We present over 350 hours of Na lidar measurements of mesospheric (~80--105 km) density perturbations obtained on 50 nights throughout the year at Urbana, Illinois. The data are used to study the spectra of perturbations exhibiting upward and downward phase progression. For perturbations with observed frequencies between ω0=2&pgr;/(120 min) and 2&pgr;/(12 min) or vertical wave numbers beween m=2&pgr;/(10 km) and 2&pgr;/(1 km), on average, two thirds of the density variance was associated with perturbations exhibiting downward phase progression (i.e., upwardly propagating waves) and one third of the variance was associated with perturbations exhibiting upward phase progression. The nightly variances were highly correlated which suggests that the downward and upward phase perturbations originate from the same low-altitude gravity wave sources. Theoretical models incorporating Doppler effects of the background winds were derived by assuming an intrinsic spectrum of upwardly propagating gravity waves embedded in a uniform horizontal flow. These models were used to predict the m and ω0 spectral indices and magnitudes associated with upward and downward phase perturbations.

For the ω0 spectra the measured upward phase spectral indices (mean=-1.6) were consistently shallower than the downward phase spectral indices (mean=-1.9). This characteristic is compatible with the predictions of the scale independent diffusive filtering theory of Gardner <1994> but not with the separable gravity wave model of Gardner et al. <1993a> which predicts the spectral indices associated with the upward and downward phase ω0 spectra will both be approximately equal to the spectral index of the intrinsic spectra. Both models predict that the upward phase variances will approach 20 to 30% of the total variance for horizontal wind velocities between 30 and 50 m/s. These are typical values for the mean horizontal winds in the upper mesosphere above Urbana. The data and model predictions strongly suggest that the majority of the wave energy observed in the mesopause region at Urbana is associated with upwardly propagating gravity waves generated by sources in the lower atmosphere. ¿American Geophysical Union 1994