NASA's Parker Solar Probe has made contact with the sun for the first time in history as it flew through the sun's upper atmosphere, known as the corona, and sampled particles and magnetic fields there.

The new milestone is a significant step forward for the Parker solar probe and a giant leap forward for solar science. Touching the very stuff the sun is made of will help scientists uncover critical information about our nearest star and its influence on the solar system, just as landing on the moon allowed scientists to understand how it was formed.

"Parker Solar Probe touching the Sun is a monumental moment for solar science and a truly remarkable feat," said Thomas Zurbuchen, the associate administrator for the Science Mission Directorate at NASA Headquarters in Washington. 

"Not only does this milestone provide us with deeper insights into our sun's evolution and its impacts on our solar system, but everything we learn about our own star also teaches us more about stars in the rest of the universe."

 

 

 

 

 

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Parker is making discoveries as it circles closer to the solar surface that other spacecraft were too far away to see, including from within the solar wind – the flow of particles from the sun that can influence us on earth. 

The probe discovered in 2019 that magnetic zigzag structures in the solar wind, known as switchbacks, are abundant close to the sun. But where and how they formed remained a mystery. Since then, the distance between the earth and the sun has been cut in half, and the Parker Solar Probe has passed close enough to identify one location where they originate: the solar surface.

The first flyby of the corona — and the promise of more to come – will continue to provide data on phenomena that are impossible to study from afar.

"Flying so close to the sun, Parker Solar Probe now senses conditions in the magnetically dominated layer of the solar atmosphere — the corona — that we never could before," said Nour Raouafi, the Parker project scientist at the Johns Hopkins Applied Physics Laboratory in Laurel, Maryland. 

"We see evidence of being in the corona in magnetic field data, solar wind data, and visually in images. We can actually see the spacecraft flying through coronal structures that can be observed during a total solar eclipse."

Closer than ever before

Parker Solar Probe was launched in 2018 to investigate the sun's mysteries by traveling closer to it than any previous spacecraft. Parker has finally arrived, three years after its initial conception and decades after its initial launch.

The sun, unlike earth, does not have a solid surface. It does, however, have a superheated atmosphere composed of solar material held together by gravity and magnetic forces. As rising heat and pressure push that material away from the sun, gravity and magnetic fields become ineffective at containing it.

The Alfvén critical surface marks the end of the solar atmosphere and the beginning of the solar wind. Solar material with enough energy to cross that boundary becomes solar wind, which carries the sun's magnetic field with it as it races across the solar system to earth and beyond. Importantly, beyond the Alfvén critical surface, the solar wind moves so fast that waves within the wind can never travel fast enough to reconnect with the sun, severing their link.

 

 

 

 

 

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Until now, researchers were unsure of the exact location of the Alfvén critical surface. Estimates based on remote images of the corona put it between 10 and 20 solar radii from the sun's surface, which is equivalent to 4.3 to 8.6 million miles. 

Parker's spiral trajectory gradually brings it closer to the sun, and during the last few passes, the spacecraft was consistently below 20 solar radii (91 percent of earth's distance from the sun), putting it in position to cross the boundary — if the estimates were correct.

On April 28, 2021, during its eighth flyby of the sun, Parker Solar Probe encountered the specific magnetic and particle conditions at 18.8 solar radii (around 8.1 million miles) above the solar surface that told scientists it had crossed the Alfvén critical surface for the first time and finally entered the solar atmosphere.

"We were fully expecting that, sooner or later, we would encounter the corona for at least a short duration of time," said Justin Kasper, lead author on a new paper about the milestone published in Physical Review Letters, deputy chief technology officer at BWX Technologies, Inc. and University of Michigan professor. "But it is very exciting that we've already reached it."  

Into the Eye of the Storm 

During the flyby, the Parker Solar Probe passed into and out of the corona several times. This proves what some predicted — that the Alfvén critical surface is not shaped like a smooth ball. Rather, it has spikes and valleys that wrinkle the surface. 

Discovering where these protrusions align with solar activity coming from the surface can help scientists learn how events on the sun affect the atmosphere and solar wind.

On encounter nine, the Parker Solar Probe flew by structures known as coronal streamers as it flew through the corona. These structures can be seen as bright features moving upward in the upper images and angled downward in the lower row. 

This view is only possible because the spacecraft flew above and below the streamers inside the corona. Until now, streamers could only be seen from afar. They are visible from the earth during total solar eclipses.

 

 

 

 

 

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As the Parker Solar Probe approached 15 solar radii (roughly 6.5 million miles) from the sun's surface, it passed through a feature in the corona known as a pseudostreamer. Massive structures that rise above the sun's surface and can be seen from the earth during solar eclipses are known as pseudostreamers.

Flying through the pseudostreamer is similar to flying into the eye of a hurricane. The conditions inside the pseudostreamer quieted, particles slowed, and the number of switchbacks decreased — a dramatic change from the usual barrage of particles the spacecraft encounters in the solar wind.

For the first time, the spacecraft found itself in an area where magnetic fields were strong enough to dominate particle movement. These conditions proved beyond a shadow of a doubt that the spacecraft had passed the Alfvén critical surface and entered the solar atmosphere, where magnetic fields shape the movement of everything in the region.

The mission's first passage through the corona, which lasted only a few hours, is one of many planned. Parker will continue to spiral closer to the sun, eventually getting as close to the surface as 8.86 solar radii (3.83 million miles). Upcoming flybys, the next of which is scheduled for January 2022, will most likely send the Parker Solar Probe through the corona once more.

"I'm excited to see what Parker finds as it repeatedly passes through the corona in the years to come," said Nicola Fox, division director for the Heliophysics Division at NASA Headquarters. "The opportunity for new discoveries is boundless."

Solar activity also influences the size of the corona. The outer edge of the corona will expand as the sun's 11-year activity cycle – the solar cycle – ramps up, giving Parker Solar Probe a better chance of staying inside the corona for longer periods of time.

"It is a really important region to get into because we think all sorts of physics potentially turn on," Kasper said. "And now we're getting into that region and hopefully going to start seeing some of these physics and behaviors."

Narrowing down switchback origins

Even before the first trips through the corona, some unexpected physics emerged. Parker Solar Probe collected data on recent solar encounters that pinpointed the origin of zigzag-shaped structures in the solar wind known as switchbacks. The data revealed that one location where switchbacks originate is on the sun's visible surface — the photosphere.

The solar wind is an unrelenting headwind of particles and magnetic fields by the time it reaches earth, 93 million miles away. The solar wind, however, is structured and patchy as it escapes the sun. 

The NASA-European Space Agency mission Ulysses flew over the sun's poles in the mid-1990s and discovered a handful of strange S-shaped kinks in the magnetic field lines of the solar wind, which detoured charged particles on a zigzag path as they escaped the sun. For decades, scientists assumed that these sporadic switchbacks were confined to the sun's polar regions.

Parker discovered switchbacks in the solar wind in 2019 at a distance of 34 solar radii from the sun. This piqued the public's interest in the features and raised new questions: Where did they come from? Were they formed on the sun's surface, or were they shaped by a process that kinked magnetic fields in the solar atmosphere?

The new findings, published in the Astrophysical Journal, confirm that one origin point is near the solar surface.

Parker's clues came as it orbited closer to the sun on its sixth flyby, which was less than 25 solar radii away. Data showed that switchbacks occur in patches and contain a higher percentage of helium — which is known to originate in the photosphere – than other elements. 

The origins of the switchbacks were narrowed even further when the scientists discovered the patches aligned with magnetic funnels that emerge from the photosphere between convection cell structures known as supergranules.

In addition to being the origin of switchbacks, scientists believe the magnetic funnels are the source of one component of the solar wind. The solar wind is divided into two types: fast and slow, and the funnels may be where some of the particles in the fast solar wind originate.

"The structure of the regions with switchbacks matches up with a small magnetic funnel structure at the base of the corona," said Stuart Bale, professor at the University of California, Berkeley, and lead author on the new switchbacks paper. "This is what we expect from some theories, and this pinpoints a source for the solar wind itself."

Understanding where and how the fast solar wind components emerge and whether they are linked to switchbacks could help scientists solve a long-standing solar mystery: how the corona is heated to millions of degrees, far hotter than the solar surface below.

While the new findings pinpoint the location of switchbacks, the scientists are unable to confirm how they are formed. According to one theory, they are caused by plasma waves that roll through the region like ocean waves. Another theory holds that they are formed by an explosive process known as magnetic reconnection, which is thought to take place at the boundaries where the magnetic funnels meet.

 

 

 

 

 

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"My instinct is, as we go deeper into the mission and lower and closer to the Sun, we're going to learn more about how magnetic funnels are connected to the switchbacks," Bale said. "And hopefully resolve the question of what process makes them."

Parker's closer passes may reveal even more clues about switchbacks and other solar phenomena now that researchers know what to look for. The upcoming data will provide scientists with a glimpse into a region critical for superheating the corona and propelling the solar wind to supersonic speeds. Such corona measurements will be critical for understanding and forecasting extreme space weather events that can disrupt communications and damage satellites all over the world.

"It's really exciting to see our advanced technologies succeed in taking Parker Solar Probe closer to the Sun than we've ever been, and to be able to return such amazing science," said Joseph Smith, Parker program executive at NASA Headquarters. "We look forward to seeing what else the mission discovers as it ventures even closer in the coming years."

The Parker Solar Probe is part of NASA's Living with a Star program, which aims to investigate aspects of the Sun-Earth system that have a direct impact on life and society. NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the Living with a Star program for the agency's Science Mission Directorate in Washington. 

The Parker Solar Probe mission is managed by the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, which also designed, built, and operated the spacecraft.