The Aurora Borealis is one of the world’s seven natural wonders, and with the exotic array of colours found in it, it isn’t at all a wonder why so many people wish to see it.
Both the Aurora Borealis (named by Pierre Gassendi, a French artist, after the Roman goddess of the dawn, Aurora, and the Roman god of the northern wind, Boreas) and the Aurora Australis (named Australis, meaning “Southern”1 ) have caught the attention of hundreds of men for hundreds of years, and records date back from the Vikings (The King’s Mirror, written in 1250).
In The King’s Mirror, a number of ideas on the formation of the Aurora Borealis, such as “the frost and glaciers have become so powerful there that they are able to radiate forth these flames.”2
Not only the Vikings had their ideas on the Northern Lights, but also the Romans; Seneca the Younger classified the Northern Lights into a number of different categories depending on how they looked – there was the well (putei), casks (pithaei), chasms (chasmata), bearded (pogoniae) and cypresses (cyparissae).3
Much later in history, from 1902 to 1903, a Norwegian scientist by the name of Kristian Birkeland [figure 1] did extraordinary amounts of research into the Aurora Borealis. His theory was that the auroral electrojets (which are found in the auroral ionosphere)4 were connected to currents named in honour of him (Birkeland currents) that streamed along geomagnetic lines, flowing between the magnetosphere and high latitude ionosphere away from the polar region of the Arctic.
Birkeland’s theory of the auroral electrojets and Birkeland currents were a source of controversy when he was alive and even a number of years after his death. However, his theory was proved in 1967 when the USA sent a probe into space.5
Actual Causes of the Aurorae
Both of the Aurorae, the Borealis and Australis, are caused by solar particles in the solar wind (numbering in the hundreds of millions) colliding with the atmospheric shielding. These solar particles, without the atmospheric shielding, would make the Earth an inhospitable place to live. The solar particles are electrically charged when they collide with the atmospheric shielding surrounding the Earth. The energy resulting from these crashes is released as photons, innumerable particles of light, giving the intense colours of the Aurorae.
Seneca the Younger was right when he categorised the aurorae into different groups by how they looked, as they can vary vastly. The shimmering effect in most aurorae is produced by the fading particle explosions at the exact same moment that new collisions and explosions occur.
The colours of the aurorae are caused by two things:
1) The height of the collisions
2) The gases in the atmosphere
The green in the aurorae, the most common of all colours, is caused by low height collisions of the solar particles with oxygen, from heights of one hundred kilometres above the Earth’s surface. At greater heights of around 250 kilometres these collisions with oxygen produce red aurorae.6
The blues are found at the very bottom of the “aurora zone”, at only ninety-six kilometres from the Earth’s surface. They are caused by collision of solar particles with nitrogen found in the atmospheric shielding. However, at alternate altitudes nitrogen can also cause some pink and red colours as well. Purple can be seen when really energetic particles pierce deep into the atmospheric shielding about eighty kilometres above the surface of the Earth.
Solar storms can also cause aurorae. This can change the course of the aurorae, shifting them towards the equator due to the magnetic disturbance of the Earth by the sun.7
Aurorae found on other Planets
Just like aurorae on Earth [figure 2], other planets have their own versions. On Jupiter, the Jovian Aurora is found. These are caused by the same effect as that on Earth, by the solar particles colliding with an atmospheric shield. Even more similar is that Jupiter’s aurorae are at its poles, just like that of Earth.
Not only Jupiter and Earth have aurorae though. Saturn is another planet that has its own aurorae, caused by the same effect as that on Earth and Jupiter. However, Saturn’s aurorae have only recently been found by camera – the Cassini camera in 2008. Again, like Earth, Saturn’s aurorae are at its poles due to the magnetic fields, found on every planet, which force them either northward or southward.7