#3.       The Polar Aurora

Typical aurora (Dick Hutchinson ©)

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.

Aurora with comet Hale-Bopp (Dick Hutchinson ©)

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

Further reading:

--"The Aurora" by Candace Savage, Sierra Club, 1995.