Study on the variation of low-frequency time code signals during medium to large solar flare events

This study is based on the 68.5 kHz signal transmitted by China's low-frequency time code time service system (BPC) and systematically researches the effects of medium to large solar flares (M/X-class) on low-frequency time code signals. By analyzing the field strength and timing deviation data of the BPC signal during 20 typical flare events, the study reveals the variation patterns of low-frequency time code signals under disturbances from medium to large solar flares. Case analyses indicate that, during such flares, the BPC signal intensity exhibits two response patterns: a single-valley structure and a double-valley structure. The BPC signal response is divided into two stages—a rapid change phase and a gradual change phase—which show a strong linear relationship with the development of the solar flare. Meanwhile, the BPC timing deviation displays a bipolar pulse characteristic, and after the flare, the instability in signal performance is closely associated with the double-valley response in field strength. These phenomena suggest that the changes in the BPC time code signal are closely related to the effects of ionospheric disturbances during solar flares on the superposition characteristics of the BPC ground-wave and sky-wave signals. This first systematic investigation analyzes low-frequency time-code signal variation during medium-to-large solar flares, revealing their response characteristics. It provides significant insights into the low-frequency time-code signal propagation-solar activity association and lays a solid theoretical foundation for improving time-service stability and reliability.