Density‐Temperature Synchrony in the Hydrostatic Thermosphere

The paper presents a detailed analysis of the density‐temperature ( ρ ‐ T ) synchrony in the thermosphere using a hydrostatic general circulation model. The numerical models in general offer not only great potential for forecasting the transient response of the thermosphere but also are excellent tools for understanding the driving mechanisms of various thermospheric trends and features. This study investigates and isolates the dependency of the ρ ‐ T synchrony on the season, altitude, space weather, high‐latitude electrodynamics, and the lower atmospheric tidal spectrum. The results demonstrate that the previously reported ρ ‐ T synchrony begins around 300‐km (350‐km) altitude at the equator (high latitudes). The effect of the lower atmospheric tidal spectrum on the ρ ‐ T synchrony patterns seems to be only marginal and more noticeable during the equinox months. The study demonstrates that the ρ ‐ T phase lag is larger in the high latitudes of the summer hemisphere and evolves through the day and is attributable to ion drag and temperature fluctuations via soft particle precipitation. The study provides physical insights into how the winds contribute to the ρ ‐ T synchrony. In addition, the results show that geomagnetic activity contributes significantly to the ρ ‐ T synchrony; the underlying mechanism may be related to temperature enhancements in the high latitudes via Joule heating and associated nonlinear interactions. While the ρ ‐ T phase lags attributable to different solar activity levels are modest, the solar heating is the primary source that maintains the ρ ‐ T synchrony in the low/middle latitudes via upward propagating thermal tides.