Long term trends in sporadic E layers and electric fields over Fortaleza, Brazil

Long term trends in the sporadic E-layer occurrences and associated electric fields over the equatorial station, Fortaleza are analysed using 16 years (1975-1990) of ionosonde data. The secular drift of the magnetic equator and hence that of the equatorial electrojet current (EEJ) to northward of Fortaleza has resulted in marked long term changes in the occurrence characteristics of the different types of Es layers: the q-type (arising from electrojet instability processes) and the l, f, c and h types (resulting from wind/wind shear mechanisms). Systematic decrease in the occurrence rates of the q-type Es, accompanied with increases in the remaining types of Es, was registered from 1975 to 1990, during which: period the magnetic equator drifted to north of Fortaleza by similar to 400km. The long term trend manifests competing roles of the equatorial electric field and wind systems, in the generation of the different types of Es layers as a function of the distance from the electrojet center. The F-layer dynamo electric field is seen to exercise strong control of Es layer formation near sunset. Solar cycle variations, represented by the F10.7cm flux, seems to be present in elctrojet intensity and the lower thermospheric wind system.