Fig. 2

An illustration of the Pc selection method. High-pass-filtered magnetogram is shown at left panel, log-log PSD spectrum \(\Pi (F)\) and its linear approximation is given at right upper panel. Log-log spectrum and its linear approximation are shown in the upper right panel with solid magenta and dashed blue lines, respectively. A dash-dotted gray line at the upper right panel shows the lower boundary of maximal PSD logarithm \(\Pi (F)\) at \(F=F_{\max }\). A linear detrended (whitened) log-log spectrum \(\Pi _w(F)\) is shown at the right lower panel (magenta) and its parabolic approximation in the vicinity of maximum is shown with a blue dashed line. Boundaries of allowed F interval for a maximum of \(\Pi _w\) are shown with black triangles, and the magenta triangle shows the frequency logarithm \(F_{\max }\) of the actual \(Pi_w(F)\) maximum