Electromagnetics of substorm onsets in the near‐geosynchronous plasma sheet

Abstract

A search of the CRRES database identified 20 events in which the satellite was located within the local‐time sector spanned by the substorm current wedge (SCW) as it formed. Poynting vectors for low‐frequency waves are derived from the electric and magnetic field measurements. In 19 of the events, data are inconsistent with the notion that the SCW initiates from the braking of earthward bulk flows emanating from a near‐Earth X line. Rather, the data support drift‐Alfvén ballooning in the near‐geosynchronous plasma sheet as being responsible for initiation of the SCW and substorm onset. Dipolarization at CRRES is preceded by eastward excursions of the electric field (trigger waves), at which time the first significant electromagnetic energy is observed flowing toward the ionosphere. Dipolarization and the SCW appear before ground onset, following one or more of these trigger waves. The so‐called “explosive growth phase” occurs in association with explosive growth of the trigger waves soon after onset. Seven characteristic features of substorm onsets and expansions observed at CRRES are described. Among these are two stages of expansion. The first expansion stage is initiated by the trigger waves (ballooning) in the near‐geosynchronous plasma sheet. Approximately 10 minutes later a second stage begins consistent with the arrival of earthward bulk flows emanating from a near‐Earth X line. Near‐geosynchronous substorm onsets can explain the observed increase in the occurrence rate of fast bulk flows earthward of its minimum value near X = −12 RE. Drift‐Alfvén ballooning also provides a possible causal link between observed reductions of the solar wind driver and substorm onsets.