Spatial occurrence of decameter midlatitude E region backscatter

This paper provides a statistical analysis of the spatial occurrence of midlatitude E region decameter backscatter. Measurements were made using the Valensole HF (high frequency) radar located in southern France during the summer of 1995 when it operated simultaneously at four frequencies. On the basis of the premise that E region scattering is fully magnetic aspect sensitive, the spatial occurrence statistics show that the aspect sensitive region moves toward the radar (southward) with respect to line of sight propagation calculations, with the lower-frequency echoes being closer toward the radar than the higher frequency ones are, in agreement with refraction theory predictions. Ray tracing inside nighttime midlatitude electron density profiles augmented with dense sporadic E, layers was used to calculate the expected echoing region, and good agreement with the observed region was found. Another finding is the angular distribution of backscatter inside the wide azimuthal sector covered by the radar scan. The spatial distribution of echo occurrence has its maximum at small azimuths at and about the geomagnetic north, suggesting that statistically, the meridional direction is strongly preferred for backscatter. Under the postulation that these are secondary decameter waves, we concluded that the observed angular anisotropy in spatial occurrence is at odds with the concept of strong isotropic plasma turbulence [Sudan, 1983] but in general agreement with the two-step gradient drift instability theory of secondary-wave generation proposed-by Sudan et al. [1973].