What causes the northern lights?

Charged particles from the Sun colliding with gases in Earth's upper atmosphere, guided to the polar regions by the planet's magnetic field.

The sequence:

The Sun emits the solar wind — a continuous stream of charged particles, mostly electrons and protons — and occasionally far larger bursts from coronal mass ejections and solar flares.

Earth's magnetic field deflects most of it. The magnetosphere acts as a shield, which is why the atmosphere has not been stripped away as Mars's largely was.

Some particles are funnelled to the poles. The magnetic field lines converge at the poles, and particles spiral along them into the upper atmosphere. This is why aurorae form in oval bands around the magnetic poles rather than uniformly.

Collisions excite atmospheric atoms. Incoming particles transfer energy to oxygen and nitrogen, raising their electrons to higher energy levels. When those electrons return to their normal state, they emit a photon at a wavelength specific to the atom and transition.

The colours are diagnostic:

Green — atomic oxygen at around 100–150km. The most common colour, because that transition is efficient and occurs where the atmosphere is dense enough for frequent collisions.

Red — atomic oxygen at higher altitudes, above roughly 200km. This transition is forbidden in quantum terms, meaning it happens slowly — the excited state lasts nearly a second, so at lower altitudes a collision de-excites the atom before it can emit. Red therefore appears only where the air is thin enough.

Blue and purple — nitrogen, usually at lower altitudes during strong activity.

Why visibility varies. Activity follows the roughly 11-year solar cycle, peaking at solar maximum, when displays are visible at much lower latitudes.

The southern equivalent is the aurora australis, and the two are near mirror images.

Cameras see more than eyes do, because long exposures accumulate light the dark-adapted eye perceives as grey.