The surface of the Sun reaches approximately 5,500 °C. Yet far above it, its atmosphere exceeds one million degrees.
This difference has long been a mystery for astronomers. Normally, we expect the temperature to drop as we move away from a heat source. But around the Sun, the corona, that immense atmosphere visible during total eclipses, is far hotter than its surface. Where does all this energy come from?
The Sun's corona can be observed with the naked eye during a total eclipse.
Luc Viatour · CC BY-SA 3.0 · Wikimedia
One of the main possibilities involves magnetic fields. The Sun is filled with gas so hot that its particles carry electric charges: plasma. Its movements drag and deform magnetic fields, which can accumulate a great deal of energy.
Sometimes, regions where magnetic fields point in opposite directions move closer together. Their arrangement then changes abruptly, releasing energy that heats the plasma. Scientists call this “magnetic reconnection.” Very small events of this kind, sometimes called nanoflares, could help heat the solar corona without displaying activity as visible as a coronal mass ejection.
But one question remains: how are these encounters between magnetic fields triggered? Researchers working in several Chinese laboratories wanted to investigate one possibility. They created highly energetic plasma in the laboratory and tracked the movements that emerge when regions with different densities meet.
In their experiment, these differences made the plasma unstable. The movements brought oppositely oriented magnetic fields closer together, eventually causing reconnection. The researchers then measured strong heating of the electrons, the small charged particles that move through the plasma.
This heating was about twice as great as in experiments where the electrons were mainly heated by a shock wave. Computer simulations also showed that the release of magnetic energy explained most of the phenomenon observed in this experiment.
Why does this interest specialists studying the Sun? Because similar movements could occur in its corona, particularly when denser matter falls back into it. Small disturbances could then be enough to bring magnetic fields closer together and trigger episodes of heating.
The experiment does not reproduce everything that happens around the Sun and, by itself, does not solve the mystery of its temperature. It nevertheless shows that a mechanism considered by physicists actually works. The remaining question is how much heat it supplies to the corona, alongside other possible phenomena.