A mechanism of midlatitude noontime f_oE long‐term variations inferred from European observations

Abstract

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.

Reply to Comment by Zhang et al. on the Paper “Long‐Term Variations of Exospheric Temperature Inferred From foF1 Observations: A Comparison to ISR Ti Trend Estimates” by Perrone and Mikhailov

“…If our earlier assumptions, namely, using MSIS‐86 T 120 and S values, were wrong, we hardly could obtain good testing results in a comparison with CHAMP/STAR neutral gas density observations (Mikhailov & Perrone, ; Perrone et al, ). Also, a good coincidence of the retrieved EUV with the EUVAC model by Richards et al () was also shown by Mikhailov et al () and by Mikhailov and Perrone (). It should be stressed that the EUVAC model has nothing common with our method; that is, this is an absolutely independent check of the method.…”

A New Method to Retrieve Thermospheric Parameters From Daytime Bottom‐Side Ne(h) Observations

“…We have not also discussed the retrieved solar EUV fluxes. On one hand, we have already presented and discussed our results on EUV earlier (Mikhailov et al, 2017;; on the other hand, this topic should be considered in relation with any geophysical problem, while this paper is devoted to the method and its testing using reliable CHAMP/STAR neutral gas density observations.…”

Comments on “Long‐Term Variations of Exospheric Temperature Inferred From foF1 Observations: A Comparison to ISR Ti Trend Estimates” by Perrone and Mikhailov

“…Recent studies by Perrone and Mikhailov (; hereafter designated PM17) and Mikhailov et al () used a single ionospheric measurement parameter, foF1, to derive a set of four neutral atmospheric parameters (exospheric temperature T e x , neutral concentrations [O], [O 2 ], and [N 2 ]) and one solar EUV flux factor ( f ). They used these results to analyze long‐term trends in the thermosphere and ionosphere.…”

“…In particular, PM17 studied T e x over multiple solar cycles and compared them to existing temperature trends from multiple observation sets using ISRs. The study states that “routine ISR observations based on a fixed model of ion composition may be not appropriate for long‐term trend analyses.” These authors have also published separate studies (Mikhailov & Perrone, , ; Mikhailov et al, ) examining long‐term trends in the ionosphere and thermosphere using results from the same derivation technique. We assert in this commentary that several aspects of the technique as described introduce large analysis difficulties and that the resulting ambiguities in derived parameters make them unsuitable for precise quantitative ionospheric and thermospheric long‐term trend studies.…”

“…Laštovička () has commented on the inconsistency of their foF1 trend analysis reported in Mikhailov and Perrone (). These F1‐region trend controversies increase the uncertainty regarding the validity of trend study results in those derived parameters reported not only by PM17 but also by Mikhailov and Perrone () and Mikhailov et al (), particularly with regard to whether the original foF1 data and/or the deriving method have led to any unrealistic trend determinations.…”