A. A. Chernyshov

Development of large eddy simulation for modeling of decaying compressible magnetohydrodynamic turbulence

The large eddy simulation technique is developed for the study of decaying compressible magnetohydrodynamic turbulence. In the present paper the obtained results of numerical computations for large eddy simulation are compared with the results of direct numerical simulation of three-dimensional compressible magnetohydrodynamic turbulence under various similarity parameters, namely, magnetic Reynolds numbers, hydrodynamic Reynolds numbers, and Mach numbers. The comparison of five subgrid-scale closures of large eddy simulation for the magnetohydrodynamic case is made. The comparison between large eddy simulation and direct numerical simulation is carried out regarding the time evolution of kinetic and magnetic energy, cross helicity, subgrid-scale and molecular dissipations for kinetic and magnetic energy, turbulent intensities and quantities that describe anisotropy of flow, that is, skewness and kurtosis of velocity and magnetic field. It is shown that some subgrid-scale models proposed in the paper prov...

Large-eddy simulation of magnetohydrodynamic turbulence in compressible fluid

In the present article, the large eddy simulation (LES) technique for the study of compressible magnetohydrodynamic turbulence is developed. The filtered equations of magnetohydrodynamics of compressible fluid are obtained with the use of a mass-weighted filtering procedure (Favre filtering). Favre-filtered equations for large-scale components of turbulence include subgrid-scale terms describing subgrid phenomena. Different models for closure of subgrid terms are suggested. In this work numerical simulation of filtered magnetohydrodynamic equations and an analysis of the received characteristics of turbulent flow is carried out. The obtained results of numerical computations for different LES models are compared with the results of direct numerical simulation.

Branches of electrostatic turbulence inside solitary plasma structures in the auroral ionosphere

The excitation of electrostatic turbulence inside space-observed solitary structures is a central topic of this exposition. Three representative solitary structures observed in the topside auroral ionosphere as large-amplitude nonlinear signatures in the electric field and magnetic-field-aligned current on the transverse scales of ∼102–103 m are evaluated by the theories of electrostatic wave generation in inhomogeneous background configurations. A quantitative analysis shows that the structures are, in general, effective in destabilizing the inhomogeneous energy-density-driven (IEDD) waves, as well as of the ion acoustic waves modified by a shear in the parallel drift of ions. It is demonstrated that the dominating branch of the electrostatic turbulence is determined by the interplay of various driving sources inside a particular solitary structure. The sources do not generally act in unison, so that their common effect may be inhibiting for excitation of electrostatic waves of a certain type. In the pre...

Inhomogeneities of plasma density and electric field as sources of electrostatic turbulence in the auroral region

Inhomogeneities of plasma density and non-uniform electric fields are compared as possible sources of a sort of electrostatic ion cyclotron waves that can be identified with broadband extremely low frequency electrostatic turbulence in the topside auroral ionosphere. Such waves are excited by inhomogeneous energy-density-driven instability. To gain a deeper insight in generation of these waves, computational modeling is performed with various plasma parameters. It is demonstrated that inhomogeneities of plasma density can give rise to this instability even in the absence of electric fields. By using both satellite-observed and model spatial distributions of plasma density and electric field in our modeling, we show that specific details of the spatial distributions are of minor importance for the wave generation. The solutions of the nonlocal inhomogeneous energy-density-driven dispersion relation are investigated for various ion-to-electron temperature ratios and directions of wave propagation. The relev...

Modeling of compressible magnetohydrodynamic turbulence in electrically and heat conducting fluid using large eddy simulation

Many electrically and heat conducting fluid flows cannot be described within the framework of incompressible medium or by compressible magnetohydrodynamic equations on the assumption of polytropic (or adiabatic) process. Therefore, we consider a heat conducting compressible fluid with the use of an energy equation. Application of large eddy simulation approach to heat conducting compressible magnetohydrodynamics is considered. The system of the filtered magnetohydrodynamic equations with the total energy equation using the mass-weighted filtering procedure has been obtained. It is shown that novel subgrid-scale terms arise in the Favre-filtered equations due to the presence of a magnetic field in the total energy equation. Parametrizations of these extra terms are developed. In order to derive these subgrid-scale terms, we use an approach based on generalized central moments. Computations at various Mach numbers are made for decaying compressible magnetohydrodynamic turbulence. The obtained numerical larg...

Subgrid-scale modeling of compressible magnetohydrodynamic turbulence in heat-conducting plasma

A large-eddy simulation (LES) approach for compressible magnetohydrodynamic (MHD) turbulence in heat-conducting plasma is developed for the first time. Subgrid-scale models for new terms appearing due to the presence of magnetic field are suggested. Results of modeling for decaying compressible MHD turbulence are presented. Comparison and testing with results obtained by direct numerical simulation are made. The efficiency of the developed LES technique for compressible MHD turbulence in heat-conducting plasma is shown.

Influence of inhomogeneities of the plasma density and electric field on the generation of electrostatic noise in the auroral zone

In order to study instabilities caused by inhomogeneities of the electric field and plasma density in the auroral zone, numerical algorithms are developed and numerical simulations are performed for different conditions in the background plasma. To this end, a nonlocal dispersion relation for a given type of wave is analyzed. It is shown that the dispersion relation has unstable solutions in a wide range of frequencies and wavenumbers. These solutions manifest themselves in satellite observations as a broadband spectrum of electrostatic perturbations. Two mechanisms of broadband noise generation related to the gradients of the density and electric field are compared.

Features of wave excitation of the electrostatic ion cyclotron type in the auroral ionosphere

Broadband electrostatic noise in the auroral ionosphere can be identified as a version of waves of an electrostatic ion cyclotron type, excited by plasma instability resulting from an inhomogeneous distribution of wave energy density. Broadband waves are generated due to both electric field inhomogeneities and plasma density inhomogeneities. The effect of the form of the distribution of electric field and plasma density inhomogeneities on the excitation of instabilities is studied. Also there is shown the role of the characteristic scale of inhomogeneities in the generation of electrostatic ion cyclotron waves due to the development of instability of this kind. The study of these issues, which are important for understanding the processes in the auroral region, is the subject of this paper. The work presents also a comparison of numerical results obtained using both satellite data and model approximations.

Forced turbulence in large-eddy simulation of compressible magnetohydrodynamic turbulence

We present the large-eddy simulation method for studying forced compressible magnetohydrodynamic turbulence. The proposed method is based on a solution of the filtered basic equations of magnetohydrodynamics by finite-difference methods and on a linear representation of the driving forces in the momentum conservation equation and the magnetic induction equation. These forces supply the production of kinetic and magnetic energies. The emphasis is placed upon the important, and not investigated, question about the ability of the large-eddy simulation approach to reproduce Kolmogorov and Iroshnikov–Kraichnan scale-invariant spectra in compressible magnetohydrodynamic flows.

Reverse flow events and small-scale effects in the cusp ionosphere

We report in-situ measurements of plasma irregularities associated with a reverse flow event (RFE) in the cusp F region ionosphere. The Investigation of Cusp Irregularities 3 (ICI-3) sounding rocket, while flying through a RFE, encountered several regions with density irregularities down to meter-scales. We address in detail the region with the most intense small-scale fluctuations in both the density and in the AC electric field, which were observed on the equatorward edge of an flow shear, and coincided with a double-humped jet of fast flow. Due to its long-wavelength and low-frequency character, the Kelvin-Helmholtz instability (KHI) alone cannot be the source of the observed irregularities. Using ICI-3 data as inputs we perform a numerical stability analysis of the inhomogeneous energy-density-driven instability (IEDDI) and demonstrate that it can excite electrostatic ion cyclotron waves in a wide range of wavenumbers and frequencies for the electric field configuration observed in that region, which can give rise to the observed small-scale turbulence. The IEDDI can seed as a secondary process on steepened vortices created by a primary KHI. Such an interplay between macro-processes and micro-processes could be an important mechanism for ion heating in relation to RFEs.