Colors in the Atmosphere
A bright display of aurora borealis is photographed over Yellowknife, northern Canada. This curtain of color extends vertically through the atmosphere; its nearly vertical rays trace out the Earth’s magnetic field lines and point toward the magnetic zenith. The red light is created by glowing oxygen more than 200 km above our planet’s surface, while the green originates much lower―only about 100 km up. The purple originates with nitrogen.