Fig. 1

Lomb–Scargle Periodograms of the residual time series data at all three analyzed Intermagnet stations ASC (top row), CZT (middle row) and SJG (bottom row)). In each column a different removal technique was applied: the subtraction of the model prediction of the CHAOS model (left), differencing of subsequent data points (middle) and the substraction of a smoothing spline (right). Dotted red lines indicate an estimation of the noise level. The dotted blue lines visualize the frequency of the prinicipal lunare tide M2. Other prominent frequency peaks or well-known tidal frequencies are marked with their tidal code, e.g., Sa, Ssa or Mf. The halved dotted blue lines in the right column indicate the targeted cutoff frequency for the smoothing spline (\(10^{-1}\) d\(^{-1}\)). The amplitudes, noise level and distribution of significant peaks vary as expected with the location due to differing coastal, oceanic and geographical conditions. However, the distribution of detected periodic signals and their signal strength varies with the chosen processing method. Histograms of the residual data obtained from the presented data processing after subtracting smoothing splines are included in the Additional file 1: Figures S1-S3.