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824 results on '"Ruderman, M."'

123. Phase mixing of Alfvén waves in axisymmetric non-reflective magnetic plasma configurations.

Author

Petrukhin, N. S., Ruderman, M. S., and Shurgalina, E. G.

Subjects
*PLASMA Alfven waves, *MAGNETIC fields, *PLASMA waves, *WAVE energy, *VISCOSITY, *DAMPING (Mechanics)
Abstract

We study damping of phase-mixed Alfvén waves propagating in non-reflective axisymmetric magnetic plasma configurations. We derive the general equation describing the attenuation of the Alfvén wave amplitude. Then we applied the general theory to a particular case with the exponentially divergent magnetic field lines. The condition that the configuration is nonreflective determines the variation of the plasma density along the magnetic field lines. The density profiles exponentially decreasing with the height are not among non-reflective density profiles. However, we managed to find non-reflective profiles that fairly well approximate exponentially decreasing density.We calculate the variation of the total wave energy flux with the height for various values of shear viscosity. We found that to have a substantial amount of wave energy dissipated at the lower corona, one needs to increase shear viscosity by seven orders of magnitude in comparison with the value given by the classical plasma theory. An important result that we obtained is that the efficiency of the wave damping strongly depends on the density variation with the height. The stronger the density decrease, the weaker the wave damping is. On the basis of this result, we suggested a physical explanation of the phenomenon of the enhanced wave damping in equilibrium configurations with exponentially diverging magnetic field lines. [ABSTRACT FROM AUTHOR]

Published
2018
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134. Rayleigh-Taylor instabilities with sheared magnetic fields in partially ionised plasmas.

Author

Ruderman, M. S., Ballai, I., Khomenko, E., and Collados, M.

Subjects
*SOLAR magnetic fields, *RAYLEIGH-Taylor instability, *SOLAR corona, *WAVENUMBER, *PLASMA flow
Abstract

Aims. In the present study we investigate the nature of the magnetic Rayleigh-Taylor instability appearing at a tangential discontinuity in a partially ionised plasma when the effect of magnetic shear is taken into account. Methods. The partially ionised character of the plasma is described by the ambipolar diffusion in the induction equation. The dynamics of the plasma is investigated in a single-fluid approximation. After matching the solutions on both sides of the interface we derive a dispersion equation and calculate the instability increment using analytical methods for particular cases of parameters, and numerical investigation for a wide range of parameters. Results. We calculated the dependence of the instability increment on the perturbation wavenumber. We also calculated the dependence of the maximum instability increment on the shear angle of the magnetic field for various values of the ionisation degree. Conclusions. Our results show that the Rayleigh-Taylor instability becomes sensitive to the degree of plasma ionisation only for plasmas with small values of plasma beta and in a very weakly ionised state. Perturbations are unstable only for those wavenumbers that are below a cut-off value. [ABSTRACT FROM AUTHOR]

Published
2018
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145. Non-axisymmetric oscillations of stratified coronal magnetic loops with elliptical cross-sections

Author

Morton, R. J., Ruderman, M. S., Morton, R. J., and Ruderman, M. S.

Abstract

We study non-axisymmetric oscillations of a straight magnetic tube with an elliptic cross-section and density varying along the tube. The governing equations for kink and fluting modes in the thin tube approximation are derived. We found that there are two kink modes, polarised along the large and small axes of the elliptic cross-section. We have shown that the ratio of frequencies of the first overtone and fundamental harmonic is the same for both kink modes and independent of the ratio of the ellipse axes. On the basis of this result we concluded that the estimates of the atmospheric scale height obtained using simultaneous observations of the fundamental harmonic and first overtone of the coronal loop kink oscillations are independent of the ellipticity of the loop cross-section.

Published
2010
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147. On the validity of nonlinear Alfven resonance in space plasmas

Author

Clack, C. T. M., Ballai, I., Ruderman, M. S., Clack, C. T. M., Ballai, I., and Ruderman, M. S.

Abstract

In the approximation of linear dissipative magnetohydrodynamics (MHD) it can be shown that driven MHD waves in magnetic plasmas with high Reynolds number exhibit a near resonant behaviour if the frequency of the wave becomes equal to the local Alfven (or slow) frequency of a magnetic surface. This near resonant behaviour is confined to a thin region, known as the dissipative layer, which embraces the resonant magnetic surface. Although driven MHD waves have small dimensionless amplitude far away from the resonant surface, this near-resonant behaviour in the dissipative layer may cause a breakdown of linear theory. Our aim is to study the nonlinear effects in the Alfven dissipative layer. In the present paper, the method of simplified matched asymptotic expansions developed for nonlinear slow resonant waves is used to describe nonlinear effects inside the Alfven dissipative layer. The nonlinear corrections to resonant waves in the Alfven dissipative layer are derived and it is proved that at the Alfven resonance (with isotropic/anisotropic dissipation) wave dynamics can be described by the linear theory with great accuracy., Comment: A&A accepted for publication

Published
2008
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148. On the steady state of nonlinear quasiresonant Alfven oscillations in one-dimensional magnetic cavity

Author

NOCERA L and RUDERMAN M S

Subjects
solar atmosphere, flux tubes, forced hydromagnetic cavity, Calcium-K grains, resonance absorption
Abstract

We study the steady state of nonlinear, small-amplitude, quasiresonant Alfven oscillations in a homogeneous dissipative hydromagnetic cavity which is forced by the shear motion of its boundaries. It is shown that, even in the case of strong nonlinearity, these oscillations can be represented, to leading order, by a sum of two solutions in the form of oppositely propagating waves with permanent shapes. An infinite set of nonlinear equations for the Fourier coefficients of these solutions is derived which, in general, admits multiple solutions, depending on the re-scaled total Reynolds number, R, and mistuning, Delta, between the frequency of the boundary forcing and the first Alfven eigenmode of the cavity. Two types of solutions are found. On the one hand, low-modal solutions set in over the entire parameter range studied, which can be represented, with a remarkable accuracy, by very few Fourier modes even at very large R. For a fixed a the time-averaged energy, epsilon, that can be stored in the cavity is saturated, as R increases, to a value which is approximately proportional to epsilon(2), epsilon(3) << 1 being the Alfven Mach number of the boundary motions. The time-averaged absorbed power (the Poynting flux S) scales as 1/R. For suitable values of R and a catastrophic transitions occur between these solutions, in which the average power released scales as R, provided R < epsilon(-1/2). The second type of solutions sets in for a narrow window of Delta and develops large gradients (shocks) which need to be represented by many Fourier modes. For a fixed a the build-up of these gradients takes place starting from a low-modal solution in either a continuous way, by increasing R, or in a sudden catastrophic way as R becomes smaller than a critical value. Both epsilon and S are saturated as R is increased. It is suggested that both types of solutions can explain bright events in the solar atmosphere

Published
1998

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