Thermospheric parameters' long-term variations over the period including the 24/25 solar cycle minimum. Whether the CO2 increase effects are seen?. (15th October 2021) - Mikhailov, Andrey V.
Abstract: The CO2 concentration has been increasing for more than five decades reaching ~29 % at present with respect to the pre-industrial era. The largest CO2 cooling effects in the thermosphere are predicted for solar minimum conditions. A comparison of solar minima in 1954/1964 to the recent one in 2019 was used to check at the quantitative level the theoretical predictions and the validity of the CO2 cooling hypothesis. June monthly median noontime ionospheric observations at Moscow, Rome, and Slough/Chilton were used to infer neutral gas density ρ, exospheric temperature Tex, height of the F2 -layer maximum hm F2, and total solar EUV flux for the (1954–2020) period. Solar and geomagnetic activity was shown to explain ~99 % of the whole variability in the retrieved neutral gas density and Tex during the (1958–2020) period resulting in statistically insignificant residual linear trends. A comparison of 1954/1964 to 2019 solar minima does not confirm the theoretically predicted decrease of ~21 % in ρ, ~15 K in Tex, and ~7 km in hm F2 related to a 29 % increase of the CO2 abundance. The main conclusion: despite continuous CO2 increase in the Earth's atmosphere long-term variations of thermospheric parameters are controlled by solar and geomagnetic activity. Highlights: Neutral density and exospheric temperature long term variations are explained for 99% by solar and geomagnetic activity variability It is not confirmed, for solar minima, the predicted decrease of ~21 % in ρ, ~15 K in Tex, and ~7 km in hm F2 related to a 29 % increase of the CO2 abundance. No distortion of the observed EUV versus F10.7 dependence has been revealed in the solar cycle 24.