83,726 research outputs found
A Divergence-Free Upwind Code for Multidimensional Magnetohydrodynamic Flows
A description is given for preserving {\bmsy\nabla}\cdot{\vec B}=0 in a
magnetohydrodynamic (MHD) code that employs the upwind, Total Variation
Diminishing (TVD) scheme and the Strang-type operator splitting for
multi-dimensionality. The method is based on the staggered mesh technique to
constrain the transport of magnetic field: the magnetic field components are
defined at grid interfaces with their advective fluxes on grid edges, while
other quantities are defined at grid centers. The magnetic field at grid
centers for the upwind step is calculated by interpolating the values from grid
interfaces. The advective fluxes on grid edges for the magnetic field evolution
are calculated from the upwind fluxes at grid interfaces. Then, the magnetic
field can be maintained with {\bmsy\nabla}\cdot{\vec B}=0 exactly, if this is
so initially, while the upwind scheme is used for the update of fluid
quantities. The correctness of the code is demonstrated through tests comparing
numerical solutions either with analytic solutions or with numerical solutions
from the code using an explicit divergence-cleaning method. Also the robustness
is shown through tests involving realistic astrophysical problems.Comment: 15 pages of text, 8 figures (in degraded gif format), to appear in
The Astrophysical Journal (Dec. 10, 1998), original quality figures available
via anonymous ftp at ftp://ftp.msi.umn.edu/pub/users/twj/mhddivb5.uu or
ftp://canopus.chungnam.ac.kr/ryu/mhddivb5.u
Spectral reflectances of natural targets for use in remote sensing studies
A collection of spectral reflectances of 156 natural targets is presented in a uniform format. For each target both a graphical plot and a digital tabulation of reflectance is given. The data were taken from the literature and include laboratory, field, and aircraft measurements. A discussion of the different measurements of reflectance is given, along with the changes in apparent reflectance when targets are viewed through the atmosphere. The salient features of the reflectance curves of common target types are presented and discussed
Plane-wave impulse approximation extraction of the neutron magnetic form factor from quasielastic ^3He(e,e') at Q^2=0.3 to 0.6 (GeV/c)^2
A high precision measurement of the transverse spin-dependent asymmetry A_T' in ^3He(e,e') quasielastic
scattering was performed in Hall A at Jefferson Lab at values of the squared four-momentum transfer, Q^2,
between 0.1 and 0.6 (GeV/c)^2. A_(T') is sensitive to the neutron magnetic form factor, G_M^n . Values of G_M^n at
Q^2 = 0.1 and 0.2 (GeV/c)^2, extracted using Faddeev calculations, were reported previously. Here, we report
the extraction of G_M^n for the remaining Q^2 values in the range from 0.3 to 0.6 (GeV/c)^2 using a plane-wave
impulse approximation calculation. The results are in good agreement with recent precision data from experiments
using a deuterium target
Transverse Asymmetry A_T′ from the Quasielastic ^3He(e,e′) Process and the Neutron Magnetic Form Factor
We have measured the transverse asymmetry A_T′ in ^3He(e,e′) quasielastic scattering in Hall A at Jefferson Laboratory with high precision for Q^2 values from 0.1 to 0.6 (GeV/c)^2. The neutron magnetic form factor GMn was extracted based on Faddeev calculations for Q^2 = 0.1 and 0.2 (GeV/c)^2 with an experimental uncertainty of less than 2%
Tricritical behavior in itinerant quantum ferromagnets
It is shown that the peculiar features observed in the low-temperature phase
diagrams of ZrZn_2, UGe_2, and MnSi can be understood in terms of a simple
mean-field theory. The nature of the ferromagnetic transition changes from
second order to first order at a tricritical point, and in a small external
magnetic field surfaces of first-order transitions emerge which terminate in
quantum critical points. This field dependence of the phase diagram follows
directly from the existence of the tricritical point. The quantum critical
behavior in a nonzero field is calculated exactly.Comment: 4pp., 4 eps figure
The turbulent pressure support in galaxy clusters revisited
Due to their late formation in cosmic history, clusters of galaxies are not
fully in hydrostatic equilibrium and the gravitational pull of their mass at a
given radius is expected not to be entirely balanced by the thermal gas
pressure. Turbulence may supply additional pressure, and recent (X-ray and SZ)
hydrostatic mass reconstructions claim a pressure support of of
the total pressure at . In this work we show that, after carefully
disentangling bulk from small-scale turbulent motions in high-resolution
simulations of galaxy clusters, we can constrain which fraction of the gas
kinetic energy effectively provides pressure support in the cluster's
gravitational potential. While the ubiquitous presence of radial inflows in the
cluster can lead to significant bias in the estimate of the non-thermal
pressure support, we report that only a part of this energy effectively acts as
a source of pressure, providing a support of the order of of the
total pressure at .Comment: 5 pages, 5 pages, accepted, to appear in MNRAS Letter
Confirmation and Analysis of Circular Polarization from Sagittarius A*
Recently Bower et al. (1999b) have reported the detection of circular
polarization from the Galactic Center black hole candidate, Sagittarius A*. We
provide an independent confirmation of this detection, and provide some
analysis on the possible mechanisms.Comment: 14 pages, to appear in Astrophysical Journal Letter
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