Large-scale structure of magnetospheric plasma

Recent investigations of magnetospheric plasma structure are summarized under the broad categories of empirical models, transport across boundaries, formation, and dynamics of the plasma sheet. This report reviews work in these areas during the period 1991 to 1993. Fully three-dimensional empirical models and simulations have become important contributors to our understanding of the magnetospheric system. Some new structural concepts have appeared in the literature: the entry boundary and geopause, the plasma sheet region 1 vortices, the low-energy layer, the adiabaticity boundary or wall region, and a region in the tail to which we refer as the injection port. Traditional structural concepts have also been the subject of recent study, notably the plasmapause, the magnetopause, and the plasma sheet. Significant progress has been made in understanding the nature of plasma sheet formation and dynamics, but the acceleration of electrons to high energy remains somewhat mysterious.     

Short-period VLF emissions as solitary envelope waves in a magnetospheric plasma maser

Space and ground-based experiments have shown evidence of natural short-period VLF emissions in which separate spectral elements are repeated with a periodicity of 2–7 s. Their basic morphological properties are found on the basis of original experimental data. In our opinion, excitation of such emissions is the result of quasi-linear relaxation effects that compensate for natural spectral dispersion. The quasi-linear relaxation of the energetic electron distribution function incrementally changes wave cyclotron instability and hence the VLF emission spectral forms. Some properties of the quasi-linear interaction of whistler waves with magnetospheric radiation belt electrons are studied. It is shown that quasi-linear relaxation can increase the cyclotron instability at the leading edge of an electromagnetic pulse. This effective saturation of absorption facilitates the division of VLF hiss-like emission into separate electromagnetic pulses without spectral modification from one pulse to the next. Some features and manifestations of this effective saturation of absorption are discussed. The results are important for a better understanding of temporal and spatial structures of VLF whistler-mode emissions and energetic electron fluxes.     

Generation of auroral kilometric radiation in inhomogeneous magnetospheric plasma

The generation of auroral kilometric radiation in a narrow 3D plasma cavity, in which a weakly relativistic electron flow is propagated along the magnetic field against a low-density cold background plasma, is studied. The time dynamics of the propagation and intensification of waves are analyzed using geometric optics equations. The waves have different wave vector components and start from the cavity center at an altitude of about the Earth’s radius at plasma parameters typical for the auroral zone at this altitude. It is shown that the global inhomogeneity of the Earth’s magnetic field is of key importance in shaping the auroral kilometric radiation spectra.     

The adequacy of the ionospheric source in supplying magnetospheric plasma

More than 30 years after the prediction of the polar wind outflow from the high latitude ionosphere, the exact magnitude and ultimate fate of the ionospheric plasma supply remains unknown. Estimates made more than a decade ago suggested that the polar ion outflow might well be of sufficient strength to populate the different regions of the Earth’s magnetosphere. Direct measurements in the high altitude magnetosphere became possible only with the launch of the Polar spacecraft. The combination of the Thermal Ion Dynamics Experiment and the Plasma Source Instrument has revealed the presence of low energy (<10 eV) ions moving through the polar regions and into the lobes of the magnetotail. These ions would have been invisible to previous un-neutralized satellites because of the high positive spacecraft potentials. Through the use of a recently developed single particle trajectory and energization code, the movement and energy transformation of these measured particles can be estimated. They are found to move into the plasma sheet region and to be energized to typical plasma sheet energies. The magnitude of the flux of the highly variable out-flowing ions mapped to 1000 km altitude is 1 − 3 × 10⁸ ions/cm² s in agreement with the original estimates. Future observations by the TIDE/PSI instruments will be required to determine the extent of the total ionospheric contribution.     

Some evidence of ground power enhancements at frequencies of global magnetospheric modes at low latitude

A statistical analysis of the power spectra of the geomagnetic field components H and D for periods ranging between 3 min and 1 h was conducted at a lowlatitude observatory (Aquila, L = 1.6) at the minimum and maximum of the solar cycle. For both components, during daytime intervals, we found evidence of power enhancements at frequencies predicted for global modes of the Earths magnetosphere and occasionally observed at auroral latitudes in the F-region drift velocities (approximately at 1.3, 1.9, 2.6, and 3.4 mHz). Nighttime observations reveal a relative low frequency H enhancement associated with the bay occurrence together with a peak in the H/D power ratio which sharply emerges at 1.2 mHz in the premidnight sector. The strong similarity between solar minimum and maximum suggests that these modes can be considered permanent magnetospheric features. A separate analysis on a two-month interval shows that the observed spectral characteristics are amplified by conditions of high-velocity solar wind.     

A magnetospheric specification model validation study: Geosynchronous electrons

The Rice University Magnetospheric Specification Model (MSM) is an operational space environment model of the inner and middle magnetosphere designed to specify charged particle fluxes up to 100 keV. Validation test data taken between January 1996 and June 1998 consist of electron fluxes measured by a charge control system (CCS) on a defense satellite communications system (DSCS) spacecraft. The CCS includes both electrostatic analyzers to measure the particle environment and surface potential monitors to track differential charging between various materials and vehicle ground. While typical RMS error analysis methods provide a sense of the models overall abilities, they do not specifically address physical situations critical to operations, i.e., how well does the model specify when a high differential charging state is probable. In this validation study, differential charging states observed by DSCS are used to determine several threshold fluxes for the associated 20–50 keV electrons and joint probability distributions are constructed to determine Hit, Miss, and False Alarm rates for the models. An MSM run covering the two and one-half year interval is performed using the minimum required input parameter set, consisting of only the magnetic activity index Kp, in order to statistically examine the model's seasonal and yearly performance. In addition, the relative merits of the input parameter, i.e., Kp, Dst, the equatorward boundary of diffuse aurora at midnight, cross-polar cap potential, solar wind density and velocity, and interplanetary magnetic field values, are evaluated as drivers of shorter model runs of 100 d each. In an effort to develop operational tools that can address spacecraft charging issues, we also identify temporal features in the model output that can be directly linked to input parameter variations and model boundary conditions. All model output is interpreted using the full three-dimensional, dipole tilt-dependent algorithms currently in operational use at the Air Force 55th Space Weather Squadron (55 SWXS). Results indicate that both diurnal and seasonal activity related variations in geosynchronous electrons are reproduced in a regular and consistent manner regardless of the input parameter used as drivers. The ability of the MSM to specify DSCS electrons in relation to thresholds indicative of spacecraft charging varies with the combination of input parameters used. The input parameter of greatest benefit to the MSM, after the required Kp index, is the polar cap potential drop as determined by DMSP spacecraft. Regarding the highest electron flux threshold, the model typically achieves high HIT rates paired with both high False Alarm rates and higher RMS error. Suggestions are made regarding the utilization of proxy values for the polar cap potential parameter and Kp-dependent model boundary conditions. The importance of generating accurate real-time proxy input data for operational use is stressed.     

基于深度学习的内磁层等离子体密度动态演化模型

等离子体密度作为空间环境的重要参量,它的全球实时分布信息不仅对理解内磁层带电粒子时空演化过程具有重要意义,对于预报和防范灾害性空间天气过程也有着潜在应用价值.利用范阿伦双星的高质量等离子体密度观测数据,本文基于机器学习算法训练得到一个稳定的深度神经网络模型: 包含五个隐藏层; 激活函数包括Sigmoid和ReLU函数; 以太阳风参数、地磁指数以及卫星对应的位置信息作为输入.在测试集上,该模型输出值和观测值之间的线性相关系数约为0.93,均方根误差(RMSE)约为0.3,表明该模型性能良好.通过使用该模型对2012年4月24日磁暴事件中等离子体密度的全球动态变化进行模拟,我们成功重构了磁暴期间内磁层等离子体密度的全球变化过程,包括等离子体层的侵蚀和恢复,以及羽流的形成和消失.该内磁层等离子体密度的深度神经网络模型将有助于推动内磁层波粒相互作用的深入研究.

     

The evolving concept of a magnetospheric substorm

A magnetospheric substorm is an episode of energy transport and dissipation in the Earth|s ionosphere and mag- netosphere which takes place in response to a time limited increase in energy input from the solar wind to the magnetosphere. For the past few decades, scientists have tried to understand the physical processes which take place that are responsible for the substorm disturbances of the geospace environment. In this paper, The development of the substorm concept is reviewed from its origins at the beginning of the 20th century to the present time. The theoretical framework in which substorm physics is normally presented is then discussed, and an outline is given of how that framework has changed in recent times. This paper concludes by posing two questions which need to be answered if further progress is to be made in solving the substorm problem.     

On the lamb frequencies of a spherical gaseous star

Abstract

It is pointed out that the Lamb frequencies which are known in the case of a plane isothermal atmosphere, exist locally in a spherical gaseous star. The isothermal atmosphere appears to be a limiting case in which the local frequencies become equal at all points of the layer, so that they give rise to frequencies of the global layer. This limiting case is made possible by the absence of curvature and of any variability in the velocity of sound.     

ELF plasma waves in hot and cold plasma fluxes observed by Prognoz-8 in the magnetospheric tail

We present Prognoz-8 observations of low-frequency plasma waves (2-105 Hz) associated with plasma fluxes near the outer boundary of the plasma sheet. These plasma fluxes were different from the regular plasma sheet boundary layer and consisted of tailward flowing warm proton and cold oxygen beams accompanied by rather cold electrons (T _e less than 100 eV). Observed plasma characteristics were used in the numerical solution of the dispersion relation for the ion-beam acoustic instability. Detailed analysis shows that this instability can be a source of observed emissions at frequencies up to 25 Hz.     

Geomagnetic activity as a reflection of processes in the magnetospheric tail: 2. Plasma convection model

A model of plasma convection in the magnetospheric tail was developed. Although highly simplified, the model adequately describes the main characteristics of the process. We have calculated the physical parameters characterizing the magnetotail, as well as described the convection of fluxtubes in it and the process of electron dropout. The model explains the semiannual variation in magnetic activity.     

Seismic reflection coefficients of faults at low frequencies: a model study

We use linear slip theory to evaluate seismic reflections at non‐welded interfaces, such as faults or fractures, sandwiched between general anisotropic media and show that at low frequencies the real parts of the reflection coefficients can be approximated by the responses of equivalent welded interfaces, whereas the imaginary parts can be related directly to the interface compliances. The imaginary parts of low frequency seismic reflection coefficients at fault zones can be used to estimate the interface compliances, which can be related to fault properties upon using a fault model. At normal incidence the expressions uncouple and the complex‐valued P‐wave reflection coefficient can be related linearly to the normal compliance. As the normal compliance is highly sensitive to the infill of the interface, it can be used for gas/fluid identification in the fault plane. Alternatively, the tangential compliance of a fault can be estimated from the complex‐valued S‐wave reflection coefficient. The tangential compliance can provide information on the crack density in a fault zone. Coupling compliances can be identified and quantified by the observation of PS conversion at normal incidence, with a comparable linear relationship.     

Nonstationary pearl pulsations as a signature of magnetospheric disturbances

We analyse long-lasting (several hours) Pc1 pearl pulsations with decreasing, increasing or constant central frequencies. We show that nonstationary pearl events (those with either decreasing or increasing central frequency) are observed simultaneously with increasing auroral magnetic activity at the nightside magnetosphere while the stationary events (constant central frequency) correspond to quiet magnetic conditions. Events with decreasing central frequency are observed mostly in the late morning and daytime whereas events with increasing central frequency appear either early in the morning or in the afternoon. We explain the diurnal distribution of the nonstationary pearl pulsations in terms of proton drifts depending on magnetic activity, and evaluate the magnetospheric electric field based on the variation of the central frequency of pearl pulsations.     

Nonlinear modification of magnetospheric plasma under the action of the ULF-wave ponderomotive force on the dayside

Nonlinear modification of the plasma magnetospheric density near the dayside magnetospheric boundary, caused by the ponderomotive force induced by ULF geomagnetic pulsations, has been studied analytically and numerically. An expression for the ponderomotive force, which differs from the previous similar results, has been obtained. It has been indicated that the well-known Pitaevskii formula for magnetic moment is incomplete. The action of the ponderomotive, gravitational, and centrifugal forces on magnetospheric plasma modification in a two-dipole geomagnetic field according to the (Antonova and Shabanskii, 1968) model has been considered.     

On the natural frequencies of infilled frames

Three approximate models are considered for the evaluation of the first few natural frequencies and associated mode shapes of infilled frames, a commonly occurring composite structural system formed by the combination of plane frames and filler walls. The reasonableness of the models is checked with the available experimental results and with the corresponding finite element solutions. The multiple strut model, wherein the infills are replaced by a set of equivalent multiple struts, can account for the frame–infill separation and infill openings and this model is seen to be an improvement over the single equivalent diagonal strut models proposed by earlier investigators. The shear—flexure cantilever analogy, by rationally evaluating the fundamental frequency, aids the equivalent lateral load procedures of the earthquake analysis. For dealing with the frames with central infill openings an equivalent plane frame model is discussed. Besides presenting the approximate models, the effect of frame—infill separation in reducing the fundamental frequency is investigated and an empirical relation is presented in this regard. Also some of the factors influencing the frame—infill contact lengths are studied and this indicates that the contact lengths are highly sensitive and indeterminate. Some constructional methods for avoiding separation and ensuring effective composite action between the frame and infill are also suggested.     

Nonlinear dynamics of 3D beams of fast magnetosonic waves propagating in the ionospheric and magnetospheric plasma

On the basis of the model of the three-dimensional (3D) generalized Kadomtsev-Petviashvili equation for magnetic field h = B _~/B the formation, stability, and dynamics of 3D soliton-like structures, such as the beams of fast magnetosonic (FMS) waves generated in ionospheric and magnetospheric plasma at a low-frequency branch of oscillations when β = 4πnT/B ² ? 1 and β > 1, are studied. The study takes into account the highest dispersion correction determined by values of the plasma parameters and the angle θ = (B, k), which plays a key role in the FMS beam propagation at those angles to the magnetic field that are close to π/2. The stability of multidimensional solutions is studied by an investigation of the Hamiltonian boundness under its deformations on the basis of solving of the corresponding variational problem. The evolution and dynamics of the 3D FMS wave beam are studied by the numerical integration of equations with the use of specially developed methods. The results can be interpreted in terms of the self-focusing phenomenon, as the formation of a stationary beam and the scattering and self-focusing of the solitary beam of FMS waves. These cases were studied with a detailed investigation of all evolutionary stages of the 3D FMS wave beams in the ionospheric and magnetospheric plasma.     

The contribution of magnetospheric currents toSq

Since the discovery of the magnetosphere, it has been known that the currents flowing in the magnetosphere contribute toSq, the regular daily variation in the earth's surface magnetic field. The early models, however, were not very accurate in the vicinity of the earth. The magnetospheric contribution toSq has therefore been recalculated by direct integration over the three major magnetospheric current systems; magnetopause, tail and ring. The finite electrical conductivity of the earth, which increases the horizontal and decreases the vertical components of the magnetospheric field at the earth's surface, has been taken into account. The magnetospheric currents are found to contribute 12 nanotesla to the day to night difference in the mid-latitudeSq pattern for steady, quiet magnetospheric conditions. They also contribute to the annual variation in the surface field and must be considered an important source of the observed day to day variation in theSq pattern.     

地磁扰动期间等离子体层顶结构的模拟研究

本文选取2001年6月8-10日的一个亚暴事件,模拟了在这期间等离子体层的结构演化过程.选取Weimer(2001模式)电场和Tsyganenko(1996模式)磁场作为背景电磁场,基于E×B的漂移运动计算磁赤道面内的带电粒子分布,模拟磁扰期间的等离子体层变化.模拟了等离子体层顶的结构和形状,结果有羽状、肩状和通道状结构,与同一时间点的EUV/IMAGE探测结果一致.