Whatever happened to superrotation?

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 m s-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 nightime polarization fields generated by the F-layer dynamo. In the eighties, satellite-borne instruments measured prevailing zonal winds of only 20 -30 m s-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 m s-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. © 2002 Elsevier Science Ltd. All rights reserved.