The auroras are among the most spectacular and enchanting light phenomena taking place on Earth. Behind the beauty of these colorful veils hides the defense mechanism deployed by our planet to resist the solar winds that relentlessly assail our atmosphere.
The polar aurora, called aurora borealis in the northern hemisphere and aurora australis in the southern hemisphere, are optical phenomena that seem to have been described for the first time by the Greek explorer Phyteas of Massalia, in the 4th century BC. At the same time, the Roman philosopher Seneca Le Jeune included a whole chapter on auroras in his first natural philosophy book Naturales Quaestiones .
It classifies them into different types according to their shapes, positions and colors. Subsequently, many peoples will mention the auroras and incorporate them into various mythologies, including Aboriginal and Norse mythologies.
However, it was not really until the early 1600s that the phenomenon was scientifically studied. French astronomer Pierre Gassendi described the aurora in 1621 and called it the "aurora borealis". In the 18th century, the British astronomer Edmond Halley suggested that the auroras had a link with the Earth's magnetic field, a hypothesis reinforced by Henry Cavendish in 1768. In 1896, the Norwegian physicist Kristien Birkeland reproduced the phenomenon in the laboratory.
From the 1950s, the development of space exploration made it possible to better understand the mechanism of formation of terrestrial auroras, and to detect these phenomena on other planets of the Solar System.
Although actively studied, the exact mechanisms at the origin of the auroras are not yet fully known, even if the implication of an interaction between the solar wind and the Earth's magnetosphere is undeniable. When the solar wind, that is to say the ejection by the Sun of a plasma of energetic particles during an eruption or a solar burst for example, strikes the magnetosphere (magnetic shield surrounding the Earth), several scenarios can occur.
When the solar wind interacts with the magnetosphere, solar particles can intrude through the open lines of the geomagnetic field—while these lines are closed in the opposite hemisphere—and cause the particles to scatter through the shock arc (area between the magnetosphere and the interstellar medium). The phenomenon can also cause atmospheric precipitation of particles trapped in the Van Allen belt.
Another mechanism involves the generation of geomagnetic disturbances (magnetic vortices) in the magnetotail. These vortices appear when interconnections are created between the interstellar and terrestrial magnetic fields. This causes the solar wind to move the magnetic fluxes (magnetic lines forming tubes, imbricated between them by a plasma) from the day side towards the magnetotail, contracting it on the night side.
This displacement can be accompanied by magnetic reconnections (magnetic lines that break and reconnect) and plasmoids (intertwined plasma structure and magnetic lines) injecting many particles into the trapped plasma around the Earth.
Finally, the third mechanism involves geomagnetic disturbances leading to the acceleration of particles by the solar wind. The disturbances, due to wave-particle interactions caused by strong electric fields, cause charged particles to accelerate along magnetic lines. The emergence of pulsating electromagnetic and electrostatic waves precipitate accelerated particles in the Earth's atmosphere.
In any case, each of these scenarios results in the excitation of atoms in the ionosphere. When these are de-excited, an electron goes down an energy level by emitting a photon whose wavelength depends directly on the energy of the level. Depending on the altitude (between 80 and 1000 km), the ions involved are not the same, the energies are therefore not the same, thus giving rise to auroras of different colors.
The color red only appears at high altitudes where excited oxygen atoms emit at the wavelength of 630 nm (red), thus giving hues of carmine, scarlet and crimson. The green color is the dominant color of the auroras, and involves oxygen atoms as well as molecular nitrogen; it appears at lower altitude, where collisions between particles neutralize the mode at 630 nm, leaving a dominant emission at 557.7 nm (green).
The blue color, mainly involving molecular nitrogen, appears at an even lower altitude; nitrogen emits in the red and blue bands of the spectrum, with a dominant mode at 428 nm (blue). Other ranges such as ultraviolet, infrared, yellow or pink are possible.
The auroras appear mainly in the auroral zone (3° to 6° wide in latitudes and 10° to 20° in longitudes) and are visible in the night sky. An area where an aurora takes place is called an "auroral oval" and a map of these auroral ovals is regularly updated. Thanks to a study of more than 12,000 auroras, astrophysicist Carl Størmer and his colleagues have established that these appear mainly between 90 and 150 km altitude.
Auroras can occur on any planet with a magnetic field. However, the mechanisms behind their formation may differ. This is particularly the case of Jupiter where auroras appear following the interruption of the rotation of the plasma by the magnetic field around the planet, because of the widening of the training speeds.
In addition, Jupiter's satellites cause "auroral spots" by generating electric fields as they move through the planetary magnetic field. Auroras have also been detected on Saturn, Neptune, Uranus, Venus, Mars and even on exoplanets.
