Plasma physics of magnetic island coalescence during magnetic reconnection

Abstract

A large-scale two-dimensional electromagnetic particle-in-cell simulation was employed to study the magnetic island coalescence/merging process during magnetic reconnection with guide field. The merged island after coalescence is characterized by strong core field and plasma density dip at the island center. We found 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. There is an in-plane electric current loop, which is mainly carried by electrons, circumventing the enhanced core field region. 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 toward 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 antiparallel 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.

Tripolar electric field Structure in guide field magnetic reconnection

“…J x reaches its peak around B x = 0 without a guide field, while it moves toward the edge of the current sheet in the guide field case. This is consistent with the result of Zhou et al (2014) that the outflow electron jet is deflected away from the neutral sheet as a result of the in-plane Lorentz force. The profile of B y also varies significantly because of the superposition of the guide field.…”

“…The Hall electromagnetic fields are asymmetric across the current sheet. Zhou et al (2014) identified a super-Alfvénic electron jet extended to 30 s ion inertial length away from the X line in a reconnection region with a weak guide field. This jet was deflected from the neutral sheet owing to the Lorentz force j L × B g ; here, j L is the electric current in the outflow direction and B g is the guide field.…”

Magnetic Reconnection at a Thin Current Sheet Separating Two Interlaced Flux Tubes at the Earth's Magnetopause

“…Their typical signatures are an enhancement of the total magnetic field strength and a magnetic bipolar signature (Teh et al, ). In addition, plasma density dips have been reported at their center (Zhou et al, ). The core region is bounded by an electric current loop mainly carried by electrons (Zhou et al, ).…”

Coalescence of magnetic flux ropes observed in the tailward high‐speed flows

“…Generally, the plasma density is significantly enhanced in the center of flux ropes [e.g., Khurana et al ., ; Chen et al ., ; Wang et al ., ]. Thus, the occasionally observed density dip within a flux rope [ Retinò et al ., ; and Wang et al ., ] is supposed to be caused by two coalescing flux ropes, as shown in the simulations [ Retinò et al ., ; Zhou et al ., ]. However, if this dip is due to the coalescence, its spatial scale would be thereby comparable to the scales of the flux rope.…”

“…Therefore, the magnetic field lines of the two flux ropes can reconnect. Although the coalescence of magnetic flux ropes has been extensively studied by both theories and simulations [ Finn and Kaw , ; Biskamp and Welter , ; Pritchett , ; Oka et al ., ; Zhou et al ., ; Cazzola et al ., ], in situ evidence has not been reported until recently by Wang et al . [].…”