Aurora (Also known as the Northern Lights
and Southern Lights
There are two types of astronomical aurora visible on earth: the Aurora Borealis (Northern Lights) and the Aurora Australis (Southern Lights). The Aurora Borealis is visible in the higher latitudes of the northern hemisphere, while the Aurora Australis is visible in the lower latitudes of the southern hemishpere.
Other planets also display auroras, and they are visible from spacecraft and also from some astronomical observatories. Like the terrestrial auroras, these extra-terrestrial displays are linked with the magnetic fields of the planets concerned.
Viewed from the planet's surface, the aurora takes many forms, but a strong aurora is perfectly described by Carl Weyprecht, leader of an Austro-Hungarian expedition to Franz-Joseph Land in the 1870s. This description matches all modern, literal descriptions, and adds some poetry as well.
Scientifically speaking, the terrestrial aurora is the visible emission
s from ionised
atoms originating some 100 to 200 km above the earth's surface. The strongest emissions occur at the lower end of this band: around 110 km. Even at those altitude
s, air is comprised mainly of oxygen
. The visible colours are usually a yellow-green colour (557.7 nm
), which is the first, (low-energy) oxygen emission line
. When the display is especially active, the next oxygen
emission line (630 nm) comes into play. This is in the orange part of the spectrum. Nitrogen
contributes some red
colour (661.1 nm up to 686.1 nm). It also contributes blue
colours, not often visible to the naked eye
s of 391.4 nm and 427.8 nm.
The ionisation energy comes from the solar wind: charged particles originating in the sun's corona, and rushing toward earth at 500 km/sec or more. These particles are very energetic, and have enough oomph to knock electrons off the atoms and molecules of air high in the atmosphere. Once an electron has been knocked off, the atom becomes electrically conducting (an ion). Enough of these charged atoms, and the whole upper atmosphere can become a huge electrical conductor. Because the pressure is low at those altitudes, the resulting low-pressure plasma can generate light much like a standard fluorescent tube light. (Note, The plasma in a fluorescent tube generates ultra-violet, and this stimulates a phosphor to give the nice white colour, but it's the same principle). Below around 90 km, the pressure is too high for good electrical conductivity, so this marks the lower limit of the auroral display.
Near the magnetic poles, the earth's magnetic field lines dip down from the magnetosphere and funnel into the ground. Furthermore, these lines are packed tighter together (which means a stronger field). Charged particles can spiral down these lines by the motor effect (magnetism, current and force).
These elements together give the conditions for an active aurora. A powerful solar wind which ionises the upper atmosphere and a strong magnetic field, with the field lines pointing down to earth.
Seen from space the earth's aurora appears as a circular or elliptical glow around the magnetic poles. The affected zone varies enormously in size, according to the power of the aurora, but is typically 200 to 300 km in diameter, although can be as large as 2000 km in a particularly active display. The light is visible from a further 2000 to 3000 km, before the curvature of the earth puts the light below the horizon. The best time to see an auroral display is close to local midnight
Although the human eye can only detect the auroral light at night time, it continues during the daylight hours, but is only visible to special sensors at those times.
Because a strong display depends on a large number of energetic particles coming from the sun, and striking the earth's magnetosphere, the best time to see a display is a couple of days after a strong magnetic storm on the sun (it takes that long for the particles to reach the earth). The strength and power of the solar wind is noted on a number of websites, and those who want to see an auroral display can sign up to newsletters which alert them to a powerful magnetic outburst.
Myths and legends
Some caves in southern France, where cro-magnon man lived 30 000 years ago have drawings thought to depict the northern lights. Chinese astronomers have systematic lists of displays starting in 687 BC. The Hebrew Bible (Old Testament) (Ezekiel 1) contains what I take to be a clear description of the aurora. There are many other descriptions in history and pre-history.
Indigenous people have their own myths. The Finnish word, revontulet, recalls a fire fox shedding sparks from its fur and brushing its tail in the snow, to cast the light into the sky. Other native people call upon the spirits. In Canada, the Inuit say that anyone who met a violent death will light his dangerous way to a final resting place with torches, which appear as the aurora. Some Canadian Inuit believe a fast-moving aurora is the dance of death.
In the Americas, one tribe referred to the lights as the souls of defeated enemies wishing to revenge their death. The Mandan Indians thought the aurora was a fire on which the spirits cooked their dead enemies.