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Lookup NU author(s): Yasin Qazi, Professor Anvar ShukurovORCiD, Dr Devika Tharakkal, Dr Fred Gent
This work is licensed under a Creative Commons Attribution 4.0 International License (CC BY 4.0).
© 2025 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group. We study the nonlinear evolution of the magnetic buoyancy instability in rotating and non-rotating gas layers (with emphasis on the parameter range typical of spiral galaxies) using numerical solutions of non-ideal, isothermal MHD equations. The unstable magnetic field is either imposed through the boundary conditions or generated by an imposed α-effect. In the case of an imposed field, we solve for the deviations from the background state which satisfy periodic boundary conditions. We also include cosmic rays as a weightless fluid which exerts a dynamically significant pressure and somewhat amplifies magnetic buoyancy. This version of the instability is known as the Parker instability. Without rotation, systems with an imposed magnetic field evolve to a state with a very weak magnetic field, very different from the marginally stable eigenfunction, where the gas layer eventually becomes very thin as it is supported by the thermal and turbulent pressures alone. However, this does not happen when the magnetic field is maintained by the α-effect. Rotation fundamentally changes the development of the instability. A rotating system develops nonlinear oscillations, and the magnetic field direction changes even with an imposed magnetic field. We demonstrate that this is caused by the secondary α-effect at large altitudes as the gas flow produced by the instability becomes helical. The secondary α-effect has an anomalous sign with the α-coefficient being negative in the northern hemisphere, whereas the Coriolis force produces a positive α. The mean-field dynamo action outside the original gas layer can also lead to a switch in the magnetic field parity from quadrupolar (typical of the mean-field dynamo action in a thin layer) to dipolar. Altogether, the magnetic buoyancy instability and the mean-field dynamo action become separated as distinct physical effects in a nonlinear rotating system. We show that none of the assumptions used in analytic studies of the Parker instability is corroborated by numerical results.
Author(s): Qazi Y, Shukurov A, Tharakkal D, Gent FA
Publication type: Article
Publication status: Published
Journal: Geophysical and Astrophysical Fluid Dynamics
Year: 2025
Pages: Epub ahead of print
Online publication date: 11/09/2025
Acceptance date: 05/08/2025
Date deposited: 06/10/2025
ISSN (print): 0309-1929
ISSN (electronic): 1029-0419
Publisher: Taylor and Francis Ltd
URL: https://doi.org/10.1080/03091929.2025.2545114
DOI: 10.1080/03091929.2025.2545114
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