Suppression of the collisionless tearing mode by flow shear: implications for reconnection onset in the Alfv\'enic solar wind

TL;DR

This paper investigates how strong shear flows suppress collisionless tearing modes in current sheets, potentially explaining the scarcity of magnetic reconnection events observed in the Alfvénic solar wind near the Sun.

Contribution

It demonstrates that shear flow can completely stabilize tearing modes and alters their scaling behavior, providing new insights into magnetic reconnection suppression.

Findings

Shear flow decreases tearing mode growth rate, stabilizing it at Alfvénic speeds.

Growth rate decreases with higher ion-to-electron temperature ratio under shear flow.

Small shear flows significantly change the tearing mode's scaling behavior.

Abstract

We analyse the collisionless tearing mode instability of a current sheet with a strong shear flow across the layer. The growth rate decreases with increasing shear flow, and is completely stabilized as the shear flow becomes Alfv\'enic. We also show that in the presence of strong flow shear, the tearing mode growth rate decreases with increasing background ion-to-electron temperature ratio, the opposite behaviour to the tearing mode without flow shear. We find that even a relatively small flow shear is enough to dramatically alter the scaling behaviour of the mode, because the growth rate is small compared to the shear flow across the ion scales (but large compared to shear flow across the electron scales). Our results may explain the relative absence of reconnection events in the near-Sun Alfv\'enic solar wind observed recently by NASA's Parker Solar Probe.