However, most aurorae there occur over uninhabited areas. Many observers said that the Northern Lights they saw on April 6 were predominately red. Energetic particles striking oxygen molecules, about 200 miles up, is what causes these unusual, all-red auroras. Oxygen at lower altitudes, about 60 miles high, produces a brilliant yellow-green, the brightest and most common color. Nitrogen molecules also add color to the display.
Ionized nitrogen gives off blue light while neutral nitrogen glows red.
The nitrogens create the purplish-red lower borders and ripple edges seen in many auroras. Although auroral lights sometimes seem very low to the ground, they are in fact higher in the atmosphere than the highest of jet planes. The lowest fringes are at least 40 miles above the Earth, while the uppermost reaches of the aurora extend 600 miles above our planet. For comparison, the space shuttle orbits near 190 miles altitude. The farther you live from the auroral oval, the smaller your chances of seeing aurora.