Magnetized Space Plasmas Research Articles

Wave instabilities in an anisotropic magnetized space plasma

We study wave instability in an collisionless, rarefied hot plasma (e.g. solar wind or corona). We consider the anisotropy produced by the magnetic field, when the thermal gas pressures across and along the field become unequal. We apply the 16-moment transport equations (obtained from the Boltzmann-Vlasov kinetic equation) including the anisotropic thermal fluxes. The general dispersion relation for the incompressible wave modes is derived. It is shown that a new, more complex wave spectrum with stable and unstable behavior is possible, in contrast to the classic fire-hose modes obtained in terms of the 13-moment integrated equations.

Diagnosis of magnetic structures and intermittency in space-plasma turbulence using the technique of surrogate data

Intermittency is usually identified in turbulent flows as non-Gaussian tails of the probability density functions (PDFs) of the turbulent field derivatives. Here we investigate the role of phase coherence among the Fourier modes in creating intermittency in magnetized space plasmas using the technique of surrogate data. We apply the technique to two examples: (i) synthetic data and (ii) magnetic field fluctuations recorded in the terrestrial magnetosheath by the Cluster spacecraft. We use a set of four series of data, one observed and three surrogate, and their PDFs and moments (q < or = 4) as discriminating statistics. We show that the technique allows for detecting coherent structures and estimating their scales. We show furthermore that the phases, but not the amplitudes, create the non-Gaussian tails of the PDFs. We show also that the surrogate data used cannot account for asymmetries of the PDFs of the observed data. This enables us to confirm a scenario of turbulent cascade of mirror structures proposed in previous publications, by showing the existence of an approximately constant energy flux in the inertial range.

Theory for two-dimensional electron and ion Bernstein–Greene–Kruskal modes in a magnetized plasma

AbstractA theory for two-dimensional electron and ion Bernstein–Greene– Kruskal (BGK) modes in a magnetized space plasma is presented. The BGK modes are constructed using the energy and the canonical angular momentum of the particles, which are conserved in a cylindrically symmetric potential. The typical length scale of the BGK modes is of the same order or larger than the thermal gyroradius of the particles. The results are relevant for understanding the properties of observed localized structures in the Earth's magnetosphere and auroral zone, as well as in laboratory magnetoplasmas.

Transient dynamics of secondary radiation from an HF pumped magnetized space plasma

In order to systematically analyze the transient wave and radiation processes that are excited when a high-frequency (HF) radio wave is injected into a magnetized space plasma, we have measured the ...

Oblique propagation of electromagnetic waves in a kappa-Maxwellian plasma

Space plasmas are often observed to contain more particles in the high-energy tail than the usual Maxwellian distributions, and are well modeled by kappa distributions. The hybrid kappa-Maxwellian distribution and associated generalized plasma dispersion function ZκM were recently introduced to model magnetized space plasmas. The susceptibility tensor for a kappa-Maxwellian plasma component is derived, making use of ZκM. This enables one to make general studies of obliquely propagating electromagnetic waves in a magnetoplasma. The susceptibility and dielectric tensors reduce to the Maxwellian expressions in the limit κ→∞. As an illustration, the formalism is applied to the lower branch of the R mode and its off-parallel variant. For low κ values, low-wavenumber, low-frequency parallel whistler waves are shown to be stable, unlike the Maxwellian case, which is unstable if the perpendicular temperature exceeds the parallel temperature. A numerical study is made of the effects of the value of kappa, the propagation angle, and the temperature anisotropy ratio on dispersion and damping. The kappa-Maxwellian distribution with very low κ is found to be unstable in an overdense plasma near the electron-cyclotron frequency even when the parallel and perpendicular temperatures are equal, because of the anisotropy of the contours in velocity space.

The effects of inelastic collisions on waves in partially ionized plasma

An analytical description is presented for the excitation of electrostatic modes in partially ionized magnetized space and laboratory plasmas. The neutrals introduce some effects that are responsible for the creation of ions, which in a stationary plasma is balanced by a number of phenomena in which ions and electrons are lost. The behaviour of several modes is discussed, such as the ion acoustic, drift, ion cyclotron and the Farley–Buneman mode. The instability conditions are obtained, showing that inelastic collisions can either modify some of these modes or make them unstable. The formation of a global nonlinear structure in a spatially bounded laboratory plasma is discussed. The structure is experimentally observed, and an analytical model is presented showing that the charge exchange plays a decisive role in its formation.

A gravity‐driven electric current in the Earth's ionosphere identified in CHAMP satellite magnetic measurements

A gravity field acting on a collisionless, magnetized space plasma causes electrons and ions to drift into opposite directions, in addition to gyrating around the magnetic field lines. This sets up an electric current which flows perpendicular to the gravity and magnetic fields in the eastward direction. Here we present the first observational evidence for such a gravity‐driven current system in the Earth's low‐latitude ionosphere. Its magnetic field signal, although 10,000 times smaller than the ambient Earth's magnetic field, is clearly visible in CHAMP satellite magnetic measurements. We find a current ribbon of more than 50 kA, about 66° wide in latitude, which moves with the sun northward in summer and southward in winter. Correcting magnetic measurements for this current's signature should lead to a better agreement between low‐orbiting satellite and ground‐based observations.

Effects of Superthermal Particles on Waves in Magnetized Space Plasmas

Distributions with excess numbers of superthermal particles are common in space environments. They are well modelled by the isotropic kappa distribution, or, where magnetic effects are important, the kappa-Maxwellian. This paper presents a review of some studies of electrostatic and electromagnetic waves in such plasmas, based on the associated generalized plasma dispersion functions, Zκ and ZκM. In particular, the effects of low values of κ are considered, i.e. strongly accelerated distribution functions. Recently the full susceptibility tensor for oblique propagation of electromagnetic waves in a kappa-Maxwellian magnetoplasma has been established and has been applied to the study of whistler waves.

Simulation study of surfing acceleration in magnetized space plasmas

We present a numerical study of the surfing mechanism in which electrons are trapped in Bernstein–Greene–Kruskal (BGK) modes, and are accelerated across the magnetic field direction by the Lorentz force in magnetized space plasmas. The BGK modes are the product of an ion-beam Buneman instability that excites large-amplitude electrostatic upper-hybrid waves in the plasma. Our study, which is performed with particle-in-cell (PIC) and Vlasov codes, reveals the stability of the BGK mode as a function of the magnetic field strength and the ion beam speed. It is found that the surfing acceleration is more effective for a weaker magnetic field owing to the longer lifetime of the BGK modes. The importance of our investigation to electron acceleration in astrophysical environments has been emphasized.

Nonlinear compressional electromagnetic ion-cyclotron wavepackets in space plasmas

Abstract. The parametric coupling between large amplitude magnetic field-aligned circularly polarized electromagnetic ion-cyclotron (EMIC) waves and ponderomotively driven ion-acoustic perturbations in magnetized space plasmas is considered. A cubic nonlinear Schrödinger equation for the modulated EMIC wave envelope is derived, and then solved analytically. The modulated EMIC waves are found to be stable (unstable) against ion-acoustic density perturbations, in the subsonic (supersonic, respectively) case, and they may propagate as "supersonic bright" (`"subsonic dark", i.e. "black" or "grey")type envelope solitons, i.e. electric field pulses (holes, voids),associated with (co-propagating) density humps. Explicit bright and dark (black/grey) envelope excitation profiles are presented, and the relevance of our investigation to space plasmas is discussed.

Nonlinear effects associated with dispersive Alfvén waves in plasmas

Large amplitude Alfvén waves are frequently found in magnetized space and laboratory plasmas. Our objective here is to discuss the linear and nonlinear properties of dispersive Alfvén waves (DAWs) in a uniform magnetoplasma. We first consider finite frequency (ω/ωci) and ion gyroradius effects on inertial and kinetic Alfvén waves, where ωci is the ion gyrofrequency. Next, we focus on nonlinear effects caused by DAWs. Such effects include plasma density enhancement and depression by the Alfvén wave ponderomotive force, electron Joule heating by the thermal Alfvén wave force, the generation of zonal flows due to the shear Alfvén wave mode couplings as well as the formation of localized Alfvénic structures and Alfvénic vortices. The relevance of our investigation to the appearance of nonlinear Alfvén waves in the Earth's auroral acceleration region, in the solar corona and in the large plasma device at UCLA is discussed.

Theoretical and numerical studies of density modulated whistlers

Recently, observations from laboratory experiments, which are relevant to space observations as well, have conclusively revealed the amplitude modulation of whistlers by low‐frequency perturbations. Our objective here is to present theoretical and simulation studies of amplitude modulated whistler packets on account of their interaction with background low‐frequency density perturbations that are reinforced by the whistler ponderomotive force. Specifically, we show that nonlinear interactions between whistlers and finite amplitude density perturbations are governed by a nonlinear Schrödinger equation for the modulated whistlers, and a set of equations for arbitrary large amplitude density perturbations in the presence of the whistler ponderomotive force. The governing equations are solved numerically to show the existence of large scale density perturbations that are self‐consistently created by localized modulated whistler wavepackets. Our numerical results are found to be in good agreement with experimental results, as well as have relevance to observations from magnetized space plasmas.

3D electron‐acoustic solitary waves introduced by phase space electron vortices in magnetized space plasmas

We derive a multi‐dimensional nonlinear equation for electron‐acoustic waves in magnetized plasmas composed of stationary ions, magnetized cold electrons with a significant fraction of electron beams, and magnetized hot electrons which have a non‐isothermal vortex‐like distribution. We find three‐dimensional (3D) electron‐acoustic solitary waves which have a stronger nonlinearity (in comparison with that associated with the traditional harmonic generation) due to the trapping of hot electrons in the localized electron‐acoustic wave potential. The relevance of our investigation to nonlinear structures (e.g., the bipolar parallel electric fields) in the Earth's magnetosphere is discussed.

Solitary waves in the Earth's magnetosphere: Nonlinear stage of the lower‐hybrid Buneman instability

A drift‐kinetic theory for electron phase‐space vortices in magnetized space plasmas is formulated in the frequency range of lower‐hybrid waves excited by the Buneman instability in the presence of an electron beam. The nonlinear model accounts for the effects of the electron polarization, anisotropic electron temperature and ion mobility, and it provides a theoretical explanation for some of the electrostatic and weakly electromagnetic bipolar structures observed by the Cluster, Polar and FAST spacecrafts in the Earth's magnetosheath, magnetopause and in the auroral zone. Quasi‐3‐D electron holes have the form of either elongated cylinders oblique to the magnetic field, or spheroids.

Localized Kinetic Structures in Magnetized Plasmas

We present a kinetic description of localized potential structures in a magnetized plasma. It is shown that the Vlasov equation admits solutions in the form of non-Maxwellian particle distributions. The general form of these particle distributions is derived, and some special solutions are employed into Poisson's equation to obtain numerical solutions comprising localized potential humps and electron and ion density holes. The relevance of our investigation to magnetized space and laboratory plasmas is discussed.

Nonlinear model for electron phase‐space holes in magnetized space plasmas

We formulate a drift‐kinetic theory of electron phase‐space vortices in magnetized space plasmas. It provides the theoretical explanation for the spatial and temporal scales of the electrostatic bipolar structures that have been observed by the Geotail and FAST spacecrafts in Earth's magnetotail and in the auroral zone. Our new nonlinear model accounts for the effects of both the electron polarization and the anisotropic electron temperature. Several different types of quasi‐three‐dimensional electron holes are found in the form of either cylinders or ellipsoids. Although topologically different, they produce similar signals on the spacecraft under typical FAST and Geotail observational conditions and cannot be distinguished with the use of the existing scarce satellite observational data.

The dielectric tensor of simple-pole distribution functions in magnetized plasmas

The dielectric tensor of a magnetized plasma described by a simple-pole particle distribution function is presented. Such model distributions give a new tool for understanding waves in collisionless non-Maxwellian plasmas. These distributions can model high-energy tails of the distribution in a way that is not possible with Maxwellians. An advantage over Lorentzian distributions is that an expression for the elements of the dielectric tensor can be obtained that does not involve any integrals over the perpendicular velocity. As an example, a simple-pole model of a magnetized space plasma is studied numerically and it is found that particle populations with the same density and mean particle energy, but with somewhat different distribution functions, correspond to different propagation properties that should be observable.

An example of resonances, coherent structures and topological phase transitions - the origin of the low frequency broadband spectrum in the auroral zone

Abstract. We consider the phenomena of intermittent turbulence in magnetized space plasmas from the point of view of topological phase transitions involving the merging and interactions of anisotropic coherent structures. The stochastic behaviour of these coherent plasma structures can undergo complex changes as the dynamic system evolves, similar to those commonly observed in (first and second order) equilibrium phase transitions. When conditions are favourable, such topological entities can evolve into a state of forced and/or self-organized criticality (FSOC). As an example, we apply these ideas to the understanding of the origin of the commonly observed broadband power-law low frequency electric field spectral densities and the characteristic filamentary current structures in the auroral zone. The broadband turbulence can provide efficient resonant energization of the ionospheric oxygen ions.

Langmuir turbulence in moderately magnetized space plasmas*

Beam-driven Langmuir turbulence is studied in two moderately magnetized (Ωe≊ωe) space-plasma regimes: regions of the lower solar corona and the Earth’s auroral ionosphere. The turbulence is modeled using modified Zakharov equations, which are employed in two-dimensional numerical simulations. For coronal parameters, highly anisotropic coherent wave packets form and collapse when Ωe&amp;lt;ωe. By contrast, the turbulence is phase incoherent when Ωe≳ωe, as a result of change in the topology of the Langmuir dispersion relation. In the auroral ionosphere, intense Langmuir waves (up to 500 mV/m) have been measured, in conjunction with field-aligned electron streams and nonthermal electron tails. Approximate agreement with high-time-resolution electric-field measurements, is found in the simulations. However, because of strong damping on nonthermal electrons, wave collapse is inhibited, irrespective of the ordering of Ωe and ωe.