Dynamics of the Earth's thermosphere across a range of spatial and temporal scales

Abstract

This dissertation presents an investigation of aspects of the dynamics of Earth's thermosphere that do not harmonize with the current understanding. Three distinct thermospheric phenomena that correspond to different spatial and temporal scales were examined using Fabry-Perot interferometer measurements of wind and temperature to investigate aspects of thermospheric dynamics. Earth's thermosphere has very high kinematic viscosity and is highly convectively stable. Theory and physics-based models suggest that structures in the horizontal wind on small spatial scales (∼500 km or less) are unlikely to occur in the upper thermosphere. By contrast, a large-scale, persistent, strong wind flow that transports air parcels from the dayside of the polar cap to the nightside, known as cross-polar jet, was found to stall abruptly upon exiting the polar cap above Alaska. Stalling was observed most frequently around mid-winter during periods of low solar activity. The stalling of the crosspolar jet is considerably more abrupt than the first principle models would predict. This phenomenon was investigated as an example of dynamics occurring at intermediate spatial and temporal scales. Along with this meso-scale phenomena, oscillatory perturbations in observed winds and temperatures were examined to test whether these could be the signatures of gravity wave activity. These waves represent a local to synoptic scale behavior. Wave periods ranging from ∼30 min to ∼3 hr were observed. Our data show that gravity wave activity was a nearly ubiquitous feature of winds observed at ∼240 km altitude at auroral latitudes. The actual wave field appears to be complicated, presumably resembling ocean surface waves if they could be visualized. Oscillation amplitudes were typically found to increase with increasing geomagnetic activity and the wave response to geomagnetic activity was similar in both hemispheres. Periods and horizontal wavelengths of the observed oscillations fall within the previously reported range for thermospheric gravity waves. In addition, we examined the thermospheric neutral temperature data to see whether there is any temporal trend in measured temperatures on a time scale of decades. Studies suggest that the Earth's troposphere is warming globally because of anthropogenic emission of greenhouse gases. First principle models suggest that, unlike in the troposphere, greenhouse gases are expected to cool the thermosphere. Consistent with model expectations our data also show a thermospheric cooling trend and the rate of cooling is -27.4 ± 6.2 K/decade. The estimated rate of cooling is more than four times the corresponding uncertainty, indicating that the cooling is statistically significant. However, the observed cooling rate is up to an order of magnitude higher than suggested by simulation studies considering only the effect of CO₂. Nevertheless, the observed trend agrees well with observations of ionospheric temperature at these altitudes using incoherent scatter radar. This long-term temperature trend is an indicator of behavior at the largest spatial and temporal scales. Overall, these three studies together suggest significant shortcomings in our current paradigm for understanding the behavior of the thermosphere. Specifically, stalling of the cross-polar jet shows that spatial structures can arise on much more localized scales than currently appreciated. The ubiquitousness of gravity waves suggests that their role in thermospheric dynamics is probably much more significant than currently appreciated. The effect of anthropogenic changes in atmospheric chemistry appear to be much more substantial than expected. The lowest altitudes at which spacecraft can orbit for an operational useful length of time occur in the Earth's thermosphere. Conditions in the thermosphere impact spacecraft orbits. It is thus necessary to account for the thermospheric dynamics in order to predict the orbit with lowest possible uncertainty and for collision avoidance.