木星Io通量管中等离子体湍流及低频无线电波的激发和功率

本文对木星极区（1000-6000公里）Io通量管中的等离子体湍流和低频无线电波的激发机制进行了理论分析,并对它们的能量密度作了数值估计。指出:在Io通量管中可能存在着两种主要的不稳定性,一是电流驱导不稳定,这种不稳定可以激发离子声湍流,离子声湍流通过离子的非线性散射可以转化为哨声湍流。这两种湍流的能量密度大致相等,约为10^-17-10^-16焦耳/厘米³。另外一种不稳定性是由于高能粒子沉降引起的束流不稳定,可以激发朗缪尔湍流和迴旋频率等离子体湍流,能量密度分别为10^-17焦耳/厘米³和10^-14焦耳/厘米³的量级。这两种湍流可转化为低频无线电波,最大的能通量可达10^-22-10^-20瓦/厘米²·赫兹。     

MHD evolutionary processes in the Earth’s model magnetic resonator

Specific features of the spatial-temporal dynamics of LF disturbances of the Earth’s magnetosphere have been analyzed by the method of numerical simulation of magnetic hydrodynamic equations taking into account plasma temperature variations. The effects related to the appearance of long-living magnetospheric jumps of density and temperature, MHD wave scattering on such inhomogeneities, Alfvén wave reflection from the near-Earth region, and magnetospheric plasma heating as a result of dissipative processes at a repeated propagation of MHD waves between magnetically conjugate regions have been considered. The problems of conformity of the discrete mathematical model with continuous equations of magnetic hydrodynamics are discussed.     

Electromagnetic waves and electrostatic oscillations in an inhomogeneous plasma structure at the geomagnetic equator

The measurements of the broadband wave radiation in the region of the geomagnetic equator in the topside ionosphere are presented (the APEX satellite experiment). It has been indicated that the electromagnetic radiation, observed in a plasma density pit, could be related to the formation of a large-scale plasma cavern in the vicinity of the geomagnetic equatorial surface. A large-scale plasma density pit could be formed in the region of heating during damping of plasma vortical structures and electrostatic oscillations, propagating across geomagnetic field lines and crossing the geomagnetic equatorial surface. Brightness of the electromagnetic radiation, observed at frequencies higher than the plasma eigenfrequencies and local plasma and/or upper hybrid frequencies, decreases with increasing eigenfrequencies.     

Computation of the key parameters of radio signals propagating through a perturbed ionosphere in the land-satellite channel

An analysis of the key parameters of HF/UHF radio signals was carried out for land-satellite radio channels, which determine the effects of fading in a perturbed ionosphere. Using the parameters of the perturbed plasma, the effects of the absorption and phase fluctuations of radio signals are analyzed for a channel with fading. For the evaluation of the effect of scattering of a radio signal by ionospheric inhomogeneities in an approximation of small-scale scintillations, expressions for the root-mean-square (RMS) magnitude of signal intensity and phase scintillations are presented. Scintillation index σ _I ² that corresponds to variations in a signal under the conditions of multipath propagation with fading is investigated by using experimental data. It is shown that roughly ～10% of inhomogeneities of the electron concentration in the F region of the ionosphere, perturbed during a magnetic storm, yield strong quickly fading radio signals in the VHF/UHF range with significant fluctuations (up to 1%) in the intensity of the signal and phase fluctuations (up to hundreds of radians). The calculated magnitudes of the scintillation index are in good agreement with experimentally observed data.     

Parametric effects in inhomogeneous plasma during injection of electron pulses into the ionosphere

The injection of electron pulses from the rocket into the ionospheric plasma in the lower hemisphere relative to the initial pitch angles of electrons during the substorm recovery phase provoked the generation of parametric processes (the ARAKS experiment). The electron flux observations, obtained using a wide-angle detector, and the whistler wave emission intensity measurements were compared. A wide-angle detector of electrons was mounted on the rocket, and a broadband wave receiver was installed on a nasal cone separated from the rocket. Bursts of the electron flux and wave emission were observed in pauses between electron pulses. It has been indicated that a clearly defined anisotropy of the observed parametric effects of the pitch angle of injected electron pulses is related to resonance characteristics of a wave emitted by electron fluxes in a magnetized plasma. Precipitation of ring current electrons was caused by a change in the magnetic moment of electrons, trapped by the magnetosphere, in the region of magnetic mirror points in the fields of electrostatic oscillations excited during decay of whistlers.     

Studying the fine structure of coherent echo spectra using data from Irkutsk incoherent scatter radar

Studying the processes generating different-scale inhomogeneities is one of the challenging problems of ionospheric physics. Plasma instabilities are one of the physical mechanisms by which small-scale inhomogeneities are formed. The main forms of instability in the ionospheric E-layer are two-stream and gradient-drift ones. The inhomogeneities generated by them lead to an abnormally intense radio scattering of different wavelengths (known as coherent echo (CE) or radio aurora) in the E-layer. Therefore, the method of radiowave backscattering is among the widely used methods for studying such inhomogeneities. The CE phenomenon has been investigated most intensely at high and equatorial latitudes, where the conditions for the CE origination are formed rather regularly. For the last decade, CE has also been intensely studied at midlatitudes, where it is observed less frequently and its formation conditions are less known. In 1998–2006, the purposeful studies of the midlatitude CE peculiarities were performed at the Irkutsk incoherent scatter (IS) radar, with a particular emphasis on its coherent properties. It was for the first time found out that the spectra of some data sets had a fine comb-shaped structure, which generated well-known single-humped CE spectra as a result of statistical averaging. In the scope of this study, unique coherent methods for processing individual data sets of CE signals were developed, making it possible to reveal the peculiarities of unaveraged CE-signal spectra. To describe these peculiarities, we proposed a new model of the inhomogeneity spectrum, which is the superposition of the discrete set of spatial harmonics with close wave numbers. The model was shown to adequately describe the scattered signal characteristics observed experimentally.     

Trends in planetary wave activity in the upper middle atmosphere inferred from the nighttime LF radio wave absorption measurements

Summary The nighttime LF radio wave absorption in the lower ionosphere measured at two frequencies in central Europe over 1963–1985 is used to infer planetary wave activity and its long-term trend in the upper middle atmosphere (∼90–100 km). The observed positive trend is roughly consistent with results based on daytime absorption. Nighttime results are less pronounced and less statistically significant probably due to perturbing effects of geomagnetic activity. The observed trends, which are probably of anthropogenic origin, are together with the daytime results [3,4] the first evidence of long-term trends in planetary wave activity in the upper middle atmosphere.     

Dynamics of accelerated electron beams and X rays in solar flares with sub-THz radiation

Unique measurements by a solar submillimeter radio telescope (SST) have been carried out in the sub-THz radiation at 212 and 405 THz over the past decade. The spectrum of RF radiation in this region increased with frequency for the three flares of November 2 and 4, 2003, and December 6, 2006, and the flux value reached 5 × 10³?2 × 10⁴ sfu at 405 GHz (Kaufman et al., 2009). In this work, we consider a set of nonlinear equations for an accelerated electrons beam and the Langmuir wave energy density. The distribution functions of the accelerated electron beam and wave energy density are calculated taking into account Coulomb collisions, electron scattering by waves, and wave scattering by plasma ions. In addition, the source of accelerated particles and the heat level of the Langmuir turbulence are specified. The beam and plasma parameters are chosen based on the aims of a problem. The plasma concentration varies from n = 10¹³ to 10¹⁵ cm^?3, the electron plasma frequency f _p = (3 × 10¹⁰?3 × 10¹¹) Hz in this case. The ratio of plasma and beam concentrations, sufficient to explain the value of the radio flux at a frequency of 300 GHz, is n _b/n = 10^?3. The Langmuir turbulence is excited due to the instability of the accelerated electron beam with an initial distribution function of the ??bump-in-tail?? type. Then, the parameters of radiowaves are calculated in the sub-THz range under the assumption of coalescence of two plasma waves. The calculation results show that a sub-THz radio flux can be obtained under the condition of injection of accelerated electrons. The fine time structure of radio flux observed is easily simulated based on this statement by the pulsed time structure of electron beams and their dynamics in overdense plasma. X-ray and gamma radiation was recorded during the events under study. Hard X-ray radiation is bremsstrahlung radiation from accelerated electron beams.     

Waveguide radio-wave propagation in the equatorial ionosphere according to the Interkosmos-19 data

Additional strongly remote (up to 2000 km) radio-signal reflection traces on Intercosmos-19 ionograms obtained in the equatorial ionosphere have been considered. These traces, as a rule, begin at frequencies slightly lower than the main trace cutoff frequencies, which indicates that an irregularity with a decreased plasma density exists here. The waveguide stretched along the magnetic-field line is such an inhomogeneity in the equatorial ionosphere. The ray tracing confirm that radio waves propagate in a waveguide and make it possible to determine the typical waveguide parameters: ?δN _e ≥ 10%, with a diameter of 15–20 km. Since the waveguide walls are smooth, an additional trace is always recorded distinctly even in the case in which main traces were completely eroded by strong diffusivity. Only one additional trace (of the radio signal X mode) is usually observed one more multiple trace is rarely recorded. Waveguides can be observed at all altitudes of the equatorial ionosphere at geomagnetic latitudes of ±40°. The formation of waveguides is usually related to the formation of different-scale irregularities in the nighttime equatorial ionosphere, which result in the appearance of other additional traces and spread F.     

Propagation of radio waves reflected from the ionosphere during a solar eclipse

Summary The article first describes in chronological order the observations on the propagation of radio waves during the solar eclipses, and the development of the conflicting results on the similar transmissions of radio signals. The general theory of the absorption of radio waves in the ionosphere is briefly described and therefrom a method is derived to estimate the variation of the absorption of obliquely incident radio waves during a solar eclipse.The variation of field-strength can be studied in terms of the relationship between the vertical incident equivalent frequency of the signals and the critical frequencies of the ionospheric layers at the regions of reflection. The total absorption of radio waves consists of the non-deviative absorption in theD region and the deviative absorption in the higher layers of the ionosphere. During the eclipse, theD region absorption decreases in phase with the progress of the eclipse, but the variation of deviative absorption may differ in each observation. The deviative absorption is large when the equivalent frequency is close to the critical frequency of the layer reflecting the waves or of the layer just penetrated by the waves. The changes in the deviative absorption during an eclipse can be estimated on the basis of the variation of the critical frequencies of the ionospheric layers. The resultant changes in the total absorption during a solar eclipse may thus be estimated. The different types of field strength variation expected during an eclipse are given.The observations of the vertical incident absorption of radio waves and the field strength variations of obliquely incident continuous wave radio signals during the solar eclipse are described and the changes are explained on the basis of the above theory.     

不同散射模式下S波包络合成

为揭示地震波在地壳小尺度非均匀介质中的散射过程,更准确地描述地震波的包络展宽现象,本文基于多次各向异性散射理论,采用离散波数法求解能量密度积分方程,选取高斯型自相关函数表征的散射模式,得到S波能量密度包络。基于此,本文首先分析了单次散射和多次散射在形成S波能量密度包络中的贡献规律;然后探讨了吸收系数和总散射系数对合成S波能量密度包络的影响;最后对比了在不同散射模式下合成的S波能量密度包络的差异。结果显示:① 不同的散射模式下单次散射和多次散射对地震波散射过程的贡献规律是一致的,对于近震（震源距小于100 km）,单次散射模型可以近似合成S波能量密度包络;随着震源距增大,多次前散射模型可以更快地接近总能量密度包络;② 吸收系数增大会降低直达S波和尾波幅值,总散射系数增大会降低直达S波幅值,但使得S波尾波幅值升高;③ 前散射模式下S波能量密度包络随震源距的增大会导致峰值延迟,包络展宽,尾波衰减一致性更快等现象产生。      

The fractal nature of the inhomogeneities in the lithosphere evidenced from seismic wave scattering

In this paper we show evidences of the fractal nature of the 3-D inhomogeneities in the lithosphere from the study of seismic wave scattering and discuss the relation between the fractal dimension of the 3-D inhomogeneities and that of the fault surfaces. Two methods are introduced to measure the inhomogeneity spectrum of a random medium: 1. the coda excitation spectrum method, and 2. the method of measuring the frequency dependence of scattering attenuation. The fractal dimension can be obtained from the inhomogeneity spectrum of the medium. The coda excitation method is applied to the Hindu-Kush data. Based on the observed coda excitation spectra (for frequencies 1–25 Hz) and the past observations on the frequency dependence of scattering attenuation, we infer that the lithospheric inhomogeneities are multiple scaled and can be modeled as a bandlimited fractal random medium (BLFRM) with an outer scale of about 1 km. The fractal dimension of the 3-D inhomogeneities isD ₃=31/2–32/3, which corresponds to a scaling exponent (Hurst number)H=1/2–1/3. The corresponding 1-D inhomogeneity spectra obey the power law with a powerp=2H+1=2–5/3. The intersection between the earth surface and the isostrength surface of the 3-D inhomogeneities will have fractal dimensionD ₁=1.5–1.67. If we consider the earthquake fault surface as developed from the isosurface of the 3-D inhomogeneities and smoothed by the rupture dynamics, the fractal dimension of the fault trace on the surface must be smaller thanD ₁, in agreement with recent measurements of fractal dimension along the San Andreas fault.     

Plasma inhomogeneities in the topside ionosphere in the region of the geomagnetic equator and wave radiation according to the APEX satellite data

The measurements of the broadband wave radiation in the topside ionosphere in the region of the geomagnetic equator (the APEX satellite experiment) are presented. The region of unstable plasma with increased density was observed in the nightside topside ionosphere. This region could be formed by heating of the ionosphere from below. An asymmetric distribution of the frequency band width and electrostatic radiation intensity relative to the geomagnetic equator was registered. It has been indicated that a substantial effect of the geomagnetic equator on plasma diffusion from the heating region could be related to the generation, propagation, and damping of electrostatic oscillations and large-scale (as compared to the Larmor ion radius) plasma vortices. The anisotropy in the temperature of the plasma electron component can increase in the regions where the transverse electric field of disturbances damps. The intensity of the electromagnetic radiation, caused by the external sources, apparently, of an artificial origin at frequencies higher than the local plasma frequency, decreases to the radiation detection threshold level in the region of increased plasma density.     

大功率高频电波加热电离层的非线性物理过程

地基大功率电波加热电离层是通过地基大功率短波发射机向电离层发射无线电波,通过波-粒和波-波的相互作用将无线电波的能量注入电离层.通过这种有目的可操控的方式改变电离层电子密度和温度的分布,可以深入研究电离层中等离子体能量和物质的非线性演化过程,特别是电离层电子的非平衡态分布和加速问题.本文通过对电离层加热中几个比较重要物理过程的评述,对过去20年来我国研究学者在这一研究方向上取得的重要进展进行了介绍.     

大功率高频电波加热电离层中人工沿场不均匀体散射特性的理论与数值模拟研究

大功率高频电波与电离层的相互作用会引起电子密度扰动,进而产生人工沿场不均匀体,其对无线电波特别是超短波信号有强的定向散射能力,可形成一种新型的超短波通信方式.基于各向异性介质的散射理论,首先通过求解电子密度扰动产生的附加极化势获得电子密度扰动散射方程,然后对电子密度扰动进行高斯自相关处理,并结合入射波/散射波与地磁场方向的空间几何关系,获得电子密度扰动的波数谱表达式,建立了人工沿场不均匀体的散射系数理论模型.利用模型对Platteville站实验中产生的人工不均匀体散射截面积进行了数值模拟,并通过与实测值对比验证了模型的正确性.根据人工沿场散射原理给出了利用其进行无线电信号传输的约束条件.重点仿真分析了人工沿场不均匀体的散射系数和散射覆盖区范围,结果表明：同等条件下,相对于高纬度地区,低纬度地区人工沿场不均匀体的散射系数小5~10 dB,但其散射覆盖区的地面范围大,东西向可达3000 km,南北向可达1500 km,完全可用于超短波信号的超远距离传输.本文结果为中低纬度地区开展相关实验研究提供了理论指导,对利用人工沿场不均匀体进行无线信号的超远距离传输应用研究具有重要意义.     

Unstable plasma irregular structures in the topside ionosphere and spread-<Emphasis Type="Italic">F</Emphasis>

The results of the Cosmos-900 satellite observ ations of plasma density inhomogeneities in the geomagnetic equator region and the longitudinal distributions of the equatorial spread-F, according to the Intercosmos-19 satellite data are presented. It is show n that the dependence of radiosignal propagation in the ionosphere on geophysical parameters is related to development of the electrostatic instability of the inhomo-geneous ionospheric plasma. The longitudinal dependence of the spread-F, can reflect the influence of the energetic sources, located outside the ionospheric layer that scatters a radio pulse, on the ionosphere. The manifestation of the longitudinal effect in the equatorial spread-F, in the Atlantic region can be explained by the influence of the cone instability on the plasma electrodynamics in the South Atlantic geomagnetic anomaly.     

空间尘埃等离子体电磁散射特性研究

基于尘埃等离子体Mie-Debye散射模型,研究平衡态下尘埃等离子体中带电尘埃的电磁散射特性.根据输运理论计算高层大气环境中尘埃等离子体层的电磁散射场,给出电磁波垂直入射时尘埃等离子体层的反射函数,同时分析不同浓度、不同粒径分布状态下尘埃等离子体层对电磁波传播的影响.结果表明:尘埃粒子尺度对其电磁散射特性影响非常大;在对数正态分布情况下,当入射波长远大于Debye半径时,尘埃等离子体的散射主要表现为Debye散射,而入射波长远小于Debye半径时,Mie散射占主要部分;并且尘埃等离子体层对电磁波的衰减随尘埃粒子浓度以及尘埃粒子半径的增大而明显增强.     

An Overview of Long-Term Trends in the Lower Ionosphere Below 120 km

The increasing concentration of greenhouse gases in the atmosphere is expectedalso to modify the mesosphere and lower thermosphere (MLT region). However,the greenhouse cooling – instead of heating – at these heights is revealed by modelsand generally confirmed by observations. This should more or less affect variousionospheric parameters at these heights. The spatial and temporal structure oftemperature trends in the MLT region is quite complex and, therefore, such structureshould occur for trends in the lower ionosphere as well. In the lower part of theionosphere below about 90 km, the rocket measurements of electron density, theindirect phase reflection height measurements and the A3 radio wave absorptionmeasurements reveal trends corresponding to cooling and shrinking of the mesosphere,while riometric measurements of cosmic noise absorption provide inconclusive results.The radio wave absorption and rocket electron density measurements clearly display asubstantial dependence of trends on height. Ionosonde data show that there is amodel-expected trend in the maximum electron concentration of the E region ionosphere;foE is slightly increasing. On the other hand, the height of the normal E layer, h'E, isslightly decreasing. The nighttime LF radio wave reflection height measurements near95 km support an idea of increasing electron density. However, rather scarce rocketmeasurements display a negative trend in electron density at 90–120 km. The role ofthe solar cycle and other longer-term variability of natural origin in the determinationof observational trends must not be neglected. In spite of the general qualitativeagreement with model expectations, there is still some controversy between variousobservational trend results (hopefully, apparent rather than real), which needs to beclarified.     

Particle energy fluxes in an unstable plasma with vortex structures in an inhomogeneous geomagnetic field in the topside ionosphere

Plasma inhomogeneities extending along geomagnetic field lines in the ionosphere and magnetosphere can have a vortex structure. Electromagnetic waves can propagate in plasma inhomogeneities in the waveguide channel mode. It has been indicated that energy and particle fluxes related to the development of small-scale electrostatic turbulence in a magnetized plasma with an unstable electron component promotes an increase in plasma density gradients in the walls of waveguide channels and an enhancement in plasma vortices. At low L shells in the region of the geomagnetic equator, the development of plasma electrostatic instability and the damping of drifting plasma vortices in the inhomogeneous geomagnetic field in the topside ionosphere can be the main mechanism by which large-scale (∼1000 km) regions with a decreased plasma density are formed.     

Phase fluctuations of radio waves experiencing total reflection from a randomly inhomogeneous plasma layer

We examine the problem of small-angle scattering of radio waves experiencing total reflection from a randomly inhomogeneous layer of plasma. We consider the waves to be normally incident on the layer. To take into account the scattering peculiarities in the neighborhood of the reflection point, we introduce an analytical transformation for the eikonal equation solution derived by the perturbation method. This transformation permits calculations of radio-wave phase fluctuations for any monotonous profile of the regular dielectric permittivity of the plasma in the layer. Using this approach, we have derived analytical formulas for the variance and two-dimensional spatial spectrum of phase fluctuations, depending on the three-dimensional power spectrum of plasma fluctuations. We have also estimated a contribution of reflection point fluctuations to the phase fluctuations and determined the limits of applicability of the derived formulas. The presented analytical transformation of the eikonal equation solution can be used to calculate other statistical moments of the radio wave phase in many problems of solar–terrestrial physics where scattering and reflection of radio waves by plasma formations are important.