What Is the Aurora Borealis?

Auroras form when the solar wind — a stream of charged particles (mainly electrons and protons) from the Sun — interacts with Earth magnetosphere. During solar storms, bursts of charged particles are funnelled along magnetic field lines toward the poles, where they collide with oxygen and nitrogen atoms in the upper atmosphere (100-300 km altitude). These collisions excite the atoms, which release energy as photons of specific colours: oxygen emits green (557.7 nm, the most common aurora colour) at lower altitudes and red (630 nm) at higher altitudes, while nitrogen produces blue and purple. The aurora oval (a ring around the magnetic pole where auroras are most frequent) typically sits between 65 and 72 degrees latitude, but during intense geomagnetic storms it can expand equatorward, making auroras visible from places like London, New York, or Tokyo. The southern counterpart is called the aurora australis. Auroras also occur on Jupiter, Saturn, and even some exoplanets.