## 4 Discussion  
To estimate the influence of vehicular noise on the 1 Hz measurements of the Z and F data at the HYB Observatory, we analyzed 100 days of data before and during lockdown. Raw variations in Z and F before and during lockdown showed quantifiable differences in the noise levels, with a reduction, to more than half, in the occurrence frequency of spikes. The spike amplitudes of first differences reduced from \(\pm\)4 nT before lockdown to \(\pm\)2.5 nT during lockdown in both Z and F.  
Figure 6: Plots of HYB 1 Hz F 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 F component. Plots of HYB 1 Hz F component data during the lockdown period: (**f**) raw data; (**g**) noise in the data; (**h**) noise spectra; (**i**) filtered data of F component; (**j**) CPL raw data of F component. A noticeable difference is observed in the occurrence frequency of spikes and the distribution density of spikes, which decreased in both the Z and F components during the lockdown period. We observed that the noise had an amplitude of about 8 nT in the raw data of the Z and F components before lockdown and 4 nT during lockdown, when there was no vehicular movement or metro rail. Similarly, at the KEL station (South Moravia), which is 25 m away from the local street, 5-10 nT spikes in the data were observed, attributable to the vehicle movements. Spike amplitudes increase for vans and buses; furthermore, a clear correlation between the spikes and traffic is established by experimental recording using another instrument placed 5 m away from the variometer and closer to the street [11]). Ref. [12] reported that at the EBR Observatory (Spain), which was located 0.5 km away from the railway, there were perturbations of about 3 nT and 15 nT in the H and Z components, similar to the effects of anthropogenic noise in the Z component noticed at HYB. In addition, comparing the EBR data with the SPT and AQU stations, which are located in noise-free places, 100 km and 200 km from EBR, respectively, magnetic disturbances appear in the EBR data, but not at the other two observatories, and are statistically estimated to be 89% due to train movements and 11% due to other sources. Similarly, at HYB, the noise was reduced by 60% during lockdown, in the complete absence of metro rail and all vehicular movement; the remaining 40% of noise would be due to the power lines and power station, which are at a distance of 500 m-1 km. Ref. [6] identified the influence of two DC power lines, Baltic Cable, operated by Baltic Cable AB, and Kontek, operated by Energinet ([[https://en.energinet.dk/](https://en.energinet.dk/)]([https://en.energinet.dk/](https://en.energinet.dk/))) (accessed on 27 November, 2021), on magnetic H and Z elements at Broffelde (BFE) Geomagnetic Observatory, and found that the H component is increased by about 0.0040 nT per megawatt of increased power by Baltic Cable, and about 0.0022 nT per ampere of increased current by Kontek Cable. The Z component is decreased by about 0.0013 nT for every megawatt of power increase at Baltic Cable, and by about 0.0016 nT for every ampere of current increase at Kontek Cable. Ref. [8] discussed the noise signatures of planes in the geomagnetic data during take-offs and landings (for a few minutes noticed perturbances in the geomagntic measurements about a few nT) which was about 100 m away from the new Easter Island observatory located at the end of Mataveri airport. Ref. [13] identified the noise in the Duronia (DUR) Observatory data during 2018 by computing the signal-to-noise ratio (SNR) on hourly averaged power spectra of both horizontal components (H and D). The observed noise peak at DUR during morning hours was ascribed to the contribution of Pc3-Pc4, as well the contribution of higher levels of local noise in the data.