Airglow

In the case of Earth's atmosphere, this optical phenomenon causes the night sky never to be completely dark, even after the effects of starlight and diffused sunlight from the far side are removed.

Since then, it has been studied in the laboratory, and various chemical reactions have been observed to emit electromagnetic energy as part of the process.

The free atoms are available for this process, because molecules of nitrogen (N2) and oxygen (O2) are dissociated by solar energy in the upper reaches of the atmosphere and may encounter each other to form NO.

Other chemicals that can create air glow in the atmosphere are hydroxyl (OH),[3][4][5] atomic oxygen (O), sodium (Na), and lithium (Li).

In order to calculate the relative intensity of airglow, we need to convert apparent magnitudes into fluxes of photons; this clearly depends on the spectrum of the source, but we will ignore that initially.

[7] If we take the example of a V = 28 star observed through a normal V band filter (B = 0.2 μm bandpass, frequency ν ≈ 6×1014 Hz), the number of photons we receive per square centimeter of telescope aperture per second from the source is Ns: (where h is the Planck constant; hν is the energy of a single photon of frequency ν).

This calculation shows that reducing the view field size can make fainter objects more detectable against the airglow; unfortunately, adaptive optics techniques that reduce the diameter of the view field of an Earth-based telescope by an order of magnitude only as yet work in the infrared, where the sky is much brighter.

Scientific experiments have been conducted to induce airglow by directing high-power radio emissions at the Earth's ionosphere.

[8] These radiowaves interact with the ionosphere to induce faint but visible optical light at specific wavelengths under certain conditions.

Airglow over Auvergne, France
Yellow, green and red bands of airglow along Earth's limb as seen from space.
Types and layering of airglow above Earth
Airglow as pinkish orange sodium line at just below one hundred kilometers and a faint green line, at the edge of space and the lower edge of the thermosphere (invisible), sandwiched between green and red bands of aurorae stretching over several hundred kilometers upward and the pink mesosphere , white and blue stratosphere , as well as orange troposphere afterglow and silhouettes of clouds at the bottom.
Two images of the sky over the HAARP Gakona facility using the NRL-cooled CCD imager at 557.7 nm. The field of view is approximately 38°. The left-hand image shows the background star field with the HF transmitter off. The right-hand image was taken 63 seconds later with the HF transmitter on. Structure is evident in the emission region.