The N2 a-X Lyman-Birge-Hopfield (LBH) bands are a prominent emission system extending throughout the far-ultraviolet spectrum, produced in the Earth's dayglow and aurora. These molecular bands have long been used as a diagnostic workhorse for characterizing the state of thermosphere, both in limb scanning techniques that retrieve N2 and O2 densities and in nadir viewing methods that interpret O/N2 column ratios. Over the years, a wide range of vibrational population distributions have been inferred from a number of remote sensing experiments, including direct excitation, direct excitation with cascade, and even some peculiar distributions. No clear consensus has been reached concerning the appropriate LBH vibrational distribution for the dayglow; the Franck-Condon/direct excitation distribution is often assumed. Recently, we have performed extensive analysis of LBH dayglow emission from several space experiments: Hopkins Ultraviolet Telescope data from the 1990 Astro-1/STS-35 mission, 1991 Ultraviolet Limb Imaging experiment spectra from STS-39, and High Resolution Airglow and Aurora Spectroscopy experiment data from 1999 to the present from the Advanced Research and Global Observation Satellite. The spectra have been uniformly analyzed using a Levenberg-Marquardt maximum likelihood solver customized for Poisson statistics. In all cases (including both down-looking and limb-viewing observing geometries), we observe non-Franck-Condon vibrational populations in the dayglow suggesting a cascade contribution to LBH.
AGU Spring Meeting Abstracts
- Pub Date:
- May 2002
- 0310 Airglow and aurora;
- 0355 Thermosphere: composition and chemistry