GEOMAGNETIC-FIELD VARIATIONS AT LOW LATITUDES AND IONOSPHERIC ELECTRIC-FIELDS

The solar cycle, seasonal and daily variations of the geomagnetic H fi eld at an equatorial station, Kodaikanal, and at a tropical latitude s tation, Alibag, are compared with corresponding variations of the E-re gion ionization densities. The solar cycle variation of the daily rang e of H at either of the stations is shown to be primarily contributed to by the corresponding variation of the electron density in the E-reg ion of the ionosphere. The seasonal variation of the DELTAH at equator ial stations, with maxima during equinoxes, is attributed primarily to the corresponding variation of the index of horizontal electric field in the E-region. The solar daily variation of DELTAH at the equatoria l station is attributed to the combined effects of the electron densit y with the maximum very close to noon and the index of electric field with the maximum around 1030 LT, the resulting current being maximum a t about 1110 LT. These results are consistent with the ionosphere E-re gion electron horizontal velocity measurements at the equatorial elect rojet station, Thumba in India.