On the dispersion of two coexisting nongyrotropic ion species

Space observations in the solar wind and simulations of high Mach number bow-shocks have detected particle populations with two coexisting nongyrotropic ion species. We investigate the influence of these two sources of free energy on the stability of parallel (with respect to the ambient magnetic field) and perpendicular propagation. For parallel modes, we derive their dispersion equation in a magnetoplasma with protons and alpha particles that may exhibit stationary nongyrotropy (SNG) and discuss the characteristics of its solutions. Kinetic simulations study the behaviour of perpendicular electrostatic (Bernstein-like) waves in a plasma whose ion populations (positrons and fictitious singly-charged particles with twice the electron mass, for the sake of simulation feasability) can be time-varying nongyrotropic (TNG). The results show that the coexistence of two gyrophase bunched species does not significantly enhance the parallel SNG instability already found for media with only one nongyrotropic species, whereas it strongly intensifies the growth of Bernstein-like modes in TNG plasmas.     

Influence of electron nongyrotropy and anisotropy on parallel wave propagation: Numerical solution of dispersion relation

This work investigates the influence of a nongyrotropic electron beam on the coupling between the electromagnetic modes of parallel propagation in a background gyrotropic plasma. We explore the importance of the relevant parameters in driving instabilities: the ratio of electron plasma frequency to electron cyclotron frequency, the gyrophase angle, and the temperature anisotropy. We confirm previous results that nongyrotropy can lead to mode coupling for specific values of relevant parameters. More important was to observe that when the anisotropy in the system provides sufficient free energy to start instabilities, the electron nongyrotropy plays an important role to change the growth rate.     

Stability of stationary and time-varying nongyrotropic particle distributions

The ubiquity of nongyrotropic particle populations in space plasmas warrants the study of their characteristics, in particular their stability. The unperturbed nongyrotropic distribution functions in homogeneous media without sources and sinks (closed phase space) must be rotating and time-varying (TNG), whereas consideration of open phase spaces allows for the occurrence of homogeneous and stationary distributions (SNG). The free energy brought about by the introduction of gyrophase organization in a particle population can destabilize otherwise thoroughly stable magnetoplasmas (or, a fortiori, enhance pre-existing gyrotropic instabilities) and feed intense wave growth both in TNG and SNG environments: The nongyrotropic (electron or ion) species can originate unstable coupling among the gyrotropic characteristic waves. The stability properties of these two types of homogeneous nongyrotropy shall be contrasted for parallel (with respect to the ambient magnetic field) and perpendicular propagation, and their potential role as wave activity sources shall be illustrated resorting to solutions of the appropriate dispersion equations and numerical simulations.     

Nongyrotropy in magnetoplasmas: simulation of wave excitation and phase-space diffusion

Nongyrotropic (gyrophase bunched) ion distributions in a magnetoplasma are studied by analytical methods and by two-dimensional hybrid code simulations. Nongyrotropy may not occur in a plasma being simultaneously homogeneous, stationary, and solenoidal in phase space. A detailed study is performed for a homogeneous and stationary plasma with sources and sinks in phase space. The analytical investigation cast in the framework of linearized Maxwell-Vlasov theory yields a coupling of low-frequency left-handed, right-handed, and longitudinal modes. Nongyrotropic ion distributions are unstable; they excite left-handed waves. The growth rate is comparable to that of the ion ring instability. The hybrid code simulation study confirms the expected propagation direction parallel to the background magnetic field. Three diffusion processes are studied: arc lengthening, arc broadening, and arc radius decreasing corresponding to particle energy diffusion. The characteristic diffusion time-scales are found to be of the order of 101 wave cycles.     

Nongyrotropic particle distributions in space plasmas

In nonstationary, strong inhomogeneous or open plasmas particle orbits are rather complicated. If the nonstationary time scale is smaller than the gyration period, if the inhomogeneity scale is smaller than the gyration radius, i.e. at magnetic plasma boundaries, or if the plasma has sources and sinks in phase space, then nongyrotropic distribution functions occur. The stability of such plasma configurations is studied in the framework of linear dispersion theory. In an open plasma nongyrotropy drives unstable waves parallel and perpendicular to the background magnetic field, whereas in the gyrotropic limit the plasma is stable. In nonstationary plasmas nongyrotropy drives perpendicular unstable waves only. Temporal modulation couples a seed mode with its side lobes and thus it renders unstable wave growth more difficult. As an example of an inhomogeneous plasma a magnetic halfspace is discussed. In a layer with thickness of the thermal proton gyroradius a nongyrotropic distribution is formed which may excite unstable parallel and perpendicular propagating waves.     

Large-Scale Magnetostatic Structures in the Solar Corona and a Model of the Polar Coronal Hole

Geomagnetism and Aeronomy - A method for the theoretical calculation of large-scale magnetoplasma solar coronal structures with a rotational symmetry in the spherical coordinate system is...     

The plasma-like properties of an aerosol of consensation nuclei

Summary An aerosol in charge equilibrium is theoretically and numerically investigated for plasma-like behaviour. It is shown that in sufficiently large containers the common type aerosols should manifest the properties of a plasma as regards their behaviour in an electric field. Such aerosols do not unambiguously possess a plasma frequency nor can they be termed a magnetoplasma. The results of this investigation may help to understand certain phenomena in atmospheric electricity.     

Observation and interpretation of cometary low frequency waves

The first observations of cometary wave activity were carried out in 1985/1986 by several space missions (ICE, VEGAs 1 and 2, Suisei, Sakigake, Giotto) in the environments of comets Giacobini-Zinner and Halley. The interpretation of thesein situ field (and particle) measurements fostered investigations on (among other topics) wave generation that, leaving aside the inherently nonlinear (but related) problem of the eventual formation of a cometary bow shock wave, explored the free energy available in two specific features of the velocity distributions of the newborn particle populations: their parallel (with respect to the IMF direction) drift in the solar wind frame and perpendicular ring-like organization. Analytical and simulation works looked into the influence of the solar wind and cometary newborn parameters on the instabilities and the ensuing, or associated (as evidenced by wave observations), nonlinear phenomenology. Comprehensive reviews have described the experimental and theoretical results obtained in this cometary wave research until 1992 and identified outstanding problems warranting further attention. Here, only a cursory revisit to the Giacobini-Zinner/Halley era of low frequency wave observation and interpretation shall be made: rather, attention shall be predominantly focussed on the new implications to cometary wave research of the recent Giotto encounter with comet Grigg-Skjellerup on July 10 of 1992. The three visited comets, starting with their gas production rates, had different characteristics that showed up in thein situ observations. Yet, with the important exception of the Grigg-Skjellerup encounter, the interpretation of the wave activity measurements could be made in terms of common basic generation mechanisms adapted to the relevant properties of the appropriate plasma environment. New aspects emerged in the last Giotto cometary mission: the smaller gas production rates yield a scale length for the neutral gas density that is not (much) larger than the gyration distance of a heavy newborn ion (estimated by the product of the solar wind speed and the ion cyclotron period). As a consequence of this inhomogeneity, the velocity distribution of the heavy newborn ions exhibits gyrophase organization, i.e. nongyrotropy. This new source of free energy, albeit briefly mentioned in a few studies preceding the Grigg-Skjellerup mission, was not investigated in the context of the Giacobini-Zinner and Halley encounters. Since the last Giotto observations strongly suggest that nongyrotropy plays a prominent role in wave generation as the comet Gigg-Skjellerup nucleus is approached and its stability characteristics have only seldomly been analyzed, the review shall emphasize the wave generation capabilities of particle populations with gyrophase organization.     

Slow Z-mode radiation from sounder-accelerated electrons

Quasi-electrostatic Z-mode waves created by the transmitting part of the OEDIPUS-C payload were measured on the receiving part at 1200 m distance. Solutions of the complete electromagnetic dispersion relation for a hot magnetoplasma reveal, however, that there is no solution that provides direct ray paths along the transmitter-receiver separation direction with the observed signal group delays. An interpretive model is therefore proposed in which sounder-accelerated electrons (SAE) radiate incoherently as they spiral along the magnetic field direction in the general direction of the receiving subpayload. Test-particle theory combined with the hot-plasma dispersion solution is used to predict the total electric field for previously reported SAE flux levels. It is found that voltage levels measured on the receiving dipoles have about the same order of magnitude as the predicted ones.     

The plasma sheet source groove

A test particle study of the ionospheric source of plasma in the Earth’s plasma sheet has been performed, in an effort to understand an apparent inconsistency between the results of forward and backward (in time) test particle calculations. Most, if not all, forward calculations of polar wind ion outflows result in energetic plasma sheet ion populations; yet most, if not all, backward trajectory calculations from typical plasma sheet ion populations lead elsewhere than to low energy polar cap outflows. Using a trajectory discovered through forward calculation to connect these two regions, we found that the trajectory was only accurately reversible within an extremely narrow range of energy, pitch angle and gyrophase angle in the plasma sheet, referred to herein as ‘the source groove’. This implies that ionospheric plasma tends to appear in the plasma sheet within narrow regions of velocity space, but is effectively diffused by fluctuations to form the observed more isotropic plasma sheet populations. The implications for backtracking test particle studies are discussed, and it is concluded that test particle backtracking from highly chaotic regions is impractical and should be supported by forward modeling of plasma flows up to the boundaries of such regions.     

Electron cyclotron waves in the presence of parallel electric fields in the Earth’s auroral plasma

The electron cyclotron waves that originate at low altitudes (<0.5 R_E) and observed by ground facilities have been studied in the presence of a weak parallel electric field in auroral magnetoplasma consisting of trapped energetic auroral electrons and cold background electrons of ionospheric origin. The model distribution for auroral trapped electrons is taken as Maxwellian ring distribution. An expression for the growth rate has been obtained in the presence of parallel electric field assuming that the real frequency in the whistler mode is not affected by the presence of the electric field. The results show that waves grow (or damp) in amplitude for a parallel (or antiparallel) electric field. The influence of the electric field is more pronounced at a shorter wavelength spectrum. An increase in population of energetic electrons increases the growth rate and thus, plays a significant role in the wave excitation process in the auroral regions.     

Nonlinear shock structure in a weakly ionised magnetoplasma

The formation of a neutral induced weak nonlinear shock structure in a weakly ionised magnetoplasma has been analytically investigated. Using the reductive perturbation method, basic dynamical equations of a three-component (electron, ion and neutral) plasma have been reduced to a well-known Burger equation which can support a weak shock solution. Its stationary and initial value solutions have been derived to describe the characteristics of the weak shock profile. Asymptotic behavior of the Burger solution results in a saw-tooth structure which has a practical implication to predict the nonlinear steepened structure of the nighttime irregularity in the lower portion of the Earth’s ionosphere. Accordingly, it is suggested that the observation of saw-tooth shape of nighttime irregularity at 92 km could be attributed to the nonlinear saturation of the NILF mode instability as proposed by Dwivedi and Das in 1992. However, exact experimental verification of this suggestion requires more data on nighttime irregularity in the mesosphere and lower thermosphere (80-95 km) for a wide range of scale sizes extending up to about 1 km and above.     

The use of near-antenna artificial density ducts for increasing the power of VLF radiation in space plasma

We investigate the radiation from an artificial density duct excited by an annular electric current in the VLF frequency range. It is assumed that the duct possesses the form of a quasi-cylindrical density enhancement which smoothly reduces with distance from the source. We have found that the field in the duct is represented with a good accuracy in terms of eigenmodes whose propagation is supported in density enhancements. The radiation field in the background medium arises due to reemission of these eigenmodes from the duct. We calculate the excitation coefficients of eigenmodes and study the characteristics of a radiation field. We show that the presence of a duct with enhanced density can cause a considerable increase in the power radiated from the source. In particular, we study the distribution of radiated power over the spatial spectrum of waves excited in a surrounding magnetoplasma. It is concluded that under certain conditions an appreciable increase in the power radiated into the long wavelength part of the spatial spectrum can be achieved. We give concrete estimations for the conditions appropriate to the Earth's ionosphere.     

东郭井水位与水温成组异常及其映震特征

分析了东郭井自2008年1月观测以来出现的水位与水温成组异常及其与周围中等地震活动的关系,研究结果显示:异常与地震有较好的对应关系,其异常的基本特征是水位阶降与水温缓升的形态配套,异常的幅度与震级大小有一定关系,异常持续的时间与井震距有一定的关系,还讨论了异常的机理。     

Magnetospheric substorm: Loss of the magnetoplasma equilibrium as a nonlinear dynamical bifurcation

The fast onset of a substorm—a substorm “explosion”—is usually associated with the moment of stability loss of the magnetoplasma equilibrium in the geomagnetic tail. The origination of such a process either from the near-Earth part of the plasma sheet or from its remote part, which is highly stretched into the tail, is now being studied theoretically and verified experimentally (at the present time, in the THEMIS project). In the first case, the resulting disturbance must have the form of a ballooning mode; in the second case, of tearing perturbation. However, in both cases, this stability loss, i.e., a quick breakdown in the balance, replacing the slow quasi-static evolution of configuration, can only occur as a nonlinear process. Taking into account the specific properties of the configuration and possible disturbances in it, we indicate why such a process cannot be the previously proposed “substorm detonation.” It is shown that a suitable mathematical model is a nonlinear dynamical bifurcation occurring on a small time scale, with a delay relative to the moment of passing the marginally stable state.     

Subsolar elevation of the equatorial electrojet

The subsolar elevation of the equatorial electrojet has been produced from satellite solstitial data available from 09 to 15 hr LT using a new approach with the general style of the overhead equivalent current system. It shows the bunching of the current around the dip equator; the return currents of the equatorial electrojet close to the flanks of the dip equator; the fast growth of the electrojet to its diurnal peak followed by a slow decay; and the contraction of its latitudinal extent around the meridian of its highest intensity. Comparison with the results of other workers using ground data suggests that the elevation from satellite data agrees better with that from ground data when the worldwideSq is removed from the ground data.     

Transient Electric Field in the Mesosphere above a Γ-shape Lightning Stroke

We describe the space–time distribution of the pulsed electric field in the middle atmosphere above a positive Γ-shaped lightning stroke. The channel of such a discharge contains a vertical and a horizontal section. The current wave moves initially vertically and then turns horizontally so that radiation appears from the vertical electric dipole followed by that from the horizontal dipole. Combined with reflection from the perfectly conducting ground, the source provides three subsequent pulses in the atmosphere, with the lag being determined by the finite velocity of the current wave in the Γ-shaped stroke. The pulses are reproduced by reflections from the air-ground and the air-ionosphere interfaces and the waveform resembles the M-component, which is often noted in the negative strokes (e.g. Yashunin et al., J Geophys Res 112:D10109, 2007). The non-stationary fine structure appears in the spatial distribution of electric field, which persists for 2 ms or even more and exceeds the runaway electron threshold. Estimates support the idea of free electron bunching in the mesosphere by the pulsed electric field. Focusing may occur about 10 km away from the point of electron- field interaction; it is delayed by a few ms from the moment of interaction. The data presented might be helpful in realistic modeling of the red sprite formation.     

新疆和田台阵PSD与PDF分析

和田台阵是我国第一个自主建成并运行至今的小孔径台阵,承担着监测印巴地区核试验以及我国西部地震活动的重要使命.台阵波形数据中充斥着背景噪声,直接影响着数据质量.为了评估台阵噪声水平,本文利用Welch平均周期法对9个子台记录的数十万条噪声样本进行功率谱估计,对出现的谱异常进行了总结归纳,通过绘制概率密度函数图以及单频曲线来研究背景噪声变化范围和规律,最后针对台阵降噪提出了建设性意见.研究结果表明,中心台长周期噪声功率谱密度随季节变化显著,具有周期性;受温度和气压影响,水平分量长周期噪声变化幅度较大,局部频段超出新高噪声模型,建议改善仪器安装条件,或者利用数学方法进行校正.所有子台短周期噪声变化规律与长周期相反,受到采石场影响,谱密度曲线在4~8 Hz之间出现形态规则的高频尖刺,A1、B3、B4子台最为明显,可以通过窄带滤波或者聚束予以压制.本文取得的研究成果为台阵运维提供重要依据,除此之外,总结出的不同地震频谱特征也为地震解释工作提供重要参考.     

Solar variability and its implications for the human environment

Solar variability can affect human activities in a variety of ways, from changing our climate to disrupting power distribution facilities and shortening the orbital lifetime of satellites. This tutorial paper will be concerned only with effects on the surface environment that can have a direct impact on our everyday life, such as variations in the stratospheric ozone layer that shields us from harmful ultraviolet radiation, and changes in global climate that can hinder or delay the detection of climate changes that might result from our own technological activities. The emphasis is on potential mechanisms, rather than on reported correlations between solar and terrestrial parameters, but reference to certain observations will be made. Realization of a potential impact of solar variability on our local environment has progressed a long way in the last few decades, from denial to partial acceptance, but a complete assessment of its reality and magnitude remains a distant goal.     

Effects of local site conditions on the seismic response of municipal solid waste landfills

Over the last decade, the seismic response of landfills made of municipal solid waste has drawn attention mainly due to the environmental and public-health issues that could be raised in the event of a failure. Nevertheless, there are several associated technical issues that have not been adequately investigated. One of these is the impact of local site conditions on the earthquake-induced accelerations and, thereby, on the seismic design of a landfill. This study presents the results of a parametric numerical simulation that has been performed in order to examine the effects of local site conditions on the dynamic response of a typical landfill. Emphasis is given on the special characteristics of ground motion, while the material nonlinearity of both soil and waste is taken into account by an equivalent-linear procedure. Results indicate that local site conditions may play a significant role in the seismic response of a landfill. However, this role cannot be judged a priori as beneficial or detrimental, as it depends not only on soil conditions and seismic excitation, but also on the material and geometric characteristics of the landfill.