The Conditions for the Onset of Magnetic Reconnection in an Asymmetric Current Sheet

Magnetic reconnection in the dayside magnetopause region is of immense interest, as various spacecraft missions over the years have provided direct evidence. Data from these missions, traversing the asymmetric magnetopause, reveal regions both with and without magnetic reconnection exhausts, raising questions about the onset conditions of asymmetric magnetic reconnection. Numerical studies of magnetic reconnection generically employ perturbation to an equilibrium, in which reconnection almost always occurs. However, there is ample observational evidence that the plasma beta plays an important role in the onset of reconnection. In fact, recent studies show that when considering an initially non-equilibrium current sheet that pinches down to smaller scales, the plasma beta acts as a controller of the final thickness of the current sheet, which, if reduced to kinetic scales, may spontaneously trigger reconnection [1]. Here we extend this concept to asymmetric reconnection by considering an initially non-equilibrium asymmetric current sheet. In the presence of a driver such as the solar wind, it is shown that the plasma beta may indeed be a controller of the degree of pinching of the asymmetric current sheet and thus controls the onset of reconnection. This idea is tested with a series of particle-in-cell simulations.

[1] Y. D. Yoon, T. E. Moore, D. E. Wendel, M. Laishram, G. S. Yun, Geophys. Res. Lett. 51, e2024GL112126 (2024)