DYNAMICS AND SPECTRA OF MAGNETICALLY CUSHIONED RADIATIVE SHOCKS

This paper investigates the effects of magnetic fields on the evolutio n and spectral appearance of unstable radiative shocks. In this work, a detailed treatment of the ionization evolution, the photoionizing ra diation and its transfer through the gas, is coupled to a one-dimensio nal magnetohydrodynamics scheme. The cushioning effect of the magnetic field on the formation of secondary shocks in the postshock cooling f low is clearly demonstrated by our results. The field strengths requir ed to suppress shock formation are at least a factor two greater than predicted by linear stability analysis. Thus a field strength, transve rse to the flow, of 9-mu-G is required to suppress shock formation beh ind a 175 km s-1 shock, travelling into a density of 1 cm-3. Inclusion of the magnetic field results in realistic pressures and densities in the cool, postshock photoionized shell. This allows us to follow the dynamics of the evolving photoionization zone where much of the low ex citation optical radiation is emitted. We discuss characteristic featu res in the spectra of unstable shocks and describe spectral diagnostic s, based on the optical and UV line ratios and spatial intensity distr ibutions, for their identification.