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		Airglow Spectrum - Green light from excited oxygen atoms dominates the glow. The atoms are 90-100 km (56-62 mile) high in the thermosphere. The weaker red light is from oxygen atoms further up. Sodium atoms, hydroxyl radicals (OH) and molecular oxygen add to the light.           The airglow is the light of electronically and/or vibration-rotationally excited atoms and molecules 80 km or higher.       Airglow vs Aurorae   Aurorae are at similar heights and are also the light of excited atoms. There is a difference, however, auroral excitation is by collisions with energetic particles whereas daytime short wavelength solar radiation produces the airglow via chemical excitation of which electronically excited oxygen atoms are the main component.      Production by sun's EUV radiation   The sun’s extreme ultraviolet light excites oxygen and nitrogen atoms and molecules in the thermosphere*. The energetic products collide and interact with other atmospheric components, including hydroxyl radicals (OH), to eventually produce light emission by chemiluminescence** and and decay of excited atoms and molecules.      Atomic oxygen green radiation   The brightest emission is green 558nm light from oxygen atoms in a layer 90-100 km high. The emission layer is clearly visible from earth orbit.      radiation vs collisional de-excitation   The excited atoms take about a second to decay to another lower energy excited state***. By atomic emission standards this is extremely slow and in that time many excited atoms lose their energy instead by collisions, mainly with nitrogen molecules. The emission does not occur at lower altitude because the collisional quenching is so severe, the extreme UV sunlight is less intense and there are fewer oxygen atoms      Atomic oxygen red light   The red radiation of atomic oxygen is from a lower energy excited state whose radiative half-life is an immensely long, 110 seconds^. This red airglow is only found at 150 - 300 km where collisions are so infrequent that the excited atoms have time to radiate away their energy. See also the red emission from OH radicals below..      Sodium   Another airglow component is the familiar yellow light from sodium atoms^^ in a layer at 92 km.          O2   There are weak blue emissions from excited molecular oxygen at ~95 km.       OH   Vibrationally and rotationally excited OH radicals emit red (image) and infra-red in a narrow layer (6-10 km FWHM) centered at ~ 86-87 km^^^.      Non uniformities Airglow is not always uniform. It can have bands and patches which shift and vary over minutes. Gravity waves propagating from the lower atmosphere modulate the atmospheric density, temperature and composition at airglow altitudes and thus the airglow intensity.      Diurnal changes Solar 11 year cycle   The airglow is brightest on Earth's day side where the original excitation occurs. The night airglow is (fortunately!) only one thousandth as bright and varies through the night. On a much longer timescale the airglow varies with the 11 year cycle of solar activity.              *  Above 100 km the atmosphere is mainly oxygen atoms and nitrogen molecules, molecular oxygen is dissociated into atoms by the solar extreme ultraviolet light.   **  Chemiluminescence is light emitted during a chemical reaction or later from the excited products of a reaction.   ***  The transition is O 1S to 1D. The singlet S to singlet D state transition is not allowed by quantum selection rules for dipole transitions. The transition probability is consequently low and the decay slow. The radiation is said to be 'forbidden'.    ^  The two red photons are from O 1D to ground state 3P transitions, also forbidden.     ^^  In contrast, the yellow sodium Na 2P to 2S transitions are selection rule permitted and occur very rapidly indeed. LIDAR studies show the sodium to be of meteoric origin rather than upwards transport of sea salt as previously thought. The sodium is present as gas phase bicarbonate (NaHCO3) and is activated by reaction with oxygen atoms.   ^^^  The O2 and OH emissions are (Meinel) bands of many closely spaced wavelengths because the transitions involve changes in vibrational energy together with smaller changes in rotational eneergy. The excited OH source is the Bates-Nicolet reaction between ozone and hydrogen atoms. The OH airglow is limited at higher altitudes by the rapid fall off in ozone concentration with height and at lower levels by the onset of rapid quenching of the excited products by collisions more frequent at the higher atmospheric pressures. The balance between the two limiting processes creates the narrow OH airglow layer.