We report the first Jicamarca Radio Observatory (JRO) observations of two-stream irregularities under daytime counter equatorial electrojet (CEEJ) conditions, made with a VHF radar. This type of irregularity, although predicted by linear theory, has been observed only very few times at other sites (once in Africa and once in India). In the first 40 years of the existence of JRO, these irregularities were not observed. Since June 2000 and November 2001, we have observed them, surprisingly on seven different days. Our observations reveal that CEEJ irregularities appear only when large reversed zonal electric fields exist. We have used the horizontal component of the magnetic field due to the equatorial electrojet (H) as a proxy of the zonal electric fields. Initially, the CEEJ echoes are weak, with a Doppler velocity comparable to the ion-acoustic speed, and a relatively narrow spectral width and coming only from small elevation angles. As the ΔH value gets more negative (∼−200 nT around noon), very strong CEEJ echoes appear, still with ion-acoustic velocities at low elevation angles but with large spectral widths. The Doppler shifts of the spectra follow a dependence on the cosine of elevation angle. Weak CEEJ echoes are again observed when the irregularities are about to disappear (i.e., when H is just slightly negative of the threshold value). In order to get a better understanding of the scattering mechanism responsible for these echoes, we have devised a simple statistical model and compared its spectra to the spectral radar data. The CEEJ spectral characteristics can be explained on the basis of turbulent plasma breakdown of pure two-stream instabilities. These echoes are thought to be generated by nonlinear mode coupling and, when sufficiently strong, by secondary wave instabilities driven by pure two-stream waves at the density maximum and the topside of the E region.
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