Case study on low-frequency time-code signal during the 10–15 November 2025 geomagnetic storm: observations and analysis

Introduction Geomagnetic storms are typical solar eruption activities. When high-speed solar wind or coronal mass ejections from solar eruptions strike the Earth, they trigger rapid severe geomagnetic disturbances and further induce ionospheric variations. For low-frequency time-code signals that depend on the ionosphere as a reflective propagation medium for long-distance transmission, geomagnetic activities degrade the signal signal-to-noise ratio, cause signal attenuation and phase delay, and seriously interfere with the normal spatial propagation and reception of low-frequency time-code signals. Methods Taking the geomagnetic storm event during November 10–15, 2025 as the research object, this paper adopts a low-frequency time-code signal monitoring system to experimentally investigate the influence of geomagnetic storms on low-frequency time-code signals. Signal field strength and time deviation data during the event were measured and analyzed, combined with solar activity and geomagnetic field parameters to explore the variation mechanisms and evolutionary characteristics of low-frequency time-code signals. Results A prominent geomagnetic fluctuation erupted on November 12. Correspondingly, the signal field strength decreased by more than 2.3 dBμV/m, and the time deviation presented dramatic fluctuations with an increment exceeding 2.4 ms and a highly discrete distribution state. Discussion The underlying physical mechanism is that geomagnetic storms raise the ionization level of the ionosphere. When low-frequency time-code signals propagate through the D-region ionosphere, the increased particle concentration enhances radio wave absorption, which leads to signal attenuation and phase delay, and further enlarges time deviation. This work provides a reference for improving the space weather resilience of low-frequency time-code systems, and offers a typical case for revealing the coupling relationship between radio signals and the ionosphere.