Unprecedented Power to the Magnetosphere: May 4, 1998

C.J. Farrugia-L.F.Burlaga-V.K. Jordanova-M.P.Freeman-C.C. Cocheci- R.L.Arnoldy-J.D. Scudder-K.W.Ogilvie-R.P.Lepping

We argue that as far as the magnetosphere is concerned, the single most significant feature of May 4, 1998 is the very high level of powering of the magnetosphere realized in a 3-hour burst early in the day when from 0230 to 0540 UT the north-south component of the IMF was about -30 nT. To characterize the interplanetary configurations involved, we examined solar wind streams over 3 months and find that the period May 1-4, 1998 had both corotating and transient features. May 4 represented a very fast (~900 km/s) and hot (2 x 10^6 K) stream overtaking a magnetic cloud (2-3 May) which was itself trailing a corotating steam. The episode of strongly southward-pointing IMF, expected to lead to large-scale dayside erosion, and the episode of high solar wind dynamic pressures (~10-60 nPa) were by and large sequential, permitting a clean separation of their respective effects on the magnetosphere, as discussed in paper 2. 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 0.7 J m^-2) was equal to that deposited during the previous 2-day period, itself a very geoeffective interval. To put the May 4 event into a broader perspective, we compared its energy and power supply to the magnetosphere with 7 other events, all magnetic clouds, spread over 3 solar cycles and encompassing different phases of the solar cycle. Specifically, we examine (a) the total energy input over a 3-day period, and (b) the average power over a 3-hour period near maximum power. As regards (a), we find a clear trend with solar cycle. The energy accumulation on May 4 alone is already equal to that of the strongest events at solar maximum. As regards (b), we find that the power on May 4 exceeds all the others by a wide margin and may represent a large fluctuation from the norm.