The Reduction of Magnetic Reconnection Outflow Jets to Sub-Alfv\'enic Speeds

TL;DR

This paper demonstrates that ion exhaust temperature can reduce magnetic reconnection outflow speeds below the Alfvén speed, with a derived scaling law validated by simulations and observations, highlighting the role of firehose instability.

Contribution

It introduces a scaling relationship linking outflow velocity to ion temperature and Alfvén speed, explaining the reduction mechanism via firehose instability.

Findings

Ion exhaust temperature reduces outflow speed.

Scaling law verified in simulations and observations.

Firehose instability explains the speed reduction.

Abstract

The outflow velocity of jets produced by collisionless magnetic reconnection is shown to be reduced by the ion exhaust temperature in simulations and observations. We derive a scaling relationship for the outflow velocity based on the upstream Alfv\'en speed and the parallel ion exhaust temperature, which is verified in kinetic simulations and observations. The outflow speed reduction is shown to be due to the firehose instability criterion, and so for large enough guide fields this effect is suppressed and the outflow speed reaches the upstream Alfv\'en speed based on the reconnecting component of the magnetic field.