Tail current surge: New insights from a global MHD simulation and comparison with satellite observations

Abstract

The present study examines the tailward propagation of substorm-associated variations of the tail current intensity. In the substorm event of 24 November 1996, the Interball and IMP 8 satellites were located in the midnight sector at X= -26 and -36 RE, respectively, and observed an increase and a decrease of the lobe magnetic field strength corresponding to the storage and release of the lobe magnetic energy. Both spacecraft observed B2 to decrease initially and then increase in the course of the decrease in |B X|, a feature that was reported previously as a manifestation of the tailward expansion of the current disruption region. The delay of the signatures between the two satellites confirms that the associated current system moved tailward. Motivated by this fortuitous coordination of the satellite observation, the present study revisits a global MHD simulation previously conducted specifically for this substorm event [Raeder et al., 2001]. The most noticeable feature of the modeled tail dynamics is the repeated occurrence of tail current surges, that is, temporal intensifications of the tail current that propagate tailward. The first tail current surge is accompanied by the stretching of the tail magnetic field, which starts in the inner magnetosphere and extends tailward. The associated tailward flow redistributes the plasma pressure in such a way that the tail current is reduced in its intensity in the near-Earth region, while the pressure gradient increases at the propagation front, which intensifies the local current. The last major tail current surge is caused by the near-Earth reconnection. Inside a plasmoid, the pressure gradient current is intensified on the tailward side of the 0-line, and it propagates tailward as the plasmoid grows and is released. For each tail current surge, irrespective of its cause, the intensification of the tail current is followed by the reduction, and its tailward propagation creates the aforementioned phase relationship between BX and BZ. It is probably difficult to determine based on lobe magnetic field observations whether it is caused by the tail stretching or a neutral line motion. The present study not only sheds new light about tail substorm dynamics but also provides a good exercise for evaluating the potential of the modeling effort for substorm study in general. Copyright 2004 by the American Geophysical Union.