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-timeresolution, 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 pro®les of signal-tonoise 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 smallscale structure for studies of the aspect sensitivity of radar returns are discussed, and the changes in wind®eld at the bifurcation level are compared with `wind corners' observed in rocket studies of the mesosphere at polar latitudes.