The Structure of Transverse Hydromagnetic Shocks in Regions of Low Ionization

Abstract

SUMMARY The compression of a magnetic field by a shock in a predominantly neutral gas is discussed. By solving the fluid equations, a steady-state structure is determined for a one-dimensional shock front propagating through a partially ionized gas in a direction perpendicular to the field lines. Temperatures and velocities of ions, electrons, and atoms are calculated as a function of the spatial coordinate moving with the shock frame. When the degree of ioniza-tion is small, field compression occurs at the expense of the momentum carried by the neutral atoms. The neutral momentum is transferred indirectly to the field by way of the ions. To effect the transfer, many ion-atom collisions are required. Shock widths from several hundred to several times 10 4 ion-atom mean free paths are required. Within the broad profile, a narrow atom shock is imbedded near the hot end if the shock speed exceeds a critical value. Within the broad region of changing field, the transient momentum in the ions causes them to drift relative to the atoms, and to become hotter than the atoms. Shocks in interstellar clouds of predominantly neutral hydrogen are discussed in detail. Within such shocks, the ion temperature rises to several hundred degrees hotter than the atom temperature, and ions drift relative to atoms with speeds of the order of a few kilometres per second. Detection of these ion-atom differences may be possible in the case of weak shocks, for it is only in weak, broad shocks that radiation carries a significant fraction of the energy flux. In the weakest shock for which numerical results are available, the radiative flux amounts to about 20 per cent of the convected flux. Suggested lines for observation of ion-atom differences are the ionized carbon line at 156 microns and the neutral oxygen line at 147 microns. It is suggested that ion-atom velocity differences within shocks lead to efficient removal of the magnetic field from a contracting protostar, thereby permitting fragmentation into masses as small as one solar mass. Application of the results to a model which has been proposed for solar flares, and to conditions in laboratory experiments are noted.