## 3 Examples of Geomagnetic Variations Causing Significant GIC Bursts  
The largest magnetic disturbances on the Earth's surface are caused by an extended large-scale auroral electrojet, intensified during substorms [18]. However, the most intense GIC bursts are produced by small-scale ionospheric current structures [19]. The energy of such impulsive or quasiperiodic disturbances is much lower than the energy of magnetospheric storms or substorms, but rapidly changing fields of such disturbances can cause GIC bursts of significant magnitude [20]. Examples of such impulsive and quasiperiodic disturbances of geomagnetic field are given below based on the data from the Russian Arctic database.  
After the expansion phase of a substorm, large-scale magnetic variations are often superimposed by irregular quasiperiodic Pi3 pulsations with characteristic time scales of 3-15 min. Figure 2 shows quick-look graphs for the substorms on 9 February 2019 (left-hand panel) and 2 March 2019 (right-hand panel) from the Arctic magnetometer database. These Pi3 pulsations are not harmonic oscillations such as Pc5 waves, but a series of nonlinear magnetic pulses with steep fronts. Ps6 pulsations in the same frequency range are morphologically similar [21]. Due to steep fronts of the Pi3/Ps6 pulses, the associated field variability is high, up to dB/dt-20 nT/s. In many events with extremely large GICs, the currents reached their maximum value (up to hundreds of Amperes) during the subsequent series of Pi3/Ps6 pulses [22; 23].  
When considering the impacts of space weather, it is usually assumed that disturbed geomagnetic fields are spatially uniform throughout the area of a technological system. However, often against the background of a general field enhancement during substorms, isolated magnetic perturbation events (MPEs) are observed in the magnetic (>100 nT) and telluric (>1 V/km) fields [24]. The MPEs are spatially localized, so the peak at one station can be several times larger than the regional value of the field on average (at distances of about 500 km). Figure 3 shows examples of such isolated impulsive disturbances during moderate magnetic activity on 9 September 2019 (at \(\sim\)16 UT) and 17 December 2019 (at \(\sim\)05 UT). These magnetic pulses are usually associated with a local intensification of the auroral luminosity. Such pulses may induce sporadic intense bursts of GIC. The physical processes that determine the generation of the MPEs are poorly understood.  
There are other geomagnetic disturbances that are hazardous to power transmission and pipeline networks. Due to the high variability of the geomagnetic field of Pc5 pulsations (3-10 min), the magnitude of associated periodic GICs in power lines may increase and exceed 10 A [25]. The long-lasting Pc5 wave activity can be more dangerous than short-term bursts of GICs during substorm onsets due to cumulative effects--corrosion and premature aging of high-voltage transformers.  
Figure 2: An example of irregular quasiperiodic Pi3 pulsations against the background of the expansion phase of a substorm on 9 February 2019 (**left**) and 2 March 2019 (**right**). The top panel in both plots shows variations in the solar wind plasma density \(Np\), solar wind velocity \(V\), and vertical component of the interplanetary magnetic field \(Bz\). Stack-plot magnetograms of the H-component from stations around Yamal Peninsula are shown in the bottom panel. Geomagnetic coordinates of the stations are indicated on the lower panels next to the station code. On the magnetograms, empty triangles show local noon at each station, and black diamonds show local midnight.