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Airglow Chemistry   Intense banded airglow pictured by Yuri Beletsky at Carnegie Las Campanas Observatory in the Atacama Desert, Chile. "We just witnessed pretty amazing airglow here in Chile! It was the night of March, 17-18, and the airglow's intensity was so high that we could easily see its structure by eye - in B&W of course".   Image ©Yuri Beletsky, shown with permission



The symbols 1S, 1D and 3P are spectroscopic shorthand for an atom's precise electronic configuration.

Oxygen atoms have eight electrons. These have different states represented by ‘orbitals’. Orbitals can be thought of as shapes where most of the electron probability concentrates. They have different forms. ‘s’ orbitals are spherical, ‘p’ orbitals have two lobes and come in triplets mutually perpendicular. There are others more exotic.

Two of the oxygen’s electrons occupy an inner lowest energy 's' orbital. Two more occupy an outer 's' orbital. Things then get more interesting. The next higher energy orbitals for occupancy are three mutually perpendicular 'p' orbitals. Electrons have spin (call it up or down) and any one orbital can only have a single electron (up or down) or a pair (up and down). There are therefore three ways that electrons can arrange in oxygen’s p orbitals.

The lowest energy state (3P - 'triplet 'p'') has one orbital occupied by two electrons of opposite spin. The remaining two electrons of the same spin are each in a separate orbital.

The next most energetic state (1D - 'singlet dee') has paired electrons of opposite spin in two orbitals.

The highest energy (1S) has electrons of opposite spin singly occupying two orbitals.

These subtle electronic variations alter the atom's properties and have a major influence on upper atmosphere chemistry. Transitions between the states give us the greens and reds of airglow and the beauty of aurorae.

None of the structures and rules are arbitrary. They come wholly naturally from solutions of the Schrodinger equation (or its equivalents) for an atom with bound energy states.

Spin up and down electrons

Atomic 'p' orbitals. There are three at right angles and, in the absence of electric or magnetic fields they have the same energy

The airglow is emitted by electronically excited oxygen atoms. Its energy comes from the sun’s extreme ultraviolet radiation transferred to the oxygen by a number of absorption and collisional events.

A thin layer 90 to 100 km high emits the green light. The radiation (O 1S to 1D ) is not allowed by the quantum rules for everyday radiation and is said to be ‘forbidden’. But the quantum world has few absolutes and the radiation oozes out within about a second – a time comparable to the age of the Universe in atomic terms. The atoms can lose their energy in another way – via collisions with other atoms and molecules. The narrowness of the emitting green layer is the result of competition between radiative and collisional decay. At lower altitudes pressures are higher and collisions win. Higher up there are too few 1S atoms.

Other excited oxygen atoms emit the red light (1D to 3P radiation) at altitudes of 150 – 300km. Only at these rarefied heights can these atoms survive long enough to emit light for their radiative lifetimes are even longer – an immense 110 seconds.

The Bands result from waves (gravity waves) propagating upwards from disturbances in the troposphere. The waves modulate the density at the airglow height and switch the airglow on or off.