Spatial occurrence of decameter midlatitude E region backscatter

This paper provides a statistical analysis of the spatial occurrence of mid latitude E region decameter backscatter. Measurements were made using the V alensole HF (high frequency) radar located in southern France during the su mmer of 1995 when it operated simultaneously at four frequencies. On the ba sis of the premise that E region scattering is fully magnetic aspect sensit ive, the spatial occurrence statistics show that the aspect sensitive regio n moves toward the radar (southward) with respect to line of sight propagat ion calculations, with the lower-frequency echoes being closer toward the r adar than the higher frequency ones are, in agreement with refraction theor y predictions. Ray tracing inside nighttime midlatitude electron density pr ofiles augmented with dense sporadic E, layers was used to calculate the ex pected echoing region, and good agreement with the observed region was foun d. Another finding is the angular distribution of backscatter inside the wi de azimuthal sector covered by the radar scan. The spatial distribution of echo occurrence has its maximum at small azimuths at and about the geomagne tic north, suggesting that statistically, the meridional direction is stron gly preferred for backscatter. Under the postulation that these are seconda ry decameter waves, we concluded that the observed angular anisotropy in sp atial occurrence is at odds with the concept of strong isotropic plasma tur bulence [Sudan, 1983] but in general agreement with the two-step gradient d rift instability theory of secondary-wave generation proposed-by Sudan et a l. [1973].