Laboratory Solar Flares Reveal Clues to Mechanism Behind Bursts of High-Energy Particles

Simulating solar flares on a scale the size of a banana, researchers at Caltech have parsed out the process by which these massive explosions blast potentially harmful energetic particles and X-rays into the cosmos.

Corona loops are arches of plasma that protrude from the surface of the sun, aligned along magnetic field lines. The magnetic field lines act like highways for charged particles, guiding the motion of the electrons and ions that comprise plasma. The loops, which may project 100,000 kilometers above the sun’s surface, can persist for minutes to hours. The loops usually grow and evolve slowly but sometimes can abruptly blast a tremendous amount of energy—billions of times stronger than the most powerful nuclear explosion on Earth—into space. This sudden blast of energy is called a solar flare.

Some of the energy in the flare takes the form of charged particles and “hard X-rays,” which are high-energy electromagnetic waves like those used to image bones in a doctor’s office. The Earth’s own magnetic field and atmosphere act as a shield that protects life on the surface from getting cooked by these torrents of energy, but they have been known to disrupt communications and power grids. They also pose an ongoing threat to spacecraft and astronauts in space.

While the fact that solar flares generate energetic particles and X-ray bursts has long been known, scientists are only starting to piece together the mechanism by which they do so.

Researchers have two options for deciphering how and why the loops form and change. The first is to observe the sun and hope to capture the phenomenon in sufficiently fine detail to yield relevant information. The second is to simulate the loops in a lab. Caltech’s Paul Bellan, professor of applied physics, chose the latter.

In a lab on the first floor of the Thomas J. Watson, Sr., Laboratories of Applied Physics on Caltech’s campus, Bellan built a vacuum chamber with twin electrodes inside. To simulate the phenomenon, he charged a capacitor with enough energy to run the City of Pasadena for a few microseconds, then discharged it through the electrodes to create a miniature solar corona loop.

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 he and his colleagues the opportunity to simulate and study them at will.

“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,” says Bellan, the senior author of a new paper on solar flares that published on April 6 in Nature Astronomy. Bellan captures each loop with a camera capable of taking 10 million frames per second, and he then studies the resulting images.