## 4 Correlations between Different Magnetometers
### Hypothesis Based on the Meridian Coordinates of the Magnetometers  
It is evident that the fluctuations in the averaged SRCIs vary significantly across different locations (Figure S3). Nevertheless, the curves derived from magnetometer data collected in California and Canada exhibit greater similarity in shape compared to those reconstructed in Lithuania and New Zealand. Therefore, it is interesting to observe the statistical correlation between different magnetometers. Correlations were calculated as the standard similarity measure between averaged SRCIs in different geographical locations (Tables 3-5), using Equation (5):  
\[r_{xy}=\frac{\sum\limits_{i=1}^{5}(x_{i}-\bar{x})(y_{i}-\bar{y})}{\sqrt{\sum \limits_{i=1}^{5}(x_{i}-\bar{x})^{2}(y_{i}-\bar{y})^{2}}} \tag{5}\]  
where \(x_{i}\) and \(y_{i}\) are the elements of the averaged SRCIs at locations \(X\) and \(Y\), and \(\overline{x}\) and \(\overline{y}\) are the global means of the averaged SRCIs at locations \(X\) and \(Y\). The correlation tables (Tables 3-5) confirm a surprisingly large correlation between the magnetometers in California and Canada. Such a similarity gives the foundation for the following hypothesis. \begin{table}
\begin{tabular}{l l l l l l} \hline \hline  & **CAN** & **LTU** & **CAL** & **SAU** & **NZ** \\ \hline CAN & 1 & \(-\)0.0823 & 0.6651 & 0.1535 & 0.1129 \\ \hline LTU & \(-\)0.0823 & 1 & 0.0413 & 0.5670 & 0.2117 \\ \hline CAL & 0.6651 & 0.0413 & 1 & 0.1353 & \(-\)0.0362 \\ \hline SAU & 0.1535 & 0.5670 & 0.1353 & 1 & \(-\)0.0637 \\ \hline NZ & 0.1129 & 0.2117 & \(-\)0.0362 & \(-\)0.0637 & 1 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: The Pearson correlation coefficient between the averaged SRCIs, obtained by eliminating the diel cycle, at different magnetometers within the 0–3.5 Hz frequency range. \begin{table}
\begin{tabular}{l l l l l l} \hline \hline  & **CAN** & **LTU** & **CAL** & **SAU** & **NZ** \\ \hline CAN & 1 & \(-\)0.4274 & 0.4214 & 0.1033 & 0.1395 \\ \hline LTU & \(-\)0.4274 & 1 & \(-\)0.0383 & 0.4818 & \(-\)0.1893 \\ \hline CAL & 0.4214 & \(-\)0.0383 & 1 & 0.0370 & 0.1154 \\ \hline SAU & 0.1033 & 0.4818 & 0.0370 & 1 & 0.0349 \\ \hline NZ & 0.1395 & \(-\)0.1893 & 0.1154 & 0.0349 & 1 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: The Pearson correlation coefficient between the averaged SRCIs, obtained by eliminating the diel cycle, at different magnetometers within the 0–1 Hz frequency range. \begin{table}
\begin{tabular}{l l l l l l} \hline \hline  & **CAN** & **LTU** & **CAL** & **SAU** & **NZ** \\ \hline CAN & 1 & 0.1441 & 0.9295 & 0.3814 & \(-\)0.2425 \\ \hline LTU & 0.1441 & 1 & \(-\)0.0414 & 0.6158 & 0.3684 \\ \hline CAL & 0.9295 & \(-\)0.0414 & 1 & 0.2945 & \(-\)0.3787 \\ \hline SAU & 0.3814 & 0.6158 & 0.2945 & 1 & 0.1807 \\ \hline NZ & \(-\)0.2425 & 0.3684 & \(-\)0.3787 & 0.1807 & 1 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: The Pearson correlation coefficient between the averaged SRCIs, obtained by eliminating the diel cycle, at different magnetometers within the 3.5–36 Hz frequency range. **Hypothesis 1**. : _The correlation between the local magnetic fields (with the diel cycle eliminated) measured at different locations around the globe depends on the absolute difference between the longitudes of those locations._  
The absolute differences between the meridian angles of the geographical sites of different magnetometers are shown in Table 6. The numbers in parentheses following the name of the country indicate the meridian angle of the magnetometer site in each designated country. In order to prove (or drop) Hypothesis 1, the correlation data in Tables 3-5 are coupled with the absolute differences between the meridian angles of the magnetometer sites in five different countries. Examining the correlations between the SRCIs of magnetometers located in different locations during the two-week period, we clearly observed moderate to very strong relationships among the SRCIs calculated by consecutive magnetometers.