High-Latitude Thermospheric Density and Wind Dependence on Solar and Magnetic Activity

Processes in the high-latitude thermosphere are strongly controlled by the activity of the sun and by the geomagnetic field geometry. The CHAMP satellite, with its sensitive tri-axial accelerometer, provided detailed information about thermospheric dynamics over its mission life-time (2000\textendash2010), thus contributing significantly to the CAWSES (Climate And Weather of the Sun-Earth System) programme. In this chapter, studies on thermospheric winds and density anomalies at high magnetic latitudes are presented. Thermospheric winds above the poles are directed predominantly from day to night side. Observations, however, reveal a distinct difference between winds on the dawn and dusk sides at auroral latitudes. While on the dawn side fast zonal winds towards night are prevailing, an anti-cyclonic vortex is formed on the dusk side. For the explanation of these local time dependent features various thermodynamic and electrodynamic influences have to be considered. As an example for mass density variation the cusp-related density anomaly is studied. The amplitude of this prominent local peak in mass density is influenced by the level of solar flux (F10.7) and by the solar wind input into the magnetosphere as quantified by the electric field caused by reconnection. A prerequisite for the appearance of density anomalies is the presence of soft-energy particle precipitation. By combining CHAMP and EISCAT measurements, it has been shown that Joule heating, fuelled predominantly by small-scale field-aligned currents (FACs), causes a strong increase in temperature at altitudes below 200 km. As a consequence molecular-rich air is up-welling. A density anomaly is recorded at 400 km altitude. Combining different observations and numerical model results provides a plausible chain of processes leading to the observed cusp-related density anomaly.