Dynamics of the thermosphere

Major advances in our understanding of the dynamics of Earth's thermosphere (, 90-500 km) during the past 25 years are reviewed. Since the thermosphere is primarily an externally-forced system, a broad overview of the energy input, conversion and transport mechanisms in the ionosphere-thermosphere system is first provided. This serves as background and context for the non-specialist. Then, several broad areas of progress are in turn discussed in some detail: (i) the role of solar thermal tides in imposing significant longitudinal variability in the lower thermosphere ( 100-150 km), and affecting the zonal mean circulation at these altitudes; (ii) the zonal mean circulation of the thermosphere, the changes in O and N-2 relative densities that accompany it, and the competing roles of solar radiative heating and Joule (ohmic) heating in determining the overall structure of this circulation; (iii) polar and auroral thermosphere dynamics, and connections to relevant magnetosphere and ionosphere processes; and (iv) the global response to geomagnetic disturbances, i.e., relatively sudden injections of energy and momentum from the magnetosphere. The paper concludes with a personal assessment of future research directions and scientific questions that remain to be addressed in forthcoming decades.