基于NWC甚低频信号的日出效应的观测与分析

频率在3~30 kHz的甚低频（VLF，Very Low Frequency）波具有较小的传播损耗和较高的趋肤深度，可以在地球-低电离层波导中实现长距离传输，广泛应用于航海导航、对潜通信等领域，且在电离层遥测方面具有十分重要的意义.基于武汉大学自主研发的VLF接收机在武汉接收的NWC（North West Cape）台站信号，本文通过分析2018年4月23日—2020年7月22日的观测数据研究了日出期间NWC信号的幅度响应及其特点和规律.结果表明NWC信号日出期间的幅度响应主要包括两种极小值结构：2个幅度极小值（SR1、SR2）的Type I结构和3个幅度极小值（SR1、SR2、SR3）的Type II结构.在以SR1出现时间为时间零点进行时序叠加分析后发现，Type I结构比Type II具有更强的规律性和稳定性.在Type I结构下，SR2出现时间的波动范围、平均值、标准差分别为43~65 min、54.2 min、4.4 min，而在Type II结构下，SR2和SR3出现时间的波动范围分别为48~93 min、80~120 min，平均值分别为64.7 min、96.4 min，标准差分别为10.2 min、11.7 min.在27个月的观测期内，3—7月份Type I结构的出现概率100%，未出现Type II结构，而在1—2月和8—12月Type I结构出现的概率明显下降，最低降至1月份的20.7%，而Type II在1月、2月、11月的出现概率均高于70%.按春秋分交替变化（周期1和周期2）的统计结果，在周期1内Type I和Type II结构出现的概率分别为91.5%、8.5%，而在周期2内Type I结构出现的概率降至41.9%，Type II结构出现概率则升至58.1%，这表示观测期间内Type II结构主要出现在秋冬季，春夏季发生概率较低.

Observations and analyses of the sunrise effect for NWC VLF transmitter signals

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.