Magnetic double gradient instability and flapping waves in a current sheet

TL;DR

This paper analyzes a new magnetohydrodynamic instability and wave phenomena in current sheets influenced by magnetic field gradients, explaining observed flapping motions in Earth's magnetotail with theoretical and observational consistency.

Contribution

It introduces a novel instability mechanism related to magnetic field gradients and connects it to observed flapping waves in Earth's magnetotail.

Findings

Identifies conditions for stability and instability based on magnetic gradient products.

Derives wave velocities consistent with observed magnetotail flapping motions.

Proposes a new interpretation of flapping waves as kink-like modes.

Abstract

A new kind of magnetohydrodynamic instability and waves are analyzed for a current sheet in the presence of a small normal magnetic field component varying along the sheet. These waves and instability are related to existence of two gradients of the tangential and normal magnetic field components along the normal and tangential directions with respect to the current sheet. The current sheet can be stable or unstable if the multiplication of two magnetic gradients is positive or negative. In the stable region, the kink-like wave mode is interpreted as so called flapping waves observed in the Earth's magnetotail current sheet. The kink wave group velocity estimated for the Earth's current sheet is of the order of a few tens kilometers per second. This is in good agreement with the observations of the flapping motions of the magnetotail current sheet.