87 research outputs found
A finite volume method for two-moment cosmic-ray hydrodynamics on a moving mesh
We present a new numerical algorithm to solve the recently derived equations
of two-moment cosmic ray hydrodynamics (CRHD). The algorithm is implemented as
a module in the moving mesh Arepo code. Therein, the anisotropic transport of
cosmic rays (CRs) along magnetic field lines is discretised using a
path-conservative finite volume method on the unstructured time-dependent
Voronoi mesh of Arepo. The interaction of CRs and gyroresonant Alfv\'en waves
is described by short-timescale source terms in the CRHD equations. We employ a
custom-made semi-implicit adaptive time stepping source term integrator to
accurately integrate this interaction on the small light-crossing time of the
anisotropic transport step. Both the transport and the source term integration
step are separated from the evolution of the magneto-hydrodynamical equations
using an operator split approach. The new algorithm is tested with a variety of
test problems, including shock tubes, a perpendicular magnetised discontinuity,
the hydrodynamic response to a CR overpressure, CR acceleration of a warm
cloud, and a CR blast wave, which demonstrate that the coupling between CR and
magneto-hydrodynamics is robust and accurate. We demonstrate the numerical
convergence of the presented scheme using new linear and non-linear analytic
solutions.Comment: 24 pages, 15 figures, submitted to MNRAS, comments are welcome
Coupling multi-fluid dynamics equipped with Landau closures to the particle-in-cell method
The particle-in-cell (PIC) method is successfully used to study magnetized
plasmas. However, this requires large computational costs and limits
simulations to short physical run-times and often to setups in less than three
spatial dimensions. Traditionally, this is circumvented either via hybrid-PIC
methods (adopting massless electrons) or via magneto-hydrodynamic-PIC methods
(modelling the background plasma as a single charge-neutral
magneto-hydrodynamical fluid). Because both methods preclude modelling
important plasma-kinetic effects, we introduce a new fluid-PIC code that
couples a fully explicit and charge-conservative multi-fluid solver to the PIC
code SHARP through a current-coupling scheme and solve the full set of
Maxwell's equations. This avoids simplifications typically adopted for Ohm's
Law and enables us to fully resolve the electron temporal and spatial scales
while retaining the versatility of initializing any number of ion, electron, or
neutral species with arbitrary velocity distributions. The fluid solver
includes closures emulating Landau damping so that we can account for this
important kinetic process in our fluid species. Our fluid-PIC code is
second-order accurate in space and time. The code is successfully validated
against several test problems, including the stability and accuracy of shocks
and the dispersion relation and damping rates of waves in unmagnetized and
magnetized plasmas. It also matches growth rates and saturation levels of the
gyro-scale and intermediate-scale instabilities driven by drifting charged
particles in magnetized thermal background plasmas in comparison to linear
theory and PIC simulations. This new fluid-SHARP code is specially designed for
studying high-energy cosmic rays interacting with thermal plasmas over
macroscopic timescales.Comment: 35 pages, 11 figures, submitted to JPP. Comments are welcom
Faraday rotation maps of disk galaxies
Faraday rotation is one of the most widely used observables to infer the
strength and configuration of the magnetic field in the ionised gas of the
Milky Way and nearby spiral galaxies. Here we compute synthetic Faraday
rotation maps at for a set of disk galaxies from the Auriga
high-resolution cosmological simulations, for different observer positions
within and outside the galaxy. We find that the strength of the Faraday
rotation of our simulated galaxies for a hypothetic observer at the solar
circle is broadly consistent with the Faraday rotation seen for the Milky Way.
The same holds for an observer outside the galaxy and the observed signal of
the nearby spiral galaxy M51. However, we also find that the structure and
angular power spectra of the synthetic all-sky Faraday rotation maps vary
strongly with azimuthal position along the solar circle. We argue that this
variation is a result of the structure of the magnetic field of the galaxy that
is dominated by an azimuthal magnetic field ordered scales of several kpc, but
has radial and vertical magnetic field components that are only ordered on
scales of 1-2 kpc. Because the magnetic field strength decreases exponentially
with height above the disk, the Faraday rotation for an observer at the solar
circle is dominated by the local environment. This represents a severe obstacle
for attempts to reconstruct the global magnetic field of the Milky Way from
Faraday rotation maps alone without including additional observables.Comment: 10 pages, 10 figures, accepted by MNRA
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