Dissimilar forms of the ionospheric equatorial anomaly observed in East Asia and India

Abstract

A longitudinal comparison has been undertaken of electron content-latitude profiles at low latitudes obtained in East Asia and India from measurements of the Faraday rotation of transiting beacon satellite signals. Very appreciable differences in the extents of the development of the equatorial anomaly have been found (1) between the East Asia and India regions (longitude difference 40°) for two particular months and (2) between similar months in two consecutive years for the India region alone. A comparison of the daily ranges of the horizontal magnetic field component (ΔH) measured near the magnetic equator in these two regions showed that ΔH was less (by about 25γ) during the period of poor development of the equatorial anomaly in the Indian region. It is considered that changes in ΔH are related to changes in the magnitude of the eastward electric field E overhead the magnetic equator and these in turn affect the development of the ionospheric equatorial anomaly via corresponding variations of the magnitude of the ionisation uplifting, (E × B) mechanism. The correlation between ΔH and the equatorial anomaly development was found to be poor in the northern summer but throughout the rest of the year it is thought that ionospheric predictions at low latitudes could be improved by considering the noon values of H monitored at appropriate longitudinally-sited stations. The day-to-day and longitudinal variabilities of ΔH and the equatorial anomaly development are thought to have their origin in the lower atmosphere. The upward propagation of semi-diurnal tides and planetary waves are affected by the temperature-height profiles in the mesosphere and also the structure of the middle atmosphere jet, these being most variable in local winter. These tides in turn would produce a variability of local winds in the upper atmosphere and by dynamo action create electric fields in the vicinity of the magnetic equator affecting the electrojet and the development of the ionospheric equatorial anomaly. It is proposed that a search should be made for planetary and tidal waves in both the lower and higher atmosphere at low latitudes.