Imaging radar observations and theory of type I and type II quasi‐periodic echoes

Midlatitude E region plasma irregularities have been investigated using the Middle and Upper Atmosphere (MU) radar in Shigaraki, Japan, and the Clemson 30 MHz radar in South Carolina, USA. A new in‐beam imaging technique has been incorporated in the data analysis. Radar images reveal that the coherent backscatter associated with quasi‐periodic (QP) echoes mainly arrives from spatially localized, elongated scattering regions and that striations in radar range‐time‐intensity (RTI) maps are the signatures of the migrations of these regions through the sparsely filled illuminated volume. The scattering regions in question appear to maintain altitude as they drift. Refraction and finite aspect angle sensitivity permit field‐aligned irregularities in the regions to be detected by radars over a broad range of zenith angles. Circulation observed within the scattering regions is consistent with simulations of polarized E region plasma clouds described in a companion paper, as is the occasional appearance of type I echoes. Several long, continuous bands of scatterers are also evident in the radar images.