Electromagnetics of substorm onsets in the near-geosynchronous plasma sheet

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 we dge (SCW) as it formed. Poynting vectors for low-frequency waves are derive d from the electric and magnetic field measurements. In 19 of the events, d ata are inconsistent with the notion that the SCW initiates from the brakin g of earthward bulk flows emanating from a near-Earth X line. Rather, the d ata support drift-Alfven ballooning in the near-geosynchronous plasma sheet as being responsible for initiation of the SCW and substorm onset. Dipolar ization 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 appea r before ground onset, following one or more of these trigger waves. The so -called "explosive growth phase" occurs in association with explosive growt h of the trigger waves soon after onset. Seven characteristic features of s ubstorm 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. Approxi mately 10 minutes later a second stage begins consistent with the arrival o f earthward bulk flows emanating from a near-Earth X line. Near-geosynchron ous substorm onsets can explain the observed increase in the occurrence rat e of fast bulk flows earthward of its minimum value near X = -12 R-E. Drift -Alfven ballooning also provides a possible causal link between observed re ductions of the solar wind driver and substorm onsets.