Damping of nonlinear magneto-acoustic waves

1978 ◽  
Vol 20 (1) ◽  
pp. 125-136 ◽  
Author(s):  
G. L. Kalra ◽  
V. B. Bhatia
Keyword(s):  
Electrical Conductivity ◽  
Numerical Investigation ◽  
Acoustic Waves ◽  
Finite Amplitude ◽  
Turbulent Plasma ◽  
Mhd Equations ◽  
Space Plasmas ◽  
Fast Mode ◽  
Mode Propagation

Nonlinear magneto-acoustic waves in a turbulent plasma are simulated by collisional MHD equations. Damping of these waves due to electrical conductivity arising from micro-instabilities and collisional viscosity are analyzed. Numerical investigation of competing effects due to non-linearity and dissipation has been carried out. It is found that finite amplitude perturbation leads to the formation of a shock in both the slow and the fast mode propagation. Collisional viscosity plays an important role in the damping of nonlinear magneto-acoustic waves in the astrophysical and space plasmas.

Evolution of current sheets following the onset of enhanced resistivity

1982 ◽  
Vol 27 (1) ◽  
pp. 157-176 ◽  
Author(s):  
T. G. Forbes ◽  
E. R. Priest ◽  
A. W. Hood
Keyword(s):  
Current Sheet ◽  
Acoustic Waves ◽  
Wave Speed ◽  
Mhd Equations ◽  
Fast Mode ◽  
Current Sheets ◽  
Flow Speeds ◽  
Sheet Width ◽  
Flux Model ◽  
A Current

An important aspect of pre-flare current sheets in the solar atmosphere is the sudden enhancement of the effective electrical resistivity in the sheet due to the onset of a plasma micro-instability. Numerical and analytical solutions to the isothermal MHD equations are here presented that describe the evolution of a current sheet subsequent to such an enhancement in the resistivity. The solutions show that, if the initial width of the current sheet is less than the acoustic-diffusion length obtained by dividing the resistivity by the sound speed, then isomagnetic shocks are formed. These shocks propagate outward from the the centre of the current sheet and are transformed into fast-mode magneto-acoustic waves when they reach the edges of the current sheet. The fast-mode waves thus formed continue to propagate outward beyond the confines of the current sheet. In contrast to a previous study by Cheng, the present solutions demonstrate that flow speeds several times greater than the local fast-mode wave speed can be produced if the plasma beta parameter and the initial sheet width are sufficiently small. The results may be relevant to the triggering of a solar flare, as in the emerging flux model of flares.

scholarly journals Inferring the chromospheric magnetic topology through waves

2007 ◽  
Vol 3 (S247) ◽  
pp. 78-81
Author(s):  
S. S. Hasan ◽  
O. Steiner ◽  
A. van Ballegooijen
Keyword(s):  
Magnetic Field ◽  
Wave Propagation ◽  
Energy Density ◽  
Acoustic Wave ◽  
Acoustic Waves ◽  
Phase Speed ◽  
Time Correlation ◽  
Propagation Time ◽  
Fast Mode ◽  
The Magnetic Field

AbstractThe aim of this work is to examine the hypothesis that the wave propagation time in the solar atmosphere can be used to infer the magnetic topography in the chromosphere as suggested by Finsterle et al. (2004). We do this by using an extension of our earlier 2-D MHD work on the interaction of acoustic waves with a flux sheet. It is well known that these waves undergo mode transformation due to the presence of a magnetic field which is particularly effective at the surface of equipartition between the magnetic and thermal energy density, the β = 1 surface. This transformation depends sensitively on the angle between the wave vector and the local field direction. At the β = 1 interface, the wave that enters the flux sheet, (essentially the fast mode) has a higher phase speed than the incident acoustic wave. A time correlation between wave motions in the non-magnetic and magnetic regions could therefore provide a powerful diagnostic for mapping the magnetic field in the chromospheric network.

Parametric instabilities of finite-amplitude, circularly polarized Alfvén waves in an anisotropic plasma

1993 ◽  
Vol 49 (1) ◽  
pp. 29-39 ◽  
Author(s):  
Hiromitsu Hamabata
Keyword(s):  
Finite Amplitude ◽  
Alfven Waves ◽  
Mhd Equations ◽  
Alfvén Waves ◽  
Linear Perturbation ◽  
Plasma Parameters ◽  
Circularly Polarized ◽  
Pressure Anisotropy ◽  
Parametric Instabilities ◽  
Fifth Order

A class of parametric instabilities of finite-amplitude, circularly polarized Alfvén waves in a plasma with pressure anisotropy is studied by application of the CGL equations. A linear perturbation analysis is used to find the dispersion relation governing the instabilities, which is a fifth-order polynomial and is solved numerically. A large-amplitude, circularly polarized wave is unstable with respect to decay into three waves: one sound-like wave and two side-band Alfvén-like waves. It is found that, in addition to the decay instability, two new instabilities that are absent in the framework of the MHD equations can occur, depending on the plasma parameters.

scholarly journals Subcritical Instabilities in Neutral Fluids and Plasmas

Fluids ◽  
2018 ◽  
Vol 3 (4) ◽  
pp. 89 ◽  
Author(s):  
Maxime Lesur ◽  
Julien Médina ◽  
Makoto Sasaki ◽  
Akihiro Shimizu
Keyword(s):  
Steady State ◽  
Random Noise ◽  
Finite Amplitude ◽  
Analytical Models ◽  
Space Plasmas ◽  
Fusion Plasmas ◽  
Potential Well ◽  
Small Perturbations ◽  
History Of ◽  
The 1960S

In neutral fluids and plasmas, the analysis of perturbations often starts with an inventory of linearly unstable modes. Then, the nonlinear steady-state is analyzed or predicted based on these linear modes. A crude analogy would be to base the study of a chair on how it responds to infinitesimaly small perturbations. One would conclude that the chair is stable at all frequencies, and cannot fall down. Of course, a chair falls down if subjected to finite-amplitude perturbations. Similarly, waves and wave-like structures in neutral fluids and plasmas can be triggered even though they are linearly stable. These subcritical instabilities are dormant until an interaction, a drive, a forcing, or random noise pushes their amplitude above some threshold. Investigating their onset conditions requires nonlinear calculations. Subcritical instabilities are ubiquitous in neutral fluids and plasmas. In plasmas, subcritical instabilities have been investigated based on analytical models and numerical simulations since the 1960s. More recently, they have been measured in laboratory and space plasmas, albeit not always directly. The topic could benefit from the much longer and richer history of subcritical instability and transition to subcritical turbulence in neutral fluids. In this tutorial introduction, we describe the fundamental aspects of subcritical instabilities in plasmas, based on systems of increasing complexity, from simple examples of a point-mass in a potential well or a box on a table, to turbulence and instabilities in neutral fluids, and finally, to modern applications in magnetized toroidal fusion plasmas.

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