Interplanetary field and plasma during initial phase of geomagnetic storms

Twenty-three geomagnetic storm events during 1966-70 have been studied by using simultaneous interplanetary magnetic field and plasma parameters. Explorer 33 and 35 field and plasma data have been analyzed on large-scale (hourly) and small-scale (3 min) during the time interval coincident with the initial phase of the geomagnetic storms. The solar-ecliptic Bsub(z) component turns southward at the end of the initial phase, thus triggering the main phase decrease in Dsub(st) geomagnetic field. When the Bsub(z) is already negative, its value continues to remain negative. The Bsub(y) component also shows large fluctuations along with Bsub(z). On the small-scale behavior of the magnetic field and electric field (E=-VxB) studied in details for the three events, it is found that the field fluctuations in Bsub(y) Bsub(z) and Esub(y) and Esub(z) are present in the initial phase. These fluctuations associated with the passage of the shock waves, continue to exist during the initial phase. In the large-scale behavior field remains quiet because the small-scale variations are averaged out. The power spectrum analysis for few events using 5.11 sec data shows the steepening of the spectrum after the passage of the shock wave associated with the ssc. It appears that large as well as small time-scale fluctuations in the interplanetary field and plasma may play a role in altering the internal electromagnetic state of the magnetosphere so that a ring current could start causing a geomagnetic storm decrease. (auth.)