Cross Meridian Currents in Storms and Sawtooth Events
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Automated Forward Modeling (AFM) is an inversion technique based on magnetic data alone, which can indicate physical parameters associated with electrojets and field-aligned currents. From perturbations along a meridian, the total electric current crossing the meridian may be determined, as well as the latitudes between which it flowed. The technique is based on nonlinear optimization of the parameters of a forward model. It is possible to compare model output to the original input to ensure that the routine has functioned well and that output parameters are reliable and presumably have physical meaning. Characteristic behaviors of substorms are readily seen in modeling output: the current strengthens rapidly and considerably at an expansive phase onset, following a growth phase during which the electrojet borders move equatorward, usually with some strengthening of current. At onset the poleward border is often seen to move poleward rapidly. Poleward border activity may be noted then and also at other times. After an onset, the recovery phase is often considered to be marked by a retreat of the equatorward border, taken to indicate shrinkage of the auroral oval then. When applied to an ensemble of substorms, average characteristics can be determined, while there is considerable variation among the substorms. The current rises for approximately 30 minutes, but poleward border expansion progresses slightly faster. Recovery is accompanied by a current decrease, but, surprisingly, no poleward retreat of the auroral oval on up to a two-hour timescale. Average characteristics of the current closely follow those of the AL index, while in individual events there are many discrepancies, at least partly due to the non-locality of the index. Electrojet boundary motion is similar to that deduced for the electron aurora from satellite studies. Complete cycles of activity are absent in sawtooth events. Our output parameters can be diagnostic of onsets and useful in determining their location and role in sawtooth events. These have many of the characteristics of expansive phase onsets, but maximum poleward expansion of the poleward border is followed by equatorward movement reminiscent of a growth phase. Since this is often correlated with the interplanetary magnetic field remaining southward, the difference from common expansive phase phenomenology may simply be the lack of a recovery phase. In addition, we find that the extent of the electrojets in latitude is very large in sawtooth events, something which the AE-type indices cannot reveal. This large latitudinal width is accompanied by currents which greatly exceed those of typical substorms.