Ionospheric Characterization Over Iran Using Multi‐Frequency IMB HF Transmitters and Global WSPRNET Reception During the 12 November 2025 Geomagnetic Storm

Abstract The Ionospheric Monitoring Beacon network in Iran provides continuous, multi‐frequency high‐frequency (HF) transmissions using WSPRNET modulation, enabling global reception and near–real‐time monitoring of ionospheric propagation conditions. In this study, we analyzed HF signals transmitted from Shiraz, Tabriz, Mashhad, and Birjand across several frequencies spanning 7–18 MHz. Reception reports from worldwide WSPRNET stations were used to characterize frequency‐dependent diurnal behavior, spatial coverage, and temporal variability under geomagnetically quiet conditions and during the ionospheric disturbances associated with the intense geomagnetic storm of 12 November 2025. The observations revealed systematic dependencies of HF propagation on transmission frequency, transmitter location, and storm‐time ionospheric electron density modifications. Lower frequencies exhibited delayed, absorption‐dominated responses consistent with enhanced D‐ and lower F‐region ionization, whereas higher frequencies showed rapid enhancements or disruptions associated with F‐region restructuring and electron density redistribution. Transmitter‐dependent differences reflected variations in latitude, background ionospheric structure, and propagation geometry across Iran. To interpret these results, three‐dimensional ray‐tracing simulations were conducted using physics‐based ionospheric models constrained by GNSS‐derived total electron content. Modified electron density distributions incorporating storm‐time tilts and gradients reproduced the observed multi‐frequency propagation anomalies, including changes in hop structure and geographic reach. These results demonstrated that distributed, multi‐frequency HF beacon networks effectively constrain regional ionospheric structure and its response to geomagnetic forcing, supporting space weather monitoring and HF communication applications.