The Earth produces a magnetic field through a dynamo effect. With no external influence, this magnetic field might resemble a large apple, whose centre is the Earth. The area controlled by this field is what is known as the magnetosphere. But there is an external influence, and it is a considerable one: the Sun. Or rather, its winds, charged with particles, which take the sun’s magnetic field with them. At the front, this solar wind compresses the Earth’s magnetosphere. At the back, the terrestrial fields lines are very stretched out and form the magnetotail. This solar wind and the solar magnetic field it is transporting therefore encompass the Earth, surrounding it and taking its magnetosphere with it on its way. Once it has passed this obstacle, it continues on its course towards the far reaches of the solar system. Reconnection of the field lines occurs in the furthermost part of the magnetotail. During these reconnections, part of the solar wind is caught in the magnetosphere. The magnetic field is deformed and stretched, almost like an elastic, thus providing the system with energy. "If you stretch an elastic", Denis Grodent explains, "it will eventually snap back and hit your fingers. The energy transferred from the elastic to you fingers will cause pain." The formation of polar aurorae is caused by the same transfer of energy. "The energy from the magnetic field is deformed and transferred to the magnetotail. This energy will heat up the particles there and excite them. They will travel along the Earth’s magnetic field lines and up to the poles." When they come into contact with the atmosphere, they will excite its atoms and molecules (oxygen, hydrogen, nitrogen). These atoms, which can’t remain in a state of excitement, will release energy by emitting photons, the elementary particles of light, and form aurorae.

Therefore, the photons are only emitted in the atmosphere. And the aurorae only appear once the interaction between the different particles of the magnetotail and solar magnetic field lines has taken place. They are nothing more than the expression of a loss of energy. The signature of an interaction between ions, electrons and a deformed magnetic field, the sign of an exchange of energy happening outside the atmosphere, but which occurs because of a topological change in the magnetic field lines. "And an influx of energy is never free. You always have to pay for it. In this case, it’s the Sun that pays the bill. The energy comes from the solar wind and it reappears in the ions."

An internal process for gas giants such as Jupiter

But this reconnection between the solar wind and the magnetic field in the magnetotail isn't the only cause of the appearance of aurorae. On some planets, these processes are internal, and don’t depend on the Sun. This is the case of Jupiter. "The effect of the solar wind is very slight", explains Denis Grodent. "Jupiter’s magnetic field is so enormous that it reinforces the magnetosphere and prevents reconnection between its magnetic field lines and those of the sun". However, the aurorae observed on Jupiter are of an unimaginable intensity. The power they generate is measured in terawatts, and is much greater than anything we could ever produce on Earth with all our nuclear plants in operation.  "To such an extent that it is mainly the aurorae that heat Jupiter’s atmosphere", the two planetologists explain, "and by several hundred degrees. It’s not the Sun."

But the causal process of these auroral footprints is internal and linked to Jupiter’s very fast rotation, which takes its magnetic field and the charged particles it contains with it. It is another of the magnetosphere’s ingredients, also swept along by the planet’s rotation, and unable to keep up with the pace, which upsets the physical mechanics of the process: plasma. In the case of Jupiter’s magnetosphere, it is the volcanic moon Io that is the main source of plasma. Part of the molecules ejected during the continuous volcanic eruptions (Io is probably the most volcanic object in the solar system) are found in Io’s atmosphere. These sulphur oxide molecules can be broken and ionised by the Sun’s UV rays or by collisions with the ambient plasma and escape Io’s gravitational force, thus filling Jupiter’s magnetosphere with (plasma) ions and electrons.

"It doesn’t turn fast enough, and according to the laws of electromagnetism, it isn’t possible. Or, in any case, the system tends to make it return to the same rotation speed. Mechanisms are set up to attempt to remedy this corotation lag. That’s the principle of corotation enforcement. A tiny part of Jupiter’s rotational energy which is transferred to this plasma by electrical currents, which will generate a force capable of accelerating the movement of the plasma. It is these electical currents that will travel along the magnetic field lines, excite the atmosphere and emit very high intensity rays."