MIDDAY REVERSAL OF EQUATORIAL IONOSPHERIC ELECTRIC-FIELD

A comparative study of the geomagnetic and ionospheric data at equator ial and low-latitude stations in India over the 20 year period 1956-19 75 is described. The reversal of the electric field in the ionosphere over the magnetic equator during the midday hours indicated by the dis appearance of the equatorial sporadic E region echoes on the ionograms is a rare phenomenon occurring on about 1% of time. Most of these eve nts are associated with geomagnetically active periods. By comparing t he simultaneous geomagnetic H held at Kodaikanal and at Alibag during the geomagnetic storms it is shown that ring current decreases are obs erved at both stations. However, an additional westward electric field is superimposed in the ionosphere during the main phase of the storm which can be strong enough to temporarily reverse the normally eastwar d electric field in the dayside ionosphere. It is suggested that these electric fields associated with the V x Bz electric fields originate at the magnetopause due to the interaction of the solar wind and the i nterplanetary magnetic field.