Abstract:
On May 1st, 2020, at the Geomagnetic Observatory in Panagjurishte, Bulgaria, the unusual ultra-low frequency (ULF) geomagnetic noise was recorded by a tri-axial (X, Y, and Z directions) induction magnetometer. The time series from April 1, 2020 till May 1, 2020 are examined. Applying the detrended fluctuation analysis (DFA), the DFA exponent is calculated for the time series along the three geomagnetic directions, and the results show an unusual decrease in values of the DFA exponent in time scales 10-180s and 10-900s that starts after April 29, 2020. Through Magnitude-Squared Coherence analysis, the correlation and phases between every two geomagnetic components are obtained for the time series on May 1, 2020, under very low geomagnetic activity levels. The high values of the coherence and phase full sync for the X and Z components are observed, in the time period 04:00–20:00 UTC, at a frequency range 0.125-0.25 mHz. The present study does not rule out that the considered disturbances have a lithospheric origin.