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.     

Wave induced energetic particle generation and space plasma modeling

An increasing number of high-resolution spacecraft observations provide access to details of energetic electron and ion velocity-space distribution structures. Since resonant wave-particle interaction processes depend considerably on the distribution function details, space plasma modeling is of particular interest for studies of a variety of plasma environments as planetary magnetospheres, the interplanetary medium or solar flares. After summarizing the most popular particle acceleration processes we focus on wave-powered energization mechanisms induced by Landau interaction and demonstrate from a time-evolutionary scenario that power-law distributions, highly favored by observations in recent years, are generated resonantly by an Alfvén wave spectrum and possibly saturate. This process is further stimulated in non-uniform magnetic field configurations where multiple wave packets at different phase velocities provide the energy source for a continuous acceleration process. Moreover, in this conjunction we demonstrate that in particular κ-distributions are a consequence of a generalized entropy concept, favored by nonextensive statistics, which provides the missing link for power-law plasma models from fundamental physics. With regard to in situ space observations examples are provided illuminating that for non-thermal plasma characteristics the particular structure of the velocity-space distribution dominates as regulating mechanism for the wave-particle interaction process over effects related to changes in space plasma parameters. This revised version was published online in August 2006 with corrections to the Cover Date.     

Nonlinear dust Alfvén modes

Nonlinear modes are investigated in magnetized dusty plasmas, where the dust dynamics is modelled by a number of cold, highly negatively charged and very massive fluids, besides ordinary electrons and protons. Several low-frequency motions occur which are typical for the dust components, some of them described by model equations such as the derivative nonlinear Schrödinger equation for electromagnetic waves. One can include equilibrium drifts and even fluctuations in the grain charges. Most of the preceding conclusions are relevant for different kinds of astrophysical and heliospheric plasmas.     

Model-independent large-scale magnetohydrodynamic quantities in magnetic clouds

Magnetic clouds are the interplanetary manifestation of coronal mass ejections, which are transient expulsions of major quantities of magnetized plasma, from the Sun toward the heliosphere. The magnetic flux and helicity are two key physical magnitudes to track solar structures from the photosphere-corona to the interplanetary medium. To determine the content of flux and helicity in magnetic clouds, we have to know their 3D structure. However, since spacecrafts register data along a unique direction, several aspects of their global configuration cannot be observed. We present a method to estimate the magnetic flux and the magnetic helicity per unit length in magnetic clouds, directly from in situ magnetic observations, assuming only a cylindrical symmetry for the magnetic field configuration in the observed cross-section of the cloud. We select a set of 20 magnetic clouds observed by the spacecraft Wind and estimate their magnetic flux and their helicity per unit length. We compare the results obtained from our direct method with those obtained under the assumption of a helical linear force-free field. This direct method improves previous estimations of helicity in clouds.     

调制电子束弛豫长度对哨声波辐射的影响

在发射调制电子束的主动空间试验中,电子束与背景等离子体的线性和非线性相互作用将产生哨声波辐射.影响辐射特性的因素很多,调制电子束的弛豫长度是其中一个重要因素.本文研究了呈指数衰减的调制电子束弛豫长度对电子束产生的哨声波辐射特性的影响.结果表明,当电子束弛豫长度与背景等离子体非均匀特征尺度相当时,有利于提高调制电子束产生的波辐射强度.     

Non-Linear Growth of Trapped Particle Modes in Linearly Stable, Current-Carrying Plasmas – A Fundamental Process in Plasma Turbulence and Anomalous Transport

The two-stream instability as a fundamental process in a current-carrying plasma is reconsidered. Its well-established linear version, based on kinetic Landau theory, predicts a threshold for the drift velocity between both species below which the plasma should be stable. We report on simulations which, however, show that a plasma as a non-linearly responding medium can be destabilized well below this threshold. Responsible for this unexpected behaviour are coherent, electrostatic, trapped particle structures such as phase space vortices or holes which can grow non-linearly out of thermal noise receiving their energy from the net imbalance of loss of electron kinetic energy and gain of ion kinetic energy. The birth of predominantly zero-energy holes is shown numerically being associated with initial, non-topological fluctuations. The latter are not subject to Landau damping, as they lie outside the realm of linear wave theory. For a pair plasma a typical scenario is presented, which encompasses several regimes such as non-linear growth of multiple holes, saturation and fully developed structural turbulence as well as an asymptotic approach to a new collisionless equilibrium. During the transient, structural state the plasma transport appears to be highly anomalous.     

Hysteresis Provides Self-Organization in a Plasma Model

The magnetosphere is a multi-scale spatio-temporal complex dynamical system. Self-organization is a possible solution to the seemingly contradicting observation of the repeatable and coherent substorm phenomena with underlying complex behavior in the plasma sheet. Self-organization, through spatio-temporal chaos, emerges naturally in a plasma physics model with sporadic dissipation.     

Compressive and Rarefactive Electron-Acoustic Solitons and Double Layers in Space Plasmas

Space observations in several near-Earth environments have revealed the presence of positive-potential, large-amplitude electrostatic structures, associated with high-frequency disturbances, and indicative of electron dynamics. Earlier models proposed in terms of electron-acoustic solitary waves in a two-electron-temperature plasma were inadequate, because only negative potential structures could thus be obtained, whereas the observations point to positive potential structures. In this paper, it is shown that the theoretical restriction to negative potential solitons is due to the neglect of the inertia of the hot electrons, implicitly or explicitly assumed in previous papers. If hot electron inertia is retained, however, there exists a parameter range where positive potential solitary waves are formed, which can have important consequences for the re-interpretation of several astrophysical phenomena involving two-electron-temperature plasmas. PACS: 52.35.Mw, 52.35.Sb, 96.50.Ry