487 research outputs found
3D Magneto-Hydrodynamic Simulations of Parker Instability with Cosmic Rays
This study investigates Parker instability in an interstellar medium (ISM)
near the Galactic plane using three-dimensional magneto-hydrodynamic
simulations. Parker instability arises from the presence of a magnetic field in
a plasma, wherein the magnetic buoyant pressure expels the gas and cause the
gas to move along the field lines. The process is thought to induce the
formation of giant molecular clouds in the Galaxy. In this study, the effects
of cosmic-ray (CR) diffusion are examined. The ISM at equilibrium is assumed to
comprise a plasma fluid and a CR fluid at various temperatures, with a uniform
magnetic field passing through it in the azimuthal direction of the Galactic
disk. After a small perturbation, the unstable gas aggregates at the footpoint
of the magnetic fields and forms dense blobs. The growth rate of the
instability increases with the strength of the CR diffusion. The formation of
dense clouds is enhanced by the effect of cosmic rays (CRs), whereas the shape
of the clouds depends sensitively on the initial conditions of perturbation.Comment: 4 pages, Computer Physics Communications 2011, 182, p177-17
Theory and Applications of Non-Relativistic and Relativistic Turbulent Reconnection
Realistic astrophysical environments are turbulent due to the extremely high
Reynolds numbers. Therefore, the theories of reconnection intended for
describing astrophysical reconnection should not ignore the effects of
turbulence on magnetic reconnection. Turbulence is known to change the nature
of many physical processes dramatically and in this review we claim that
magnetic reconnection is not an exception. We stress that not only
astrophysical turbulence is ubiquitous, but also magnetic reconnection itself
induces turbulence. Thus turbulence must be accounted for in any realistic
astrophysical reconnection setup. We argue that due to the similarities of MHD
turbulence in relativistic and non-relativistic cases the theory of magnetic
reconnection developed for the non-relativistic case can be extended to the
relativistic case and we provide numerical simulations that support this
conjecture. We also provide quantitative comparisons of the theoretical
predictions and results of numerical experiments, including the situations when
turbulent reconnection is self-driven, i.e. the turbulence in the system is
generated by the reconnection process itself. We show how turbulent
reconnection entails the violation of magnetic flux freezing, the conclusion
that has really far reaching consequences for many realistically turbulent
astrophysical environments. In addition, we consider observational testing of
turbulent reconnection as well as numerous implications of the theory. The
former includes the Sun and solar wind reconnection, while the latter include
the process of reconnection diffusion induced by turbulent reconnection, the
acceleration of energetic particles, bursts of turbulent reconnection related
to black hole sources as well as gamma ray bursts. Finally, we explain why
turbulent reconnection cannot be explained by turbulent resistivity or derived
through the mean field approach.Comment: 66 pages, 24 figures, a chapter of the book "Magnetic Reconnection -
Concepts and Applications", editors W. Gonzalez, E. N. Parke
Radio broadband visualization of global three-dimensional magneto-hydrodynamical simulations of spiral galaxies I. Faraday rotation at 8GHz
Observational study of galactic magnetic fields is hampered by the fact that
the observables only probe various projections of the magnetic fields.
Comparison with numerical simulations is helpful to understand the real
structures, and observational visualization of numerical data is an important
task. In this paper, we investigate 8~GHz radio synchrotron emission from
spiral galaxies, using the data of global three-dimensional
magneto-hydrodynamic simulations. We assume a frequency independent
depolarization in our observational visualization. We find that the appearance
of the global magnetic field depends on the viewing angle: a face-on view
seemingly has hybrid magnetic field types combining axisymmetric modes with
higer order modes; at a viewing angle of \sim 70\degr, the galaxy seems to
contain a ring-like magnetic field structure; while in edge-on view, only field
structure parallel to the disk can be seen. The magnetic vector seen at 8~GHz
traces the global magnetic field inside the disk. These results indicate that
the topology of global magnetic field obtained from the relation between
azimuthal angle and Faraday depth strongly depends on the viewing anglue of the
galaxy. As one of the examples, we compare our results at a viewing angle of
25\degr with the results of IC342. The relation between azimuthal angle and
Faraday depth of the numerical result shows a tendency similar to IC342, such
as the peak numbers of the Faraday depth.Comment: 10 pages, 7 figures accepted in MNRA
Magnetohydrodynamic Simulations of Disk Galaxy Formation: the Magnetization of The Cold and Warm Medium
Using magnetohydrodynamic (MHD) adaptive mesh refinement simulations, we
study the formation and early evolution of disk galaxies with a magnetized
interstellar medium. For a \msun halo with initial NFW dark matter
and gas profiles, we impose a uniform G magnetic field and follow its
collapse, disk formation and evolution up to 1 Gyr. Comparing to a purely
hydrodynamic simulation with the same initial condition, we find that a
protogalactic field of this strength does not significantly influence the
global disk properties. At the same time, the initial magnetic fields are
quickly amplified by the differentially rotating turbulent disk. After the
initial rapid amplification lasting Myr, subsequent field
amplification appears self-regulated. As a result, highly magnetized material
begin to form above and below the disk. Interestingly, the field strengths in
the self-regulated regime agrees well with the observed fields in the Milky Way
galaxy both in the warm and the cold HI phase and do not change appreciably
with time. Most of the cold phase shows a dispersion of order ten in the
magnetic field strength. The global azimuthal magnetic fields reverse at
different radii and the amplitude declines as a function of radius of the disk.
By comparing the estimated star formation rate (SFR) in hydrodynamic and MHD
simulations, we find that after the magnetic field strength saturates, magnetic
forces provide further support in the cold gas and lead to a decline of the
SFR.Comment: 13 pages, 14 figures. Higher resolution figure version can be found
at http://www.stanford.edu/~pengwang/mhddisk.pd
25 Years of Self-Organized Criticality: Solar and Astrophysics
Shortly after the seminal paper {\sl "Self-Organized Criticality: An
explanation of 1/f noise"} by Bak, Tang, and Wiesenfeld (1987), the idea has
been applied to solar physics, in {\sl "Avalanches and the Distribution of
Solar Flares"} by Lu and Hamilton (1991). In the following years, an inspiring
cross-fertilization from complexity theory to solar and astrophysics took
place, where the SOC concept was initially applied to solar flares, stellar
flares, and magnetospheric substorms, and later extended to the radiation belt,
the heliosphere, lunar craters, the asteroid belt, the Saturn ring, pulsar
glitches, soft X-ray repeaters, blazars, black-hole objects, cosmic rays, and
boson clouds. The application of SOC concepts has been performed by numerical
cellular automaton simulations, by analytical calculations of statistical
(powerlaw-like) distributions based on physical scaling laws, and by
observational tests of theoretically predicted size distributions and waiting
time distributions. Attempts have been undertaken to import physical models
into the numerical SOC toy models, such as the discretization of
magneto-hydrodynamics (MHD) processes. The novel applications stimulated also
vigorous debates about the discrimination between SOC models, SOC-like, and
non-SOC processes, such as phase transitions, turbulence, random-walk
diffusion, percolation, branching processes, network theory, chaos theory,
fractality, multi-scale, and other complexity phenomena. We review SOC studies
from the last 25 years and highlight new trends, open questions, and future
challenges, as discussed during two recent ISSI workshops on this theme.Comment: 139 pages, 28 figures, Review based on ISSI workshops "Self-Organized
Criticality and Turbulence" (2012, 2013, Bern, Switzerland
Magnetic fields and the dynamics of spiral galaxies
We investigate the dynamics of magnetic fields in spiral galaxies by
performing 3D MHD simulations of galactic discs subject to a spiral potential.
Recent hydrodynamic simulations have demonstrated the formation of inter-arm
spurs as well as spiral arm molecular clouds provided the ISM model includes a
cold HI phase. We find that the main effect of adding a magnetic field to these
calculations is to inhibit the formation of structure in the disc. However,
provided a cold phase is included, spurs and spiral arm clumps are still
present if in the cold gas. A caveat to two phase
calculations though is that by assuming a uniform initial distribution, in the warm gas, emphasizing that models with more consistent
initial conditions and thermodynamics are required. Our simulations with only
warm gas do not show such structure, irrespective of the magnetic field
strength. Furthermore, we find that the introduction of a cold HI phase
naturally produces the observed degree of disorder in the magnetic field, which
is again absent from simulations using only warm gas. Whilst the global
magnetic field follows the large scale gas flow, the magnetic field also
contains a substantial random component that is produced by the velocity
dispersion induced in the cold gas during the passage through a spiral shock.
Without any cold gas, the magnetic field in the warm phase remains relatively
well ordered apart from becoming compressed in the spiral shocks. Our results
provide a natural explanation for the observed high proportions of disordered
magnetic field in spiral galaxies and we thus predict that the relative
strengths of the random and ordered components of the magnetic field observed
in spiral galaxies will depend on the dynamics of spiral shocks.Comment: 17 pages, 14 figures, accepted by MNRA
Global Simulations of Protoplanetary Disk Outflows with Coupled Non-ideal Magnetohydrodynamics and Consistent Thermochemistry
Magnetized winds may be important in dispersing protoplanetary disks and
influencing planet formation. We carry out global full magnetohydrodynamic
simulations in axisymmetry, coupled with ray-tracing radiative transfer,
consistent thermochemistry, and non-ideal MHD diffusivities. Magnetized models
lacking EUV photons () feature warm molecular outflows
that have typical poloidal speeds . When the
magnetization is sufficient to drive accretion rates $\sim 10^{-8}\ M_\odot\
\mathrm{yr}^{-1}$, the wind mass-loss rate is comparable. Such outflows are
driven not centrifugally but by the pressure of toroidal magnetic fields
produced by bending the poloidal field. Both the accretion and outflow rates
increase with the poloidal field energy density, the former almost linearly.
The mass-loss rate is also strongly affected by ionization due to UV and X-ray
radiation near the wind base. Adding EUV irradiation to the system heats,
ionizes, and accelerates the part of the outflow nearest the symmetry axis, but
reduces the overall mass-loss rate by exerting pressure on the wind base. Most
of our models are non-turbulent, but some with reduced dust abundance and
therefore higher ionization fractions exhibit magnetorotational instabilities
near the base of the wind.Comment: 25 pages, 16 figures; submitted to Ap
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