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.