Azimuth-Time-Intensity striations of quasiperiodic radar echoes from the midlatitude E region ionosphere

The Valensole high frequency (HF) radar in the south of France is an ionospheric Doppler sounder which can perform E region coherent backscatter measurements over an azimuthal sector of 86 degrees, from 26 degrees E to 58 degrees W, with similar to 20 angular resolution. This large azimuthal coverage is taken advantage of in order to study quasiperiodic (QP) echoes in the zonal direction using azimuth-time-intensity (ATI) analysis. ATI plots show sequential sloping striations of scatter reminiscent of those detected routinely in the range-time-intensity (RTI) plots of midlatitude radars which view the medium at a fixed azimuth about the meridian. It was found that ATI striation periods range from a few minutes to less than 30 min, whereas the striation slopes are systematically negative (motions westward) prior to local midnight, and turn positive (motions eastward) in the post-midnight hours. The zonal rates, dx/dt, computed from the striation slopes take values between similar to 30 and 160 m/s. These are due to real motions of unstable plasma structures, most likely sporadic E patches that drift along with the neutral wind, that have zonal scale lengths of several tens of kilometers. The present observations imply that the mechanism responsible for QP echoes is independent of azimuth and can basically operate effectively in any direction in the horizontal plane.