Solar variability in all its forms has profound effects on the middle and upper atmosphere of the Earth, and on the near-Earth space environment. These effects increase with increasing altitude, and become dramatic above about 100 km. Considerable study has been devoted to solar-driven changes to the charged particle environment in the ionosphere and magnetosphere, but the high-altitude neutral atmosphere, or thermosphere, is also important. The problems associated with tracking and predicting the trajectories of the thousands of objects currently in Earth orbit have received recent attention; for low-Earth orbits, an important variable is the drag force applied by thermospheric atoms and molecules. In order to calculate and predict thermospheric density and composition, numerical models are employed, using solar irradiance and magnetospheric currents, together with lower atmosphere processes, to create a dynamical description of the coupled thermosphere and ionosphere. In addition to the challenges associated with applying realistic external inputs, the upper atmosphere is evolving, due to changes in thermodynamics associated with anthropogenic increases in radiatively active gases, particularly carbon dioxide. Understanding these long-term changes is, in turn, complicated by recent developments in the solar cycle, with the last solar minimum particularly long and quiet, and the current cycle apparently very weak. This seminar will give an overview of these developments, present some modeling and measurement results, and discuss the implications for solar cycle variability.
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