Abstract
Variations in the amplitude and phase of a mid-latitude low-frequency signal from the JJY (40 kHz) transmitter were analyzed during four geomagnetic storms in 2021 and 2025, which were caused by high-speed solar wind streams from coronal holes creating corotating interaction regions. The analysis has shown that nighttime anomalous disturbances in the amplitude and phase were observed for three days during a magnetic storm. The negative amplitude anomalies of the signal ranged from 4.5 to 21 dB; phase anomalies were from 41° to 124°. The sign of the phase anomalies was different on different days. We recorded both positive and negative phase anomalies, as well as wave-like disturbances with a transition from negative to positive anomalies. A statistical relationship was found between the value of the phase anomaly and the intensity of geomagnetic disturbances. The correlation coefficient with the Dst index was –0.39. The relationship between phase anomalies and the AE index is higher; the correlation coefficient in this case was 0.53. No statistical relationship was observed between geomagnetic activity and the anomaly for the sub-ionospheric signal amplitude.