Simultaneous 50‐MHz coherent backscatter and digital ionosonde observations in the midlatitude <i>E</i> region

Abstract

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 E s characteristics such as the layer's top frequency ƒ t E s (a measure of maximum E s electron density) and the apparent E s 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 E s 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 E s layer and a recently proposed mechanism for strong polarization fields at midlatitudes.