By Jenny Tindall
For centuries the source of the ethereal display of the aurora borealis, known to many as the northern lights, was up to speculation. In many places, it was proof that the gods were there watching over. In ancient China, it was a great battle taking place between good and evil. For the Greeks, Aurora, the sister goddess of the god of the sun and goddess of the moon, was racing across the sky to tell her siblings to prepare for the next day. For the Cree, their deceased loved ones tried to communicate with them through the aurora borealis. After centuries of the lights representing good or bad omens, natural philosophers of the 18th century wanted an explanation. Several years later, and many research projects later, a conclusion has been made on the source of these magical lights.
Solar winds and magnetic fields
The sun is millions of degrees celsius. Because of this, there are frequent and explosive collisions between gas molecules. These charged particles escape from the sun and hurtle across space known as solar wind. Earth’s magnetic field deflects these particles. However, this magnetic field is weaker at the southern and northern poles, allowing some particles to enter the atmosphere (the layer of gases surrounding the earth). The lights are caused by the charged solar particles striking atoms in the earth’s atmosphere. Atoms have a central nucleus with a surrounding cloud of electrons. When struck with charged particles from the sun, the electrons of an atom move farther from the nucleus to higher-energy orbits. When returning to their lower-energy orbits they release a particle of light known as a photon. These particles of light are what make up the northern lights.
Factors affecting colours
There are two main factors that affect the colours seen in the aurora borealis: element type and distance from earth. The most common colours seen in the northern lights are pale green and pink. It is also possible to see red, yellow, blue and violet. The different colours are largely due to the type of gas particles that are colliding. When particles collide with oxygen, it produces both green and yellow light. Collisions with nitrogen produce red, violet and blue. The distance above the earth is the other main factor affecting the colour. Green and red usually occur at collisions around 241km above the earth, while blue and violet generally occur around 96.5km above the earth.
When and where are they visible?
The northern lights peak every 11 years with the solar cycle. The last peak was in 2014 and currently, we are in a solar minimum (meaning the activity is much lower). With the next solar cycle beginning, we can expect to reach another solar maximum between 2023 and 2026.
The lights can be seen in the northern and southern hemispheres. In the northern hemisphere, they are called the aurora borealis, and in the south, they are called the aurora australis. Generally, winter is the best season to see them, on a clear night around midnight. The lights are most easily seen in Northern nations bordering the arctic ocean. These include Canada, Alaska, Scandinavian countries, Iceland, Greenland and Russia. The aurora australis can technically be seen in the southern hemisphere, but it is difficult and they usually occur near the pole in Antarctica.
I have been interested in these beautiful lights since I was very young. One day I hope that I will be able to see them in Iceland or Finland. Seeing the beautiful colours in the sky the way people have for centuries.