The Dynamical State of the Interstellar Gas and Field. II. Non-Linear Growth of Clouds and Forces in Three Dimensions
The growth of the two-dimensional instability of a gas in a gravitational field g confining a horizontal magnetic field B is followed into the non-linear regime to provide an illustration of the formation of discrete gas clouds from a more uniform medium. A shock forms as the gas falls together, dissipating the energy and making the process irreversible. The effective force field of an element of gas (suspended on the lines of force of a horizontal magnetic field) is calculated in three dimensions. The force field is made up of an attractive force of the same form as gravity but stronger by the factor g2/GB2, and a force which extends undiminished along the lines of force passing near the element of gas. The latter dominates the former at large distances and is attractive on the lines of force through, and above and below, the element of gas, and repulsive along the lines of force passing beside the element of gas. The over-all effect is a strong attraction, accompanied by vigorous streaming of gas both toward and away from an established cloud. Clouds are expected to accumulate gas in such a way as to increase their vertical dimension relative to the transverse dimension g X B.