Radar scatter from equatorial electrojet waves: An explanation for the constancy of the Type I Doppler shift with zenith angle

The first results from the 430 MHz Advanced Modular Incoherent Scatter Radar Prototype (AMISR-P) at the Jicamarca Radio Observatory were reported by Hysell et al. (2007). We present additional data showing that the phase velocity of Type I echoes is independent of zenith angle, an unexplained property of these waves. We interpret the results using rocket data by predicting the total line-of-sight velocity at the four zenith angles used. We find that the radars preferentially detect waves within 10% of C-s in at least four range gates for all beams and up to eight range gates for the 51 JULIA beam. This result is consistent with recent auroral observations that Type I waves are only generated with k vectors near the electron flow velocity, where the latter is the vector sum of the zero-order drift and the perturbation drift due to large-scale waves in the equatorial case.