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The auroral ionosphere

Between 500 and 100 kilometers of altitude, i.e. with altitudes of flight of the space shuttle or satellites in low Earth orbit, the precipitated particles will run up against the mixture of gas. Certain collisions are simply elastic, which return part of flow entering towards the other hemisphere along the local line of the magnetic field. Other collisions can heat the ambient electrons: under the effect of strong precipitations, the temperature of the electrons can reach 9000 K to 300 km !

 auroral oval Photographs auroral oval on November 23rd, 1981,
to 10.48 UT, by Dynamics Explorer.
On the left the image, it is the atmosphere excited
and ionized by ultraviolet solar flow

Moreover, under the effect of a collision, a neutral molecule can, like a character of cartoon, to start to vibrate in various directions, to even turn on it. It is said that it is in an excited state. It returns in its ground state by emitting electromagnetic waves, sometimes in the visible field : that gives rise to the splendid phenomenon of the polar lights.

 engraving This engraving of the 19th century carried like captions:
“ice raising itself under northern lights”

One can see them only during the night, because if the auroral oval exists permanently, its light is so thin that it is crushed by that of the day, and even by that of full moon. As they occur with the top of 80 km, one also needs a clear sky to see them. These conditions are met in particular in winter, by the dry great colds, which made wrongly associate the dawns with the arrival of the cold. These cold makes crack the polar ices, which is the source of many and terrifying legends.
Often, the phenomenon starts with beams of pale light white which seem to fall towards the Earth. The phenomenon intensifies in about fifteen minutes; it takes the form of a veil which would oscillate under the effect of a hypothetical wind. While intensifying, the dawn is variegated of red in bottom of the veil, which sometimes plank the mauve. More in altitude, one distinguishes from the fringes of green Each color identifies his transmitter perfectly (atomic or molecular oxygen, nitrogenizes etc…). At every moment creates for itself a new composition in a quiet ballet. In period of strong solar activity, the dawns can succeed the dawns, alternating the tables: diffuse, of a green whose movements seem unperceivable, red dawn, large arcs with the homogeneous milky colors, dawns striped, in which vertical scratches of colors alternate with remote regions… In all the cases, the luminous intensity is so low that one can see the stars most brilliant with through. Their length is very changing, being able to be spread out until the hundred kilometers: in the same way, their width varies few hundreds of meters (one speaks then about auroral arc) to several tens of kilometers; finally, their duration varies few minutes at a few hours.

The dynamics of the dawns is impressive, and impossible to return to the only means of photographs. Swirls are created in some tenths of a second, are propagated on distances of several tens of kilometers in space of a few seconds. There exists a nomenclature of the dawns, which goes until the dawns black, which are the zones of the sky between two auroral arcs. Let us note finally that at the time of strong solar events, the dawns can extend towards the equator. Thus one could see of it north of France in the night of the from April 6th to 7th 2000, in period of quasi maximum of solar activity.

Institut de Planétologie et d'Astrophysique de Grenoble (IPAG)