VHF radar observations of gradient instabilities associated with blanketingEslayers in the equatorial electrojet

It is observed that blanketing type sporadic E layers in the equatorial electrojet region often give rise to strong backscattering of 55-MHz radar signals. The Doppler spectra of the backscatter signals also show that the drift speeds of the irregularities are small at these times (in comparison to midday values) and that they cannot sustain gradient instabilities of 2.7 m with the normal E region electron density gradients. It is inferred from these observations that gradient instabilities of 2.7 m are generated at the sharp edges of the thin Es layers even under the conditions of small electric fields. However, the gradient instabilities are observed to disappear close to the time of the electric field reversal. It is further observed that the initial formation of the Es layers takes place only at such times when the drift velocities are small. This observational result and theoretical considerations suggest that the formation of Es layers in the electrojet region is inhibited by strong vertical drifts. This inhibiting factor and a probable diurnal variation of the ion convergence source result in the formation of blanketing Es layers in the equatorial electrojet region over Trivandrum predominantly in the late afternoon hours.