1,294 research outputs found
Vertical Structure of Stationary Accretion Disks with a Large-Scale Magnetic Field
In earlier works we pointed out that the disk's surface layers are
non-turbulent and thus highly conducting (or non-diffusive) because the
hydrodynamic and/or magnetorotational (MRI) instabilities are suppressed high
in the disk where the magnetic and radiation pressures are larger than the
plasma thermal pressure. Here, we calculate the vertical profiles of the {\it
stationary} accretion flows (with radial and azimuthal components), and the
profiles of the large-scale, magnetic field taking into account the turbulent
viscosity and diffusivity and the fact that the turbulence vanishes at the
surface of the disk.
Also, here we require that the radial accretion speed be zero at the disk's
surface and we assume that the ratio of the turbulent viscosity to the
turbulent magnetic diffusivity is of order unity. Thus at the disk's surface
there are three boundary conditions. As a result, for a fixed dimensionless
viscosity -value, we find that there is a definite relation between the
ratio of the accretion power going into magnetic disk winds to the
viscous power dissipation and the midplane plasma-, which is the ratio
of the plasma to magnetic pressure in the disk. For a specific disk model with
of order unity we find that the critical value required for a
stationary solution is , where the disk's
half thickness. For weaker magnetic fields, , we argue that
the poloidal field will advect outward while for it will
advect inward. Alternatively, if the disk wind is negligible (), there are stationary solutions with .Comment: 5 pages, 3 figure
Accretion into black holes with magnetic fields, and relativistic jets
We discuss the problem of the formation of a large-scale magnetic field in
the accretion disks around black holes, taking into account the non-uniform
vertical structure of the disk. The high electrical conductivity of the outer
layers of the disk prevents the outward diffusion of the magnetic field. This
implies a stationary state with a strong magnetic field in the inner parts of
the accretion disk close to the black hole, and zero radial velocity at the
surface of the disk. Structure of advective accretion disks is investigated,
and conditions for formation of optically thin regions in central parts of the
accretion disk are found. The problem of jet collimation by magneto-torsion
oscillations is considered.Comment: 6 pages, 4 figure
Field Evaluations of Herbicides on Vegetable, Small Fruit, and Ornamental Crops, 2000, 2001, & 2002
Field evaluations of herbicides provide the chemical industry, governmental agencies, such as IR-4, and the Arkansas Agricultural Experiment Station with an evaluation of herbicide performance on small fruit, vegetable, and ornamental crops grown under Arkansas conditions. This report provides a means for disseminating information to interested private and public service weed scientists
Three Disk Oscillation Modes of Rotating Magnetized Neutron Stars
We discuss three specific modes of accretion disks around rotating magnetized
neutron stars which may explain the separations of the kilo Hertz quasi
periodic oscillations (QPO) seen in low mass X-ray binaries. The existence of
these modes requires that there be a maximum in the angular velocity of the
accreting material, and that the fluid is in stable, nearly circular motion
near this maximum rather than moving rapidly towards the star or out of the
disk plane into funnel flows. It is presently not known if these conditions
occur, but we are exploring this with 3D magnetohydrodynamic simulations and
will report the results elsewhere. The first mode is a corotation mode which is
radially trapped in the vicinity of the maximum of the disk rotation rate and
is unstable. The second mode, relevant to relatively slowly rotating stars, is
a magnetically driven eccentric () oscillation of the disk excited at a
Lindblad radius in the vicinity of the maximum of the disk rotation. The third
mode, relevant to rapidly rotating stars, is a magnetically coupled eccentric
() and an axisymmetric () radial disk perturbation which has an inner
Lindblad radius also in the vicinity of the maximum of the disk rotation. We
suggest that the first mode is associated with the upper QPO frequency,
, the second with the lower QPO frequency, , and
the third with the lower QPO frequency, , where
is the star's rotation rate.Comment: 6 pages, 2 figure
Relativistic Jets from Accretion Disks
The jets observed to emanate from many compact accreting objects may arise
from the twisting of a magnetic field threading a differentially rotating
accretion disk which acts to magnetically extract angular momentum and energy
from the disk. Two main regimes have been discussed, hydromagnetic jets, which
have a significant mass flux and have energy and angular momentum carried by
both matter and electromagnetic field and, Poynting jets, where the mass flux
is small and energy and angular momentum are carried predominantly by the
electromagnetic field. Here, we describe recent theoretical work on the
formation of relativistic Poynting jets from magnetized accretion disks.
Further, we describe new relativistic, fully-electromagnetic, particle-in-cell
simulations of the formation of jets from accretion disks. Analog Z-pinch
experiments may help to understand the origin of astrophysical jets.Comment: 7 pages, 3 figures, Proc. of High Energy Density Astrophysics Conf.,
200
- …