Aurora Borealis

Aurora Borealis



From "The Ballad of the Northern Lights"  by Robert Service

by Dr. Neil Davis (reprint)

Northern Lights

The Aurora, also known as the Northern Lights, has mystified people down through the ages, scientists, poets, and lay persons alike. Written records of so-called "great auroral displays" date back more than two thousand years.

Sometimes the Aurora over Alaska and other auroral zones is barely visible or appears colorless and unmoving. But at other times the auroras can be incredibly bright, multihued and fast moving. Tall green curtains of lights, red tipped at their bottoms, stretch from horizon to horizon. They ripple and sway, fold and unfold, then suddenly disappear, only to reform in a new shape minutes later.

For those who live in Alaska, the Aurora is a part of northern life. Fall, winter and spring is the special season for viewing the great lights for residents and off-season visitors. Some Alaskans have "Aurora Alerts;" when a display begins, the first person to spot them begins a phone tree to get the word out.

 

Northern Lights

The "pulsating Aurora" is one form of Northern Light common during the post-midnight hours. It blinks on and off every few seconds as though controlled by some mysterious unseen hand in the sky flicking a switch.

The great auroral displays are spectacular global events during which the Aurora spreads down from the polar regions to cover as much as two thirds of the earth's skies with bright, fast-moving masses of light, often deep red in color.

The rare "great auroral displays" follow one or two days of violent solar flares in the vicinity of major sunspots. These solar flares cast out vast streams of electrically charged particles which stream down into the earth's atmosphere. These particles, mostly electrons and protons, are steered away from the tropical regions by the earth's magnetic field.

Striking the gases of the earth's high atmosphere, the charged particles glow.

It is exactly the same thing that happens in a television tube: complex streams of electrons within the TV strike the phosphor coated face of the tube, and cause it to glow, creating the colored moving patterns we see from our living room couches. Outside, at night, we can look up at that great star-studded television tube in the sky; if the Aurora is out and no clouds are about, the colored patterns will be there too.

Even though "great auroral displays" appear infrequently, spectacular displays of Aurora are common in Alaska and other auroral zone locations. This is because the sun is always sending out a stream of electrons and protons. They make up the solar wind which blows constantly toward the earth and other planets.

Enough of the solar wind particles come into the earth's high atmosphere at the auroral zone to create continuous Aurora, summer, winter, fall and spring.

 

Northern LightsAlthough much is known about the Aurora, this fascinating phenomenon still withholds some of its mysteries. For example, scientists do not yet understand why the Aurora is so highly structured. For the University of Alaska Fairbanks, this continues to be an important area of study. The most beautiful Aurora is composed of thin sheets that stretch upward a hundred miles or more and extend across the sky from horizon to horizon, like gigantic ribbons set on edge high above and parallel to the earth's surface. These arcs and bands whip and weave across the sky, and bright rays ripple along them at fantastic speed. Some of these intricate multicolored forms have a thickness of only 100 meters, about the length of a football field, yet may be several hundred kilometers tall and well over 1,000 kilometers in length.

It boggles the mind to realize—as has been proven by simultaneous observation in the two hemispheres—that these sharply defined auroral forms have an identical or nearly identical mate in the southern hemisphere, out of sight and almost a world away. Joined together by an invisible magnetic bond that arches far above the Equator, each pair of mated auroral dancers move across the cold polar skies in perfect harmony. Each motion or brightness change in one hemisphere is mirrored in the other, on a time scale of less than a fraction of a second.

A most curious and highly controversial question involves auroral sound. Although I've never heard one, my wife has and so have several scientists I know. Some reports of hearing noises associated with auroral displays may be erroneous impressions, but I am convinced many others are not. Hundreds of written reports indicate noises are sensed when a particular type of fast-Aurora is nearly overhead. Some people sense whistling and crackling noises, even when they close their eyes.

The reports of auroral sound make it certain that the unknown cause of the apparent sound is in the vicinity of the hearer, not in the Aurora itself. Most likely, the reported noises are related to electrical phenomena which accompany certain types of bright Aurora.

No matter how many auroras one may have seen, a good high-latitude display, as seen in Alaska, sends tingles up the viewer's spine. Even a moderate display seen at the auroral zone in the 49th state can be far more exciting to watch than one of the rare "great displays." The best of words and photographs fail to capture the magnificence of the high-latitude Aurora, although some of Robert Service's poetry may come close.

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 a 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, radio waves 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 strike 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 cause corrosion in pipes, including the trans-Alaska pipeline.

  • Come to Interior or Northern Alaska during the late fall or winter months, particularly March when the skies are often clear and dark. Plan to spend a few days because the Aurora is, like the weather, variable. Aurora-watching and auroral photography is best done on clear, moonless nights.
  • When flying between Seattle and Anchorage or Fairbanks in wintertime, take a night flight and sit on the right-hand side of the airplane going north (left-hand side of the airplane going south). Don't be misled by seeing Aurora below the aircraft's wing; that Aurora is high over western Canada, at least 50 miles above the ground. The places where the auroral forms appear to "touch" the ground are about 600 miles distant.
  • To be sure of seeing Aurora, plan to stay up most of the night. The best hours often are near midnight. Wear warm clothing.
  • Stay away from city or other bright lights that can obscure the views.
  • For a real treat, go to a hot springs resort where you can lay in an outdoor hot pool and watch the Aurora in comfort.
  • If you plan to photograph the Aurora, bring a tripod. Bright as it is, the Aurora still requires time exposures of several seconds.
  • Summer visitors to Fairbanks can view and purchase a video tape showing actual auroral motions at the University Museum. Each evening at nearby Ester, Alaska, Photosymphony Productions puts on a unique musical and photographic show entitled "The Crown Of Light" containing some of the finest photographic images of Aurora available anywhere.

Links for More Information:

University of Alaska Articles on the Aurora Borealis

Aurora Forecasts from the University of Alaska Fairbanks Geophysical Institute

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