东西传播路径上JJI台站甚低频信号的日出效应研究

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.     

基于LWPC和IRI模型的NWC台站信号传播幅度建模分析

频率为3~30 kHz的甚低频（VLF,Very Low Frequency）电磁波具有波长长、传播距离远的特点,能够沿地面-低电离层波导进行传播,在通信、导航等许多领域都被广泛应用.基于波导模理论的长波传播模型（LWPC,Long-Wavelength Propagation Capability）能够用于计算甚低频波的传播路径及幅度,进而研究耀斑、磁暴、地震等事件对电离层的扰动.本文利用国际电离层参考模型（IRI,International Reference Ionosphere）对LWPC中电子密度和碰撞频率进行改进,并将模拟结果与武汉大学VLF接收机实际观测到的NWC （North West Cape）台站信号幅度进行比较分析,结果表明改进后LWPC模型得到的幅度及变化趋势与实际值更加接近.LWPC模型给出的电子密度与IRI模型得到的电子密度在日间基本一致,但是在夜间存在差异,造成夜间部分区域NWC台站信号幅度的差异性,验证了电离层电子密度对于VLF信号传播具有的重要影响.传播路径上的晨昏变化也可以引起VLF信号幅度分布的突变,在日出和日落时间段内存在明显的过渡区域.基于IRI模型的LWPC,改善了VLF电波传播过程的预测分析效果,提供了一种长波导航通信质量的评估方法.     

基于范阿伦双星EMFISIS观测数据的NWC和NAA人工甚低频台站信号的内磁层全球统计分布

地球表面的人工甚低频台站信号可以穿透电离层泄漏进地球磁层导致内辐射带电子沉降到两极大气.因此研究人工甚低频台站信号的空间全球分布特性对于分析辐射带电子的损失具有重要科学意义.本文使用范阿伦双星从2013年到2018年共计6年的高质量的波动观测数据,统计了 NWC(19.8 kHz)、NAA(24.0 kHz)两个人工VLF台站信号的全球分布,分析了台站信号的电场功率谱密度对地理经纬度、磁壳值L、磁地方时MLT、地磁活动水平的依赖性.结果表明,在内磁层中,人工台站VLF信号主要沿着台站位置对应的磁力线传播,夜侧强度高于日侧,冬季高于夏季.这种日夜和夏冬差异的形成是因为夜侧和冬季的日照强度较弱,电离层电子密度较低,VLF信号较容易穿透电离层进入磁层.此外人工VLF台站信号的全球分布受地磁活动的影响很弱.这些统计观测结果给出了 NWC和NAA两个重要人工VLF台站信号强度的全球分布特征,为进一步分析人工VLF台站信号与地球辐射带电子的波粒相互作用提供了关键信息.     

地面VLF波穿透电离层的能量衰减变化

地面VLF人工源可以导致辐射带高能粒子沉降.为研究辐射带粒子加速和沉降机制,乃至实现对辐射带电子"人工控制"的设想,需要在精确计算得到VLF人工源在电离层激发电磁场能量分布的基础上,计算哨声波对辐射带电子的调制作用.以往计算VLF人工源在电离层中激发的电磁场能量分布多基于Cary给出的波导中VLF波衰减率和Helliwell提出的经典电离层吸收曲线,但近期研究表明,这些模型结果存在较为明显的误差.本研究建立了地面VLF信号穿透电离层传播的全波计算模型,将计算结果与DEMETER卫星记录的NWC通讯台激发的电磁响应进行对比.虽然模型没有考虑电离层参数水平不均匀性,但模型计算结果与卫星观测值也有较好的一致性.利用经过验证的全波模型计算了不同地磁参数、电离层参数情况下,不同辐射特性的地面VLF辐射在波导中的衰减和穿透电离层时D/E区的吸收值,探讨了上述参数对电磁波能量在波导和D/E区中衰减的影响规律.     

与日夜交替线移动相关的中尺度电离层扰动GPS网观测

本文选取由国际GPS服务中心(IGS)提供的北美中纬度地区GPS网TEC观测数据,通过多通道最大熵频谱分析方法研究了2005年（太阳活动低年）地磁活动平静期间日出和日落时由于日夜交替线的移动而激发的中尺度电离层扰动(MSTID),并统计分析其季节变化特性.结果表明：(1) 日出或日落期间,在中纬度地区经常观测到由日夜交替线移动激发的中尺度电离层扰动.扰动主要沿日夜交替线运动方向传播,平均持续时间约2～3 h;振幅在0.2～0.8 TECU之间,水平波长,水平相速度和周期分别为300±150 km,150±80 m/s和25±15 min;(2) 由日夜交替线移动激发的中尺度电离层扰动在春秋分出现率较少;在夏季,扰动在黄昏时出现率达最大值,在日出后少量出现;而冬季则日出后的扰动效应更为明显.分析表明,在中纬地区,这种扰动出现率随季节的变化与不同季节的日出日落时刻太阳EUV辐射通量变化过程的快慢,以及电离层中离子损失过程快慢有关.     

低电离层调制激励VLF/ELF辐射参数优化理论分析

针对低电离层幅度调制加热过程中甚低频/极低频（VLF/ELF）激发效率较低的问题,该文利用低电离层调制加热模型分析方波幅度调制加热过程中占空比和加热频率对VLF/ELF辐射效率的影响,在此基础上获得最大VLF/ELF辐射效率下的优化占空比和加热频率选择范围.研究表明,随着调制波占空比的增大,激发的VLF/ELF等效辐射源强度先增大后减小,占空比的优化范围为40%~70%;随着加热频率的增大,激发的VLF/ELF等效辐射源强度先增大后减小,加热频率的优化范围为（0.8~0.9）倍低电离层临界频率.     

连续太阳耀斑对甚低频信号传播影响分析

甚低频(VLF)是在地-低电离层之间形成的波导内进行传播的.太阳耀斑可引起电离层突然扰动,VLF信号传播出现异常现象,对导航造成影响,同时也会造成短波通讯中断.为了研究连续太阳耀斑与VLF信号传播的关系,观测2002-04-15连续太阳耀斑爆发时,Alpha导航系统西副台VLF信号传播时的幅度和相位变化情况,解释急始型和缓变型太阳耀斑的不同之处,并且分析单个太阳耀斑与连续太阳耀斑的区别,然后根据观测数据和美国GOES卫星观测数据比较和分析,以此来研究分析连续太阳耀斑爆发对VLF信号传播造成的影响.研究结果显示连续太阳耀斑爆发与VLF信号相位的异常变化情况之间有较好的关联性.     

确定尼泊尔地震震前电磁前兆的多种实验方法

实验数据来自三种不同的技术:(i)利用钻孔天线测量的地下固定频率(VLF)电场;(ii)利用全球定位系统(GPS)接收机测量的电离层总电子含量(TEC)和(iii)SoftPAL监测的电离层下固定频率(f=19.8kHz)海军通信台站NWC发射信号的VLF振幅,分析了2015年4月25日(M=7.8)和5月12日(M=7.3)发生的两次尼泊尔大地震。VLF电场数据和VLF发射信号数据取自印度的马图拉台站和阿格拉台站,而TEC数据取自三个互联网全球服务(IGS)台站,其中一个位于印度的勒克瑙(Lck3),其他位于中国的拉萨(Lhaz)和武汉(Wuhn)。除了武汉的台站位于2 865km外,其余均在距两次地震震中800km以内。分析结果表明,VLF电场和TEC数据显示,在4月25日地震前8～15天内,在正常变化中出现了异常变化,在5月12日地震前5～7天内也出现了类似的异常。电离层VLF振幅的变化在震前4天也出现了异常。根据当时的雷电和磁扰因素对这些异常进行了严格的检查。然而,这些因素并不影响观测数据。文中简述了孕震辐射的产生和传播机制。     

基于人工源的甚低频电磁波空间传播特征统计分析

利用2005年1月至2010年11月DEMETER卫星记录的NWC发射站的VLF电场功率谱数据,采用指数拟合的方法,分析了VLF电磁波在卫星高度激发的电场空间分布和衰减特征.研究结果表明:(1)VLF电场在发射站上空及其磁共轭区有着很强的对应关系,存在南、北2个强电场中心涡;(2)相对于发射站的位置,VLF电场中心点具有经度和纬度偏移,日侧地磁经度偏移均值大于夜侧,而地磁纬度偏移均值则小于夜侧;(3)日侧VLF电场强度呈现出周期性的年变化;(4)在VLF电场中心10°范围内,电场强度随距离快速衰减,衰减常数b在长达6年的时间内保持稳定.在以上研究结果基础上初步构建的卫星高度人工源电磁波空间分布特征,将为研究地表-电离层电磁波传播机理提供基础技术支撑.     

不同方位甚低频电磁波衰减的初步研究*

利用2005—2010年DEMETER卫星记录NWC发射站的19.8kHz电场功率谱数据,采用统计和线性拟合等方法,研究了NWC站发射的电磁波在顶部电离层及磁共轭区激发的电场效应及其在不同方位的衰减特性。结果发现:(1)在NWC发射站上空,卫星记录电场呈椭圆状扩散分布,电场最强中心点位置相对地面人工源位置有一定偏移;(2)研究区上空电场北部衰减梯度最小,东部衰减梯度最大;(3)在其磁共轭区,南部电场衰减梯度最小,北部衰减梯度最大。综合分析认为人工源(NWC)发射的VLF电磁波传播到电离层高度后,受地磁场影响,电磁波主要沿磁力线方向传播,空间电场最强中心点位置相对发射站的位置发生偏移,向磁赤道方向倾斜,而且偏于磁赤道方向的电场衰减梯度最小。     

Variations in thunderstorm VLF emissions propagating over epicenters of earthquakes

In this study, the variations of average amplitude of the very-low-frequency (VLF) signal thunderstorm origin propagating over areas of strong (magnitude >5) earthquakes are considered. Despite the non-stationary state of thunderstorms, a decrease in the amplitude of its VLF signals was usually detected 3–6 days before an earthquake, with subsequent recovery on the day of the event. The effect is similar to the attenuation of the signal amplitude of low-frequency radio stations that is observed for several days before the occurrence of an earthquake. These influences of earthquakes on thunderstorm VLF signals are recorded along different paths (different azimuths and different distances to the earthquake epicenter). The probability that an earthquake of strong magnitude will induce changes in the thunderstorm VLF signal is 60–70%.     

A Study of Early/Slow VLF Perturbations Observed at Agra,India

We present here the results of sub-ionospheric VLF perturbations observed on NWC (19.8 kHz) transmitter signal propagating in the Earth-ionosphere waveguide, monitored at our low latitude station Agra. During the period of observation (June-December 2011), we found 89 cases of VLF perturbation, while only 73 cases showing early character associated with strong lightning discharges. Out of 73 events, 64 (~84%) of the early VLF perturbations are found to be early/slow in nature; the remaining 9 events are early/fast. The onset duration of these early/slow VLF perturbations is up to ~ 5 s. A total of 54 observed early events show amplitude change lying between ± 3.0 dB, and phase change ± 12 degree, respectively, and found to occur mainly during nighttime. One of the interesting results we found is that the events with larger recovery time lie far away from the VLF propagation path, while events with smaller duration of recovery are within the ± 50–100 km of signal path. The World Wide Lightning Location Network (WWLLN) data is analysed to find the location of causative lightning and temporal variation. The lightning discharge and associated processes that lead to early VLF events are discussed.     

The wave-like structures of ionospheric perturbation associated with Sumatra earthquake of 26 December 2004, as revealed from VLF observation in Japan of NWC signals

The observations of subionospheric VLF waves from the Australian VLF transmitter NWC (frequency=19.8 kHz) at the Japanese receiving stations Chofu, Chiba and Kochi have been utilized to identify a possible precursor of ionospheric perturbations to the huge Sumatra earthquake of 26 December 2004. The VLF amplitude data at Japanese stations have indicated the depression in amplitude and also the enhancement in nighttime amplitude fluctuation before the earthquake. The nighttime fluctuation is composed of wave-like structures, and the wavelet analysis and cross-correlation analyses have been performed for those fluctuations. A significant enhancement in the fluctuation spectra in the period 20–30 min to ∼100 min (the frequency range of atmospheric gravity waves) is observed only before the earthquake. Then, the wave-like structures tend to propagate from the NWC–Kochi path to NWC–Chiba path with the time delay of ∼2 h, and so the wave propagation speed is estimated as ∼20 m/s. This finding might be important when we think of lithosphere–ionosphere coupling mechanism.     

A new type of daytime high-frequency VLF emissions at auroral latitudes (“bird emissions”)

This paper is concerned with a new, previously unknown type of high-frequency (above 4 kHz) VLF emissions that were detected during winter VLF campaigns in Kannuslehto (L ~ 5.5), Finland. These previously unknown emissions have been discovered as a result of the application of special digital filtering: it clears the VLF records from pulse signals of intensive atmospherics, which prevent other kinds of VLF emissions in the same frequency range from being seen on spectrograms. As it appears, aside from wellknown bursts of auroral hisses and discrete quasiperiodic emissions, a previously unknown type of daytime right-hand polarized VLF waves is also present at frequencies above 4 kHz. These emissions can persist for several hours as series of separate short discrete wideband (from 4 to 10 kHz and higher) signals, each with a duration between one and several minutes. It has been found that such signals can be observed almost daily in winter. These emissions sound like bird’s chirping to a human ear; for that reason, they were called “bird emissions.” The dynamic spectra of individual signals often resemble flying birds. The signals are observed during daytime, more often in magnetically quiet conditions preceded by geomagnetic disturbances. As a rule, the occurrence of these bird emissions is accompanied by a slight increase in electron density in the lower ionosphere, which is evidence of the precipitation of energetic (>30 keV) electrons. This raises a number of questions as to where and how the VLF bird emissions are generated and how such emissions, at frequencies greatly exceeding half the electron equatorial gyrofrequency at L ~ 5.5, can reach the Earth’s surface.     

Earthquake effects along paths of VLF radio noise and impulsive signals as observed at Yakutsk

An analysis of amplitude variations in the noise and storm-induced impulsive VLF electromagnetic signals recorded at Yakutsk along paths above earthquakes is reported. It is shown that amplitude characteristics of storm-induced VLF signals can usefully supplement signals of radio stations as used in the radio monitoring of regions above earthquakes. The effect on these signals due to earthquakes of magnitude greater than 5 is observed as an amplitude increase within the three days following an earthquake. The pre-seismic variations in the storm-induced signal amplitude which can be regarded as precursors are generally observed as increased amplitudes (within 10 days before the earthquake) with a subsequent minimum.     

JJI甚低频台站信号对太阳耀斑事件的响应特性

太阳耀斑发生时,日地空间的X射线通量会随之增大,进而影响到地球电离层的电子密度分布,导致地球-电离层波导状态发生改变,因此接收到的甚低频(VLF,Very Low Frequency)信号会表现出对应的扰动现象.2017年9月8日,位于湖北省内武汉和随州两站点的VLF接收机分别监测到与X射线太阳耀斑相关的来自日本宫崎县(130°49′E,32°04′N)的JJI甚低频台站信号(22.2 kHz)的振幅异常事件.分析当日的数据发现JJI信号的振幅对不同的太阳耀斑出现不同的响应类型,而且对于同一个耀斑,两地的信号响应类型不尽相同.通过统计2017—2019年间与太阳耀斑相关的JJI信号振幅扰动事件,发现两接收站点的JJI信号响应类型都与耀斑强度及其发生时间存在一定的关系,且呈现出四种响应类型,即两次上升下降型、先上升后下降型、下降型和上升型,但是四种响应类型的事件占比不同.拟合结果表明信号的扰动幅度与X射线通量的积分成正相关,但是两站点的线性拟合斜率存在差异.JJI信号到武汉和随州均属于近似沿纬度方向的短距离传播,且两接收站点相距较近,因此两传播路径大致相似.研究两路径上JJI信号对太阳耀斑响应的差异性有助于理解VLF信号的传播以及探索其在太阳活动监测方面的应用.     

Numerical simulation of LEP Trimpis observed at Poitiers,France, on signals from VLF transmitters

Transient amplitude and phase perturbations on subionospheric VLF signals, known as the Trimpi effect, are caused by the scattering of VLF radiation from localized ionization enhancements in the nighttime D-region. The patches of ionization are due to precipitation from the radiation belts of keV electrons, that is induced by lightning-generated whistlers.This work is concerned with the numerical simulation of such VLF perturbations, termed LEP (lightning-induced electron precipitation) or classic Trimpis. Two different codes are used to compute the VLF propagation in the Earth-ionosphere waveguide in the presence of a D-region inhomogeneity. The first is based on mode theory, and the second on the FDTD (finite-difference time-domain) method. Both codes are two-dimensional and, therefore, relevant only to LEP events lying on the transmitter-receiver great circle path (TRGCP). A method of simulation is proposed to interpret quantitatively VLF amplitude and phase changes in terms of the approximate location and size of the associated ionospheric perturbation along the TRGCP. The method is applied to LEP Trimpis observed at Poitiers (L = 2) on signals from the NAA and GQD transmitters. Results are discussed in the light of the information that may be deduced from the high-resolution analysis of VLF temporal signatures. A sequence of simultaneous LEP Trimpis observed at Poitiers on three widely separated transmission paths is studied; two alternative interpretations are proposed.