Power to the magnetosphere: May 4, 1998

An extraordinary powering of the magnetosphere by the solar wind occurred in a 3-hour burst early on May 4 when the IMF was very intense and pointed south (≈-35 nT; “erosion phase”). Examining solar wind streams over 3 months, we found that May 4 represented a very fast, hot, non-corotating stream overtaking an interplanetary coronal mass ejection (ICME), thus forming a compound stream. By integrating the “epsilon” parameter over time, we find that the energy deposited in the magnetosphere during the erosion phase on May 4 (of order 7.5 J m−2) was higher to that deposited during the previous 3-day period, itself a very geoeffective interval. We compare the energy and power supply to the magnetosphere on May 4 with 13 other events, mainly ICMEs and magnetic clouds, during the period 1995–2000. Specifically, we examine (a) the total energy input over 3 days, and (b) the average power over a 3-hour period near maximum power of the respective configurations. As regards (a), we find the energy of the May 4 stream to be comparable to that of the strong events observed during the 6-year period. As regards (b), we find May 4 to represent a large fluctuation from the norm, exceeded only by the Bastille Day event (July 15, 2000). The ability to predict a concentration of electromagnetic power and energy such as that in the May 4 fast stream poses a challenge to our ability to predict space weather.