Comparison of secondary islands in collisional reconnection to Hall reconnection

TL;DR

This study uses large-scale resistive Hall-MHD simulations to distinguish the effects of secondary islands from collisionless effects in magnetic reconnection, revealing their different roles and implications for energy release.

Contribution

It is the first to separate secondary island effects from collisionless effects in large-scale Hall-MHD simulations of magnetic reconnection.

Findings

Secondary islands occur without collisionless effects when dissipation regions exceed ion gyroscales.

Reconnection with secondary islands is faster than Sweet-Parker but slower than Hall reconnection.

Hall reconnection triggers the ejection of secondary islands and accounts for most energy release.

Abstract

Large-scale resistive Hall-magnetohydrodynamic (Hall-MHD) simulations of the transition from Sweet-Parker (collisional) to Hall (collisionless) magnetic reconnection are presented, the first to separate effects of secondary islands from collisionless effects. Three main results are described. There exists a regime in which secondary islands occur without collisionless effects when the thickness of the dissipation regions exceed ion gyroscales. The reconnection rate with secondary islands is faster than Sweet-Parker but significantly slower than Hall reconnection. This implies that secondary islands are not the cause of the fastest reconnection rates. Because Hall reconnection is much faster, its onset causes the ejection of secondary islands from the vicinity of the X-line. These results imply that most of the energy release occurs during Hall reconnection. Coronal applications are discussed.