Astronomy and Astrophysics – Astrophysics
Scientific paper
Dec 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998phdt........18c&link_type=abstract
Thesis (PHD). THE UNIVERSITY OF TEXAS AT AUSTIN , Source DAI-B 59/06, p. 2801, Dec 1998, 132 pages.
Astronomy and Astrophysics
Astrophysics
Scientific paper
Magnetohydrodynamics in various astrophysical systems has been investigated. In particular, the effect of rotation on the magnetic buoyancy and the general relativistic effect on the magnetohydrodynamics have been studied in detail. The analyses and the simulations are compared with the observations of the solar X-ray active regions, the spots of remote fast-rotating stars, and the jets in black hole candidates. The dynamics of magnetic flux tubes embedded in the solar or stellar atmosphere has been studied by a linear analysis and nonlinear 3D simulations. The linear analysis shows two major effects of a fast rotation on emerging magnetic fluxes: first, it stabilizes the Parker instability of a toroidal flux sheet located near to the equator, and second, it accounts for the large patches observed in the polar region of fast rotating stars. The linear stability analysis also provide hints as to what signatures might be observed in the courses of the evolution of G-type and M-type stars. The size of stellar spots may decrease when a star becomes older, due to the decreasing of the magnetic field. Three dimensional simulations of magnetic flux sheets (and tubes) under buoyancy instability have been carried out in co-rotating Cartesian coordinates. The combination of the buoyancy instability and the Coriolis effect gives rise to a mechanism for twisting the emerging magnetic-flux sheet (tube) embedded in the solar (or stellar) atmosphere into a helical structure. This can be a model accounting for the S-shaped active regions in the low latitudes of the Sun shown in the soft X-ray picture taken by the Yohkoh satellite. Qualitative comparisons of simulation results with several characteristic features of the sunspots are presented. Very close to the horizon of a black hole, the gravitational acceleration becomes so large that a vacuum can radiate (the Hawking effect). The temperature of this radiation can exceed the rest masses of electrons and positrons. It is shown that an electron-positron plasma is realized and self-sustained and there is an opaque layer around the horizon so that the apparent temperature of a black hole may be lower than the Hawking temperature. Outside the vicinity of this (Hawking) plasma but still inside the traditional accretion disk of a black hole, it is demonstrated that equilibrium solutions exist. This plasma is studied using the 3+1 paradigm of general relativistic magnetohydrodynamics. It is shown that the plasma is subject to the convective instability when the magnetic field is absent, and to the magnetic buoyancy instability when toroidal fields exist. These instabilities are stabilized by poloidal magnetic fields. Therefore, when a poloidal magnetic field is changed to a toroidal field by rotation, the plasma becomes unstable and jets are forming. This model can be applied to study the recent observations of the black hole candidate GRO J1655-40.
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