Large-scale Geomagnetic Disturbances during May 4, 1998

C. J. Farrugia, V. K. Jordanova, M. P. Freeman, C. C Cocheci, R. L. Arnoldy, M. Engebrettson, J. D. Scudder, G. Rostoker, P. Stauning, K. Yumoto, M. Thomsen, and G. Reeves We study large-scale inner-magnetospheric disturbances, concentrating

on (a) modeling the Dst variation and (b) studying the effects of the erosion and compression phases (see paper 1) on May 4, 1998 from 5 meridional chains of magnetometers covering all local times, supplemented by readings as geosynchronous orbit. May 4 substantially enhanced pre-existing Dst activity to produce the largest storm in the preceding four years (min corrected Dst ~ -290 nT). We find it useful in the analysis to separate the effects due to the 'erosion' from the 'compression' phases. After correcting raw Dst values for magnetopause currents, a correction which was often several tens nT, we follow the evolution of the ring current by applying the kinetic model of Jordanova et al. (1996, 1997), which includes both charge exchange and Coulomb collisions on ring current ions H^+ and O^+ drifting in a Volland-Stern convection electric field. Geostationary measurements are used to provide the initial and boundary conditions. The Dst variation over 1-4 May is reproduced well, but the rapidity and peak strength of the short-lived Dst growth on May 4 is somewhat underestimated, suggesting the importance of taking into account the magnetotail current and radial diffusion, not included here. The ground magnetometers captured evidence of (a) large enhancements of magnetopause Chapman - Ferraro currents; (b) substorm onsets, some of which were triggered; (c) a convection reversal boundary at relatively low latitudes; and (d) what might be omega bands associated with substorm recovery. An unprecedented measurement at Halley Bay station of an approximately 10% change in the ambient magnetic field strength is (a) related to a sharp dynamic pressure rise (from 8 to 40 nPa) toward the end of the erosion phase, and (b) followed in the other magnetograms and shown to be propagating tailward.