NASA’s Parker Solar Probe has now flown through the Sun’s upper atmosphere, the Corona. A feat achieved first time in the history of the mankind. It sampled particles and magnetic fields present on the outer atmosphere of the Sun.

Considered as one giant leap in the domain of solar science, the new milestone marks one major step for Parker Solar Probe. Just like landing on the Moon allowed scientists to know more about the Moon was formed, Parker Probe touching the very stuff the Sun is made of will help scientists uncover critical information about our closest star and its influence on the solar system. 

As it circles closer to the solar surface, Parker is making new discoveries that other spacecraft were too far away to see, including from within the solar wind. Solar wind is the flow of particles from the Sun that can influence us at Earth.

Back in 2019, Parker discovered that magnetic zigzag structures in the solar wind, called switchbacks, are plentiful close to the Sun, but how and where they form remained a mystery. Travelling a substantial distance to the Sun since then, Parker Solar Probe has now passed close enough to identify one place where they originate – the solar surface.

The first passage through the corona will continue to provide data on phenomena that are impossible to study from afar.

Background 

Parker Solar Probe was launched back in 2018 to explore the mysteries of the Sun by traveling closer to it than any spacecraft before. Three years after launch and decades after first conception, Parker has finally arrived.

Unlike Earth, the Sun does not have a solid surface, but it does have a superheated atmosphere made of solar material bound to the Sun by gravity and magnetic forces. As rising heat and pressure push that material away from the Sun, it reaches a point where gravity and magnetic fields are too weak to contain it.

That point is known as the Alfvén critical surface, which marks the end of the solar atmosphere and beginning of the solar wind. Solar material with the energy to make it across that boundary becomes the solar wind, which drags the magnetic field of the Sun with it as it races across the solar system to Earth and beyond. More importantly, beyond the Alfvén critical surface, the solar wind moves so fast that waves within the wind cannot ever travel fast enough to make it back to the Sun. 

Until now, researchers were unsure exactly where the Alfvén critical surface lay. Based on remote images of the corona, estimates had put it somewhere between 10 to 20 solar radii from the surface of the Sun which is about 4.3 to 8.6 million miles. Parker’s spiral trajectory brings it slowly 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 the position to cross the boundary.

Earlier this year in April, during its eighth flyby of the Sun, Parker Solar Probe encountered the specific magnetic and particle conditions above the solar surface that told scientists it had crossed the Alfvén critical surface for the first time and finally entered the solar atmosphere.

In and out of the Corona 

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

As Parker Solar Probe dipped just beneath the Sun’s surface, it transited a feature in the corona called a Pseudostreamer. Pseudostreamers are massive structures that rise above the Sun’s surface and can be seen from Earth during solar eclipses.

Passing through the pseudostreamer was like flying into the eye of a storm. Inside the pseudostreamer, the conditions quieted, particles slowed, and number of switchbacks dropped. 

For the first time, the spacecraft found itself in a region where the magnetic fields were strong enough to dominate the movement of particles there. These conditions were the definitive proof 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 size of the corona is also driven by solar activity. As the Sun’s 11-year activity cycle ramps up, the outer edge of the corona will expand, giving Parker Solar Probe a greater chance of being inside the corona for longer periods of time.

The Switchbacks

Even before the first trips through the corona, some surprising physics was already surfacing. On recent solar encounters, Parker Solar Probe collected data pinpointing the origin of zig-zag-shaped structures in the solar wind called switchbacks. The data showed one spot that switchbacks originate is at the visible surface of the Sun which is also called the photosphere. 

By the time it reaches Earth, 93 million miles away, the solar wind is an unrelenting headwind of particles and magnetic fields. But as it escapes the Sun, the solar wind is structured and patchy. In the mid-1990s, the NASA-European Space Agency mission Ulysses flew over the Sun’s poles and discovered a handful of bizarre S-shaped kinks in the solar wind’s magnetic field lines, which detoured charged particles on a zig-zag path as they escaped the Sun. For decades, scientists thought these occasional switchbacks were oddities confined to the Sun’s polar regions.  

The clues came as Parker orbited closer to the Sun on its sixth flyby. Data showed switchbacks occur in patches and have a higher percentage of helium than other elements. The switchbacks’ origins were further narrowed when the scientists found the patches aligned with magnetic funnels that emerge from the photosphere between convection cell structures called supergranules.

In addition to being the birthplace of switchbacks, the scientists think the magnetic funnels might be where one component of the solar wind originates. The solar wind comes in two different varieties – fast and slow – and the funnels could be where some particles in the fast solar wind come from. 

Understanding where and how the components of the fast solar wind emerge, and if they are linked to switchbacks, could help scientists answer a longstanding solar mystery about how the corona is heated to millions of degrees, far hotter than the solar surface below.

While the new findings locate where switchbacks are made, the scientists cannot yet confirm how they are formed. One theory suggests they might be created by waves of plasma that roll through the region like ocean surf. Another contends that they are made by an explosive process known as magnetic reconnection, which is thought to occur at the boundaries where the magnetic funnels come together.