9          Airglow Appendix

Geocoronal emission, or airglow, was always present. Excitation mechanisms include resonant scattering of Solar emission lines, fluorescence, and electron impact. For species other than hydrogen, the gas is all local to the upper atmosphere. The scale heights vary with the Solar cycle, and the strengths of the emission lines vary accordingly. FUSE launched close to the maximum of the Solar cycle, and ended the mission during the minimum, so the airglow lines were stronger early in the mission than at the end. The distribution of hydrogen extends farther than the other species, and there is a non-negligible density of HI throughout inter-planetary space. The extended distribution of HI means that the telescope line of sight always encounters sunlit HI, and thus resonantly-scattered Ly β is present even during orbital night.

9.1           Upward-Looking Airglow

During normal science exposures, the line of sight was pointed away from the Earth, through the upper atmosphere. Emission was seen primarily from neutral atomic hydrogen, helium, nitrogen, and oxygen. During orbital night, emission was usually seen only from HI. Ordinarily, only Ly β was present, but other HI Lyman lines may be present when viewing at low Earth-limb angles. Emission from HeI 584 Å was also occasionally seen (in second order at 1168 Å) during orbital night. Emission lines present during orbital day are listed in Table 9.2‑1. Emission from O I and H I shortward of 918 Å is also present, but is faint and the lines are often blended.  See Morton (2003) for wavelengths of these features. The strongest lines of H I and O I are Ly β and 988.773 Å, respectively; line strengths decrease with wavelength shortward of these two lines. Emission to the low-lying excited states of the O I ground state also decreases with wavelength, and may not be seen for the multiplets shortward of 936.630 Å.

9.2           Downward-Looking Airglow

The emission spectrum seen when looking downwards towards the Earth is much richer than that seen looking up. Some spectra were obtained while pointed downwards, but the main impact of this emission for the FUSE project was excessive exposure of the detector at spots illuminated by this emission. These effects are described in Section 4.4.2.1, and changes to operations procedures to limit these effects are described in Section 6.3.2.2. The brightest lines are listed in Table 9.2‑2; for additional information on downward-looking airglow, see Feldman et al. (2001).