Annu. Rev. Astron. Astrophys. 1991. 29: 89-127 Copyright © 1991 by Annual Reviews. All rights reserved

2.3 Radiation from Terrestrial NO

Intense terrestrial NO - and -band radiation, as well as O₂ radiation due to chemical recombination, is present in Figure 4 near the beginning of the scan, in the wavelength range 1900 to 3200 Å. The terrestrial NO spectrum and altitude dependence were measured by Tennyson et al (108) and the result is shown in Figure 6. At the UVX altitude of 340 km, terrestrial NO emission when looking away from the Earth is not expected to be a problem. The spectrometer line of sight at the beginning of the scan in Figure 4 intersected the terrestrial limb, which accounts for the strong spectrum that is seen in the lower right hand part of the diagram.

Figure 6. NO - and -band emission as measured by Tennyson et al (108) who also obtained altitude profiles for the various emissions. Above 250 km or so, NO emission should not be a problem for upward-looking observers.

Most terrestrial line emissions are quite unimportant beyond 500 km for example. It should be kept in mind that many measurements that have been hoped in the past to be of the diffuse cosmic ultraviolet background have been made from much lower altitudes. If in such cases a spectrometer is used (as in Figure 4), there is no great difficulty in separating the unwanted noise from true signal. In contrast, if broad-band photometers are used - as has often been the case - it is much more difficult to make the case that one understands the physical origin of the count rate that emerges from the apparatus.