Substorm onset viewed by a two-dimensional, global-scale hybrid code

A two-dimensional hybrid code is used to simulate processes related to the onset of the substorm expansive phase that take place in the midnight merid ian plane of the magnetosphere. The simulation domain extends from the Eart h's ionosphere to 35 Earth radii in the antisunward direction and to 11 Ear th radii along the polar axes. Simulation runs were made both with and with out a driving dawn-to-dusk electric field imposed at the boundaries. In the driven cases momentum exchange between the plasma being accelerated earthw ard in the neutral sheet and the magnetic field results in a stretching of the magnetic field lines and thinning of the plasma sheet, In the undriven cases, the induction electric field from the collapsing tail field also acc elerates plasma sheet plasma earthward. In both cases a pair of field-align ed currents develops connecting the inner edge of the plasma sheet to the a uroral ionosphere, with the downward current equatorward of the upward curr ent. The breaking of the inward plasma flow drives these currents. Next fol lows the apparent dipolarization. "Dipolarization" is carried outward by pl asma rebounding off the dipole field. Ion-ion two streaming instabilities a re excited behind the expanding dipolarization front. The effect of these i nstabilities is propagated earthward along magnetic field lines by shear Al fven waves. These are seen as multiple field-aligned current filaments abov e the auroral ionosphere. We take the upward field-aligned current filament s as proxies for amoral arcs. Results of the simulations indicate substorm effects to be a consequence of the near-Earth breaking fast earthward flows , which develop in the plasma sheet. (C) 2001 Elsevier Science Ltd. All rig hts reserved.