## 4 Correlations between Different Magnetometers  
### The Elimination of the Circadian Rhythm  
It is well known that the SRA varies throughout the day [21]. There are records showing almost clock-like accuracy of the diurnal SRA changes [47]. Therefore, the elimination of the diurnal SRA changes is performed by computing moving averages of SRCI indices and using a length of averaging window equal to 24 h. The investigation period used in this paper spans from 28 February 2015 to 11 March 2015. As mentioned previously, the calibration (the selection of \(\varepsilon\)) is based on data collected during 28 February 2015. In other words, the values of \(\varepsilon\) are fixed for the entire investigation period for each individual site, for each specific frequency range.  
It is interesting to observe the variation in the local magnetic field measured at different magnetometers with the diel cycle (the 24 h period) eliminated. This is performed by arithmetic averaging over 24 SRCIs. This averaging is performed in overlapping windows throughout the investigation period (28 February 2015 to 11 March 2015). The averaging window is shifted by one SRCI (by one hour) until the last day of the investigation period is reached (11 March 2015). An example of the SRCI computations is shown in Figure 9. All resulting moving averages are shown in Figure S3.  
Note that the horizontal axis in panel (b) denotes the number of days (28 February 2015 to 11 March 2015); the vertical axis represents the averaged SRCIs. Since every SRCI is measured in percent, the averages of the SRCIs are also measured in percent. Note that the x-axis in panel (a) denotes hours (1-24), the y-axis represents the number of days (28 February 2015, to 11 March 2015), and the vertical axis shows the SRCI (measured in percent). The color scheme used in panel (a) represents the local time of each individual location: bright yellow denotes the middle of the day, and dark blue shows the middle of the night.  
\begin{table}
\begin{tabular}{l l l l} \hline  & **3.5 –36 Hz** & **0 –3.5 Hz** & **0 –1 Hz** \\ \hline California & 0.55 & 1.1 & 1.75 \\ \hline Canada & 0.575 & 0.95 & 1.575 \\ \hline Lithuania & 0.675 & 1.49 & 2.65 \\ \hline New Zealand & 0.865 & 1.75 & 2.05 \\ \hline Saudi Arabia & 0.64 & 1.58 & 2.1 \\ \hline Average & 0.661 & 1.374 & 2.025 \\ \hline \end{tabular}
\end{table}
Table 2: The calibrated \(\varepsilon\) values for each location in different frequency ranges.  
Figure 9: SRCIs computed for the California site in the frequency range 0-1 Hz. The dynamics of the SRCI in the time period between 28 February and 11 March 2015, is shown in panel (**a**). The color scheme in panel (**a**) represents the local time (the bright yellow denotes the middle of the day and the dark blue shows the middle of the night). The time shown on the x-axis in panel (**a**) corresponds to GMT. Moving averages of SRCI (averages with the diel cycle eliminated) are depicted in panel (**b**). The dynamics of SRCI computed for the California site in the frequency range 0-3.5 Hz are shown in panel (**c**) and moving averages of the SRCI are depicted in panel (**d**). The dynamics of the SRCI computed for the Canada site in the frequency range 0-1 Hz are shown in panel (**e**) and moving averages of the SRCI are depicted in panel (**f**). The dynamics of the SRCI computed for the Canada site in the frequency range 0-3.5 Hz are shown in panel (**g**) and moving averages of the SRCI are depicted in panel (**h**).