Empirical correction of thermal responses in the Solar Occultation for Ice Experiment nitric oxide measurements and initial data validation results.

The Solar Occultation for Ice Experiment (SOFIE) makes broadband transmission measurements centered at 5.32 µm to determine the concentration profile of nitric oxide (NO). These measurements show a signal oscillation due to detector temperature variations that severely limit the accuracy of NO retrievals if corrections are not applied. An empirical correction was developed to remove this instrumental error. This paper describes the correction, its impact on the retrieval, and presents a comparison from 87 to 105 km versus coincident atmospheric chemistry experiment-Fourier transform spectrometer (ACE-FTS) measurements. The southern hemisphere (SH) shows excellent agreement between the datasets, with statistically insignificant differences. The northern hemisphere (NH) SOFIE measurements exhibit a low bias of -18.5% compared to ACE-FTS. NH measurements (sunrise observations) are still under study, and only SH NO data (sunset observations) are currently publicly available as of SOFIE data version 1.2.

High precision refraction measurements by solar imaging during occultation: results from SOFIE.

A new method for measuring atmospheric refraction angles is presented, with in-orbit measurements demonstrating a precision of +/-0.02 arcsec (+/-0.1 microrad). Key advantages of the method are the following: (1) Simultaneous observation of two celestial points during occultation (i.e., top and bottom edges of the solar image) eliminates error from instrument attitude uncertainty. (2) The refraction angle is primarily a normalized difference measurement, causing only scale error, not absolute error. (3) A large number of detector pixels are used in the edge location by fitting to a known edge shape. The resulting refraction angle measurements allow temperature sounding up to the lower mesosphere.

On-orbit calibration of HALOE detector linearity.

The Halogen Occultation Experiment (HALOE) conducted satellite solar occultation measurements for 14 years ending on 21 November 2005. HALOE contained a calibration wheel, which included three neutral density filters that were used to examine response linearity through a combination of ground and on-orbit measurements. Although measurement uncertainties preclude a confident assessment of the true extent of nonlinearity, the on-orbit data lead to the conclusion that any existing response nonlinearity has changed by less than 2% over the mission lifetime. This conclusion eliminates a potentially significant uncertainty when using HALOE data for studies of long-term atmospheric trends.