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Recent papers on astro-ph
Wind
Accretion to Dipole Disk
Accretion to Dipole The Origin of Jets Accretion
Disks Theory Extrasolar Planets
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The origin of astrophysical jets
Theory and simulations of pointing jets
Recent work favors models where the twisting of an ordered
magnetic field threading an accretion disk acts to magnetically accelerate the jets. Our papers Ustyugova et al. (1995), Romanova et al. (1997), and Ustyugova et al. (1999) have to do with the origin of jets in the hydromagnetic regime. The papers Romanova et al. (1998), Ustyugova et al. (2000), Lovelace et al. (2002), Lovelace and Romanova (2003a, 2003b) have to do with the origin of jets in the Poynting regime. THEORY AND SIMULATIONS OF POINTING JETS
[up] SELECTED PAPERS:
"Relativistic Jets from Accretion Disks"
"Relativistic Poynting jets from accretion disks" "Magnetohydrodynamic
Origin of Jets from Accretion Disks", "Poynting
Jets from Accretion Disks",
"Poynting Jets from Accretion Disks:
Magnetohydrodynamic Simulations" "Magneto-Centrifugally
Driven Winds: Comparison of MHD Simulations with Theory", "Dynamics
of Magnetic Loops in Coronae of Accretion Disks", "Formation
of Stationary MHD Outflows from a Disk by Time-Dependent
Simulations", "Magnetohydrodynamic
Simulations of Outflows from Accretion Disks", "Dinamo
Model of Double Radio Sources" [up] MAGNETO-CENTRIFUGALLY DRIVEN WINDS: COMPARISON OF MHD SIMULATIONS WITH THEORY We have studied MHD outflows from a rotating, conducting accretion disk using axisymmetric simulations. The disk was treated as a boundary condition, and the initial poloidal magnetic field was taken to be a split monopole. The main conclusions of this work are: In many different runs we observed the
formation of stationary MHD outflows from the disk. Close to the disk
the main driving force is the centrifugal force. At larger distances
the main driving force is the magnetic force ~- For the considered
conditions, the slow
magnetosonic surface lies inside the disk. Above the disk the flow
accelerates and passes through the Alfvén and fast magnetosonic
surfaces, which are almost parallel to the disk. Within the simulation
region, the outflow accelerates from thermal velocity (~cs)
to a much larger asymptotic, poloidal flow velocity of the order of
0.5 The outflow is only slightly collimated within the simulation region. The collimation distance for the outflow, over which the flow becomes collimated (with divergence less than, say, 10°) is much larger than the size of our simulation region. This "poor" collimation is similar to that found in our earlier work (Romanova et al. 1997) using a different initial magnetic field and is qualitatively similar to the very gradual collimation found by Sakurai (1987). MHD simulations using much larger computational regions are needed to determine the collimation of the outflow at large distances. Furthermore, separate simulations are also needed to study the collimating influence of an external medium (Lovelace et al. 1991; Mellema & Frank 1998). The stationarity of the MHD flows
was checked in a number of ways, including comparisons of simulation
results with predictions of theory of stationary axisymmetric flows.
We found that: (1) fluxes of mass, angular momentum, and energy across
the surface z=0.5Zmax become independent of
time with high precision at early times of simulations t < 0.1 tout
, where tout ~ 2200 ti and ti
=2p ri
/ Different outer boundary conditions on the toroidal magnetic field Bf were investigated. We analyzed simulation results and found that the collimation of the jet and other characteristics of the flow depend critically on the outer boundary condition on Bf (as well as the shape of the simulation region, as is discussed below). We observed that the outer, "free" boundary condition on Bf leads to an artificial force that can give apparent magnetic collimation of the flow. "Force-free" and "force-balance" outer boundary conditions were also investigated. The "force-free" outer boundary condition was found to give valid flow solutions if the simulation region is not narrow in r-direction (compared with z-direction). The question of the optimum shape of simulation region was investigated. We have shown that if region is narrow in the r-direction, then an essential part of the Mach cones on the outer boundaries may be directed toward the inside of the computational region. This can lead to the influence of the boundary on the calculated flow and to artificial collimation. This effect is reduced or absent if the computational region is approximately square, if it is elongated in the r-direction, or if it is spherical. In these cases the Mach cones tend to point outside of the computational region. [up]
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(rBf
)2. The pressure
gradient force is much smaller than these forces, and it has no
significant role in driving the outflows.
,
where M is the mass of the central object and ri
is the inner radius of the disk. This asymptotic velocity is much
larger than the local escape speed and is larger than fast
magnetosonic speed by a factor of ~1.75. The acceleration distance
for the outflow, over which the flow accelerates from ~0% to, say, 90%
of the asymptotic speed, occurs at a flow distance ~80ri.
