Astronomy and Astrophysics – Astrophysics
Scientific paper
Jan 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995nyasa.773...44s&link_type=abstract
Waves in Astrophysics, vol. Volume 773, p. 44-54
Astronomy and Astrophysics
Astrophysics
1
Magnetohydrodynamic Turbulence, Magnetohydrodynamic Waves, Interstellar Matter, Electron Density (Concentration), Space Plasmas, Incompressibility, Velocity Distribution, Ionized Gases, Gas Density, Anisotropy, Interstellar Magnetic Fields
Scientific paper
Fluctuations of electron number density in the ionized interstellar medium (ISM) manifest themselves through the scintillation of small angular diameter radio sources. Studies of interstellar scintillation have established that the spectrum of electron density fluctuations is a power law on scales lambda in the range 109 cm less than lambda less than 1015 cm, perhaps even extending to scales as large as 100 pc. The rms density fluctuation is similar to the Kolmogorov law for velocity fluctuations in hydrodynamic turbulence. It is thought that such a power spectrum results from the action of underlying interstellar turbulence on the electrons. The elongation of the images of strongly scattered sources suggests that the turbulence is anisotropic. This is not surprising, since the interstellar magnetic field is the dominant source of stress on the relevant length scales. Will turbulence in a predominantly ionized, magnetized plasma result in a Kolmogorov-like cascade? Recent work strongly suggests that Alfvenic turbulence in the ionized ISM exhibits an anisotropic Kolmogorov cascade of energy to small spatial scales. The power spectrum is quasi-two-dimensional; moreover, the turbulence mixes electron density variations in precisely the right manner needed to match the interstellar spectrum. We idealize the ionized ISM as a homogeneous, completely ionized plasma which, in the undisturbed state, supports a uniform magnetic field. If the gas density is rho, purely magnetic stresses transport disturbances at the Alfven speed. The sound speed corresponds to (mainly) ionized hydrogen at approximately 8000 K. The relative importance of the elasticity of the gas to the magnetic field is characterized by the plasma. We assume that the equations of magnetohydrodynamics (MHD) provide a reasonable description of the medium. We imagine that this medium is stirred by a steady stochastic force that imparts rms velocities, with isotropic correlation on spatial scale L. By analogy with hydrodynamics, we expect the fluid to exhibit a steady, turbulent structure. In this paper, we shall try to give a physical picture of those aspects we understand reasonably well. As a first step toward simplifying the analysis, we begin with the Alfven wave. The velocity field in the shear Alfven wave is divergence-free, so we might learn something useful by studying the more tractable problem of incompressible MHD (IMHD) turbulence.
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