Models of Vertical Disturbances in the Interstellar Medium

This paper describes some interesting properties of waves in, andoscillations of, the interstellar medium in the direction normal to the planeof the Galaxy. Our purpose is to examine possible reasons for four observedphenomena: the falling sky in the northern hemisphere; the apparent presence ofclouds in absorption spectra when a sightline is occupied primarily only bywarm intercloud gas; the peculiar structuring of spiral arms involving clumps,spurs, and feathering; and the existence of an abundance of high stage ions faroff the plane of the Galaxy. We explored the reaction of the interstellar medium - in the verticaldirection only - to large imposed disturbances (initial displacements,expansive velocities, and compressions), and to the introduction of smallamplitude waves via oscillation of the midplane. Our findings included: 1) theanticipated growth in amplitude of high frequency waves with height; 2) thefour lowest normal modes for the oscillation of the atmosphere as a whole, asfunctions of the height of the outer boundary; 3) the time for material to`bounce' from one unusually dense state to the next as a function of height;and 4) the tendency for the disk to develop a hot outer halo, either after thepassage of a single shock from a large event, or in response to a continuousstream of small amplitude waves.Comment: This paper contains 31 pages of text and 23 figures. This paper was submitted to The Astrophysical Journal on 6/6/2000. It was revised and resubmitted on 9/29/200