The research to be undertaken here is focused on a significant controversy in solar physics and beyond - namely the value of the solar oxygen abundance. In the past few years, advanced 3D time-dependent radiation-hydrodynamics simulations of the solar atmosphere and subsequent formation of forbidden O I and OH absorption features have pointed to a reduction in the recommended solar oxygen abundance of nearly a factor of two. This revision brings the solar value near that expected from the present-day interstellar medium and nearby B stars. However, because of the role of oxygen as a significant interior opacity source, such values are in severe conflict with models of the solar interior and their excellent agreement with helioseismology. Moreover, recent studies of carbon monoxide rovibrational bands in the thermal infrared have raised questions concerning the validity of the 3-D convection models in the mid-photosphere and support an oxygen abundance more consistent with previous high values. Here further independent observational tests of the photospheric convection simulations will be developed, primarily in the thermal infrared where the important continuum and line opacity sources have a different, simpler, but complementary character to those in the visible. A secondary objective is to continue exploration of the outer boundary of the photosphere, the so-called Magnetic Transition Zone, where plasma control in the deep atmosphere gives way to magnetic dominance in the higher layers. In this enigmatic region, the 4.66 micron CO bands indicate the surprising presence of ultra-cool (T ~ 3500 K) gas protruding into what should be the warm, molecule-hostile chromosphere. Understanding this puzzle is essential to the full description of heat balance and energy transport in the outer atmosphere. New advances in solar IR spectral imaging technology will enable the observational studies that have been carried out in prototype for a decade. In addition, current thermal profile modeling will be extended to new tracers such as OH and Ca II, and a unique opportunity to coordinate IR imaging spectroscopy with a NASA solar UV sounding rocket flight will be exploited. This project will advance our knowledge of the Sun's atmospheric structure and energy transport, which is relevant to space weather. Beyond this, the composition of the Sun is a crucial baseline for many other areas of astronomy. Furthermore, the high-altitude COmosphere" provides a laboratory for the study of processes associated with molecular cooling catastrophes." The work here will provide graduate training and the associated instrument development will serve the broader National Solar Observatory user community. In addition to dissemination through professional publications and conference proceedings, a variety of outreach projects and community service activities relevant to the NSF mission will be carried out in connection with this work.
