Green curtains of the aurora borealis (Northern Lights) appeared in the skies over Alaska in this photo from April 2001.
Click on image for full size
Image courtesy of Jan Curtis.
The polar aurora is formed
when Field-Aligned currents
(FAC's) short-circuit through the atmosphere. Particles from the Sun traveling along magnetic field lines
collide with particles in the atmosphere. The atmospheric particles become electronically excited from the collision. Relaxing into their normal state obviously requires a release of energy. We see this energy release as colored light
and call it aurora.
The aurora is also known as the northern and southern lights. From the ground, they can usually be seen where the northern and southern auroral ovals are on the Earth. The northern polar auroral oval usually spans Fairbanks, Alaska, Oslo, Norway, and the Northwest Territories.
During geomagnetic storms, however, more particles impinge upon the atmosphere, and the auroral ovals expand to absorb the excessive energy. The northern polar auroral oval can expand to latitudes of 50 or even 40 degrees. Thus there are times when the aurora can be seen as far south as Michigan, Oregon or even farther south.
Auroral classification is quite complex. In its simplest form, aurora can be broken down into these types: polar auroral (pictured and discussed above), equatorial arcs (pictured in the above image), mid-latitude aurora and the SAR arc.
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