In this paper, an experiment designed for dual-frequency azimuthal Doppler spectrum studies of decameter-scale field-aligned irregularities in the midlatitude E region ionosphere is introduced, and the first results are presented. The observations were made in July 1993 with a large HF radar facility near Valensole (43.8¿N, 6.1¿E), in the south of France. The radar system is an oblique ionospheric sounder that employes long antenna arrays of vertical monopole pairs and covers the HF frequency band from 4 to 30 MHz in 1-kHz steps. A scheme of broad beam width transmission and narrow-beam, phased-array, multireceiver coverage was used to scan with 2¿ step an azimuthal sector from about 24¿ east to 60¿ west of geomagnetic north. The 15-gate viewing region was confined in range between 100 and 370 km in order to cover an area of E region magnetic aspect sensitive backscatter near 37¿ invariant magnetic latitude (L≂1.7, magnetic dip, ~60¿). In this configuration, each azimuthal scan was completed in 80 s over a 15¿42 spatial grid with the full Doppler spectrum at each grid point recorded in real time. The experiment provided observations simultaneously at two frequencies, 9.0 MHz and 14.8 MHz, that correspond to backscatter from plasma waves with wavelengths of 16.7 and 10.1 m, respectively.

Here, we present an overview of the observations that include azimuthal and range-time echo characteristics as well as mean Doppler shift and spectrum width properties. The first results show aspect sensitive decameter-wavelength irregularities having mean phase velocities at least as large as 120 m/s to act as tracers of wavelike dynamic structures that drift westward with speeds in the 40- to 80-m/s range and have characteristic times between 10 and 30 min and typical scale lengths between 40 and 90 km. In our interpretation, we consider these structures to be sporadic Es ionization patches, possibly affected by the passage of atmospheric gravity waves, which are accompanied by intense horizontal electron density gradients and enhanced electric fields. Although secondary generation cannot be excluded entirely, the evidence supports the possibility that the gradient drift instability is the basic physical mechanism for direct generation of decameter aspect sensitive plasma waves in the midlatitude E region ionosphere. ¿ American Geophysical Union 1995