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 By0 normalized by B0 (B0 is the initial intensity of Bx field at the top and bottom boundaries of the simulation domain) using a Hall magnetohydrodynamic (MHD) code. For the case with By0 = 0 the profiles of the out‐of‐plane By component are the bipolar signature or the bipolar wavelike signature which is caused by Hall effect and independent of the external mechanism. Such By features are in line with the observed signature of a closed‐loop‐like plasmoid in the magnetotail. The bipolar and fluctuation signatures of By have an asymmetric feature in the presence of a small By0(= 0.1), and the By profile becomes a positive signature as By0 reaches or exceeds 0.3. In the case of By0 = 0.5, a By bulge appears in the By signature when the enhanced By regions caused by Hall effect take place in the plasmoid. The By bulge evolves into a peaking signature, whose maximum (By∣max) is quickly raised and approaches the lobe magnetic field strength. Such a significant enhancement of the By 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 By 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 By flux added by the plasma inflow makes a most important contribution to the growth of the core By field.