With powerful lasers and a tiny piece of plastic, researchers have for the first time emulated a violent process that may take place on the sun. The experiment could help shed light on why the outermost layer of the sun’s atmosphere is so mysteriously hot.
For decades, scientists have struggled to explain why the sun’s outer atmospheric layers – a region called the corona – are thousands of times hotter than the sun’s surface, reaching several millions of degrees. “It is as if the air grew hotter as you walked away from a campfire,” says Wei Sun at the China Institute of Atomic Energy.
Now, Sun and his colleagues have emulated some of the conditions on the sun in the lab and, for the first time, observed how tiny bursts of electromagnetic radiation, or solar nanoflares, could power the corona’s puzzling heat.
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Nanoflares are known to occur when magnetic fields within the corona abruptly rearrange themselves and emit energy in the process. But for this rearrangement to happen, the exact conditions, such as the shape of the magnetic fields, must be just right, says Sun. He and his colleagues investigated how these conditions could arise and ultimately lead to nanoflares. They fired four very powerful lasers onto a tiny plastic target, which produced a patch of plasma – a hot mixture of charged particles – about 2 millimetres across. This plasma was not an exact recreation of plasma in the sun’s corona but behaved analogously.
The team created plasmas in two different scenarios, once using a flat target and once using one with a wavy surface. In the second case, the plasma became unstable and the magnetic fields it contained rearranged themselves and ultimately produced a nanoflare.
This nanoflare made a big difference to the temperature of the plasma. It reached temperatures of 799,726°C, which was nearly double the temperature achieved in the flat target version of the experiment where no nanoflares occurred.
A similar process – instability leading to nanoflares leading to plasma heating – could feasibly happen on the sun, says Sun. In fact, the parameters that determined their tiny plasma’s behaviour matched those observed in regions of the sun where blobs of colder plasma “rain” down through the corona. He and his colleagues also completed a computer simulation that corroborated this.
Coronal holes allow more charged particles to escape the sun as solar winds, and on 29 January, NASA’s Solar Dynamics Observatory spacecraft spotted a vast new one
Rony Keppens at KU Leuven in Belgium says that these simulations support the team’s interpretation of the experiment and make for an interesting entry on to the list of effects that may be contributing to coronal heating. Coronal rain blobs really could lead to the kinds of instabilities that the researchers now spotted in the lab, he says. Yet, in Keppens’s view, the new experiment does not definitively resolve the coronal heating puzzle, as other plasma behaviours, such as certain types of waves in it, could also contribute to it.
Going forward, Sun and his colleagues want to upgrade their experiment to create more three-dimensional plasmas and use more precise techniques for diagnosing its behaviour. And they want to make their laboratory measurements more directly comparable to reality – for example, by using data about the sun collected through telescopes, says Sun. “It is important to check that the laboratory is reproducing what the sun does,” he says.