GEOMAGNETIC STORM EFFECTS ON ZONAL AND MERIDIONAL EQUATORIAL ELECTROJET CURRENTS

The sudden commencements of geomagnetic storms at the equatorial elect rojet station Kodaikanal are characterized by simultaneous increases o f both the horizontal field H and the westerly declination D(W) or the decrease of eastward field, Y. The amplitudes of the SSC in H or Y fi elds (Delta H and Delta Y) show a maximum deviation around midday hour s. The ratio of Delta Y to Delta H is about 0.23 for any hour of the d ay or night. During the main phase of the storm, the fluctuations in t he H and Y components at Kodaikanal vary in unison with each other. Oc casional large decreases of the H field at Kodaikanal at daytime hours during the main phase of the storm do not have corresponding decrease of H at Alibag or in the equatorial Dst index. The events are shown t o be associated with the sudden disappearance of the equatorial type o f sporadic E (E-sq) at Kodaikanal indicating a temporary reversal of t he ionospheric electric field over the magnetic equator to westward di rection. The observed storm time variations of the magnetic fields at an equatorial electrojet station are suggested to be due to the combin ed effects of the equatorial ring current and of the electric field ch anges at the ionospheric region transmitted from high latitudes.