Periodic Oscillation of VLF Transmitter Signals Measured in Low and Middle Latitude Regions

Measurements of Very Low Frequency (VLF) signals from navy transmitters carry direct information about the D-region ionosphere and have been widely utilized for detecting the electron density at D-region altitudes, but not frequently for the atmospheric waves therein. Atmospheric waves have been extensively studied using the total ionospheric electron content, but if and how they are correlated with the D-region ionosphere and VLF measurements still remains poorly investigated. In this study, we have conducted a comprehensive analysis using 7-year measurements (2016-2022) of VLF signals from the JJI, NWC, and VTX transmitters as being recorded in Suizhou, China. These three transmitter-receiver paths are representative and the corresponding observations constitute a valuable data set to investigate the periodicities of VLF data. Different from previous studies, we have utilized the ensemble empirical mode decomposition and Lomb-Scargle methods to determine the periodicities of these data. By contrasting these paths, prominent periodicities ranging from 2 to 730 days have been found, with clear diurnal variation and suggestive latitudinal/longitudinal dependence. Moreover, we have found that the mesospheric temperature is closely related with the annual oscillation of VLF measurements, while this oscillation has a low correlation with solar Lyman-alpha fluxes or geomagnetic activity. The oscillations with relatively shorter periods are likely atmospheric waves such as gravity waves, planetary waves, or harmonics of these waves. Our results suggest that, in addition to the electron density, the subionospheric VLF technique can be potentially utilized to remotely sense atmospheric waves that propagate up to or through the D-region ionosphere. Radio waves in the very low frequency range (VLF, 3-30 kHz) can propagate long distances within the Earth's atmosphere between the surface and the D-region ionosphere, a region at altitudes of 60-100 km that is influenced by both solar fluxes that propagate through the upper atmosphere to the D-region ionosphere and atmospheric waves from the lower atmosphere. Measurements of VLF waves from navy transmitters have been widely utilized to detect the electron density of D-region ionosphere, but not the atmospheric waves therein. Toward this goal, we have investigated the VLF measurements recorded by our detector in Suizhou, China. These signals originate from the JJI transmitter in Japan, the NWC transmitter in Australia, and the VTX transmitter in India. The results show that these VLF signals have clear periodic oscillations ranging from 2 to 730 days, exhibiting notable variation between daytime and nighttime conditions, and suggestive dependence on the latitudes/longitudes the VLF propagation path crosses. Moreover, we have analyzed the main reason for the annual oscillation of VLF measurements and found that the annual oscillation is mainly driven by the mesospheric temperature. Our work demonstrates the potential usage of the VLF technique in remotely sensing the atmospheric waves of D-region ionosphere. We have quantified the periodicities of 7-year VLF measurements using the empirical mode decomposition and Lomb-Scargle methods Periods ranging from 2 to 730 days have been found with clear diurnal dependence and latitudinal/longitudinal dependence The annual oscillation is closely correlated with mesospheric temperature, while low correlation with Lyman-alpha fluxes or geomagnetic activity