Observation and simulations of winds and temperatures in the Antarctic thermosphere for August 2–10, 1992

Optically derived upper thermospheric wind and temperature data, collected at Antarctic stations at South Pole (L = 14), Mawson (L = 9.3), and Halley (L = 4.6), and averaged over the low-activity period August 2–10, 1992, have been interpreted with the help of simulation by the National Center for Atmospheric Research thermosphere ionosphere electrodynamic general circulation model (TIEGCM) with inputs matching the average conditions of observation. The simulation provides a global background context upon which the widely-separated optical observations can be placed. The simulation shows three large-scale structures in the polar wind field: the morning vortex, the evening vortex, and the cross-polar wind jet. Each of these came within view of the group of observing stations during the diurnal cycle, providing arrival time observations and signatures which were examined relative to the TIEGCM simulation. Reasonable correspondence was found, indicating the capability of the model to agree simultaneously with observations at three widely spaced stations representative of the subauroral and auroral zones, as well as the polar cap. Simulated wind directions were in excellent agreement with observation, although wind magnitudes frequently exceeded measured values by up to 30%. Apparent divergent flows in the data from Halley and Mawson were explained as signatures of vortices from their presence in the simulated wind fields. Observed diurnal mean temperatures compared well with the simulation, confirming that heat inputs and the distribution of thermal energy in the model are, on average, reasonable. A significant and persistent difference between experimental and modeled temperatures was that the diurnal temperature variation observed at South Pole peaked at the nightside crossing of the jet and was minimum a few hours before noon magnetic local time, whereas the simulation indicated minimum temperatures on the nightside, in antiphase to the measurements. A simple calculation indicates that the observed temperature difference between the air parcels entering the polar cap, encountered on the dayside, and those leaving the polar cap on the nightside is reasonably matched to the heating due to the ion-drag acceleration process. No explanation of the lack of this temperature rise in the TIEGCM simulation is presently available.

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