On the dynamics and composition of the high-latitude thermosphere

Recent experimental measurements of the dynamics of the neutral upper thermosphere have demonstrated the important roles of ion-drag and Joule heating processes in establishing the basic neutral wind morphology and controlling neutral composition, particularly in the high-latitude region. Instruments on the Dynamics Explorer-2 spacecraft (DE 2), for example, were capable of measuring the three-dimensional vector neutral wind and ion drift in the thermosphere along the orbital track, together with constituent densities and temperatures. Ground-based optical and radar measurements of winds and temperatures from observatories in Greenland have contributed additional measurements of thermospheric neutral wind velocities and ionospheric parameters. The comprehensive nature of these various data sets has enabled more stringent experimental constraints to be placed on the numerical models of the region (thermosphere-ionosphere general circulation models, TIGCMs), leading to an improved theoretical understanding of the important physical processes that control thermospheric circulation and variability. In addition, the measurements have enabled the development of semi-empirical models of thermosphere dynamics which can be used in various theoretical studies. The Vector Spherical Harmonic (VSH) model, for example, provides a description of global thermospheric state variables (wind, temperature and density), using a combination of empirical data and NCAR-TIGCM calculations. This paper presents a brief review of some of the more recent progress made in this area by the team of researchers at the University of Michigan, with emphasis on the interpretation of experimental measurements made from DE 2 and from ground-based observatories in Thule and Søndrestrømfjord, Greenland. Comparisons between individual data sets from these sources and the VSH model are also presented.