Neutral density anomalies in the whole polar thermosphere produced by ion drag

Abstract:

Recent studies have pointed out that the upward mass transport due to heating is important for the thermospheric mass density anomalies in the cusp. The cusp is not a single area of anomalies, and there is another neutral mass density enhancement on the nightside. Density depletions also occur on the dawn- and duskside, as has been shown by the CHAMP satellite observations. In the present study, in order to understand the mechanism of the enhancement or depletion of the neutral mass density, we have studied the effect of ion drag due to ionospheric convection, which can give momentum to the neutral air over the whole area of the polar thermosphere. We have used a three-dimensional numerical model in which heat transfer from plasmas to neurals is not included. The result of the model calculations shows that the neutral mass density is enhanced at the cusp. Besides the cusp enhancement, another enhancement occurs on the sunlit side of the terminator. This terminator enhancement then spreads to the nightside due to thermal expansion, so that density enhancements can occur on the nightside. On the dawn- and duskside the ion drag drives neutral wind circulations, which make depletions in those regions. These dawn- and duskside depletions would make the cusp enhancement more prominent.