Earth's thermosphere is the region of atmosphere from E  90 km up to E  500-1,000 km altitude depending on solar-cycle conditions. Its lower edge is defined by the transition to a positive vertical temperature gradient above the mesopause, whereas the upper limit is generally taken to be the height at which the mean free path exceeds one scale height. The upper portion of the thermosphere corresponds to the altitude region occupied by low Earth-orbiting spacecraft. Understanding the details of the weather in the thermosphere is thus important for orbital prediction and for space debris collision avoidance. The dominant terms involved in orbital predictions are obtained from the direct application of well-known equations of Newtonian mechanics. However, operational responses to space debris hazards are driven by the uncertainty in these predictions, on the time scales of days to a week or so ahead. The largest contribution to this uncertainty comes from aerodynamic drag effects, due to imperfect knowledge of the (vector) wind and (scalar) mass density fields of the ambient atmosphere. Thus, in order to reduce these uncertainties, an accurate thermospheric model, including wind, is needed.

Multiple years of thermospheric wind and temperature data were examined to study gravity waves in Earth's thermosphere.Winds and temperatures were measured using all-sky imaging optical Doppler spectrometers deployed at three sites in Alaska, and three in Antarctica. For all sites, oscillatory perturbations were clearly present in high-pass temporally filtered F-region line-of-sight (LOS) winds for the majority of the clear-sky nights. Oscillations were also discernible in E-region LOS wind and F-region Doppler temperature, albeit less frequently. Oscillation amplitudes correlated strongly with auroral and geomagnetic activity. Observed wave signatures also correlated strongly between geographically nearby observing sites. Amplitudes of LOS wind oscillations were usually small when viewed in the zenith and increased approximately with the sine of the zenith angle -as expected if the underlying motion is predominantly horizontal. The SDI instruments observe in many look directions simultaneously. Phase relationships between perturbations observed in different look directions were used to identify time intervals when the oscillations were likely to be due to traveling waves. However, a portion of the instances of observed oscillations had characteristics suggesting geophysical mechanisms other than traveling waves -a recognition that was only possible because of the large number of look directions sampled by these instruments. Lomb-Scargle analysis was used to derive examples of the range of temporal periods associated with the observed LOS wind oscillations. F-region wind oscillations tended to exhibit periods typically ranging from 60 minutes and above. By contrast, E-region wind oscillation periods were as short as 30 minutes.