Evaluation of VLF electromagnetic background noise based on ground-based observations in middle and low latitudes of China

The temporal variations in Very Low Frequency (VLF) background noise at 14 kHz and 29 kHz over a two-year period (2020–2021) are investigated based on ground-based observations in the middle and low latitudes of China. Seasonal trends reveal that noise levels peak during the warmer months (May to September) and decrease during the colder months (December to March). The 14 kHz frequency consistently exhibits more pronounced seasonal peaks and stronger variability compared to 29 kHz, reflecting its greater sensitivity to natural disturbances. Diurnal variations reveal higher noise levels during nighttime (18:00–6:00 LT) compared to daytime (6:00–18:00 LT), particularly at 14 kHz. At 29 kHz, the diurnal differences are less pronounced, reflecting its reduced sensitivity to ionospheric conditions and shorter propagation range. Interannual differences show higher noise levels in 2020 than in 2021, particularly during summer. The study emphasizes the frequency-dependent behavior of VLF noise, with 14 kHz being more influenced by natural disturbances like thunderstorms and ionospheric dynamics, while 29 kHz remains relatively stable, dominated by anthropogenic noise. These findings contribute to understanding VLF noise dynamics, improving ionospheric modeling, and enhancing the reliability of communication, navigation, and timing systems relying on VLF signals.