1,735 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
Transmission Line Analogy for Relativistic Poynting-Flux Jets
Radio emission, polarization, and Faraday rotation maps of the radio jet of
the galaxy 3C 303 have shown that one knot of this jet carries a {\it
galactic}-scale electric current and that it is magnetically dominated. We
develop the theory of magnetically dominated or Poynting-flux jets by making an
analogy of a Poynting jet with a transmission line or waveguide carrying a net
current and having a potential drop across it (from the jet's axis to its
radius) and a definite impedance which we derive. Time-dependent but not
necessarily small perturbations of a Poynting-flux jet are described by the
"telegrapher's equations." These predict the propagation speed of disturbances
and the effective wave impedance for forward and backward propagating wave
components. A localized disturbance of a Poynting jet gives rise to localized
dissipation in the jet which may explain the enhanced synchrotron radiation in
the knots of the 3C 303 jet, and also in the apparently stationary knot HST-1
in the jet near the nucleus of the nearby galaxy M87. For a relativistic
Poynting jet on parsec scales, the reflected voltage wave from an inductive
termination or load can lead to a backward propagating wave which breaks down
the magnetic insulation of the jet giving . At the
threshold for breakdown, , positive and negative
particles are directly accelerated in the direction which is
approximately along the jet axis. Acceleration can occur up to Lorentz factors
. This particle acceleration mechanism is distinct from that in
shock waves and that in magnetic field reconnection.Comment: 8 pages, 6 figure
Kelvin-Helmholtz Instability of Counter-Rotating Discs
Observations of galaxies and models of accreting systems point to the
occurrence of counter-rotating discs where the inner part of the disc ()
is co-rotating and the outer part is counter-rotating. This work analyzes the
linear stability of radially separated co- and counter-rotating thin discs. The
strong instability found is the supersonic Kelvin-Helmholtz instability. The
growth rates are of the order of or larger than the angular rotation rate at
the interface. The instability is absent if there is no vertical dependence of
the perturbation. That is, the instability is essentially three-dimensional.
The nonlinear evolution of the instability is predicted to lead to a mixing of
the two components, strong heating of the mixed gas, and vertical expansion of
the gas, and annihilation of the angular momenta of the two components. As a
result the heated gas will free-fall towards the disc's center over the surface
of the inner disc
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