A statistical study of the relation of Pi 2 and plasma flows in the tail

In this paper, we examine some of the statistical properties of Pi 2 pulsations and their relation to plasma flow bursts in the tail. These statistics are based on a set of 462 expansion onsets identified by the AL index and Pi 2 pulsations. The most probable number of Pi 2 bursts per substorm is 2, but some substorms have as many as 6. The first Pi 2 burst corresponds to the expansion onset about 58% of the time, but 28% of the time, there is a pseudo‐breakup prior to the main onset. When there are multiple Pi 2 bursts per substorm, the time between bursts ranges from 5 to 50 min with 20 min as the mean separation. Multiple Pi 2 bursts are more probable in the expansion and recovery phase than in the growth phase. We have selected a subset of 48 events for which International Sun Earth Explorer 2 (ISEE‐2) was in the region of expected x‐line formation at the time of onset. Plasma flows were observed in 32 of these events. In this subset, 26 events appear to be bursty bulk flows (BBFs), while the remaining 7 events were probably ion streaming in the plasma sheet boundary layer. Thus about one half (26/48) of all Pi 2 events observed when ISEE‐2 was in the region of x‐line formation could be associated with BBFs inside the plasma sheet. An examination of the time separation between BBFs reveals a repetitive distribution which is similar to that between Pi 2 bursts. The slight discrepancy between these two distributions may be due to the localization of the plasma flows which causes some BBFs to be missed, and to the definition of BBFs which does not allow an event shorter than 10 min. The frequent association of BBF and Pi 2 and their similar distributions of time separation suggest that they are causally related. If so, the observed properties of Pi 2 pulsations suggest that most substorms are made up of a sequence of transient reconnection events.