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Most of the baryonic matter in the Universe is undergoing some form of magnetohydrodynamical (MHD) turbulence. At the scale of a few parsecs, in the cool, approximately isothermal molecular hydrogen gas of star-forming clouds, the turbulence is most likely driven on large scales, with enough energy to invoke random motions that exceed the sound speed, causing the gas to become highly compressible. However, MHD turbulence theory has been largely developed in the incompressible regime, and theorists regularly ignore compressibility effects, even though it is these effects that define the most important structures for star formation, such as filaments. In this talk I report on the first of three (magnetic field, density, energy) analytical turbulent fluctuations theories that we have been developing as part of my PhD. That is, the ubiquitous fluctuations of the magnetic field in a supersonic turbulent medium, including anisotropic affects imparted upon the flow by a local coherent (mean) magnetic field. We (1) derive an analytical model of the magnetic field fluctuations in the sub-Alfvenic field limit, (2) show how the coherent field causes anisotropic compressibility effects leading to bimodally oriented shocks in the gas, and (3) show that fundamental assumptions about the Alfven velocity and velocity scaling relations in the flow are incorrect for anisotropic flows, which in turn motivates a modified Chandrasekhar-Fermi method for highly-magnetised molecular clouds. We test our models on an ensemble of 24 MHD simulations across an order of magnitude in turbulent Mach number, and five orders of magnitude in mean-field strength. Because we form our models undimensionalised we believe that these results are generally applicable for anyone seeking to understand magnetism in any supersonic, turbulent medium, where the turbulence is being continuously stirred by astrophysical energy sources. |
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