Sunday, May 28, 2023

Lab-grown solar flares offer clues on mechanism behind bursts of high-energy particles

Simulating solar flares on a scale the size of a banana.

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Solar flares are powerful electromagnetic radiation bursts accompanied by hard X-rays and energetic particles. When magnetic flux loops explode in the solar atmosphere, they occur. Because the particle acceleration occurs at a scale smaller than the observation resolution, solar measurements can detect energetic particles and hard X-rays but cannot identify the originating mechanism.

Thus, details of the cross-scale physics that explain the generation of energetic particles and hard X-rays remain a mystery.

A new study presents observations from a laboratory experiment that simulates solar coronal loop physics. To better understand how powerful explosions on the sun’s surface send intense particles and X-rays into space, scientists have built miniature coronal loops that recreate solar flares the size of bananas in the laboratory. 

Scientists at Caltech have figured out how these enormous explosions spew potentially dangerous energetic particles and X-rays into space by simulating solar flares on a scale the size of a banana.

simulated corona loop
A simulated corona loop in the Bellan Lab.

There are two ways that researchers might explain how and why the loops form and alter. The first is to watch the sun in the hopes of capturing the phenomenon in enough detail to provide helpful information. The second is a lab simulation of the loops. Paul Bellan, a professor of applied physics at Caltech, opted for the latter.

In the lab, Caltech’s Paul Bellan- professor of applied physics- built a vacuum chamber with twin electrodes inside. He created a small solar corona loop using electrodes to imitate the phenomenon by first charging a capacitor with enough power to run the city of Pasadena briefly. Each loop lasts about 10 microseconds and has a length of about 20 centimeters (cm) and a diameter of about 1 cm. But structurally, Bellan’s loops are identical to the real thing, offering him and his colleagues the opportunity to simulate and study them at will.

Bellan said, “Each experiment consumes about as much energy as it takes to run a 100-watt lightbulb for about a minute, and it takes just a couple minutes to charge the capacitor up—each loop with a camera is capable of taking 10 million frames per second.”

Bellan then studies the resulting images.

Yang Zhang, a graduate student and lead author of the study, said, “If you dissect a piece of rope, you see that it’s made up of braids of individual strands. Pull those individual strands apart, and you’ll see that they’re braids of even smaller strands, and so on. Plasma loops appear to work the same way.”

It turns out that structure plays a crucial role in producing energetic particles and X-ray bursts linked to solar flares. Consider neon signs, which are made of plasma and light up as electricity travels through them. Plasma is a powerful electrical conductor. However, a solar corona loop becomes unstable when an excessive amount of current tries to pass through it. The loop kinks, becoming unstable in a corkscrew fashion, and the strands separate. The surviving strands then experience stress from every new broken one.

A simulated corona loop in the Bellan Lab
A simulated corona loop in the Bellan Lab.

Seth Pree, a postdoctoral scholar research associate in applied physics and materials science, said“As an elastic band stretched too tight, the loop gets longer and skinnier until the strands just snap.”

The scientists saw a negative voltage spike associated with an X-ray burst at the precise moment a strand snapped after studying the process microsecond by microsecond. This voltage spike is comparable to the pressure drop that develops at the site of a water pipe’s constriction. Charged particles are accelerated to extremely high energies by the electric field from this voltage spike, and when the energetic particles decelerate, X-rays are released.

Journal Reference:

  1. Zhang, Y., Pree, S. & Bellan, P.M. Generation of laboratory nanoflares from multiple braided plasma loops. Nat Astron (2023). DOI: 10.1038/s41550-023-01941-x
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