## 3 Noise Characteristics  
We calculated the noise spectra in the HYB data and identified the outliers, which will provide information about the deviation of these points from the mean value. With the available information from the outliers, we designed a zero-phase forward low-pass Butterworth filter of order six, with a cut-off frequency 0.01 Hz, to eliminate the noise in the Z and F components of the HYB raw data. Figures 5 and 6 show the details of the raw and filtered data before and during the lockdown period from the HYB Observatory. From Figures 5a,b,f,g and 6a,b,f,g, a noticeable difference is observed in the occurrence frequency of spikes in the Z and F components before and during the lockdown period. Even though the amplitude was found to be in the range of \(\pm\)2.5 nT in the Z and F components, the density distribution and frequency of spikes were significantly reduced in the lockdown period when compared to before lockdown. A frequency peak was observed in the noise spectra at 11 mHz in the Z and F components before lockdown and during the lockdown period, and the same frequency peak was noticed with reduced power intensity during the lockdown period in both the components (Figures 5c,h and 6c,h). Figures 5d,i and 6d,i show the raw signal after removing the noise with a low-pass Butterworth filter with a lower cut-off frequency of 0.01 Hz in the Z and F components. Figures 5e,j and 6e,j show the 1 Hz raw signal of the Z and F components from the CPL Observatory. Comparing Figure 5d,i with Figure 5e,j, and Figure 6d,i with Figure 6e,j, shows that the filtered data are consistent with the raw data from CPL, after removing the remaining sources of anthropogenic noise on the Z and F components at the HYB Observatory. We also attempted to identify the effect of vehicular noise on the H and D components from the HYB Observatory before and during the lockdown period, and our analysis shows that a negligible difference was observed in the occurrence frequency of spikes, both in the H and D components, before and during the lockdown period, within the range of \(\pm\)0.03 nT. As observed in the Z and F components, we also observed peaks in the noise spectra at 11 mHz in the H and D components during and before the lockdown period at the HYB Observatory. We also compared the 1 Hz trends of the HYB H and D components with CPL and found that these trends are consistent. To strengthen our observations with the filtered data for the Z and F components from HYB, we subtracted the corresponding variations from CPL for the days before and during lockdown periods, and the linear trend shows good synchronicity between the observatories. Further, a small deviation of 1.5 nT was observed from the zero crossing line in all the components, which could be due to the distance between the observatories, followed by different geological constraints. \begin{table}
\begin{tabular}{c c c c c c c} \hline Date & \(\mathbf{H_{B}}\) (nT) & \(\mathbf{D_{B}}\) (min) & \(\mathbf{Z_{B}}\) (nT) & \(\mathbf{\delta}\) H & \(\mathbf{\delta}\) D & \(\mathbf{\delta}\) Z \\ \hline
10.04.2020 & 39,534.7 & \(-\)36.61 & 18,166.4 & \(-\)0.55 & 0.01 & \(-\)0.06 \\ \hline
16.04.2020 & 39,534.8 & \(-\)36.65 & 18,166.3 & 0.06 & \(-\)0.04 & \(-\)0.09 \\ \hline
20.04.2020 & 39,534.9 & \(-\)36.61 & 18,166.1 & 0.15 & 0.04 & \(-\)0.2 \\ \hline
28.04.2020 & 39,534.8 & \(-\)36.59 & 18,165.8 & \(-\)0.1 & 0.02 & \(-\)0.26 \\ \hline
04.05.2020 & 39,535.1 & \(-\)36.59 & 18,165.7 & 0.33 & 0 & \(-\)0.16 \\ \hline \multicolumn{5}{c}{Average of variable amplitudes} & \(\mathbf{0.24}\) & \(\mathbf{0.03}\) & \(\mathbf{0.18}\) \\ \hline \multicolumn{5}{c}{Standard deviation of variable amplitudes} & \(\mathbf{0.32}\) & \(\mathbf{0.03}\) & \(\mathbf{0.09}\) \\ \hline \end{tabular}
\end{table} 
Table 2: _Cont._Figure 5: Plots of HYB 1 Hz Z component data before the lockdown period: (**a**) raw data; (**b**) noise in the data; (**c**) noise spectra; (**d**) filtered data; (**e**) CPL raw data of Z component.