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Steady states of the Parker instability: the effects of rotation

Lookup NU author(s): Dr Devika Tharakkal, Professor Anvar ShukurovORCiD, Dr Fred Gent, Dr Graeme Sarson, Dr Andrew Snodin



This work is licensed under a Creative Commons Attribution 4.0 International License (CC BY 4.0).


We model the Parker instability in vertically stratified isothermal gas using non-ideal MHD three-dimensional simulations.Rotation, especially differential, more strongly and diversely affects the nonlinear state than the linear stage (where we confirm the most important conclusions of analytical models), and stronger than any linear analyses predict. Steady state magnetic fields are stronger and cosmic ray energy density higher than in comparable nonrotating systems. Transient gas outflows induced by the nonlinear instability persist longer, of order 2 Gyr, with rotation. Stratification combined with (differential) rotation drives helical flows, leading to mean-field dynamo. Consequently, the nonlinear state becomes oscillatory (while both the linear instability and the dynamo are non-oscillatory). The horizontal magnetic field near the midplane reverses its direction propagating to higher altitudes as the reversed field spreads buoyantly. The spatial pattern of the large-scale magnetic field may explain the alternating magnetic field directions in the halo of the edge-on galaxy NGC 4631. Our model is unique in producing a large-scale magnetic structure similar to such observation. Furthermore, our simulations show that the mean kinetic helicity of the magnetically driven flows has the sign opposite to that in the conventional non-magnetic flows. This has profound consequences for the nature of the dynamo action and large-scale magnetic field structure in the coronae of spiral galaxies which remain to be systematically explored and understood. We show that the energy density of cosmic rays and magnetic field strength are not correlated at scales of order a kiloparsec.

Publication metadata

Author(s): Tharakkal D, Shukurov A, Gent FA, Sarson GR, Snodin A

Publication type: Article

Publication status: Published

Journal: Monthly Notices of the Royal Astronomical Society

Year: 2023

Volume: 525

Issue: 2

Pages: 2972-2984

Print publication date: 01/10/2023

Online publication date: 17/08/2023

Acceptance date: 07/08/2023

Date deposited: 16/08/2023

ISSN (print): 0035-8711

ISSN (electronic): 1365-2966

Publisher: Oxford University Press


DOI: 10.1093/mnras/stad2475


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