DYNAMICS OF THE INNER MAGNETOSPHERE NEAR TIMES OF SUBSTORM ONSETS

The electrodynamics of the inner magnetosphere near times of substorm onsets have been investigated using CRRES measurements of magnetic and electric fields, energetic electron fluxes, in conjunction with groun d-based observations. Six events were studied in detail, spanning the 2100 to 0000 MLT sector and L values from 5 to 7. In each case the daw n-dusk electric field was enhanced over typical background electric fi elds, and significant, low-frequency pulsation activity was observed. The amplitudes of the pulsations were larger than the background elect ric fields. Dusk-dawn excursions of the cross-tail electric field ofte n correlated with changes in currents and particle energies at CRRES a nd with ULF wave activity observed on the ground. Variations of the el ectric field and Poynting vectors with periods in the Pi 2 range are c onsistent with bouncing Alfven waves that provide electromagnetic comm unication between the ionosphere and plasma sheet. Magnetic signatures of field-aligned current filaments directed away from the ionosphere, presumably associated with the substorm current wedge, were observed during three orbits. In all cases, ground signatures of substorm expan sion were observed at least 5 min before the injection of electrons at CRRES. Field-aligned fluxes of counter-streaming, low-energy electron s were detected after three of the injections. We develop an empirical scenario for substorm onset. The process grows from ripples at the in ner edge of the plasma sheet associated with dusk-dawn excursions of t he electric field, prior to the beginning of dipolarization. Energy de rived from the braking of the inward plasma convection flows into the ionosphere in the form of Poynting flux. Subsequently reflected Poynti ng flux plays a crucial role in the magnetosphere-ionosphere coupling. Substorms develop when significant energy (positive feedback?) hows i n both directions, with the second cycle stronger than the initial. Ps eudobreakups occur when energy flow in both directions is weak (negati ve feedback?). ''Explosive-growth-phase'' signatures occur after onset , early in the substorm expansion phase. Heated electrons arrive at th e spacecraft while convection is earthward, during or at the end of el ectromagnetic energy flow away from the ionosphere.