ULF-Geomagnetic Response at Mid-Latitudes in Relation to the Space Weather Parameters Observed During the Extreme Geomagnetic Storm of May 2024

The study focuses on the regional inhomogeneities of the spatial and temporal structures of the ULF geomagnetic disturbances (10–600 s) recorded in four geomagnetic observatories (EBR-Spain, WIC-Austria, NCK-Hungary, and PAG-Bulgaria) located at mid-latitudes in the European sector. These were caused by the extreme storm on 10–11 May 2024, which was the most powerful event since 2003. Polar lights, an extraordinary phenomenon inherent to these latitudes, were observed. We used established and powerful classical methods, such as the cross-correlation analysis, to obtain normalized cross-correlation functions between the total vector of the interplanetary magnetic field (IMFBtotal) and the geomagnetic field’s H-component, and that between the solar wind pressure and the H-component. Their respective maximal absolute values are −0.8 and −0.7. For all observatories, we determined a time delay of 240 min corresponding to the time required for the magnetosphere to reach maximum perturbation after the initial impact. We also determined delay periods (based on the cross-correlation function and latitude) between the different observatories that varied between 1 and 14 min for each pair of observatories. Using the wavelet coherence approach, we determined the degree of coherence between the six pairs of observatories and their phase differences in terms of time of occurrence and frequency. We unveiled significant phase discrepancies (0, π/2) and a low coherence in the region of Pc3-4 pulsations (10–150 s). Moreover, we found out that Pc5 pulsations (150–600 s) show no phase or frequency changes at the recording points. In contrast, for the waveform, we discovered that by applying a band-pass filter based on the fast Fourier transform, the disturbances in the Pc3-4 range alter their waveform relative to the different measurement points and phases of the storm. However, this tendency is not detected in the Pc5 range at mid-latitudes in Europe.