Large-scale geomagnetic effects of May 4, 1998

We study large-scale magnetospheric disturbances elicited by the May4, 1998 high speed stream by modeling the Dst and studying records from 4 meridional magnetometer chains covering key local time sectors. The quasi-sequential episodes of Bz < < 0 and high dynamic pressure (10–50 nPa) allow a clean separation of their respective geoeffects. Ring current evolution is followed by the kinetic model of Jordanova et al. (1998), which includes both charge exchange and Coulomb collisions of ring current ions H+, He+ and O+ drifting in a Volland-Stern convection electric field. The overall agreement with the temporal variation of the Dst is very good, but the strength of the great storm (min Dst = -280 nT) with its rapid main phase is not reproduced fully. A very asymmetric ring current forms near minimum Dst with maximum energy density located at dusk for all ion species. The data show evidence of (a) a great geomagnetic storm; (b) large enhancements of magnetopause currents; (c) substorm onsets, some of which were triggered; (d) a convection reversal boundary at relatively low latitudes (60–65°); and (e) what might be omega bands at morning local times associated with substorm recovery. An unprecedented measurement at Halley Bay station of an approximately 10% change in the ambient magnetic field strength is related to a sharp 5-fold increase in the dynamic pressure and to a large (≈50 nT) variation in IMF Bz

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