A global synthesis model of dipolarization at substorm expansion onset

In this paper, we describe a global synthesis dipolarization model combinin g coupled processes in the midtail, inner tail and auroral ionosphere: In t he late growth phase, magnetic reconnection releases the magnetic energy st ored in the magnetotail. Magnetic flux and energy are transported earthward and tailward. As earthward flow slows down in the near-earth plasma sheet (NEPS), it compresses the magnetic field acid plasmas near and earthward of the inner edge of the NEPS and pushes them further inward (earthward and e quatorward). This sets up a favorable condition for generating the drift ba llooning mode (DBM) instability in the inner tail. The unstable DBMs genera te coupled Alfven-slow magnetosonic waves and field-aligned currents (FACs) , resulting in a turbulent state in the equatorial region and enhancing the ionospheric conductance Sigma. As soon as Sigma and FACs increase to a thr eshold level, the substorm current wedge is formed, leading to an explosive intensification of the auroral electrojet and magnetic field dipolarizatio n at substorm onset. Moreover, we regard the "substorm trigger phase" (Ohta ni et al., Planet. Space Sci. 37 (1989) 579-588) as the interval during whi ch the inner tail is being further compressed inward and the DBMs explosive ly develop to trigger magnetic field dipolarization. We suggest that the da wn-dusk electric field E-y which causes further compression of the inner ta il may either be associated with flow braking, or produced by storm SSC and other magnetospheric processes. The present model is thus applicable to th e cases either with or without neutral-line formation. Furthermore, in the former case, the enhanced E-y in the inner tail may either appear somewhat later than or simultaneously with magnetic reconnection in the midtail. It seems that a variety of expansion onset features can be explained in terms of this synthesis dipolarization model. (C) 2001 Elsevier Science Ltd. All rights reserved.