Equatorial sporadic E and cross-field instability

Abstract

The occurrence of sporadic E at an equatorial station during magnetically quiet daytime conditions corresponds almost exactly to the time during which the horizontal component of the earth's magnetic field is above the mean night time level. Any large decrease of H below the night time level is accompanied by the disappearance of equatorial E_s-q reflections precisely for the period when the value of H remains below its night time level. Such disappearance of E_s-q can be attributed to the reversal of the current equal to, or greater than, the normal eastward equatorial electrojet current. During magnetically disturbed conditions, however, the depressions in H are not always accompanied by the disappearance of E_s-q. Whenever the normal E and sporadic E reflections can be resolved on the equatorial ionograms, the minimum virtual height of the normal E is seen to be clearly greater than that of the sporadic E layer. The creation of E region irregularities at equatorial latitudes giving the appearance of an E_s-q layer in daytime ionograms is suggested to be due to cross-field (plasma gradient) instability. The horizontal magnetic field and the upward Hall polarisation (electric) field produce irregularities in the lower E-region where the rate of increase of ambient electron density is large and directed upward. A temporary reversal of the electrojet current indicated by a decrease in H below the night time level and the disappearance of E_s-q are due to the temporary reversal of the vertical Hall polarisation field making it downward instead of upward which being opposite to the direction of the gradient of plasma density inhibits the cross-field instabilities.