What causes auroras? How the northern lights get their colours

From the Sun to a glowing atmosphere

The Sun continually sends charged particles into space. This flow, called the solar wind, interacts with the region controlled by Earth’s magnetic field. Energy can accumulate there and later be released, accelerating particles towards the upper atmosphere. Collisions with atmospheric gases then produce light. [1]

There are three distinct parts to this explanation: an energy source, a magnetic environment that shapes particle motion, and gases that emit the visible light. An aurora is therefore not a beam of green sunlight arriving at Earth.

Why are northern lights green, red or purple?

Oxygen produces the familiar green and some red emissions; nitrogen contributes blue and pink tones. The gas involved and the conditions at different heights both matter. NASA places common green oxygen emissions roughly 100–200 kilometres above the surface, with red oxygen emissions higher up. These are broad ranges, not sharp layers. [1]

A colour photograph records the outcome of several processes at once. It should not be treated as a precise altitude measurement simply because one band looks red.

Why are auroras usually seen near the poles?

Earth’s magnetic field helps direct particle motion towards the polar regions. Auroral activity forms belts around the magnetic poles, rather than a single spotlight over each geographic pole. Strong activity can bring visible displays to lower latitudes. The northern display is the aurora borealis; the southern counterpart is the aurora australis. [2]

What an aurora forecast can—and cannot—tell you

A forecast describes the likelihood and extent of activity. It is not a reservation for a visible light show. Darkness, clouds, local light pollution and your position still affect the view. [2]

• First check whether activity is expected near your location.
• Then check whether your sky is dark and sufficiently clear.
• Keep the forecast time and your local time zone separate.

This checklist separates a space-weather question from a visibility question. A disappointing view does not, by itself, mean that no aurora occurred.

For a different way the atmosphere changes light, read why the daytime sky looks blue.