What are the northern lights called?

Aurora borealis - northern dawn - named by Galileo, with the southern equivalent called aurora australis. They are caused by charged particles from the Sun being funnelled down the Earth's magnetic field lines towards the poles, where they collide with atoms in the upper atmosphere and knock electrons into higher energy states. The light is emitted as those electrons fall back.

The colours are a direct read-out of which gas is being hit and how high up. Oxygen at around 100 kilometres gives the familiar green; oxygen much higher, above 200 kilometres, gives a rarer deep red; nitrogen gives blues and purples along the lower edges. Because each is a specific atomic transition, the aurora is a spectroscopy experiment running across the sky.

The reason it matters beyond being beautiful is that the same solar storms that brighten it damage infrastructure. The Carrington Event of 1859 was strong enough that auroras were seen in the Caribbean and telegraph systems sparked, shocked operators and in some cases kept working with their batteries disconnected. A storm of that size today would hit power grids, satellites and navigation, and the estimates of the cost run into trillions.