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   Glowing Gases - Aurorae


Auroral light is mostly from electronically excited oxygen atoms. Green radiation prevails at low altitudes and red at higher.

Excited nitrogen molecules and nitrogen molecular ions produce pink and red at low altitudes.

Energetic particles from the magnetotail spiral downwards along magnetic lines of force to penetrate deep into Earth’s tenuous upper atmosphere, the thermosphere. The most energetic* reach down to ~80 km (50 mile). They collide** with the upper atmosphere’s atoms and molecules producing ionisation, dissociation and excitation. The clouds of excited atoms eventually radiate their excess energy to form the glowing shifting aurorae.

Most auroral light is from excited oxygen atoms. Above 100 km the atmosphere is mainly oxygen atoms and nitrogen molecules, the molecular oxygen is dissociated into atoms by solar extreme ultraviolet light.

The auroral green light is a single extremely narrow wavelength (557.7 nm) from very energetic oxygen atoms decaying to a lower, but still excited energy level***.   The radiative lifetime of the excited atoms is about a second and the decay is slow, an eternity by ordinary electronic transition standards.  In that time many of the excited atoms lose their energy instead by collisions with other atoms and molecules. The green radiation is only possible in the near vacuum of the upper atmosphere where collisions are less frequent. Also, there are few oxygen atoms below 100 km to produce it.

Oxygen atoms are also responsible for red aurorae. If oxygen’s green radiation is emitted grudgingly, its red light is even more so. The radiation is from less excited atoms decaying to oxygen’s lowest electronic level^. Their radiative lifetime is an immense 110 seconds and the atoms only have a chance to radiate above 150 km. At lower altitudes their energy is nearly always first lost in collisions.

Green oxygen aurorae are at 100 km up to about 150 km. Red oxygen aurorae are 150 km upwards to 250 km and more rarely to 600 km plus.

The other major thermosphere constituent, molecular nitrogen N2, is exceptionally stable and there are not many nitrogen atoms below 400 km to make aurorae. The few nitrogen atoms emit a faint green masked by that of oxygen. In very intense displays there is a deep red violet border beneath the usual green curtains. This is emission from excited molecular nitrogen. Nitrogen molecular ions produce purple blue aurorae at very high altitudes.
We only see aurorae because we are looking through tens to hundreds of kilometres of glowing gas. By sea level standards that 'gas' is a vacuum. At 100 km, the altitude of green aurorae, the atmosphere's pressure is a millionth of that at sea level and the mean distance an oxygen atom travels between collisions is about a metre^^. Even so, it undergoes about 500 collisions each second and any excitation is quickly removed. At 200 km, where the red oxygen aurora glows, the vacuum has hardened. The oxygen atom will travel on average 4 to 5 kilometre between collisions and will be hit on average only once every 7 seconds. Excited atoms then have ample time time to radiate their energy and their collective light over a layer perhaps tens of kilometres thick gives us the soft and elusive auroral glow. Lights in a vacuum.. ..almost!

*   Most excitation is by collisions with electrons. proton aurorae are rarer.
**   The particle energies range from 1 to 100 KeV - far greater than that of the original solar wind particles.

The transition is O 1S to 1D. The singlet S to singlet D state transition is not allowed by the quantum selection rules for electric and magnetic dipole transitions. The consequences are a very low transition probability, slow decay and very narrow band radiation. The transition is said to be 'forbidden'.

^   1D to ground state 3P transitions, also forbidden.
^^   A gas is an ensemble of particles with a range of velocities defined by the temperature and the particle masses. The greater the temperature the greater the spread of velocities and the greater their absolute values. The distances quoted are 'mean free paths' defined by gas kinetic theory. They are approximate and depend on temperature, pressure, and the gas moleculat weight.