Island instability as a mechanism of destabilization and collapse of current sheets

TL;DR

This paper investigates how magnetic islands within current sheets lead to MHD instability, with Hall effects significantly accelerating the process, culminating in a rapid collapse relevant to magnetotail substorms.

Contribution

It demonstrates that a single elongated island causes MHD instability in current sheets, with Hall dynamics greatly enhancing growth rates and leading to rapid sheet collapse.

Findings

Hall effects increase instability growth rate to one-tenth of ion cyclotron frequency.

Non-linear phase involves fast impulsive current intensification.

Collapse of the sheet occurs after about 10 ion-cyclotron periods.

Abstract

The work shows that a single elongated island immersed in the quasi-neutral current sheet makes it MHD unstable. Typical values of growth rate are found to be several per cent of inverse Alfven time for broad sheets. Hall dynamics greatly enhance instability and growth rate reaches one-tenth of ion cyclotron frequency when sheet width is comparable to ion inertia length. A weak square-root dependence of increment on island width and length is derived both from numerical simulation and analytical analysis. At the non-linear phase of evolution a phenomenon of fast impulsive intensification of current is found. After finite time of about 10 ion-cyclotron periods it ends by a collapse of the sheet. It is shown that Hall dynamics is responsible for such a behaviour. Possible implications for magnetotail substorms are discussed.