First 24.5-MHz radar measurements of quasi-periodic backscatter from field-aligned irregularities in midlatitude sporadic E

We report three new findings from the first observations of quasi-periodic (QP) radar echoes at 24.5-MHz. Most interesting is that QP echoes produced by 6.1-m field-aligned irregularities (FAI) can occur at altitudes as high as 150 km. Because Hall currents are negligible there, the existence of these FAI require interpretation at least in terms of a generalized gradient-drift instability that allows for (1) a transition from a Hall current at low altitudes to a Pedersen current at high altitudes, and (2) ion magnetization effects at altitudes above the lower E region. The second finding is the occurrence of extreme spectral broadening that often accompanies the strongest echoes. The third finding, from data accumulated over a two-month period, is evidence that the longitudinal gradient in conductivity associated with the solar terminator plays a role in the first appearance of QP echoes in the even-ing.