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#3. The Polar Aurora

Note: Click here a longer, much more comprehensive overview of the polar aurora

(Files in red–history)

Index

2. Magnetic Field

2H. Oersted, 1820

3. Aurora

3H. Birkeland 1895

3a. Loomis & Aurora

3b. Fritz & Aurora

3c. The Terrella

4. Electrons

4H. Thomson, 1896

4a. Electric Fluid

Typical aurora (Dick Hutchinson ©)

In Alaska, Canada, Norway, Finland or northern Russia, on a clear night, a greenish glow is often seen in the sky, known as the "Northern Lights."

During magnetic storms, the glow may move southwards, and on occasion it can be seen in much of the US. It often appears as a glow on the horizon, like the glow preceding sunrise, and has therefore become known among scientists as "aurora borealis" ("aurora" for short), Latin for "northern dawn." A similar phenomenon is also seen in southern polar regions.

The aurora--a woodcut by Fridtjof Nansen
To an observer, an aurora is a fascinating spectacle, constantly moving and changing. It usually consists of many near-vertical greenish rays, forming long arcs and curtains, which stretch like ribbons across the sky, often from horizon to horizon. An example is shown on the right, a woodcut by the great polar explorer Fridtjof Nansen (1861-1930). The rays constantly fade while new ones appear, and during "magnetic substorms" (described in a later section) the arcs move rapidly and expand.

Auroral light is produced at a height of about 100 km (60 miles) when fast electrons, arriving from space, slam into atoms and molecules of the atmosphere. The computer screen displaying these words may be lit up in a similar way, by a beam of fast electrons accelerated electrically towards it, then steered and modulated so as to form letters and pictures.

Location

The location of auroras on Earth is strongly controlled by the Earth's magnetism. In the 19th century it was noticed that they occur most frequently in a narrow belt, the "auroral zone", which circles the magnetic pole (see history, below). Their arcs and ribbons are approximately aligned with that zone, too. The circles drawn on the left are centered on the northern magnetic pole, and the auroral "circle of fire" is evidently lined up with them.

The magnetic connection is also demonstrated by the fact that the rays of the aurora lie along magnetic field lines, and that the Earth's magnetic field observed beneath a bright and active aurora tends to be disturbed.

Aurora observed
by an imaging
camera aboard DE-1

Color

The green light of the aurora has a precisely defined color in the spectrum ("narrow spectral line"). Such precise colors are usually the signatures of the atoms which emit them: for instance, street lights (depending on the metal vapor they contain) usually emit either the yellow-orange light of sodium or the bluish light of mercury.

The green light of the aurora puzzled scientists for many years, since it fit no known element. It turned out to be produced by oxygen atoms, but under conditions that in our atmosphere only exist in the very rarefied upper levels. A red aurora, occasionally seen, arises at even greater heights and is also produced by electrons hitting oxygen.

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Observing the aurora from space

Satellites nowadays observe the aurora from above, using cameras more sensitive than the human eye. On dark parts of the polar cap they can "see" aurora at most times, forming a large "auroral oval" which extends around the magnetic pole.

Aurora viewed from the Space Shuttle

Auroras Galore

Dick Hutchinson lives in Circle, Alaska, on the Yukon river north-east of Fairbanks, prime aurora territory. He enjoys photographing the aurora, and his collection of auroral images can give you a better feeling of "what the aurora looks like" than anything else I have seen on the web.

Disclaimer: The following material is being kept online for archival purposes.

Although accurate at the time of publication, it is no longer being updated. The page may contain broken links or outdated information, and parts may not function in current web browsers.

#3. The Polar Aurora

Location

Color

Observing the aurora from space

Auroras Galore

Further reading:

Above is background material for archival reference only.