Signatures of the substorm recovery phase at high‐altitude spacecraft

Abstract

The substorm recovery phase typically commences ∼30 min after the substorm expansion phase onset and covers a period of roughly 1 hour. Several signatures have previously been associated with the recovery phase such as plasma sheet expansion in the midtail and magnetic field return toward the quiet time configuration. However, the detailed temporal sequence during the recovery phase is still not very well established. A total of 66 events in February‐April 1979 have been investigated where ISEE 2 observed a plasma sheet expansion associated with fast earthward flows. Of these, 50 events were clearly associated with the substorm recovery phase as identified in ground magnetic records. For this data set, energetic electron (>30 keV) and proton (>145 keV) observations from two geostationary spacecraft were available in 41 cases. Of the 41 cases, 32 of the midtail plasma sheet recoveries were associated with distinctive ion or electron flux increases at geostationary orbit. These flux increases, often in both protons and electrons, were generally observed from the predusk to the postdawn sector. However, very few enhancements were found near local noon. The lack of large energy dispersion in the flux increases and the simultaneous occurrence of both electron and ion enhancements suggests that the particles do not drift from a more distant location (as in substorm expansion phase onsets) but are accelerated or redistributed locally. These events are suggested to be associated with a large‐scale reconfiguration of the near‐Earth tail as the neutral line retreats to large distances.