Investigation of VLF/LF electromagnetic wave propagation as recorded by the receivers of the INFREP network

In recent decades, significant efforts have been devoted to understanding and interpreting the link between ionospheric perturbations and natural or anthropogenic phenomena, such as seismic activity, electrical or geomagnetic storms, and unidentified radio emissions. This is achieved through various methods among which is also the study of electromagnetic (EM) wave propagation in the very low frequency (VLF, 3–30 kHz) and low frequency (LF, 30–300 kHz) bands. These bands enable long-distance communication, navigation, and military applications, including submarine contact, AM broadcasting, lightning detection, and weather systems. Due to their long wavelengths, VLF and LF waves exhibit unique propagation characteristics. VLF waves propagate globally by using Earth-ionosphere waveguides, reflecting off the D and E layers as skywaves, and are influenced by solar and atmospheric conditions. LF waves primarily rely on ground waves for extensive coverage, although they can also utilize ionospheric reflection (skywaves) for longer-distance communication.

This paper introduces fundamental concepts related to VLF/LF electromagnetic wave emission, propagation, reception, and the perturbing factors that affect them. Additionally, it presents key findings from the European INFREP Receivers Network, which studies seismo-ionospheric anomalies linked to earthquake activity. Established in 2009, the INFREP network monitors VLF/LF signals from transmitters across Europe and neighboring regions. The network currently comprises 10 receivers, built by Elettronika (Italy), and operates at a sampling rate of one sample per minute. The Romanian segment of INFREP includes two receivers, operational since 2009 and 2017, with only brief interruptions, notably during the pandemic when travel restrictions hindered access to the observatories.

The paper discusses the current state of the INFREP network and outlines methods for providing near real-time data access. It highlights advancements in real-time electromagnetic data transmission, archiving, and the use of 2D and 3D online signal visualization and processing techniques. Data access is available through the INFREP headquarters in Graz, Austria (https://infrep.iwf.oeaw.ac.at/data-access/) and the National Institute for Earth Physics in Romania (https://mg.infp.ro/d/ch-aqZXIz/vlf-lf-radio-data?orgId=1&from=now-6M&to=now). The paper also shares findings from the detection of potential ionospheric anomalies in EM signals preceding large earthquakes that occurred between 2012 and 2024. All anomalies are analyzed in correlation with space weather events and extreme meteorological phenomena.

This paper was carried out within Nucleu Program SOL4RISC, supported by MCI, project no PN23360201, and PNRR- DTEClimate Project nr. 760008/31.12.2023, Component Project Reactive, supported by Romania - National Recovery and Resilience Plan