Midnight density and temperature maxima, and thermospheric dynamics in whole atmosphere model simulations

Hydrostatic expansion in a gravity field of an atmospheric layer with elevated temperatures, such as the long known thermospheric midnight temperature maximum (MTM), results in a total mass density increase at a given altitude above the layer. Long-term simulations with the Whole Atmosphere Model reveal a noticeable midnight density maximum (MDM), appropriately lagging behind the MTM at the same height. The MDM magnitude, timing, and variability are in good agreement with available in-situ observations. Of particular importance is the observation of a downward phase progression of the MDM peak time obtained from the San Marco satellites and closely reproduced in the model results. This is consistent with the suggestion, made over 30 years ago, that both the MTM and MDM are driven by tidal waves, in particular, the terdiurnal tide propagating upward from the lower atmosphere and interacting with a diurnally varying ion drag. The accompanying wind variations are also found in good agreement with radar observations, which first related them to the nighttime ionosphere collapse in the early 1970s.