Airglow

During the daytime, solar EUV excites neutral atoms and molecules into higher energy states, as well as ionizing some of the neutral species to produce the ionosphere.  The neutral species decay, emitting radiation ranging from IR to EUV, which is called dayglow, which can be used to infer information on the composition and temperature of the atmosphere, i.e., remote sensing.

At night, the solar stimulation goes away, as does some of the ionization through recombination with the ambient electrons.  However, atomic ions ((such as ionized oxygen, O+) persist because the  recombination rate is much lower than for molecular ions.  Although much less bright than the dayglow, this nightglow can be used for remote sensing of the ionosphere. 

GOLD is designed to monitor the dayglow to obtain information about the daytime thermosphere, especially the dominant components, atomic oxygen and molecular nitrogen.

Nightglow

For the terrestrial, once the molecular ions have decayed away, the the remaining ionization mostly consists of equal parts of O+ ions and electrons.  Radiative recombination, the dominant loss process, results in many oxygen atoms left in excited states, denoted O*, which then decay to the ground state by emitting radiation ranging from IR to EUV.  The emission rate is proportional to the ion density and the electron density.  Since the ion density is equal to the electron density (charge neutrality), the emission rate is proportional to the square of the electron density.

While most of the nightglow is produced by radiative recombination described above, a small number of electrons combine with neutral oxygen to produce negative ions, denoted O-, which can then combine with O+ ions to produce pairs of neutral atoms, one or both of which may be in excited states and produce radiation at the same wavelengths as radiative recombination.  Under most nighttime conditions, the brightness from this source is less than 10% of the radiative recombination emission.

In addition, one of the spectral features that is detected by GOLD (OI 135.6 nm) is a resonance line of atomic oxygen, meaning that it can be absorbed and re-emitted in a random direction, a form of scattering.  The result is that GOLD, looking downward, sees not only the direct emission from recombination but also radiation absorbed and re-emitted by atomic oxygen that is below the source of the direct emission.  Because the neutral oxygen is generally much lower in altitude than the peak ionization density, this enhancement of the nightglow generally amounts to less than 10% of the nightglow brightness.

Observational Considerations

Besides being less bright than the dayglow, the nightglow also has more spatial structure, reflecting the spatial structure of the ionosphere.  For the dayglow, GOLD was designed to make repeated scans of each hemisphere (North, South)