The small-scale structure of VHF mesospheric summer echo layers observed at mid-latitudes

The VHF radar system at Aberystwyth (52.4° N, 4.1° E) has been used to make high-time-resolution, multi-beam observations of mesospheric summer echo layers. These show that the altitude and the sense of vertical movement of the layers can vary over time-scales of minutes and horizontal scales of kilometres. In general, the altitude profiles of signal-to-noise ratio provide evidence of a bifurcated structure with sharp changes in the horizontal wind vector and vertical velocity, and enhanced spectral width occurring at the bifurcation level. The implications of the small-scale structure for studies of the aspect sensitivity of radar returns are discussed, and the changes in wind-field at the bifurcation level are compared with 'wind corners' observed in rocket studies of the mesosphere at polar latitudes.