Midlatitude E region coherent backscatter observed simultaneously at two HF radar frequencies

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Abstract
We present results from a dual‐frequency ionospheric backscatter experiment carried out with the Valensole HF radar facility in the south of France. The radar was operated in a mode that allowed detailed Doppler spectrum recordings of coherent echoes from midlatitude E region irregularities simultaneously at 9.0 MHz and 12.4 MHz, corresponding to aspect‐sensitive backscatter from plasma waves with 16.7‐m and 12.1‐m wavelength, respectively. In this decameter wave band and in the presence of intense nighttime sporadic Es layers that contribute strong destabilizing density gradient components perpendicular to B, the E region is susceptible to direct plasma wave generation by the gradient drift instability even if electric fields are only a few mV/m. We observe that 9.0‐MHz echoes are stronger, more frequent, and more spatially extended than the 12.4‐MHz ones, which is in line with the threshold requirements of the gradient drift instability. Further, the data supports the notion of primary and secondary waves with spectral shapes similar to those seen, for example, at the equator. In one case, a situation analogous to the equatorial electrojet was clearly seen, in which the Doppler spectrum changed from a negatively shifted narrow peak (type I‐like) to a broad type II near zero shift and then to a positively shifted narrow type I‐like component, in the radar's azimuthal sector span from 22° east to 38° west of geomagnetic north. We have studied the ratio of the observed 12.4‐MHz versus 9.0‐MHz Doppler velocities in an attempt to test the widely used hypothesis that primary gradient drift waves have their phase velocity limited at instability threshold values. For simultaneous velocities in the range from 40 to 120 m/s, the V12.1 m/V16.7 m ratio was found on the average close to 1.1 and thus quite small compared to the anticipated value of 1.6 if the waves were to propagate at instability threshold values. Instead, the evidence is more supportive of the theory's dispersion relationship which requires that the wave phase velocity matches the relative electron‐ion drift along the direction of propagation.