In this paper we present the detailed results of a series of experiments designed to study the coherent backscatter of 50--MHz radar waves from the mid-latitude F region. Data were obtained with the active phased-array MU radar in Japan and include some auxiliary E region coherent echoes as well. As in other turbulent ionospheric phenomena the intense nonthermal scatter comes from irregularities oriented parallel to B. The strongest echoes correspond to irregularities at least 20 dB stronger than thermal backscatter at the same frequency from typical F region densities at the same range. Simultaneous observations with ionosondes show that these echoes occur during strong mid-latitude spread F. As defined by ionosondes, the latter phenomenon is certainly much more widespread than the turbulent upwelling events described here, but we believe that in some sense these correspond to the most violent mid-latitude spread F. The strongest echoes occur in large patches which display away Doppler shifts corresponding to irregularity motion upward and northward from the radar. At the edges of these patches there is often a brief period of toward Doppler before the echoing region ceases. On rare occasions comparable patches of strong away and toward Doppler are detected, although in such cases the Dopple width of the toward echoes is much narrower than that of the away echoes.

The away patches often characterized by mean velocities well over 250 m/s and Doppler widths (full width at half maximum) of 50 m/s. The multiple beam capability at MU allowed us to track the patches in the zonal direction on two days. The patches moved east in both cases at velocities of 125 m/s and 185 m/s, respectively. There is a distinct tendency for the bottom contour of the scattering region to be modulated at the same period as the patch occurrence frequency as well as at higher frequencies. This higher-frequency component may correspond to substructures in the large patches and to the E region coherent scatter patches which were detected simultaneously in several multiple beam experiments. In the companion paper (Kelley and Fukao, this issue), we explore a number of possible explanations for this phenomenon in more detail. ¿1991 American Geophysical Union