Substorm development as observed by Interball UV imager and 2-D magnetic array

Results of the study of two substorms from Interball auroral UV measurement s and two-dimensional patterns of equivalent ionospheric currents derived f rom the MACCS/CANOPUS and Greenland magnetometer arrays are presented. Subs torm development in 2-D equivalent ionospheric current patterns may be desc ribed in terms of the formation of two vortices in the equivalent currents: a morning vortex related to downward field-aligned current and an evening vortex related to upward field-aligned current. Poleward propagation of the magnetic disturbances during substorm expansive phase was found to be asso ciated mainly with a poleward displacement of the morning vortex, whereas t he evening vortex remained approximately at the same position. As a result, the initial quasi-azimuthal separation of the vortices was replaced by the ir quasi-meridional separation at substorm maximum. Interball UV images dur ing this period showed the formation of a bright auroral border at the pole ward edge of substorm auroral bulge. The auroral UV images showed also that the auroral distribution in the region between the polar border and the ma in auroral oval tends to have a form of bubbles or petals growing from a br ight protuberant region on the equator-ward boundary of the auroral oval. H owever, the resolution of the UV imager was not sufficient for the reliable separation of such the structures, therefore, this result should be consid ered as preliminary. Overlapping of the auroral UV images onto equivalent c urrent patterns shows that the bright substorm surge was well collocated wi th the evening vortex whereas the poleward auroral border did not coincide with any evident feature in equivalent ionospheric currents and was located several degrees equator-ward of the morning current vortex center related to downward field-aligned current. The ground-based magnetic array allowing us to obtain instantaneous: patterns of equivalent ionospheric currents gi ves a possibility to propose a new index for substorm activity such as the magnitude of the total cui rent between the centers of the morning and even ing vortices. Such integral index would not depend on where the substorm is located and be unaffected by the migration of substorm activity poleward o r equatorward. (C) 2001 Published by Elsevier Science Ltd.