Density Fluctuation Spectrum of Solar Wind Turbulence between Ion and Electron Scales

We present a measurement of the spectral index of density fluctuations between ion and electron scales in solar wind turbulence using the EFI instrument on the ARTEMIS spacecraft. The mean spectral index at 1?AU was found to be -2.75±0.06, steeper than predictions for pure whistler or kinetic Alfvén wave turbulence but consistent with previous magnetic field measurements. The steep spectra are also consistent with expectations of increased intermittency or damping of some of the turbulent energy over this range of scales. Neither the spectral index nor the flattening of the density spectra before ion scales were found to depend on the proximity to the pressure anisotropy instability thresholds, suggesting that they are features inherent to the turbulent cascade.     

The combined toroidicity,ellipticity and triangularity effects on the energy deposition of Alfvén modes in pre-heated,low aspect ratio tokamaks

The combined plasma non-uniformity effects on the energy deposition of Alfvén waves launched by an external antenna in pre-heated spherical tokamaks are investigated. The following relevant physical processes are here possible: (a) the emergence of gaps in the shear Alfvén continuum spectrum and the generation of discrete global Alfvén eigenmodes with frequencies inside the gaps; (b) multi-wave interactions, interactions of gaps of the same kind (e.g., toroidicity induced) and of different kinds (toroidicity, ellipticity and triangularity induced) as well as of secondary order gaps arising when a pair of modes is coupled to one or more modes through other coupling parameters; (c) basic wave-plasma interactions as propagation, reflection, mode-conversion, tunneling and deposition. Thus, we solved numerically the full 2D wave equations for the vector and scalar potentials, using a quite general two-fluid resistive tensor-operator, without any geometrical limitations. The results obtained indicate the existence of antenna-launched wave characteristics for which the power is most efficiently coupled in outer regions of plasmas, which is of special interest for low aspect ratio tokamaks, e.g., for the generation of non-inductive current drive as well as for turbulence suppression and transport barriers formation.     

Twisted shear Alfvén waves with orbital angular momentum

It is shown that a dispersive shear Alfvén wave (DSAW) in a magnetized plasma can propagate as a twisted Alfvén vortex beam carrying orbital angular momentum (OAM). We obtain a wave equation from the generalized ion vorticity equation and the magnetic field-aligned electron momentum equation that couple the scalar and vector potentials of the DSAW. A twisted shear Alfvén vortex beam can trap and transport plasma particles and energy in magnetoplasmas, such as those in the Earth?s auroral zone, in the solar atmosphere, and in Large Plasma Device (LAPD) at University of California, Los Angeles.     

Alfvénic shock waves in a collisional magnetoplasma

Compressional Alfvénic shock waves in a cold collisional magnetoplasma are investigated. For this purpose, we use the hydrodynamic equations and Faraday?s law to derive the governing nonlinear equations for the compressional Alfvén waves. It is shown that the latter can appear in the form of Alfvénic shock waves.     

Decay of initial nonlinear Alfvén wave discontinuities in a collisionless plasma

A system of Whitham equations for the derivative nonlinear Schrödinger equation in the Riemann form was used to analyze possible types of decay of discontinuities which accompany the overturning of the simplest quasilongitudinal nonlinear Alfvén wave. A whole class of structures is found which are formed when the discontinuities decay into two simple collisionless shock waves without forming a plateau between them. Different types of decay of the initial discontinuities are considered for cases when the overturning of the Alfvén wave is modulationally stable.Checheno—Ingushk State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 110–117, July, 1994.     

Predictions and observations of low-shear beta-induced shear Alfvén–acoustic eigenmodes in toroidal plasmas

New global MHD eigenmode solutions arising in gaps in the low frequency Alfvén–acoustic continuum below the geodesic acoustic mode (GAM) frequency have been found numerically and have been used to explain relatively low frequency experimental signals seen in NSTX and JET tokamaks. These global eigenmodes, referred to here as Beta-induced Alfvén–Acoustic Eigenmodes (BAAE), exist in the low magnetic safety factor region near the extrema of the Alfvén–acoustic continuum. In accordance to the linear dispersion relations, the frequency of these modes shifts as the safety factor, q  , decreases. We show that BAAEs can be responsible for observations in JET plasmas at relatively low beta <2%$<2\mathrm{\%}$ as well as in NSTX plasmas at relatively high-beta >20%$>20\mathrm{\%}$. In contrast to the mostly electrostatic character of GAMs the new global modes also contain an electromagnetic (magnetic field line bending) component due to the Alfvén coupling, leading to wave phase velocities along the field line that are large compared to the sonic speed. Qualitative agreement between theoretical predictions and observations are found.     

Nonresonant and stochastic heating of ions by low-frequency wave

The present paper addresses the nonresonant and stochastic heating of minor ions by a low-frequency Alfvén wave in the solar wind. A new and whole physical mechanism that enables the heating of ions by a low-frequency parallel propagating wave with finite amplitude in a low beta plasma. The heating process has two stages: First, ions are picked up via low-frequency wave–ion nonresonant interaction and the ion energy gain depends on the ratio of the wave electric-field energy to background magnetic-field energy. Second, when the velocity approaches the threshold value, the stochastic heating is prominent. The stochastic heating of ions sustains until the average parallel speed is close to the wave velocity.     

Generating magnetic field pulses in the development of an explosive Alfvén wave instability in a flow-plasma system

The excitation of Alfvén radiation is studied when an explosive instability is developed in a heated gyrotropic plasma penetrated by a monovelocity flow of ionized gas. It is shown that magnetic field pulses are generated in the plasma under certain conditions. The amplitudes of the Alfvén wave harmonics are found and the conditions for stabilization of the explosion due to the nonlinear absorption associated with weakly damped Alfvén wave harmonics are specified.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 39, No. 8, pp. 1001–1006, August, 1996.This work was supported in part by the Russian Foundation for Fundamental Research under Grant No. 95-02-05255.     

Statistics of magnetic field fluctuations in a partially ionized space plasma

Voyager 1 and 2 data reveals that magnetic field fluctuations are compressive and exhibit a Gaussian distribution in the compressed heliosheath plasma, whereas they follow a lognormal distribution in a nearly incompressible supersonic solar wind plasma. To describe the evolution of magnetic field, we develop a nonlinear simulation model of a partially ionized plasma based on two-dimensional time-dependent multifluid model. Our model self-consistently describes solar wind plasma ions, electrons, neutrals and pickup ions. It is found from our simulations that the magnetic field evolution is governed by mode conversion process that leads to the suppression of vortical modes, whereas the compressive modes are amplified. An implication of the mode conversion process is to quench the Alfvénic interactions associated with the vortical motions. Consequently anisotropic cascades are reduced. This is accompanied by the amplification of compressional modes that tend to isotropize the plasma fluctuations and lead to a Gaussian distribution of the magnetic field.     

Integrability lost: Chaotic dynamics of classical strings on a confining holographic background

It is known that classical string dynamics on pure AdS₅×S⁵${\mathit{AdS}}_5\times S^5$ is integrable and plays an important role in solvability. This is a deep and central issue in holography. Here we investigate similar classical integrability for a more realistic confining background and provide a negative answer. The dynamics of a class of simple string configurations on AdS soliton background can be mapped to the dynamics of a set of non-linearly coupled oscillators. In a suitable limit of small fluctuations we discuss a quasi-periodic analytic solution of the system. Numerics indicates chaotic behavior as the fluctuations are not small. Integrability implies the existence of a regular foliation of the phase space by invariant manifolds. Our numerics shows how this nice foliation structure is eventually lost due to chaotic motion. We also verify a positive Lyapunov index for chaotic orbits. Our dynamics is roughly similar to other known non-integrable coupled oscillator systems like Hénon–Heiles equations.     

Multiple polarization of geodesic curvature induced modes

Dispersion relations for geodesic acoustic modes are derived by using the Grad hydrodynamic equations thereby reconciling long known but not previously explained discrepancy between the results of kinetic and fluid calculations. Extended fluid theory allows a simple analysis of mode polarization and coupling. A new type of electromagnetic modes induced by geodesic compressibility is predicted. These modes are related to Alfvén and geodesic acoustic modes. While a standard geodesic acoustic mode involves poloidally and toroidally symmetric perturbations of electrostatic potential (m=n=0) and the first poloidal side-bands of plasma pressure, new modes involve side-bands of the electrostatic and vector potential as well as pressure perturbations at zeroth and second harmonics. It is shown that there exist two different values of the adiabatic constant depending on the mode polarization. Both standard (electrostatic) geodesic acoustic modes and new electromagnetic modes involve finite perturbations of parallel viscosity, which modify an effective adiabatic (compressibility) index for a toroidal plasma.     

Redistribution of magnetospheric ions under the action of Miller force

Ponderomotive redistribution of ions along geomagnetic field lines is analyzed within the frame-work of a simple model of diffusion equilibrium of a multicomponent plasma. Miller force prevails in the case of Alfvén waves. This also takes place in the case of ion-cyclotron waves at the low-frequency limit. Qualitative analysis of the equations with allowance for satellite-based data on wave intensity shows significant redistribution of magnetospheric plasma due to Miller force.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 39, No. 4, pp. 464–471, April, 1996.This paper was supported in part by the Russian Foundation for Fundamental Research (Project Code 94-05-16173a).     

Kinetic simulations of magnetized turbulence in astrophysical plasmas

This Letter presents the first ab initio, fully electromagnetic, kinetic simulations of magnetized turbulence in a homogeneous, weakly collisional plasma at the scale of the ion Larmor radius (ion gyroscale). Magnetic- and electric-field energy spectra show a break at the ion gyroscale; the spectral slopes are consistent with scaling predictions for critically balanced turbulence of Alfvén waves above the ion gyroscale (spectral index -5/3) and of kinetic Alfvén waves below the ion gyroscale (spectral indices of -7/3 for magnetic and -1/3 for electric fluctuations). This behavior is also qualitatively consistent with in situ measurements of turbulence in the solar wind. Our findings support the hypothesis that the frequencies of turbulent fluctuations in the solar wind remain well below the ion cyclotron frequency both above and below the ion gyroscale.     

Quasiclassical approach to kinetic equations for superfluid 3hellum: General theory and application to the spin dynamics

Using the method of the quasiclassical Green function, we derive a set of kinetic equations which describe general nonequilibrium situations in the quasiclassical regime, i.e., when the external frequency and wave vector, ω and q are small compared to the atomic scale (ω ? μ, ∥ q ∥ ? pf. The equations consist of a Boltzmann equation for the quasiparticle distribution function, labeled by the energy and the direction of the momentum (particle representation), coupled to a time dependent Ginzburg-Landau equation for the order parameter. We discuss extensively the properties of these equations, and apply them to orbital and spin dynamics. Solving the Boltzmann equation in a well defined approximation, we are able to derive the expressions for the linewidths for all temperatures, with the correct identification of the phenomenological relaxation times. Furthermore, we discuss the connection between various relaxation times used in non-equilibrium situations, and we give a detailed comparison of the particle representation with the excitation representation which is used frequently in other work on non-equilibrium phenomena in superfluid ³He and in superconductors.     

S = −1 resonances in two meson-one baryon systems

We study the two meson-one baryon systems by solving Faddeev equations, using chiral dynamics. The calculations, carried out for the $\pi \overline{K} N$ system and its coupled channels for the case of strangeness = ?1, in the S-wave, lead to a dynamical generation of many strangeness = ?1 resonances in the 1500–1800?MeV region. While building the formalism, we found important cancellations between different sources of three-body forces.     

Acceleration of dust particles by low-frequency Alfvén waves

We investigate the efficiency of acceleration of charged dust particles by low-frequency Alfvén waves in nonlinear approximation. We show that the longitudinal acceleration of dust particles is proportional to the square of the soliton amplitude O(²|bm|), while the transversal acceleration is of O(|bm|). In the conditions of the interstellar medium the resulting velocity of dust particles can reach 0.3 to 1 km s⁻¹.     

Impact of trailing wake drag on the statistical properties and dynamics of finite-sized particle in turbulence

We study by means of an Eulerian-Lagrangian model the statistical properties of velocity and acceleration of a neutrally-buoyant finite-sized particle in a turbulent flow statistically homogeneous and isotropic. The particle equation of motion, besides added mass and steady Stokes drag, keeps into account the unsteady Stokes drag force-known as Basset-Boussinesq history force-and the non-Stokesian drag based on Schiller-Naumann parametrization, together with the finite-size Faxén corrections. We focus on the case of flow at low Taylor-Reynolds number, Re_λ?31, for which fully resolved numerical data which can be taken as a reference are available [Homann H., Bec J. Finite-size effects in the dynamics of neutrally buoyant particles in turbulent flow. J Fluid Mech 651 (2010) 81-91]. Remarkably, we show that while drag forces have always minor effects on the acceleration statistics, their role is important on the velocity behavior. We propose also that the scaling relations for the particle velocity variance as a function of its size, which have been first detected in fully resolved simulations, does not originate from inertial-scale properties of the background turbulent flow but it is likely to arise from the non-Stokesian component of the drag produced by the wake behind the particle. Furthermore, by means of comparison with fully resolved simulations, we show that the Faxén correction to the added mass has a dominant role in the particle acceleration statistics even for particles whose size attains the integral scale.     

Overturning of nonlinear Alfvén waves in a collisionless plasma

An analysis is made of the overturning of nonlinear Alfvén waves in a collisionless plasma. It is shown that overturning is followed by the appearance of a region which broadens with time and consists of two collisionless shock waves which can be joined at the point s²=1. If only one Riemann invariant changes in the region of the collisionless shock waves, the waves are simple. The structures of the collisionless shock waves are constructed for different initial conditions of the nonlinear wave. The Whitham averaging method is used for this purpose. Conditions are obtained which are similar to the Rankine-Hugoniot adiabats for passage through the collisionless shock waves. The effect of overturning one of the collisionless shock waves, involving the zeroing of the density at the soliton peak on its trailing edge, is treated as a bifurcation for which a discontinuity occurs in an analog of the hydrodynamic velocity and phase of the nonlinear Alfvén wave. The width of one of the collisionless shock waves decreases with an increase in the parametera which determines the magnitude of the field discontinuity when overturning occurs.L. N. Tolstoi State University, Checheno-Ingushk. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 62–67, January, 1993.     

Magnetohydrodynamics computations with lattice gas automata

Lattice gas automata have received considerable interest for the last several years and possibly may become a powerful numerical method for solving various partial differential equations and modeling different physical phenomena, because of their discrete and parallel nature and the capability of handling complicated boundaries. In this paper, we present recent studies on the lattice gas model for magnetohydrodynamics. The FHP-type lattice gas model has been extended to include a bidirectional random walk process, which allows well-defined statistical quantities, such as velocity and magnetic field, to be computed from the microscopic particle representation. The model incorporates a new sequential particle collision method to increase the range of useful Reynolds numbers in the model, an improvement that may also be of use in other lattice gas models. In the context of a Chapman-Enskog expansion, the model approximates the incompressible magnetic hydrodynamic equations in the limit of low Mach number and high. Simulation results presented here demonstrate the validity of the model for several basic problems, including sound wave and Alfvén wave propagation, and diffusive Kolmogoroff-type flows.     

A bridge between hyperspherical and integro-differential approaches to the many-body bound states

The solution of the Schrödinger equation can be obtained from the one of a system of coupled differential equations generated from the potential harmonic expansion of the bound-state wave function of a system of identical particles governed by two-body central interactions. It is shown that the system of coupled equations can be transformed into an equivalent integro-differential equation. For three bosons inS states this equation is identical to the Faddeev equation as written by Noyes. The integro-differential equations describing the triton for non-central realisticN-N forces are explicitly given.Laboratoire associé au C.N.R.S.