NO 2 air afterglow and O and NO densities from Odin‐OSIRIS night and ACE‐FTS sunset observations in the Antarctic MLT region

The continuum spectrum produced by the NO + O(+M) → NO 2 (+M) + hv chemiluminescent reaction has been detected in the upper mesospheric dark polar regions by Optical Spectrograph and Infra‐Red Imager System (OSIRIS) on the Odin spacecraft. For the sample period of 8–9 May 2005, Southern Hemisphere, limb radiance profiles of continuum spectra, resolved from OH airglow and auroral contamination, are inverted to obtain volume emission rate altitude profiles. The maximum observed differential brightness referred to zenith viewing is 1.2 × 10 7 photons cm −2 s −1 nm −1 at 580 nm with a measurement uncertainty 5 × 10 5 photons cm −2 s −1 nm −1 , for an analysis range 80–101 km. Atomic oxygen densities [O] required by the analysis to derive NO densities [NO] are determined from OSIRIS O 2 (b 1 Σ g + − X 3 Σ g − ) 0‐0 band night airglow observations and from Atmospheric Chemistry Experiment‐Fourier Transform Spectrometer (ACE‐FTS) sunset ozone observations. The derived Southern Hemisphere [O] map that shows pronounced longitudinal variation, maximum of 7 × 10 11 cm −3 , considerably exceeding MSIS model values, and suggests significant dynamical influence. Combining the continuum observations and the derived [O], a hemispheric map of derived [NO] is assembled that also shows considerable spatial variation, maximum of 1.1 × 10 9 cm −3 , measurement uncertainty of 3 × 10 7 cm −3 . Comparing the two maps, the geographical distribution of [NO] differs considerably from that of [O]. From a qualitative comparison, the distribution of derived [NO] is similar to that in coordinated GUVI LBH1 auroral precipitation images. The OSIRIS‐derived [NO] agrees with the measured ACE‐FTS [NO] to within 1 × 10 8 cm −3 . The estimated systematic uncertainty of the NO densities derived from OSIRIS observations is approximately 30%.