The CO Fundamental Vibration‐Rotation Lines in the Solar Spectrum. II. Non‐LTE Transfer Modeling in Static and Dynamic Atmospheres

Abstract

We present a numerical method for solving radiative transfer in molecular vibration-rotation bands that allows for departures from local thermodynamic equilibrium (LTE) while accurately including a large number of lines. The method is applied to the formation of the CO fundamental vibration-rotation bands in several plane-parallel hydrostatic models and in a sequence of 20 snapshots from a radiation-hydrodynamics simulation of chromospheric dynamics. Calculations for the hydrostatic models performed with different values of the collisional coupling between different vibrational states confirm earlier results in the literature showing that the CO lines have LTE source functions in the solar atmosphere, so emergent CO intensities reflect actual temperatures therein. Only if the canonical collisional strengths are too large by more than 2 orders of magnitude would it be possible to explain the low temperatures derived from CO line core intensities at the solar limb by scattering in an atmosphere with much higher temperatures, consistent with the values derived from UV line and continuum and Ca II resonance line diagnostics. An interesting feature in the wavelength structure of the CO vibration-rotation bands is pointed out, in which pairs of lines can be found in different bands but of similar strength and wavelength. In principle such pairs provide a diagnostic for departures from LTE in the CO lines. CO line core intensity variations computed from the sequence of dynamical snapshots, which represent a typical episode in the chromospheric dynamics simulation, have an amplitude that is 2.5 times higher than observed. It is shown that this large amplitude is due in part to the up and down shift of the CO line formation region during the evolution of the atmosphere and is related to the assumption of instantaneous chemical equilibrium that was assumed to calculate CO concentrations. This suggests that the CO concentration is not in equilibrium, may be lower than would be expected on the basis of chemical equilibrium at the time-averaged mean temperature of the atmosphere, and may have reduced variations compared to instantaneous chemical equilibrium values at the local temperatures.