Geomagnetic coast and other effects deduced from the new observatory at Visakhapatnam, India

Abstract

Summary Coastal effects observed in the long‐period Sq Solar quist day ranges and short‐period changes over 30 min intervals during magnetic substorms (bays), as recorded at a new magnetic observatory at Visakhapatnam, on the east coast of India, during 1995 in the H, Z and D elements, are analysed and the preliminary results are discussed. The results are compared with those from Alibag and Hyderabad (located more or less at the same latitude) based on simultaneous 1995 records. Short‐period induction vectors are computed and the arrows presented and discussed at the three locations. While the enhancement associated with close proximity to the coast in Sq (Z) at Alibag on the west coast compared to the central inland station of Hyderabad (enhancement of 17–37 per cent) is quite prominent in all three seasons of the year, at Visakhapatnam it is only marginally observed during the d‐season. At bay frequencies, medium‐sized induction arrows point towards the nearest deep‐sea conductive body at both Alibag and Visakhapatnam, while it is towards the southeast with a considerably reduced magnitude at Hyderabad. These induction arrows and the effects on Sq (Z), Sq (H) and Sq (D) ranges are discussed in conjunction with the Parkinson arrows and Sq (Z), Sq (H) and Sq (D) ranges from Margao (west coast)–Dharwar–Bellary–Mahanandi–Chirala (east coast) at the latitude of the 16°N profile of an earlier array experiment, and the two are interpreted together.

Fuzzy clustering analysis to study geomagnetic coastal effects

“…The results agree with the graphical method found by Srivastava et al (2001) using 1995 data. The results from the analytic technique of Fuzzy-cluster analysis are highly encouraging for application of this technique to the future analysis of geomagnetic data.…”

“…It is necessary to recollect the findings of Srivastava et al (2001) that vast deposits of limestone have been discovered in the Bay of Bengal off Visakhapatnam and that a relatively deep resistive body may not allow the (Sq)-induced currents in the seawater to concentrate near the Visakhapatnam coast. In the case of Alibag on the west coast, however, such induced (Sq) currents in the Arabian Sea do concentrate along the coast line and the continental margins give rise to an enhanced daily range in Z and reduced daily ranges in H and D, as compared to those at the Hyderabad station.…”

Geomagnetic coast effect at two Croatian repeat stations

“…Further, it is assumed that X n * and Y n * are independent of Z n *, and that they can be replaced by X* and Y*, respectively [Banks 1973]. In this case, Equation (1) reduces to [Banks 1973, Viljanen et al 1995, Hitchman et al 2000, Srivastava et al 2001]:…”

“…It is assumed that under the geomagnetic observatories is a conductivity structure characterised by a lateral homogeneity and vertical stratification. On the other hand, a variometer station located close to a conductivity anomaly will measure vector sum of the normal field B n and an anomaly field B ia induced by the horizontal components of B n according to Equation (2) [Banks 1973, Hitchman et al 2000, Srivastava et al 2001]. If lateral variations of B n over the distance separating an anomalous point from the corresponding reference observatory are negligible in comparison to the induced anomaly field itself, Z ia * in Equation (2) can be identified with the difference between the vertical components of the total field measured with at the variometer station and the observatory, respectively.…”

“…This effect is very intense during geomagnetic storms and substorms, but can occur also under quiet geomagnetic conditions. Its maximum intensity is achieved on periods between 30 and 90 minutes, and its influence can be measured on distances larger than 150 km inland from a coast [Parkinson and Jones 1979, Viljanen et al 1995, Srivastava et al 2001]. The models that explain the coastal effect are based on the electromagnetic induction (by the time-varying ionospheric and magnetospheric magnetic fields) contrasts: in the crust and upper mantle under continents and oceans (due to their prominently different electrical conductivities), and/or in the oceans and continents (ocean water has much higher electrical conductivity with respect to the land masses).…”