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Hottest news from the Sun's corona
Imposing, bright and above all extremely hot, the Sun reigns supreme at the centre of our Solar System. Although observed for thousands of years with the help of ever more sophisticated instruments, our star still holds many secrets. However, one of these mysteries has now been partially elucidated.
Tahar Amari, at the Center for Theoretical Physics (CPHT)1 and his colleagues have recently published a paper in The Astrophysical Journal Letters2 on the enigma surrounding the temperature of the solar corona.
While that of the Sun's surface is only a few thousand degrees, astonishingly, the outermost part of its atmosphere, the corona – approximately 2,200 km to several tens of millions of kilometres above the surface – reaches temperatures of around one million degrees. This long-known paradox is thought to be caused by “magnetic flux ropes” that ascend from the Sun's surface and heat the corona.
Temperature gradient
Nowhere in the Solar System does the heat exceed that found at the centre of the Sun, which acts as a huge nuclear fusion reactor nearly 1.4 million kilometres across. "In the Sun's core, temperatures easily reach over ten million kelvins (K)3. And like the Earth, the Sun is made up of several layers," Amari explains. "By the time the radiation from the core reaches the bottom of the layer called the convection zone, after travelling 200,000 km of the 700,000 km (of the Sun's radius, Editor's note) needed to reach the surface, the temperature of the plasma that makes up the Sun has already fallen to 2 million kelvins. And in the final 500,000 km it drops dramatically, falling to around 6,000 K."
So far, so normal. The temperature gradient behaves in a way that could be described as classic: the further away from the primary heat source, the more the temperature tends to decrease. But it's then that things get complicated.
An atmosphere hotter than the surface
The Sun's atmosphere is made up of several different layers, just like the Earth's. The innermost layer, the photosphere, extends from the surface to an altitude of 500 km. Next comes the chromosphere, which goes up to an elevation of some 2,200 km. These two layers already exhibit heat levels higher than those found at the Sun’s surface – on the order of 4,000 K for the photosphere and as much as 25,000 K for the chromosphere.
Above them lies the corona, with a temperature of around one million kelvins. So, strangely, up to a certain point the further you move away from the Sun, the hotter things become.
This can be observed right from the bottom of the corona, which is strongly coupled to the underlying layers, suggesting that the different layers are connected by whatever causes this phenomenon. How can this be explained?
"The solar corona is governed by the magnetic field"
"Today, two theories are frequently put forward, both of them based on magnetism," Amari explains.“The first involves magnetic waves." Unlike sound waves, these don’t become weaker as the density of matter in the surrounding medium decreases. The second theory is also based on magnetic fields, in particular on the fact that they are constantly undergoing rearrangement (known as “reconnection”), which can cause solar flares.
As Amari points out, "The solar corona is governed by the magnetic field, due to the large number of collisions between atoms, as well as to the fact that the corona is an electrically conductive medium." This makes the magnetic field visible, because of the way matter is organised within it. The spectacular solar flares are a perfect example of this.
"However, we still didn't understand how energy moves from the photosphere to the chromosphere, and then to the rest of the corona nearest to the Sun," the researcher adds.
The magnetic flux rope hypothesis confirmed
In 2015, Amari and his colleagues had already designed a numerical model and put forward a hypothesis about the formation of magnetic flux ropes on the surface of the Sun, even when the latter is quiet4. As he explains, "In our model we realised that a whole bunch of small, twisted eruptive magnetic flux ropes were emerging from the Sun's surface, coupling with larger structures to form a network similar to a mangrove forest."
This “magnetic mangrove forest” was therefore able to heat the chromosphere via numerous micro-flares which, by coupling with the large structures extending up into the corona, excited a particular type of wave called Alfvén waves5. However, the physicists still needed to understand what could cause the formation of these waves. Thanks to their model, they realised that, at the base of the magnetic flux ropes, something was driving an energy transfer large enough to heat the corona.
"Magnetic fields are like guitar strings: if you pluck them at the bottom, at the level of the Sun's surface, the energy should travel all the way up the 'string'," Amari explains. Thus, little by little, the energy should reach the corona and eventually heat it up.
Using their model, the researchers also discovered that the Sun's surface is in motion at the base of the magnetic flux ropes. They believe that this is indirect evidence that something deeper down is affecting what happens at the surface.
"In the top 1,000 km beneath the surface, there is a zone made up of convection cells, rather like at the bottom of a heated pan of water," says Amari by way of illustration. "The heat coming from below is transported upwards by these cells, eventually warming the rest of the water in the pan. It is this process beneath the Sun's surface that is thought to contribute to the transfer of heat, and to the creation of this magnetic field and its flux ropes."
Evidence from observations
Compelling evidence confirming this hypothesis has now been provided by data from direct observations of the Sun's surface by the Japanese Hinode satellite, which can measure the magnetic field using a magnetic imaging technique. These images enabled the researchers to identify for the first time magnetic flux ropes in a quiet area of the Sun, thus validating their model and the predictions they had made a decade earlier.
So, whether the Sun is quiet or active, both large and small magnetic flux ropes transport enough energy to the corona to heat it to around one million kelvins. This discovery is only a first step in understanding the coronal heating process. Instruments such as the Parker Solar Probe and the Daniel K. Inouye Solar Telescope (DKIST) in Hawaii (US) should refine the direct observation of magnetic flux ropes, shedding fresh light on their interaction with the Sun's magnetic environment. ♦
See also
Solar storms ahead
Those stars that come and go
- 1. CNRS / École polytechnique.
- 2. Tahar Amari et al., "The Ubiquity of Twisted Flux Ropes in the Quiet Sun," ApJL 984 L37, April 2025: https://doi.org/10.3847/2041-8213/adb74f
- 3. The temperature in kelvins (K) is equal to the temperature in degrees Celsius (°C) + 273. Thus, 0 °C = 273 K, 100 °C = 373 K, 1000 °C = 1273 K, etc.
- 4. Amari T., Canou A. and Aly J.-J., "Characterizing and predicting the magnetic environment leading to solar eruptions", Nature 514, 465–469 (2014): https://doi.org/10.1038/nature13815
- 5. Alfvén waves: magnetohydrodynamic waves found in plasmas. They are responsible for energy transport in various astrophysical systems, such as the magnetosphere.
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