Thermospheric Molecular Oxygen Density via Stellar Occultations Measured by SORCE

The thermosphere is the uppermost layer of the Earth's atmosphere, beginning at about 100 kilometers and ending at about 600 kilometers above the surface of the Earth. The majority of the x-ray and ultraviolet radiation from the sun is absorbed here. Due to photoionization, at about 100 kilometers, molecular oxygen begins to break down into oxygen. We used stellar occultation scans collected from the Solar Stellar Irradiance Comparison Experiment (SOLSTICE) instrument, on the Solar Radiation and Climate Experiment (SORCE) satellite to look at the density of molecular oxygen in this region. These stellar occultations measure the irradiance of a star as it sets relative to the satellite. As the light passes through the atmosphere, it is absorbed by the gas in the thermosphere, and thus the spectra of the star changes. This can quantify the type and amount of any particular gas in the thermosphere when compared to the star's original spectrum.

The measurements taken by SOLSTICE included both solar and stellar occultation scans. The stellar occultation scans were taken as a calibration of SORCE, and thus span a greater time period. SORCE took 818 stellar occultation scans over a seven-year period between October 2003, and October 2010, which is a good portion of the solar cycle and includes a solar minimum. Irradiance values taken were in the far ultraviolet range at wavelength range of 138.01 nm to 139.33 nm. Molecular oxygen absorbs these wavelengths. There is a good signal to noise ratio at altitudes from 125 kilometers to 225 kilometers. We present the density of molecular oxygen in the middle and lower thermosphere and show its variability over the solar cycle.