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The present paper is devoted to the theoretical and numerical analysis of the spontaneously emitted electromagnetic fluctuations characterized by quasi-parallel wave vectors relative to the ambient magnetic field. The formulation is based upon the Klimontovich plasma kinetic theory. The comparative study is carried out between the spontaneously emitted field fluctuation spectrum constructed on the basis of a single Maxellian velocity distribution function (VDF) and the spectrum that arises from multi-component electron VDFs similar to those found in the solar wind. Typical solar wind electron VDF is composed of a Gaussian core and kappa distributions of halo and super-halo components. Of these, the halo and super-halo populations represent tenuous but energetic components. It is found that the energetic electrons make important contributions to the total emission spectrum. It is also found that the halo electrons are largely responsible for the emission spectrum in the whistler frequency range, whereas the more energetic super-halo electrons emit quasi-longitudinal fluctuations in the Langmuir frequency range, thus validating the recent quasi-steady state model of the solar wind electrons put forth by the present authors [Kim et al., Astrophys. J. 806, 32 (2015); Yoon et al., Astrophys. J. 812, 169 (2015)]. [ABSTRACT FROM AUTHOR]

Magnetic fields are an essential component of a plasma. In many astrophysical, solar, magnetospheric, and laboratory situations the magnetic field in the plasma can be very dynamic and form highly complex structures. One approach to unraveling these structures is to determine the magnetic skeleton of the field, a set of topological features that divide the magnetic field into topologically distinct domains. In general, the features of the magnetic skeleton are difficult to locate, in particular those given by numerical experiments. In this paper, we propose a new set of tools to find the skeleton of general magnetic fields including null points, spines, separatrix surfaces, and separators. This set of tools is found to be considerably better at finding the skeleton than the currently favored methods used in magnetohydrodynamics. [ABSTRACT FROM AUTHOR]

This article presents an approach to solving a special Fredholm-type integral equation of the first kind with a particular kernel containing a modified Bessel function for applications in plasma physics. From the physical point of view, the problem was defined by Bissell and Johnson (B&J) [Phys. Fluids 30, 779 (1987)] as a task to find the potential profile and the ion velocity distribution function in a plane-parallel discharge with a Maxwellian ion source. The B&J model is a generalization of the well-known Tonks–Langmuir (T&L) [Phys. Rev. 34, 876 (1929)] discharge model characterized by a “cold” ion source. Unlike the T&L model, which can be readily solved analytically, attempts to solve the B&J model with a “warm” ion source have been done only numerically. However, the validity of numerical solutions up to date remains constrained to a rather limited range of a crucial independent parameter of the B&J integral equation, which mathematically is the width of a Gaussian distribution and physically represents the ion temperature. It was solved only for moderately warm ion sources. This paper presents the exact numerical solution of the B&J model, which is valid without any restriction regarding the above-mentioned parameter. It is shown that the ion temperature is very different from the temperature of the ion source. The new results with high-temperature ion sources are not only of particular importance for understanding and describing the plasma-sheath boundary in fusion plasmas, but are of considerable interest for discharge problems in general. The eigenvalue of the problem, found analytically by Harrison and Thompson [Proc. Phys. Soc. 74, 145 (1959)] for the particular case of a cold ion source, is here extended to arbitrary ion-source temperatures. [ABSTRACT FROM AUTHOR]

The reversal mechanism of a dipole magnetic field generated by dynamo action in a rotating spherical shell is investigated by a three-dimensional nonlinear magnetohydrodynamic simulation as well as a linear stability analysis. The emphasis of the study is on understanding the relationship between dipole reversal and the symmetry properties of the dynamo solution. As a result, first, it is found that there is a threshold of the magnetic Prandtl number, below which the dipole field is never reversed, and above which the reversal occurs at irregular intervals like the paleomagnetic evolution of the geodynamo. Second, it is shown that the dynamo process responsible for the generation of a dipole field (called “a-dynamo” in this paper) consists only of the antimirror symmetric magnetic field and the mirror symmetric velocity field with respect to the equatorial plane. Third, it is found that the components of the opposite symmetry to the a-dynamo grow only during the polarity reversal events and quickly decay afterwards. This indicates that the dipole field reversal and the loss of equatorial symmetry are tightly connected. In fact, it is clearly demonstrated by numerical analyses that the a-dynamo process is linearly unstable for the perturbation of opposite symmetry when the magnetic Prandtl number exceeds the threshold for dipole reversal. Mode coupling between the longitudinal Fourier components plays a crucial role in creating the instability. Based on the above results, it is proposed that symmetry-breaking instability could be the mechanism for dipole field reversal in the geodynamo process. The energy conversion between components of different symmetry is also analyzed in the quasistable polarity phase and in the polarity reversal phase, respectively. [ABSTRACT FROM AUTHOR]

A general orbit-averaged guiding-center Fokker–Planck operator suitable for the numerical analysis of transport processes in axisymmetric magnetized plasmas is presented. The orbit-averaged guiding-center operator describes transport processes in a three-dimensional guiding-center invariant space: the orbit-averaged magnetic-flux invariant [formula], the minimum-B pitch-angle coordinate ξ0, and the momentum magnitude p. [ABSTRACT FROM AUTHOR]

An analytic expression for the stability threshold of linear tearing modes is derived. The magnetized plasma is described in terms of a standard viscoresistive magnetohydrodynamic model. The analytic derivation requires an extension of the standard layer equation that represents an approximation of the full model in the vicinity of the reconnecting layer. The analytic result is checked against numerical simulations, showing excellent agreement. [ABSTRACT FROM AUTHOR]

The conceptual development and implementation of a scalable nonlinear solver for the time-dependent three-dimensional (3D) compressible resistive magnetohydrodynamics model (MHD) is discussed. The approach is based on Jacobian-free Newton–Krylov technology, preconditioned with multigrid methods for algorithmic scalability. The key to the approach is the reformulation of the hyperbolic MHD system into a parabolic one, which is amenable to multigrid techniques. Such reformulation (parabolization) aims to render the modified system block diagonally dominant (unlike the original MHD system, which is diagonally submissive for implicit time steps Δt larger than the explicit Courant–Friedrichs–Lewy time step). The algorithm has been tested on a variety of 2D and 3D configurations which demonstrate its excellent algorithmic scalability properties. In particular, it is shown that, serially, the CPU time for a given simulation scales linearly with the number of unknowns, and that large CPU gains (∼30) are attainable with respect to an explicit approach even for moderate grids (256×256). In parallel, the algorithm features excellent scalability properties up to thousands of processors (4096) and millions of unknowns (∼1.3×108). [ABSTRACT FROM AUTHOR]

Four types of magnetohydrodynamic modes determine the overall stability of centrifugally confined plasmas: flute interchanges and the Kelvin-Helmholtz instability, in a low β system, and the magnetorotational instability (MRI) and the Parker instability, in a high β system. Analytic calculations and numerical simulations of velocity shear stabilization of flute interchanges are reviewed. The results showed strong evidence of stabilization. The Kelvin-Helmholtz instability is ideally stable if the generalized Rayleigh's inflexion criterion is satisfied. The physical mechanisms of both the MRI and the Parker instability are examined and an explanation of why the MRI mechanism is insufficient to destabilize the system while the Parker instability could occur is given. Numerical simulations of the nonlinear behavior of the Parker instability are presented. It is shown that clumping from the Parker instability could reinforce centrifugal confinement. [ABSTRACT FROM AUTHOR]