Dynamics of plasma, energetic particles, and fields near synchronous orbit in the nighttime sector during magnetospheric substorms

We discuss two phases of the substorm‐associated magnetospheric dynamics in terms of the particles and fields at synchronous orbit. The first phase corresponds to the ‘decreases’ of energetic particle flux first identified by Erickson and Winckler (1973) and discussed by Walker et al. (1976) and Erickson et al. (1979). This phase begins one‐half hour to one hour before the substorm onset and is characterized by (1) a distortion of the magnetosphere to a more taillike configuration caused by (2) an intensification and/or motion toward the earth of the cross‐tail current and of its earthward part, the partial ring current, (3) a shift of trapped particle trajectories closer to the earth on the nightside following contours of constant B causing the particle ‘decreases,’ accompanied by a change in the pitch angle distributions from ‘pancake’ to ‘butterfly’ as observed at geostationary orbit, (4) an initiation of a response of the auroral electrojet ( AE ) index. The decreases of energetic particle flux can correspond to the substorm growth phase as defined initially by McPherron (1970) or the growth or precursor phase of Erickson et al. (1979). Plasma motions and currents during decreases tend to be variable, but the description above nevertheless characterizes the large‐scale trend. It is suggested that the electric field induced by the increasing tail current near the earth acts opposite to the cross‐tail convection field and can temporarily inhibit convection near the geostationary orbit. The second phase is the conventional expansion phase that begins with the ‘onset,’ characterized in our study by (1) a sudden decrease in the tail current and a return of the inflated magnetosphere to a dipolelike configuration, (2) a sudden shift of trapped high‐energy particles toward the tail again following contours of constant B , and at the same time (3) a surge of tail plasma toward the earth as the induced electric field now increases the total convection field. Separate effects thus result in the dramatic increases of both high energy and plasma particles seen at substorm expansion phase onset, (4) an AE index response and the appearance of bays at stations near midnight local time accompanied by very active aurora as well as the precipitation of high‐energy particles. The different appearance of the responses at ATS 1 (on the magnetic equator) and ATS 6 (off the magnetic equator) can be well explained by the above description. True diamagnetic effects of the particle population are clearly evident at the ATS 6 region and must be carefully distinguished from the effects of distant currents. The use of oppositely directed detectors on ATS 6 that permit the evaluation of the guiding center particle density gradients has been of great use in this analysis.