THURSDAY, 10 JUNE 2021The Northern Lights are without a doubt one of the most beautiful sights in nature and have inspired many stories to explain their cause, from the twinkling of the Valkyrie’s shields to the spirits of the dead playing football with a walrus skull. However, my personal favourite is that which is told by physics.
Kristian Birkeland was the first to suggest a scientific theory for the Northern lights in the early 1900s but even now the details are not fully understood. The phenomenon involves a complex interplay of solar physics, electrodynamics, and atomic physics. Here I will focus on the most visually spectacular part; how do the Northern Lights get their colours?
The Northern Lights are caused by the interaction of the solar wind with the Earth’s magnetic field. The solar wind consists of a plasma of particles including protons and electrons which are directed towards the Earth’s poles by its magnetic field. When these particles reach the upper regions of the atmosphere, they collide with atmospheric atoms and molecules and gain energy, which excites them.
After a particle has been excited, it moves back to a lower energy state by emitting a photon, the colour of which is determined by the energy difference between its excited and final states. These energy differences are unique to each element meaning each one will emit a unique spectrum of colours. Most of the colours we see in the Northern lights are caused by just two elements: oxygen and nitrogen. Oxygen is responsible for the green and yellow hues whereas nitrogen causes the reds, purples and sometimes blue.
To me, this understanding makes the lights even more beautiful as not only can I marvel at their beauty but also at the complexity of the mechanisms that created them.
Lucy Hart is a BlueSci contributor. Artwork by Josh Langfield