磁尾静电离子束流-密度漂移不稳定性的数值研究

本文研究了磁尾等离子体片边界层宽频带静电噪声的产生机制.模型等离子体由暖的背景电子、暖的向地球方向的离子束流和较冷的向尾方向的离子束流组成.结果表明,静电离子束流-密度漂移不稳定性可以在比较宽的频率范围内激发,在低频区大传播角方向上增长率最大,在高频区小传播角方向上增长率也比较大。最大增长率的方向取决于离子束流和密度漂移的速度比值.这些结果与磁尾观测到的宽频带静电噪声特征符合一致.      

极光场线上上行氧离子(O+)束驱动的静电离子迥旋波和离子声波不稳定性

本文研究了由背景热电子、背景冷质子(H+)和强各向异性氧离子(O+)束组成的模型等离子体中静电O+迴旋波和离子声波不稳定性.结果表明,低频(|ω|<σ_p,σ_p表示质子迴旋频率)静电O+迴旋波和离子声波可以由极光场线上上行O+束来激发.上行O+束可能是极光场线上低频静电不稳定性一个重要的自由能源.      

Standing Alfvén waves with m ≫ 1 in a dipole magnetosphere with moving plasma and aurorae

The structure of standing Alfvén waves with large azimuthal wave numbers (m ? 1) is studied in a dipole model of the magnetosphere with rotating plasma. In the direction across magnetic shells the structure of such waves is determined by their dispersion associated with curvature of geomagnetic field lines and corresponds to the travelling wave localized between toroidal and poloidal resonant surfaces. In projection into the ionosphere (along geomagnetic field lines) this structure is similar to the structure of a discrete auroral arc. The azimuthal structure of an auroral arc is similar to azimuthal structure of Alfvén waves with m ∼ 100. Possible interaction mechanisms between the Alfvén waves and energetic electron fluxes forming auroral arcs are discussed.     

倒V电子沉降通量能谱的理论计算

沿极光区磁力线大约在2000公里到8000公里的高度范围内,存在着一个等离子体湍流和大尺度平行电场的加速区。沿磁力线运动的等离子体片中的电子通过此加速区时,受到等离子体湍流和平行电场的共同作用,形成电子沉降的倒V结构。从一维准线性的动力学方程出发,导出了沉降电子通量的能谱方程,得出了电子通量能谱的理论公式。对等离子体湍流和平行电场对沉降电子能谱的影响作了分析和讨论。本文所提出的理论可以解释目前观测到的某些基本现象。      

Electron acoustic solitary waves in the Earth’s magnetotail region

Satellite observations have revealed solitary potential structures in the Earth’s magnetotail region. These structures have both positive (compressive) and negative (rarefactive) electrostatic potentials. In this paper we study the electron-acoustic solitary waves (EASWs) in an unmagnetized plasma consisting of cold plasma electrons and isothermal ions with two different temperatures. Using the reductive perturbation method, the nonlinear evolution of such structures is studied. The numerical computations are performed to study the role of two temperature ions in the generation of EASWs. In this case, the model supports the existence of both positive and negative electrostatic potentials with bipolar pulses. The electric field associated with these positive and negative solitary structures are numerically computed. The present study could be useful to construe the compressive and rarefactive electric field bipolar pulses associated with the BEN type emissions in the magnetospheric regions where the electron beams are not present.     

Electron acceleration and parallel electric fields due to kinetic Alfvén waves in plasma with similar thermal and Alfvén speeds

We investigate electron acceleration due to shear Alfvén waves in a collissionless plasma for plasma parameters typical of 4–5R_E radial distance from the Earth along auroral field lines. Recent observational work has motivated this study, which explores the plasma regime where the thermal velocity of the electrons is similar to the Alfvén speed of the plasma, encouraging Landau resonance for electrons in the wave fields. We use a self-consistent kinetic simulation model to follow the evolution of the electrons as they interact with a short-duration wave pulse, which allows us to determine the parallel electric field of the shear Alfvén wave due to both electron inertia and electron pressure effects. The simulation demonstrates that electrons can be accelerated to keV energies in a modest amplitude sub-second period wave. We compare the parallel electric field obtained from the simulation with those provided by fluid approximations.     

DE-1卫星对千米波辐射和极光嘶声的部分观测结果及分析

对DE-1卫星等离子体波的部分观测数据进行了处理,本文主要给出关于千米波辐射及极光嘶声的观测结果及分析.仅就带宽而言,千米波辐射可以从50kHz到大于400kHz,但峰值强度处于200kHz左右.电场谱密度可达10-11V2ni-2Hz-1或更大.其频率范围和强度随着卫星的观测地点而变化,但具有一个共同特征,就是在其频率范围内强度通常存在多个峰值.峰值的相对强度在迅速变化,峰值所对应的频率也在移动.极光嘶声则有明显的上截止频率和下截止频率.上截止频率或者是等离子体频率,或者是电子迴旋频率,看哪一个更小而定.而下截止频率主要是由于传播效应造成的.      

Ion- and electron-acoustic solitons and double layers in multi-component space plasmas

A general model for the ion- and electron-acoustic solitons and double layers in a multi-component unmagnetized plasma consisting of background electrons, counter-streaming electron beams and ions is discussed. The model is based on the multi-fluid equations and the Poisson equation, and uses the Sagdeev pseudo-potential techniques. For identical counter-streaming electron beams and depending upon the plasma parameters, three types of solutions, namely, ion-acoustic, slow and fast electron-acoustic soliton/double layer, are possible. Generally, the ion acoustic solitons have positive potentials, slow-electron acoustic solitons have negative potentials and fast electron-acoustic solitons and double layers can have either positive or negative potentials depending on the core electron density. As beam speed is increased, first ion-acoustic and then slow electron-acoustic solitons disappear. At large beam speed, only fast electron-acoustic solitons/double layers survive. The results may be relevant to the observations of the electrostatic solitary waves (ESWs) observed in the Earth’s magnetosphere.     

An investigation of high frequency auroral plasma emissions with a tubular dipole antenna

A band of enhanced amplitudes which follows a local plasma frequency fn in raw high frequency (HF) noise spectra is usually related to plasma emissions in the upper hybrid band (fn, fu). The enhanced band in question occurs permanently in noise spectra recorded on the Intercosmos-19, APEX and CORONAS satellites in the altitude range of 500 km–3000 km. For moderately magnetized plasma with fn > 2fc (fc – electron gyro frequency), the band occurs below fn determined from the topside sounder and impedance data or from electron beam induced spectra. The simulations of an equivalent circuit composed of a dipole antenna in a cold plasma and its preamplifiers, determined the physical origin of the band as the passive circuit resonance, due to inductive character of the antenna in a frequency band (fc, fu). The resonance spectral content is highly structured due to an inflight variability of the circuit impedances. In this report we analyze the noise and impedance spectra which are the most typical in an auroral zone if fn > fc. We focus attention on determination of local electron plasma density, essential for provisional HF mode classification. We found that the natural plasma emission in the upper hybrid band does not manifest itself as the banded natural emission, which may be used for reliable determination of local plasma frequency in the altitude range of 500–3000 km. The fast magnetosonic mode predominates in the auroral emissions. The broadband and multi banded electromagnetic emissions extending from the fast magnetosonic band well above fn > fc are characteristic for the strong wave activity and are much less frequent.     

Active stimulation of the ionospheric plasma

Laboratory experiments in which high power, pulsed electromagnetic waves interact with an inhomogeneous plasma indicate that the generated nonlinear plasma phenomena depend on peak incident power and not on pulse length. The electromagnetic waves can penetrate beyond the cutoff and produce large, enhanced electrostatic fields at the critical layer within 100 electron plasma periods. The enhanced electric field pressure can be comparable to the thermal pressure and can accelerate ions and electrons to velocities much greater than their thermal speed. Large density cavities (with δn/n ? 10%) can be created in a time shorter than the usual ion response time because of the accelerated ion dynamics. These laboratory results have been extended to create a new and generalized concept to actively stimulate space plasmas with high power pulses of short duration. A field experiment will be used for the stimulation of auroral ionospheric plasma. The ground-based system is modular, each module consisting of a 2 MW pulsed HF transmitter designed at UCLA and a crossed-dipole antenna element. Incoherent scatter radar and optical diagnostic methods are discussed.     

Nonlinear electron-acoustic rogue waves in electron-beam plasma system with non-thermal hot electrons

The properties of nonlinear electron-acoustic rogue waves have been investigated in an unmagnetized collisionless four-component plasma system consisting of a cold electron fluid, non-thermal hot electrons obeying a non-thermal distribution, an electron beam and stationary ions. It is found that the basic set of fluid equations is reduced to a nonlinear Schrodinger equation. The dependence of rogue wave profiles on the electron beam and energetic population parameter are discussed. The results of the present investigation may be applicable in auroral zone plasma.     

Generation of kinetic Alfvén waves by velocity shear instability on auroral field lines

Ion beams observed in the plasma sheet boundary layer (PSBL), cusp, and on the auroral zone field lines are expected to have spatial gradients in their drift velocity. Generation of kinetic Alfvén waves by velocity shear of the ion beams is discussed. It is shown that a hot ion beam can excite both a resonant kinetic Alfvén wave instability and a non-resonant coupled Alfvén-ion acoustic instability. For typical parameters, observed on the auroral field lines in the altitude range of 5–7 R_E (where R_E is the Earth’s radius), the frequency of the velocity shear modes, in the satellite frame of reference, lie in the ultra-low frequency (ULF) range. The noise due to velocity shear driven Alfvén modes is electromagnetic in nature, and has a finite parallel electric field component.     

Ion-acoustic waves in unmagnetized collisionless weakly relativistic plasma of warm-ion and isothermal-electron using time-fractional KdV equation

Collisionless unmagnetized plasma consisting of a mixture of warm ion-fluid and isothermal-electron is considered, assuming that the ion flow velocity has a weak relativistic effect. The reductive perturbation method has been employed to derive the Korteweg–de Vries (KdV) equation for small – but finite-amplitude electrostatic ion-acoustic waves in this plasma. The semi-inverse method and Agrawal’s method lead to the Euler–Lagrange equation that leads to the time fractional KdV equation. The variational-iteration method given by He is used to solve the derived time fractional KdV equation. The calculations show that the fractional order may play the same rule of higher order dissipation in KdV equation to modulate the soliton wave amplitude in the plasma system. The results of the present investigation may be applicable to some plasma environments, such as space-plasmas, laser-plasma interaction, plasma sheet boundary layer of the earth’s magnetosphere, solar atmosphere and interplanetary space.     

Solitary and periodic waves in magneto–rotating plasmas: Effect of the Kappa–Cairns distributed electrons

The nonlinear propagation of ion–acoustic (IA) waves in a magneto–rotating plasma is studied by considering the Kappa-Cairns electron distribution. Employing the perturbation scheme, Korteweg–de Vries equation is derived. It is seen that both positive and negative potential solitons can be supported in the present plasma model. The numerical results reveal that the Kappa-Cairns distributed electrons modify features of the electrostatic waves significantly. The effects of non–thermal parameters, plasma rotation frequency, ion temperature, and the wave propagation angle on electrostatic solitary wave structures are also discussed here. It is found that the plasma parameters considerably influence the propagation of IA waves in rotating plasmas. Furthermore, using the bifurcation theory of planar dynamical systems to the K-dV equation, we have presented the existence of solitary and periodic traveling waves. Our study may be helpful to understand the behavior of ion–acoustic wave in the rotating plasma.     

Small amplitude ion acoustic solitons in a weakly magnetized plasma with anisotropic ion pressure and kappa distributed electrons

The Zakharov–Kuznetzov (ZK) equation is derived for nonlinear electrostatic waves in a weakly magnetized plasma in the presence of anisotropic ion pressure and superthermal electrons. The anisotropic ion pressure is defined using Chew–Goldberger–Low (CGL) while a generalized Lorentzian (kappa) distribution is assumed for the non-thermal electrons. The standard reductive perturbation method (RPM) is employed to derive the two dimensional ZK equation for the dynamics of obliquely propagating low frequency ion acoustic wave. The influence of spectral index (kappa) of non-thermal electron on the soliton is discussed in the presence of anisotropic ion pressure in plasmas. It is found that ion pressure anisotropy and superthermality of electrons affect both the width and amplitude of the solitary waves. On the other hand the magnetic field is found to alter the dispersive property of the plasma only, and hence the width of the solitons is affected while the amplitude of the solitary waves is independent of external magnetic field. The numerical results are also presented for illustrations.     

Positive ion distributions in the morning auroral zone: Local acceleration and drift effects

We report on the typical structure of the large scale ion precipitation in the morning sector of the auroral zone and associated low frequency electromagnetic waves. Data obtained during near radial passes of the AUREOL-3 satellite point to a distinction between two main precipitation regions: 1) In the poleward part of the auroral zone the latitudinal variation of the average energy (or temperature) of the precipitated ions (mainly H⁺) indicate that they are adiabatically accelerated in the outer magnetosphere. This “high energy” (? 3 to > 20 keV) precipitation is usually associated with a low energy (E < 110 eV) upward flowing 0⁺ and H⁺ component, and 2) near the boundary between discrete and diffuse electron aurorae a drastic change in the ion characteristics is observed. The flux of energetic precipitated H⁺ ions is sharply reduced, which suggests the formation of an Alfvén layer. However, intense fluxes of precipitated H⁺, O⁺, and He⁺ ions with energies < 3 keV are observed equatorward of the Alfvén layer, in coincidence with the diffuse aurora and in association with quasi-monochromatic electromagnetic waves with frequencies around the proton gyrofrequency. As the characteristic convection and bounce times of the low energy upward flowing ion component are comparable (τ > 3 hours) we suggest that the precipitation of ionospheric ions inside the diffuse aurora results from convection and corotation of the ions accelerated to suprathermal energies at higher latitudes.     

Electromagnetic ion cyclotron resonance heating in the VASIMR

Plasma physics has found an increasing range of practical industrial applications, including the development of electric spacecraft propulsion systems. One of these systems, the Variable Specific Impulse Magnetoplasma Rocket (VASIMR) engine, both applies several important physical processes occurring in the magnetosphere. These processes include the mechanisms involved in the ion acceleration and heating that occur in the Birkeland currents of an auroral arc system. Auroral current region processes that are simulated in VASIMR include lower hybrid heating, parallel electric field acceleration and ion cyclotron acceleration. This paper will focus on using a physics demonstration model VASIMR to study ion cyclotron resonance heating (ICRH). The major purpose is to provide a VASIMR status report to the COSPAR community. The VASIMR uses a helicon antenna with up to 20 kW of power to generate plasma. This plasma is energized by an RF booster stage that uses left hand polarized slow mode waves launched from the high field side of the ion cyclotron resonance. The present setup for the booster uses 2–4 MHz waves with up to 20 kW of power. This process is similar to the ion cyclotron heating in tokamaks, but in the VASIMR the ions only pass through the resonance region once. The rapid absorption of ion cyclotron waves has been predicted in recent theoretical studies. These theoretical predictions have been supported with several independent measurements in this paper. The ICRH produced a substantial increase in ion velocity. Pitch angle distribution studies show that this increase takes place in the resonance region where the ion cyclotron frequency is equal to the frequency on the injected RF waves. Downstream of the resonance region the perpendicular velocity boost should be converted to axial flow velocity through the conservation of the first adiabatic invariant as the magnetic field decreases in the exhaust region of the VASIMR. In deuterium plasma, 80% efficient absorption of 20 kW of ICRH input power has been achieved. No evidence for power limiting instabilities in the exhaust beam has been observed.     

Analysis of the turbulence observed in the outer cusp turbulent boundary layer

One of the prominent features of the cusp Turbulent Boundary Layer (TBL) is a persistent low frequency electromagnetic turbulence that extends from <1Hz up to the electron cyclotron frequency, accompanied by what appears to be purely electrostatic noise above this frequency range. The Plasma Wave Instrument onboard Polar obtained plasma wave measurements in the cusp TBL in the form of waveform captures simultaneously from 6 different sensors (3 each orthogonal electric and magnetic) in the frequency range 1 Hz up to 25 kHz. This allowed us to directly calculate the phase velocity from the measured ratio of |dE| to |dB| and compare it to theoretical values for various modes. Using this technique, we have gained some insight into the mode of the electromagnetic turbulence that extends in frequency from 1 Hz up to the electron cyclotron frequency (several hundred Hz to a few kHz) in the TBL. The whistler and kinetic Alfvén wave modes are discussed as the possible modes of this turbulence. By analyzing the high time resolution waveforms, we isolate and identify some of these modes. The electrostatic turbulence above the electron cyclotron frequency is associated with pulses and quasi-sinusoidal waveforms observed in the measured time series. These do not fit any known mode, although work is continuing in this area to show that some of them may be associated with electron holes or with downshifted Langmuir waves produced through a two-stream instability.     

Study of correlations between waves and particle fluxes measured on board the DEMETER satellite

The topic of relativistic electron dynamics in the outer radiation belt has received considerable attention for many years. Nevertheless, the problem of understanding the physical phenomenon involved is far from being resolved. In this paper, we use DEMETER observations to examine the variations of the energetic electron fluxes and ELF/VLF wave intensities in the inner magnetosphere during the intense 8 November 2004 magnetic storm. Electron flux spectra and associated wave intensity spectra are analysed throughout the magnetic storm and common characteristics or differences to other storm events are retained. The overall objective of this study is to identify and derive parameters that are relevant for particle flux modelling; the time constant characterizing the persistent decay after particle enhancement was found to be one of these important model parameters.The analysis of the 8 November 2004 event reveals that for L-shell parameter higher than 4, an electron flux dropout is observed during the storm’s main phase for electrons in the energy range 0.1–1 MeV, as has been reported from other measurements. Characteristic wave spectra accompanying this phase are analysed. They show a typical enhancement in the frequency range 0.3–10 kHz at onset for all L-shell values under consideration (2 < L < 5). During the first stage of the recovery phase, the electron fluxes are increased to a level higher than the pre-storm level, whereas the level of wave intensity in the frequency range observed below 300 Hz is at its highest. In the second stage, the particle flux decrease goes hand in hand with a global wave activity decline, the relaxation time of the latter being smaller than the former’s one. In some other cases, long-lasting electron enhancement associated with constant wave activity has been observed during this latter stage. For the above mentioned storm, while at low L values the decay time constants are higher for low energy electrons than for high energy electrons, this order is reversed at high L values. At about L = 3.6 the time constant is independent of electron energy.