CONSIDERATIONS OF VARIATIONS IN IONOSPHERIC FIELD EFFECTS IN MAPPING EQUATORIAL LITHOSPHERIC MAGSAT MAGNETIC-ANOMALIES

Longitudinal, seasonal, and altitude-dependent variability of magnetic fields is investigated in equatorial latitudes to determine their eff ect on the isolation of lithospheric Magsat magnetic anomalies. An est imate of 'ionospheric effect' was compiled by averaging the total inte nsity Magsat anomalies as a function of dip latitudes (called 'dip-lat itude averages') from dawn and dusk data sets grouped according to lon gitudes, time (months), and altitudes. Unanticipated seasonal variatio ns were observed in dusk Magsat data over the Indian sector that may c ontribute to an improved understanding of the equatorial electrojet. T he amplitudes of the dawn dip-latitude averages are small in compariso n to the dusk averages and they are of opposite sign as anticipated fr om the westward and eastward flowing currents at the dip equator at da wn and dusk time, respectively. However, longitudinal variation in the equatorial amplitudes of the dawn dip-latitude averages is not entire ly consistent with present knowledge of the electrojet field. In the p ast, dawn Magsat anomalies have been considered to be largely free of ionospheric fields. However, in the preparation of the lithospheric co mponent maps, neglecting to remove the dawn dip-latitude averages from dawn Magsat data set leaves conspicuous E-W trending anomaly artifact s in certain regions of the world. Removal of both the dawn and the du sk dip-latitude averages from their respective data sets substantially improves the agreement between the separately prepared dawn and dusk lithospheric component Magsat maps. Despite the agreement between the resultant lithospheric anomaly maps, dip-latitude averages do appear t o contain some contribution from the lithospheric fields. Thus, proces sing of the data with the aid of the dip-latitude averages unavoidably removes a small degree of E-W oriented lithospheric signal as well.