On the Value of the Reconnection Rate

TL;DR

This paper reviews why the reconnection rate in collisionless regimes often stabilizes around 0.1 times the Alfvén speed times magnetic field, emphasizing the role of the ion diffusion region length.

Contribution

It provides an overview of the conditions leading to the universal reconnection rate value and highlights the importance of the ion diffusion region length in this context.

Findings

The reconnection rate often reaches about 0.1 v_A B_u in simulations.

The value is linked to the ion diffusion region length.

Understanding the ion diffusion region is key to explaining the reconnection rate.

Abstract

Numerical simulations have consistently shown that the reconnection rate in certain collisionless regimes can be fast, on the order of $0.1 v_A B_u$, where $v_A$ and $B_u$ are the Alfv\'en speed and the reconnecting magnetic field upstream of the ion diffusion region. This particular value has been reported in myriad numerical simulations under disparate conditions. However, despite decades of research, the reasons underpinning this specific value remain mysterious. Here, we present an overview of this problem and discuss the conditions under which the "0.1 value" is attained. Furthermore, we explain why this problem should be interpreted in terms of the ion diffusion region length.