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
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
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
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'.
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.