Though often considered a minor species, nitric oxide (NO) plays several key roles in the thermosphere. It terminates the ion-neutral chemistry due to its low ionization potential. Its long lifetime enables it to be transported to the middle atmosphere where it will catalytically destroy ozone. And as the primary heteronuclear species, infrared radiation from vibrationally excited NO is an important source of thermospheric cooling, as well as a key diagnostic for solar and geomagnetic forcing of the upper atmosphere.
In the 50 years since its first measurement [Barth, 1964], our understanding of the origins and fate of thermospheric NO has improved. Many have contributed in this endeavor, but none so prominently as Charles A. Barth himself. The comprehensive research program he initiated ranged from rocket and satellite measurements of NO to the ultraviolet spectroscopic data required to interpret these measurements, as well as the development of computational models to explain these observations. This talk will review the present state of our knowledge of lower thermospheric NO and address the many contributions of Barth and his students in getting us here (e.g. excited state chemistry, photoelectrons, solar soft x-ray irradiance). Aeronomic and laboratory measurements needed to further this work are presented and it is argued that in spite of its low mixing ratio (~10-4), NO is best thought of as a major species of the upper atmosphere.