756 research outputs found
Buoyancy Instabilities in Weakly Magnetized Low Collisionality Plasmas
I calculate the linear stability of a stratified low collisionality plasma in
the presence of a weak magnetic field. Heat is assumed to flow only along
magnetic field lines. In the absence of a heat flux in the background plasma,
Balbus (2000) demonstrated that plasmas in which the temperature increases in
the direction of gravity are buoyantly unstable to convective-like motions (the
``magnetothermal instability''). I show that in the presence of a background
heat flux, an analogous instability is present when the temperature decreases
in the direction of gravity. The instability is driven by the background heat
flux and the fastest growing mode has a growth time of order the local
dynamical time. Thus, independent of the sign of the temperature gradient,
weakly magnetized low collisionality plasmas are unstable on a dynamical time
to magnetically-mediated buoyancy instabilities. The instability described in
this paper is predicted to be present in clusters of galaxies at radii from ~
0.1-100 kpc, where the observed temperature increases outwards. Possible
consequences for the origin of cluster magnetic fields, ``cooling flows,'' and
the thermodynamics of the intercluster medium are briefly discussed.Comment: 5 pages; 1 cartoon; ApJ in pres
Angular Momentum Transport in Particle and Fluid Disks
We examine the angular momentum transport properties of disks composed of
macroscopic particles whose velocity dispersions are externally enhanced
(``stirred''). Our simple Boltzmann equation model serves as an analogy for
unmagnetized fluid disks in which turbulence may be driven by thermal
convection. We show that interparticle collisions in particle disks play the
same role as fluctuating pressure forces and viscous dissipation in turbulent
disks: both transfer energy in random motions associated with one direction to
those associated with another, and convert kinetic energy into heat. The
direction of angular momentum transport in stirred particle and fluid disks is
determined by the direction of external stirring and by the properties of the
collision term in the Boltzmann equation (or its analogue in the fluid
problem). In particular, our model problem yields inward transport for
vertically or radially stirred disks, provided collisions are suitably
inelastic; the transport is outwards in the elastic limit. Numerical
simulations of hydrodynamic turbulence driven by thermal convection find inward
transport; this requires that fluctuating pressure forces do little to no work,
and is analogous to an externally stirred particle disk in which collisions are
highly inelastic.Comment: 15 pages; final version accepted by ApJ; minor changes, some
clarificatio
Convection-Dominated Accretion Flows
Non-radiating, advection-dominated, accretion flows are convectively
unstable. We calculate the two-dimensional (r-theta) structure of such flows
assuming that (1) convection transports angular momentum inwards, opposite to
normal viscosity and (2) viscous transport by other mechanisms (e.g., magnetic
fields) is weak (alpha << 1). Under such conditions convection dominates the
dynamics of the accretion flow and leads to a steady state structure that is
marginally stable to convection. We show that the marginally stable flow has a
constant temperature and rotational velocity on spherical shells, a net flux of
energy from small to large radii, zero net accretion rate, and a radial density
profile proportional to r^{-1/2}, flatter than the r^{-3/2} profile
characteristic of spherical accretion flows. This solution accurately describes
the full two-dimensional structure of recent axisymmetric numerical simulations
of advection-dominated accretion flows.Comment: final version accepted by ApJ; discussion expanded, references adde
On the Energetics of Advection-Dominated Accretion Flows
Using mean field MHD, we discuss the energetics of optically thin, two
temperature, advection-dominated accretion flows (ADAFs). If the magnetic field
is tangled and roughly isotropic, flux freezing is insufficient to maintain the
field in equipartition with the gas. In this case, we expect a fraction of the
energy generated by shear in the flow to be used to build up the magnetic field
strength as the gas flows in; the remaining energy heats the particles. We
argue that strictly equipartition magnetic fields are incompatible with a
priori reasonable levels of particle heating; instead, the plasma in
ADAFs (defined to be the gas pressure divided by magnetic/turbulent pressure)
is likely to be \gsim 5; correspondingly, the viscosity parameter is
likely to be \lsim 0.2Comment: 24 pages, ApJ submitte
Turbulence and Particle Heating in Advection-Dominated Accretion Flows
We extend and reconcile recent work on turbulence and particle heating in
advection-dominated accretion flows. For approximately equipartition magnetic
fields, the turbulence primarily heats the electrons. For weaker magnetic
fields, the protons are primarily heated. The division between electron and
proton heating occurs between and (where
is the ratio of the gas to the magnetic pressure), depending on unknown
details of how Alfv\'en waves are converted into whistlers on scales of the
proton Larmor radius. We also discuss the possibility that magnetic
reconnection could be a significant source of electron heating.Comment: 17 pages (Latex), incl. 2 Figures; submitted to Ap
Higher Spin Entanglement and W_N Conformal Blocks
Two-dimensional conformal field theories with extended -symmetry
algebras have dual descriptions in terms of weakly coupled higher spin gravity
in AdS at large central charge. Observables that can be computed and
compared in the two descriptions include R\'enyi and entanglement entropies,
and correlation functions of local operators. We develop techniques for
computing these, in a manner that sheds light on when and why one can expect
agreement between such quantities on each side of the duality. We set up the
computation of excited state R\'enyi entropies in the bulk in terms of
Chern-Simons connections, and show how this directly parallels the CFT
computation of correlation functions. More generally, we consider the vacuum
conformal block for general operators with . When two of the
operators obey , we show by explicit computation that
the vacuum conformal block is computed by a bulk Wilson line probing an
asymptotically AdS background with higher spin fields excited, the latter
emerging as the effective bulk description of the excited state produced by the
heavy operators. Among other things, this puts a previous proposal for
computing higher spin entanglement entropy via Wilson lines on firmer footing,
and clarifies its relation to CFT. We also study the corresponding computation
in Toda theory and find that this provides yet another independent way to
arrive at the same result.Comment: 56 page
Multiflow TCP, UDP, IP, and ATM Traffic Generation Module
Networking devices must be capable of processing traffic flows from multiple sources. In order to verify that such devices operates properly, a network testbench can be used to inject traffic into the device. The specification of the traffic flows can be difficult. At the low level, there are header fields, data checksums, and packet length fields that all must be formatted correctly. Further, there can be multiple flows of traffic that will arrive simultaneously. It is desirable to specify traffic at a high level of abstraction. A software program can then be written to parse the specification and generate the low-level data that is actually processed by the networking hardware. For this project, a traffic generation program was built that accepts high-level traffic flow specifications. The program generates a cell-by-cell representation of the combined traffic flows. These flows can then be read by a testbench and fed into a simulation. With a hardware module capable of sending traffic created from the above program, a hardware test can be conducted using traffic generated with this program
Galaxy Morphology from NICMOS Parallel Imaging
We present high resolution NICMOS images of random fields obtained in
parallel to other HST observations. We present galaxy number counts reaching
H=24. The H-band galaxy counts show good agreement with the deepest I- and
K-band counts obtained from ground-based data. We present the distribution of
galaxies with morphological type to H<23. We find relatively fewer irregular
galaxies compared to an I-band sample from the Hubble Deep Field, which we
attribute to their blue color, rather than to morphological K-corrections. We
conclude that the irregulars are intrinsically faint blue galaxies at z<1.Comment: 13 pages, including 4 figures. Accepted for publication in ApJ
Letter
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