Thermal bifurcation in the solar outer atmosphere

Abstract

Two distinct plasma thermal states are possible in the solar outer atmosphere, owing to 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 carbon monoxide. 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/sup -/. As a result, thermal equilibrium in a mechanically heated atmospheric zone can be attained only for temperatures above T/sub crit/roughly-equal4900 K because H/sup -/ is itself a net radiative heating agent for temperature cooler than T/sub crit/. Thermal bifurcation effects in the solar outer atmosphere are encouraged by the likelihood that mechanical energy deposition is significantly enhanced in small-scale, discrete structures, namely, magnetic flux tubes.