Superrotation of the upper atmosphere

Abstract

Some years ago D. G. King‐Hele discovered, from small changes in the orbital inclination of artificial satellites, that the upper atmosphere rotates about 20% faster than the earth. This article outlines several theories that have been advanced to explain this so‐called ‘superrotation.’ Some theories explain it as a consequence of the horizontal pressure variations that are known to exist in the thermosphere. Other theories attribute it to ionospheric or magnetospheric motions that react on the neutral air. Another group of theories treats superrotation in terms of thermally driven instabilities or oscillations in the thermosphere. It is not yet possible to make a clear choice of a ‘correct’ explanation of superrotation, but at present the following appear to be possible contributory causes: geostrophic winds, diurnal modulation of ion drag by wind‐produced polarization fields, ion drag driven by electric fields associated with magnetic substorms, the ‘moving flame’ phenomenon, and deposition by atmospheric waves of eastward momentum carried from lower levels.