Relationship between solar wind electric field and geomagnetic activity in increasing and decreasing phases of solar wind velocity

Solar wind velocity normally increases to a maximum above 600km/s from a quiet value below 400km/s in a few days and returns to the quiet one in a week. We select 445 time profiles of solar wind velocity data with simple variation in the interval from January 1964 to April 1985. Averaged ΣK_p values at the solar wind velocity of 500km/s are 28 and 19 in the increasing (leading) and decreasing (trailing) phases of solar wind velocity for the above time-profiles, respectively. The maximum ΣK_p was found to occur around the middle in the leading phase, to statistically coincide with a large southward IMF-Bz (≦-4 nT) with duration longer than 5 hours. The K_p rapidly increased with increasing the southward IMF in first 10 hours of the geomagnetic storm on July 22-25,1974. A maximum K_p=7 occurred together with a maximum westward solar wind electric field (VBz) of -6.9mV/m. Correlative variations of AE with VBz continued from 22 UT, July 22 to 11 UT, July 23 when a maximum southward IMF-Bz of -12.4 nT occurred. After 11 UT, July 23,the correlation between VBz and AE became irregular. It is concluded that, for the initial storm phase, the solar wind electric field directly penetrates into the polar magnetosphere along interconnected field lines and produces higher AE indices. After the maximum southward IMF, the magnetic merging could no longer occur at the dayside magnetopause. Rapid AE increases associated with Dst decreases are discussed in terms of the ring current and the cross-tail current. The AE activities after the maximum southward IMF seem to be related with magnetospheric processes which are independent of the solar wind electric field