Mentioned for the first time in the third millennium BC, and originally described as ‘shining lights’ or ‘large flashing arches’, northern lights are one of the most spectacular natural phenomena which can be observed on earth. Driven by charged particles emitted from the sun – the so-called solar wind – they can be seen in both the southern (aurora australis) and northern polar regions (aurora borealis), typically at latitudes of more than 60°. Their appearance is thereby restricted to the respective winter months (northern hemisphere: September to March), when the sky is dark enough during nighttime.
On its sun-shaded side, the Earth’s magnetic field has the shape of a long tail, reaching millions of light years behind. Once the solar wind spreads into space it develops its own magnetism, causing the earth’s magnetic field to deform. Charged particles travel along the tail verging towards the nocturnal side of the Earth where they enter the pre-atmospheric plasma sphere. The majority of the particles are bent towards the poles and ping pong back and forth between them – which in turn again increases the magnetic field strength, forming somewhat like a magnetic trap.
Especially in the polar regions, some of the charged particles are fast enough to enter the atmosphere. They collide with molecules such as oxygen, nitrogen, and hydrogen. The collision kicks out electrons sitting in the outer atom’s shell from their initial place. This process causes energy to be released in the form of light – the aurora! Quite often, free electrons can cause even further stimulations of molecules, hence light emissions, in the atmosphere.
Northern lights appear in several different types of colours and shapes. Ranging from stable arcs winding in east-west direction to curved bands or even spirals. Their shape mainly depends on the Earth’s magnetic field and the resulting movement of the charged particles. The colours of the aurora as we observe them are determined by the atmospheric compounds and the amount of energy they emit: Green light, for instance, is radiated by free oxygen atoms, while nitrogen ions typically release blue and purple rays.