Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Magnetohydrodynamic Density Waves in a Composite Disk System of
  Interstellar Medium and Cosmic-Ray Gas   Multi-wavelength observations from radio to soft X-ray bands of large-scale
galactic spiral structures offer synthesized and comprehensive views of nearby
disk galaxies. In the presence of a massive dark-matter halo, the density-wave
dynamics on galactic scales involves the stellar disk, the gas disk of
interstellar medium (ISM), the magnetic field, and the cosmic-ray gas (CRG). In
this paper, we explore the dynamic and electromagnetic interplay between the
magnetized ISM disk and CRG disk so that structural and diagnostic features of
optical, infrared, and synchrotron radio-continuum emissions from a spiral
galaxy can be physically understood. On timescales of galactic density waves,
cosmic rays collectively may be treated as a relativistically hot tenuous gas
fluid that is tied to the large-scale mean magnetic field in transverse bulk
motions but moves otherwise differently along the magnetic field relative to
the ISM. For both fast and slow magnetohydrodynamic (MHD) density waves in a
composite disk system of magnetized ISM and CRG, the minute CRG mass density
enhancement is phase shifted relative to the enhancement of parallel magnetic
field. Owing to an extremely small number of cosmic rays, the large-scale
magnetic field enhancement dominates in synchrotron radio-continuum emissions
(as if the CRG is almost unperturbed) for spiral structural manifestations. In
addition to the fast and slow MHD density waves, there also exists a
suprathermal MHD wave mode by which CRG adjusts itself with an effective
suprathermal sound speed close to the speed of light c.

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The categories of this paper are: cond-mat
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The categories of this paper are: gr-qc
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The categories of this paper are: nlin
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The categories of this paper are: q-fin
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The categories of this paper are: atom-ph
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The categories of this paper are: cs
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The categories of this paper are: mtrl-th
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The categories of this paper are: hep-ph
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The categories of this paper are: comp-gas
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The categories of this paper are: acc-phys
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The categories of this paper are: hep-lat
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The categories of this paper are: nucl-ex
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The categories of this paper are: bayes-an
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The categories of this paper are: nucl-th
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The categories of this paper are: cmp-lg
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The categories of this paper are: adap-org