Longitudinal structure of the F-region equatorial anomaly—a review

Abstract

A review has been undertaken of various ground-based and satellite measurements of the equatorial anomaly. The particular methods employed affect the credibility of the results. Transit satellite measurements cannot give the true seasonal variations of the equatorial anomaly diurnal variations. Bottomside sounding is very much better in this respect but in this case there is the difficulty of obtaining sufficient data from stations situated closely in dip location near the magnetic equator and which are also confined to a narrow range of longitude. Considering various longitude zones, such as the American, African and East Asian, an attempt has been made to relate the observed differences of the forms of the equatorial anomaly, throughout the day and from season to season in these zones, with differences of (1) magnetic equator location relative to the geographic, (2) magnetic declination and (3) electric and magnetic fields in the vicinity of the magnetic equator. Attention has also been drawn to the very significant differences in equatorial anomaly development which may also occur over relative small changes in longitude (40°). These may possibly be due to the upward propagation of semi-diurnal tides or planetary waves from the lower atmosphere affecting the magnitude of the eastward electric field over the magnetic equator. It is proposed that further efforts should be made towards more careful comparison measurements of the equatorial anomaly, neutral winds and electric fields at varying longitudes. The provision of a sun synchronized satellite with transit periods of 1300–1500 LT and 0100–0300 LT would seem very desirable for ionization measurements.