Solar Flux Dependence of Upper Thermosphere Diurnal Variations: Observed and Modeled

Upper thermosphere mass density over the declining phase of solar cycle 23 is investigated using a day‐to‐night ratio (DNR) of thermosphere properties to evaluate how much relative change occurs climatologically between day and night. Challenging Minisatellite Payload (CHAMP) observations from 2002 to 2009, MSIS 2.0 output, and TIEGCM V2.0 simulations are analyzed to assess their relative response in DNR. The CHAMP observations demonstrate nightside densities decrease more significantly than dayside densities as solar flux decreases. This causes a steadily increasing CHAMP mass density DNR from around two to greater than four with decreasing solar flux. The MSIS 2.0 nightside densities decrease, with decreasing solar flux, more significantly than the dayside, resulting in the same trend as CHAMP. TIEGCM V2.0 displays an opposite trend in density DNR with decreasing solar flux due to dayside densities decreasing more significantly than nightside densities. A sensitivity analysis of the two models reveals the TIEGCM V2.0 to have greater sensitivity in temperature to levels of solar flux, while MSIS 2.0 displayed a greater sensitivity in mean molecular weight. The pressure DNR from both models contributed the most to the density DNR value at 400 km. As solar flux decreases, the two models' estimate of pressure DNR deviate appreciably and trend in opposite directions. The TIEGCM V2.0 dayside temperatures during middle‐to‐low solar flux are too cold relative to MSIS 2.0. Increasing the dayside temperature values by about 50–100 K and decreasing the nightside temperature slightly would bring the TIEGCM V2.0 into better agreement with MSIS 2.0 and CHAMP observations. Plain Language Summary The mass density of the upper thermosphere varies daily as the atmosphere thermally expands and contracts due to dayside heating and nightside cooling, respectively. However, the magnitude of change in mass density from day‐to‐night is not well described. The general consideration is that this day‐night ratio (DNR) in mass density is constant regardless of solar flux levels. This study demonstrates through observations and modeling that the mass density DNR varies from a value around two during solar maximum to a factor greater than four during solar minimum. This has implications on how the thermosphere responds to geomagnetic storms under these two phases of the solar cycle, and the level of drag a spacecraft in low Earth orbit will experience. The cause of such change is sug