Tail dynamics during the growth phase of the 24 November 1996, substorm event: Near‐Earth reconnection confined in the plasma sheet

Abstract

The present study examines a substorm event on 24 November 1996, with an emphasis on magnetic field and plasma flow measurements in the magnetotail during the growth phase. The unique and most intriguing feature of this event is a systematic change of the flow direction from earthward to tailward observed by the Geotail satellite at X ∼ −21 RE. Good correlation between VX and BX is found for the intervals of both earthward and tailward flows, which suggests that observed variations of the flow velocity were caused by satellite motion relative to the plasma sheet rather than the temporal change of the velocity itself. It is inferred that the earthward and tailward flows lasted at least 9 and 5 min, respectively. The signs of BZ and BY variations also changed systematically along with the sign of VX. These results are consistent with the idea that the observed flow reversal was caused by the earthward movement of a neutral line, which might be formed at a preceding pseudobreakup or even earlier. It is inferred that the associated reconnection did not reach the lobe magnetic field, and that the neutral line is convected toward the Earth along with the background plasma. Interestingly, during the interval of the flow reversal, other satellites observed typical growth‐phase features. The geosynchronous magnetic field continued to be stretched in the midnight sector. The Interball and IMP8 satellites observed a continuous increase of the lobe field strength at X = −20 and −34 RE, respectively, in the premidnight sector. It is therefore concluded that the tail current intensification during the growth phase is a global process, whereas the near‐Earth reconnection is a local process, which, at its early stage, can proceed independently from the overall storage of the tail magnetic energy.