Other
Scientific paper
Nov 2000
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000phdt........10k&link_type=abstract
Thesis (PhD). CALIFORNIA INSTITUTE OF TECHNOLOGY, Source DAI-B 61/05, p. 2577, Nov 2000, 133 pages.
Other
3
Scientific paper
The launching, acceleration and stability of MHD outflows were studied numerically in two and three dimensions using a parallelized version of the ZEUS code. Launching from Keplerian accretion disks was investigated using time-dependent simulations, to determine parameter dependence and stability to 3D perturbations. The most important parameters controlling cold outflows from a Keplerian disk surface are the poloidal magnetic field profile and the mass discharge rate: they determine the acceleration of gas away from the disk, the Alfvén radius, the angular momentum loss and the asymptotic speed. These outflows were remarkably stable in 3D against perturbations of the initial conditions, at least in the formation region of the jet before the Alfvén surface. This is surprising in the context of previous studies. Intermittent flows are found when the mass discharge rate is too large for a given magnetic field profile. This intermittency can be suppressed by reducing mass loading, for instance using the angular dependence suggested by the magnetocentrifugal mechanism which turns off the mass discharge when q<~30°. This intermittency may explain the observed episodic sources, allows transitions between intermittent and steady regimes by varying the mass discharge, and implies an upper bound for the mass discharge of the observed steady jets. These results can be compared with recently published simulations presenting intermittency for very small mass discharges, suggesting that there may be also a lower bound. Launching from disks was simulated using a cold disk and atmosphere. The number of boundary conditions that was imposed on the disk surface is that necessary and sufficient to take into account information propagating upstream from the fast and Alfvén critical surfaces, avoiding flow overdetermination and unphysical effects, such as numerical ``boundary layers.'' The solar wind provides another example of an MHD outflow. The simulations performed here allowed an estimate of the mean value of the azimuthal velocity, a quantity not directly accessible to measurement, and necessary to estimate the torque of the solar wind. High solar latitude observations by the satellite Ulysses were used. Similar outflows from stars in faster rotation were simulated, and found to be collimated along the rotational axis.
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