Characteristics of nighttime VLF/ELF emissions with frequency drift and estimation of the large-scale electric field from the frequency drift observed at a low latitude Indian ground station, Jammu

Characteristics of nighttime VLF/ELF emissions are examined on the basis of the data obtained at our low latitude ground station Jammu (geomagnetic latitude, 22°26′ N; L = 1.17), India during our VLF/ELF campaign. From the detailed analysis of a huge amount of acquired VLF/ELF data at Jammu we have found three remarkable events which clearly exhibit a rise in their frequency in pre and past midnight sectors during magnetically quiet and substorm periods. Our analysis shows that the frequency drift in VLF/ELF emissions seems to be a rare phenomenon at low latitudes during magnetically quiet and substorm periods in pre and post midnight sectors. This property of temporal frequency drift (regular frequency increases with time) in VLF/ELF emissions observed at our station Jammu are interpreted in terms of a quasi-linear electron cyclotron instability model for wave excitation. The initial frequency increase is believed to be due to a combind effect of L-shell drift of energetic electrons. Further, the frequency drifts in VLF/ELF emissions observed at Jammu have been used to estimate the large scale electric field during quiet and substorm periods in pre and post midnight sectors. This investigation would be most useful for the study of the wave-particle interaction processes, magnetospheric plasma structure and particle dynamics, especially during quiet periods in premidnight sector at low latitudes.     

低纬电离层人工调制所激发的ELF波射线追踪

通过大功率极低频(ELF)/甚低频(VLF)高频调幅波能有效地扰动低电离层电流, 形成等效的ELF/VLF电离层虚拟天线,辐射ELF/VLF波, 所辐射出的低频信号能够传播进入到磁层,对其传播特性的研究对于理解辐射带高能电子沉降具有重要意义.本文基于磁层射线追踪理论,通过数值模拟得到在低纬地区所激发出的ELF波在磁层中的射线路径,并对其特征进行分析.数值模拟结果表明, 从低纬激发的ELF波在南北半球来回弹跳,并逐渐传播到更远处,对于不同频率的ELF波, 频率越高,传播距离越近,频率越低,传播距离越远,在传播过程中, ELF波会逐渐倾向于在一个固定的磁层区域附近来回反射,在此过程中波法向角也逐渐变为90°,射线方向倾向于沿着背景磁场方向传播.     

Observation of signals of VLF radio stations and VLF noise during the solar eclipse on March 29, 2006

Effects of the solar eclipse on March 29, 2006 on signals of VLF radio stations, regular radio-noise intensity in the VLF range 0.3–10 kHz, and the number of atmospherics registered in Yakutsk mainly from the western direction are considered. The full lunar shadow zone consistently shaded the daytime section of the radio-signal propagation path from the western coast of Africa to Altai. A multichannel parallel analyzer-recorder (operating in the frequency range 0.47–8.7 kHz), one-point thunderstorm direction-and-range finder (0.3–100 kHz), narrow-sector radio-noise direction finder (0.3–10 kHz), recorder of VLF radio-station signals, and broadband radio-noise recorder (0.3–100 kHz) were used for observations. Time synchronization of the recorder of VLF radio-station signals was carried out with the help of GPS-clocks. The effect was observed in radio-station signals, radio-noise, and number of atmospherics from a direction of (270 ± 20)°, counted from the north clockwise, in the last stage of the eclipse (∼11–12 UT), when the lunar shadow approached the night terminator and crossed the signal propagation path. The effect was manifested through a ∼20% gain of the received signals, ∼40% increase in the number of atmospherics, and radio-station phase changes. The registered effects are similar to those we observed during the eclipse on March 9, 1997. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 84–87, June, 2007.     

Experimental study of low-frequency emission from a mobile ionospheric source

The results of an investigation of VLF radiation generated by a mobile ionospheric source are presented. Radiation at frequency F appears due to the operation of two powerful HF transmitters at frequencies f and f±F fed to two space-separated antennas. The intensity of the signals received at the ground depends on the location of the source with respect to the receiving point and is determined by the sign and value of the frequency shift F. The proposed model of the phenomena satisfactorily explains the experimental results.Scientific-Research Institute of Radiophysics. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 37, No. 6, pp. 783–792, June, 1994.     

Studies on the anomalies in the behaviour of transmitted subionospheric VLF electromagnetic signals and the changes in the fourth Schumann resonance mode as signatures of two pending earthquakes

Some anomalies are observed in the subinospheric VLF electromagnetic (EM) signals at 19.8 kHz transmitted from North West Cape, Australia (lat: 21.82° S; long: 114.16° E) and 40 kHz from Fukushima, Japan (lat: 36.18° N, long: 139.85° E), recorded by VLF receivers near Kolkata (lat: 22.56° N, long: 88.5° E) during the occurrence of the two earthquakes at Andaman Island (lat:14.018° N, long: 92.92° E), India and other at South Coast of Honsu (lat: 34.78° N, long: 138.27° E), Japan. The analyses of these seismo-ionospheric VLF EM anomalies at these two frequencies during some days before and after the occurrence of earthquake on August 11, 2009 will be presented here. VLF-LF transient variations of significant magnitude in the form of spikes are observed few days prior to the day of occurrence of the earthquakes that continued several days more, then decaying gradually and finally ceased. Signals are examined to describe their connectivity with earthquakes. The enhancements of the amplitude and frequency of the fourth mode of Schumann resonance spectra have been detected during the occurrence of the two earthquakes, which will also be reported here.     

Multifractal estimates of monofractality in RR-heart series in power spectrum ranges

Two popular estimators of multifractal properties: the Wavelet Transform Modulus Maxima method and Multifractal Detrended Fluctuation Analysis are applied to investigate signals consisting of normal RR-series in 39 healthy subjects and 90 patients suffering from systolic dysfunction of the left ventricle. However, differently from standards for multifractal analysis the scaling is performed separately in intervals corresponding to standard power spectral bands: low (LF), very low (VLF) and ultra low frequencies (ULF). Tests on fractional Brownian motions (fBm) are done to quantify properties of the estimators as detectors of monofractality in LF, VLF and ULF bands. Arguments are given that multifractal analysis of RR-series performed in these bands has a physiological meaning. The increased activation of the sympathetic nervous system caused by heart disease is detected evidently only by analysis in LF. The transition in multifractal characteristics between diurnal and nocturnal activity takes place when the analysis moves from LF and VLF to ULF. Only in ULF, the diurnal heart rate variability can be approximated by fBm with a self-similarity index of H=0.20.     

地基人工VLF 电波对辐射带电子的调制

辐射带电子的加速与沉降机理是空间物理研究的重要课题.法国DEMETER电磁卫星观测到了美国NPM发射站VLF信号及与之相关的高能电子沉降事例.本研究工作将根据基于回旋共振相互作用的准线性扩散理论,通过对局域投掷角扩散系数的计算,来说明受VLF影响的高能电子的投掷角分布与电子的能量及所处位置的关系.理论计算较好地解释了DEMETER卫星在NPM实验期间所观测到的电子沉降事例.在此基础上进一步讨论了通过人工方式对辐射带高能电子施加影响的效率问题. 关键词： 回旋共振 投掷角散射 电子沉降     

Observation of very low frequency emissions at Indian Antarctic station,Maitri

Recently, we have succeeded in recording VLF emissions at the Indian Antarctic station, Maitri (geom. lat. 62°S, geom. long. 57.23°E,L = 45) using a T-type antenna, pre/main amplifiers and digital audio tape recorder. VLF hiss in the frequency ranges 11–13 kHz and 13–14.5 kHz and some riser-type emissions in the frequency range 3–5 kHz and magnetospheric lines at about 6.2, 8.0 and 9.2 kHz are reported for the first time. The generation and propagation mechanism of these emissions are discussed briefly.     

Separation of physiological very low frequency fluctuation from aliasing by switched sampling interval fMRI scans

Anesthetized children have dominant blood-oxygen-level-dependent (BOLD) signal sources presenting high-power fluctuations at very low frequencies (VLF <0.05 Hz). Aliasing of frequencies higher than critically sampled has been regarded as one probable origin of the VLF fluctuations. Aliased signal frequencies change when the sampling rate of the data is altered. In this study, the aliasing of VLF BOLD signal fluctuation was analysed by switching the repetition time (TR) of magnetic resonance (MR) images. Eleven anesthetized children were imaged at 1.5 T using TRs of 500 and 1200 ms. The BOLD signal sources were separated with independent component analysis (ICA). Occipital cortex signal sources had nonaliased VLF fluctuation ( approximately 0.03 Hz) in 9 of 11 subjects. Arterial signal sources failed to present stable power peaks at frequencies lower than 0.42 Hz presumably due to aliasing. Cerebrospinal fluid (CSF)-related signal sources showed nonaliased VLF in four subjects. In conclusion, the VLF BOLD signal fluctuation in the occipital cortex is a true physiological fluctuation, not a result of signal aliasing.     

典型甚低频电磁波对辐射带高能电子的散射损失效应

辐射带中高能电子与空间甚低频电磁波由于波粒共振相互作用发生投掷角散射, 进而沉降入稠密大气而损失. 为研究甚低频电磁波对辐射带中高能电子的散射作用机制, 本文基于准线性扩散理论, 利用库仑作用和波粒共振相互作用扩散系数的物理模型, 得到了两组典型甚低频电磁波与高能电子波粒共振相互作用的赤道投掷角弹跳周期平均扩散系数, 并分析了甚低频电磁波共振散射作用与大气库仑散射作用对不同磁壳及不同能量的辐射带电子扩散损失的影响规律. 以磁壳参数L=2.2, 能量E=0.5 MeV的辐射带电子作为算例, 采用有限差分方法数值求解扩散方程, 计算分析了电子单向通量和全向通量随时间的沉降损失演化规律. 研究结果表明: 当电子能量大于0.5 MeV, 磁壳参数大于1.6时, 甚低频电磁波的共振散射作用显著; 随着磁壳参数或电子能量的增大, 斜传播甚低频电磁波引起的高阶共振相互作用越来越大; 电子全向通量近似随时间呈指数函数形式衰减.     

电离层调制加热产生极低频/甚低频波定向辐射的理论分析

通过低电离层调制加热能够产生极低频/甚低频(ELF/VLF)波. 基于调制加热理论, 并引入相控阵天线思想, 建立了通过双波束幅度调制模式(DAM)和圆形几何调制模式(CGM)产生ELF/VLF波的定向辐射模型, 并通过与实验数据的对比验证了模型的正确性. 据此模型, 采用HARRP加热阵参数, 对比分析了上述两种定向辐射模式与常规幅度调制模式(AM)之间的特性差异, 并研究了调制频率(f_ELF/VLF)和加热波束与垂直方向倾角(ψ)对各模式的影响. 结果表明: 相对AM模式, 通过合理设置初始相位、f_ELF/VLF和ψ, DAM 模式和CGM模式在实现ELF/VLF信号定向辐射的同时还可以提高其辐射强度, 相对AM模式, CGM模式信号强度最大提高约11.3 dB. 关键词： 定向辐射 双波束幅度调制 圆形几何调制     

Relationship between variations in thunderstorm very-low-frequency radio noise and variations in the solar wind density (1979–1994)

The relationship between long-period variations in the intensity of very-low-frequency (VLF) radio noise of thunderstorm origin with variations in the solar wind density has been investigated. The data obtained by continuous analog measurement of the intensity of 8.7-kHz VLF radio noise performed at the Shafer Institute of Cosmophysical Research and Aeronomy, Siberian Division, Russian Academy of Sciences, for the period 1979–1994 were used. The intensity of VLF radio noise characterizes the thunderstorm activity in the East of Siberia and the African global thunderstorm center. A significant relationship between the intensity of VLF radio noise of thunderstorm origin and the solar wind density is established. The result obtained can be explained by the action of solar and cosmophysical factors on the global atmospheric electric circuit.     

Determine the physical mechanism and source region of beat wave modulation by changing the frequency of high-frequency waves

This paper introduces a new approach for the determination of the source region of beat wave(BW)modulation.This type of modulation is achieved by transmitting high-frequency(HF)continuous waves with a frequency difference f,where f is the frequency of modulated ELF/VLF(extremely low frequency/very low frequency)waves from two sub-arrays of a high power HF transmitter.Despite the advantages of BW modulation in terms of generating more stable ELF/VLF signal and high modulation efficiency,there exists a controversy on the physical mechanism of BW and its source region.In this paper,the two controversial theories,i.e.,BW based on D-E region thermal nonlinearity and BW based on F region ponderomotive nonlinearity are examined for cases where each of these two theories exists exclusively or both of them exist simultaneously.According to the analysis and simulation results presented in this paper,it is found that the generated VLF signal amplitude exhibits significant variation as a function of HF frequency in different source regions.Therefore,this characteristic can be utilized as a potential new approach to determine the physical mechanism and source location of BW.     

Electromagnetic and seismoacoustic signals revealed in karst caves (Central Italy)

Summary Since 1988–89 equipment for detecting electric, magnetic and seismoacoustic signals has been running inside the Amare cave. The Amare cave is placed on the southern slope of the Gran Sasso chain, that is one of the largest karst areas of the Italian Apennines. In 1992, a similar equipment was installed inside the Cervo cave. This cave is located in another karst area of the Central Apennines, at about 50 km southwestwards of the Amare cave. In both these measurements sites, the signals are recorded every ten minutes in a digital form; the equipment is able to record signals, the frequency of which ranges from some hundred Hz to some hundred kHz. The data collected up to now seem to identify two different states that we call ?quiet? and ?perturbed? state. In the quiet state only electric and magnetic signals with the highest frequencies appear. These signals are connected with radio broadcastings and with the general lightnings activity of the Earth. A perturbed state is characterized by the sudden appearance of seismoacoustic signals coupled with electric and magnetic ones. This phenomenology is connected with local processes. Rainfall, atmospheric-pressure variations and some thermal effects are responsible for these local processes. A possible model is proposed to justify the observed phenomenology: micromovements of the limestone blocks that constitute the roof of the caves are invoked for the production of seismoacoustic signals. The electrification generated by these movements is invoked for the production of electric and magnetic signals.     

基于多径分集的啁啾扩频正交频分复用水声通信系统

针对传统正交频分复用系统在具有复杂多途和深度频率选择性衰落特点的水声信道中性能下降的问题,提出了啁啾扩频正交频分复用水声通信方案.该方案对原信息码扩频,子载波变为相同调频斜率、不同中心频率、频带相互重叠的正交啁啾信号.经过水声相干多途信道后在接收端解扩,使多个途径信号在频域上拓展,多径信号彼此分离.结合虚拟时间反转镜技术,聚焦多途信道能量,完成信道多径分集接收,不仅可以抑制频率选择性衰落的影响,还充分利用信道多径分集增益提高系统性能.通过仿真研究和湖试验证,表明该方案具有较好的有效性和可靠性.     

Recent developments in underwater imaging using the ultrasonic image converter tube

The Sokolov ultrasonic image converter provides a simple real time imaging system capable of good resolution and range in turbid water conditions. The tube is operated with anode potential target stabilisation and capacitive signal collection. Spurious signals at the ultrasound frequency are generated because of the stray capacitance of the scanned quartz surface. These signals are separated by modulating the electron beam at a different frequency and employing a head amplifier tuned to the intermediate frequency. Displayed images are free from fringes due to spurious signals and are unaffected by Doppler frequency shift. Factors which affect the resolution of an ultrasonic visualisation system are discussed.     

Midazolam sedation increases fluctuation and synchrony of the resting brain BOLD signal

The blood oxygen level-dependent (BOLD) magnetic resonance signal of functional brain cortices is dominated by very low frequency (VLF) fluctuations in anesthetized child patients. The temporal synchrony of the BOLD signal is also higher in anesthetized children compared with awake adults. The origin of the synchronous fluctuations can be related to maturation, pathological status or the anesthesia used in the imaging. Two of the three confounding variables (maturation and pathology) were controlled in this study. The effect of midazolam (4+/-0.8 mg) sedation on the BOLD signal was assessed in 12 healthy adults (aged 24+/-1.5 years) at 1.5 T. The VLF fluctuation power and temporal synchrony of the BOLD signal increased significantly after the sedation in the auditory and visual cortices. The fast Fourier transformation power spectral baseline fit parameters of the BOLD signal were also found to change significantly after sedation. It is concluded that the VLF fluctuation and temporal synchrony of the BOLD signal become increased after sedation in functional brain regions.     

基于试验粒子模拟的电离层人工调制激发的极低频和甚低频波对磁层高能电子的散射效应

电离层调制加热能够有效激发极低频和甚低频(ELF/VLF)波,其中向上传播进入磁层的ELF/VLF波能够与高能电子发生共振相互作用,具有人工沉降高能电子、消除辐射带等潜在实际用途.本文综合运用射线追踪和试验粒子方法模拟电离层人工激发的单频ELF/VLF波在电离层和磁层的传播,以及在外辐射带层与高能电子的共振相互作用过程,通过投掷角和能量散射系数评估人工ELF/VLF波对磁层高能电子的共振散射效应.研究表明,电离层人工ELF/VLF波传播到磁层后呈现高倾斜性,传播所能跨域的空间范围主要取决于加热的纬度位置和调制频率.在内辐射带,与～100 keV到几个MeV高能电子发生一阶共振相互作用的为10 kHz的VLF波段;在外辐射带,为几百Hz到1 kHz的ELF波段.对于L=4.5的外辐射带,试验粒子模拟结果显示,单个粒子在人工ELF波作用下投掷角和能量(α,E)的改变具有随机性,而所有试验粒子平均化的?α2和?E2随时间呈现出近似线性的增大,说明波粒共振散射过程体现出整体性.基于试验粒子模拟得到的共振散射系数表明,幅度为10 pT的人工ELF波可在外辐射带的磁赤道局地对1 MeV电子产生较强的投掷角散射效应,进而影响高能电子的损失、沉降等动力学过程.当人工ELF/VLF波在传播过程中变得高度倾斜,不仅最基本的一阶共振十分重要,高阶共振散射也具有较大效应.这些定量分析结果表明,通过电离层加热激发人工ELF/VLF哨声波来沉降、消除辐射带高能电子具有可行性.     

Study of long-wave radio signal disturbances due to lightning-induced energetic electron precipitation

Observing long-wave radio signals at short distances simultaneously with the broadband reception of VLF radiations, we obtained an abrupt (by more than 10%) amplitude variation of signals from the Moscow long-wave transmitter which can be identified as the reflection of long-wave signals from lightning-induced energetic electron precipitation regions in the ionosphere. Some amplitude variation events were accompanied by single-hop whistlers which arrived from the southern hemisphere and multihop whistlers originating from the northern hemisphere. Physical and Engineering Institute, Moscow, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 40, No. 4, pp. 472–478, April, 1997.     

Dynamics of Single-Pulse Echo Signals under the Conditions of Zeeman Switching

The signals of a single-pulse echo (SE) generated on nonresonant excitation of an inhomogeneously broadened two-level system by a radio-frequency pulse (RFP), against the background of which a magnetic video pulse (MVP) of certain length has been switched on, have been described analytically. It is established that under the conditions of such excitation several SE signals are formed whose generation moments are determined by the duration and amplitudes of the RFP and MVP. The amplitudes of some of these signals depend on the Rabi frequency according to a quadratic law, and the amplitudes of others depend on this frequency according to a biquadratic law. It has been established that the mechanism underlying the appearance of these signals is associated with zero beats arising as a result of superposition of the magnetization fluctuations at variable frequencies and at a frequency of detuning from resonance. It is shown that in the limiting case, where the RFP duration considerably exceeds the duration of the MVP or is comparable with it, the number of SE signals decreases to two. Theoretical results on the formation of SE and the dependence of the amplitude of these signals on the MVP amplitude agree with the experimental data on NMR pulsed signals in ferromagnetic materials.