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The Polar Aurora In Alaska, Canada, Norway, Finland or northern Russia, on a clear night, a greenish glow is often seen in the sky, known as the "Northern Lights."During magnetic storms, the glow may move southwards, and on occasion it can be seen in much of the US. It often appears as a glow on the horizon, like the glow preceding sunrise, and has therefore become known among scientists as"aurora borealis" ("aurora" for short), Latin for "northern dawn." A similar phenomenon is also seen in southern polar regions. |
  The aurora--a woodcut by Fridtjof Nansen |
    To an observer, an aurora is a fascinating spectacle, constantly moving and changing. It usually consists of many near-vertical greenish rays, forming long arcs and curtains, which stretch like ribbons across the sky, oftenfrom horizon to horizon. An example is shown on the right, a woodcut by the great polar explorer Fridtjof Nansen (1861-1930). The rays constantly fade while new ones appear, and during "magnetic substorms" (described in a later section) the arcs move rapidly and expand.Auroral light is produced at a height of about 100 km (60 miles) when fast electrons, arriving from space, slam into atoms andmolecules of the atmosphere. The computer screen displaying these words may be lit up in a similar way, by a beam of fast electrons accelerated electrically towards it, then steered and modulated so as to form letters and pictures.LocationThe location of auroras on Earth is strongly controlled by the Earth's magnetism. In the 19th century it was noticed that they occur most frequently in a narrowbelt, the "auroral zone", which circles the magnetic pole (see history, below). Their arcs and ribbons are approximately aligned with that zone, too. The circles drawn on the left are centered on the northern magnetic pole, and the auroral "circle of fire" is evidently lined up with them. |
Aurora with comet 
Hale-Bopp (Dick
Hutchinson ©) |
The magnetic connection is also demonstrated by the factthat the rays of the aurora lie along magnetic field lines, and that the Earth's magnetic field observed beneath a bright and active aurora tends to be disturbed. |
    Aurora observed 
    by an imaging 
  camera aboard DE-1 |
ColorThe green light of the aurora has a precisely defined color in the spectrum ("narrow spectral line"). Such precise colors are usually the signatures of the atomswhich emit them: for instance, street lights (depending on the metal vapor they contain) usually emit either the yellow-orange light of sodium or the bluish light of mercury.The green light of the aurora puzzled scientists for many years, since it fit no known element. It turned out to be produced by oxygen atoms, but under conditions that in our atmosphere only exist in the very rarefied upperlevels. A red aurora, occasionally seen, arises at even greater heights and is also produced by electrons hitting oxygen. |

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Aurora on 6 January 1998 (Dick Hutchinson ©) |

Observing the aurora from spaceSatellites nowadays observe the aurora from above, using cameras more sensitive than the human eye. On dark parts of the polar cap they can "see" aurora at most times, forming a large "auroraloval" which extends around the magnetic pole. |

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Aurora viewed from the Space Shuttle |

Leticular Cloud

Lenticular clouds (Altocumulus lenticularis) are stationary lens-shaped clouds that form at high altitudes, normally aligned perpendicular to the wind direction. Lenticular clouds can be separated into altocumulus standing lenticularis (ACSL), stratocumulus standing lenticular (SCSL),and cirrocumulus standing lenticular (CCSL). Due to their shape, they have been offered as an explanation for some Unidentified Flying Object (UFO) sightings.

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. Formation
Where stable moist air flows over a mountain or a range of mountains, a series of large-scale standing waves may form on the downwind side. If the temperature at the crest of...