One of the most spectacular sights of nature is the aurora borealis, also called the northern lights. They occur at heights between about 80km and 650km above the ground and are formed when charged particles of the solar wind from the Sun interact with our planet’s atmosphere.
If it weren’t for the magnetic field of the Earth, the surface of our planet would be constantly bombarded by these particles, making life on land impossible. It is also possible that our atmosphere would have been stripped away, as has happened to Mars, which has no magnetic field.
Charged particles can be deflected by magnetic fields. The Earth’s magnetic field forces the particles into areas of space around the planet called the Van Allen Belts. Since the magnetic field comes from the poles, the particles are funneled into our atmosphere above the North and South poles where they collide with gas molecules.
The most common colour is green, caused by the particles colliding with oxygen molecules 80 to 100 km up. Red is formed by collisions with higher altitude oxygen molecules whilst rarer blues and purples are created by collisions with nitrogen molecules.
The best displays occur in cycles of about 11 years, following the natural rythym of the sunspot cycle.