An aurora, called aurora borealis in the northern hemisphere and aurora australis in the Southern Hemisphere, a luminous phenomenon is characterized by a kind of extremely colorful sails into the night sky. These bright veils appear as a diffuse light curtain resembling a cloud brighter than the others that will begin to move very quickly and escalate until you see lines of light impressive several thousand kilometers, "said Christopher Perez (photographer hunter aurora borealis). This veil is caused by the interaction between charged particles of solar wind and the upper atmosphere, aurorae occur primarily in areas near the magnetic poles in an annular area aptly named "auroral zone" (between 65 and 75° magnetic latitude). It will thus form in regions of high latitudes of the Earth and appear in many different forms. The length of the aurora may be several thousands of kilometers, but its width can not exceed 100 meters. The underlying mechanisms for creating such auroras are a subject of study that take in breath scientists for years where THEMIS below.
nota: The Sun ejects not only photons, but also protons and electrons that are extremely energetic solar wind.
Image: Circle of the auroras australis in the southern hemisphere
Image: Circle of the auroras borealis in the north hemisphere
Formation of the aurora
It is in the area several thousand kilometers of our planet that these phenomena arise. The solar corona makes it extremely hot plasma wind in the form of energetic particles (electrons and ions) moving around 450 km/s. This phenomenon is due to the arrival of charged particles ejected from the sun colliding with the magnetic shield of the Earth. These high-energy charged particles are then captured and channelled by the lines of the magnetic field into the polar circles. The electrons and protons sometimes excite or ionize the atoms of the upper atmosphere (the ionosphere). The excited atoms, cannot remain in this state, an electron which exchange layer, releases a photon. This ionization causes the formation of the aurora arc, the color depends on ionized atoms and altitude, from where the color variations that we see in the sky at altitudes between 80 and 1000 km.
Image: Aurora by Gilles Boutin, hunter of Northern Lights of the Quebec.
Image: Occurring at an altitude between 80 and 1000 km, auroras are also visible from space.
The mission Themis satellites were launched in February 2007 by NASA to develop the heart of the Earth's magnetosphere. In order to study the explosive phenomena at the origin of the aurora borealis, called substorms. The solar wind particles causing the aurora are emitted by the sun in the form of an extremely hot plasma sprayed towards the Earth. This is the solar corona emits the wind plasma in the form of energetic particles (electrons and ions) moving about 450 km/s. This plasma is interacting with objects in the solar system and in particular with the magnetic field of the Earth. This wind plasma in the interplanetary travel, meeting the Earth's magnetic shield that will compress and most of the plasma is deflected, and circumnavigate the Earth.
Image: Wind plasma meeting the Earth's magnetic shield.
The magnetic shield is porous and the plasma accumulates in the suburbs of the Earth, it is in this vast reservoir of plasma that are triggered substorms. Several times a day bursts of particles are thus projected to Earth and trigger auroras. To attend the live trigger auroras, scientists at the University of California, Berkeley, sent five small satellites at different distances from the Earth to have a multi-vision of the phenomenon.
Since December 2007, these satellites do not miss anything this natural spectacle. Satellites align all four days over North America in the extension of the axis Sun - Earth, i.e. in the planet's magnetic tail as below.
Themis satellites are designed to follow the flow of energy from one satellite to another. Meteorologists do the same by placing buoys in the ocean, big waves to track moving a buoy to another. So follow these satellites substorms and trigger the solar wind to the poles using sensors and magnetometers on board. Particles that propagate along the magnetic lines of the Earth distort by measuring variations of the magnetic field at different locations, satellites follow the movement of the particles. Twenty terrestrial observatories are also studying the aurora from the mainland. Thanks to the vision of multi Themis and vision terrestrial observatories, scientists have a more spatial data collected. The end of Themis mission in 2012. It should elucidate the secret training of the aurora and thus predict aurora in the atmosphere cause electromagnetic interference in the different grids, GPS, radio communications, satellites.
Image: The solar wind compresses the Earth's magnetic field, but the wind plasma, essentially bypassing the Earth's magnetic shield.
Image: 5 Themis mission satellites were launched in February 2007 by NASA. To attend the live trigger auroras, scientists at the University of California, Berkeley, sent five small satellites at different distances from the Earth, to have a vision multipoint phenomenon.
Aurora on other planets
So that the polar auroras are visible on a planet, it has to be surrounded with a magnetic field, to divert the particles of the solar wind towards the magnetic poles, the dawns will seem then on an oval everything around a magnetic pole, that's why we have to can observe them only in certain latitudes, notably near the poles. Furthermore it has to have an atmosphere, so that there is emission of light by electric shock with the constituents of this atmosphere, the color depending on the nature of the met gas. The effects are not the same on the other planets. For example, on Jupiter, the dawns are ultraviolet while on Earth they are green or red. The dawns are predictable between one and four days before, but the forecasts remain less precise than the atmospheric weather report. When a solar flare takes place, we measure the intensity of X-rays emitted during this eruption, we are going to deduct an estimate from it of the solar wind speed (X-rays of range X: 1 day, of range M: 2 days, of range C between 3 and 4 days). The more the eruption is powerful, the more the solar wind risks to be fast. The solar wind will have to move in the direction of the Earth, to have dawns. We are going to measure also the density, the speed and the solar wind energy thanks to a satellite (ACE) situated between the sun and the earth.
If the solar wind is very powerful (range X), the auroral oval (centered on the magnetic pole) will be very wide and there will be then chances to see the height of the dawns since the average latitudes, in France for example (in 2003, a dawn was seen since Greece). The solar wind will also have to be very dense so that the luminosity is maximum. The Earth being round, if we are far from the auroral oval we shall see only the height of the "curtain". The color depends on the composition of the atmosphere. If there is electric shock with a gas, there is emission of light. And every element emits its own color. The atomic oxygen emits of the green between 100 and 200 km in height and some red between 200 and 500 km. The molecular nitrogen emits several red and purple between 60 and 100 km. Both conditions to see dawns: to be surrounded with a magnetic field and to have an atmosphere, thus for all the planets answering these conditions, we can see dawns there. The spatial telescope Hubble and the Cassini probe followed the south pole of Saturn simultaneously as Cassini approached the gaseous giant in January, 2004, Hubble took images in ultraviolet light, whereas Cassini recorded emissions radio and followed the solar wind. As on Earth, the dawns of Saturn form total or partial rings around the magnetic pole. However, contrary to the Earth, the dawns of Saturn owed days, against some minutes on Earth.
The dawns of Saturn, although certainly created by particles in charge entered.
Image: The sequence above shows three images of Saturn taken by Hubble in two days of interval some of the others. Credit: J. Clarke (Boston U.) and Z. Levay ( STScI), ESA, NASA