Characteristics of plasma flows at the inner edge of the plasma sheet

摘要

All known types of auroral zone magnetic activity are associated with closure of open magnetic flux in the magnetotail. As closure is caused by magnetic reconnection we expect to observe fast flows during geomagnetic activity. We have scanned the ion flow data during the first pass of the THEMIS D spacecraft through the tail (December 2007 to May 2008), identifying all flows with |V_(⊥X)|> 150 km/s. These flows generally occur in a sequence of several short bursts (bursty bulk flows). Earthward flows are much more common than tailward flows and are faster than tailward flows. Earthward flows have a longer duration; tailward flows are seen alone or after an earthward flow. Both directions of flow are associated with an increase in tail B_z, (dipolarization). Fast flows in either. direction are rarely seen inside of 9 R_E. Earthward flows are strongly localized in the local time sector 2100-0100 and have a probability distribution identical to that seen in auroral substorm expansions by the IMAGE spacecraft. Tailward flows are also localized but with a peak shifted to 2330 LT. Very close to midnight the flows are slowed and reflected. At other local times they appear to be deflected around the Earth. Fast flows often follow a reduction in E, (GSM VB_s) and occur close to the time of a sudden decrease in the AL index. Generally, the first flow burst in a sequence is most closely associated with the AL onset, and its peak follows the AL onset by about 2 min. The probability of observing a fast flow at THEMIS D during steady magnetospheric convection (SMC) events is quite low compared with the probability during an interval before the SMC. Since most of the fast flows carry magnetic flux earthward and are associated with substorm onset seen in the aurora by IMAGE and in the AL index, we interpret them as evidence that magnetic reconnection has occurred in the tail. Near 30 RE in the tail plasmoid ejection has also been associated with substorm onset, so we conclude that the fast flows are created by a new X line formed outside the 11.9 apogee of THEMIS D some time earlier than they are seen at THEMIS D. During SMC it appears that fast flows due to reconnection are deflected around the Earth outside the apogee of the satellite.