On the origin of quasi-periodic radar backscatter from midlatitude sporadic E

We describe a newly discovered polarization process that appears to be induced by an atmospheric gravity wave (AGW) when it altitude–modulates a sporadic E (Es) layer in the nighttime, midlatitude ionosphere. This large–scale polarization process appears capable of accounting for three as yet unexplained features found in radar backscatter from field–aligned irregularities in Es layers: (1) kilometer–scale, wavelike variations in the mean Doppler velocity, (2) mean Doppler velocities much larger than background ionospheric motion, and (3) quasi–periodic patterns in backscatter power when plotted as a function of range and time. We show that the polarization electric field develops as a result of the altitude modulation and that its properties are similar to those of the AGW. The novel feature in this process is the spatial modulation that is produced in the field line–integrated Pedersen conductivity by the variation in the ion–neutral collision frequency associated with the varying altitude of the Es layer. The resultant electric field together with the altitude–modulated Es layer then drive the production of secondary plasma waves via the gradient–drift instability. The three puzzling features are shown to be associated with the characteristics of these secondary waves.