Aurora

Aurora is a natural display of light in the sky that can be seen with the unaided eye only at night. An auroral display in the Northern Hemisphere is called the aurora borealis, or the northern lights. A similar phenomenon in the Southern Hemisphere is called the aurora australis. Auroras are the most visible effect of the sun's activity on the earth's atmosphere.

Most auroras occur in far northern and southern regions. They appear chiefly as arcs, clouds, and streaks. Some auroras move, brighten, or flicker suddenly. The most common color in an aurora is green. But displays that occur extremely high in the sky may be red or purple. Most auroras occur about 60 to 620 miles (97 to 1,000 kilometers) above the earth. Some auroras extend lengthwise across the sky for thousands of miles or kilometers.

Auroral displays are associated with the solar wind, a continuous flow of electrically charged particles from the sun. When these particles reach the earth's magnetic field, some are trapped by the field. Many of the trapped particles travel toward the earth's magnetic poles. When they strike atoms and molecules in the atmosphere, energy is released. Some of this energy appears in the form of auroras.

Auroras occur most frequently during the most intense phase of the 11-year sunspot cycle. During this phase, dark patches on the sun's surface, called sunspots, increase in number. Violent eruptions on the sun's surface, known as solar flares, are associated with sunspots. Electrons and protons released by solar flares add to the number of solar particles that interact with the earth's atmosphere. This increased interaction produces extremely bright auroras. It also results in sharp variations in the earth's magnetic field called magnetic storms. During these storms, auroras may shift from the polar regions toward the equator.

Contributor: C. R. O'Dell, Ph.D., Prof. of Space Physics and Astronomy, Rice Univ.

Aurora

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