The northern and southern lights are luminous manifestations that occur in the upper atmosphere . Their shape and intensity vary rapidly and they are generally observed at the poles . This is a very common phenomenon and there are a lot of photos of the northern and southern lights on the internet.

But where do the polar aurora come from?

Photo of an aurora borealis taken in Canada. The latter looks like a curtain of light coming from space and which suddenly dims at a certain altitude above the ground.

Physical properties of the polar aurora

The polar aurorae appear in the Earth’s ionosphere, that is to say the highest part of the Earth’s atmosphere at about 100 km altitude. These are charged particles which interact with the atmospheric constituents (mainly nitrogen and oxygen).

This interaction then results in a generally green or yellow light emission , but sometimes red and more rarely blue.

Contrary to what one might think, the polar lights also form during the day . They are only seen at night because the light emitted by the interaction between charged particles and the atmosphere is far too weak. Finally, the polar lights generally last several hours .

The origin of the polar aurora

Against all odds, the polar auroras do not come from the Earth but from the Sun.

Indeed, the charged particles which interact with the Earth’s atmosphere generally come from violent solar flares . These charged particles are expelled by the solar magnetic field and travel at extremely high speeds towards the Earth.

When they are close enough to Earth, they are – because of their electrical charge – forced to follow the Earth’s magnetic field lines . The only possible trajectories for these particles are to go either to the North Pole or to the South Pole depending on their electrical charge.

This diagram represents the interaction between the charged particles emitted by the Sun (green spots) and the lines of terrestrial magnetic field (blue lines). We can see that the field lines are concentrated either at the North Pole or at the South Pole. This is normal, we can consider the Earth as a magnetic dipole. The charged particles then tend to follow these field lines.

The polar lights on other planets?

And yes, Earth is not the only planet to experience this kind of phenomenon. In reality, any planet with a sufficiently dense atmosphere can be subject to the polar aurora. This is in particular what has been observed on Mars , on Jupiter or even on Saturn .

Here is an image of Saturn in X-rays taken by Hubble in 2009. We observe at the poles a double polar aurora characterized by two bright blue rings. Credits: Nasa / Esa

Here is this time an aurora borealis taken at the north pole of Jupiter in X-rays by the Hubble telescope. It also has the shape of a ring, sign of the convergence of the magnetic field lines at the poles. Credits: ESA / NASA

Conclusion:The Northern and Southern Lights are nothing other than the manifestation of solar flares. Thanks to the Earth’s magnetic field, charged particles from the Sun only enter the atmosphere near the poles and fortunately at high altitude. However, when the solar flares are huge, this is called a geomagnetic storm and the particles have such high energy that they no longer necessarily follow the lines of the Earth’s magnetic field. We can then observe aurorae outside the poles and the latter can even interfere with certain telecommunications systems. This is also where this famous obsession of certain comes from experiencing a global breakdown of communication systems due to an extremely violent geomagnetic storm. May I reassure you,

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For further

You will find many details on the polar auroras on Wikipedia.

I also give you a link to a very interesting experience: Planeterrella. This project consists of simulating an aurora in the laboratory. The experimental device consists of an electron gun which represents the Sun, and a magnetized ball which represents a planet, all in a vacuum enclosure. The research team can then test the northern lights in many possible configurations (tilt axis, amount of charged particles) and deduce real consequences for the planets of the solar system.