#3. The Polar Aurora

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."

Click here for a full size version of this image.

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.

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 left, 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.

Location

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 is probably 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.

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.

Aurora observed by an imaging camera aboard the DE-1 (Dynamics Explorer 1) spacecraft.

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.

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.

Red Aurora

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.

View from Shuttle

#3. The Polar Aurora

Location

Color

Observing the aurora from space

Further reading: