## 2 Data and Results  
In order to achieve the aim of the study, we studied and analyzed the 1 Hz vector variation HDZ data acquired from a tri-axial fluxgate magnetometer and the 1 Hz scalar variation (F) data acquired from a GSM90-F1 magnetometer for 50 days before and 50 days during lockdown (1 February to 10 May 2020). Quiet days were chosen in March (before lockdown) and compared with April 2020 (during lockdown) for the analysis. Further, we used the vector and scalar data from the Choutuppal (CPL) Magnetic Observatory of this institute, 65 km away from HYB, for comparison. The CPL Observatory is located in a 100 acre campus, and is relatively far from sources of anthropogenic and cultural noises. International quiet (IQ) days vary from the highest to the lowest order of quietness and disturbance, respectively [10]. This classification leads to the study of the different physical processes in the ionosphere and magnetosphere, which cause variations in the geomagnetic field. The K-index is defined as a quasi-logarithmic measure, ranging in steps of one, from zero to nine, of the range of geomagnetic disturbance at a geomagnetic observatory in a three-hourly UT interval (00-03, 03-06,..., 21-24) [1]. The K-sum is the sum of all 3 h interval K-index values of one day. The IQ days of March 2020 were 14, 11, 5, 10 and 7 prior to lockdown (K-sum was 4, 4, 8, 7 and 6, respectively), and in April 2020, they were 30, 29, 06, 23 and 17 during lockdown (K-sum was 1, 3, 3, 5 and 5, respectively). The diurnal geomagnetic field variations in H, D, Z and F (for quiet days, 11 March (DoY 71) and 6 April 2020 (DoY 97)) are shown in Figure 1 (top and bottom panel). In Figure 1, the top panel indicates the raw variations in the geomagnetic field at HYB (vector and scalar) on quiet days before lockdown, i.e., 11 March 2020, and the bottom panel indicates the data during lockdown, i.e., 6 April 2020. Blue and red colors indicate the H and D components. Visually, it appears that there is no significant change in data quality before and during lockdown. The green and black lines indicate the \(Z\) and F components'  
Figure 1: Diurnal variation in H, D, Z and F on one of the best quiet days of the respective months along with their K-index values, on (a) 11 March 2020 (before lockdown) and (b) 6 April 2020 (during lockdown). For plotting convenience, dH, dZ and dF are subtracted by 50 nT and 80 nT and 43,620 nT, which represent the deviation in 1 Hz value data from the base values. variation data. Comparing the Z and F components before and during lockdown periods shows a visibly reduced frequency of noise peaks during lockdown. We also noticed spikes with an amplitude of around 7 nT in the Z and F components in the period before lockdown, whereas an amplitude of around 4 nT is observed during the lockdown period. The maximum amplitude of first difference is \(\pm\)0.2 nT in the H and D components both before and during lockdown, while it is \(\pm\)4 nT in Z before lockdown, which is reduced to \(\pm\)3 nT during lockdown (Figure 2, top panel in green). The spikes with an amplitude between 3 and 4 nT are reduced by eight times in the lockdown period, when compared to before lockdown. Similarly, the amplitude of spikes between 1 and 3 nT in the lockdown period are reduced by half when compared to before lockdown. The maximum amplitude of first difference of the F component is noted as \(\pm\)2 nT, both before and during lockdown. The range of first difference values in each component is independent of the magnetic activity levels. Additional case studies for before and during lockdown are provided in Figures S1 and S2 in the Supplementary Information. We also tried to assess whether the reduction in noise during lockdown affected the quality of absolute measurements. These absolute observations are performed manually on a weekly basis using a DI single-axis fluxgate magnetometer, producing spot values of D and I specific to a time at that location.