98,101 research outputs found
Alfven wave transport effects in the time evolution of parallel cosmic-ray modified shocks
Some of the issues associated with a more complete treatment of Alfven transport in cosmic ray shocks are explored qualitatively. The treatment is simplified in some important respects, but some new issues are examined and for the first time a nonlinear, time dependent study of plane cosmic ray mediated shocks with both the entropy producing effects of wave dissipation and effects due to the Alfven wave advection of the cosmic ray relative to the gas is included. Examination of the direct consequences of including the pressure and energy of the Alfven waves in the formalism began
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
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
Preliminary design of a geologic sample acquisition and transport device Final report, May - Oct. 1965
Design concept for breadboard model of geological sample and transport device for Surveyor projec
Precessing Jets and Molecular Outflows: A 3-D Numerical Study
We present 3-D numerical hydrodynamical simulations of precessing supersonic
heavy jets to explore how well they serve as a model for generating molecular
outflows from Young Stellar Objects. The dynamics are studied with a number of
high resolution simulations on a Cartesian grid (128x128x128 zones) using a
high order finite difference method. A range of cone angles and precession
rates were included in the study. Two higher resolution runs (256x256x256
zones) were made for comparison in order to confirm numerical convergence of
global flow characteristics. Morphological, kinematical and dynamical
characteristics of precessing jets are described and compared to important
properties of straight jets and also to observations of YSOs. In order to
examine the robustness of precessing jets as a mean to produce molecular
outflows around Young Stellar Objects, ``synthetic observations'' of the
momentum distributions of the simulated precessing jets are compared to
observations of molecular outflows. It is found that precessing jets match
better the morphology, highly forward driven momentum and momentum
distributions along the long axis of molecular outflows than do wind-driven or
straight jet-driven flow models.Comment: Accepted by ApJ, 31 pages, using aasms.sty, Also available in
postscript with figures via a gzipped tar file at
ftp://s1.msi.umn.edu/pub/afrank/3DJet/3DJet.tar.gz . For information contact
[email protected]
- …