OBSERVATIONS OF SOLAR CARBON-MONOXIDE WITH AN IMAGING INFRARED SPECTROGRAPH .1. THERMAL BIFURCATION

We describe long-slit spectroscopy of the solar 4.7 mu m carbon monoxi de (CO) Delta upsilon = 1 bands at the Main spectrograph of the NSO Mc Math-Pierce telescope. We utilized stigmatic imaging of the temperatur e- and velocity-sensitive CO absorptions to map quiet regions near dis k center and at the extreme limb. At Sun center the dominant long-live d spatial structures are small-scale hot spots associated with fragmen ts of the supergranulation network seen in cotemporal Ca II filtergram s. Oscillatory thermal and velocity fluctuations of the global p-mode interference pattern are a pervasive feature of the maps, but the rms amplitudes (approximate to 70 K and approximate to 240 m s(-1)) are pe rturbations on a relatively unstructured outer photosphere. We occasio nally see small-scale transient cooling episodes-longer lived than the p-mode wavepackets-that might be overshooting granules or rising magn etic flux ropes. The events are too rare, however, to influence the gl obal properties of the CO fundamental bands. Seeing-selected frames of the off-limb CO emissions show a typical extension of 0.'' 6 for the strongest lines, with little variation along the limb. The off-limb ex tensions indicate the presence of cool gas up to 350 km above the ''T- min'' of popular reference models of the solar chromosphere. We carrie d out two-dimensional model atmosphere simulations to study the effect s of thermal inhomogeneities on the disk-center, extreme-limb, and off -limb behavior of the CO lines. The models are spherically symmetric, static, and in LTE. Our data favor a scenario in which the bulk of the low chromosphere-below the base of the magnetic ''canopy''-is in real ity a ''COmosphere'' dominated by gas colder than the minimum temperat ure in conventional models. The moderate-scale (approximate to 5 ''), mild thermal perturbations of the p-mode pattern have little influence on the CO Delta upsilon = 1 spectra. Small-scale (approximate to 1 '' ) hot regions embedded in a cool average atmosphere are strongly ''sha dowed'' at the extreme limb. The shadowing is of little consequence, h owever, because the atmosphere already is dominated by the cool compon ent. The opposite scenario-small-scale cold regions in a warm average atmosphere-can produce effective shadowing at the limb for granule-siz e (approximate to 1 ''-2 '') dark points only if the covering fraction is relatively large (f greater than or similar to 0.2). That scenario is ruled out: it predicts high-contrast dark spots at disk center, co ntrary to our observations. We also argue against the possibility of s hadowing by even smaller, subresolution (approximate to 1 '') cold spo ts with f greater than or similar to 0.1. We show that multistep react ions, rather than direct radiative associations, dominate the gas-phas e chemistry of CO molecules under conditions typical of the outer phot osphere. The CO formation and radiative cooling timescales are fast en ough that low-temperature plasma conditions can be restored following disruption by a localized heating event such as a Ca II K-2V ''flash.' ' In cool giant stars, the chemical formation timescales are much long er than in dwarfs like the Sun. Nevertheless, the density dependence i s such that the molecular cooling proceeds proportionately more rapidl y than the gas dynamics ensuring an even more important role for autoc atalyzed ''thermal bifurication.''