The recently developed high-quality WHU ELF/VLF receiver system has been deployed in Suizhou, China （geomagnetic lati- tude 21.81°N, longitude 174.44°E, L=1.16） to detect low latitude extremely-low-frequency （ELF： 0.3-3 kHz） and very-low-frequency （VLF： 3-30 kHz） emissions originating from either natural or artificial sources since February 2016. Dur- ing the first-month operation of the receiver system, a total of 3039 clear whistlers have been recorded at this low latitude sta- tion with the majority （97.0%） occurring on 28 February and 1 March 2016. Observed whistlers manifest various types includ- ing single one-hop, echo train, multi-flash, and multi-path. They tend to intensify after local midnight, reach the peak around 04435 LT, and then weaken quickly. Both features of lower cutoff frequencies of most whistlers below -1.6 kHz and almost uniform dispersion for many successive multi-flash whistlers suggest that these whistlers propagate along the geomagnetic field lines in the duct mode. The computed dispersion varies between -15 s1/2 and 23 s1/2 for observed one-hop whistlers and is greater than 50 sm for three-hop echo train whistlers, indicating that the whistlers observed at the Suizhou station are low lati- tude whistlers.
As a companion paper to Zhou RX et al.(2020),this study describes application of the automatic detection and analysis module to identify all the tweek atmospherics detectible in the WHU ELF/VLF receiver data collected at Suizhou station during the period of 3 February through 29 February 2016.Detailed analysis of the identified low-latitude tweek events reveals that the occurrence rate varies considerably—from 800 to 6000 tweeks per day,and exhibits a strong diurnal and local time dependence,the peak occurring before local midnight.The diurnal variation of identified tweeks was similar to that of the lightning data obtained by the World-Wide Lightning Location Network(WWLLN)..Estimates of the propagation distance and ionospheric reflection height of tweek atmospherics suggest that the majority(~92%)of the low latitude tweeks originate from the lightning activity within a radius of 4000 km and that they are very likely to reflect from the lower ionospheric D-region at the height range of 75–85 km.At these lower ionospheric reflection altitudes,~74%of the corresponding electron densities from the tweek spectral measurements are within 24.5–27.5 cm^-3.The daily variation of estimated D-region electron densities in the considered period(February 2016)also exhibits a small overall increasing trend from early to later in the month.
甚低频(VLF)信号被广泛应用于地球空间环境地基监测、全球航海通信与导航等领域。该系统在传统甚低频接收天线的基础上,进行了天线小型化的改进,通过阻抗匹配,改善了系统灵敏度。该系统包括磁环天线,低噪声模拟前端,数据采集与传输模块以及同步模块。磁环天线采用罗德施瓦茨公司生产的HZ-10型磁环天线。模拟前端分为基于差分结构的低噪放大器和巴特沃斯滤波器两部分,在带宽1~50 kHz内增益高达80 d B。数据采集和传输模块采用基于FPGA和USB 2.0总线的方法。在武汉(30.54°N,114.37°E)测得宽带甚低频波谱结果初步验证了该地基甚低频接收系统的有效性和可行性。对人工甚低频MSK调制信号进行幅度解调处理,得到其幅值的日变化曲线图。
As a dispersive wave mode produced by lightning strokes, tweek atmospherics provide important hints of lower ionospheric(i.e., D-region) electron density. Based on data accumulation from the WHU ELF/VLF receiver system, we develop an automatic detection module in terms of the maximum-entropy-spectral-estimation(MESE) method to identify unambiguous instances of low latitude tweeks.We justify the feasibility of our procedure through a detailed analysis of the data observed at the Suizhou Station(31.57°N, 113.32°E) on17 February 2016. A total of 3961 tweeks were registered by visual inspection;the automatic detection method captured 4342 tweeks, of which 3361 were correct ones, producing a correctness percentage of 77.4%(= 3361/4342) and a false alarm rate of 22.6%(= 981/4342).A Short-Time Fourier Transformation(STFT) was also applied to trace the power spectral profiles of identified tweeks and to evaluate the tweek propagation distance. It is found that the fitting accuracy of the frequency–time curve and the relative difference of propagation distance between the two methods through the slope and through the intercept can be used to further improve the accuracy of automatic tweek identification. We suggest that our automatic tweek detection and analysis method therefore supplies a valuable means to investigate features of low latitude tweek atmospherics and associated ionospheric parameters comprehensively.
