See also: Dawn

A polar lights (also called northern light in the northern Hemisphere and aurora australis in the Southern hemisphere) is a luminous phenomenon characterized by kinds of veils extremely coloured in the night sky.

Caused by the interaction between the particles charged with the Solar wind and high the atmosphere, the dawns occur mainly in the areas close to the magnetic poles, in an annular zone precisely called “auroral zone” (between 65 and 75° of magnetic Latitude).

Principle

At the time of under polar storm accompanying a Magnetic storm making following a chromospheric eruption or a solar Start important, an surge of particle S charged ejected by the Sun enters in collision with the shield which the Magnétosphère constitutes. Electrified particles with high energy can then be collected and channeled by the lines of the Terrestrial magnetic field on the side harms of the magnetosphere (the tail) and to lead in the polar horns . These particles (electron S and sometimes Proton S) excite (change of layer of an electron) or ionize the Atome S of the upper atmosphere (the Ionosphère). The excited atom, cannot remain in this state, an electron changes layer then, releasing with the passage a little energy, by emitting a Photon. Like the nature of these ions (Oxygen, Hydrogen, Nitrogen,…) depends on altitude, this partly explains the variations of colors of the clouds, draperies, curtains, arcs, rays… which are spread in the sky at altitudes ranging between 80 and 1.000 km. The Ion isation resulting from this surge of particles causes the formation of ionized clouds reflecting the waves radio.

The spectographic study of the emitted light watch the presence of oxygen (green line with 557 Nm and red doublet with 630 and 636 Nm) between 120 and 180 km of altitude, of the Nitrogen and its compounds and of hydrogen (656 Nm) at the time of the dawns to protons . With the lowest latitudes, the color observed most frequently is the red (altitudes from 90 to 100 km).

Observation

The spectacle is very changing and can begin with the formation from an arc (auroral arc) perpendicular to the magnetic meridian line from the place, then to be accompanied by sometimes animated rays of a more or less fast pulsation (0,05 to 15 hertz) or to move more or less quickly. One observes sometimes gleams resembling a curtain or a drapery agitated by the breeze.

The luminosity can vary much and the phenomenon can last of a few minutes to a few tens of minutes. It is very rare to observe dawns with magnetic latitudes lower than 50 degrees. That occurs only for the period of maximum solar activity of the cycle 11 years, at the time of the most important solar eruptions. The photograph opposite was taken in Belfort the November 20th 2003 with 19:15 at the time of a visible dawn exceptional until in the south of Europe.

History

Northern lights were observed since always but were studied scientifically only as from the 17th century. In 1621, the French astronomer Pierre Gassendi describes this phenomenon observed until in the South of France and the name of northern light gives him. At the 18th century, the British astronomer Edmond Halley suspects the terrestrial magnetic field of playing a part in the formation of northern lights. Henry Cavendish, in 1768, manages to evaluate the altitude to which the phenomenon occurs but it will be necessary to await 1896 so that this one either reproduced in laboratory by Birkeland. Work of Carl Stormer on the movements of the particle S electrified in a Magnetic field facilitated the comprehension of the mechanism of formation of the dawns. Starting from 1957, the space Exploration allowed not only one better knowledge of the terrestrial polar lights, but also the observation of auroral phenomena on the grosses Planet S like Jupiter or Saturn.

The ionized cloud that the polar lights constitute reflects the electromagnetic waves in the field of very the high frequencies (VHF and beyond). The Radioamateur S use this phenomenon to carry out experimental connections at long distance. The Ondes radio in fact are diffused more than considered what produces a strong deformation of the modulation. Telegraphy Morse is practically the only mode of transmission usable. A harmful effect of this phenomenon is the disturbance of the communications on these frequencies.

Another planets

Polar lights also occur on another planets:

  • On Jupiter, Saturn, Uranus and Neptune, planets having a Magnetic field, their origin is identical to what occurs on Ground.
  • On Venus, which does not have a planetary magnetic field, the atmospheric molecules are ionized directly by the Solar wind.
  • On Mars, of the dawns can appear close to local magnetic anomalies in the planetary bark, supposed remainders of an ancestral magnetic field nowadays disappeared.

See too

Internal bonds

External bonds

  • Photographs of dawns by Gilles Boutin
  • Northern light
  • AuroresBoreales.com
  • Northen-lights.no: contest of photographs of northern lights
  • Northern Lights Pictures
  • solar wind and polar lights
  • formation of the terrestrial Polar lights

Simple: Aurora

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