October202008

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Nature Photos

October 20, 2008

Auroras!

Auroras are natural light displays in the sky, usually observed at night, particularly in the polar zones. They typically occur in the ionosphere. In northern latitudes, the effect is known as the aurora borealis, named after the Roman goddess of dawn, Aurora, and the Greek name for north wind , Boreas. Aurorae seen near the magnetic pole may be high overhead, but from further away, they illuminate the northern horizon as a greenish glow or sometimes a faint red, as if the sun was rising from an unusual direction. The aurora borealis most often occurs from September to October and from March to April. The Cree indians call this phenomenon the Dance of the Spirits.

Its southern counterpart, the aurora australis has similar properties, but is only visible from high southern latitudes. Benjamin Franklin first brought attention to the "mystery of the Northern Lights." He theorized the shifting lights to a concentration of electrical charges in the polar regions intensified by the snow and other moisture.

Aurorae are produced by the collision of charged particles from Earth's magnetosphere, mostly electrons but also protons and heavier particles, with atoms and molecules of Earth's upper atmosphere (at altitudes above 50 miles). They originate from the Sun and arrive at the vicinity of Earth in the relatively low-energy solar wind. When the trapped magnetic field of the solar wind is favourably oriented (principally southwards) it reconnects with Earth's magnetic field, and solar particles enter the magnetosphere and are swept to the magnetotail. Further magnetic reconnection accelerates the particles towards Earth.

The collisions in the atmosphere electronically excite atoms and molecules in the upper atmosphere. The excitation energy can be lost by light emission or collisions. Most aurorae are green and red emission from atomic oxygen . Molecular nitrogen and nitrogen ions produce some low level red and very high blue/violet aurorae. The light blue colors are produced by ionic nitrogen and the neutral nitrogen gives off the red and purple color with the rippled edges. Different gases interacting with the upper atmosphere will produce different colors, caused by the different compounds of oxygen and nitrogen. The level of solar wind activity from the Sun can also influence the color of the aurorae.

Typically the aurora appears either as a diffuse glow or as "curtains" that approximately extend in the east-west direction. At some times, they form "quiet arcs"; at others ("active aurora"), they evolve and change constantly. Each curtain consists of many parallel rays, each lined up with the local direction of the magnetic field lines, suggesting that aurora is shaped by Earth's magnetic field. Indeed, satellites show electrons to be guided by magnetic field lines, spiraling around them while moving towards Earth.

The similarity to curtains is often enhanced by folds called "striations." When the field line guiding a bright auroral patch leads to a point directly above the observer, the aurora may appear as a "corona" of diverging rays, an effect of perspective.

A corona.

An Aurora Borealis from space!