Comparison of UV optical signatures with the substorm current wedge as predicted by an inversion algorithm

Optical images of the auroral bulge as seen by the Viking UV imager were compared in several cases with the substorm current wedge (SCW) upward and downward field-aligned currents (FAG) whose positions were determined using the inversion algorithm based on the substorm-related magnetic variations observed at midlatitudes. With reasonable accuracy (better than 0.5 hours MLT) the estimated longitudes of the upward FAC generally pointed to the surge or to the brightest luminousity region in the western half of the bulge. The latter feature may imply a more complicated structure of the net FACs than can be described by the simple substorm current wedge scheme. Similarly, the estimated positions of the downward FAC pointed close to the eastern termination of the bulge. The associated optical signatures of this current system ranged from the well-defined emission depletion regions to new auroral intensifications. The downward current appears to correspond in some cases at least with the division between the morning sector portion of the double oval and the nightside portion connected more directly to the substorm bulge. The results in general confirm the expected association between the auroral bulge and the SCW, as well as showing a reasonably good results from the inversion algorithm based on midlatitude magnetic observations. Our results, however, also indicate that one must be careful in interpreting the apparent motion of SCW-related field-aligned currents inferred from midlatitude observations in terms of a true westward or eastward expansion of the SCW or of the auroral bulge. The observed changes may instead sometimes be related to the redistribution of the net FACs within than a shift or expansion of the simple current system.