The colour of the auroral emissions varies considerably over most of the electro-magnetic spectrum. Typically, the colours vary between white, greenish-white, reddish, pink and blue. The most prominent colours seen in aurorae, is a white-greenish glow, at 557.7 nanometres, that is caused by electrically excited oxygen atoms. The reddish-pink and blue emissions are derived by the element nitrogen. Reddish aurorae are also produced by another ion of oxygen, at a wavelength of 630.0 nanometers.
Most of the colours we see are dependant on the height where the fluorescent process is happening - which is mainly based on number of light-emitting collisions occurring by the atoms or molecules in the atmosphere. Where the atmosphere is denser, some 100 kilometres up, much of the red and blue colours are produced by nitrogen atoms.
Rising between 100km and 250 km., the atmosphere is even thinner. As the atoms are further apart, the collisions average about 0.5 to 0.8 seconds, and the fluorescence process begins to interact with the oxygen atoms instead. At a certain atomic energy level, the oxygen atoms start emitting a green light. The wavelength emitted is produced at a so-called forbidden line It is named this because fluorescence process cannot happen in the lower Earth’s dense atmosphere - being far too dense for “light-reaction” to take place. It is also the reason why all fluorescent lights have to have the air evacuate from the tube for them to glow. At low magnetic latitudes this is second process is seen far more often than the nitrogen atoms producing the reds or blues. This is also because there is more energy available to ionise these oxygen atoms.
At even greater heights, between 250km and 1000km, another third fluorescent process starts. This time the collision between atoms is now even less, and this allows another forbidden line light colour to be produced - in this instance again emitted in the red wavelengths, at xxx nm. Atom collision at this height will only happen one every twenty to thirty seconds.
With all these reactions are happening at various heights, it is clearly obvious that all three levels can occur at the same time. It is this reason that aurora events can be so bright and multicoloured.
Other fluorescence forbidden lines make different colour variations, which are found in the extreme ultra-violet, ultra-violet and infra-red wavelengths, but most are absorbed by the Earth’s lower atmosphere.
When an aurora occurs other phenomena can be observed. The significant disruption to the upper atmosphere and the ionosphere can produce changes in the position of both magnetic south and north by as much a 5o in an hour. This a be simply observed using a hand compass. A caused for this is that it produces great disturbances happening in the magnetic field, by so-called magnetic storms. Others observers have also noted the radio communications (like short-wave) and the telephone are badly disrupted, or in some instances even completely cut-off! Indeed, it is fortunate that this has happened, because during the Cold War the interruptions meant that visual sights of the aurorae became paramount.
However, the true physical cause of the aurorae is definitely related to the solar activity and during high sunspot numbers. Visual observers have also noted that sometimes sound has been heard during the time the aurora is at its brightest. Most report a sort of ‘swishing’, like the material of a silk dress or curtain moving through the air. The explanation to why this happens is thought to be caused by electrical discharge effects that are sometimes noted with electric motors and the like.
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