4,641 research outputs found
Quantitative Susceptibility Mapping: Contrast Mechanisms and Clinical Applications.
Quantitative susceptibility mapping (QSM) is a recently developed MRI technique for quantifying the spatial distribution of magnetic susceptibility within biological tissues. It first uses the frequency shift in the MRI signal to map the magnetic field profile within the tissue. The resulting field map is then used to determine the spatial distribution of the underlying magnetic susceptibility by solving an inverse problem. The solution is achieved by deconvolving the field map with a dipole field, under the assumption that the magnetic field is a result of the superposition of the dipole fields generated by all voxels and that each voxel has its unique magnetic susceptibility. QSM provides improved contrast to noise ratio for certain tissues and structures compared to its magnitude counterpart. More importantly, magnetic susceptibility is a direct reflection of the molecular composition and cellular architecture of the tissue. Consequently, by quantifying magnetic susceptibility, QSM is becoming a quantitative imaging approach for characterizing normal and pathological tissue properties. This article reviews the mechanism generating susceptibility contrast within tissues and some associated applications
High-resolution magnetohydrodynamics simulation of black hole-neutron star merger: Mass ejection and short gamma-ray burst
We report results of a high-resolution numerical-relativity simulation for
the merger of black hole-magnetized neutron star binaries on Japanese
supercomputer "K". We focus on a binary that is subject to tidal disruption and
subsequent formation of a massive accretion torus. We find the launch of
thermally driven torus wind, subsequent formation of a funnel wall above the
torus and a magnetosphere with collimated poloidal magnetic field, and high
Blandford-Znajek luminosity. We show for the first time this picture in a
self-consistent simulation. The turbulence-like motion induced by the
non-axisymmetric magnetorotational instability as well as the Kelvin-Helmholtz
instability inside the accretion torus works as an agent to drive the mass
accretion and converts the accretion energy to thermal energy, which results in
the generation of a strong wind. By an in-depth resolution study, we reveal
that high resolution is essential to draw such a picture. We also discuss the
implication for the r-process nucleosynthesis, the radioactively-powered
transient emission, and short gamma-ray bursts.Comment: 8 pages, 8 figures, to be appeared in PR
Turbulence in Global Simulations of Magnetized Thin Accretion Disks
We use a global magnetohydrodynamic simulation of a geometrically thin
accretion disk to investigate the locality and detailed structure of turbulence
driven by the magnetorotational instability (MRI). The model disk has an aspect
ratio , and is computed using a higher-order Godunov MHD
scheme with accurate fluxes. We focus the analysis on late times after the
system has lost direct memory of its initial magnetic flux state. The disk
enters a saturated turbulent state in which the fastest growing modes of the
MRI are well-resolved, with a relatively high efficiency of angular momentum
transport . The accretion stress
peaks at the disk midplane, above and below which exists a moderately
magnetized corona with patches of superthermal field. By analyzing the spatial
and temporal correlations of the turbulent fields, we find that the spatial
structure of the magnetic and kinetic energy is moderately well-localized (with
correlation lengths along the major axis of and respectively),
and generally consistent with that expected from homogenous incompressible
turbulence. The density field, conversely, exhibits both a longer correlation
length and a long correlation time, results which we ascribe to the importance
of spiral density waves within the flow. Consistent with prior results, we show
that the mean local stress displays a well-defined correlation with the local
vertical flux, and that this relation is apparently causal (in the sense of the
flux stimulating the stress) during portions of a global dynamo cycle. We argue
that the observed flux-stress relation supports dynamo models in which the
structure of coronal magnetic fields plays a central role in determining the
dynamics of thin-disk accretion.Comment: 24 pages and 25 figures. MNRAS in press. Version with high resolution
figures available from
http://jila.colorado.edu/~krb3u/Thin_Disk/thin_disk_turbulence.pd
Three-dimensional MHD Simulations of Jets from Accretion Disks
We report the results of 3-dimensional magnetohydrodynamic (MHD) simulations
of a jet formation by the interaction between an accretion disk and a large
scale magnetic field. The disk is not treated as a boundary condition but is
solved self-consistently. To investigate the stability of MHD jet, the
accretion disk is perturbed with a non-axisymmetric sinusoidal or random
fluctuation of the rotational velocity. The dependences of the jet velocity
, mass outflow rate , and mass accretion rate
on the initial magnetic field strength in both non-axisymmetric cases are
similar to those in the axisymmetric case. That is, ,
and where is the
initial magnetic field strength. The former two relations are consistent with
the Michel's steady solution, , although
the jet and accretion do not reach the steady state. In both perturbation
cases, a non-axisymmetric structure with appears in the jet, where
means the azimuthal wave number. This structure can not be explained by
Kelvin-Helmholtz instability and seems to originate in the accretion disk.
Non-axisymmetric modes in the jet reach almost constant levels after about 1.5
orbital periods of the accretion disk, while all modes in the accretion disk
grow with oscillation. As for the angular momentum transport by Maxwell stress,
the vertical component, , in the wide range of initial magnetic field
strength.Comment: Accepted for publication in ApJ. The pdf file with high resolution
figures can be downloaded at
http://www.kusastro.kyoto-u.ac.jp/~hiromitu/3j050806.pd
Full Issue: Volume 13, Issue 1 - Winter 2018
Full Issue: Volume 13, Issue 1 - Winter 201
Turbulent Mixing and the Dead Zone in Protostellar Disks
We investigate the conditions for the presence of a magnetically inactive
dead zone in protostellar disks, using 3-D shearing-box MHD calculations
including vertical stratification, Ohmic resistivity and time-dependent
ionization chemistry. Activity driven by the magnetorotational instability
fills the whole thickness of the disk at 5 AU, provided cosmic ray ionization
is present, small grains are absent and the gas-phase metal abundance is
sufficiently high. At 1 AU the larger column density of 1700 g/cm^2 means the
midplane is shielded from ionizing particles and remains magnetorotationally
stable even under the most favorable conditions considered. Nevertheless the
dead zone is effectively eliminated. Turbulence mixes free charges into the
interior as they recombine, leading to a slight coupling of the midplane gas to
the magnetic fields. Weak, large-scale radial fields diffuse to the midplane
where they are sheared out to produce stronger azimuthal fields. The resulting
midplane accretion stresses are just a few times less than in the surface
layers on average.Comment: to appear in the Astrophysical Journal; 25 pages, 10 figure
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