2,320 research outputs found
Flow dynamics and magnetic induction in the von-Karman plasma experiment
The von-Karman plasma experiment is a novel versatile experimental device
designed to explore the dynamics of basic magnetic induction processes and the
dynamics of flows driven in weakly magnetized plasmas. A high-density plasma
column (10^16 - 10^19 particles.m^-3) is created by two radio-frequency plasma
sources located at each end of a 1 m long linear device. Flows are driven
through JxB azimuthal torques created from independently controlled emissive
cathodes. The device has been designed such that magnetic induction processes
and turbulent plasma dynamics can be studied from a variety of time-averaged
axisymmetric flows in a cylinder. MHD simulations implementing
volume-penalization support the experimental development to design the most
efficient flow-driving schemes and understand the flow dynamics. Preliminary
experimental results show that a rotating motion of up to nearly 1 km/s is
controlled by the JxB azimuthal torque
Historical perspective on astrophysical MHD simulations
This contribution contains the introductory remarks that I presented at IAU
Symposium 270 on ``Computational Star Formation" held in Barcelona, Spain, May
31 -- June 4, 2010. I discuss the historical development of numerical MHD
methods in astrophysics from a personal perspective. The recent advent of
robust, higher order-accurate MHD algorithms and adaptive mesh refinement
numerical simulations promises to greatly improve our understanding of the role
of magnetic fields in star formation.Comment: 11 pages, 5 figures, in "Computational Star Formation" held in
Barcelona, Spain, May 31 - June 4, 2010", Eds. J. Alves, B. G. Elmegreen, J.
M. Girart, V. Trimbl
Measuring magnetism in the Milky Way with the Square Kilometre Array
Magnetic fields in the Milky Way are present on a wide variety of sizes and
strengths, influencing many processes in the Galactic ecosystem such as star
formation, gas dynamics, jets, and evolution of supernova remnants or pulsar
wind nebulae. Observation methods are complex and indirect; the most used of
these are a grid of rotation measures of unresolved polarized extragalactic
sources, and broadband polarimetry of diffuse emission. Current studies of
magnetic fields in the Milky Way reveal a global spiral magnetic field with a
significant turbulent component; the limited sample of magnetic field
measurements in discrete objects such as supernova remnants and HII regions
shows a wide variety in field configurations; a few detections of magnetic
fields in Young Stellar Object jets have been published; and the magnetic field
structure in the Galactic Center is still under debate.
The SKA will unravel the 3D structure and configurations of magnetic fields
in the Milky Way on sub-parsec to galaxy scales, including field structure in
the Galactic Center. The global configuration of the Milky Way disk magnetic
field, probed through pulsar RMs, will resolve controversy about reversals in
the Galactic plane. Characteristics of interstellar turbulence can be
determined from the grid of background RMs. We expect to learn to understand
magnetic field structures in protostellar jets, supernova remnants, and other
discrete sources, due to the vast increase in sample sizes possible with the
SKA. This knowledge of magnetic fields in the Milky Way will not only be
crucial in understanding of the evolution and interaction of Galactic
structures, but will also help to define and remove Galactic foregrounds for a
multitude of extragalactic and cosmological studies.Comment: 19 pages, 2 figures; to appear as part of 'Cosmic Magnetism' in
Proceedings 'Advancing Astrophysics with the SKA (AASKA14)', PoS(AASKA14)09
Three dimensional evolution of differentially rotating magnetized neutron stars
We construct a new three-dimensional general relativistic
magnetohydrodynamics code, in which a fixed mesh refinement technique is
implemented. To ensure the divergence-free condition as well as the magnetic
flux conservation, we employ the method by Balsara (2001). Using this new code,
we evolve differentially rotating magnetized neutron stars, and find that a
magnetically driven outflow is launched from the star exhibiting a kink
instability. The matter ejection rate and Poynting flux are still consistent
with our previous finding (Shibata et al., 2011) obtained in axisymmetric
simulations.Comment: 12 pages, 14 figures, accepted by PR
Constraining the regular Galactic Magnetic Field with the 5-year WMAP polarization measurements at 22 GHz
[ABRIDGED] The knowledge of the regular component of the Galactic magnetic
field gives important information about the structure and dynamics of the Milky
Way, as well as constitutes a basic tool to determine cosmic rays trajectories.
It can also provide clear windows where primordial magnetic fields could be
detected. We want to obtain the regular (large scale) pattern of the magnetic
field distribution of the Milky Way that better fits the polarized synchrotron
emission as seen by the 5-year WMAP data at 22 GHz. We have done a systematic
study of a number of Galactic magnetic field models: axisymmetric, bisymmetric,
logarithmic spiral arms, concentric circular rings with reversals and
bi-toroidal. We have explored the parameter space defining each of these models
using a grid-based approach. In total, more than one million models are
computed. The model selection is done using a Bayesian approach. For each
model, the posterior distributions are obtained and marginalised over the
unwanted parameters to obtain the marginal 1-D probability distribution
functions. In general, axisymmetric models provide a better description of the
halo component, although attending to their goodness-of-fit, the rest of the
models cannot be rejected. In the case of disk component, the analysis is not
very sensitive for obtaining the disk large scale structure, because of the
effective available area (less than 8% of the whole map and less than 40% of
the disk). Nevertheless, within a given family of models, the best-fit
parameters are compatible with those found in the literature. The family of
models that better describes the polarized synchrotron halo emission is the
axisymmetric one, with magnetic spiral arms with a pitch angle of ~24 degrees,
and a strong vertical field of 1 microG at z ~ 1 kpc. When a radial variation
is fitted, models require fast variations.Comment: 14 pages, 9 figures. Accepted for publication in A&
On the Formation of Cluster Radio Relics
(abridged) We present detailed 3-dimensional magneto-hydrodynamical
simulations of the passage of a radio plasma cocoon filled with turbulent
magnetic fields through a shock wave. Taking into account synchrotron, inverse
Compton and adiabatic energy losses and gains we evolved the relativistic
electron population to produce synthetic polarisation radio maps. On contact
with the shock wave the radio cocoons are first compressed and finally torn
into filamentary structures, as is observed in several cluster radio relics. In
the synthetic radio maps the electric polarisation vectors are mostly
perpendicular to the filamentary radio structures. If the magnetic field inside
the cocoon is not too strong, the initially spherical radio cocoon is
transformed into a torus after the passage of the shock wave. Very recent,
high-resolution radio maps of cluster radio relics seem to exhibit such
toroidal geometries in some cases. This supports the hypothesis that cluster
radio relics are fossil radio cocoons that have been revived by a shock wave.
For a late-stage relic the ratio of its global diameter to the filament
diameter should correlate with the shock strength. Finally, we argue that the
total radio polarisation of radio relic should be well correlated with the
3-dimensional orientation of the shock wave that produced the relic.Comment: accepted by MNRAS, 10 pages, 13 figures, some modifications due to
comments of a refere
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