Hall MHD simulations of collisionless multiple X line reconnection

A Hall MHD simulation of multiple X line reconnection, in which a diffusion region around the standing X line and the moving plasmoids are involved, is presented. The observed features of Hall magnetic and electric fields in the diffusion region and the direction reversal of the electron flow in the vicinity of the magnetic separatrices are displayed in case 1 with the guide field B_y0 = 0. During the passage of a tailward moving plasmoid the bipolar B_y waveform signatures caused by the Hall current and the Hall electric field signatures in case 1 are qualitatively in line with the observations from the Geotail and Cluster spacecraft. Case 2 with B_y0 = 0.15 is carried out to investigate the reconnection configuration on the earthward side of the X line. The peaking signature of the positive B _y correlated with the bipolar (-/+) B_z is observed while an earthward moving plasmoid passes though the given point below the current sheet and close to the midplane; but the negative B_y is recorded at the given point above the current sheet. Such positive and negative B _y signatures, which are comparable with the observations from Cluster 3 and other Cluster spacecraft, are attributed to the B_y asymmetric structures associated with the Hall effect. The enhancement and variation of the electric field detected by Cluster 3 might be related to the -V′ × B electric field associated with the earthward movement of the X line.