Steady states of the Parker instability : the effects of rotation

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A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
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Monthly Notices of the Royal Astronomical Society, Volume 525, issue 2
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 is higher than in comparable non-rotating 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 mid-plane 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 that 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 1 kiloparsec.
Funding Information: We are grateful to Axel Brandenburg and Kandaswamy Subramanian for useful discussions. GRS would like to thank the Isaac Newton Institute for Mathematical Sciences, Cambridge, for support and hospitality during the programme ‘Frontiers in dynamo theory: from the Earth to the stars’, where work on this paper was undertaken. This work was supported by EPSRC grant no. EP/R014604/1. We are thankful to the anonymous referee for a very insightful review. Publisher Copyright: © 2023 The Author(s). Published by Oxford University Press on behalf of Royal Astronomical Society.
cosmic rays, dynamo, galaxies: magnetic fields, instabilities, ISM: evolution, MHD
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Tharakkal, D, Shukurov, A, Gent, F A, Sarson, G R & Snodin, A 2023, ' Steady states of the Parker instability : the effects of rotation ', Monthly Notices of the Royal Astronomical Society, vol. 525, no. 2, pp. 2972-2984 .