Considerations of variations in ionospheric field effects in mapping equatorial lithospheric Magsat magnetic anomalies

SUMMARY Longitudinal, seasonal, and altitude‐dependent variability of magnetic fields is investigated in equatorial latitudes to determine their effect on the isolation of lithospheric Magsat magnetic anomalies. An estimate of ‘ionospheric effect’ was compiled by averaging the total intensity Magsat anomalies as a function of dip latitudes (called ‘dip‐latitude averages’) from dawn and dusk data sets grouped according to longitudes, time (months), and altitudes. Unanticipated seasonal variations were observed in dusk Magsat data over the Indian sector that may contribute to an improved understanding of the equatorial electrojet. The amplitudes of the dawn dip‐latitude averages are small in comparison to the dusk averages and they are of opposite sign as anticipated from the westward and eastward flowing currents at the dip equator at dawn and dusk time, respectively. However, longitudinal variation in the equatorial amplitudes of the dawn diplatitude averages is not entirely consistent with present knowledge of the electrojet field. In the past, dawn Magsat anomalies have been considered to be largely free of ionospheric fields. However, in the preparation of the lithospheric component maps, neglecting to remove the dawn dip‐latitude averages from dawn Magsat data set leaves conspicuous E‐W trending anomaly artifacts in certain regions of the world. Removal of both the dawn and the dusk 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 to contain some contribution from the lithospheric fields. Thus, processing of the data with the aid of the dip‐latitude averages unavoidably removes a small degree of E‐W oriented lithospheric signal as well.