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Aurora

he beautiful and often eerie curtainsof light in the night-time sky knownas aurora have been enjoyed bypeople for millennia. Called the au-

rora borealis or “northern lights” (fig. 1), auro-ra also occur in the Southern Hemisphere andare called the aurora australis.

Many legends, myths and superstitions have re-volved around the aurora throughout the histo-ry of mankind. The early dragon legends ofChina and Europe are said to have originatedfrom the aurora. Some cultures have regardedthe sighting of the aurora as a sign of royalbirth; to others it suggests ghosts of the dead or

Figure 1.—Aurora borealis taken in the Copper River Delta, Alaska; �1990 by Dave Parkhurst, Alaska Naturally.

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the precursor for war. The Eskimos of NorthAmerica believed that if you whistled at the au-rora it would sweep down and take you from theearth; by clapping your hands you could forceit to retreat.

Magnets Are the Key

The origin of the aurora is 93 million miles (149million km) from Earth at the Sun. Energeticparticles from the Sun are carried out into spacealong with the ever-present hot solar wind. Thiswind sweeps supersonically toward Earththrough interplanetary space at speeds rangingfrom 300 to over 1000 km per second, carryingwith it the solar magnetic field. The solar winddistorts the Earth’s magnetic field to create thecomet-shaped magnetosphere (fig. 2).

The terrestrial magnetic shield acts as a barrier,protecting the Earth from energetic particlesand radiation in the hot solar wind. Most ofthese energetic particles are deflected aroundthe Earth by the magnetosphere, but some gettrapped. Electrons trapped in the Earth’s mag-netic field are accelerated along the magnetic

field toward the polar regions and then strikethe atmosphere to form the aurora.

Figure 3. Energetic electrons spiral down the geo-magnetic field lines towards the polar regions, strik-ing the upper atmosphere, resulting in the display ofauroral lights.

Auroral oval

MagneticNorth

GeographicNorth

The particles, which stream down the magneticfield of the Earth, reach the neutral atmospherein a rough circle called the auroral oval. Thiscircle, or annulus, is centered over the magneticpole and is around 3000 km in diameter duringquiet times. The annulus grows larger when themagnetosphere is disturbed. The location of theauroral oval is generally found between 60 and70 degrees north and south latitude (fig. 3).

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Figure 2.—A “side view” of the Earth and magnetosphere showing some of the important regions.

Acceleration Region

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Auroral features come in many shapes andsizes. Tall arcs and rays start brightly 100 kmabove the Earth’s surface and extend upwardalong its magnetic field for hundreds of km.These arcs or curtains can be as thin as 100 me-ters while extending from horizon to horizon.Auroral arcs can nearly stand still and then, asthough a hand has been run along a tall curtain,the aurora will begin to dance and turn. Aftermidnight, the aurora can take on a patchy ap-pearance and the patches often blink on and offonce every 10 seconds or so until dawn.

Most of the auroral features are greenish yellowbut sometimes the tall rays will turn red at theirtops and along their lower edge. On rare occa-sions, sunlight will hit the top part of the auroralrays creating a faint blue color. On very rare oc-casions (once every 10 years or so) the auroracan be a deep blood red color from top to bot-tom. In addition to producing light, the energet-ic auroral particles deposit heat. The heat is dis-sipated by infrared radiation or transportedaway by strong winds in the upper atmosphere.

The Chemistry of the Aurora

The aurora is caused by the interaction of high-energy particles (usually electrons) with neu-tral atoms in the Earth’s upper atmosphere.These high-energy particles can “excite” (bycollisions) valence electrons that are bound tothe neutral atom. The excited electrons can thenreturn to their initial, lower energy state, and inthe process release photons (light particles).This process is similar to the discharge in a neonlamp (fig. 4).

Any particular color of the aurora depends ona specific atmospheric gas and its electricalstate, and on the energy of the particle that hitsthe atmospheric gas. Atomic oxygen is respon-sible for the two main colors of green (wave-length of 557.7 nm) and red (630.0 nm).

Variations on the Sun

The Sun is a highly variable star that changes ontime scales of hours to hundreds of years. Theinterplanetary magnetic field direction and so-lar wind speed and density are driven by the ac-tivity on the Sun. They can change drasticallyand influence the geomagnetic activity. As geo-magnetic activity increases, the southern edgeof the aurora borealis usually moves to lowerlatitudes. Similarly, solar mass ejections coin-

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The Cause of the Aurora

Figure 4.—Fast electrons from space travel alongthe magnetic field and strike oxygen atoms or nitro-gen molecules in the atmosphere. Energy from anatom or molecule excited by fast electrons is re-leased as different colors of light.

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Red Light(630 nm)

Green Light(557.7 nm)

Red Light(600–700 nm)

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cide with larger auroral ovals. If the interplane-tary magnetic field is in the opposite directionof the Earth’s magnetic field, there can be in-creased energy flow into the magnetosphereand thus, increased energy flow into the polarregions of the Earth. This will result in an inten-sification of the auroral displays.

Disturbances in the Earth’s magnetosphere arecalled geomagnetic storms. These, in turn, canproduce sudden changes in the brightness andmotion of the aurora called auroral substorms.The magnetic fluctuations of these storms andsubstorms may cause surges in electric powerlines and occasional equipment failures in thepower grid, resulting in wide-spread power out-ages. They can also impact the performance ofsatellite-to-ground communications and naviga-tion systems. Magnetospheric storms can lastseveral hours or even days, and auroral sub-storms can occur several times a day. Each sub-storm can deliver several hundred terajoules ofenergy—as much as the electrical energy con-sumed in the entire United States over 10 hours.

Measuring the Geomagnetic Field

The geomagnetic field can be measured withinstruments called magnetometers. Data frommany magnetometers allow observers to trackthe current state of the geomagnetic conditions.The magnetometer data are often given in theform of 3-hourly indices that give a quantitativemeasure of the level of geomagnetic activity.One such index is called the K-index.

The K-index value ranges from 0 to 9 and is di-rectly related to the amount of fluctuation (rela-tive to a quiet day) in the geomagnetic fieldover a 3-hour interval. The higher the K-indexvalue, the more likely it is that an aurora will oc-cur. The K-index is also, necessarily, tied to aspecific observatory location. For locationswhere there are no observatories, one can onlyestimate what the local K-index would be bylooking at data from the nearest observatory. Aglobal average of auroral activity is convertedto the Kp index. This index is available on a dai-ly basis over the World Wide Web.

______________________________________________________________________________Resources

There are many sources for beautiful picturesof the aurora. A few are suggested here, and alonger list can be obtained from Space Environ-ment Center.

Gift Shop, Geophysical InstituteUniversity of AlaskaFairbanks, AK 99775–0800

Astronomical Society of the Pacific390 Ashton Ave.San Francisco, CA 94112

National Geophysical Data CenterNOAA, E/GC2 Dept. 945325 Broadway, CO 80303–3328info@ngdc.noaa.gov

Remember also that many resources exist onthe World Wide Web. Space Environment Cen-ter Web page is a good starting point for some

of these resources:http://www.sec.noaa.gov

References

Many beautiful pictures have been published inarticles and books. Here are two:

Majestic Lights, Robert H. Eather, AmericanGeophysical Union, Washington, D.C., 1980

Aurora Borealis, S.-I. Akasofu, Alaska GeographicSociety, vol. 6, no. 2, Anchorage, AK, 1979.

Do-It-Yourself Resources

You can take your own photos of the aurora ifyou are in a location that permits good viewingof the event. This is an excellent article that willhelp you to do that:

“How to Photograph the Northern Lights,” FloydDamron, Alaska magazine, November, 1973.

Written by Larry Combs and Rodney Viereck, 1996