Earthward expansion of tail current disruption: Dual-satellite study

The present paper addresses the spatial development of tail current disruption in the near-Earth tail by making use of magnetic field measurements made by two satellites located close to each other. The paper consists of two parts: a statistical study and a case study of a substorm event that took place on August 30, 1986. In the first part the relative timing of the onsets of tail magnetic field reconfiguration (dipolarization) at the AMPTE/CCE (r < 8.8 R-E) and GOES geosynchronous satellites is statistically examined. The result shows that the tail current disruption region expands azimuthally outside geosynchronous orbit. Another, and most unexpected, finding is that the tail current disruption region often expands earthward. The substorm event of August 30, 1986, observed by the AMPTE/CCE and SCATHA satellites provides a good opportunity to examine the earthward expansion of the tail current disruption region. In this event, AMPTE/CCE was located on the earthward side of SCATHA by 0.4 R-E and observed the onset of magnetic fluctuations 15 s after the SCATHA onset. The earthward expansion velocity is estimated at 180 km/s, much slower than the propagation velocity of the fast magnetosonic mode, indicating that the observed earthward expansion is not an effect of the wave propagation. The very initial signature observed by AMPTE/CCE in this event is a decrease in the north-south magnetic component accompanied by the enhancement of the flux of energetic ions having their guiding centers tailward. This result suggests that the tail current is intensified by injected particles at the front of the earthward expanding region of the tail current disruption. This enhanced current may trigger the local disruption of the tail current, making the disruption region expand further earthward.