Tides and the midnight temperature anomaly in the thermosphere

Spectral analysis of the midnight temperature anomaly observed on AE‐E indicates that the phenomenon can be adequately represented with wave numbers 1<m<5. Theoretical results from a multi‐constituent circulation model suggest that this feature and associated variations in composition are the result of a complex interaction between the semidiurnal tide generated in the lower atmosphere and ion‐neutral momentum coupling associated with the diurnal variation of the ion density which in turn also generates higher order tidal modes. The semidiurnal tide from the lower atmosphere and the ion drag induced component excited in the thermosphere are out of phase and of comparable magnitudes. Variability in the tidal excitation source and the prevailing wind system of the lower atmosphere as well as in the ionospheric density (which itself is sensitive to tide‐generated dynamo electric fields) are presumably responsible for the observed lability of the midnight temperature anomaly.