Fields and flows at GEOTAIL during a moderate substorm

The behavior of the tail field and plasma during a moderate substorm with onset at ∼1120 UT on Dec. 13, 1994 is examined using data from the ISTP spacecraft GEOTAIL and WIND, synchronous orbit and the ground. Five substorms were observed on this day while GEOTAIL was located near the center of the tail at Xgsm = −46 R e. In each substorm the field and plasma variations were similar to those observed during substorms by spacecraft closer to the earth. A southward turning of the IMF caused accumulation of lobe flux, development of a more tail-like field, and eventually an expansion phase and its consequences. The ∼1120 UT onset immediately followed a northward turning which ended a lengthy interval in which the IMF was alternately northward and southward. Shortly after the onset a flux rope passed GEOTAIL with a delay consistent with its formation at 20–30 R e several minutes earlier than a Pi 2 burst began at midnight. Immediately after the onset the lobe field decreased and the plasma sheet disappeared. During the substorm recovery phase the plasma sheet reappeared with plasma moving earthward. The plasma data show that the tailward flow is a combination of convecting and streaming plasma. All of the substorms exhibited multiple onsets. The main onset of each can be determined by a combination of negative bay onsets, Pi 2 bursts, synchronous field-aligned currents, dispersionless particle injection, and midlatitude positive bays. In one event the flux rope at 46 R e arrived before dispersionless injection at synchronous orbit suggesting that reconnection in the tail begins at or before major onsets. In fact most of the major onsets were preceded by pseudo breakups early in the growth phase, and weak tailward flows carrying a weak vertical field fluctuating about zero. These observations suggest that reconnection begins in the middle tail early in the growth phase and that it is substantially intensified or begins again at another location at the expansion onset.