Midday reversal of equatorial ionospheric electric ®eld

A comparative study of the geomagnetic and ionospheric data at equatorial and low-latitude stations in India over the 20 year period 1956±1975 is described. The reversal of the electric ®eld in the ionosphere over the magnetic equator during the midday hours indicated by the disappearance of the equatorial sporadic E region echoes on the ionograms is a rare phenomenon occurring on about 1% of time. Most of these events are associated with geomagnetically active periods. By comparing the simultaneous geomagnetic H ®eld at Kodaikanal and at Alibag during the geomagnetic storms it is shown that ring current decreases are observed at both stations. However, an additional westward electric ®eld is superimposed in the ionosphere during the main phase of the storm which can be strong enough to temporarily reverse the normally eastward electric ®eld in the dayside ionosphere. It is suggested that these electric ®elds associated with the V Bz electric ®elds originate at the magnetopause due to the interaction of the solar wind and the interplanetary magnetic ®eld.