That should bring Parker into the solar corona again and again, solar physicist Nour Raouafi of the Johns Hopkins Applied Physics Laboratory in Laurel, Md., said in the news briefing.

Parker is still orbiting the sun and planning to make several more close approaches over the next few years, eventually getting within 6 million kilometers of the solar surface. “That has consequences that we don’t know yet, but are likely to be profound,” he says. That structure could influence everything from the way solar eruptions leave the sun to the way the solar wind interacts with itself farther out from the sun, DeForest says. “The surface clearly has some structure and warp to it,” Kasper said. The first dip lasted about five hours, the last only half an hour. But during the spacecraft’s close approach to the sun in April, it crossed in and out of the boundary three times. The surface is smooth enough that the moment of crossing was noticeable, Kasper said. Neither of those images turned out to be correct. More recently, some thought it would be so ragged that it wouldn’t be apparent when the spacecraft crossed it. Decades ago, scientists imagined the boundary as a smooth sphere surrounding the sun like a snow globe. “There was some debate in the community about whether the Alfvén surface would exist as a surface at all,” he says.

“That was one of the big outstanding questions,” says solar physicist Craig DeForest of the Southwest Research Institute in Boulder, Colo., who is a member of the Parker probe team but was not part of this measurement. Surprisingly, Parker’s measurements showed that the Alfvén critical surface is wrinkly. NASA, Johns Hopkins APL, Naval Research Laboratory These structures, called streamers, are visible from Earth during a total solar eclipse, but this is the first time they’ve been observed in their birthplace. Parker Solar Probe’s WISPR camera took pictures of structures in the solar corona as the spacecraft flew through the sun’s atmosphere for the first time. But below the Alfvén critical surface, some of those waves bend back toward the surface of the sun. Out in the solar wind, waves of charged particles gush away from the sun. With the boundary identified, “we’ll now be able to witness directly how coronal heating happens,” Kasper said.Īs Parker crossed the invisible boundary, its instruments recorded a marked increase in the strength of the local magnetic field and a drop in the density of charged material. But until now, scientists didn’t know where this frontier began. Solar physicists have suspected that what happens as those waves climb higher and meet the Alfvén critical surface might play a role in heating the corona. It took until 2009 to directly observe such waves, in the lower corona, but they didn’t carry enough energy there to explain all the heat ( SN: 3/19/09). In 1942, physicist Hannes Alfvén proposed a solution to the mystery: A type of magnetic wave might carry energy from the solar surface and heat up the corona. But the sun’s corona sizzles at more than a million degrees Celsius, while the surface is only a few thousand degrees. With most heat sources, temperatures drop as you move farther away. The Alfvén critical surface also may hold the key to one of the biggest solar mysteries: why the sun’s corona, its wispy outer atmosphere, is so much hotter than the sun’s surface ( SN: 8/20/17). Scientists want to pinpoint exactly how the wind gets started to better understand how it can impact Earth. The solar wind and other, more dramatic forms of space weather can wreak havoc on Earth’s satellites and even on life ( SN: 2/26/21). The Alfvén critical surface is important because it marks where packets of plasma can separate from the sun and become part of the solar wind, the speedy stream of charged particles that constantly emanates from the sun ( SN: 8/18/17). Finding this crucial layer was one of Parker’s main goals when it launched in 2018 ( SN: 7/5/18).

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