On Petschek's mechanism for dissipating interstellar magnetic fields

Abstract

Petschek's solution for the hydromagnetic flow at a magnetic neutral plane is applied to the interior of interstellar clouds. The stationary flow between the opposed magnetic fields involves a streaming speed of about 3 km/s and a gas density of roughly 102atoms/cm3. At most one-tenth of all the gas resides in these streams, but their low temperatures should make them observable in absorption at 21 cm; they may even dominate the interstellar absorption lines of Ca+and of Na, since the abundance of these absorbing ions, proportional to the recombination rate of the dominant ions with electrons, depends on the square of the gas density. Optical and 21-cm lines may, therefore, represent somewhat different samples of the interstellar velocity and density distributions.In analogy to the geomagnetic field, which is separated from the interplanetary field, interstellar clouds may be separated from the general galactic magnetic field. Such magnetic bubbles can move through the galactic field without twisting it, but they can maintain their identity only if they contain a twisted magnetic field. This field leads to the operation of Petschek's Mechanism and makes these clouds prominent in the optical absorption lines. A full evaluation of Petschek's Mechanism must involve some plasma instability to reconnect the lines of force.Details of this work have appeared elsewhere (Wentzel 1966).