MHD Simulations of Parker Instability Undergoing Cosmic-Ray Diffusion - Astrophysics > High Energy Astrophysical PhenomenaReport as inadecuate




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Abstract: Parker instability arises from the presence of magnetic fields in a plasma ina gravitational field such as the interstellar medium ISM, wherein themagnetic buoyant pressure expels the gas and causes the gas to move along thefield lines. The process of mixing of this instability in the ISM near theGalactic plane is investigated. The initial ISM is assumed to consist of twofluids: plasma gas and cosmic-ray particles, in hydrostatic equilibrium,coupled with a uniform, azimuthally-aligned magnetic field. The evolution ofthe instability is explored in two models: an isothermal exponential-decliningdensity model and a two-layered, hyperbolic tangent temperature model. After asmall perturbation, the unstable gas aggregates at the bottom of the magneticloops and forms dense blobs. The growth rate of the instability decreases asthe coupling between the cosmic rays and the plasma becomes stronger meaning asmaller CR diffusion coefficient. The mixing is enhanced by the cosmic-raydiffusion, while the shape of the condensed gas depends sensitively on theinitial equilibrium conditions. The hyperbolic tangent temperature modelproduces a more concentrated and round shape of clumps at the foot points ofrising magnetic arches, like the observed giant molecular cloud, whereas theexponential density model gives rise to a filamentary morphology of the clumpystructure. When considering a minimum perpendicular or cross field diffusion ofcosmic rays, which is often substantially smaller than the parallel coefficient$\kappa {\|}$, around $2%-4%$ of $\kappa {\|}$, the flow speed is significantlyincreased such that the magnetic loops extend to a greater altitude. Wespeculate that the galactic wind flow perpendicular to the galactic disk may befacilitated by Parker instabilities through the cross field diffusion of cosmicrays.



Author: Chih-Yueh Wang, Ying-Yi Lo, Chung-Ming Ko

Source: https://arxiv.org/



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