NASA probe is the first spacecraft to visit the sun’s atmosphere

For the first time, a spacecraft comes into contact with the sun. During a recent flyby, NASA’s Parker solar probe entered the sun’s atmosphere.

“We’re finally here,” Nicola Fox, director of NASA’s Heliophysics Science division in Washington, DC, said Dec. 14 during a press briefing at the American Geophysical Union’s fall meeting. “Mankind has touched the sun.”

Parker left interplanetary space and entered solar territory on April 28, 2021, during one of his close encounters with the sun. On site, the probe took the first measurements of the exact location of this boundary, known as the Alfven critical surface. It was about 13 million kilometers above the surface of the sun, reported the physicists at the meeting, held online and in New Orleans, and in Physical examination letters December 14.

“We knew the Alfvén critical surface had to exist,” solar physicist Justin Kasper of the University of Michigan at Ann Arbor said at the press conference. “We just didn’t know where it was.”

Finding that crucial layer was one of Parker’s main goals when it launched in 2018 (NS: 5/7/18). Alfven’s critical area is important because it marks where the plasma packets can separate from the sun and become part of the solar wind, the rapid stream of charged particles that constantly emanates from the sun (NS: 08/18/17). Solar wind and other more dramatic forms of space weather can wreak havoc on Earth’s satellites and even life (SN: 02/26/21). Scientists want to figure out exactly how the wind sets in to better understand how it can impact Earth.

Alfvén’s critical surface may also hold the key to one of the greatest solar mysteries: why the solar corona, its vaporous outer atmosphere, is so much hotter than the surface of the sun (NS: 08/20/17). With most heat sources, temperatures drop the further away you are. But the solar corona sizzles at over a million degrees Celsius, while the surface is only a few thousand degrees.

In 1942, physicist Hannes Alfvén proposed a solution to the mystery: a type of magnetic wave could carry energy from the solar surface and heat the corona. It was not until 2009 to directly observe such waves, in the lower corona, but they did not carry enough energy there to explain all the heat (NS: 03/19/09). Solar physicists have suspected that what happens when these waves rise higher and hit the critical Alfven surface could play a role in the corona warming. But until now, scientists have not known where this frontier begins.

Once the limit is identified, “we will now be able to directly observe how crown heating occurs,” Kasper said.

As Parker crossed the invisible frontier, his instruments recorded a marked increase in the strength of the local magnetic field and a decrease in the density of the charged material. In the solar wind, waves of charged particles shoot out from the sun. But below Alfvén’s critical surface, some of these waves fold back towards the surface of the sun.

six black and white images of structures in the solar corona
Parker Solar Probe’s WISPR camera took photos of structures in the solar corona as the spacecraft flew over the sun’s atmosphere for the first time. These structures, called streamers, are visible from Earth during a total solar eclipse, but this is the first time they have been seen in their birthplace.NASA, Johns Hopkins APL, Naval Research Laboratory

Surprisingly, Parker’s measurements showed that Alfven’s critical surface is wrinkled. “That was one of the big unanswered questions,” says solar physicist Craig DeForest of the Southwest Research Institute in Boulder, Colorado, who is a member of the Parker probe team but was not part of this measure.

“There was a debate in the community as to whether the Alfvén surface would exist as a surface,” he says. Decades ago, scientists imagined the border as a smooth sphere surrounding the sun like a snow globe. More recently, some believed it would be so jagged that it wouldn’t be apparent when the spacecraft passed through it.

None of these images turned out to be correct. The surface is smooth enough that the moment of the crossing is noticeable, Kasper said. But during the spacecraft’s close approach to the sun in April, it entered and exited the border three times. The first dive lasted about five hours, the last only half an hour.

“The surface clearly has some structure and is warping,” Kasper said.

This structure could influence everything from how solar flares leave the sun to how the solar wind interacts with itself further from the sun, DeForest says. “This has consequences that we do not yet know, but which are likely to be profound,” he says. ” It’s very exciting. It is a terra incognita.

Parker is still orbiting the sun and plans to make several closer approaches over the next few years, eventually approaching within 6 million kilometers of the solar surface. This should bring Parker into the solar corona over and over again, solar physicist Nour Raouafi of the Johns Hopkins Applied Physics Laboratory in Laurel, Md., Said at the press conference. The spacecraft may have made another trip beyond the critical area of ​​Alfvén in August and will have another opportunity in January.

“It is expected that as we fly closer and closer to the sun, we will continue to cross this border,” Raouafi said. But the boundary may not be in the same place every time. As the activity of the sun changes, the level of Alfven’s critical surface should rise and fall as if the crown is breathing and exhaling, he said.

This is another thing that scientists hope to observe for the first time.

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