Geomagnetic Secular Variation at the Indian Observatories

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 Il-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.