Fig. 2
From: On the characterization of tidal ocean-dynamo signals in coastal magnetic observatories
Comparison of highly resolved periodograms of the M2 TODS peaks at all three analyzed Intermagnet stations ASC (top row), CZT (middle row) and SJG (bottom row). While the left column shows the spectra obtained after first order differencing, the right column shows the results after smoothing spline subtraction. The dotted blue lines mark the frequency of the diurnal principal lunar tide M2. Solid red lines mark side band frequencies associated with an annual variation (Sa) and red dot–dash lines mark side band frequencies associated with a semiannual variation (Ssa). When comparing the left and the right column, we find the hpyerfine structure around the M2 TODS peaks to be independent of the method used for trend removal