The co fundamental vibration-rotation lines in the solar spectrum. I. Imaging spectroscopy and multidimensional LTE modeling

Spectroscopic imaging observations of the CO fundamental vibration-rotation transitions at 4.6 pm, obtained at the Kitt Peak McMath-Pierce facility, show that the dynamics of both the solar granulation and, to a lesser extent, the 5 minute oscillations play an important role in CO line formation. Spectroheliograms made in the cores of strong CO lines display an inverted granular contrast with dark areas corresponding to granule centers and a bright network corresponding to the intergranular lanes. This observation is confirmed by multidimensional radiative transfer modeling of CO line formation in a solar convection-simulation snapshot. Unfortunately, current granulation simulations do not extend to high enough layers in the atmosphere to model formation of CO lines into the chromosphere and close to the solar limb where they exhibit their anomalous temperature behavior. The presented transfer calculations facilitate the interpretation of the observed pattern, predicting that the darkest CO line cores at disk center are associated with the strong adiabatic expansion and cooling that occurs over granule centers when warm upflowing material runs into the steep density gradient of the stable layer above the photosphere. The calculated granulation intensity contrast in the CO line cores is considerably higher than observed, and the calculated spatially averaged line profiles at disk center are deeper than the observed ones. It is speculated that both discrepancies result from the assumption of instantaneous chemical equilibrium which may not be valid in the convective flows. If the CO concentration in the hot convective upflow cannot increase fast enough to adjust to the lower temperatures in the radiatively cooled layer above the photosphere, CO lines would form deeper in the atmosphere, have higher core intensities, and show less contrast, more in agreement with observations.