## 3 Calculation of Results and Discussion
### Detection of Short-Period Geomagnetic Disturbances, Minute-Resolution Data Processing  
Minute-resolution data were obtained using the method defined by INTERMAGNET standards ([51], Section 2.4). Filtering, with Gaussian weight coefficients, was applied to the second data on the interval of \(-45\dots\)\(+45\) s. Minute value was centered at the beginning of a minute. When some second values were missing, weight renormalization was carried out. Figures 9 and 10 show the results of the method applied for a moderate magnetic storm on 12 May 2021. To estimate the near-Earth space state, the upper panels of Figure 9 show SWS (Figure 9a), IMF Bz (Figure 9b), and geomagnetic activity Dst-index (Figure 9c). According to the resource [50] SWS was about 300 km/s on 11 May, IMF Bz fluctuations were within \(\pm\)2 nT. The application of operations (8) and (9) to the PET minute data are illustrated in Figure 9e.f. In operation (8) threshold coefficient \(U_{2}=2.5\) was used (positive and negative disturbances exceeding the threshold \(T_{2,s}\) are illustrated in Figure 9 by pink and by blue, respectively). Based on the ratio (8), application of the coefficient \(U_{2}=2.5\) allows one on each scale \(s\) to detect wavelet coefficient fluctuations exceeding in amplitude the 2.5 RMSD from their characteristic values (\(T_{2,s}=2.5\times St_{s}\)) within the time window under analysis (window length \(l=720\) counts, corresponding to 12 hours, was used). To detect weaker geomagnetic disturbances, threshold coefficient \(U_{2}=1.5\) was used (positive and negative disturbances exceeding the threshold \(T_{1,s}\) are illustrated in Figure 9 by yellow and light blue, respectively). We should note that the times of weak geomagnetic disturbance occurrences before the event correlate with the corresponding small IMF Bz fluctuations times. Moreover, a clear increase in disturbance intensity is observed as the magnetic storm beginning is approaching. A similar character of pre-storm short-period geomagnetic disturbances, according to the meridional station network and auroral zone stations, was mentioned in the papers [31; 32]. Strong magnetic storms on 7 January and 17 March 2015 [32] and weak magnetic storms on 9 July and 27 September 2017 were under analysis [31]. Based on space weather data [50], an inhomogeneous accelerated flux from a coronal hole and coronal mass ejection (CME from 9 May) arrived at 05:49 UT on 12 May. During that period, SWS suddenly increased (Figure 9a), and IMF BZ fluctuations increased to Bz = \(\pm\)20 nT (Figure 9b). The time of the storm's sudden commencement is evident on the magnetic field H-component graph (Figure 9d, red vertical line). The results of data processing (Figure 9e,f) show that geomagnetic disturbances exceeding the threshold occurred at the Paratunka observatory from 00:00 UT (12:00 LT) to the time of storm beginning on 12 May. Several hours before the storm, during SWS sudden increase (Figure 9a) and IMF Bz southward turn (Figure 9b), disturbance intensity increased significantly and reached \({I_{k}}^{2\pm}(k)=4000\) (Figure 9f). That is likely to be associated with the beginning of compression region formation. Strong geomagnetic disturbances were observed during the IMF Bz southward turns and fluctuation intensification during the storm initial phase. IMF Bz northward direction accompanies a geomagnetic activity decrease. During the IMF Bz long southward turn, the storm main phase occurred. It was characterized by a sudden decrease in Dst-index (Figure 9c) and significant increase in H-component fluctuations (Figure 9d). We should note that the Paratunka observatory was in the night sector (00:00-02:00 LT) during that period. Based on the processing data, the intensity reached the values \({I_{k}}^{2\pm}(k)=12,000\) near the H-component minimum values. That is likely to be associated with ring current effects and intensification of the magnetosphere tail currents. The results agree well with those represented in the papers [52; 53].