Abstract:
In this study, the spectral characteristics of irregular ULF (ultra-low frequency) geomagnetic pulsations – Pi2 with periods of 40-150 s (6.7-25 mHz) during the main phase of the extreme geomagnetic storm on May 10-11, 2024 – are presented. Data records with resolution 1s of the magnetometer located at the Geomagnetic Observatory Panagjurishte are analysed. By using a band-pass filtering procedure, based on the fast Fourier transform, fifteen frequency ranges within the definition of this type of pulsation have been obtained. It was found that Pi2 pulsations accompany the main phase of the storm and, at mid-latitudes at the point of recording, it is most intensive in the frequency range 8-20 mHz during 18-02 UT (21-05 MLT).