The auroras—known as the aurora borealis (or northern lights) in the Northern Hemisphere and the aurora australis (or southern lights) in the Southern Hemisphere—are breathtaking natural phenomena that light up the night sky, especially in higher-latitude regions. Unlike other celestial events like meteors and comets, auroras occur in the atmosphere. But what causes them?
Although auroras appear in Earth’s atmosphere, they are powered by forces from space. These forces, however, are not entirely mysterious. The Sun’s corona—the outermost layer of the Sun’s atmosphere, made up of hot ionized gas (plasma)—releases a continuous stream of charged particles, called the solar wind. This stream contains protons and electrons that travel through space. When some of these high-energy particles encounter Earth’s magnetic field, they are directed along magnetic field lines toward the atmosphere near the North and South magnetic poles.
Earth’s atmosphere is mainly composed of nitrogen and oxygen. When the solar particles collide with nitrogen and oxygen atoms, they strip away electrons, leaving the atoms in an excited state. As these atoms return to a stable state, they emit light, creating the auroras’ characteristic colors. Collisions with oxygen produce red or green light, while collisions with nitrogen result in green or purple hues.
During times of low solar activity, which are often associated with fewer sunspots, fewer solar particles reach Earth. As a result, the colorful auroras shift closer to the poles. On the other hand, during periods of high solar activity, when more plasma erupts from the Sun, larger amounts of solar particles reach Earth’s atmosphere. This can push the auroras farther south, occasionally reaching mid-latitudes. For example, the aurora borealis has been seen as far south as 40° latitude in the United States.
Auroras typically occur at altitudes of around 100 km (60 miles) but can form anywhere between 80 and 250 km (50 to 155 miles) above the Earth’s surface.