Why does Aurora Borealis occur?

One of the most spectacular phenomena in nature is without doubt the amazing games of light, shades and colors called Aurora Borealis and Australis, depending on which of the two poles it is perceived at.


Those who live at the extreme north and south of Earth might at times experience this colored spectacular lights shimmering across the night sky. But what makes these lights  appear?

Well, it may sound weird but everything begins from the sun.

The temperature above its surface is millions of degrees Celsius. At this temperature, collisions between gas molecules are frequent and explosive. Free electrons and protons are thrown from the sun’s atmosphere by its rotation and escape through holes in the magnetic field. Blown towards the earth by the solar wind, the charged particles get in contact first of all with our planet’s magnetic field which may be thought as been generated by a giant rectangular calamite positioned at the centre of the Earth. The structure of a rectangular calamite’s magnetic field is well known and is based on closed field lines getting out of the south pole and entering the north one. Exactly the same happens on Earth where we have to imagine a giant magnetic shield protecting the whole planet surface, except for the source (south pole) and the pit (north pole) of the field lines which are necessariauroraly more exposed.

The charged particles scattered all around by solar wind are largely deflected by the earth’s magnetic field. In particular these charges are trapped by the force of the magnetic field and they start following the force lines being channeled either towards the south or the north pole.  Therefore some particles enter the earth’s atmosphere and collide with gas atoms or molecules at various heights. These collisions  excite gas particles causing them to light up. Sounds something similar to phosphorescence… 

What does it mean for an atom to be excited? Atoms consist of a central nucleus and a surrounding cloud of electrons encircling the nucleus at increasing distances from the centre. When charged particles from the sun strike atoms in Earth’s atmosphere, electrons move to higher-energy orbits, further away from the nucleus. Then when an electron moves back to a lower-energy orbit, in order to lose the amount of energy it has gained, it releases a particle of light or photon. The color of emitted light depends on the atom and on the size of the inner electron’s jump, but the result is absolutely amazing as it involves billions and billions of particles emitting light at the same time.

aurora3What happens in an aurora is similar to what occurs in the neon lights we see on many business signs. Electricity is used to excite the atoms in the neon gas within the glass tubes of a neon sign. That’s why these signs give off their brilliant colors. The aurora works on the same principle – but at a far more vast scale.

The aurora often appears as curtains of lights, but they can also be arcs or spirals, often following lines of force in Earth’s magnetic field. Most are green in color but sometimes you’ll see a hint of pink, and strong displays might also have red, violet and white colors. The lights typically are seen in the far north – the nations bordering the Arctic Ocean – Canada and Alaska, Scandinavian countries, Iceland, Greenland and Russia. And of course, the lights have a counterpart at Earth’s south polar regions.

The most common auroral color, a pale yellowish-green, is produced by oxygen molecules located about 60 miles above the earth. Rare, all-red auroras are produced by high-altitude oxygen, at heights of up to 200 miles. Nitrogen produces blue or purplish-red aurora.

Several fascinating  myths and legends are connected to the phenomenon of auroras.

aurora2In Finnish, the name for the aurora borealis is “Revontulet”, which literally translated means “Fox Fires.” The name comes from an ancient Finnish myth, a beast fable, in which the lights were caused by a magical fox sweeping his tail across the snow spraying it up into the sky. The Lapps, or the Saami, a people who are a close relative ‘race’ of the Finns, who live in Lapland — that is, north of the Arctic Circle, in what officially are Northern Finland, Sweden, and Norway — traditionally believed that the lights were the energies of the souls of the departed. In Norwegian folklore, the lights were the spirits of old maids dancing in the sky and waving.  Several of the Eskimo tribes also connected the lights with dancing. Eskimos in Eastern Greenland attributed the northern lights to the spirits of children who died at birth.

 

That’s it! Cool, isn’t it?

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by Francesco Pochetti

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