## 4 Modeling Disturbances of the Geoelectric Field
The selected example corresponds to the substorm with Pi3 activity recorded at a local latitudinal profile of stations at Yamal. The magnetograms from stations along the latitudinal profile 150\({}^{\circ}\) during the event on 6 January 2019 are shown in Figure 4. The peak magnetic disturbance in the \(H\) component is reached at BEY station \(\Delta H\)-350 nT. To calculate the geoelectric fields that could be excited by this geomagnetic disturbance, we used information on the impedances of the Earth's crust calculated on the basis of the model from [30]. This global 3D model was compiled using the results of many regional MT sounding projects, as well as current ideas about the structure of the outer solid shells of the Earth and their electrical properties. The model describes the structure of the electrical conductivity of the Earth in the depth range of 0-100 km, including seawater, crust, and partly the upper mantle. This model was augmented by the data of the upper 10 km of the SMAP conductivity model of Fennoscandia [31], which was not originally included in [30]. To describe the conductivity distribution at depths larger than 100 km, the global 1D conductivity profile from [32] was used. Figure 5 (left-hand panel) demonstrates the vertical conductivity profiles \(\sigma(z)\) at magnetometer stations' locations at Yamal, whereas Figure 5 (right-hand panel) shows the corresponding frequency-dependent impedance \(Z(f)\). At station SKD, the dependence is close to a power law, which corresponds to the homogeneous conductivity. At other stations, the impedances at \(f>0.5\) mHz deviates noticeably from a power law dependence. Figure 6 demonstrates the results of the synthesis of electric field time series during the substorm event on 6 January 2019 for station BEY. The superposed Pi3 pulsations on a magnetic bay are most evident in the eastward magnetic component E. These pulsations provide a high variability of the geomagnetic field, \(\text{d}H/\text{d}t\sim 5\) nT/s, \(\text{d}E/\text{d}t\sim 6.5\) nT/s. The synthetic electric field is also highly variable, though its spectral content is evidently different from \(\text{d}B/\text{d}t\) variations. The highest peak-to-peak variations in the telluric field are \(E_{\text{H}}\)-\(E_{\text{E}}\sim 0.4\) V/km. The occurrence of electric field peaks does not coincide with the moment of maximum magnetic field disturbance (cf. Figure 4). Figure 4: Magnetograms (\(H\) component) from stations along the latitudinal profile 150\({}^{\circ}\): VIZ-BEV-AMD-NAD during 14–22 UT on 6 January 2019. Black diamonds denote local midnight. Figure 5: The conductivity profiles \(\sigma\)(z) and impedance dependence on frequency \(Z(f)\) for stations at Yamal derived from the global conductivity model. Below 100 km, profiles for each stations merge into one. The spectra of magnetic and telluric field variations at station BEV during the 6 January 2019 event (shown in Figure 6) are presented in Figure 7. The spectral amplitude of magnetic field variations decays with frequency and has a spectral peak of both components in the band 2-3 mHz caused by Pi3 pulsations. The electric field has similar behavior with spectral peaks in the same band. In contrast, the spectra of magnetic field variability \(\mathrm{d}H/\mathrm{d}t\) and \(\mathrm{d}E/\mathrm{d}t\) have a weakly decaying dependence on frequency with only a slightly evident spectral peak at the Pi3 frequency. Thus, this comparison shows that in the telluric field variations, the high-frequency part of the spectrum is substantially suppressed as compared with dB/dt fluctuations [33]. Indeed, the spectral power of horizontal magnetic components drops \(\sim\)12.5 times from 1 mHz to 6 mHz, whereas that of the electric field drops \(\sim\)8 times, and that of dB/dt drops \(\sim\)3.5 times only. The considered event and other observations [22; 23] showed that the most intense GICs in power transmission lines are excited by geomagnetic fluctuations in the Pi3/P86 band.