Signatures of storm sudden commencements in geomagnetic H, Y and Z fields at Indian observatories during 1958-1992

The work describes an intensive study of storm sudden commencement (SSC) im pulses in horizontal (H), eastward (Y) and vertical (Z) fields at four Indi an geomagnetic observatories between 1958-1992. The midday maximum of Delta H has been shown to exist even at the low-latitude station Alibag which is outside the equatorial electrojet belt, suggesting that SSC is associated with an eastward electric field at equatorial and low latitudes. The impuls es in Y field are shown to be linearly and inversely related to Delta H at Annamalainagar and Alibag. The average SC disturbance vector is shown to be about 10-20 degrees W of the geomagnetic meridian. The local time variatio n of the angle is more westerly during dusk hours in summer and around dawn in the winter months. This clearly suggests an effect of the orientation o f shock front plane of the solar plasma with respect to the geomagnetic mer idian. The Delta Z at SSC have a positive impulse as in Delta H. The ratio of Delta Z/Delta H are abnormally large exceeding 1.0 in most of the cases at Trivandrum. The latitudinal variation of Delta Z shows a tendency toward s a minimum over the equator during the nighttime hours. These effects are explained as (1) resulting from the electromagnetic induction effects due t o the equatorial electrojet current in the subsurface conducting layers bet ween India and Sri Lanka, due to channelling of ocean currents through the Palk Strait and (2) due to the concentration of induced currents over exten ded latitude zones towards the conducting graben between India and Sri Lank a just south of Trivandrum.