Computer Science
Scientific paper
Jan 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993phdt........55m&link_type=abstract
Thesis (PH.D.)--THE UNIVERSITY OF WISCONSIN - MADISON, 1993.Source: Dissertation Abstracts International, Volume: 54-06, Sectio
Computer Science
9
Parker Instability, Spiral Waves
Scientific paper
Upon consideration of recent observations consistent with a more distended vertical distribution of the galactic system of interstellar gas, magnetic field and cosmic rays than previously assumed, we present simple hydrostatic structures that model the equilibrium of the system in the gravitational field from stars in the solar neighborhood. The response of this thick layer of gas and nonthermal forms of pressure to various perturbations is studied utilizing a three dimensional magnetohydrodynamic code that solves the nonlinear fluid equations in the frozen-field approximation. We studied first the effect of undular and interchange modes of perturbation. The nonlinear development of the Parker instability is followed in two and three dimensions. It is found that the timescale for the instability in the thick layer is an order of magnitude longer than the timescale obtained in previous studies. The system achieves energy reduction by corrugating the central disk and forming discrete sheets of material that extend perpendicularly to the galactic midplane. This new equilibrium appears to be stable in our calculations. The theoretical prediction that the shortest waves perpendicular to the plane containing the magnetic field and gravity vectors have the largest growth rate is confirmed numerically. The response of the gas to a density wave perturbation is next simulated. When the gas in differential rotation encounters the spiral pattern, shocks develop. Even for weak shocks, interesting vertical motions of gas are produced. The transition resembles the hydraulic jumps that occur in incompressible fluids in that the scale-height of the gas changes through the front. The various flow regimes that result as a function of the values of three free parameters that determine the problem are numerically explored. Finally, a linear analysis is given for the system of partial differential equations that the code solves. A wave equation is derived.
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