Piyanate Chuychai

STATISTICAL ANALYSIS OF DISCONTINUITIES IN SOLAR WIND ACE DATA AND COMPARISON WITH INTERMITTENT MHD TURBULENCE

The comparison between Advanced Composition Explorer (ACE) solar wind data and simulations of magnetohydrodynamic (MHD) turbulence shows a good agreement in the waiting-time analysis of magnetic field increments. Similarity between classical discontinuity identification and intermittency analysis suggests a dynamical connection between solar wind discontinuities and intermittent MHD turbulence. Probability distribution functions of increments in ACE data and in simulations reveal a robust structure consisting of small random currents, current cores, and intermittent current sheets. This adds to evidence that solar wind magnetic structures may emerge fast and locally from MHD turbulence.

Spectral Properties and Length Scales of Two-dimensional Magnetic Field Models

Two-dimensional (2D) models of magnetic field fluctuations and turbulence are widely used in space, astrophysical, and laboratory contexts. Here we discuss some general properties of such models and their observable power spectra.Whilethefieldlinerandomwalkinaone-dimensional(slab)modelisdeterminedbythecorrelationscale,for 2Dmodels,itischaracterizedbyadifferentlengthscale,theultrascale.Wediscusspropertiesofcorrelationscalesand ultrascales for 2D models and present a technique for determining an ultrascale from observations at a single spacecraft, demonstrating its accuracy for synthetic data. We also categorize how the form of the low-wavenumber spectrum affects the correlation scales and ultrascales, thus controlling the diffusion of magnetic field lines and charged test particle motion. Subject headingg diffusion — magnetic fields — turbulence

Separation of Magnetic Field Lines in Two-Component Turbulence

The problem of the separation of random magnetic field lines in collisionless astrophysical plasmas is closely related to the problem of the magnetic field line random walk and is highly relevant to the transport of charged particles in turbulent plasmas. In order to generalize treatments based on quasi-linear theory, here we examine the separation of nearby magnetic field lines by employing a nonperturbative technique based on the Corrsin independence hypothesis. Specifically, we consider the case of two-component turbulence in which the magnetic field fluctuations are a mixture of one-dimensional (slab) and two-dimensional ingredients, as a concrete example of anisotropic turbulence that provides a useful description of turbulence in the solar wind. We find that random field trajectories can separate in general through three regimes of the behavior of the running diffusion coefficient: slow diffusive separation, an intermediate regime of superdiffusion, and fast diffusive separation at large distances. These features are associated with the gradual, exponential divergence of field lines within islands of two-dimensional turbulence, followed by diffusive separation at long distances. The types of behavior are determined not by the Kubo number but rather a related ratio that takes the turbulence anisotropy into account. These results are confirmed by computer simulations. We discuss implications for space observations of energetic charged particles, including ‘‘dropouts’’ of solar energetic particles.

Random Walk of Magnetic Field Lines in Nonaxisymmetric Turbulence

The random walk of turbulent magnetic field lines strongly affects transport of energetic particles in astrophysical plasmas, but is not well understood for general configurations that lack rotational symmetry. Here we derive nonperturbative field-line diffusion coefficients for magnetic fluctuations that are nonaxisymmetric with respect to the mean magnetic field. We consider a superposition of slab plus two-dimensional fluctuations, a model that has proven useful in heliospheric studies. Two independent parameters are introduced to allow polarization of the slab component and stretching of the two-dimensional component. With the assumptions of homogeneity, the diffusion approximation, and Corrsins independence hypothesis, we derive two coupled biquadratic equations for the diffusion coefficients. The results and underlying assumptions are confirmed by numerical simulations. Special cases of interest include the counterintuitive results that enhanced fluctuations in one direction lead to decreased diffusion in the other direction, and that extreme nonaxisymmetry leads to diffusion coefficients proportional to the rms two-dimensional fluctuation.

Perpendicular Transport of Energetic Charged Particles in Nonaxisymmetric Two-Component Magnetic Turbulence

We examine energetic charged particle diffusion perpendicular to a mean magnetic field B0 due to turbulent fluctuations in a plasma, relaxing the common assumption of axisymmetry around B0 and varying the ratio of two fluctuation components, a slab component with parallel wavenumbers and a two-dimensional (2D) component with perpendicular wavenumbers. We perform computer simulations mostly for 80% 2D and 20% slab energy and a fluctuation amplitude on the order of B0. The nonlinear guiding center (NLGC) theory provides a reasonable description of asymptotic perpendicular diffusion as a function of the nonaxisymmetry and particle energy. These values areroughlyproportionaltotheparticlespeedtimesthefieldlinediffusioncoefficient,withaprefactorthatismuchlower than in the classical field line random walk model of particle diffusion. NLGC predicts a prefactor in closer agreement with simulations. Next we consider extreme fluctuation anisotropy and the approach to reduced dimensionality. For 99% slab fluctuation energy, field line trajectories are diffusive, but the particle motion is subdiffusive. For 99% 2D fluctuation energy, both field lines and particle motions are initially subdiffusive and then diffusive, but NLGC gives unreliable results. The time dependence of the running particle diffusion coefficient shows that in all cases asymptotic diffusionisprecededbyfreestreamingandsubdiffusion,but thelatterdiffersfromstandardcompoundsubdiffusion.We can model the time profiles in terms of a decaying negative correlation of the perpendicular velocity due to the possibility of backtracking along magnetic field lines.

INTERPLANETARY MAGNETIC TAYLOR MICROSCALE AND IMPLICATIONS FOR PLASMA DISSIPATION

The Taylor microscale, a measure of mean square spatial derivatives, is evaluated for interplanetary magnetic field fluctuations from single- and multiple-point data using Cluster and ACE spacecraft data. The Taylor scale is compared to the measured inner scale, which for hydrodynamics would correspond to the Kolmogorov scale. The results are not consistent with dissipation of the hydrodynamic type, and indicate that solar wind dissipation involves kinetic plasma physics at both proton and electron scales.

Trapping and Diffusive Escape of Field Lines in Two-Component Magnetic Turbulence

Recent studies have shown that transport along magnetic field lines in turbulent plasmas admits a surprising degree of persistent trapping due to small-scale topological structures. This underlies the partial filamentation of magnetic connection from small regions of the solar corona to Earth orbit, as indicated by the observed dropouts (i.e., inhomogeneity and sharp gradients) of solar energetic particles. We explain the persistence of such topological trapping using a two-component model of magnetic turbulence with slab and two-dimensional (2D) fluctuations, which has provided a useful description of transport phenomena in the solar wind. In the presence of slab turbulence, the diffusive escape of field lines from 2D orbits is suppressed by either a strong or an irregular 2D field. For slab turbulence superposed on a 2D field with a single, circular island, we present an analytic theory, confirmed by numerical simulations, for the trapping length and its dependence on various parameters. For a turbulent 2D+slab field, we find that the filamentation of magnetic connectivity to the source is sharply delineated by local trapping boundaries, defined by a local maximum in the mean squared field along the 2D orbit, because of a similar suppression effect. We provide a quasi-linear theory for field-line diffusion in a turbulent 2D+slab field, which indicates that irregularity of the 2D orbit enhances the suppression of slab diffusion. The theory is confirmed by computer simulations. These concepts provide a physical explanation of the persistence and sharpness of dropouts of solar energetic particles at Earth orbit.

Suppressed Diffusive Escape of Topologically Trapped Magnetic Field Lines

Many processes in astrophysical plasmas are directly related to magnetic connection in the presence of turbulent fluctuations. Even statistically homogeneous turbulence can contain closed topological structures that inhibit otherwise random transport of field line trajectories, thus temporarily trapping certain trajectories. When a coherent random field perturbation is added, the trapped field lines can escape diffusively but at a suppressed rate that is much lower than what would be estimated based on the perturbation field alone. Here we demonstrate both trapping and escape, and show, using a novel quasi-linear theory, how to compute the suppressed diffusion that affects the escape from the trapping structure. The effect is relevant to understanding filamentary magnetic connection in interplanetary space and the observed dropouts in moderately energetic particles from impulsive solar flares. Expressed here in terms of a magnetic field line random walk, this phenomenon also has analogies in a broad range of dynamical systems that evolve as an incompressible flow in phase space with a coherent perturbation.

Charged particle diffusion in isotropic random magnetic fields

The investigation of the diffusive transport of charged particles in a turbulent magnetic field remains a subject of considerable interest. Research has most frequently concentrated on determining the diffusion coefficient in the presence of a mean magnetic field. Here we consider diffusion of charged particles in fully three-dimensional isotropic turbulent magnetic fields with no mean field, which may be pertinent to many astrophysical situations. We identify different ranges of particle energy depending upon the ratio of the Larmor radius of the charged particle to the characteristic outer length scale of the turbulence. Two different theoretical models are proposed to calculate the diffusion coefficient, each applicable to a distinct range of particle energies. The theoretical results are compared with those from computer simulations, showing good agreement.

Simulations of Lateral Transport and Dropout Structure of Energetic Particles from Impulsive Solar Flares

We simulate trajectories of energetic particles from impulsive solar flares for 2D+slab models of magnetic turbulence in spherical geometry to study dropout features, i.e., sharp, repeated changes in the particle density. Among random-phase realizations of two-dimensional (2D) turbulence, a spherical harmonic expansion can generate homogeneous turbulence over a sphere, but a 2D fast Fourier transform (FFT) locally mapped onto the lateral coordinates in the region of interest is much faster computationally, and we show that the results are qualitatively similar. We then use the 2D FFT field as input to a 2D MHD simulation, which dynamically generates realistic features of turbulence such as coherent structures. The magnetic field lines and particles spread nondiffusively (ballistically) to a patchy distribution reaching up to 25 degrees from the injection longitude and latitude at