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Existence Of The Magnetorotational Instability
By posing and solving a global axisymmetric eigenvalue problem on an infinite domain with modes vanishing at zero and infinity for a differentially rotating MHD plasma, the conditions for the occurrence of a purely growing low-frequency mode known as the magnetorotational instability (MRI) are mapped. It is shown that the MRI criterion drawn from the "local dispersion relation" is at best inadequate and may even be misleading. The physics of the MRI is rather nuanced. It is dictated by the details of the radial profile of the rotation velocity Omega(r) and not just by the sign and the magnitude of its gradient, Omega'. The salient features of the class of profiles for which the MRI-like eigenmodes may occur are given along with the eigenspectrum. For a variety of other profiles, it is shown that an unstable magnetorotational mode is not a valid eigensolution.Institute for Fusion Studie
Acceleration of Plasma Flows Due to Reverse Dynamo Mechanism
The "reverse-dynamo" mechanism - the amplification/generation of fast plasma
flows by micro scale (turbulent) magnetic fields via magneto-fluid coupling is
recognized and explored. It is shown that macro-scale magnetic fields and flows
are generated simultaneously and proportionately from micro scale fields and
flows. The stronger the micro-scale driver, the stronger are the macro-scale
products. Stellar and astrophysical applications are suggested.Comment: 16 pages including 3 figures. The Astrophys. J. (accepted);
additional material is given for clarification; terminology is change
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