In the Earth's far northern and far southern regions, dazzling displays of colored light sometimes appear in the night sky. These displays are called auroras. In the Northern Hemisphere such a display is known as aurora borealis, or the northern lights. In the Southern Hemisphere it is called aurora australis, or the southern lights. Auroras are named after Aurora, the ancient Roman goddess of the dawn.

Cause

Auroras are caused by the sun, but they are not sunlight. In addition to light, the sun also sends out electrically charged particles called electrons and protons (see atom). This stream of particles is called the solar wind. The solar wind travels from the sun toward the Earth at great speed.

The Earth is a huge magnet. It is surrounded by a magnetic field, and it has north and south magnetic poles that act like the ends of a bar magnet (see magnet and magnetism). (The magnetic poles are not the same as the North and South poles, though they are close to them.) The Earth's magnetic field deflects the charged particles in the solar wind to the magnetic poles. As the particles move down toward the Earth, they crash into atoms and molecules in the highest layer of the Earth's atmosphere. These collisions cause the atoms and molecules, mostly of the gases oxygen and nitrogen, to give off light. This light is an aurora.

Appearance

Auroras occur every day at both the north and south magnetic poles. Because more people live in the far north than in the far south, the aurora borealis is seen more frequently than the aurora australis. People in Alaska, Canada, and other northern regions enjoy the most spectacular views. Occasionally, however, the aurora borealis is visible as far south as Southern Europe and Mexico.

From space an aurora looks like an oval ring of light over one of the Earth's magnetic poles. From the ground a person can see only a small part of the ring. Auroras take many shapes and forms, with arcs and rays being the most common. The light may also look like shimmering curtains, bands, waves, or clouds. An aurora constantly changes shape as the light moves across the sky. The light also may brighten and fade from second to second.

The colors of an aurora depend on the different gases that give off the light. They also depend on the height of the gases above the Earth's surface. An aurora usually begins as white light. Then, as the aurora becomes brighter, the light takes on a yellowish green color. This color is produced by oxygen molecules. Higher in the atmosphere, oxygen gives off red light. Nitrogen produces blue or violet light.