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Whip-cracking flux in Sun’s magnetic field could explain slow solar wind

New images from a Sun-orbiting spacecraft may explain strange phenomena within the Sun’s atmosphere that have puzzled solar physicists for decades. The data—released yesterday from the European Space Agency’s Solar Orbiter—shed literal light on shifts in the Sun’s magnetic field known as switchbacks. They could even help explain other solar oddities, such as why the solar wind blows at two different speeds.

“I would consider this a stunning example of the Solar Orbiter mission succeeding in its objective,” says Justin Kasper, a solar physicist at the University of Michigan, Ann Arbor, who was not involved with the work. “It’s really tying what’s happening in the Sun to consequences in the solar wind.”

The Sun’s outer atmosphere—the corona—expels a swarm of charged particles into space. The plasma travels out along the Sun’s magnetic field lines, which radiate away from the star in all directions. This roiling sea of charged particles, known as the solar wind, causes space weather, including geomagnetic storms that can interfere with radio communications and power grids, and the visible aurorae on Earth.

But scientists don’t fully understand many aspects of the solar wind, such as where it originates and why it blows at two distinct speeds. Two spacecraft circling close to the Sun aim to clarify these and other questions: NASA’s Parker Solar Probe, named for Eugene Parker, an American physicist who correctly predicted the existence of the solar wind in 1958, and the Solar Orbiter.

In 2018, the Parker probe observed thousands of unusual, perplexing fluctuations in the direction of the Sun’s magnetic field lines. These lines typically all point in the same direction, but the probe revealed they sometimes spontaneously changed direction before quickly switching back to their original orientation. The probe’s instrumental readings of these changes in direction resembled a zig-zagging mountain trail, giving them the name “switchbacks.”

“Everybody who had a theoretical bone in their body wanted to try to figure out what these were,” says Gary Zank, a theoretical physicist at the University of Alabama, Huntsville, who co-authored the new study.

The Sun’s magnetic field mostly consists of closed loops of magnetism that arc off its surface before curving back. Less commonly, some magnetic field lines, known as open field lines, stream straight out into space. These are thought to accelerate the Sun’s ejecting plasma, generating the solar wind’s fastest gales, traveling about 800 kilometers per second. The looped field lines pump the brakes on escaping plasma and release relatively slower solar wind, streaming at roughly 500 kilometers per second. These slower solar winds are the primary source of space weather in our Solar System, but it’s so far been unclear how plasma escapes from these loops in the first place.

As the Sun spins, the open field lines get dragged around, and sometimes they cross paths with the closed loops. A straight line may briefly slam into and combine with a closed loop, crossing the plasma streams. In 2020, Zank and others predicted that such perturbations could create a kink in the straight magnetic field line, producing an S-shaped wave, like a cracking whip, which would explain the mysterious switchbacks seen in the Sun’s magnetic field and free up the plasma that drives slower solar wind. But they lacked the evidence to prove it.

Then in March, Daniele Telloni, a physicist at the National Institute for Astrophysics at the Astrophysical Observatory of Torino, spotted a ghostly S-shaped vortex in the Sun’s corona in data collected by the Solar Orbiter. He immediately recalled Zank’s paper from the year before.

Telloni sent Zank the images, and the duo confirmed they matched predictions for the magnetic field’s whip-cracking behavior. The results strongly suggest the crossed plasma streams and kinking of the solar magnetic field lines produces switchbacks, and that they may be responsible for the slow solar wind, Zank, Telloni, and colleagues reported on 12 September in The Astrophysical Journal Letters.

“It was sort of the physical manifestation of stuff that we had been inferring from plasma measurements,” Zank says, “but it was so much nicer to see it in person.”

Telloni says further proof would come from linking switchback observations with measurements of slower solar wind in the same spot. He may be able to see it soon. The Solar Orbiter will make its next close approach to the Sun in October, and the Parker Solar Probe is emerging from its most recent close encounter, with new data expected in a few weeks.

The new result shows one plausible explanation for switchback formation, says James Drake, a physicist at the University of Maryland, College Park, he isn’t convinced it’s the only one. Other mechanisms may generate S-curve kinks in the magnetic field, he says, such as atmospheric turbulence in the Sun’s ambient magnetic field. “Is this representative of all switchbacks?” Drake asks. “We don’t totally know the answer to that.”

The new results could help scientists predict solar storms that can interfere with electronics on Earth, says Nicholeen Viall, a solar physicist at NASA’s Goddard Space Flight Center and an investigator on the Parker Solar Probe team. “If you want to know how the big solar storms are going to propagate, you need to understand the solar wind.”

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