Geomagnetic Secular Variation at the Indian Observatories.

Abstract

Annual mean values of geomagnetic field components D, H and Z for all days and quiet days at six observatories are analysed to investigate the secular variations and geomagnetic jerks in the Indian region. Secular trends show a region of demarkation between equatorial and low latitude stations. The residual D, H and Z curves, obtained by removing polynomial fits, do not show any parallelism with the 11-year sunspot cycle. However, the D residual has a periodicity of 2 solar cycles, whereas H and Z residuals indicate a quasi-periodicity of 3 solar cycles at Alibag. For the period 1958 to 1990, D and Z residuals show a periodicity of nearly 2 solar cycles, while H shows out-of-phase variations with the sunspot cycle for all the six stations. The secular jerk around 1969-70, noted at many observatories over the globe, is not seen in D but is noted in the H and Z components at some of the six Indian stations. A comparison between the observed annual means and IGRF models indicates very low secular variation anomaly in the Indian region.

Secular trend of geomagnetic elements in the Indian region

“…Three out of the six stations-TRD, KOD and ANNare under the influence of the daytime equatorial electrojet. This causes an enhancement of the daily variation and shortperiod fluctuations in H, and though the electrojet does not contribute in any measurable way to the secular trends of H or D, it may introduce significant departures from the trend (Bhardwaj and Rangarajan, 1997). The diurnal variation in the vertical component close to the dip equator is expected to be small, but the analysis of south India magnetic array data suggests a significant internal contribution due to the channeling of induced currents through a subsurface conductor between India and Sri Lanka (Rajaram et al, 1979) and a regional south Indian offshore conductivity anomaly (Arora and Subba Rao, 2002).…”

“…A sudden change in the slope of the magnetic secular variation is known as a secular impulse, or geomagnetic jerk, that arises from sources inside the Earth (Cafarella and Meloni, 1995;Macmillan, 1996;Le Huy et al, 1998). Recently, these jerks have been suggested as geomagnetic rapid secular fluctuations (Olsen and Mandea, 2008;Mandea and Olsen, 2009;Qamili et al, 2013) that have periods ranging from several months to a few years (Macmillan, 2007).…”

“…Moos (1910) studied the secular variation of the geomagnetic field components at Bombay for the period 1871-1905 data to Alibag, Pramanik (1952) extended these secular variation curves up to 1949. Rao and Bansal (1969) fitted polynomials of a third order to the observed annual mean values of H, Z and D for Alibag from 1905-1965. Bhardwaj and Rangarajan (1997 further extended these curves up to 1990 for Alibag.…”

Geomagnetism

“…Observed annual mean values for quiet days at six Indian stations from 1958-1990, together with their best-fitting curves of (a) D, westerly-increasing D is plotted downwards, (b) H-component and (c) Z-component(Bhardwaj and Rangarajan, 1997).…”