Most of the photographs I see of the aurora borealis are green. Is this due to the film used?

As the friends who joined me on the TravelQuest/Astronomy magazine Norway aurora tour in October can tell you, the northern lights are one of the few night-sky phenomena that look better in person than they do through a heavily processed image. The lights are truly green. They also can appear red, orange, yellow, magenta, and a variety of combinations. Green is the most common color.

The classic aurorae flash across the sky and move in curtains and arcs. These are called discreet aurorae, and they can be bright enough to read by. Diffuse aurorae are harder to see and typically look like fuzzy clouds, which are better detected by long camera exposures.

In either case, northern lights are just emissions of photons. The solar wind accelerates particles during their final 10,000-kilometer-plunge into the atmosphere, and then they roughly distribute perpendicularly along Earth’s magnetic field lines. That creates the arcs. Earth’s atmosphere is mostly nitrogen and oxygen. Aurorae occur when ionized nitrogen picks up an electron and smashes into oxygen and then they both return to a ground state. That is, the oxygen radiates away the energy in greenish wavelengths.

When oxygen is excited in the atmosphere, it typically happens at lower altitudes. But in intense storms, it also can happen at higher altitudes and with other gases, which creates a plethora of colors.

As you watch the northern lights dance, they also appear to have a bottom level. This is because atomic oxygen is less common below 62 miles (100 kilometers) altitude in Earth’s atmosphere.