THE SOUTHERN AURORAE : PART 2

OBSERVATION of AURORAL COLOURS

The colour of the auroral emissions varies considerably over most of the electro-magnetic spectrum. Typically, the colours vary anywhere between white, greenish-white, reddish, pink and blue. The most prominent colours seen in aurorae, is the white-greenish glow at 557.7nm. (nanometres), which is caused by electrically excited oxygen atoms. Reddish-pink and blue emissions are derived by the element nitrogen. Reddish aurorae are also produced by another ion of oxygen, radiating at the wavelength of 630.0 nm.

Most of the colours we see are also dependant on the height where this fluorescent process is happening. This is mostly based on actual numbers per unit time of light-emitting collisions occurring by the atoms or molecules in that level of the atmosphere. Some 100 kilometres up, where the atmosphere is denser (10¹² particles per cm^-3), much of the red and blue (or crimson) colours are produced by the slightly more plentiful nitrogen molecules.

Rising into heights between 100 km and 250 km., the atmosphere is much thinner (10⁸ to 10⁶ particles per cm^-3). As the atoms are further apart, the collisions average once every 0.5 to 0.8 seconds or so, where the fluorescence process begins to interact with the oxygen atoms instead. At certain atomic energy levels, the oxygen atoms start emitting yellow-green colours instead of red light. The wavelength emitted is produced at the so-called forbidden line. This is named this way because the 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 in vacuum tubes for them to glow. At low magnetic latitudes it is often the second process that is seen far more frequently than the nitrogen atoms producing the visible reds or blues. This is also because there is more energy available to ionise these oxygen atoms.

At even greater heights between 250 km and 1000 km (10⁶ to 10⁴ particles per cm^-3), another third fainter fluorescent process can start. This time the collisions between atoms is very much less, and this allows another forbidden line light colour to be produced — in this instance again emitted in several red wavelengths at around 700 nm. Here atomic collision at this height will only happen once every twenty to thirty seconds. This difference in time can mean that the aurora may seem to change colours or show borders between different layers, curtains or in tall columns of rays. As seen in the lower layers, sometimes the green colours seem to be chasing the magenta or purple colours, caused by the delay in the ionic reactions of the atoms and molecules.

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. Usually we can visually see all the primary colours (red, blue and yellow) and green. Sometimes they appear separately, sometimes not. When we see whites and yellow together we may be seeing mixtures of these colours. If you think back to your schooldays, you may remember the experiment in combining equal quantities of primary colours — principally by overlapping three red, green and blue coloured filters over three separate light sources. At the intersection of all three coloured circles was the combined colour — which happened to be white. (You might also remember that between the two of the colours I.e. red and blue, red and green, blue and yellow produced the secondary colours, being respectively, magenta (or purples), yellow and cyan.

Other fluorescence forbidden lines make different colour variations, which are found in the extreme ultra-violet (EUV), ultra-violet (UV) and infra-red (IR) wavelengths, but most are absorbed by the 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 5° 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 are more notable during times of high sunspot numbers. Visual observers have also reported that sometimes audible sounds have 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 observed with electric motors and the like.

Last Update : 13th September 2014

Southern Astronomical Delights © (2014)

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