Hall current system around the magnetic neutral line in the magnetotail: Statistical study

Abstract

Recent magnetotail observations clarified the ion‐electron decoupling processes for the magnetic reconnection. One of the decoupling processes is the Hall current system generated around the magnetic neutral line. In this paper we study the current structure on the magnetotail meridian plane when Geotail observed a simultaneous reversal of north‐south magnetic field Bz and earthward‐tailward ion flows. The currents are evaluated by examining the cross‐tail magnetic field By′ variation. Two case studies of reversal events indicate that the By′ component varied consistently with the quadrupolar magnetic field structure induced by the Hall current loops. This tendency is confirmed statistically on the basis of all reversal events defined by the simultaneous reversal of Bz and ion bulk velocity Vx and also by the negatively correlated number densities of earthward flow (NEarth) and tailward flow (Ntail). Here NEarth and Ntail are calculated from the ion velocity distribution within earthward and tailward flowing parts of the velocity space, respectively. The number of selected reversal events are 4022 in the region of 0 ≥ XGSM ≥ −50 RE and −20 ≤ YGSM ≤ 20 RE from 1993 to 2001. The mean By′ variation is found to make a quadrupole structure consistent with the Hall generated magnetic field. The amplitude of By′ becomes larger for increasing the magnitude of Bz. It also increases with the electric field Ey component. These characteristics also can be interpreted in terms of the Hall current system created around the neutral line.