Vertical velocities associated with gravity waves measured in the mesosphere and lower thermosphere with the EISCAT VHF radar

The EISCAT VHF radar (69.4°N, 19.1°E) has been used to record vertical winds at mesopause heights on a total of 31 days between June 1990 and January 1993. The data reveal a motion ®eld dominated by quasi-monochromatic gravity waves with representative apparent periods of 30±40 min, amplitudes of up to 2.5 m s and large vertical wavelength. In some instances waves appear to be ducted. Vertical pro®les of the vertical-velocity variance display a variety of forms, with little indication of systematic wave growth with height. Daily mean variance pro®les evaluated for consecutive days of recording show that the general shape of the variance pro®les persists over several days. The mean variance evaluated over a 10 km height range has values from 1.2 ms to 6.5 ms and suggests a semi-annual seasonal cycle with equinoctial minima and solsticial maxima. The mean vertical wavenumber spectrum evaluated at heights up to 86 km has a slope (spectral index) of )1.36 ‹ 0.2, consistent with observations at lower heights but disagreeing with the predictions of a number of saturation theories advanced to explain gravity-wave spectra. The spectral slopes evaluated for individual days have a range of values, and steeper slopes are observed in summer than in winter. The spectra also appear to be generally steeper on days with lower mean vertical-velocity variance.