Nonequilibrium CO Chemistry in the Solar Atmosphere

Investigating the reliability of the assumption of instantaneous chemicalequilibrium (ICE) for calculating the CO number density in the solar atmosphereis of crucial importance for the resolution of the long-standing controversyover the existence of `cool clouds' in the chromosphere, and for determiningwhether the cool gas owes its existence to CO radiative cooling or to ahydrodynamical process. Here we report the first results of such aninvestigation in which we have carried out time-dependent gas-phase chemistrycalculations in radiation hydrodynamical simulations of solar chromosphericdynamics. We show that while the ICE approximation turns out to be suitable formodeling the observed infrared CO lines at the solar disk center, it maysubstantially overestimate the `heights of formation' of strong CO linessynthesized close to the edge of the solar disk, especially concerning vigorousdynamic cases resulting from relatively strong photospheric disturbances. Thishappens because during the cool phases of the hydrodynamical simulations the COnumber density in the outer atmospheric regions is smaller than what isstipulated by the ICE approximation, resulting in decreased CO opacity in thesolar chromosphere. As a result, the cool CO-bearing gas which produces theobserved molecular lines must be located at atmospheric heights not greaterthan 700 km, approximately. We conclude that taking into account thenon-equilibrium chemistry improves the agreement with the available on-disk andoff-limb observations, but that the hydrodynamical simulation model has to beeven cooler than anticipated by the ICE approximation, and this has to be thecase at the `new' (i.e. deeper) formation regions of the rovibrational COlines.Comment: 4 pages, 3 figures, accepted for publication in ApJ Letter