A study of the thermospheric forces at a high latitude site on two days of differing geomagnetic activity

Data from the Fabry-Perot Interferometer and Dynasonde at Halley (75.5°S, 26.6°W, L ∼ 4.2), Antarctica, have been used to calculate the forces acting on the high latitude thermosphere. Two case studies of the forces have been undertaken to study why the thermospheric zonal wind speeds are typically so different on nights with different geomagnetic activity. One case study analyses the forces on a geomagnetically active night and the other analyses them on a geomagnetically quiet night. Even on the geomagnetically active night, it is found that the ion drag force is not necessarily the largest force at any one time. Simple comparison of the magnitudes of the forces does not make it very clear which ones dominate in controlling the motion of the thermosphere. This can be seen more clearly by rewriting the momentum equation so that the neutral velocity is expressed in terms of the ion velocity, and the other forces normalized by the ion density. It then becomes clear that, in the evening, the differences in the neutral velocity are due to increases in both ion density and ion velocity, while in the morning, only changes in ion density are important. Thus, although the ion drag force is often not the largest force, it appears that changes in it can account for the variations in neutral velocity between the two nights that were studied. It has also been shown as part of the analysis that whether or not the viscosity needs to be considered when calculating the ion drag force at an altitude of 240 km depends on the ion density profile. If the profile has a single peak then it is only necessary to consider the ion density at 240 km. It is, however, possible that just considering the ion density at this altitude may lead to an underestimate of the effective ion drag force if more than one peak is present.

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