日侧极隙纬度低频波及其与地磁扰动的关系

利用挪威斯瓦尔巴德地区的ＥＬＦ／ＶＬＦ观测资料和世界数据中心公布的Ｋｐ指数，分析了日侧极隙纬度低频波不同季节里的周日变化和１９８４年５月－１９８６年４月的年度变化及这些波与地磁扰动的某些关系．主要结果有：１．夏季ＥＬＦ波出现率极大值最大，而冬季的最小，前者约为后者的３倍，而且此极大值无论哪个季节都出现在磁正午时段．２．ＶＬＦ波出现率在夏季较小冬季较大；其极大值出现在磁午后至磁子夜前的２－４ｈ时段里．３．夏季ＥＬＦ波指数∑Ａ与地磁指数∑ＫＰ日变化较相吻合，在太阳自转周内两者相关系数约为０．６．４．冬季ＶＬＦ波指数∑Ａ与∑Ｋｐ日变化相吻合，在太阳自转周内两者相关系数可达０．８．这些结果表明日侧极隙纬度ＥＬＦ波和ＶＬＦ波各有不同的产生机制和源区．     

Estimation of global lightning activity and observations of atmospheric electric field

Variations in the global atmospheric electric circuit are investigated using a wide range of globally spaced instruments observing VLF (∼10 kHz) waves, ELF (∼300 Hz) waves, Schumann resonances (4–60 Hz), and the atmospheric fair weather electric field. For the ELF/VLF observations, propagation effects are accounted for in a novel approach using established monthly averages of lightning location provided by the Lightning Image Sensor (LIS) and applying known frequency specific attenuation parameters for daytime/nighttime ELF/VLF propagation. Schumann resonances are analyzed using decomposition into propagating and standing waves in the Earth-ionosphere waveguide. Derived lightning activity is compared to existing global lightning detection networks and fair weather field observations. The results suggest that characteristics of lightning discharges vary by region and may have diverse effects upon the ionospheric potential.     

Scientific heritage of Ya. I. Likhter,a pioneer of the studies of VLF emission (Devoted to centenary of Ya. Likhter)

This work presents the most important results, some of which have been previously unknown, of long-term experimental studies of signals and emission in the ELF and VLF ranges carried by Ya.I. Likhter during ground-based and satellite observations. In addition, the possibility of using the research results obtained for the diagnostics of parameters and the state of the near Earth’s plasma is shown.     

Electromagnetic anomalies associated with 1995 Kobe earthquake

Occurrences of anomalous electro-magnetic phenomena at varied frequency ranges, covering ELF to VHF, have been reported in relation to the 17 January 1995 Kobe earthquake (M7.2), by several independent research groups. Prominent pre-seismic peaks, which could have been emitted from the focal area, were observed on 9-10 January in ELF, VLF, LF and HF ranges. Whether these changes were truly related to the earthquake is not certain, because atmospheric (thunderbolt discharge) activities also peaked on 9-10 January. The nomalous changes were markedly enhanced toward the catastrophe in agreement with many reports on unusual radio/TV noise. Anomalous transmission of man-made electromagnetic waves in VLF and VHF ranges was also detected from a few days before the earthquake, indicating the possibility that the ionosphere above the focal zone was disturbed at the final stage of the earthquake preparation process.     

基于人工电离层扰动对地-电离层波导传播的VLF波的影响

基于低电离层加热理论和甚低频电波在地-电离层波导中传播理论,建立低电离层扰动对甚低频电波传播影响的分析模型,并利用实验数据验证了该模型的正确性.据此模型,研究了加热功率、加热波极化以及背景参数所导致的低电离层扰动对不同频率甚低频电波传播的影响.结果表明,低电离层扰动越强,则通过该区域内甚低频波幅度和相位的相对变化越强,通过研究地-电离层波导甚低频信号通过人工扰动区域后幅度和相位的变化,可望用于诊断人工电离层扰动强度.     

A generation mechanism for chorus emission

A chorus generation mechanism is discussed, which is based on interrelation of ELF/VLF noise-like and discrete emissions under the cyclotron wave-particle interactions. A natural ELF/VLF noise radiation is excited by the cyclotron instability mechanism in ducts with enhanced cold plasma density or at the plasmapause. This process is accompanied by a step-like deformation of the energetic electron distribution function in the velocity space, which is situated at the boundary between resonant and nonresonant particles. The step leads to the strong phase correlation of interacting particles and waves and to a new backward wave oscillator (BWO) regime of wave generation, when an absolute cyclotron instability arises at the central cross section of the geomagnetic trap, in the form of a succession of discrete signals with growing frequency inside each element. The dynamical spectrum of a separate element is formed similar to triggered ELF/VLF emission, when the strong wavelet starts from the equatorial plane. The comparison is given of the model developed using some satellite and ground-based data. In particular, the appearance of separate groups of chorus signals with a duration 2–10 s can be connected with the preliminary stage of the step formation. BWO regime gives a succession period smaller than the bounce period of energetic electrons between the magnetic mirrors and can explain the observed intervals between chorus elements.     

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.     

电离层人工调制在水平分层电离层中所激发的ELF波辐射

通过大功率ELF/VLF调幅高频波能有效地扰动低电离层,形成等效的ELF/VLF电离层虚拟天线,用来辐射ELF/VLF波,所辐射出的低频信号可以进入中性大气层形成地球-电离层波导.本文基于调制加热模型,采用全波有限元算法计算由人工调制电离层所形成的电偶极矩所辐射出的ELF波在水平分层电离层中的波场,计算结果将与地面观测结果进行比较.模拟结果表明,所辐射出的ELF波在电离层中形成一个窄的准直波束,海面所能接收到的ELF信号强度为pT量级,并且频率越低,海面所接收到的场强就越小,与HAARP实验数据一致.结果还表明,低纬电离层对低频信号的传播衰减较大,并且所能透射出电离层的角度小,因此高纬地区更适合地球-电离层波导的激发.     

ELF and VLF wave generation by HF heating: A comparison of AM and CW techniques

Antenna field 2 of the Tromsø Heating facility consists of six rows of six horizontal crossed full wave dipoles aligned with the rows running geographic east-west. In previous experiments on ELF/VLF wave generation it has been the practice to feed the rows in parallel, with HF radiation amplitude modulated at the ELF/VLF frequency it was desired to radiate (AM configuration). Here we describe how the antenna array was also configured so that it could be fed with a continuous wave (CW) input power but still carry information at an ELF/VLF frequency. To effect this the three southern most rows of the antenna array were driven with a CW signal at 4.04 MHz and the three northernmost rows with a CW signal at frequency greater than 4.04 MHz by the ELF/VLF frequency it was desired to radiate (CW configuration). Experiments were performed with modulation/difference frequencies of 565 and 2005 Hz and the signals were received at the Lycksele Geophysical Observatory, 500 km south of the heating facility. The signals were typically 11 dB greater in the AM than the CW configuration, despite the fact that the average power delivered to the ionosphere in the CW configuration was four times that in the AM configuration. Significant harmonic radiation (both odd and even) was produced in the AM configuration but no harmonic radiation was detected in the CW configuration. A simple theory has been developed to model the fields produced by HF heating using the two techniques (AM,CW). A good agreement has been obtained between the experimental observations and model computations. The model has been extended to show how the relative efficiency of generation, AM/CW, varies with the frequency of the ELF/VLF radiation and HF antenna element spacing. Radiation patterns for the ‘ionospheric ELF/VLF antenna’ have also been derived for the two generation techniques.     

On the Use of VLF Narrowband Measurements to Study the Lower Ionosphere and the Mesosphere–Lower Thermosphere

The ionospheric D-region (~60 km up to ~95 km) and the corresponding neutral atmosphere, often referred to as the mesosphere–lower thermosphere (MLT), are challenging and costly to probe in situ. Therefore, remote sensing techniques have been developed over the years. One of these is based on very low frequency (VLF, 3–30 kHz) electromagnetic waves generated by various natural and man-made sources. VLF waves propagate within the Earth–ionosphere waveguide and are extremely sensitive to perturbations occurring in the D-region along their propagation path. Hence, measurements of these signals serve as an inexpensive remote sensing technique for probing the lower ionosphere and the MLT region. This paper reviews the use of VLF narrowband (NB) signals (generated by man-made transmitters) in the study of the D-region and the MLT for over 90 years. The fields of research span time scales from microseconds to decadal variability and incorporate lightning-induced short-term perturbations; extraterrestrial radiation bursts; energetic particle precipitation events; solar eclipses; lower atmospheric waves penetrating into the D-region; sudden stratospheric warming events; the annual oscillation; the solar cycle; and, finally, the potential use of VLF NB measurements as an anthropogenic climate change monitoring technique.     

高压输电系统引起的空间电磁扰动

随着全球工业化进程的大力推进,高压输电系统逐渐成为电磁环境监测的主要人为干扰源之一。本文总结了以法国DEMETER卫星电磁场数据为主开展的高压电力线辐射研究的最新成果,对电力线产生的各类空间电磁扰动现象及其时空分布特征进行了归纳分析,其中包括电力线谐振辐射(PLHR)、磁层线辐射(MLR)、ELF/VLF辐射、高能粒子沉降等,并对与其耦合机制相关的电磁波传播及波粒相互作用模型进行了总结讨论,旨在对中国电磁监测试验卫星(CSES-1)数据处理方法、干扰识别和地震弱信号提取分析提供更多的参考信息。     

Latest progress on interactions between VLF/ELF waves and energetic electrons in the inner magnetosphere

Interactions between very/extremely low frequency (VLF/ELF) waves and energetic electrons play a fundamental role in dynamics occurring in the inner magnetosphere. Here, we briefly discuss global properties of VLF/ELF waves, along with the variability of the electron radiation belts associated with wave-particle interactions and radial diffusion. We provide cases of electron loss and acceleration as a result of wave-particle interactions primarily due to such waves, and particularly some preliminary results...     

Physical mechanisms of man-made influences on the magnetosphere

Since the discovery of the Luxembourg effect in the 1930s, it is clear that man-made activities can perturb the ionosphere and the magnetosphere. The anthropogenic effects are mainly due to different kinds of waves coming from the Earth's surface. Acoustic-gravity waves are generated by large explosions, spacecraft launches, or flight of supersonic planes. Electromagnetic waves are active in different frequency ranges. Power line harmonic radiation which is radiated in the ELF range by electrical power systems can be observed over industrial areas. At VLF and HF, the ground-based transmitters used for communications or radio-navigation heat the ionosphere and change the natural parameters. A large variety of phenomena is observed: wave-particle interaction, precipitation of radiation belt electrons, parametric coupling of EM whistler waves, triggered emissions, frequency shift, and whistler spectrum broadening. This paper will review the different physical mechanisms which are relevant to such perturbations. The possibility of direct chemical pollution in the ionosphere due to gas releases is also discussed.     

Aspects regarding the use of the INFREP network for identifying possible seismic precursors

In the last decades, one of the main research directions in identifying seismic precursors involved monitoring VLF (Very Low Frequency) and LF (Low Frequency) radio waves and analysing their propagation characteristics. Essentially this method consists of monitoring different available VLF and LF transmitters from long distance reception points. The received signal has two major components: the ground wave and the sky wave, where the sky wave propagates by reflection on the lower layers of the ionosphere. It is assumed that before and during major earthquakes, unusual changes may occur in the lower layers of the ionosphere, such as the modification of the charged particles number density and the altitude of the reflection zone. Therefore, these unusual changes in the ionosphere may generate unusual variations in the received signal level.The International Network for Frontier Research on Earthquake Precursors (INFREP) was developed starting with 2009 and consists of several dedicated VLF and LF radio receivers used for monitoring various radio transmitters located throughout Europe. The receivers’ locations were chosen so that the propagation path from these VLF/LF stations would pass over high seismicity regions while others were chosen to obtain different control paths.The monitoring receivers are capable of continuously measuring the received signal amplitude from the VLF/LF stations of interest. The recorded data is then stored and sent to an INFREP database, which is available on the Internet for scientific researchers. By processing and analysing VLF and LF data samples, collected at different reception points and at different periods of the year, one may be able to identify some distinct patterns in the envelope of the received signal level over time. Significant deviations from these patterns may have local causes such as the electromagnetic pollution at the monitoring point, regional causes like existing electrical storms over the propagation path or even global causes generated by high-intensity solar flares. As a consequence, classifying these perturbations and minimizing them (when possible) would represent an important step towards identifying significant pattern deviations caused by seismic activities.Taken into consideration some of the issues mentioned above, this paper intends to present some aspects meant to improve the overall performance of the existing INFREP network. The signal-to-noise ratio improvement of the monitoring receiver may be achieved by relocating the antenna (or even the entire monitoring system if possible) in areas with less electromagnetic pollution within the VLF and LF bands. Other solution may involve replacing the existing electric “whip” antennas with magnetic loop antennas.Regarding the measuring method, long-term averaging of the received signal to reduce the electromagnetic noise should be carefully applied. If the averaging time is too long, there is a risk that, during a seismic event, the details of the received signal envelope would be lost. Moreover, this may reduce the possibility of making correlations between the monitored stations and INFREP receivers in case of sudden ERP (Effective Radiated Power) variations of the VLF/LF stations. For the same reason, the time synchronization of the recorded data using (for instance) GPS technology is highly recommended.Other aspects related to the overall performance improvement of the INFREP network consist of monitoring other VLF/LF stations such as the Krasnodar station (south of Russia), part of the ALPHA/RSDN-20 VLF navigation system, or the 77.5 kHz DCF77 time signal transmitter (near Frankfurt am Main, Germany). Moreover, the installation of a new reception point in Romania (near Cluj-Napoca) for monitoring the Vrancea area (within the Carpathians Mountains) and the Adriatic region will provide complementary scientific data within the network.     

电离层人工调制激发的下行ELF/VLF波辐射

通过大功率ELF/VLF调幅高频波对电离层进行加热,形成电离层虚拟天线,可以作为发射ELF/VLF波的一种有效手段.本文使用汪枫(2009)的调制加热模型,计算高频加热电离层产生的低频辐射源强度,采用全波解算法分析辐射的低频波向下传播过程中的衰减和反射问题,并采用HAARP实验参数,模拟出在海面上接收到的低频信号强度为...     

低纬地区电离层电流的人工调制数值模拟

利用高频泵波能对低电离层进行有效的人工扰动.采用ELF/VLF调幅高频电波对电离层进行加热,电子温度会随着调制频率振荡,并引起电导率周期性变化,从而使加热区内电离层电流周期性变化,形成等效的ELF/VLF电离层虚拟天线,辐射调制频率范围内的无线电波.早期的电离层人工调制研究主要集中在高纬和极区,本文讨论低纬地区电离层人工调制的可能性.本文的理论研究和数值模拟结果表明,低纬地区低电离层电导率在周期性加热的条件下能有效地被调制,使加热区域形成ELF/VLF波的电流辐射源,并分析了不同加热参数和入射条件对调制效果的影响.     

Probing the lower ionospheric perturbations associated with earthquakes by means of subionospheric VLF/LF propagation

There have been reports for many years that the ionosphere is very sensitive to seismic effects, and the detection of ionospheric perturbations associated with earthquakes (EQs) attracts a lot of attention as a very promising candidate for short-term EQ prediction. In this review we present a possible use of VLF/LF (very low frequency (3–30 kHz)/low frequency (30–300 kHz)) radio sounding of seismo-ionospheric perturbations. In order to avoid the overlapping with my own previous reviews, we first show some pioneering results for the Kobe EQ and we try to present the latest results including the statistical evidence on the correlation between the VLF/LF propagation anomalies (ionospheric perturbations) and EQs (especially with large magnitude and with shallow depth), medium-distance (6-8 Mm) propagation anomalies, the fluctuation spectra of subionospheric VLF/LF data (the effect of atmospheric gravity waves, the effect of Earth's tides, etc.), and the mechanism of lithosphere-atmosphere-ionosphere coupling. Finally, we indicate the present situation of this kind of VLF/LF activities going on in different parts of the globe and we suggest the importance of international collaboration in this seismo-electromagnetic study.     

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.     

Time–frequency analysis of VLF for seismic-ionospheric precursor detection: Evaluation of Zhao-Atlas-Marks and Hilbert-Huang Transforms

This work investigates the application of two post-processing methods of extracting spectra from VLF signals in order to detect disturbances that could be attributed to seismic-ionospheric precursory phenomena. Although precursory phenomena have been investigated in detail in past studies, a different application of time–frequency analysis methods may produce distinct patterns, which reveal disturbances in the VLF spectra received from stations that are in the propagation path over preparation zones, and also pinpoint disturbances that could be attributed to seismic-ionospheric precursors.To this purpose, three different methods of post processing are compared. These are the Wavelet Transform as a benchmark method in the form of the Continuous Wavelet Transform, a noise-assisted variant of the Hilbert-Huang Transform and the Zhao-Atlas-Marks Distribution. Comparative diagrams are presented and the advantages and weaknesses of each method are presented.     

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.