Thermospheric winds in the geomagnetic polar cap for solar minimum conditions

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Abstract
A Fabry‐Perot interferometer located at Thule, Greenland (Λ = 86) has monitored the F region thermospheric neutral wind over the northern hemisphere geomagnetic polar cap during the 1985/1986 solar minimum, winter solstice period. The wind observations were obtained by determining the Doppler shift of the (O I) 15,867‐K (630.0‐nm) emission. We present a subset of the measurements made during December 1985 to January 1986. Three factors make this data set unique and particularly valuable for a study of the effects of the deposition of energy and momentum from the magnetosphere into the high‐latitude neutral thermosphere. These factors are (1) the proximity of the observing station to the geomagnetic pole, (2) the continuous nature of the coverage due to the high geographic latitude and polar night conditions, and (3) the fact that the data set was obtained near solar minimum. The measured winds are compared with the simulations of the NCAR thermospheric general circulation model (TGCM). The results show that winds in the geomagnetic polar cap have a fundamental diurnal character, in accord with model predictions, with typical speeds of ∼200 m/s, generally in an antisunward direction. A large degree of variability, however, in both the magnitude and direction of the winds is observed, including evidence for curvature in the neutral flow within the instrumental region of observation (∼400 km diameter). Acceleration of the meridional component across Thule is observed at times. This acceleration is ascribed to regions of ion‐drag forcing associated with the magnetospheric input of energy and momentum. Characteristic asymmetric wind signatures were seen that were well correlated with positive or negative changes in the By component of the interplanetary magnetic field.