The Aurora Explained - Printer Friendly Version

Aurora Facts -

THE AURORA has a curtain-like shape, and the altitude of its lower edge is sixty or seventy miles, about ten times higher than a jet aircraft flies.

AURORAS OCCUR along ring-shaped regions around the north and south geomagnetic poles. Fairbanks, Alaska, is a good place for aurora watching because it is under this region in the north. where people see aurora borealis, or northern lights: the southern aurora is aurora australis.

LIKE A NEON SIGN, auroral light is produced by a high-vacuum electrical discharge. It is powered by interactions between the sun and earth. The light is glow from atoms and molecules in the earth's upper atmosphere.

THE SUN IS a ball of gases that is so hot its outermost part blows away as the solar wind. Consisting of charged particles. this tenuous gas travels to earth in about three days. Because the earth's magnetic field prevents the solar wind from penetrating our atmosphere, its solar particles stream around our planet, encasing earth and its magnetic field within a comet-shaped cavity called the magnetosphere.

THE SOLAR WIND powers the gigantic electrical discharge process, causing the magnetosphere to behave as a generator that produces up to ten million megawatts of electrical power.

THE UPPER ATMOSPHERE contains, at the lower edge of the aurora, a thin and partly ionized layer called the ionosphere. Reflected by the ionosphere, radiowaves can propagate great distances by bouncing between it and the ground.

AURORAL DISPLAYS INDICATE that the ionosphere and our protective atmosphere are being energized by the electric power generated in the magnetosphere. As these electrical currents are discharged in the ionosphere. many phenomena are produced including the visible emissions we recognize as the aurora and magnetic storms.

AURORAS ARE SIMILAR to color television images. In the picture tube, a beam of electrons controlled by electric and magnetic fields strikes the screen, making it glow in colors that vary with the screen's phosphor. Auroral color depends on the type of atoms and molecules struck by the energetic particles, particularly electrons, that rain down along earth's magnetic field lines in the discharge process. Each atmospheric gas glows with a specific color, depending on whether it is ionized or neutral, and on the energy of the particle hitting the atoms and air molecules.

THE BRIGHTEST and most common auroral color. a brilliant yellow-green, is produced by oxygen atoms at roughly 60 miles altitude. High-altitude oxygen atoms (about 200 miles) produce rare, all-red auroras. Ionized nitrogen molecules produce blue light; neutral nitrogen molecules create purplish-red lower borders and ripple edges.

AURORAL INTENSITY varies from night to night and during a single night, with best viewing usually from late evening through the early morning hours. Strong auroras can be seen in the continental U.S., particularly in the north during sunspot maximum years. The number of sunspots (a sign of solar activity) varies according to an eleven-year cycle: a few years after a maximum sunspot year (such as 1991), auroras in high-latitude are more numerous. There's also a slight tendency for more auroras in spring and fall.

THE MAGNETOSPHERE protects us from direct effects of the solar wind, but auroras can seriously disrupt radio communications, radio navigation. some defense-related radar systems, and power transmission lines. Current created by changing magnetic fields accompanying aurora causes corrosion in pipes, including the trans-Alaska pipeline.
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Selected Bibliography on the
Aurora Borealis

Akasofu, S.-I.. 1979. Aurora Borealis, the Amazing Northern Lights. Alaska Geographic. 6(2). 95 pp. Click Here to find out how to order this book!

Akasofu, S.-I. 1981. "The Aurora." American Scientist, 69(5), pp. 492-499.

Akasofu, S.-I. 1989. "The Dynamic Aurora." Scientific American, 260(5), pp. 90-97.

Anderson, A. 1989. "Auroral Satellite: Japan Continues in an Old Furrow." Nature, 338(6210), p. 8.

Anonymous. 1987. "Above and Beyond the Auroras." Natural History, 96(9), pp. 6-8.

Anonymous. 1990. "An Active Sun Sparks Aurorae." Astronomy, 18(1), pp. 104-105.

Anonymous. 1990. "Aurora Borealis: Photographing the Northern Skies Ablaze." Petersen's Photographic Magazine, 19, pp. 42-46.

Anonymous. 1989. "Brighter than the Lights of Big Cities." Science News, 135(20), p. 310.

Anonymous. 1985. "Challenger Snaps a Dazziing Aurora." Science Digest, 93, p. 11.

Anonymous. 1985. "Energy from the Arctic Sky." Environment, 27(3), p. 23.

Anonymous. 1988. "Estimate of Electromagnetic Quantities in Space from Ground Magnetic Records." Science, 241, p. 328.

Anonymous. 1986. "Giotto Tracks Aurora Borealis Through the Home Countries." New Scientist, 109(1497), p. 21.

Anonymous. 1989. "Lights of Wonder." Alaska, 55(Oct. 89), pp. 38-46.

Anonymous. 1989. "Low Glow." Life, 12(6), p. 7.

Anonymous. 1989. "March's Spectacular Aurora." Astronomy, 17(7), pp. 92-93.

Anonymous. 1989. "Nature's Incredible Night Show (Aurora Borealis)." Reader's Digest, 135(Dec. 1989), pp. 86-92.

Anonymous. 1987. "A New Light on Auroras." Sky and Telescope, 74(4), pp. 344-345

Anonymous. 1989. "Researchers Bottle the Aurora Borealis." Earth Science, 42, p. 5.

Anonymous. 1987. "Solar Waves Cause Aurora." Astronomy, 15(10), pp. 89-90.

Anonymous. 1990. "Spectacular Aurorae Visible." Astronomy, 18(3), pp. 81-84.

Anonymous. 1990. "Star Turn." Life, 13(Jul. 1990), pp. 42-46.

Anonymous. 1987. "Those Mysterious Lights in the Sky." National Geographic World, 148, pp. 18-23.

Anonymous. 1985. "What Turns On the Aurora." Science '85, 6(4), pp. 11-14.

Bagnall, D. M. 1988. "Observe the Naked-Eye Sky Glows." Astronomy, 16(6), pp. 46-51.

Bernhardt, P. A., Duncan, L. M. and Tepley, C. A. 1988. "Artificial Airglow Excited by High-Power Radio Waves." Science, 242(4881), pp. 1022-1028.

Brown, B. 1985. "Shedding Light on the Aurora." National Wildlife, ' -(2), pp. 50-53.

Byrd, D. 1990. "An Aurora Alert." Astronomy, 18(3), p. 52.

Eather, R. H.. 1980. Majestic Lights, the Aurora in Science, History, and the Arts. American Geophysicai Union. 323 pp.

Eberhart, J. 1986. "The Night Skies of Venus: Another Kind of Aurora?" Science News, 130(23), pp. 364-365.

Eicher, D. J. 1989. "Brilliant Aurorae Produced by Solar Flares." Astronomy, 17, p. 95+.

Falck - Ytter, H.. 1985. Aurora: The Northern Lights in Mythology, History, and Science. New York: Anthroscopic Press. 167 pp.

Fienberg, R. T. 1988. "Dragons in the Sky." Sky & Telescope, 76, pp. 494-495.

Free, J. 1986. "New Probes of the Mysterious Northern Lights." Popular Science, 228(4), p. 41.

Keller, W. E. 1986. "Why the Northern Lights Have Pleats." Environment, 28(3), p. 23.

Kerr, R. A. 1987. "A New Source of Power to Drive the Aurora." Science, 237(4818), p. 974.

Livesey, R. J. 1989. "A Jam-Jar Magnetometer as "Aurora Detector." Sky and Telescope, 78(4), pp. 426-427.

Mackowski, M. J. 1989. "Home on the Esrange: Swedes are Looking at Space from the Top of the World." Ad Astra, 1(5), pp. 39-44.

MacRobert, A. 1986. "The Aurora." Sky and Telescope, 71(1), pp. 30-32.

McPherron, R. L. 1987. "What Drives the Aurora? (Discussion of Kerr, R. A., A New Source of Power to Drive the Aurora)." Science, 238(4832), p. 1340.

Nash, J. M. 1989. "Fury on the Sun: Once Worshiped as a God, Earth's Star is Revealing the Secrets of its Awesome Power." Time, 134(1), pp. 46-55.

Nicholson, T. D. 1989. "The Northern Lights Head South." Natural History, (11), pp. 104-108.

Rennicke, J. 1990. "The Ghost Fires: Are the Northern Lights Science in the Sky or Windows of the Soul." Backpacker, 18(Apr. 1990), pp. 18-20.

Rich, F. 1989. "Looking Down on the Aurora." Sky and Telescope, 78, p. 243.

Sadar, A. J. 1987. "The Awesome Aurora." Weatherwise, 40(2), pp. 76-77.

Schaaf, F. 1989. "Return of the Northern Lights." The Mother Earth News, 119, pp. 75-77.

Sullivan, W. 1987. "Curtains of Light, Horsemen of Night." Audubon, 89(11), pp. 40-51.

Tsurutani, B., McPherron, R. and Gonzalez, W. 1988. "Aurora Hypothesis (Discussion of Kerr, R. A., A New Source of Power to Drive the Aurora)." Science, 239(4845), p. 1228.  Top of Page

Aurora Milestones!

1600 English physician William Gilbert shows that earth is a gigantic magnet.

1774 Jean Jacque Dortous de Mairan of France relates auroral displays to solar activity.

1868 Elias Loomis of Yale University identifies the auroral zone.

1868 Anders Jonas Angstrom of Norway uses a prism to show that auroral light differs from sunlight.

1910 Carl Stormer of Norway uses triangulation to measure auroral heights.

1925 Merle Tuve and others at the Carnegie Institution of Washington announce discovery of the ionosphere. an electrically conducting layer of the high atmosphere (starting at roughly 50 miles altitude).

1830 In Alaska. measurements by Veryl Fuller (1930-1934) confirm that auroras occur at the same altitudes throughout the northern auroral zone.

1939 World War II intensifies study of auroral effects on radio communication, navigation, and detection systems.

1957 During the International Geophysical Year (IGY). extensive auroral studies occur and all-sky camera networks simultaneously record auroral displays throughout the arctic auroral zone. The first satellite, Sputnik I, orbits earth measuring density and other upper atmosphere features. The U.S. Explorer I satellite shortly follows.

1964 Geophysical Institute scientists identify the auroral substorm. an intermittent surge of auroral activity. Several other important advances result from IGY data

1967 Scientists at the Geophysical Institute showed that electrons causing northern and southern auroras come from the same source, creating simultaneous and often mirror-image auroras in the north and south polar regions.

1974 Institute scientists observe evidence that electric fields exist parallel to the magnetic field. They also lead a multinational expedition to the eastern Arctic to observe the daytime aurora and its direct relationship to the solar wind.

How Aurora Generate Electrical Currents On The Earth.

Aurora range in altitude from 115 to 260 kilometers above the surface of the earth. As they change in brightness electric currents associated with and near to them change and create a changing magnetic field. This changing magnetic field generates electric currents in conductive materials such as the earth and in electric power lines and pipelines.

Demonstration of how a changing electrical current creates a magnetic field
   The instrument with the ball-shaped magnet, battery, coil of wire and switch is built to demonstrate how changing electric currents create magnetic fields.

The compass inside the ball will point to the earth's magnetic north pole. While holding the compass instrument in the palm of one hand use a finger from your other hand to push the electrical switch button located above the battery.

Pushing this switch completes the electrical circuit that supplies electrical current from the battery to the coil of wire that surrounds the magnetic compass. This electrical current will create a magnetic field so much stronger than the earth's magnetic field that the compass ball will quickly rotate and align itself with the created magnetic field.

If the ball does not seem to change position when you push the switch there are three possible problems. First, you may be holding the compass so that when the switch is pushed the magnetic field generated is in the same direction as the earth's natural magnetic field. While holding the unit in the outstretched palm of your hand turn to your left about ninety degrees and push the button again. If the compass ball still doesn't quickly rotate try a new battery. The coil uses very little electricity when the switch is pushed so the battery should last several months. If the instrument still does not work, the battery may have become magnetized because it got too close to the cow magnet that will be used in the second half of this demonstration for too long. Try a new battery.

The magnetic field generated by the changing electrical currents in the aurora travel quickly to the surface of the earth. Scientists monitor the magnetic field of the earth and the disturbance of the earth's magnetic field by aurora with an instrument called a magnetometer. Using the magnetometer it is possible even during the daytime to determine whether aurora are taking place.

Demonstration of how a changing magnetic field creates an electrical current.
   To demonstrate how a changing magnetic field can create an electrical current, pick up the unit that consists of a meter and a coil of wire, and one of the cow magnets*. The cow magnet is about three-inches long and a half-inch in diameter.
   The cow magnet is a very powerful magnet. Keep it away from computer disks!

Hold the meter and coil in one hand and the cow magnet in your other hand so that you can push the cow magnet back and forth inside of the coil of wire. Now push and pull the cow magnet back and forth inside of the coil of wire. Note that the needle of the meter will move only when the cow magnet is moving. If the cow magnet is inside of the coil and not moving the needle of the meter stands still. When you change the magnetic field inside the coil by pushing and pulling the cow magnet back and forth, you are generating electricity shown by the movement of the needle of the meter.
   In the 1970s scientists at the University of Alaska Fairbanks Geophysical Institute during aurora activity measured ten amperes of direct electrical current generated by the aurora in local power lines. Almost all electrical power lines are built to carry only alternating current. Direct current generated in these lines by the aurora causes heating and other problems in the equipment that is used to distribute alternating electrical power. As aurora occurred Geophysical Institute scientists also measured over 2000 amperes of direct current flowing in the Trans-Alaska Pipeline. This electrical current flows in and out of the pipeline where the pipeline touches the ground which causes increased corrosion or loss of metal from the pipeline at that point. Although the electrical current is large the voltage is only a few thousandths of a volt per kilometer, which is not enough to feel if you touch the pipe.

*What is a cow magnet? Cows, if they get a chance, will eat most anything including cardboard boxes that have metal staples. After chewing what they eat they swallow it into the first of their three stomachs. The muscles in this stomach work to mix acids from the stomach to digest food. The heart of a cow lies next to this first stomach. Many cows have died because a metal staple or other piece of metal in the first stomach pushed through the wall of the stomach into the heart. So the people who care about cows invented cow magnets. They get the cows to swallow them but I don't know how. Note that they have nice round ends and are very powerful magnets. Any ferrous (iron and steel are ferrous and aluminum and brass are not) metal the cow swallows into its first stomach will be attracted to and held by the magnet so the metal cannot injure the cow. Cow magnets are one of the most inexpensive powerful magnets you can buy for use in demonstrations.

TODAY new observation techniques and theoretical work at the Geophysical Institute and elsewhere continue to increase understanding of the processes that produce the beautiful aurora.  Top of Page

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© Copyrighted 1998 by Neal B. Brown
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