3 research outputs found
Cosmic rays and random magnetic traps
The spatial distribution of cosmic ray (CR) particles in the interstellar
medium (ISM) is of major importance in radio astronomy, where its knowledge is
essential for the interpretation of observations, and in theoretical
astrophysics, where CR contribute to the structure and dynamics of the ISM.
Local inhomogeneities in interstellar magnetic field strength and structure can
affect the local diffusivity and ensemble dynamics of the cosmic ray particles.
Magnetic traps (regions between magnetic mirrors located on the same magnetic
line) can lead to especially strong and persistent features in the CR spatial
distribution. Using test particle simulations, we study the spatial
distribution of an ensemble of CR particles (both protons and electrons) in
various magnetic field configurations, from an idealized axisymmetric trap to
those that emerge in intermittent (dynamo-generated) random magnetic fields. We
demonstrate that both the inhomogeneity in the CR sources and the energy losses
by the CR particles can lead to persistent local inhomogeneities in the CR
distribution and that the protons and electrons have different spatial
distributions. Our results can have profound implications for the
interpretation of the synchrotron emission from astronomical objects, and in
particular its random fluctuations.Comment: 19 pages, 18 figure
Steady states of the Parker instability: the effects of rotation
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
Steady states of the Parker instability : the effects of rotation
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.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.Peer reviewe