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A mechanism of midlatitude noontime f o E long‐term variations inferred from European observations
Author(s) -
Mikhailov A. V.,
Perrone L.,
Nusinov A. A.
Publication year - 2017
Publication title -
journal of geophysical research: space physics
Language(s) - English
Resource type - Journals
eISSN - 2169-9402
pISSN - 2169-9380
DOI - 10.1002/2017ja023909
Subject(s) - middle latitudes , daytime , atmospheric sciences , environmental science , physics
Manually scaled June noontime monthly median f o E values at three European stations Rome, Juliusruh, and Slough/Chilton were used to understand the mechanism of f o E long‐term variations. The 11 year running mean smoothed f o E 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 ( f o E ave ) 11y and ( R 12 ) 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 ( f o E ave ) 11y long‐term variations. After removing solar activity long‐term variations the residual ( f o E ave ) 11y trend is very small (~0.029% per decade) being absolutely insignificant. This means that all ( f o E ave ) 11y variations are removed with one solar activity index, ( R 12 ) 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 ( f o E ave ) 11y variations. Therefore, the observed daytime midlatitude f o E 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.