Spatial field structure and polarization of geomagnetic pulsations in conjugate areas

The ultra-low-frequency (ULF) geomagnetic pulsations observed at two nearly conjugate mid-latitude sites are examined to study their spatial structure and polarization, and learn about the role of ionospheric conductivity in forming their ground signatures. The data of 1999–2002 from Antarctica and New England (L of 2.4) are compared with the numerical results obtained in a simple plane model of ULF wave propagation through the ionosphere and atmosphere. The multi-layered model environment includes an anisotropic and parametrically time-dependent ionosphere, a uniform magnetosphere and a conducting Earth, all placed in a tilted geomagnetic field. The measured diurnal and seasonal variations in the orientation angle of the polarization ellipse are interpreted as effects of hydromagnetic wave propagation through the ionosphere and conversion to an electromagnetic field below. Essentially, the phase, amplitude and polarization of ULF waves observed at the ground are controlled by the wave's spatial structure in the magnetosphere and ionospheric transverse conductivities. The differences shown by the characteristics of simultaneous pulsations in conjugate areas arise mainly from different local ionospheric conditions, while the source waves of the pulsations are common to both sites.     

Earthquake-related Electric Field Changes Observed in the Ionosphere and Ground

The changes of the ionospheric electric field before and after four huge earthquakes, which include the Ms 8.7 earthquake of 2004 and the Ms 8.5 earthquake of 2005 in Sumatra of Indonesia, the Ms 8.0 Wenchuan earthquake of 2008 in China, the Ms 8.8 earthquake of 2010 in Chile, and their strong aftershocks are studied in this paper. The significant results revealed that the power spectral density of low-frequency electric field below 20 Hz in the ionosphere, a kind of electromagnetic radiation phenomena, increased abnormally before and after the earthquakes and partially corresponded to the increased power spectral density of the low-frequency geoelectric field in time. This research preliminarily indicates that the low-frequency electromagnetic radiation during the imminent stages before such earthquakes could be detected by the observation of the ionospheric electric field. However, the spatial, temporal, and intensive complexities of the electric field anomalies in the ionosphere before earthquakes have come in sight also.     

Excitation of Rossby waves by HF electromagnetic seismic origin emissions in the earth's mesosphere

Interaction of high-frequency seismo-electromagnetic emissions with the weakly ionized gas of the ionospheric D-layer is considered. It is shown that through the earth's ionosphere weakly damped high-frequency electron cyclotron electromagnetic waves can propagate. These new type of waves easily reach the ionospheric D-layer where they interact with the existing electrons and ions. Acting on electrons ponderomotive force is taken into account and corresponding modified Charney equation is obtained. It is shown that only nonlinear vortical structures with negative vorticity (anticyclone) can be excited. The amplitude modulation of electromagnetic waves can lead to the excitation of Rossby waves in the weakly ionized gas. The corresponding growth rate is defined. Depending on the intensity of the pumping waves generated by seismic activity different stable and unstable branches of oscillations are found. Detection of the new oscillation branches and energetically reinforcing Rossby solitary vortical anticyclone structures may be serve as precursors to earthquake.     

Diffusion spreading of middle-latitude ionospheric plasma irregularities

In contrast to the way that the spreading of irregularities in a plasma is usually considered, the diffusion spreading of irregularities stretched along the geomagnetic field B is examined using a three-dimensional rigorous numerical model of quasi-neutral diffusion in the presence of a magnetic field, in conjunction with the actual height variations of the diffusion and conductivity tensors in the ionosphere. A comparison with the earlier constructed approximate model of unipolar diffusion was made. As in the previous case, the same peculiarities of irregularity spreading in the inhomogeneous background ionospheric plasma were observed. The accuracy of the approximate model for describing the process of spreading of anisotropic ionospheric irregularities is established. Time relaxation effects of real heating-induced ionospheric irregularities on their scale transverse to B are presented using the approximate analytical model for the case of a quasi-homogeneous ionospheric plasma. The calculated results have a vivid physical meaning and can be directly compared with experimental data on the radiophysical observations of artificial heating-induced irregularities created by powerful radio waves in the ionosphere.     

Plasma wave radiation in the main ionospheric trough in the region of the terminator from the APEX satellite data

The data of measurements of broadband wave radiation in the main ionospheric trough in the subauroral zone of the topside ionosphere in the region of the day-night terminator (APEX satellite experiment) are presented. It is shown that the observed attenuation of electrostatic radiation in a broad frequency band and fluctuations (variations) in the cutoff frequency of the electrostatic mode spectrum at the level of the local plasma or upper hybrid frequency are related to plasma heating by damping electrostatic oscillations in the ionospheric trough. Waveguide channels for propagation of electromagnetic whistler-mode waves observed on the satellite can be generated during the propagation of a gravity-thermal disturbance from the terminator.     

Electromagnetic and plasma effects during a carrier rocket flight

The disturbance generation model for the total electron content of the ionosphere and formation of the narrowband spectrum of electromagnetic disturbance on the Earth during a rocket flight along the horizontal leg of the trajectory has been considered. It has been indicated that a change in the total electron content is caused by the propagation of an acoustic gravity wave pulse, generated during a rocket flight along the horizontal trajectory leg, in the ionosphere. This pulse forms horizontal inhomogeneities of ionospheric conductivity in the bottomside ionosphere. Electric currents, induced by the background electromagnetic field in these inhomogeneities, are emitters of discrete modes of coherent gyrotropic waves propagating horizontally in a conductive layer of a finite thickness in the bottomside ionosphere. The line spectrum of electromagnetic disturbances has been calculated. The calculation results agree with the observational data.     

Dynamics of the large-scale ULF electromagnetic wave structures in the ionosphere

The present article displays the results of theoretical investigation of the planetary ultra-low-frequency (ULF) electromagnetic wave structure, generation and propagation dynamics in the dissipative ionosphere. These waves are stipulated by a spatial inhomogeneous geomagnetic field. The waves propagate in different ionospheric layers along the parallels to the east as well as to the west and their frequencies vary in the range of (10–10⁻⁶) s⁻¹ with a wavelength of order 10³ km. The fast disturbances are associated with oscillations of the ionospheric electrons frozen in the geomagnetic field. The large-scale waves are weakly damped. They generate the geomagnetic field adding up to several tens of nanotesla (nT) near the Earth's surface. It is prescribed that the planetary ULF electromagnetic waves preceding their nonlinear interaction with the local shear winds can self-localize in the form of nonlinear long-living solitary vortices, moving along the latitude circles westward as well as eastward with a velocity different from the phase velocity of the corresponding linear waves. The vortex structures transfer the trapped particles of medium, as well as energy and heat. That is why such nonlinear vortex structures can be the structural elements of the ionospheric strong macro-turbulences.     

Application of the method of numerical simulation of ionospheric filtration of Pc1 signals to solving the inverse problem of ionospheric modelling

Summary The numerical method of simulating ionospheric filtration of ULF signals in the range of Pc1 frequencies has been applied to French geomagnetically conjugate observations of ULF signals made by the GEOS-1 satellite and at the observatory of Husafell (Iceland) [2]. The experimentally obtained variable values of the transmissivity of the Pc1 signal through the ionosphere [5] in the course of the micropulsation distrubance of 13.7. 1977 have been compared with the results of the numerical simulation taking into consideration the fundamental physical parameters of the high-latitude external ionosphere. This approximate form of solving the inverse problem of ionospheric modelling yielded quantitative estimates of the rapid variations of the concentration of charged particles in connection with the expected changes of their temperature. It is assumed that nonstationary states of the ionospheric plasma are caused by the very ion-cyclotron waves penetrating the ionosphere at high latitudes ( 70°) along the plasmapause.     

Recent Advances and Difficulties of Infrasonic Wave Investigation in the Ionosphere

Acoustic waves have a remarkable ability to transfer energy from the ground up to the uppermost layers of the atmosphere. On the ground, there are many permanent sources of infrasound, and also pulsed and/or sporadic sources (e.g., sea waves, infrasonic and sonic noise of cities, lightning, earthquakes, explosions, etc.). The infrasonic waves carry away the major part of their energy upwards through the atmosphere. What are the consequences of the upward energy transfer? What heights of the atmosphere are supplied by energy from various sources of an infrasonic wave? In most cases, the answers to these questions are not well known at present. The only opportunity to monitor the propagation of an infrasonic wave to high altitudes is to watch for its influence on the ionospheric plasma. Unfortunately, most of standard equipment for ionospheric sounding, as a rule, cannot detect plasma fluctuations in the infrasonic range. Besides, the form of an infrasonic wave strongly varies during propagation due to nonlinear effects. However, the development of the Doppler method of radiosounding of the ionosphere has enabled progress to be made. Simultaneously, the ionospheric method for sensing aboveground and underground explosions has been developed. Its main advantage is the remote observation of an explosion in the near field zone by means of short radio waves, i.e., the radio sounding of the ionosphere directly above the explosion. The theory of propagation of an acoustic pulse produced by an explosion on the ground up to ionospheric heights has been developed better than the theory for other sources, and has been quantitatively confirmed by experiments. A review of some advances in the area of infrasound investigations at ionospheric heights is given and some current problems are presented.     

Penetration to the Earth’s surface of standing Alfvén waves excited by external currents in the ionosphere

The problem of boundary conditions for monochromatic Alfvén waves, excited in the magnetosphere by external currents in the ionospheric E-layer, is solved analytically. Waves with large azimuthal wave numbers m≫1 are considered. In our calculations, we used a model for the horizontally homogeneous ionosphere with an arbitrary inclination of geomagnetic field lines and a realistic height disribution of Alfvén velocity and conductivity tensor components. A relationship between such Alfvén waves on the upper ionospheric boundary with electromagnetic oscillations on the ground was detected, and the spatial structure of these oscillations determined.     

地震前兆研究的某些新进展

本文简要介绍了当前国际上在地震前兆研究方面取得的某些新进展。所介绍的前兆类别包括测震学前兆中的地震活动时空强分布特征、地震活动波与形变波、地震波特征量、高频地脉动、慢地震与“前兆波”,以及非测震学前兆中的应力与应变、地磁场与宇宙辐射、臭氧、重力脉冲、天体运动与电离层等。     

Transionospheric radiosounding (Review)

The transionospheric radiosounding (TIS) method has naturally brought together the two most precise methods for scanning and monitoring the ionosphere (ionospheric radiosounding by airborne and ground ionosondes) into a single system. The subsequent development of TIS equipment has led to a qualitative change in the structure and operation of the ionospheric observatory, which greatly broadened the diagnostic capabilities of the ionospheric monitoring and expanded the ionospheric region monitored by the ground station. In fact, it can be said that a closed radiosounding system has been developed. It uses three branches of ionospheric multifrequency ray (or radio wave) fans to monitor the inner and outer ionosphere and to control both of these regions via transillumination of the ionosphere at the boundary of its radio transparency. The advantage of such a system is the full use of the entire range of radiated radio waves, each part of which is responsible for certain components of the diagnostic circuit. The paper presents the results of scientific studies obtained based on TIS data, which have led to the appearance of new and, to some extent, unexpected and previously unknown phenomena and effects. Special attention is paid to the modern stage of development of the TIS concept, which has good prospects for continuous monitoring of the polar ionosphere. It is questioned whether it is expedient to replace the term sounding with the term transillumination. It is noted that TIS was and remains the most precise method of ionosphere diagnostics.     

几次大震前的地面和空间电磁场变化

本文应用非线性谱估计方法(MEM方法)、电/磁场脉冲能量统计方法,处理了四川汶川MS8.0 (2008)、新疆于田MS7.3 (2008)和青海玉树MS7.1 (2010) 地震及强余震附近的地面电/磁场和Demeter卫星电离层磁场观测数据,研究了地震电/磁场变化,得到:在震中附近及周围出现了震前地电/地磁场极低频成...     

Effects on the Ionosphere Due to Phenomena Occurring Below it

The terrestrial thermosphere and ionosphere form the most variable part of theEarth's atmosphere. Because our society depends on technological systems thatcan be affected by thermospheric and ionospheric phenomena, understanding,monitoring and ultimately forecasting the changes of the thermosphere–ionosphere system are of crucial importance to communications, navigation and the exploration of near-Earth space. The reason for the extreme variability of the thermosphere–ionosphere system isits rapid response to external forcing from various sources, i.e., thesolar ionizing flux, energetic charged particles and electric fields imposed via the interaction between the solar wind, magnetosphere and ionosphere, as well as coupling from below (meteorological influences) by the upward propagating, broad spectrum,internal atmospheric waves (planetary waves, tides, gravity waves) generated in thestratosphere and troposphere. Thunderstorms, typhoons, hurricanes, tornadoes andeven seismological events may also have observable consequences in the ionosphere.The release of trace gases due to human activity have the potential to cause changes inthe lower and the upper atmosphere.A brief overview is presented concerning the discoveries and experimentalresults that have confirmed that the ionosphere is subject to meteorologicalcontrol (especially for geomagnetic quiet conditions and for middle latitudes).D-region aeronomy, the winter anomaly of radiowave absorption, wave-liketravelling ionospheric disturbances, the non-zonality and regional peculiaritiesof lower thermospheric winds, sporadic-E occurrence and structure, spread-Fevents, the variability of ionospheric electron density profiles and Total ElectronContent, the variability of foF2, etc., should all be considered in connection withtropospheric and stratospheric processes. Ionospheric weather, as a part of spaceweather, (i.e., hour-to-hour and day-to-day variability of the ionospheric parameters)awaits explanation and prediction within the framework of the climatological, seasonal,and solar-cycle variations.     

Influence of the ionosphere, modified by powerful oblique radiowaves, on HF radio wave propagation

Variations of HF probe radio wave parameters caused by the influence on the ionosphere of oblique powerful HF radiowaves are analysed. The analysis is made on the data obtained from the original experiments carried out on single-hop paths in the middle and subauroral latitudes. Powerful and probe waves with some difference in frequency were chosen near MUF and were transmitted in the same direction. The polar diagrams of both transmitting systems overlapped in both the horizontal and vertical planes. The results obtained indicate that the ionospheric plasma parameters can be varied by powerful oblique HF radiowaves under certain geophysical conditions.     

Alfven波在低纬电离层中的传播研究

Alfven波在低纬地区电离层的传播有其特殊性,一方面,低纬地区同样存在Alfven速度梯度的巨大变化,导致电离层Alfven谐振器(Ionospheric Alfven resonator, IAR)的形成;另一方面,由于在低纬地区磁倾角很小,所以剪切Alfven波在传播的过程中纬度方向跨度很大,不同纬度电离层参数将共同对其产生影响;并且,由于电离层水平分层,故磁力线与电离层不正交.本文选取双流体力学模型,在忽略场向电场的条件下,利用非正交坐标系,结合IRI07模型与MSISE00模型模拟低纬地区Alfven波的传播,得到其反射及耦合特性.结果表明,低纬地区同样存在电离层Alfven谐振现象,由耦合产生的压缩模有向磁赤道方向传播的趋势,夜间电离层状态相对于白天更适合IAR的形成,谐振频率沿磁力线L值增大单调递增.     

Electromagnetic waves in the auroral and subauroral regions of the topside ionosphere

The density and temperature of the plasma electron component and wave emission intensity in the topside ionosphere were measured by the INTERCOSMOS-19 satellite. In the subauroral ionosphere, a decrease in the plasma density correlates with an increase in the plasma electron component temperature. In this case, the additional increase in the electron component temperature was measured in regions with increased plasma density gradients during the substorm recovery phase. In a linear approximation, the electromagnetic wave growth increments are small on electron fluxes precipitating in the auroral zone. It has been indicated that Bernstein electromagnetic waves propagating in the subauroral topside ionosphere can intensify in regions with increased plasma density gradients on electron fluxes orthogonal to the geomagnetic field, which are formed when plasma is heated by decaying electrostatic oscillations of the plasma electron component. This can be one of the most important factors responsible for the intensification of auroral kilometric radiation.     

Amplification and energy transformation of the magnetized Rossby waves in the ionosphere with an inhomogeneous zonal wind. II. Analysis of numerical solution

The specific features of the amplification and mutual transformation of wave modes in a linear regime have been revealed based on an analysis of the numerical solution to the set of equations describing the interaction of the magnetized Rossby wave and the inertial wave with the spatially inhomogeneous zonal wind (shear flow) in the ionospheric D, E, and F regions. It has been established that the presence of the geomagnetic field and Hall and Pedersen currents in the ionosphere improves the interaction and energy exchange between the waves and shear flow.     

NWC通信台在电离层中激发电磁响应的时变特征

本文利用DEMETER卫星VLF频段电场和磁场频谱数据对DEMETER卫星运行期间2005年至2009年澳大利亚甚低频(Very Low Frequency)通信台NWC发射的通信信号造成的电离层电磁响应的日变化、季节变化及年变化特征进行了统计分析,统计结果表明电磁响应日变化显著,夜间电场强度明显增强可达40dB,磁场变化略小也可为15dB左右,而季节变化不显著,年变化主要受太阳活动的影响,太阳活动越强,电磁响应越小.为解释数据分析结果,对地-电离层电磁波传播过程采用传递矩阵方法进行了模拟计算,模拟结果与数据分析的结果一致.我们认为这种随时间变化的特点可能由250km以下电离层电子密度分布特征导致,因此研究250km以下的电离层电子密度变化可能对寻找地震电离层电磁异常有重要意义.