基于NWC甚低频信号的日出效应的观测与分析

Very-low-frequency (VLF) waves at frequencies of 3~30 kHz can propagate within the Earth-ionosphere waveguide over a long distance due to the small propagation loss and high skin depth. VLF signals are widely used in various fields including navigation, submarine communication and ionospheric remote sounding. In this paper, the self-developed Wuhan University VLF receiver system is adopted to detect the NWC (North West Cape, Australia) VLF transmitter signals (19.8 kHz) at Wuhan, Hubei propagating over a long vertical path >6000 kilometers. By analyzing the ground-based NWC signal data obtained during the period from April 23, 2018 to July 22, 2020, the sunrise effect on the amplitude variation of the NWC VLF signals is investigated in detail. The amplitude responses are categorized to include Type I with two amplitude minima (SR1 and SR2) and Type II with three amplitude minima (SR1, SR2 and SR3). By performing the superposed epoch analysis with the SR1 occurrence time as the zero epoch time, we find that Type I is more stable than Type II. For Type I, the SR2 occurrence time varies between 43~65 minutes, with the mean value and standard deviation of 54.2 minutes and of 4.4 minutes, respectively. In contrast, for the Type II events, the variation range, mean value and standard deviation are 48~93 minutes, 64.7 minutes and 10.2 minutes, and 80~120 minutes, 96.4 minutes and 11.7 minutes for SR2 and SR3, respectively. During the 27-month period, the occurrence rate is 100% for Type I and zero for Type II during the seasonal interval from March to July. The Type I occurrence reduces significantly in other months with the minimum of 20.7% in January. The occurrence probability of Type II is >70% in January, February, and November. The statistical results within the period of equinox-solstice (Period 1 and Period 2) indicate that Type I (Type II) occurs with the probability of 91.5% (8.5%) in Period 1, and in Period 2 the occurrence probability of Type II (Type I) increases (decreases) to 58.1% (41.9%). Consequently, the Type II amplitude variation of NWC VLF transmitter signals frequently occurs in autumn and winter, and occurs in summer and spring with a much lower probability.