By C. Farrugia
In the early hours of May 4, 1998, between 0230 UT and 0520 UT, the leading edge of a high speed stream powered the magnetosphere at an estimated 0.8mW/m^2 and dumped a total energy of about 7J/m^2 in less than 3 hours. It caused the largest geomagnetic storm in the last four years (Dst < -200 nT) with a Kp index peaking at 8. To better appreciate the uniqueness of this event in studies of solar wind-magnetosphere coupling, we first discuss transient and corotating aspects of the interplanetary observations made by the Wind spacecraft on May 1-4, 1998. We then compare the power and energy inputs into the magnetosphere with those resulting from other well-known and strongly geoeffective interplanetary configurations seen at different phases of the solar cycle: (a) the 2-day-long, expanding magnetic cloud ahead of this high speed stream; (b) October 18-20, 1995, itself the strongest event near the last solar minimum; (c) January 13-15, 1988; and (4) September 29-30, 1978. Some unusual geomagnetic effects to be looked for will be mentioned.