Solar Rotation Effects in Earth's and Mars' Thermospheric Densities as Revealed by Concurrent MAVEN, Swarm‐C, and GOES Observations

The responses of Earth's and Mars' thermospheric densities to quasi‐periodic (∼27‐day) solar rotation variations in flux were measured contemporaneously by the Mars Atmosphere and Volatile EvolutioN, Geostationary Operational Environmental Satellites, and Swarm‐C satellites. While large solar rotation variability is found in both planetary thermospheres, correlation analyses performed on over 6 years of observations reveal that, independently of extreme ultraviolet flux level, Earth's daytime density response is about 10%–50% larger than Mars' at a similar density level. Important altitude dependencies in the density sensitivity to the solar rotation in solar flux are found in the Martian thermosphere, while the terrestrial thermosphere is shown to exhibit only small (±5%) day/night and latitude variations in the response. Detailed analyses focused on correlative periods in 2015–2016 and 2020 indicate important solar cycle effects in the sensitivities of both planetary thermospheres, with increased slopes under low solar flux conditions. These results provide important new insights into processes relevant to the interpretation of the sources of short‐term density variability in Mars' and Earth's thermospheres associated with solar drivers and point to the need for targeted modeling efforts along with dedicated data analyses to help resolve current unknowns in thermal balance processes. Plain Language Summary The quasi‐periodic change in solar extreme ultraviolet (EUV) radiation from active regions on the Sun rotating with a period of about 27 days is one of the largest sources of short‐term variability in the upper atmospheres of both Mars and Earth. This work investigates the response of total mass densities in the upper atmospheres of Mars (∼160–260 km altitude) and Earth (∼450–480 km altitude) to the solar rotation variation in EUV flux, as a tracer of short‐term solar‐driven impacts on neutral densities. We use simultaneous density and solar flux observations from the Mars Atmosphere and Volatile EvolutioN, Swarm‐C, and Geostationary Operational Environmental Satellites from 2014 through 2021. Earth's daytime densities in the middle thermosphere are found to be about 10%–50% more responsive than Mars' for an equivalent altitude. Important dependencies on height are found at Mars, while small day/night and latitude dependencies are found at Earth. Correlation analyses also suggest strong solar cycle effects on the sensitivities, with significantly highe