The semi-annual variation of the residual H and Z components of geomagnetic field was studied by analyzing magnetograms with sixty-one years data. Mean monthly residual H- and Z-component field (Hdiff and Zdiff) variations were computed using appropriate technique. Semi-annual variation exhibited Hdiff peaks in the equinoxes, which for low latitudes, was higher during March/April than in September/October. In the mid and high latitude stations also, Hdiff exhibited clear semi-annual variation albeit with less intensity during the equinoxes. Mid latitude broad peaks in the said variation suggest longitudinal asymmetric contributions to the ring currents. Zdiff in all the latitudes exhibited greater asymmetry in its distribution in comparison with Hdiff. Zdiff variation was largest in the high latitude regions, showing strong solar activity dependence. It is suggested that asymmetric component of the ionospheric coupling to the magnetospheric current may be responsible for the observed irregularities in the low- and mid-latitude Zdiff semi-annual variation, while the East-West and West-East currents in the auroral oval region may be modulating the ring current effect on residual field at high latitudes.
Key words: Geomagnetic field, magnetograms, H and Z components.
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