A two-dimensional hybrid code is used to simulate the midnight meridian plane of the magnetosphere. The simulation domain extends from the Earth's ionosphere to 35 Earth radii in the antisunward direction and to 11 Earth radii along the polar axes. The simulation is driven by a dawn-to-dusk electric field imposed at the boundaries. The sequence of events follows those associated with the growth and expansive phases of the substorm.

Momentum exchange between the plasma being accelerated earthward in the neutral sheet and the magnetic field results in a stretching of the magnetic field lines and thinning of the plasma sheet. A pair of field-aligned currents develops connecting the inner edge of the plasma sheet to the auroral ionosphere, with the downward current equatorward of the upward current. The breaking of the inward plasma flow drives these currents. Next follows the apparent dipolarization. "Dipolarization" is carried outward by plasma rebounding off the dipole field. Ion-ion two streaming instabilities are excited behind the expanding dipolarization front. The effect of these instabilities is propagated earthward along magnetic field lines by shear Alfvén waves. These are seen as multiple field-aligned current filaments above the auroral ionosphere. We take the upward field-aligned current filaments as proxies for auroral arcs.

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