MgII as a Solar Proxy to Filter F2-Region Ionospheric Parameters

The estimation of long-term variations in F2 region ionospheric parameters is sensitive to the proxy used to filter solar activity effects, in addition to other factors such as the period considered and filtering methods. In this work, foF2 trends are estimated using the MgII index as an EUV proxy in the usual filtering procedure that considers the residuals of a linear regression for foF2 versus MgII. The sensitivity of these trend values to the foF2 residual estimation and to a multiple linear regression is analyzed, together with a comparison to trends obtained when F10.7, Lyman α and Rz are used instead of MgII. The study is carried out for three Japanese stations, namely Wakkanai, Kokubunji and Syowa, for the period 1979–2020. The results obtained suggest that MgII is a more stable and suitable EUV solar proxy to filter out variation in solar activity from foF2, followed by F10.7, at least for the 1979–2020 period. In addition, while Kokubunji and Wakkanai presented significant decreasing foF2 trends qualitatively, in agreement with the expected consequences of increasing greenhouse gas concentrations, the foF2 increase at Syowa was qualitatively explained by the secular variation in the Earth’s magnetic field at its location.