generate Alfvénic turbulence during magnetic reconnection.

Contribution

It introduces a detailed analysis of streaming tearing, sausage, and kink modes in reconnection jets, highlighting their growth rates and dependence on plasma parameters, including the Hall effect and guide field.

Findings

Identified three unstable modes: tearing, sausage, and kink.

Growth rates vary with magnetic Reynolds number and guide field strength.

Unstable modes can exist in both super- and sub-Alfvénic flows.

Abstract

The magnetohydrodynamic linear stability with the localized bulk flow oriented parallel to the neutral sheet is investigated, by including the Hall effect and the guide magnetic field. We observe three different unstable modes: a "streaming tearing" mode at a slow flow speed, a "streaming sausage" mode at a medium flow speed, and a "streaming kink" mode at a fast flow speed. The streaming tearing and sausage modes have a standard tearing mode-like structure with symmetric density fluctuations in the neutral sheet, while the kink mode has an asymmetric fluctuation. The growth rate of the streaming tearing mode decreases with increasing magnetic Reynolds number, while the growth rates of the sausage and kink modes do not depend strongly on the Reynolds number. The sausage and kink modes can be unstable for not only super-Alfv\'enic flow but also sub-Alfv\'enic flow when the lobe density is low. The wavelengths of these unstable modes are of the same order of magnitude as the thickness of the plasma sheet. Their maximum growth rates are higher than that of a standard tearing mode, and under a strong guide magnetic field, the growth rates of the sausage and kink modes are enhanced, while under a weak guide magnetic field, they are suppressed. For a thin plasma sheet with the Hall effect, the fluctuations of the streaming modes can exist over the plasma sheet. These unstable modes may be regarded as being one of the processes generating Alfv\'enic turbulence in the plasma sheet during magnetic reconnection.