Why does steady-state magnetic reconnection have a maximum local rate of order 0.1?

TL;DR

This paper explains why steady-state magnetic reconnection typically occurs at a maximum local rate around 0.1, linking it to MHD-scale constraints and showing it is near the maximum efficiency predicted by scaling analysis.

Contribution

It provides a scaling analysis that predicts a maximum reconnection rate close to 0.2, aligning with simulation results and explaining the universal fast reconnection rate.

Findings

Predicted maximum reconnection rate is about 0.2.

Simulation rates are close to the predicted maximum.

Reconnection rate is insensitive to opening angle.

Abstract

Simulations suggest collisionless steady-state magnetic reconnection of Harris-type current sheets proceeds with a rate of order 0.1, independent of dissipation mechanism. We argue this long-standing puzzle is a result of constraints at the magnetohydrodynamic (MHD) scale. We perform a scaling analysis of the reconnection rate as a function of the opening angle made by the upstream magnetic fields, finding a maximum reconnection rate close to 0.2. The predictions compare favorably to particle-in-cell simulations of relativistic electron-positron and non-relativistic electron-proton reconnection. The fact that simulated reconnection rates are close to the predicted maximum suggests reconnection proceeds near the most efficient state allowed at the MHD-scale. The rate near the maximum is relatively insensitive to the opening angle, potentially explaining why reconnection has a similar fast rate in differing models.