Sunrise Effect of Very-Low-frequency JJI Transmitter Signal Propagating over an East-West Path

Very-low-frequency (VLF) waves with the frequency range of 3 similar to 30 kHz are widely used in the remote sounding of the lower ionosphere due to their small propagation loss and a long propagation distance in the Earth-ionosphere waveguide. In this paper, based on the WHU VLF receiver system observations at Suizhou (31. 57 degrees N, 113. 32 degrees E), Hubei, we investigate in detail the sunrise effect on mid-latitude VLF signals from the JJI transmitter (32. 04 degrees N, 130. 81 degrees E) at Japan propagating over an east-west path. The statistical analysis of JJI VLF transmitter signal amplitude minima during sunrise indicates that the occurrence time of the minima corresponds favorably to the sunrise time at the JJI transmitter or the Suizhou receiver station. The amplitude responses of the JJI signal during sunrise mainly include two types of structures, type I and Type II, corresponding to two amplitude minima and three amplitude minima, respectively. The type I structure mainly occurs in spring and summer, with the last amplitude minima appearing after the sunrise time at Suizhou with an averaged time delay of 24 minutes, while the type II structure mainly occurs in autumn and winter. In terms of the interference theory, the two simulation models are used to investigate the influence of high order modes on the sunrise effect of artificial VLF signals transmitted over a short distance. Our simulation results of signal intensity are consistent with the observations when the first three order modes are included but become inconsistent when only considering the first two order modes. It is demonstrated that the higher order modes play an essential role in the VLF wave propagation along a relatively short path, which needs to be considered in studies of the sunrise effect.