An Interpretation of the Induction Arrows at Indian Stations

Abstract

Induction arrows (Wiese vectors) for night-time SSCs and Bays are determined and discussed along with geological and other geophysical data at eight Indian magnetic observatories, from Sabhawala in the Himalayan foothills to Trivandrum on the seacoast in the extreme south.A medium-sized induction arrow at Sabhawala pointing northwards indicates that the region of higher subsurface conductivity lies to the south of Sabhawala, possibly along the Aravalli Hills, and not beneath the Kashmir Himalaya, where the conductive upper mantle appears to be depressed. East-south-east pointing arrows at Jaipur and Ujjain are again indicative of a high electrical subsurface conductivity on the west along the Aravalli Hills and the Cambay region, where high heatflow and gravity values have also been reported, and the upper mantle appears to be elevated.At Alibag, a medium-sized induction arrow points eastward, with the conductive seawater lying on the west. Hyderabad is the only station where small induction arrows (small Z variations), characteristic of an inland station, are observed. Negative Z variations for positive H variations along the Alibag-Hyderabad-Kalingapatnam profile, become positive about 100km east of Hyderabad due to ocean effect from the east coast.Unusually large induction arrows (very large positive Z variations) in the extreme south Peninsular tip (largest one at Trivandrum), so close to the dip equator, are indicative of very strong induced current concentrations near and along this coast. These oceanic induced currents causing the anomalous geomagnetic variations in Peninsular India, appear to concentrate along the coastline and the continental shelf, and flow from north to south along the east coast. The currents concentrate further as they pass through the narrow Palk Strait and the Gulf of Manar between India and Sri Lanka on to Kanyakumari and the Trivandrum coast, and turn northwards along the west coast due to the obstruction provided by the volcanic ridge of Lakshadweep, Minicoy and Amindivi islands lying to the west in the Arabian Sea, about 300km off the Trivandrum coast. The current concentration would turn westwards near Calicut (11°N) and pass north of these islands in a diffuse form. Another current concentration is expected to flow along the Alibag west coast from north to south (so as to explain the reversed coast effect at Alibag), and turn westwards near Calicut to merge with the Bay of Bengal current system. There is no need to postulate a couductor and current channelling beneath the sea near the Trivandrum coast without any supporting geological evidence. There could be a channelling of the induced currents at the interface of the conductive upper mantle beneath the ocean and the less conductive upper mantle beneath the Peninsular India (100-200km), in a step-structure around the coast, which will account for the induction anomalies observed in long-period variations like Sq.