The auroras—the aurora borealis (or northern lights) in the Northern Hemisphere, and the aurora australis (the southern lights) in the Southern Hemisphere—are brilliant natural spectacles that can be seen in the evening sky especially at higher latitudes. Unlike other phenomena of the night sky, such as meteors and comets, the auroras are atmospheric phenomena, but what causes them?
Although auroras appear in the atmosphere, they are the result of extraterrestrial forces; however, these forces are not particularly alien. The Sun’s corona—the outermost region of the Sun’s atmosphere, consisting of plasma (hot ionized gas)—drives the solar wind (a particle flux of protons and electrons) away from the Sun. Some of these high-energy particles strike Earth’s magnetic field and follow magnetic field lines down into Earth’s atmosphere at the North and South magnetic poles.
Earth’s atmosphere is mostly made up of nitrogen and oxygen. Once the solar particles reach Earth’s atmosphere, they collide with atoms of nitrogen and oxygen, stripping away their electrons to leave ions in excited states. These ions emit radiation at various wavelengths, creating the characteristic colors. Collisions of solar particles with oxygen produce red or green light; collisions with nitrogen produce green and purple light.
During periods of low solar activity—which are often associated with periods where the Sun has fewer sunspots—fewer of these high-energy particles are emitted from the Sun, and the shimmering sheets of color that characterize Earth’s auroral zones shift poleward. When the Sun is more active and larger amounts of plasma are erupting from the Sun’s surface, more particles reach Earth’s atmosphere, and the auroras occasionally extend to the middle latitudes. For example, the aurora borealis has been seen as far south as 40° latitude in the United States. The auroras typically occur at altitudes of about 100 km (60 miles); however, they may occur anywhere between 80 and 250 km (about 50 to 155 miles) above Earth’s surface.