Equatorial Sporadic E Layer during Geomagnetic Storms

Abstract

The decrease of the average value of foEs at equatorial stations on disturbed days as compared to that on quiet days is shown to be due to frequent disappearance of ES(f0Es<1MHz) during geomagnetic active periods rather than due to the decrease of individual values of foEs. The study of the q-type of ES during major SC-type geomagnetic storms indicates that there is no effect of a storm on ES-q so long as the change in the H field is less than that of the Dst(H) value. Only during large decreases in the H field exceeding the corresponding change in the Dst(H) value, the ES-q is shown to disappear completely.There is no apparent time lag between the decrease of H and the disappearance of Es. The ES-q can disappear even for a short period, for example, as five minutes. These effects are explained on the basis that ES-q is generated by the cross-field instability mechanism and its disappearance is due to temporary reversal of the electrojet current because of temporary imposition of a strong westward electric field over the normal Sq field.

Unique examples of solar flare effects in geomagnetic H field during partial counter electrojet along CPMN longitude sector

“…On CEJ day H showed a minimum over the magnetic equator and the latitudinal profile of Z was reversed with respect to that during an EEJ day, with a maximum in northern and a minimum in southern fringe region of the electrojet. Rastogi (1972) showed number of examples of the disappearances of Es-q at Huancayo and Kodaikanal during the depressions of H on quiet as well as on disturbed days. Thus, the presence (or absence) of equatorial Es-q during daytime was found to be an alternate parameter to the presence of EEJ (or CEJ).…”

Local time dependent response of postsunset ESF during geomagnetic storms

“… Rastogi [1972a] has explained that Es‐q configuration in the ionograms at equatorial latitudes is due to scattering by irregularities generated by gradient drift instability mechanism when the electron density gradient (dN/dh) and Hall field are in the same direction. On certain occasions when the primary horizontal electric field reverses to westward direction such as during geomagnetic storm activity, the Hall field becomes downward and the Es‐q irregularities disappear [ Rastogi , 1972b]. Further, the post sunset enhancement of h ′ F at Trivandrum is reduced compared to its quiet day average value (Figure 2f), and consequently, the occurrence of Spread‐F and scintillations is inhibited on this night.…”

Midday reversal of equatorial ionospheric electric field

“…However during the midday hours, i s -q had occurred over 90±99% of times. Rastogi (1972a) had shown that during a geomagnetic storm, i s -q at an equatorial station does not indicate any decrease of the maximum i s -q frequency, but it does disappear completely during very large decreases of Dr at the particular station.…”