Whatever happened to superrotation?

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

Modelling F2-layer seasonal trends and day-to-day variability driven by coupling with the lower atmosphere

This paper presents results from the TIME-GCM-CCM3 thermosphere–ionosphere–lower atmosphere flux-coupled model, and investigates how well the model simulates known F2-layer day/night and seasonal behaviour and patterns of day-to-day variability at seven ionosonde stations. Of the many possible contributors to F2-layer variability, the present work includes only the influence of ‘meteorological’ disturbances transmitted from lower levels in the atmosphere, solar and geomagnetic conditions being held at constant levels throughout a model year.In comparison to ionosonde data, TIME-GCM-CCM3 models the peak electron density (NmF2) quite well, except for overemphasizing the daytime summer/winter anomaly in both hemispheres and seriously underestimating night NmF2 in summer. The peak height hmF2 is satisfactorily modelled by day, except that the model does not reproduce its observed semiannual variation. Nighttime values of hmF2 are much too low, thus causing low model values of night NmF2. Comparison of the variations of NmF2 and the neutral [O/N2] ratio supports the idea that both annual and semiannual variations of F2-layer electron density are largely caused by changes of neutral composition, which in turn are driven by the global thermospheric circulation.Finally, the paper describes and discusses the characteristics of the F2-layer response to the imposed ‘meteorological’ disturbances. The ionospheric response is evaluated as the standard deviations of five ionospheric parameters for each station within 11-day blocks of data. At any one station, the patterns of variability show some coherence between different parameters, such as peak electron density and the neutral atomic/molecular ratio. Coherence between stations is found only between the closest pairs, some 2500 km apart, which is presumably related to the scale size of the ‘meteorological’ disturbances. The F2-layer day-to-day variability appears to be related more to variations in winds than to variations of thermospheric composition