These days, northern lights start to appear in the sky above us, leaving us mesmerized by their beauty – and wondering how exactly this natural spectacle is formed.
Text: Cora Dieterich
Over thousands of years, as the days began to become shorter and the nights longer towards the winter season, people of various cultures were mesmerized by strange lights dancing across the dark sky. Many myths evolved concerning the impressive phenomenon, and it was even said that ‘God is angry when the aurora flames’. The Sámi here in northern Europe call the northern lights “guovssahas” or “the light you can hear”. Nowadays, the reason for the Aurora borealis is well understood, but nevertheless the northern lights still withhold their mysterious beauty.
The physics behind the northern lights is surely as exciting as the phenomenon itself, and will broaden your amazement of the Aurora borealis. Please don’t be scared by the term ‘physics’ – this will only be a basic explanation you can impress your family and friends with (or your crush on a romantic date) – so keep reading!
Although the northern lights are seen at times when the sun is absent in our sky, they are caused by ‘the star’ of our solar system. Activity in the sun can result in the emission of clouds of hot plasma gas, mostly consisting of charged particles – pretty cool, right? These ejections lead to the withdrawal of the plasma gas from the sun, which is blown into our solar system as solar wind. It takes the wind only a few days to travel past Mercury and Venus before it reaches Earth. Here the charged particles come in contact with the magnetic field of our planet. Since the particles are charged, they flow alongside the magnetic field to the magnetic poles. As they travel closer and closer to the poles, they eventually collide with particles in the atmosphere surrounding Earth. The charged particles transfer their energy to these atmospheric particles and lift them into what is called an ‘excited state’. This is comparable to you eating a lot of granola bars to gain enough energy to study or work out – exciting, isn’t it? However, in the same way that you lose your energy over time, the excited particles in our atmosphere also fall back to their normal state, the so called ‘ground state’. This is what happens when you see the northern lights! As atmospheric particles make the transition back to their ground state, they lose the energy they got from the charged particles in the solar wind, and this energy can be seen as light – the northern lights.
Moreover, beside the light itself, the varying colours of the Aurora borealis – from green to red to purple – surely add to the magic of this spectacle. Now as I said, the sun particles collide with particles from our atmosphere. Our atmosphere consists of air, or more precisely mostly of nitrogen and oxygen. If nitrogen is lifted into an excited state, it emits red light. If oxygen is lifted into an excited state, it can emit different colours depending on the height of the collision: red light above 300 km, or green light between 100 – 300 km. Blue and purple colouration is caused by hydrogen and helium respectively, but can only be seen faintly by the human eye.
The Aurora borealis only occurs up here in the north, but Aurora australis can be seen near the south pole. Astonishingly, there is even Aurora on Jupiter. Similar to the Aurora on earth, it is also caused by charged particles interacting with gases near the poles of this gas giant, but can only be seen in far-ultraviolet images of the planet.
Now you know the basic facts about the northern lights. Congratulations! Feeling like a scientist, right? So besides all this knowledge, where is the mystery? Well, one big question remains. Scientist are not able to explain the different shapes of the lights… maybe these shapes are caused by spirits – who knows!