Why Are Auroras Different Colors?
QUICK ANSWER
Auroras come in different colors based on which gas in the atmosphere is excited and at what altitude. Green (the most common color) comes from oxygen at 60 to 180 miles altitude. Red comes from oxygen above 180 miles. Blue and purple come from nitrogen at lower altitudes. Pink usually mixes red and green.
Auroras display a remarkable range of colors, from familiar greens to rare reds, blues, and purples. The colors aren't random: each one comes from a specific gas in Earth's atmosphere being excited at a specific altitude. The same physics that makes neon signs glow different colors gives auroras their dramatic palette. Understanding aurora colors reveals where in the atmosphere each part of the display is happening.
Why are most auroras green?
Oxygen at typical aurora altitudes glows green. According to NASA, the colors of an aurora reveal both where the lights are formed and what gases create them. Most auroras appear bright green because oxygen atoms at altitudes between about 60 and 180 miles release green light when excited. This altitude range is where most aurora activity happens, since it's where solar particles deposit the most energy. The eye is also more sensitive to green than other aurora colors, making green displays appear especially vivid.
Why are red auroras rare?
Red oxygen emission requires special conditions. Red light comes from oxygen atoms at altitudes above about 180 miles. At these high altitudes, atoms are spread out enough that they can stay in an excited state long enough to emit red light without being knocked out of it by collisions. The red emission takes several seconds to release after excitation, making it slow compared to green. Red auroras typically appear during strong solar storms when energy reaches higher altitudes. They often appear as a faint reddish glow above the brighter green displays.
What about blue and purple?
Nitrogen produces those colors. Blue and purple auroras come from nitrogen atoms and molecules at lower altitudes, typically below 60 miles. The colors appear when very energetic particles penetrate deeper into the atmosphere than the typical green altitude range. Blue is rarer than green and red because nitrogen requires more energy to excite. Pink and magenta colors often come from mixing red, blue, and other emissions, typically appearing along the bottom edges of green auroras. Strong solar storms produce the most colorful displays.
Why does my photo show more color than my eye?
Camera sensors are more sensitive than the human eye in low light. Auroras often appear paler to the human eye than in photographs because human color vision works poorly in dim light. Our rods (sensitive to low light) don't detect color, so faint auroras can appear ghostly white or pale green to direct viewing. Camera sensors capture the full color regardless of brightness. Aurora photographs often show vivid colors that observers may have seen as just faint glows.
Auroras come in different colors based on which atmospheric gas is excited and at what altitude. Green (the most common) comes from oxygen at 60-180 miles up. Red comes from oxygen above 180 miles. Blue and purple come from nitrogen below 60 miles. Camera sensors typically show more color than the human eye can in dim light. The most colorful displays occur during strong solar storms.
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