Long-lived polar thermospheric vortices: a combined radar and optical study

A Doppler Imaging System has been used to map the polar thermospheric circulation in the vicinity of the high-latitude station of Kiruna (67°N, 20°E), in conjunction with extended coverage of ionospheric plasma flow patterns obtained simultaneously by the EISCAT radar facility. Combined wind and radar results were obtained during the nights of 24/25 and 25/26 November 1987. On 24 November 1987, a strong anti-cyclonic vortex was generated within the region of the dusk auroral oval and polar cap, associated with sunward wind velocities in the dusk auroral oval, of up to 400 m s −1. These winds were generated by the combination of ion drag and the resonant response of the thermosphere. The observations of this sustained anti-cyclonic vortex on the night of 24/25 November confirmed a concept predicted earlier by theoretical modelling. Following the intense activity which led to their generation, such vortices may survive during several hours of decreasing geomagnetic activity. The decrease of viscosity with decreasing altitude and the decrease of plasma density in the ionosphere below 250 km limits the viscous and the electrodynamic dissipation of the thermospheric ‘flywheel’.