Aurora Borealis
Aurora Borealis, or Northern Lights, a glow in the night sky in the regions near the north magnetic pole. A similar glow occurs in the area of the south magnetic pole at the same time. It is called Aurora Australis, or Southern Lights.
The Aurora Borealis is brightest and most frequent in a circle about 23 from the north magnetic pole. It decreases in brilliance and frequency both north and south of this circle.
The brightest auroras are about as bright as the full moon. The auroral glow is most often a pale greenish color, but sometimes includes pure white or shades of pink, red, purple, blue, or gray. Auroras appear in various forms—rays, flames, arcs, bands, shapeless glows, "draperies," or crown-shaped coronas. They may last only briefly, or all night. Some remain motionless; others weave, shift, and dance.
Auroras usually occur in the upper atmosphere 60 to 200 miles (95 to 320 km) above the earth; sometimes they reach altitudes of 500 miles (800 km) or more. Although their exact nature is not clearly understood, auroras are known to be the result of bombardment of the atmosphere by electrically charged particles—protons and electrons—from space.
The particles that bombard the atmosphere are part of the solar wind, a constant outpouring of matter from the sun in all directions. The earth's magnetic field deflects most of the charged particles that reach the vicinity of the earth, but some find their way into the atmosphere, especially near the north and south magnetic poles. When the particles collide with molecules of oxygen, nitrogen, and other gases in the atmosphere, the molecules gain energy which they then release as light. The effect is similar to that in neon lamps, where atoms of neon give off light when they are struck by electrically charged particles.
Auroras are most intense during solar maximum, the most active phase in the 11-year sunspot cycle. During this period, violent eruptions on the sun's surface called solar flares occur frequently. During a solar flare, large numbers of high-speed protons and electrons are ejected, and a fluctuating magnetic field travels outward with the solar wind. The fluctuating field can interact with the earth's magnetic field and cause many charged particles to enter the earth's atmosphere, producing auroras that extend farther toward the Equator than usual.