Stability of the magnetotail current sheet with normal magnetic field and field-aligned plasma flows

TL;DR

This paper analyzes the stability of magnetotail current sheets with normal magnetic fields and field-aligned plasma flows, revealing the stabilizing effects of finite $B_z$ and counter-streaming ion flows, relevant for understanding substorm onsets.

Contribution

It provides a detailed multi-fluid stability analysis of current sheets with transient ions, highlighting the stabilizing role of finite $B_z$ and specific ion flow configurations.

Findings

Finite $B_z$ stabilizes the current sheet.

Counter-streaming ion flows with zero net flow are most stable.

Results align with recent spacecraft observations.

Abstract

One of the most important problems of magnetotail dynamics is the substorm onset and the related instability of the magneotail current sheet. Since the simplest 2D current sheet configuration with monotonic $B_z$ was proven to be stable to the tearing mode, the focus of the instability investigation moved to more specific configurations, e.g. kinetic current sheets with strong transient ion currents and current sheets with non-monotonic $B_z$ (local $B_z$ minima or/and peaks). Stability of the latter current sheet configuration has been studied both within kinetic and fluid approaches, whereas the investigation of the transient ion effects were limited to kinetic models only. This paper aims to provide detailed analysis of stability of a multi-fluid current sheet configuration that mimics current sheets with transient ions. Using the system with two field-aligned ion flows that mimic the effect of pressure non-gyrotropy, we construct 1D current sheet with a finite $B_z$. This model describes well recent findings of very thin intense magnetotail current sheets. The stability analysis of this two-ion model confirms the stabilizing effect of finite $B_z$ and shows that the most stable current sheet is the one with exactly counter-streaming ion flows and zero net flow. Such field-aligned flows may substitute the contribution of the pressure tensor nongyrotropy to the stress balance, but cannot overtake the stabilizing effect of $B_z$. Obtained results are discussed in the context of magnetotail dynamical models and spacecraft observations.