According to a study presented at the British astronomy meeting, Europe’s Solar Orbiter probe has managed to closely observe the “coronal rains”, a phenomenon that explains the temperatures of over a million degrees in the sun’s crown.
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This observation “is a major step for solar physics,” astronomer Patrick Antolin of the University of Northumbria is quoted as saying in a press release. “Because it gives us important clues about the Sun’s most important mysteries,” and specifically the way its corona is heated.
This temperature “gives us cause for concern,” astronomer Frédéric Auchère of the Institute for Space Astrophysics, who co-authored the study conducted by Patrick Antolin and to be published in Astronomy & Astrophysics, tells AFP.
The Sun’s internal temperature, which reaches fifteen million degrees at its core, drops to about 5,000 degrees on the star’s surface, the photosphere. The concern is that it extends about a million degrees further back, into the corona, the outermost layer of the Sun’s atmosphere, which is made up of plasma, a highly ionized gas.
“If we assume that the surface of the sun is a radiator, it should be colder as we move away from it, not hotter,” summarizes Frédéric Auchère.
The explanation for this warming is based on the unprecedented observation of coronal rainfall.
This phenomenon of corona plasma condensation, which cools in places and causes plasma droplets to fall on the sun’s surface like rain, was already known to physicists.
Like a “beautiful rain”
It is the effect of this rain that the Solar Orbiter imaging instruments launched in 2020 have been able to observe.
Their precision reveals plasma clusters more than 200 kilometers wide that “fall like a fine rain, while we could see heavy showers there,” says Mr. Auchère.
Due to gravity, these droplets fall at a phenomenal speed of over 200,000 km/h, following the coronal loops, the magnetic field lines that structure the entire solar atmosphere.
Solar Orbiter shows that coronal raindrops are compressed and heated in the corona’s ultralight atmosphere before hitting the Sun’s surface like shooting stars (these particles of matter heat up to incandescence as they enter Earth’s atmosphere). The impact then throws material and shock waves upwards, which heat up the corona plasma.
“It seems to confirm that the warming is not happening in the corona,” but from further down, on the Sun’s surface, according to astronomer Frédéric Auchère.
It remains to explain the physical mechanism of this warming, which requires “a lot of theoretical modeling work”.
But so did other observations with Solar Orbiter, which ventured closer to the star in 2022, just over two-thirds the distance that separates our Earth from the Sun. An unprecedented feat for a probe with telescopes and instruments to operate under the protection of a heat shield heated by the sun’s rays to 500 degrees Celsius.