GEOEFFECTIVENESS OF 3 WIND MAGNETIC CLOUDS - A COMPARATIVE-STUDY

We compare the large-scale goemagnetic response to the three magnetic clouds observed by Wind in October 1995 (OCT95), May 1996 (MAY96), and January 1997 (JAN97), studying specifically storm and substorm activi ty, and other global effects due to untypically large and variable sol ar wind dynamic pressures. Since the temporal profiles of the interpla netary parameters of the three clouds resemble one another closely, th e comparison is meaningful. Using the integrated Poynting flux into th e magnetosphere as a rough measure of energy input into the magnetosph ere, we find relative energy inputs to be OCT95:JAN97:MAY96 = 22:11:4, with most of the accumulation in the 3-day periods occurring during p assage of the B-z < 0 cloud phase. The peak Dst ring current indices, corrected for magnetopause currents, were in the ratio -138:-87:-38, a nd hence OCT95 caused a major, JAN97 a moderate, and MAY96 a weak stor m. The empirical criterion derived from studies near solar maximum tha t a solar wind dawn-dusk electric field greater than or equal to 5 m V m(-1) lasting for at least 3 hours is necessary and sufficient to gen erate major storms does not hold for JAN97. Storm main phase onset coi ncides with cloud arrival in all three cases. The number of substorm o nsets during the cloud periods were OCT95:JAN97:MAY96 = 5:3:2, with pe ak AL values in the ratio -1180:-1750:570. The dayside magnetosphere w as variably compressed, the largest amplitude of variation being on JA N97, where the dynamic pressure change spanned 2 orders of magnitude. MAY96 showed the least variation. The interaction of the individual cl ouds with the faster trailing flows had two major effects on the magne tosphere: (1) a compression of the cavity during passage of the B-z > 0 cloud phase and the leading edge of the fast stream; and (2) a weake ning of the control of the cloud field on magnetosheath flow during th e B-z > 0 cloud phase. In summary we find that under most of the aspec ts considered, OCT95 is the most geoeffective. The buffetting of the m agnetospheric cavity by dynamic pressure changes was, however, stronge st on JAN97. The profound differences in the magnetospheric response e licited by the clouds is found to be due to the amplitude, duration an d rapidity of change of the relevant interplanetary parameters. At pre sent, interplanetary monitors are indispensable for understanding the geomagnetic response to interplanetary structures.