Imprints of Intense Geomagnetic Storm on Very Low Frequency (VLF) Radio Waves Over the Mediterranean Region

The study of solar and geomagnetic activity effects on the D-region ionosphere (60–90 km altitude) using signals from very low frequency and low frequency (VLF, 3–30 kHz) transmitters has garnered considerable attention over the past decades. This research investigates the impacts of the geomagnetic storm (GMS) of 23–24 March 2023, utilizing VLF measurements from a monitoring station in the Mediterranean region. A variety of analytical approaches, including the traditional wavelet analysis for detecting events such as traveling ionospheric disturbances (TIDs) and or gravity waves (GWs), and the novel cross-wavelet coherence (WTC) correlation from cross wavelet transform methods, were employed to identify both apparent and subtle effects of the storm on the D-region ionosphere. Key findings include signal enhancement and attenuation of up to several tens of decibels during different phases of the storm's progression, observation of wave-like signatures (WLS) with time scales in the periods of GWs. The study also highlights significant delays in the D-region's response to magnetosphere-ionosphere energy exchanges driven by solar wind-magnetic reconnection during the storm. These delays, ranging from minutes to tens of minutes, were found to vary depending on the analytical technique used. The WTC analysis demonstrated maximum correlation values between amplitude deviations and the SYM-H geomagnetic index, ranging from ±0.9084 to ±0.9364 on the N/S signal channel and ±0.9188 to ±0.9363 on the E/W channel. These results not only corroborate previous research but also offer new insights into the dynamics of the D-region ionosphere during geomagnetic disturbances.