A multitude of "campfires" heating the crown of the sun could explain how its temperature far exceeds that of its surface, a real physical enigma that the Solar Orbiter is trying to unravel.

To date, "we have no coherent explanation for this phenomenon," astronomer Frédéric Auchère, from the Institute of Space Astrophysics, told AFP.

It has been going on for more than 70 years, when we realized that the solar corona - the atmosphere of the sun, whose white disk heats the Earth - exceeded one million degrees Celsius as the surface temperature of the sun peaks. at some 5,500 degrees.

"When you move away from it, the temperature drops a little, like a radiator which heats less when you move away from it", but very quickly, this temperature rises very high, says the astronomer, who co-authored two studies on the subject to appear in Astronomy & Astrophysics.

A new phenomenon

This is where Solar Orbiter, the European Space Agency and NASA probe that took its first images of the sun last year. A five-minute observation that was aimed simply at checking the proper functioning of his instruments. Taken about 77 million kilometers from the Sun, halfway between the Earth and its star, they showed a new phenomenon: mini solar flares, omnipresent on the surface, called "campfires".

Since then, a team of European, American and Russian scientists have had access to calibrated, ie “clean” data taken by an instrument of the probe which takes images in the extreme ultra-violet.

Thanks to him, we "were able to see very fine structures in the atmosphere of the sun", explains Frédéric Auchère, and determine that they are "small-scale copies of larger events that we already knew" .

With an observed length of 400 to 4,000 km (tiny on the scale of the Sun), they arise between 1,000 and 5,000 km above the surface of the Sun - the photosphere.

Ephemeral, they appear over very short durations, ranging from 10 to 200 seconds.

But by reaching in this very short period of time a "coronal" temperature of over a million degrees Celsius.

Matches

Scientists assume that in this way "the atmosphere would be heated by this myriad of small flashes occurring all over the place", explains Frédéric Auchère who co-signed his study with David Berghmans, of the Royal Observatory of Belgium, and David Long , from the UK Mullard Space Science Laboratory.

A second study, led by Chinese scientist Yajie Chen and Prof Hardi Peter, from the German Max Planck Institute, modeled the phenomenon using observations from Solar Orbiter.

She comes to a similar conclusion, suggesting that "this process contributes significantly to the heating of the crown".

But all hasten to add that it will be necessary to wait for other more detailed observations to confirm a scenario, a priori improbable, in which "we try to heat the atmosphere of this big star with matches", as summarized. nicely Frédéric Auchère.

An atmosphere that rises millions of kilometers from the surface.

Patience

"We are trying to find out if there are enough of these small eruptions and if they carry enough energy to bring the crown to its temperature," he adds. You have to arm yourself with a little patience. Solar Orbiter will deploy all its talents in the fall, when its battery of instruments is fully operational. With the SPICE spectrograph - for which Frédéric Auchère is scientific director - we expect precise measurements of the temperature and density of the solar atmosphere, and therefore of its campfires.

The PHI polarimeter will draw ephemeral "maps" of the innumerable magnetic fields which traverse its surface and whose interaction is supposed to be the driving force behind campfires.

These interactions are at the heart of larger solar flares.

It will be necessary to wait until next February-March, when the probe will venture twice as close to the Sun as during its first passage, to see there even more clearly, and especially smaller, with details of only 200 km, against 400 today. 'hui.

And who knows, maybe smaller fires, even more.

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