What causes the Northern Lights?
The aurora borealis has occurred since the dawn of our planet. Dinosaurs walked under it, just as we do today. It is a constant of our world. But what creates this awe-inspiring phenomenon?
Why does the aurora occur?
The Northern Lights are caused by electrically charged particles entering the Earth’s upper atmosphere at a very high speed. Known as the solar wind, these particles originate from the Sun, which is constantly emitting waves of particles that travel between 300 and 500 km per second in all directions.
As the Earth travels around the Sun, a small fraction of particles from the solar wind collide with our planet. Around 98% of these particles are deflected by the Earth’s magnetic field and continue their journey into deep space. But a small percentage leak through the Earth’s magnetic field and are funnelled downwards towards the magnetic North and South poles.
When these charged particles hit the atoms and molecules high up in our atmosphere, the atoms become excited and then emit distinctive colours as they decay back to their original state. This creates two glowing rings of auroral emission around the North and South magnetic poles, known as Auroral Ovals.
What causes the different colours of the Northern Lights?
The Earth’s atmosphere is made up of different atoms at different levels in the atmosphere. It’s these atoms that become excited as particles from the sun collide with them, causing the colours we see in the Northern Lights.
The most common colour seen in the Northern Lights is green. When the solar wind hits millions of oxygen atoms in the Earth’s atmosphere at the same time, they emit the green hue we see from the ground.
The red light we sometimes see in the aurora is also caused by oxygen atoms. These particles are found higher up in the atmosphere and are subject to a lower-energy red-light emission. So although the red colour is always present, our eyes are five times less sensitive to red light than green, so we can’t always see it.
A large part of the Earth’s atmosphere is made up of nitrogen, but particles from the solar wind must hit these atoms much harder for them to become excited. Once this happens and the nitrogen atoms begin to decay, they emit a purple-coloured light. This is quite a rare colour to see, however, and usually only happens during a particularly active auroral display.