Why are auroras green, red, pink and purple?

Different gases produce different light

Auroral colors begin with atoms and molecules in the atmosphere. Energetic particles excite those gases, which release light as they lose the added energy. The light is not an arbitrary rainbow filter: particular emissions are associated with particular atmospheric constituents and conditions.

Oxygen contributes the familiar green and also red auroral light. Nitrogen contributes blue, pink and reddish-purple emissions. Several emissions can overlap along your line of sight, so a photograph or a bright display can contain mixtures rather than neatly separated stripes.

Altitude helps arrange the colors

NASA's aurora overview associates common green emissions with oxygen roughly 100 to 200 kilometers up, while red oxygen emissions occur higher, around 200 kilometers and above. Nitrogen's emissions can add blue light and pink lower borders. These are broad guides to the physical layers, not a scale you can measure by holding a photograph next to the horizon.

A useful way to watch is to notice whether color changes accompany a change of shape or brightness. Does the lower edge become sharper? Does a red glow remain after an active curtain fades? Record what you actually observe without assuming every color requires a different event.

Your eyes and camera tell different stories

Human night vision is not equally sensitive to brightness, fine detail and color. Faint aurora can look pale or grayish even when a camera records green or red. A bright display may show much clearer color to the eye. Your experience can therefore differ substantially from the saturated images used to advertise an aurora trip.

If you are observing with friends, compare descriptions gently rather than insisting there is one correct color. Give yourself time away from bright screens and watch changes in the display. The motion, scale and structure may be more striking than the color, especially on a quieter night.

Map colors are a legend, not a sky preview

The colors on our aurora map encode modeled activity. A purple area does not predict that the sky above that place will be purple, and a brighter color is not a measurement of the shade your eyes will see. Read the legend as a forecast display, then use the sky itself to observe the actual emissions.

This distinction is useful when looking at any aurora product. Before interpreting a color, ask what the creator encoded: activity, intensity, a camera image, or a satellite measurement. Similar-looking graphics can answer very different questions. Keeping that question in mind makes both the science and the evening's observations easier to understand.

Common questions