High-precision solar occultation for sounding of the thermosphere and mesosphere

The Solar Occultation for Ice Experiment (SOFIE) is one instrument proposed for the recently selected Aeronomy of Ice in the Mesosphere (AIM) satellite. SOFIE will characterize profiles of temperature, particle extinction, and gas concentration using differential solar occultation measurements with eight channel pairs covering wavelengths (λ) from 0.290 to 5.32 μm. Six of these broadband (~2% filter width) channels are designed to measure gaseous absorption (O 3 , CO 2 , H 2 O, CH 4 , and NO), and two are dedicated to particle absorption. Each channel uses two detectors, one that measures the target gas in a strongly absorbing spectral region and one that measures in an adjacent weakly absorbing spectral region. In addition to direct measurements, simultaneous difference signals measured for the eight channel pairs allow extraordinarily precision, resulting in a limb-path extinction of 10 -6 or less. Measurements in two CO 2 bands will be used to simultaneously retrieve profiles of temperature and CO 2 mixing ratio. In addition, temperature profiles will be retrieved in the lower stratosphere using measurements of the solar refraction angle versus height, an approach conceptually similar to that used in Global Positioning Satellite (GPS) refraction angle temperature retrievals. SOFIE measurements will accurately characterize temperature, gas mixing ratios, and particle extinction at altitudes from the tropopause into the lower thermosphere. Implementation and design issues, plus proof-of-concept are presented.