Ionospheric E-region irregularities during equatorial counter electrojet

130-137The paper describes a coordinated study of the geomagnetic fields at Trivandrum and Alibag, digital vertical incidence ionograms and oblique VH F backscatter radar echoes at Thumba on a day with a large counter electrojet (CEJ) event in the afternoon hours. It is shown that the cusp type of sporadic-E (Esc), above the normal E-layer continues to descend with time after the start of CEJ up to about 110 km, indicating downward Hall electric field. When the Esc layer descends to around 110 km, the downward Hall polarization field due to CEJ, acting on the downward gradient of plasma density in the top side of the layer, generates irregularities by the cross-field instability mechanism. These irregularities give the signature of the q-type of sporadic-E (Esc) on the ionograms and type-2 VHF Doppler spectra of scatter echoes. With continued descent, Esc disappears and no VHF echoes are recorded. With intensification of CEJ, the eastward streaming velocity of the electrons exceed the ion acoustic velocity and strong type -l echoes are observed on VHF backscatter radar. With the decrease of CEJ even type-2 echoes disappear and no scatter echoes are observed by HF ionosonde or on VHF radar