Signatures of storm sudden commencements in geomagnetic H, Y and Z ®elds at Indian observatories during 1958±1992

The work describes an intensive study of storm sudden commencement (SSC) impulses in horizontal (H), eastward (Y) and vertical (Z) ®elds at four Indian geomagnetic observatories between 1958±1992. The midday maximum of DH 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 ®eld at equatorial and low latitudes. The impulses in Y ®eld are shown to be linearly and inversely related to DH at Annamalainagar and Alibag. The average SC disturbance vector is shown to be about 10±20°W of the geomagnetic meridian. The local time variation of the angle is more westerly during dusk hours in summer and around dawn in the winter months. This clearly suggests an e€ect of the orientation of shock front plane of the solar plasma with respect to the geomagnetic meridian. The DZ at SSC have a positive impulse as in DH. The ratio of DZ/DH are abnormally large exceeding 1.0 in most of the cases at Trivandrum. The latitudinal variation of DZ shows a tendency towards a minimum over the equator during the nighttime hours. These e€ects are explained as (1) resulting from the electromagnetic induction e€ects due to the equatorial electrojet current in the subsurface conducting layers between India and Sri Lanka, due to channelling of ocean currents through the Palk Strait and (2) due to the concentration of induced currents over extended latitude zones towards the conducting graben between India and Sri Lanka just south of Trivandrum.