## 2 Initial Data, Their Preliminary Analysis and Preprocessing  
The initial data in this research are 1-min values of the GIC recorded at the Vykhodnov transformer substation (VKH) and 10-s data from the IMAGE magnetometers (Figure 1, Table 1). These observational data have been augmented by 1-min values of the geomagnetic IE index (regional proxy of the global AE index). The analysis of the samples was carried out for the period from 2015 close to the maximum activity of the 24 th solar cycle (January 2009-May 2020). Table 2 demonstrates the results of assessing the completeness of the raw time series, as well as of their reliability indicators. The gap occurrence is regarded as a failure of the data source, that is, its transition to an inoperative state [14]:  
\[T_{F}=T-T_{W}, \tag{1}\]  
where \(T_{F}\) is total downtime of the data source, \(T\) is an operating time, \(T_{W}\) is number of informative values (total operating time) for the time period \(T\). In this example \(T\) = 525,600 min (365 days). The average time to restore the operating state (equivalent to the expectation of the size of the missing fragment) and the mean time between failures of the system (equivalent to the average size of the fragment without gaps) are determined by the following expressions:  
\[\langle T2R\rangle=\frac{1}{N_{F}}\sum_{i=1}^{N_{F}}T2R_{i}=\frac{T_{F}}{N_{F}}; \tag{2}\]  
\[\langle T2F\rangle=\frac{1}{N_{W}+k}\sum_{i=1}^{N_{W}+k}T2F_{i}=\frac{T_{W}}{N_ {W}+k}, \tag{3}\]  
here, \(T2R_{i}\) and \(T2F_{i}\) are the time until the \(i\)-th system recovery after a failure and the time before the \(i\)-th system failure, respectively; \(N_{F}\) and \(N_{W}\) are the number of system failures and the number of failover recoveries, respectively; \(k=1\) or \(k=0\), if at the time of the start of observation the system was in a working or inoperative state, respectively. The magnetometer data have been downsampled to a common sampling step \(\Delta t=1\) min. The data detrending--exclusion of the baseline (daily component of the geomagnetic variations)--is carried out in accordance with the methodology described in [15]. Further, using the two-sided difference method, the absolute values of the first derivative at each point of the time series are determined as follows: \(\left\lvert\,dB/dt\,\right\rvert_{i}=\left\lvert\,B_{i+1}-B_{i-1}\,\right\rvert/ 2\Delta t\). At the final stage, to minimize the stochastic component, the time series are averaged over 15-min \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**No.**} & \multirow{2}{*}{**Data Source**} & \multicolumn{4}{c}{**Coordinates**} \\ \cline{3-8}  & & & \multicolumn{2}{c}{**Geographic**} & \multicolumn{2}{c}{**Geomagnetic**} \\ \cline{3-8}  & & **Station** & **Code** & **Latitude, [N]** & **Longitude, [E]** & **Latitude, [N]** & **Longitude, [E]** \\ \hline
1 & Vykhodnov & VKH & 68.83 & 33.08 & 64.41 & 125.95 \\
2 & Ivalo & IVA & 68.56 & 27.29 & 64.97 & 121.14 \\
3 & Kevo & KEV & 69.76 & 27.01 & 66.25 & 121.60 \\
4 & Sodankylá & SOD & 67.37 & 26.63 & 63.96 & 119.54 \\
5 & Lovozero & LOZ & 67.97 & 35.02 & 64.77 & 114.60 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Coordinates of the GIC recording station VKH and selected IMAGE magnetometers. Figure 1: Geography of data sources: magnetometers (green circles) and GIC recording station (red circle). Black solid lines denote the 330/440 kV power lines.