Measurements of thermospheric molecular oxygen from the Solar Ultraviolet Spectral Irradiance Monitor

We present a new data set of thermospheric O₂ density profiles retrieved from solar occultation measurements made by the Solar Ultraviolet Spectral Irradiance Monitor (SUSIM) instrument on board the UARS satellite. SUSIM is nominally a solar experiment whose mission was to measure the magnitude and variability of the UV solar irradiance. However, it was also capable of remotely sensing the Earth's upper atmosphere using occultation. SUSIM measurements of solar attenuation in the O₂ Schumann Runge continuum are used to retrieve O₂ density profiles between 110 and 240 kin. Between October 1991 and February 2005, SUSIM performed solar occultation measurements up to one day per week, measuring full-disk solar extinction as a function of tangent altitude at three nominal wavelengths (144, 161 and 171 nm). These data have been inverted using an optimal estimation algorithm to produce altitude profiles of O₂ density. This unique data set comprises approximately 1550 measurements, which span a wide range of solar and geomagnetic activity and latitudes up to 75° in each hemisphere. We present a discussion of the SUSIM instrument, the measurement technique employed for occultations, and the operational retrieval algorithm. A full error analysis and retrieval characterization study demonstrates that the SUSIM retrievals have uncertainties of 10-15% and a vertical resolution of 5-15 km, depending on altitude and measurement wavelength. The data thus have sufficient accuracy and resolution to provide scientifically useful constraints on the O₂ abundance and variability, and initial comparisons show good overall agreement between the NRLMSIS-00 model and the SUSIM measurements.