Comparison of secondary islands in collisional reconnection to Hall reconnection

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.