Abstract
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.
Highlights
Ecklund and Balsley (1981) were the ®rst to notice the extraordinary nature of mid-summer VHF radar echoes from the polar mesosphere, which have become known as polar mesosphere summer echoes (PMSEs) (RoÈ ttger et al, 1988)
Most of the altitude pro®les of signal-to-noise ratio (S/N) for mesospheric summer echo layers observed from Aberystwyth show evidence of a bifurcation
The large vertical wind shears and enhanced vertical spectral widths at the bifurcation level suggest that it is turbulent and that backscatter is responsible for the radar returns from this level
Summary
Ecklund and Balsley (1981) were the ®rst to notice the extraordinary nature of mid-summer VHF radar echoes from the polar mesosphere, which have become known as polar mesosphere summer echoes (PMSEs) (RoÈ ttger et al, 1988) They are observed predominantly between altitudes of 80 and 90 km, in the vicinity of the mesopause, and have radar signal strengths which are typically 20±30 dB stronger than those of non-summer polar mesospheric echoes (Ecklund and Balsley, 1981; Balsley et al, 1983; Czechowsky et al, 1989). Thomas: The small-scale structure of VHF mesospheric summer echo layers study will concentrate on MSEs observed by the UK Natural Environment Research Council MST radar facility at Aberystwyth 52X4 N, 4X1 W). These observations will be compared with apparently analogous phenomena reported at polar latitudes, in Sect. 6, certain dierences have already been noted and it remains to be established to what extent the fundamental natures of the polar and mid-latitude echoes and mesopause regions are similar
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