Auroras borealis and australis

A polar aurora called aurora borealis in the north hemisphere and the aurora australis in the southern hemisphere, is a brilliant phenomenon characterized by sorts of veils extremely colored in the night-sky. This veil is provoked by the interaction between particles loaded with the solar wind and the high atmosphere, the dawns occur mainly in regions close to magnetic poles, in annular zone exactly called " auroral zone " (between 65 and 75 ° of magnetic latitude). It thus form in the regions of high latitudes of the Earth and appear under numerous different forms. The length of the dawn can measure several thousand kilometers, but its width can not exceed 100 meters. The deep mechanisms of creation of such dawns, are a subject of study which hold in breath the scientists, for years.

Circle of the auroras australis in the southern hemisphere

Circle of the auroras borealis in the north hemisphere

The auroras on 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.

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

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Astronomy - october 15th 2007