This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: From its visible surface, the sun radiates heat at a relatively cool temperature of 5,800 Kelvin. Yet within the thin outer layer of the sun's atmosphere known as the corona, electrons can reach temperatures of millions of degrees, for reasons astronomers have yet to fully explain.
Through new research published in Physical Review Letters, a team led by Wei Sun at the China Institute of Atomic Energy has recreated corona-like conditions in the lab. Their experiment shows how this dramatic heating could be driven by instabilities in the corona's plasma, which trigger tiny bursts of energy called 'nanoflares.' The leading explanation for coronal heating begins at the sun's surface, where churning plasma slowly twists and stretches the magnetic fields reaching up into the corona, storing energy within them. Eventually, inside thin sheets of electrical current, magnetic field lines pointing in opposite directions can break and rejoin in a process called 'reconnection,' suddenly releasing this energy.
Each nanoflare is small by solar standards, but could launch jets of plasma and briefly heat its surroundings to extreme temperatures. Separately, observations have revealed finger-like structures forming where dense drops of cooler plasma fall through the thinner plasma around them, in a process astronomers call 'coronal rain.' Much like a heavy liquid resting on top of a lighter one, this arrangement is unstable. Simulations have suggested that these fingers can squeeze magnetic fields together, triggering reconnection and small plasma jets.
So far, however, no direct evidence has connected these effects to nanoflare heating. To create their analog of the corona, Sun's team used China's Shenguang-II laser facility. They fired powerful laser beams at tiny plastic targets with wavy surfaces.
This produced a plasma just millimeters across, with instabilities seeded in a precisely controlled way. Although far smaller than anything on the sun, the plasma was tuned to behave in a similar way to the plasma around coronal rain. The researchers then tracked the plasma's structure and magnetic fields using optical imaging, and estimated its electron temperatures by analyzing the X-rays it emitted.
For comparison, they repeated the experiment using flat targets, where no instabilities could form. The result was the first-ever lab demonstration of nanoflare heating driven by plasma instabilities. With the wavy targets, the team reproduced the finger-like structures seen in the real corona, which pushed oppositely directed magnetic fields together and drove reconnection.
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