SESCAT, a coherent backscatter radar system located in Crete, Greece, was operated for one summer together with a (CADI) digital ionosonde observing nearly the same scattering volume. The purpose of the experiment was to further investigate the origin of midlatitude E region VHF echoes which occur almost exclusively during summer nighttime. It was found that 50-MHz midlatitude backscatter always occurs in association with sporadic E layers. A statistical analysis indicated significant correlations between SESCAT total echo power and Es characteristics such as the layer's top frequency ftEs (a measure of maximum Es electron density) and the apparent Es trace spread which results from range spreading due to oblique reflections from a nonuniform and horizontally inhomogeneous layer. Similar correlations were obtained for SESCAT spectral width and the same sporadic E characteristics. The experiment confirmed that the presence of an Es layer in the scattering volume, which could provide destabilizing electron density gradients perpendicular to the magnetic field, is necessary but not sufficient for the occurrence of 50-MHz backscatter. We suggest that in addition there is need for an enhanced electric field to be present inside the layer as well, a notion that is in line with the observed correlation of backscatter with a dense but strongly inhomogeneous Es layer and a recently proposed mechanism for strong polarization fields at midlatitudes. ¿ 1998 American Geophysical Union