Fig. 4

Mauesberger–Lowes spectra of the secular variation at the Earth’s surface in 2014.3. The scale on the y axis is logarithmic. Solid blue line: the spectrum of the initial (non-regularized) model constructed from the Swarm data. Solid black, magenta and cyan lines: the spectra of the estimated secular variation, assuming values of the inflation factor β equal to 3.8, 95.7 and 2393.5, respectively. The dotted lines are the spectra of the difference between the initial model and the estimated secular variation (same color code). The light gray area is the one corresponding to harmonic degree n≤8, the IGRF truncation