Auroras borealis and australis
What is a aurora?
|Automatic translation||Updated June 01, 2013|
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.
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.
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.
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.
Themis satellites are designed to follow the flow of energy from one satellite to another.
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.
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.
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