On a Possible Interstellar Galactic Corona.

The physical conditions in a possible interstellar galactic corona are analyzed Pressure equilibrium between such a rarefied, high-temperature gas and normal interstellar clouds would account for the existence of such clouds far from the galactic plane and would facilitate the equilibrium of spiral arms in the presence of strong magnetic fields. Observations of radio noise also suggest such a corona. At a temperature of 106 degrees K, the electron density in the corona would be 5 X 10-4/cm3 the extension perpendicular to the galactic plane, 8000 pc; the total number of electrons in a column perpendicular to the galactic plane, about 2 X 1019/cm2 the total mass, about 108 Mo. The mean free path would be 4 pc, but the radius of gyration even in a field of 10-15 gauss would be a small fraction of this. Such a corona is apparently not observable optically except by absorption measures shortward of 2000 A. Radiative cooling at 106 degrees would dissipate the assumed thermal energy in about 10 years. Cooling by conduction can apparently be ignored, especially since a chaotic magnetic field of only 10-15 gauss will sharply reduce the thermal conductivity At 3 X 106 degrees, near the maximum value consistent with confinement by the Galaxy's gravitational field, radiative cooling is unimportant, and a corona at this temperature might be primeval. The energy source needed at the lower temperatures may be provided by material ejected at high speed from stars or possibly by compressional waves produced by the observed moving clouds Condensation of cool matter from the corona may perhaps account for the formation of new spiral arms as the old ones dissipate.