Interplanetary Gas. III. a Hydrodynamic Model of the Corona.

The ionized interplanetary gas is viewed as an outer eztension of the solar corona and is discussed with the scalar equations of hydrodynamic motion, continuity, and the first law of thermodynamics with hea t conduction. This approach is contrasted with the evaporative theory of Paper II. Although there are differences in the results of the two theories arising from the different assumptions inherent in their formulations, similar solutions appear in the two cases. Specifically, the hydrodynamic treatment allows a solution in which both the ezpansion velocity and the density vanish at infinite distance. Indeed, it is argued that the high expansion velocities necessary for a steady "solar wind" require the basic assumption of an accelerating mechanism in the corona and that a solar wind is not an inevitable phenomenon Further, coronal observations offer no basis for suspecting an outward expansion of the interplanetary gas faster than several kilometers per second at the earth's orbit-corresponding to a gentle "solar breeze." At the earth's orbit the temperature is governed by adiabatic equilibrium and is predicted to be in the neighborhood of K. Closer to the sun, conduction appears to become important, as advocated by Chapman, giving a much smaller temperature gradient. The density at the earth is computed to be around 30 electmns/cm3.