Aurora Borealis: What Causes the Northern Lights

The aurora is caused by charged particles from the Sun — the solar wind — interacting with Earth’s magnetic field and atmosphere. When those particles collide with oxygen and nitrogen atoms at high altitude, they excite the atoms into emitting light.

Oxygen at around 100 km altitude produces the most common green colour. At higher altitudes it produces red. Nitrogen produces blue and purple hues. The dance of colours depends on solar activity, atmospheric density, and your viewing angle.