Thermal bifurcation in the solar outer atmosphere

It is suggested that two distinct plasma thermal states are possible in the solar outer atmosphere because of the bifurcated character of the low-temperature cooling function at small optical depths. In radiative equilibrium, the plasma is strongly cooled to temperatures well below 4000 K by surface emission in the Delta V = 1 fundamental vibration-rotation bands of CO. However, when significant mechanical energy deposition is present in addition to the radiative heating component, the only effective cooling channel available to stabilize the plasma is optically thin emission in the recombination continuum of H(-). Consequently, thermal equilibrium in a mechanically heated atmospheric zone can be attained only for temperatures above the critical temperature of 4900 K because H(-) is itself a net radiative heating agent for temperatures cooler than the critical temperature.