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  • 1
    Publication Date: 2011-08-24
    Description: The growth of parametric instabilities, which may lead to the development of a turbulent cascade, is studied using an MHD code that permits nonlinear couplings in the parallel direction to the ambient magnetic field and one perpendicular direction. Compressibility is included in the form of a polytropic equation of state. Parametric instabilities associated with a parallel-propagating decay instability are found to dominate the low-beta case. An obliquely propagating filamentationlike instability dominates the high-beta case. The nonlinear growth of the nth harmonic of a daughter wave growing as a factor of n times the fundamental's growth rate is found in both cases. Nonlinear saturation is caused by the parallel decay instability in the low-beta case and by the oblique filamentationlike instability in the high-beta case.
    Keywords: PLASMA PHYSICS
    Type: Journal of Geophysical Research (ISSN 0148-0227); 98; A9; p. 15,561-15,570.
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  • 2
    Publication Date: 2011-08-24
    Description: The nonlinear dynamics following saturation of the parametric instabilities of a monochromatic field-aligned large-amplitude circularly polarized Alfven wave is investigated via direct numerical simulation in the case of high plasma beta and no wave dispersion. The magnetohydrodynamic (MHD) code permits nonlinear couplings in the parallel direction to the ambient magnetic field and one perpendicular direction. Compressibility is included in the form of a polytropic equation of state. Turbulent cascades develop after saturation of two coupled oblique three-wave parametric instabilities; one of which is an oblique filamentationlike instability reported earlier. Remnants of the parametric processes, as well as of the original Alfven pump wave, persist during late nonlinear times. Nearly incompressible MHD features such as spectral anisotropies appear as well.
    Keywords: PLASMA PHYSICS
    Type: Journal of Geophysical Research (ISSN 0148-0227); 99; A10; p. 19,289-19,300
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  • 3
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    In:  Other Sources
    Publication Date: 2011-08-19
    Description: The stability of the super-Alfvenic flow of a two-fluid plasma model with respect to the Mach number and the angle between the flow direction and the magnetic field is investigated. It is found that, in general, a large scale chaotic region develops around the initial equilibrium of the laminar flow when the Mach number exceeds a certain threshold value. After reaching a maximum the size of this region begins shrinking and goes to zero as the Mach number tends to infinity. As a result high Mach number flows in time independent astrophysical plasmas may lead to the formation of 'quasi-shocks' in the presence of little or no dissipation.
    Keywords: PLASMA PHYSICS
    Type: Astronomy and Astrophysics (ISSN 0004-6361); 148; 1, Ju
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  • 4
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    Publication Date: 2011-08-19
    Description: An investigation is conducted on how low-frequency MHD oscillations in a warm plasma may undergo a transition from a coherent state to one of turbulence. A driven/dissipative derivative nonlinear Schroedinger equation is derived from the fluid equations. The time evolution of an arbitrary spectrum of waves is analyzed in the case where one k-mode is unstable, with the rest damped. It is found that the transition from order to chaos in the driven/dissipative system is correlated with the existence or absence of 'breathing' solitons in the associated conservative system.
    Keywords: PLASMA PHYSICS
    Type: Physics of Fluids (ISSN 0031-9171); 30; 1371-138
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  • 5
    Publication Date: 2011-08-24
    Description: The nature of turbulent cascades arising from the parametric instabilities of a monochromatic field-aligned large-amplitude circularly polarized Alfven wave is investigated via direct numerical simulation for the case of low plasma Beta and no wave dispersion. The magnetohydrodynamic code permits nonlinear couplings in the parallel direction to the ambient magnetic field and one perpendicular direction. Compressibility is included in the form of a polytropic equation of state. Anisotropic turbulent cascades, similar to those found in early incompressible two-dimensional simulations, occur after nonlinear saturation of the parallel propagating decay instability. The turbulent spectrum can be divided into three regimes: the lowest wave numbers are dominated by lower sideband remnants of the parametric process, intermediate wave numbers display nearly incompressible dynamics, and the highest wave numbers are dominated by acoustic turbulence.
    Keywords: PLASMA PHYSICS
    Type: Journal of Geophysical Research (ISSN 0148-0227); 99; A7; p. 13,351-13,362
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  • 6
    Publication Date: 2019-07-12
    Description: The compressible extensions of time asymptotic relaxation states of incompressible two-dimensional magnetohydrodynamic turbulence are studied. A polytropic equation of state is used with viscous and resistive dissipation. The incompressible case is known to allow three distinct time asymptotic types of behavior: magnetic energy dominated relaxation, kinetic energy dominated relaxation, and cross helicity dominated relaxation. At low Mach numbers the incompressible scenario is reproducible from the compressible simulations, and compressibility plays only a secondary role. At moderate, but still subsonic, Mach numbers the distinct incompressible processes are still recognizable, but strong compressibility features dominate the high-wave-number regime of several simulations. In particular, the magnetic and kinetic energy dominated simulations display regions of strong acoustic turbulence near the dissipation scale.
    Keywords: PLASMA PHYSICS
    Type: Physics of Fluids B (ISSN 0899-8221); 2; 1520-153
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  • 7
    Publication Date: 2019-07-12
    Description: The paper presents a series of incompressible two-dimensional simulations of driven/dissipative MHD turbulence where the amount of correlation between the kinetic and magnetic forcing is regulated, thereby controlling the amount of cross helicity injection. It is shown that correlated forcing provides a strong source of magnetofluid cross helicity. The clear appearance of the 'minority species effect,' which is the most striking and systematic effect during this driven dynamic alignment process, is discussed.
    Keywords: PLASMA PHYSICS
    Type: Physics of Fluids (ISSN 0031-9171); 31; 2171-218
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