SEMIANNUAL VARIATION OF DST-RELATED DISTURBANCE AND ASSOCIATION WITH SOLAR AND GEOMAGNETIC ACTIVITY
A study of the semiannual variation of the Monthly mean Dst-related disturbance in H and Z components of the geomagnetic field has been carried out. Magnetograms from fifteen (15) geomagnetic stations in the high- mid- and low latitudes, and which includes three of the stations used in computing the Dst index have been analyzed. Sixty-one years data (1950-2010) was utilized. The H-component (Hdiff) and Z-component (Zdiff) of the Dst-related disturbance were computed by removing the solar quiet day (Sq) contribution to the monthly means in the geomagnetic stations used. The Sq monthly contributions to the Dst and Dst* was removed to obtain Dstall-Sqand Dst*all-Sq. We observed that the removal of the Sq monthly Dst mean value from Dst and Dst* (which represents the Dst after internal sources have been removed) generally improves the correlation of Dst with Dst-related disturbance fields. Hdiff showed strong correlation with Dstall-Sq. The semiannual variation showed Hdiff peaks in the equinoxes. The semiannual peak is greater in March/April than in September/October for low latitudes. The mid and high latitude stations Hdiff also exhibited clear semi-annual variation albeit with less intensity during the equinoxes. Mid latitude broad peaks in the semiannual variation suggest longitudinal asymmetric contributions to the ring rcurrents. Zdiff in all the latitudes exhibited greater asymmetry in its distribution when compared with the Hdiff. The variation in Zdiff is largest in the high latitude which showed 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 semiannual variation, while the East-West and West-East currents in the auroral oval region may be modulating the ring current effect on Dst-related disturbances at high latitudes.