Does the Sun Have a Full-Time COmosphere?

Off-limb emissions of solar 4.7 μm rovibrational bands of carbon monoxide, recorded under excellent observing conditions with the Infrared Imaging Spectrograph at the McMath-Pierce telescope, are compared with theoretical translimb CO simulations based on time slices from the Carlsson & Stein dynamical model of chromospheric "K grains." In the Carlsson-Stein view, the solar outer atmosphere—in nonmagnetic internetwork regions—is a spatially and temporally intermittent wave-driven phenomenon, yielding an average thermal profile in the classical low chromosphere that is cool, not hot. Recent papers by Kalkofen and collaborators have criticized the dynamical model in favor of traditional "layered" stratifications, in which temperatures vary smoothly with altitude and are warm throughout the chromosphere. The present work sharpens the controversy by reiterating that traditional models with warm thermal profiles in the altitude range 500-1000 km fail two key "infrared CO" tests. The same tests reveal that the Carlsson-Stein dynamical model—which Kalkofen et al. argue is too cool in the low chromosphere—is not cold enough. Equally important, there need not be any contradiction between the existence of cool gas above the classical temperature minimum and observations of ubiquitous ultraviolet emission from the solar outer atmosphere, a central criticism by Kalkofen and collaborators of a full-time cold "COmosphere."