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We report the first lidar observations of neutral Fe layers with gravity wave signatures in the thermosphere from 110–155 km at McMurdo, Antarctica in May 2011. The thermospheric Fe densities are low, ranging from ∼200 cm−3 at 120 km to ∼20 cm−3 at 150 km. The measured temperatures from 115–135 km are considerably warmer than MSIS and appear to be related to Joule heating enhanced by aurora. The observed waves originate in the lower atmosphere and show periods of 1.5–2 h through 77–155 km. The vertical wavelength increases from ∼13 km at 115 km to ∼70 km at 150 km altitude. These wave characteristics are strikingly similar to the traveling ionospheric disturbances caused by internal gravity waves. The thermospheric Fe layers are likely formed through the neutralization of vertically converged Fe+ layers that descend in height following the gravity wave downward phase progression.

On 34 nights between December 1980 and May 1986, 171 monochromatic gravity waves were observed in the mesospheric Na layer at Urbana, Illinois, using a lidar system. The characteristics of the waves are compared with radar measurements of gravity wave activity and theoretical models of wave saturation and dissipation phenomena. The measured vertical wavelengths (λz) range from 2 km to 17 km, the observed periods (Tob) range from 25 min to 800 min and the horizontal wavelengths (λx) range from 20 km to almost 3000 km. The most apparent seasonal characteristic of the waves is the absence in summer of periods greater than 200 min and horizontal wavelengths greater than 400 km. The wave kinetic energy distributions are approximately proportional to kz−3, kx−15/14 and ƒ−5/3. Because the average amplitude growth length for the data is only 19 km and linear saturation theory predicts a kz−2 dependence for the kinetic energy distribution of saturated waves, the observed waves are not strongly damped and appear to be influenced more by dissipation rather than by saturation effects. Both λz and λx show a strong tendency to increase with Tob. The measured power law relationships are approximately λz ∝ Tob5/9 and λx ∝ Tob14/9. The wave induced mean flow acceleration ranges from 0 to −200 m s−1 day−1 and the average value is −27.2 m s−1 day−1. The effective viscosity limiting wave amplitudes ranges from 0 to 140 m² s−1 and the average value is 37 m² s−1.

The nightly and seasonal variations in gravity wave activity in the mesopause region are examined by analyzing 60 nights of Na lidar observations obtained during a 5‐year period at Urbana, Illinois. The lidar data were used to calculate the atmospheric density perturbations and their spectra. The atmospheric density variances, density vertical shear variances, vertical wind variances, ω spectra magnitudes, and m spectra magnitudes all exhibit considerable nightly variability as well as strong annual and semiannual variations with the largest values in summer. The annual mean values of the rms density perturbations, density shear variance, and rms vertical wind velocity are respectively 5.6%, 37 (%/km)2, and 1.3 m/s. The midsummer values for these parameters are typically 2 to 3 times larger than the annual means. the equivalent Richardson number for the wave field varies between 1/2 and 2 for most of the year. However, during summer, Ri decreases appreciably and sometimes falls well below 1/4. The ω spectra exhibit power law shapes with slopes varying between −1.28 and −2.45. The m spectra also exhibit power law shapes with slopes varying between −2.20 and −3.55. The annual mean slopes are −1.82 for the ω spectra and −2.90 for the m spectra. The magnitudes of both the ω spectra and m spectra vary by more than a factor of 10 throughout the year at all periods between 5 min and 4 hours and vertical scales between 1 and 10 km, with the largest values in summer. The observed variability of the m spectra slopes and magnitudes is not consistent with the predictions of linear instability theory and the concept of a universal vertical wave number spectrum. The characteristic periods (T* = 2π/ω*) vary between 3 and 50 hours with an annual mean of 9.7 hours. The characteristic vertical wavelengths (λz* = 2π/m*) vary between 8.9 and 27 km with an annual mean of 14.1 km. The characteristic wavelengths λz* exhibit a weak seasonal variation with smaller values in summer ( ) compared to winter ( ). The rms bandwidths of the wave field have mean values of and . The two‐dimensional density spectra are not separable.

The Collaborative Observations Regarding the Nightglow (CORN) campaign took place at the Urbana Atmospheric Observatory during September 1992. The instrumentation included, among others, the Aerospace Corporation narrowband nightglow CCD camera, which observes the OH Meinel (6–2) band (hereafter designated OH) and the O2 atmospheric (0–1) band (hereafter designated O2) nightglow emissions; the University of Illinois Na density/temperature lidar; and the University of Illinois MF radar. Here we report on observations of small‐scale (below 10‐km horizontal wavelength) structures in the OH airglow images obtained with the CCD camera. These small‐scale structures were aligned perpendicular to the motion of 30‐ to 50‐km horizontal wavelength waves, which had observed periods of about 10–20 min. The small‐scale structures were present for about 20 min and appear to be associated with an overturned or breaking atmospheric gravity wave as observed by the lidar. The breaking wave had a horizontal wavelength of between 500 and 1500 km, a vertical wavelength of about 6 km, and an observed period of between 4 and 6 hours. The motion of this larger‐scale wave was in the same direction as the ≈30‐ to 50‐km waves. While such small‐scale structures have been observed before, and have been previously described as ripple‐type wave structures [Taylor and Hapgood, 1990], these observations are the first which can associate their occurrence with independent evidence of wave breaking. The characteristics of the observed small‐scale structures are similar to the vortices generated during wave breakdown in three dimensions in simulations described in Part 2 of this study [Fritts et al., this issue]. The results of this study support the idea that ripple type wave structures we observe are these vortices generated by convective instabilities rather than structures generated by dynamical instabilities.