Plasmoid‐like structures in multiple X line Hall MHD reconnection

Abstract

Driven by a waveform inflow, multiple X line reconnection is initiated in a long current layer, and the plasmoids are bound between two neighboring reconnection sites. We investigate the behaviors of the plasmoid‐like structures in the absence and presence of an initial guide field B y 0 normalized by B 0 ( B 0 is the initial intensity of B x field at the top and bottom boundaries of the simulation domain) using a Hall magnetohydrodynamic (MHD) code. For the case with B y 0 = 0 the profiles of the out‐of‐plane B y component are the bipolar signature or the bipolar wavelike signature which is caused by Hall effect and independent of the external mechanism. Such B y features are in line with the observed signature of a closed‐loop‐like plasmoid in the magnetotail. The bipolar and fluctuation signatures of B y have an asymmetric feature in the presence of a small B y 0 (= 0.1), and the B y profile becomes a positive signature as B y 0 reaches or exceeds 0.3. In the case of B y 0 = 0.5, a B y bulge appears in the B y signature when the enhanced B y regions caused by Hall effect take place in the plasmoid. The B y bulge evolves into a peaking signature, whose maximum ( B y ∣ max ) is quickly raised and approaches the lobe magnetic field strength. Such a significant enhancement of the B y component in the central region of the plasmoid might be representative of the observed strong core field in the magnetic flux rope. The present results indicate the following implications: (1) Hall effect and a preexisting cross‐tail component B y are two important factors controlling the occurrence of various plasmoid‐like structures in the magnetotail. (2) In the later phase the nonlinear interaction between Hall effect and the B y flux added by the plasma inflow makes a most important contribution to the growth of the core B y field.