16,146 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
Three-Dimensional Simulations of the Parker Instability in a Uniformly-rotating Disk
We investigate the nonlinear effects of uniform rotation on the Parker
instability in an exponentially-stratified disk through high-resolution
simulations. During the linear stage, the speed of gas motion is subsonic and
the evolution with the rotation is not much different from that without the
rotation. This is because the Coriolis force is small. During the nonlinear
stage, oppositely-directed supersonic flows near a magnetic valley are under
the influence of the Coriolis force with different directions, resulting in
twisted magnetic field lines near the valley. Sheet-like structures, which are
tilted with respect to the initial field direction, are formed with an 1.5
enhancement of column density with respect to its initial value. Even though
uniform rotation doesn't give much impact on density enhancement, it generates
helically twisted field lines, which may become an additional support mechanism
of clouds.Comment: 3 pages, uses rmaa.cls, to appear in Proc. of the Conference on
"Astrophysical Plasmas: Codes, Models and Observations", Eds. J. Franco, J.
Arthur, N. Brickhouse, Rev.Mex.AA Conf. Serie
Cluster Accretion Shocks as Possible Acceleration Sites for Ultra High Energy Protons below the Greisen Cutoff
Three-dimensional hydrodynamic simulations of large scale structure in the
Universe have shown that accretion shocks form during the gravitational
collapse of one-dimensional caustics, and that clusters of galaxies formed at
intersections of the caustics are surrounded by these accretion shocks.
Estimated speed and curvature radius of the shocks are 1000-3000 \kms and about
5 Mpc, respectively, in the CDM universe. Assuming that energetic
protons are accelerated by these accretion shocks via the first-order Fermi
process and modeling particle transport around the shocks through Bohm
diffusion, we suggest that protons can be accelerated up to the {\it Greisen
cutoff energy} near eV, provided the mean magnetic field
strength in the region around the shocks is at least of order a microgauss. We
have also estimated the proton flux at earth from the Virgo cluster. Assuming a
few (1-10) \% of the ram pressure of the infalling matter would be transferred
to the cosmic-rays, the estimated flux for eV is consistent
with observations, so that such clusters could be plausible sources of the UHE
CRs.Comment: 14 pages, uuencoded compressed postscript file. Accepted for Jan. 1,
1996 issue of Ap
Simulating Electron Transport and Synchrotron Emission in Radio Galaxies: Shock Acceleration and Synchrotron Aging in Three-Dimensional Flows
We present the first three-dimensional MHD radio galaxy simulations that
explicitly model transport of relativistic electrons, including diffusive
acceleration at shocks as well as radiative and adiabatic cooling in smooth
flows. We discuss three simulations of light Mach 8 jets, designed to explore
the effects of shock acceleration and radiative aging on the nonthermal
particle populations that give rise to synchrotron and inverse-Compton
radiations. We also conduct detailed synthetic radio observations of our
simulated objects. We have gained several key insights from this approach: 1.
The jet head in these multidimensional simulations is extremely complex. The
classical jet termination shock is often absent, but motions of the jet
terminus spin a ``shock-web complex'' within the backflowing jet material of
the head. 2. Understanding the spectral distribution of energetic electrons in
these simulations relies partly upon understanding the shock-web complex, for
it can give rise to distributions that confound interpretation in terms of the
standard model for radiative aging of radio galaxies. 3. The magnetic field
outside of the jet itself becomes very intermittent and filamentary in these
simulations, yet adiabatic expansion causes most of the cocoon volume to be
occupied by field strengths considerably diminished below the nominal jet
value. Thus population aging rates vary considerably from point to point.Comment: 44 pages, 6 figures; to be published in the Astrophysical Journal
(August 2001); higher-quality figures can be found at
http://www.msi.umn.edu/Projects/twj/radjet/radjet.htm
Quasiscarred modes and their branching behavior at an exceptional point
We study quasiscarring phenomenon and mode branching at an exceptional point
(EP) in typically deformed microcavities. It is shown that quasiscarred (QS)
modes are dominant in some mode group and their pattern can be understood by
short-time ray dynamics near the critical line. As cavity deformation
increases, high-Q and low-Q QS modes are branching in an opposite way, at an
EP, into two robust mode types showing QS and diamond patterns, respectively.
Similar branching behavior can be also found at another EP appearing at a
higher deformation. This branching behavior of QS modes has its origin on the
fact that an EP is a square-root branch point.Comment: 5 pages, 5 figure
Hydrodynamics of Cloud Collisions in 2D: The Fate of Clouds in a Multi-phase Medium
We have studied head-on collisions between equal-mass, mildly supersonic
(Mach number 1.5) HI clouds, in a standard Two-phase ISM (T_cl = 74 K, n_cl =
22 cm^-3, \chi = 100). We explore the role of various factors, including the
radiative cooling parameter \eta = t_rad/t_coll (t_coll=R_c/v_c), evolutionary
modifications on the cloud structure (by colliding clouds ``evolved'' through
independent motion within the intercloud medium (WIM)), and the symmetry of the
problem (by colliding initially identical clouds, evolved to different ages
before impact). The presence of bow shocks and ram pressure from material in
the cloud wake, developed during such evolution through the WIM, significantly
alters these interactions with respect to the standard case of non-evolved
clouds.
In general, in our adiabatic collisions the clouds are disrupted and convert
their gas into a few low density contrast clumps. By contrast, for symmetric
radiative cases we find that the two clouds coalesce, with almost all the
initial kinetic energy radiated away. On the other hand, for both adiabatic and
radiative collisions, asymmetric collisions have a much greater tendency to
disrupt the two clouds. Fragmentation of the clouds may occur, and
instabilities are in general enhanced. In addition, radiative cooling is less
efficient in our asymmetric interactions, so that those parts of the clouds
that initially seem to merge are more likely to re-expand and fade into the
WIM. Since the majority of real cloud collisions should be asymmetric for one
reason or another, we conclude that most gasdynamical diffuse cloud collisions
will be disruptive, at least in the absence of significant self-gravity or of a
significant magnetic field.Comment: Revised version, published in The Astrophysical Journal; 26 pages
Latex + 9 figures, mpeg animations available at
http://www.msi.umn.edu/Projects/twj/Cloud-Collision.htm
Winds and Shocks in Galaxy Clusters: Shock Acceleration on an Intergalactic Scale
We review the possible roles of large scale shocks as particle accelerators
in clusters of galaxies. Recent observational and theoretical work has
suggested that high energy charged particles may constitute a substantial
pressure component in clusters. If true that would alter the expected dynamical
evolution of clusters and increase the dynamical masses consistent with
hydrostatic equilibrium. Moderately strong shocks are probably common in
clusters, through the actions of several agents. The most obvious of these
agents include winds from galaxies undergoing intense episodes of starbursts,
active galaxies and cosmic inflows, such as accretion and cluster mergers. We
describe our own work derived from simulations of large scale structure
formation, in which we have, for the first time, explicitly included passive
components of high energy particles. We find, indeed that shocks associated
with these large scale flows can lead to nonthermal particle pressures big
enough to influence cluster dynamics. These same simulations allow us also to
compute nonthermal emissions from the clusters. Here we present resulting
predictions of gamma-ray fluxes.Comment: 12 pages, uses aipproc.cls and aipproc.sty, to appear in Proc. of the
International Symposium on "High Energy Gamma-Ray Astrophysics" (published as
a volume of AIP Conference Series) eds. F. Aharonian and H. Voel
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