Models of vertical disturbances in the interstellar medium

This paper describes some interesting properties of waves in, and oscillati ons of, the interstellar medium (ISM) in the direction normal to the plane of the Galaxy. Our purpose is to examine possible reasons for four observed phenomena: the falling sky in the northern hemisphere; the apparent presen ce of clouds in absorption spectra when a sight line is occupied primarily only by warm intercloud gas; the peculiar structuring of spiral arms involv ing clumps, spurs, and feathering; and the existence of an abundance of hig h-stage ions far off the plane of the Galaxy. We explored the reaction of t he interstellar medium-in the vertical direction only-to large imposed dist urbances (initial displacements, expansive velocities, and compressions) an d to the introduction of small-amplitude waves via oscillation of the midpl ane. Our findings included (1) the anticipated growth in amplitude of high- frequency waves with height; (2) the four lowest normal modes for the oscil lation of the atmosphere as a whole, as functions of the height of the oute r 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 dis k to develop a hot outer halo, either after the passage of a single shock f rom a large event or in response to a continuous stream of small-amplitude waves. We discovered that three of the four observed phenomena targeted are likely to be closely connected. Following a large expansion, material near the plane falls back first, with material at higher z then falling in upon it. This provides precisely the sort of velocity segregation observed in t he northern sky, at about 50 Myr after the event. In addition, this bounce time (and/or the period of the subsequent smaller oscillations, which is ab out twice the bounce time) may be linked to structure in the spiral arms, w ith vertical oscillations having been provoked by initial compressions in t he arms. Oscillations of the fundamental symmetric (breathing) mode of the ISM also produce substantial disturbances in the outer atmosphere. This can result in the production of an extensive layer of hot gas overlying the co oler disk material, i.e., a hot Galactic halo with a significant population of high-stage ions. Hence, three of the four phenomena may be natural resu lts of the simple existence of strong local compressions at the spiral arms and the associated vertical motions in a thick Galactic disk. Finally, the somewhat mysterious appearance of clouds in some absorption spectra can be produced by small-amplitude waves in the ISM. Under the right conditions, clouds will seem to appear through "velocity crowding," when in fact there are no density concentrations in space.