A global synthesis model of dipolarization at substorm expansion onset

Abstract

In this paper, we describe a global synthesis dipolarization model combining coupled processes in the midtail, inner tail and auroral ionosphere: In the late growth phase, magnetic reconnection releases the magnetic energy stored 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 and plasmas near and earthward of the inner edge of the NEPS and pushes them further inward (earthward and equatorward). This sets up a favorable condition for generating the drift ballooning mode (DBM) instability in the inner tail. The unstable DBMs generate coupled Alfvén-slow magnetosonic waves and field-aligned currents (FACs), resulting in a turbulent state in the equatorial region and enhancing the ionospheric conductance Σ. As soon as Σ and FACs increase to a threshold level, the substorm current wedge is formed, leading to an explosive intensification of the auroral electrojet and magnetic field dipolarization at substorm onset. Moreover, we regard the “substorm trigger phase” (Ohtani et al., Planet. Space Sci. 37 (1989) 579–588) as the interval during which the inner tail is being further compressed inward and the DBMs explosively develop to trigger magnetic field dipolarization. We suggest that the dawn-dusk electric field E y which causes further compression of the inner tail may either be associated with flow braking, or produced by storm SSC and other magnetospheric processes. The present model is thus applicable to the 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.