other: Aurora (disambiguation)

Auroras, sometimes called the northern and southern (polar) lights or aurorae (singular: aurora), are natural light displays in the sky, usually observed at night, particularly in the polar regions. They typically occur in the ionosphere. They are also referred to as polar auroras. 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 by Pierre Gassendi in 1621. The aurora borealis is also called the northern polar lights, as it is only visible in the sky from the Northern Hemisphere, the chance of visibility increasing with proximity to the North Magnetic Pole, which is currently in the arctic islands of northern Canada. 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 northern lights have had a number of names throughout history. The Cree people call this phenomenon the "Dance of the Spirits." Aurorae can be spotted throughout the world. It is most visible closer to the poles due to the longer periods of darkness and the magnetic field.

Its southern counterpart, the aurora australis or the southern polar lights, has similar properties, but is only visible from high southern latitudes in Antarctica, South America, or Australasia. Australis is the Latin word for "of the South."

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.

Auroral mechanism

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 80 km (50 miles)). The particles have energies of 1 to 100 keV. 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 connects 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 electrically excite electrons to take quantum leaps (a mechanism in which the electron's kinetic energy is converted to visible light); and molecules in the upper atmosphere. The excitation energy can be lost by light emission or collisions. Most aurorae are green and red emissions from atomic oxygen. Molecular nitrogen and nitrogen ions produce some low level red (pink) and very high blue/violet aurorae. The light blue and green colors are produced by ionic nitrogen and the neutral helium gives off the purple colour whereas Neon is responsible for the rare orange flares with the rippled edges. Different gasses 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.fact: date=July 2008