UPPER THERMOSPHERE WINDS AND TEMPERATURES IN THE GEOMAGNETIC POLAR-CAP - SOLAR-CYCLE, GEOMAGNETIC-ACTIVITY, AND INTERPLANETARY MAGNETIC-FIELD DEPENDENCIES

Ground-based Fabry-Perot interferometers located at Thule, Greenland ( 76.5 degrees N, 69.0 degrees W, Lambda = 86 degrees) and at Sondre Str omfjord, Greenland (67.0 degrees N, 50.9 degrees W, Lambda = 74 degree s) have monitored the upper thermospheric (similar to 240-km altitude) neutral wind and temperature over the northern hemisphere geomagnetic polar cap since 1983 and 1985, respectively. The thermospheric observ ations are obtained by determining the Doppler characteristics of the (O I) 15,867-K (630.0-nm) emission of atomic oxygen. The instruments o perate on a routine, automatic, (mostly) untended basis during the win ter observing seasons, with data coverage limited only by cloud cover and (occasional) instrument failures. This unique database of geomagne tic polar cap measurements now extends over the complete range of sola r activity. We present an analysis of the measurements made between 19 85 (near solar minimum) and 1991 (neat solar maximum), as part of a lo ng-term study of geomagnetic polar cap thermospheric climatology. The measurements from a total of 902 nights of observations are compared w ith the predictions of two semiempirical models: the vector spherical harmonic (VSH) model of Killeen et al. (1987) and the horizontal wind model (HWM) of Hedin et al. (1991). The results are also analyzed usin g calculations of thermospheric momentum forcing terms from the thermo sphere-ionosphere general circulation model (TIGCM) of the National Ce nter for Atmospheric Research (NCAR). The experimental results show th at upper thermospheric winds in the geomagnetic polar cap have a funda mental diurnal character, with typical wind speeds of about 200 m s(-1 ) at solar minimum, rising to up to about 800 m s(-1) at solar maximum , depending on geomagnetic activity level. These winds generally blow in the antisunward direction, but are interrupted by episodes of modif ied wind velocity and altered direction often associated with changes in the orientation of the interplanetary magnetic field (IMF). The cen tral polar cap (> similar to 80 magnetic latitude) antisunward wind sp eed is found to be a strong function of both solar and geomagnetic act ivity. The polar cap temperatures show variations in both solar and ge omagnetic activity, with temperatures near 800 K for low Kp and F-10.7 and greater than about 2000 K for high K-p and F-10.7. The observed t emperatures are significantly greater than those predicted by the mass spectrometer/incoherent scatter model for high activity conditions. T heoretical analysis based on the NCAR TIGCM indicates that the antisun ward upper thermospheric winds, driven by upstream ion drag, basically ''coast'' across the polar cap. The relatively small changes in wind velocity and direction within the polar cap are induced by a combinati on of forcing terms of commensurate magnitude, including the nonlinear advection term, the Coriolis term, and the pressure gradient force te rm. The polar cap thermospheric thermal balance is dominated by horizo ntal advection, and adiabatic and thermal conduction terms,