Risbeth, H. (2002) Whatever happened to superrotation? Journal of Atmospheric and Solar-Terrestrial Physics, 64 (12-14), 1351-1360. (doi:10.1016/S1364-6826(02)00097-4).
Abstract
In the 1960s, it was deduced from observations of satellite orbits that the thermosphere rotates about 20% faster than the Earth; i.e., there is a prevailing west-to-east wind of order 100 ms?1. In the seventies, this ‘superrotation’ was explained as a consequence of the day-to-night variation of ion-drag at low latitudes, caused by the strong nighttime polarization fields generated by the F-layer dynamo. In the eighties, satellite-borne instruments measured prevailing zonal winds of only 20–30 ms?1 at low latitudes. In the 1990s, global coupled thermosphere–ionosphere models indicate similar prevailing wind speeds. Can all these be reconciled?
The paper briefly reviews the observations and the theory, discussing the essentials of the ion-drag explanation of superrotation. It is now clear that the local time variation of neutral air pressure is not the simple day/night variation that was assumed in the early F-layer dynamo calculations. The present-day thermospheric models can account for a prevailing west-to-east wind of 30–40 ms?1 at the magnetic equator, agreeing reasonably well with the wind measurements; the discrepancy with the satellite orbital data has been reduced but not eliminated.
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