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What causes the Aurora Borealis?

Introducing a fascinating guest blog series from astronomer and researcher, Dr. Robin Catchpole, who reveals how the Aurora Borealis form.

In 2014 & 2015, Golden Eagle Luxury Trains hosted the esteemed astronomer and researcher Dr. Robin Catchpole as a guest astronomer on the Arctic Explorer rail adventure. In a series of guest blogs we will be sharing some of Robin’s expert knowledge and viewing tips of the Aurora Borealis , including ‘How to photograph the Northern Lights?’ and ‘When and where is best to see the Aurora Borealis?’, but first let’s address the question ‘What causes the Aurora?’.

What causes the Aurora Borealis?

The aurora occurs between 60 and 1000km above the ground and is caused by electrons and some protons, ultimately originating from the Sun, striking molecules and atoms of oxygen and nitrogen in the Earth’s atmosphere.

The electrons are channelled along the Earth’s magnetic field and converge towards the North and South poles where the field lines come closer together. If there was no atmosphere their open spiral motion along the lines of magnetic force would become tighter and they would reflect back, bouncing from pole to pole.

Occasionally one of these high-energy electrons strikes an atom. Some of the electron’s energy raises one of the atom’s electrons into a higher energy orbit around the nucleus and the remaining energy increases the atoms speed.

High above the earth, the atmosphere is very thin and a molecule or atom can travel a long way and for many seconds, before colliding with another atom. This gives the excited electron inside the atom, time to drop back to a lower energy and emit a photon, before it loses energy in a collision.

The rules of quantum physics, mean the emitted photon has a very precise energy, which means it has a very precise wavelength or colour. In the case of oxygen we see the colour green. Red and purple are also produced by oxygen and nitrogen. These colours can combine to give yellow and white. Protons from the Sun can also combine with electrons to form hydrogen and give a red glow at 656 nm.

The writhing curtains arise because moving electrons create magnetic fields, which interact with the Earth’s field and the resulting magnetic fields tell electrons how to move. The downward motion of the electrons creates the vertical structure. The vast extent of the aurora, stretching hundreds of kilometres into the sky, creates dramatic perspective effects.

The sharp base of the curtains is caused by the increase in density of the Earth’s atmosphere. This results in more frequent collisions, which take away the atom’s energy before its electron has time to radiate. Also, as the density increases, the oxygen atoms responsible for the green glow combine to form molecules. The red light from oxygen appears above the green, because the red emitting atoms must survive much longer between collisions, before emitting their light.

Aurorae also occur in the southern hemisphere but are seen less often because the oval of activity is close to Antarctica, where few people live. Aurorae are also observed near the poles of Jupiter and Saturn.

Read part 2 of Robin’s blog: ‘When and where is best to see the Aurora Borealis?’

This article was written by astronomer and researcher Dr. Robin Catchpole from the Institute of Astronomy, Cambridge, UK

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