Monitoring of subionospheric radio wave propagation within the Romanian part of INFREP European network

The Romanian INFREP network, operating since 2009, monitors very low and low frequency (VLF/LF) radio signals that propagate through subionospheric reflections. The main goal is to observe and analyze variations in these signals in order to study ionospheric disturbances of natural or anthropogenic origin. Such data are important for understanding how the lower ionosphere reacts to solar and geomagnetic activity and may also provide insight into possible electromagnetic precursors of earthquakes. After more than ten years of continuous operation, some instruments in the Romanian stations began to show signs of aging and malfunction. In October 2025, a series of strong autumn storms with intense lightning affected the power supply at the Dobrogea Seismological Observatory, where an Elettronika receiver operated as part of the European INFREP network. Once power was restored, the receiver started to fail intermittently. At first, data interruptions occurred sporadically, but gradually the gaps became longer, until the receiver stopped recording completely. Errors appeared simultaneously on both the VLF and LF channels. Although the unit remained accessible and continued to produce daily log files showing frequency and time, the data were replaced by error messages. The instrument was later retrieved for inspection, but the source of the malfunction has not yet been identified. Therefore, in December 2025, the Elettronika receiver was replaced with a new one, developed jointly by the National Institute for Earth Physics (NIEP) and Integrated Project and Process Tools (IPPT). The presentation will describe the Romanian INFREP network, outline the main features of the new equipment, and compare sample recordings from both receiver generations. Early results show that the new instrument operates stably, with a high signal-to-noise ratio and uninterrupted data flow, thus improving the overall reliability of subionospheric radio wave monitoring in the region.

This paper was carried out within Nucleu Program SOL4RISC, supported by MCI, project no PN23360201, and PNRR- DTEClimate Project nr. 760008/30.12.2022, Component Project Reactive, supported by Romania - National Recovery and Resilience Plan