Auroral forms
Throughout the ages, people have sought to describe the mysterious, ever-changing shapes and movement of the auroral curtain. Some descriptions saw the source of this light as a glow from distant fires, fire in the air, or luminous gases escaping from rips in the sky. More creative descriptions capture the wonder auroral displays evoke in their viewers. Aurora have appeared as warriors, bridges to heaven, and even a "snake of electric light".
Auroral forms can be divided into broad categories based on activity level and the viewer's perspective.
Homogeneous arc
At its least active, the auroral curtain forms diffuse,
glowing streaks hanging quietly in the sky. This form has no distinct
structure.
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Rayed arc
When the aurora becomes slightly more active, vertical
stripes or striations, called 'rays', form. These are actually fine pleats
in the auroral curtain.
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Active aurora
Active aurora may form folds anywhere from 10
to 100 miles wide as they swirl and move.
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Rising vapor column
The auroral curtain sometimes appears to touch a
distant mountain top or even rise like smoke. This illusion occurs because
you are seeing a several hundred mile long aurora near the horizon where
perspective gives the illusion that it is touching the ground.
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Corona
The aurora may appear as rays shooting out in
all directions from a single point in the sky. This dramatic form occurs
when you are directly beneath the swirls and folds of an active curtain.
The rays are actually hundreds of miles long and perspective makes them
appear to converge.
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This graphic illustrates how the
auroral curtain changes appearance
depending on where the viewer
is standing in relation to it.
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Photograph illustrating perspective
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Geophysical Institute
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site last modified: July 2003 maintained by Asahi Aurora
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