Many of us admire the spectacle that is the Northern Lights. Many chase them across the night sky. I’ve been lucky enough to visit both Iceland and Norway but didn’t see them on either trip. They are one of nature’s wonders, various colours of light dancing across the sky, but what causes them?
The Northern Lights are technically known as the aurora borealis, where aurora is Latin for ‘dawn, morning light’. Originally it was thought auroras were the start of a new day, the very early morning sunrise or light. There is an auroral zone over the poles (e.g. above the arctic circle in the north) where the majority of auroras occur, and are most clearly seen at night in a cloudless sky. (Image courtesy of Wikipedia)
On a very clear night, and with a strong aurora, we can be lucky enough to spot auroras near the equator. The last event recorded such as this for the Northern Lights was spotted in Honolulu in 1859. However, the aurora australis (i.e. Southern Lights) have been seen near the equator more recently in Samoa in 1929.
It’s all to do with the sun. Despite being 93 million miles away, the sun has more influence than you know. As well as producing light and heat for our day to day lives, the sun also has solar storms which are sudden releases of stored magnetic energy (i.e. solar flare). These lead to gusts of charged solar particles being ejected from the sun’s surface which can then travel across the solar system. If Earth is in the pathway of these particles, the Earth’s magnetic field and atmosphere of our planet will interact with them. The charged particles from the sun excite atoms in our atmosphere, causing them to light up. What does that mean?
To explain this we need to understand what makes up an atom. There are 3 major parts: the nucleus, which consists of protons and neutrons, and electrons which orbit the nucleus. Neutrons are neutral, protons are positively charged, and electrons are negatively charged. When charged particles from a solar flare come into contact with electrons in the atoms of the Earth’s atmosphere, this excites the electrons. This means the electrons gain energy and move further away from the nucleus of the atom to which they belong. However, as in the case of a child who has had too much sugar, electrons will have to come down from their excited state. When electrons lose this extra energy it must to go somewhere (i.e. the first law of thermodynamics – energy cannot be created or destroyed, only transferred). The excess energy is emitted as light energy and we see the aurora.
The larger the solar storm (i.e. geomagnetic storm), the further towards the equator the auroras can be seen. This is due to a stronger storm having a greater influence over the pull it has on our Earth’s magnetic field. Most aurora are seen as ‘curtains’ of light but can be arcs or spirals, and this is because the charged particles follow the Earth’s magnetic field. Now we know why auroras happen, what causes those various, beautiful colours? The colours stem from the energy emitted by the electrons, providing different wavelengths of light and is determined by the gases found in our atmosphere. The atmosphere mainly consists of nitrogen and oxygen, which emit characteristic colours dependant on their charge. Most auroras are green/yellow in colour due to oxygen presence, but aurora can also be red/blue, caused by nitrogen. The differing colours also depend on the altitude of the aurora.
In 2016, a new purple aurora-like phenomenon was spotted in the night sky over Canada and the Alberta Aurora Chasers group named it Steve. More recently however, it has been proven that Steve is not an aurora but in fact a mystery for now. The aurora used to be mysterious things, not fully understood and worshipped by some. Today we understand a great deal more about their origin and the science behind them, but that doesn’t mean we marvel at them any less. With their dancing colours, and magical appearance, the auroras still draw people towards the poles to witness them in all their natural beauty.