Seven Natural wonders of the world
Seven Natural wonders of the world : Grand
Canyon | Great
Barrier Reef | Harbor
of Rio de Janeiro | Mount
Everest | Polar Aurora
| Paricutin volcano | Victoria
Falls
Polar Aurora
An aurora is a bright glow observed in the night sky, usually in the polar zone. For this reason some scientists call it a "polar aurora" (or "aurora polaris"). In northern latitudes, it is known as the aurora borealis , which is named after the Roman goddess of the dawn, Aurora, and the Greek name for north wind, Boreas. Especially in Europe, it often appears as a reddish glow on the northern horizon, as if the sun were rising from an unusual direction. The aurora borealis is also called the northern lights since it is only visible in the North sky from the Northern Hemisphere. The aurora borealis most often occurs from September to October and from March to April. Its southern counterpart, aurora australis, has similar properties. Australis is the Latin word for "of the South".
Auroral mechanism
Auroras are now known to be caused by the collision of charged particles (e.g. electrons), found in the magnetosphere, with atoms in the Earth's upper atmosphere (at altitudes above 80 km). These charged particles are typically energized to levels between 1 and 15 keV and, as they collide with atoms of gases in the atmosphere, the atoms become energized. Shortly afterwards, the atoms emit their gained energy as light . Light emitted by the Aurora tends to be dominated by emissions from atomic oxygen, resulting in a greenish glow (at a wavelength of 557.7 nm) and - especially at lower energy levels and at higher altitudes - the dark-red glow (at 630.0 nm of wavelength). Both of these represent forbidden transitions of electrons of atomic oxygen that, in absence of newer collisions, persist for a long time and account for the slow brightening and fading (0.5-1 s) of auroral rays. Many other colors - especially those emitted by atomic and molecular nitrogen (blue and purple, respectively)[1] - can also be observed. These, however, vary much faster and reveal the true dynamic nature of auroras.
As well as visible light, aurorae emit infrared (NIR and IR) and ultraviolet (UV) rays as well as X-rays (e.g. as observed by the Polar spacecraft). While the visible light emissions of aurorae can easily be seen on Earth, the UV and X-ray emissions are best seen from space, as the Earth's atmosphere tends to absorb and attenuate these emissions.
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