SEASONAL-VARIATIONS OF GEOMAGNETIC D, H AND Z FIELDS AT LOW LATITUDES

The extensive set of geomagnetic data at nine observatories in India c overing region of magnetic dip angle 0-degree to 50-degrees-N and a pe riod of 1975 to 1986 have shown that the equinoctial maxima in the geo magnetic field are observed only on the horizontal and vertical compon ents at the electrojet observatories, for the local hours around the p eak current density and are consequent to the seasonal and daily chang es of the equatorial electrojet current. The eastward component of the geomagnetic field shows predominantly annual variation with maximum d uring summer months at any of the stations. These observations exclude the seasonal changes of solar ionisation effects to either axial theo ry of Cortie (1912) or the equinoctial theory of Bartels (1932). The e quatorial stations are suggested to be affected by the imposition of e ast-west electric field in addition to that generated by the dynamo ac tion of the earth's ionosphere. The most plausible source of these add itional electric field arise out of the action of solar wind and inter planetary magnetic field with the magnetosphere.