Last week we walked through the gallery of the universe to observe the majestic broad strokes found strewn across the canvas of interstellar space. This week we’re going to wander slightly closer to home, and take a look at the beautiful natural lights put on for us by our very own skies. However, they do get a little bit of help from a certain life-giving star, not to mention a lot of physics…
Those well-travelled of you may have had the opportunity to witness firsthand the lights that shine above the Earth’s two poles, known as the auroras. Located in bands of irregular oval shape centered over the northern and southern poles, they are called the aurora borealis and aurora australis respectively. Loosely translated from Latin, these are the northern lights and southern lights, although in Roman mythology Aurora is also the name for the goddess of dawn, and they believed that she flew across the sky in the early morning to announce the arrival of the sun and the dawning of a new day. For now, however, I’m going to tentatively rule out the possibility that this phenomena is caused by a Roman deity because, you know, I have my doubts.
So who exactly is it then that puts on such a spectacular display for us? The Romans weren’t too far off. Our very own Sun, which is considered a god in almost every ancient mythology, is responsible. The intense heat from the Sun creates a plasma of charged particles and, when great storms brew on the surface of the Sun, these charged particles can be sent streaming towards the Earth in ‘solar winds’. These particles are largely deflected by the Earth’s magnetic field which envelops the planet. However, at the poles, the strength of this magnetic field is weaker and so some particles are able to enter into the Earth’s atmosphere. There they interact with the molecules in our upper atmosphere, which are primarily oxygen and nitrogen. When the particles collide, the gas particles in our atmosphere can become excited if enough energy is transferred, and are left in an unstable state. They de-excite very rapidly, releasing that energy in the form of a photon, which we subsequently observe as coloured light. The concept of energy states is covered briefly in an early post on fluorescent lights. The actual colour we see depends on the amount of the energy absorbed and emitted by the molecule, and this is partly dependent on the characteristic energy levels of a certain element. In the case of oxygen and nitrogen, we get a respective green or red, and blue or purple.
Auroras are not unique to Earth – they have been observed on many other planets, and even moons, in the solar system. The most noticeable of these are found on Jupiter, supposedly caused by solar winds as well, although volcanic activity from its moon, Io, also contributes.
If you’re looking for a chance to catch one of these views for yourself, it is best to try at a spot with little light pollution, far away from urban civilisation. Winter is also generally a better season due to the long, dark, clear nights. It is also definitely worth checking the solar cycle, to find a time period in which the Sun is particularly active and is more likely to emit large solar winds. I’m certainly going to be trying to catch a glimpse for myself, hopefully not in the far too distant future.