Atmospheric Science

Auroras

December 9, 2010, 5:53 am
Source: NOAA
Content Cover Image

Aurora borealis in vicinity of Anchorage, AK. Credit: NOAA

Understanding Auroras

caption Aurora australis over South Pole. Credit: NOAA. The eerie beauty of the northern and southern lights (aurorae) has evoked visions of the supernatural for centuries: foxes of fire whisking their tales, the fighting souls of dead warriors or ancestors dancing around a ceremonial fire.

The English poet Sir Walter Scott in 1805 conjured up otherworldly beings when he wrote, "He knew, by the streamers that shot so bright, That spirits were riding the northern light."

But it was a French scientist, not a poet, who named the sight after Roman gods. In 1621, Pierre Cassendi paired Aurora, goddess of dawn, with Boreas, god of the north wind, to christen the northern lights “aurora borealis.” Those centered above the South Pole are called aurora australis for “southern dawn.”

The Anatomy of an Aurora

Deep within the Sun, 93 million miles away, roiling plasma rises and bursts through the solar atmosphere, sometimes thrusting highly charged protons and electrons our way. When this so-called solar wind arrives near Earth, it energizes protons and electrons trapped in the planet’s magnetic field.

These charged particles then travel down magnetic field lines, like beads slipping along a string, into Earth’s upper atmosphere near the poles. There the particles in turn excite atoms and molecules of oxygen, nitrogen, and other atmospheric gases. As these atoms relax back down into their normal state, they release the excess energy as visible light, forming an aurora oval loosely centered on the magnetic pole.

caption Aurora Australis. Credit: NOAA. During an aurora, vivid arcs, curls, waves and bands of green, red, and sometimes blue dance across the sky for minutes or hours, peaking near midnight — all between 60 and 600 miles above the ground.

Many people think auroras are rare events, but there’s almost always an aurora of some size in the sky near the poles. Seeing one is another matter.

Auroras are most often visible in regions bordering the Arctic Circle: Canada, Alaska, northern Greenland, the Scandinavian coast, and Siberia. In the south, you need to be visiting Antarctica to see an aurora frequently. But the larger the solar storm reaching Earth’s upper atmosphere, the farther the aurora extends from the poles. Residents of New England or southern Chile might see an aurora every few years. If you live in Florida or Italy, you’d be lucky to see an aurora once in your lifetime.


How Space Weather Affects Us

One of the nation’s critical operations centers, NOAA’s Space Weather Prediction Center keeps a close eye on solar activity that precedes an aurora. When a major storm explodes on the sun, followed by a suddenly intensified solar wind heading toward Earth, the center alerts airlines, the military, the communications industry, power companies and the media that a storm is on its way.

Visit the Space Weather Prediction Center’s aurora Web site to view the current shape and size of the auroras around the two poles. If the auroras shown there are exceptionally large and you’re in a far northern or far southern latitude, look for those spirits hurtling across the midnight sky!

Tips on Seeing an Aurora

  • Best time of night: 10:00 p.m. to 2:00 a.m.;
  • Best conditions: clear night with no moon and far from light pollution;
  • Best season: mid-winter;
  • Best phase of the solar cycle:maximum;
  • Best years in the sun’s current cycle:2012 to 2013; and
  • Best position on Earth: far northern or southern latitudes.
Glossary

Citation

(2010). Auroras. Retrieved from http://www.eoearth.org/view/article/51cbf0257896bb431f6a075f

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