东西传播路径上JJI台站甚低频信号的日出效应研究

Sunrise effect of very-low-frequency JJI transmitter signal propagating over an east-west path

甚低频(VLF, Very Low Frequency)波的频率范围为3~30 kHz,因其在地球-电离层波导中传播损耗小、传输距离远而被用于低电离层遥测的相关研究.本文基于武汉大学自主研发的甚低频探测系统在湖北随州(31.57°N,113.32°E)的观测数据,详细研究了中纬度地区来自日本JJI(32.04°N,130.81°E)甚低频台站信号沿东西方向传播的日出效应.在对日出期间接收信号的幅度极小值进行统计分析后发现,幅度极小值出现时间的分布与随州或JJI日出时间的变化趋势基本一致.日出期间JJI信号的幅度响应主要包括type Ⅰ和type Ⅱ两种结构,分别对应2个幅度极小值和3个幅度极小值的情况.其中,type Ⅰ结构主要发生在春夏两季,幅度极小值出现的平均时间落后随州日出时间24 min左右;type Ⅱ结构主要发生在秋冬两季.基于波模干涉理论对日出期间两种不同模式的JJI甚低频信号进行模拟研究,结果表明,在JJI-随州的东西向传播路径上,若只考虑前两阶模的影响,模拟结果与观测结果差异明显;若考虑前三阶模的影响,那么模拟结果与观测结果基本一致.由此可见,JJI甚低频信号在短路径上传播时高阶模会对信号的传播产生较大的影响,分析此类路径上甚低频台站信号的传播特征时需要考虑前三阶模甚至更高阶模的作用.

Very-low-frequency(VLF) waves with the frequency range of 3~30 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°N, 113.32°E), Hubei, we investigate in detail the sunrise effect on mid-latitude VLF signals from the JJI transmitter(32.04°N, 130.81°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.