Trimpi perturbations from large ionisation enhancement patches

A number of increasingly sophisticated and realistic models have been developed in order to investigate the interaction between sub-ionospherically propagating VLF waves and regions of ionisation enhancement (LIE¹) in the D-region caused by lightning-induced electron precipitation enhancements (LEP). This LEP-produced LIE can result in phase and amplitude perturbations on received VLF radio signals that are referred to as Trimpis or more precisely, classic Trimpis, to distinguish them from “early/fast Trimpis” or “VLF sprites” which are not caused by LEP and are not considered here. It is important, for comparison with experimentally observed Trimpi effects, that the spatial extent of the D-region electron density (N_e) perturbation is modeled accurately. Here, it is argued that most previous modeling has used patch (LIE) sizes that are typically up to 100 km in both latitudinal and longitudinal extent, which are generally smaller than those that actually occur for real lightning induced electron precipitation events. It would also appear that maximum ΔN_e values assumed have often been too large, and the patches (LIEs) have been incorrectly modelled as circular rather than elliptical in horizontal extent. Consequently, in the present work, Trimpi perturbations are determined for LIEs with smaller maximum ΔN_e, larger spatial extent and elliptical shape. Calculations of VLF Trimpis have been made as a function of the horizontal coordinates of the LIE centre, over the whole rectangular corridor linking transmitter and receiver. The Trimpi modelling program is fully 3D, and takes account of modal mixing at the LIE. The underlying theory assumes weak Born scattering, but the code calculates a non-Born skin depth attenuation function for the LIE in question. The LIE is modelled as an electron density enhancement with a Gaussian profile in all coordinates. Results for a large elliptical LIE ∼ 200 × 600 km show that significant Trimpis, ∼−0.4 dB in amplitude and ∼+4° in phase are predicted, using modest maximum ΔN_e values ∼ 1.5 el/cc. Such an electron density enhancement is well within the range that would be expected to result from experimentally observed fluxes of electron precipitation following wave particle interactions with whistler-mode waves.     

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.     

根据VLF的相位变化确定D层宇宙线强度变化的可能性

银河宇宙线是电离层D层的重要电离源之一,它的急剧变化会使D层电子密度发生改变,从而影响VLF波的夜间传播。本文根据在西安接收英国GBR台的VLF波（16kc/s）的相位变化,讨论了在有宇宙线暴（Forbush下降）和强磁暴时,中纬D层电子浓度的变化和相应的VLF波的相位漂移;并根据VLF的相位变化,估计了相应的宇宙线强度变化。由于D层中的宇宙线强度变化通常难于观测,通过VLF波的相位漂移来估计它,这是很有意义的。所以,VLF波的传播效应可能成为间接探测宇宙线强度变化的有用工具。     

VLF phase and amplitude: daytime ionospheric parameters

Experimental observations of the daytime variations of VLF phase and amplitude over a variety of long subionospheric paths have been found to be satisfactorily modelled with a D-region ionosphere, described by the two traditional parameters, H′ and β (being measures of the ionospheric height and the rate of increase of electron density with height, respectively). This VLF radio modelling uses the NOSC Earth–ionosphere waveguide programs but with an experimentally deduced dependence of these two ionospheric parameters on solar zenith angle. Phase and amplitude measurements from several VLF Omega and MSK stations were compared with calculations from the programs LWPC and Modefinder using values of H′ and β determined previously from amplitude only data. This led to refined curves for the diurnal variations of H′ and β which, when used in these programs, give not only calculated amplitudes but also, for the first time, calculated phase variations that agree well with a series of observations at Dunedin, New Zealand, of VLF signals from Omega Japan, Omega Hawaii, NPM (Hawaii) and NLK (Seattle) covering a frequency range of 10–25 kHz.     

Maintenance of the middle-latitude nocturnal D-layer by energetic electron precipitation

Summary Energetic electrons are continually removed from the radiation belts by resonant pitch-angle scattering with ELF turbulence. A realistic simulation of the concomitant precipitation loss of such electrons to the atmosphere shows it to be a significant source for the nocturnal ionospheric D-region. During geomagnetically quiet (non-storm) periods, precipitating electrons are expected to provide the dominant nocturnal ionization source at medium invariant latitudes corresponding to field lines just inside the plasmapause. When the level of scattering turbulence is high the quiet time precipitation can dominate for an extended range of latitudes ( 55° to 65°). Observed fluctuations in the level of scattering turbulence should produce modulations in the concentration of nocturnal middle latitude D-region electrons which may be detected using radio probing techniques.     

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.     

Solar–terrestrial,ionospheric and natural phenomena studies using the South America VLF network (SAVNET)

The South America VLF Network (SAVNET) has been installed in April 2009, and is composed of eight tracking receivers spread over South America, in Brazil, Peru and Argentina, and the Antarctica Peninsula. SAVNET is monitoring the properties of subionospheric propagating waves that reveal changes of the electrical properties of the ionospheric diurnal D-region or nocturnal E-region. In this paper, we will show the ability of the diagnostic obtained by SAVNET to discuss the monitoring of the solar activity on short timescales related to ionization due to solar flares. The sensitivity of flare detection as a function of the solar activity level will be discussed. On longer timescales related to the solar cycle, SAVNET is also able to provide information on the solar Lyman-α radiation. Finally we show that the VLF technique is well suited to search for of seismic-electromagnetic effects, and to provide a genuine diagnostic of high-energy astrophysical phenomena.     

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.     

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.     

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%.     

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.     

Experimental study of the features of VLF, MF, HF and VHF electromagnetic radiation accompanying rock fracture

In order to investigate the physical mechanism of seismo-electromagnetic signals, many scholars from China and other countries have carried out laboratory and field experiments of rock samples by loading them up to fracture in recent ten-odd years. In each of these experiments, the phenomenon of electromagnetic radiation was observed during the process of rock sample fracture. However, the experimental results can not explain the non-synchronous electromagnetic radiation in actual observation records. In these records, some stations displayed anomalies but some others did not, and the signals of different frequency bands observed by the same station did not occur simultaneously. This has brought about many difficulties to the application of electromagnetic radiation in earthquake prediction. The experimental result of this paper can explain the phenomenon of non-synchronous electromagnetic radiation. In the experiment of this paper, rock samples are uniaxially compressed up to fracture. The antennas of different frequency bands are installed two meters away from the rock sample in different directions. The electromagnetic signals during the whole process of rock sample fracture are recorded synchronously by a 14-channel tape recorder. The experimental results are as follows: (1) signals of the four frequency bands: VLF, MF, HF and VHF, are all recorded during the main fracturing process of rock samples, but signals of different frequencies may somtimes occur non-synchronously; (2) the intensity of electromagnetic radiation is the highest at the moment when the main fracture occurs, but signals in different directions are of different intensities. The above results are consistent with the non-synchronous electromagnetic radiation observed in real earthquakes. It seems possible to predict the orientation of future seismic source by the electromagnetic method of the LF or VLF frequency bands. Contribution No. 95A0075, Institute of Geophysics, SSB, China. The paper is translated by Prof.Jie-Fan HUANG, Peking University from the Chinese into English version.     

Whistler–electron interactions in the magnetosphere: new results and novel approaches

Attention is focused here on the quasilinear and nonlinear physics of cyclotron interactions between magnetospheric whistler mode waves and energetic electrons on dipolar geomagnetic flux tubes. These interactions can lead to the generation of noise-like emissions or phase-coherent discrete signals in the frequency-time domain. In the magnetosphere noise-like emissions called hiss are accompanied by a smooth electron precipitation pattern. Examples of discrete emissions are ELF/VLF chorus or VLF emissions triggered by whistlers from lightning or by radio transmitters on the ground. The rapid temporal variations of these signals are associated with fine structure of the distribution function of the radiation belt electrons, such as a transient step-like deformation or a well-organized beam, which are prepared by initial noise-like emissions or by a quasimonochromatic whistler–wave packet, respectively. These cause the properties of the electrons, which may be observed on a satellite, to evolve rapidly in time and on relatively short spatial scales. Bursts of precipitating electrons occur, and can contribute significantly to depleting the radiation belts. Recent results on improvements in the theoretical understanding of such processes and on new observations of magnetospheric electrons and whistler-mode waves are presented.     

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.     

Changes in animal activity prior to a major (M = 7) earthquake in the Peruvian Andes

During earthquake preparation geophysical processes occur over varying temporal and spatial scales, some leaving their mark on the surface environment, on various biota, and even affecting the ionosphere. Reports on pre-seismic changes in animal behaviour have been greeted with scepticism by the scientific community due to the necessarily anecdotal nature of much of the evidence and a lack of consensus over possible causal mechanisms. Here we present records of changes in the abundance of mammals and birds obtained over a 30 day period by motion-triggered cameras at the Yanachaga National Park, Peru, prior to the 2011 magnitude 7.0 Contamana earthquake. In addition we report on ionospheric perturbations derived from night-time very low frequency (VLF) phase data along a propagation paths passing over the epicentral region. Animal activity declined significantly over a 3-week period prior to the earthquake compared to periods of low seismic activity. Night-time ionospheric phase perturbations of the VLF signals above the epicentral area, fluctuating over the course of a few minutes, were observed, starting 2 weeks before the earthquake. The concurrent observation of two widely different and seemingly unconnected precursory phenomena is of interest because recently, it has been proposed that the multitude of reported pre-earthquake phenomena may arise from a single underlying physical process: the stress-activation of highly mobile electronic charge carriers in the Earth’s crust and their flow to the Earth’s surface. The flow of charge carriers through the rock column constitutes an electric current, which is expected to fluctuate and thereby emit electromagnetic radiation in the ultralow frequency (ULF) regime. The arrival of the charge carriers can lead to air ionization at the ground-to-air interface and the injection of massive amounts of positive airborne ions, known to be aversive to animals.     

Anomalous enhancement in daytime 40-kHz signal amplitude accompanied by geomagnetic storms,earthquakes and meteor showers

Anomalous propagational characteristics, daytime signal levels greater than night-time, were observed. The amplitude records of a 40-kHz signal propagated over a distance of 5100 km from Sanwa, Japan to Calcutta along a low-latitude path show higher signal strength at midday compared to the midnight level on days preceded by principal geomagnetic storms, earthquakes and major meteor showers. This is explained by the increased ionization in the D-region following geophysical events. The storm after-effects only have a duration of a single day in this low-latitude path.     

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.     

Neural network technique for identifying prognostic anomalies from low-frequency electromagnetic signals in the Kuril–Kamchatka region

In this paper, we suggest a technique for forecasting seismic events based on the very low and low frequency (VLF and LF) signals in the 10 to 50 Hz band using the neural network approach, specifically, the error back-propagation method (EBPM). In this method, the solution of the problem has two main stages: training and recognition (forecasting). The training set is constructed from the combined data, including the amplitudes and phases of the VLF/LF signals measured in the monitoring of the Kuril-Kamchatka region and the corresponding parameters of regional seismicity. Training the neural network establishes the internal relationship between the characteristic changes in the VLF/LF signals a few days before a seismic event and the corresponding level of seismicity. The trained neural network is then applied in a prognostic mode for automated detection of the anomalous changes in the signal which are associated with seismic activity exceeding the assumed threshold level. By the example of several time intervals in 2004, 2005, 2006, and 2007, we demonstrate the efficiency of the neural network approach in the short-term forecasting of earthquakes with magnitudes starting from M ≥ 5.5 from the nighttime variations in the amplitudes and phases of the LF signals on one radio path. We also discuss the results of the simultaneous analysis of the VLF/LF data measured on two partially overlapping paths aimed at revealing the correlations between the nighttime variations in the amplitude of the signal and seismic activity.     

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.     

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。