Daily variation of the geomagnetic field at the dip equator

Abstract

The range _ΔH_A of the daily variation of the horizontal component of the geomagnetic field at Alibag (A), a low latitude station outside the influence of the equatorial electrojet, is largely caused by weakening of the ambient field on the night side rather than an enhancement on the day side. The effect may be caused by magnetospheric drift currents which, being more than 30,000 km above the surface of the Earth, would produce almost identical effects at Alibag and Trivandrum (T), situated on the dip equator 1,100 km south of Alibag, if differences in the induced field arising from local ground conditions are ignored. On the other hand, the well known enhancement of ΔH at the dip equator is caused by the influence of the equatorial electrojet. Consequently _ΔH_T-H_A must be related to purely ionospheric currents.