SIMULTANEOUS 50-MHZ COHERENT BACKSCATTER AND DIGITAL IONOSONDE OBSERVATIONS IN THE MIDLATITUDE-E REGION

SESCAT, a coherent backscatter radar system located in Crete, Greece, was operated for one summer together with a (CADI) digital ionosonde o bserving nearly the same scattering volume. The purpose of the experim ent 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 s uch as the layer's top frequency f(t)E(s), (a measure of maximum E-s e lectron density) and the apparent E-s trace spread which results from range spreading due to oblique reflections from a nonuniform and horiz ontally inhomogeneous layer. Similar correlations were obtained for SE SCAT spectral width and the same sporadic E characteristics. The exper iment confirmed that the presence of an E-s layer in the scattering vo lume, which could provide destabilizing electron density gradients per pendicular to the magnetic field, is necessary but not sufficient for the occurrence of 50-MHz backscatter. We suggest that in addition ther e is need for an enhanced electric field to be present inside the laye r as well, a notion that is in line with the observed correlation of b ackscatter with a dense but strongly inhomogeneous E-s layer and a rec ently proposed mechanism for strong polarization fields at midlatitude s.