Thermospheric parameters' long-term variations over the period including the 24/25 solar cycle minimum. Whether the CO2 increase effects are seen?

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 hmF2, 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 hmF2 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

A mechanism of midlatitude noontime foE long-term variations inferred from European observations

Manually scaled June noontime monthly median foE values at three European stations Rome, Juliusruh, and Slough/Chilton were used to understand the mechanism of foE long-term variations. The 11 year running mean smoothed foE manifests long-term (for some solar cycles) variations with the rising phase at the end of 1960–1985 and the falling phase after 1985. A close relationship (even in details) between (foEave)11y and (R12)11y variations with the correlation coefficient of 0.996 (absolutely significant according to Fisher F criterion) suggests that the Sun is the source of these (foEave)11y long-term variations. After removing solar activity long-term variations the residual (foEave)11y trend is very small (~0.029% per decade) being absolutely insignificant. This means that all (foEave)11y variations are removed with one solar activity index, (R12)11y, i.e., this means that long-term variations are fully controlled by solar activity. Theory of midlatitude daytime E region tells us that long-term variations of solar EUV in two lines λ = 977 Å (CIII) and λ = 1025.7 Å (HLyβ) and X-ray radiation with λ<100 Å (both manifesting the same long-term variations with the rising phase at the end of 1960–1985 and the falling phase after 1985) are responsible for the observed (foEave)11y variations. Therefore, the observed daytime midlatitude foE long-term variations have a natural (not anthropogenic) origin related to long-term variations of solar activity. No peculiarities in relation with the last deep solar minimum in 2008–2009 have been revealed.Published4466–44732A. Fisica dell'alta atmosferaJCR Journa

Solar Extreme and Far Ultraviolet Radiation Modeling for Aeronomic Calculations

Modeling the upper atmosphere and ionospheres on the basis of a mathematical description of physical processes requires knowledge of ultraviolet radiation fluxes from the Sun as an integral part of the model. Aeronomic models of variations in the radiation flux in the region of extreme (EUV) and far (FUV) radiation, based mainly on the data of the last TIMED mission measurements of the solar spectrum, are proposed. The EUVT model describes variations in the 5–105 nm spectral region, which are responsible for the ionization of the main components of the earth’s atmosphere. The FUVT model describes the flux changes in the 115–242 nm region, which determines heating of the upper atmosphere and the dissociation of molecular oxygen. Both models use the intensity of the hydrogen Lyman-alpha line as an input parameter, which can currently be considered as one of the main indices of solar activity and can be measured with relatively simpler photometers. A comparison of the results of model calculations with observations shows that the model error does not exceed 1–2% for the FUVT model, and 5.5% for EUVT, which is sufficient for calculating the parameters of the ionosphere and thermosphere