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B.G. Elmegreen (IBM)
Interstellar turbulence makes fractal structure in the gas density as a result of non-linear processes related to magnetic wave interactions and shocks. These structures can have high and low density regions with considerable contrast. While the high density regions are usually identified with interstellar clouds and clumps, the low density regions can have observable forms as well. We propose that the low density regions of turbulent fractals can be identified with the intercloud medium. They have a large total volume filling factor, given by the fractal dimension and the density contrast, that exceeds 80%-90%, both inside individual clouds that are fractal and in the general medium. The mean free path in this intercloud medium can be larger than the inverse cloud or clump spacing, because the clouds in a fractal are highly correlated and clustered together. As a result, the interclump mean free path in a molecular cloud can be comparable to the cloud radius, allowing great penetration to ambient uv light, and the intercloud mean free path in the general medium can exceed the gaseous scale height in the Galactic disk, allowing stray Ly continuum photons from midplane HII regions to reach and ionize the Galaxy halo. The distribution of density versus size in the ionized intercloud medium follows the prediction of this theory, according to a recent study by Berkhuijsen. Computer simulations of hierarchical structure in 2D MHD turbulence illustrate the properties of fractal holes.
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