What is the Aurora?

(Aurora photo by Dr. Syun Akasofu.)

People who live in the far north have tried to figure out what the aurora, or northern lights, is all about for thousands of years. The native peoples of the arctic have many legends about the aurora, and often see the rare red aurora as a source of danger, believing that it comes down to snatch lone travelers. Medieval Europeans also regarded the aurora as an ominous portent, believing that the red aurora foretold war, and interpreting other auroral displays as supernatural signs.

As scientific inquiry began to replace superstition, many theories about the aurora were proposed. Some thought the aurora was the reflection of distant fires. One theory that lasted almost to the twentieth century was that the aurora was caused by sunlight reflecting off ice crystals high in the arctic sky.

A Scientific Detective Story

1600

English physician William Gilbert shows that Earth is a gigantic magnet. Nobody realized that this fact is crucial to understanding the aurora.

1774

French scientist Jean Jacque Dortous de Mairan relates auroral displays to solar activity.

1860

Elias Loomis of Yale University identifies the auroral zone, the region of greatest auroral activity. Eight years later, Anders Jonas Angstrom of Norway uses a prism to show that auroral light differs from sunlight. So much for the reflected sunlight theory!

1910

Norwegian scientist Carl Stormer uses triangulation (observing the same aurora from different locations) to measure auroral heights. Measurements in Alaska (1930-1934) by Veryl Fuller confirm that auroras occur at the same altitudes throughout the northern auroral zone (typically around 100 km high).

1925

Discovery of the ionosphere, an electrically conducting layer of the upper atmosphere starting at about 80 km above the earth, is announced by Merle Tuve and others at the Carnegie Institution. This means that the aurora is in the ionosphere!

1939

World War II intensifies research on auroral effects on communication, navigation, and detection systems. Auroras are known to wipe out radio communications, and they show up on some radar displays, obvious concerns during wartime.

1957

Extensive auroral studies take place during the International Geophysical Year (IGY, 1957-1959); all-sky camera networks simultaneously record auroral displays from horizon to horizon throughout the arctic auroral zone. The first artificial satellite, Sputnik I, orbits Earth measuring density and other upper atmosphere features; the U.S. Explorer I satellite soon follows.

1964

Information gathered during the IGY enables Geophysical Institute scientists to identify the auroral substorm, an intermittent surge of auroral activity. Several other important advances result from IGY data and satellite measurements.

1967

Geophysical Institute research shows that electrons causing northern and southern auroras come from the same source, creating simultaneous and often mirror-image auroras in each hemisphere.

1969

Barium releases from rockets fired from the new Poker Flat Research Range paint the Earth's magnetic field in a way that Gilbert would never have dreamed of, creating a sort of artificial aurora seen across Alaska.

1974

Scientists at the Geophysical Institute acquire observational evidence for electric fields existing parallel to the magnetic field, which provide current driving a massive ionospheric electrical circuit. They also lead a multinational expedition to the eastern Arctic to observe the daytime aurora (visible only during the high-arctic winter, when the sun does not rise for weeks) and its direct relationship to the solar wind.

And through the present...

Scientists at the Geophysical Institute are using rockets launched from Poker Flat, numerous instruments scattered around Alaska and the rest of the world, satellite observations, and supercomputer simulations of mathematical models to discover how the sun, the aurora, Earth's climate, and magnetic fields that fill interplanetary space interact and affect our life on Earth.

So What is the Aurora???

• Auroras occur above Earth's north and south geomagnetic poles in regions known as auroral ovals. Southern auroras are called aurora australis; northern ones, aurora borealis.
• The aurora is higher in the atmosphere than the highest jet plane flies. The lowest fringes are at least 40 miles above the Earth, while the uppermost reaches of the aurora extend 600 miles above the Earth. The space shuttle flies near 300 miles altitude.
  • Although there are stories about the aurora seeming to reach down into the clouds or to the tops of mountains, these are illusions. Only astronauts can fly through the aurora!
  • Some people believe that the aurora makes sound that accompanies the ripples and flow of the light. If the aurora does make sound, the sound would have to be generated here on Earth by some electromagnetic effect. Any noise generated by the aurora would take a long, long time to travel all the way to Earth, and the air up by the aurora is much too thin to carry sound. So does the aurora make noises? Nobody knows for sure!
• Auroras occur because Earth's magnetic field interacts with the solar wind, a tenuous mix of charged particles blowing away from the sun. This wind from the sun sweeps by Earth in the interplanetary magnetic field which is produced by the sun. We are protected from the solar wind's direct effects by Earth's comet-shaped magnetosphere, where the Earth's magnetic field is distorted by the interplanetary magnetic field and the solar wind. The electrical energy generated by the charged particles blowing across the Earth's magnetic field send charged particles down into the Earth's upper atmosphere.
• Auroral light is similar to light from color television. In the picture tube, a beam of electrons controlled by electric and magnetic fields strikes the screen, making it glow in different colors, according to the type of chemicals (phosphors) that coat the screen. Auroral light is the from the air glowing as charged particles, particularly electrons, rain down along the Earth's magnetic field lines. The color of the aurora depends on the type of atom or molecule struck by the charged particles.
• Each atmospheric gas glows with a particular color, depending on its electrical state (ionized or neutral) and on the energy of the particle that hits the atmospheric gas. High-altitude oxygen, about 200 miles up, is the source of the rare, all-red auroras. Oxygen at lower altitudes, about 60 miles up, produces a brilliant yellow-green, the brightest and most common auroral color. Ionized nitrogen molecules produce blue light; neutral nitrogen glows red. The nitrogens create the purplish-red lower borders and ripple edges of the aurora.
• Auroral displays vary from night to night and during a single night. Usually, if sun-earth conditions produce an auroral substorm, a diffuse patch of glowing sky will be seen first, followed by a discrete arc that brightens, perhaps a thousand-fold in a minute. As an arc moves toward the equator, new ones may form on its poleward side. Appearing within arcs are upward-reaching striations aligned with the magnetic field, giving the impression of curtains of light. Ripples and curls dance along the arc curtains and pulsating patches of light may appear in the morning hours.

To learn more about the aurora, read The Aurora Watcher's Handbook by T. Neil Davis (University of Alaska Press, 1992; ISBN 0-912006-60-9).