Hydromagnetic astrophysical outflows

Citation

Krasnopolsky, Ruben (2000) Hydromagnetic astrophysical outflows. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/j2ak-0b24. https://resolver.caltech.edu/CaltechETD:etd-09172008-092907

Abstract

NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. The launching, acceleration and stability of MHD outflows were studied numerically in two and three dimensions using a parallelized version of the ZEUS code. The launching from Keplerian accretion disks was investigated using time-dependent simulations, to determine parameter dependence and stability to 3D perturbations. The most critical factors controlling the cold outflows from the disk surface were found to be the poloidal magnetic field profile and the mass discharge rate: together they determine the acceleration of gas away from the disk, and the location of the Alfven surface, which, in turn, fixes the angular momentum loss and the asymptotic speed. The flows were found to be remarkably stable in 3D against perturbations of the initial conditions, at least in the formation region of the jet before the Alfven 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 may be relevant to some observed episodic sources. This intermittency can be suppressed if the mass loading has the angular dependence suggested by the magnetocentrifugal mechanism, namely that the discharge is a function of the angle θ between the poloidal fieldline threading the disk and the rotation axis, turning off when θ[...]30°. The mechanism of intermittency sets up a maximum mass loading to the observed smooth jets, may explain those that are pulsed, and shows a possible transition back and forth between both regimes. The result presented here may be compared to some recently published papers which suggest that intermittency could occur if the mass loading is too small: here it occurs when it is too large. If both results are generic, mass loading is bracketed for steady flows. Launching from disks was simulated using a cold disk and atmosphere. The number of boundary conditions that was imposed on the disk surface is what is necessary and sufficient to take into account information propagating upstream from the fast and Alfven critical surfaces, avoiding over-determination of the flow and unphysical effects, such as numerical "boundary layers" that otherwise isolate the disk from the flow, produce impulsive accelerations and confuse the connection between the disk parameters and the flow. The solar wind provides another example of an MHD outflow, using the high solar latitude observations by the satellite Ulysses. The simulations performed here allowed an estimate of the mean value of the azimuthal velocity, which is not directly accessible to measurement, and is necessary to estimate the torque of the solar wind. The Alfven point was found to be located at ~11R[...]. Similar outflows from faster rotators were simulated, and found to be collimated along the rotational axis.

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