Magnetic Island Dynamics and Energy Dissipation in Multiple X-line Reconnection

Abstract:

Magnetic islands are intrinsic products of multiple X-line magnetic reconnection. They are closely related to the energy dissipation in reconnection, such as the electron energization. Generally there are three type islands: the primary island formed between two preexisting X-lines; the secondary island formed as a result of tearing instability in an elongated thin current sheet; merged island due to island coalescence between two islands. In this study, we performed a series large-scale two-dimensional electromagnetic Particle-In-Cell simulation to study the magnetic island dynamics and energy dissipation during multiple X-line reconnection.

The merged island after coalescence is characterized by strong core field and plasma density dip at the island center. We find that the core field enhancement is caused by the out-of-plane magnetic field pileup, as well as the field line twisting due to Hall effect. Total force points away or tangentially to the surface of density dip within the merged island, which prevents electrons from higher density region entering the lower density region between two merging islands. This is contrary to the force in the secondary island, inside which the total force points towards the island center and constrains plasma there. The coalescence process involves a reconnection at the merging sheet between two islands. Electron frozen-in condition is violated locally along the merging sheet. It is contributed by both the divergence of electron pressure tensor and electron inertial. Energy dissipation is concentrated on the merging line during coalescence, while a train of dynamo (j’•E’<0) and dissipation (j’•E’>0) regions are distributed along the merging sheet after coalescence. Electrons and ions within the density dip at the island center are accelerated anti-parallel to the ambient magnetic field, i.e., along the out-of-plane direction. The resultant distribution can excite the Buneman instability and two-streaming instability, which probably account for the electrostatic solitary waves observed by satellite. The energy and energy flux partition among the different types of islands are also discussed.