A spacecraft has reached the Sun for the first time in history. NASA's Parker Solar Probe has now collected particles and magnetic fields in the Sun's upper atmosphere, known as the corona.
The latest achievement is a huge step forward for the Parker Solar Probe and a giant leap forward for solar research. Touching the same substance the Sun is composed of will help scientists unearth crucial knowledge about our nearest star and its effect on the solar system, much as landing on the Moon helped scientists to understand how it was formed.
A major milestone and new results from NASA’s Parker Solar Probe were announced on Dec. 14 in a press conference at the 2021 American Geophysical Union Fall Meeting in New Orleans. The results have been published in Physical Review Letters and accepted for publication in the Astrophysical Journal.
“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 it’s 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.”
Parker is finding new discoveries that other spacecraft couldn't observe because they were too far away, including from within the solar wind, which is the flow of particles from the Sun that may affect us on Earth. Parker revealed in 2019 that magnetic zig-zag formations in the solar wind, known as switchbacks, are abundant near the Sun. However, it remained a mystery as to how and where they formed. Parker Solar Probe has now traveled near enough to pinpoint one spot where they originate: the solar surface, after halving the distance to the Sun since then.
The first flyover into the corona – and the promise of many to come – will continue to deliver information on processes that are difficult to examine 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 with the goal of delving deeper into the secrets of the Sun by journeying closer to it than any previous mission. Parker has finally come three years after its introduction and decades after its creation.
The Sun, unlike Earth, does not have a solid surface. It does, however, have a superheated atmosphere consisting of solar material that is gravitationally and magnetically bonded to the Sun. As the material is pushed away from the Sun by increased heat and pressure, it reaches a point where gravity and magnetic fields are no longer strong enough to retain it.
The end of the solar atmosphere and the start of the solar wind are marked by the Alfvén critical surface. Solar material having enough energy to pass that limit creates the solar wind, which carries the Sun's magnetic field with it as it speeds through the solar system to Earth and beyond. Importantly, the solar wind flows so quickly beyond the Alfvén critical surface that waves inside the wind can never travel fast enough to return to the Sun, breaking their link.
Researchers had no idea where the Alfvén critical surface was until today. Estimates based on distant views of the corona put it between 10 and 20 solar radii – 4.3 to 8.6 million miles – from the Sun's surface. Parker's spiral trajectory pushes it closer to the Sun with time, and over the past few visits, it was continuously below 20 solar radii (91 percent of Earth's distance from the Sun), putting it in position to breach the threshold — assuming the estimations were true.
Parker Solar Probe encountered specific magnetic and particle conditions at 18.8 solar radii (around 8.1 million miles) above the solar surface on April 28, 2021, during its eighth flyby of the Sun, indicating 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, and 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
Parker Solar Probe flew into and out of the corona many times throughout the flyby. This shows that the Alfvén critical surface is not formed like a smooth ball, as some had assumed. Rather, the surface is wrinkled by spikes and troughs. Scientists will be able to learn more about how events on the Sun influence the atmosphere and solar wind if they can figure out where these protrusions match up with solar activity coming from the surface.
As the Parker Solar Probe approached 15 solar radii (about 6.5 million miles) from the Sun's surface, it passed through a structure in the corona known as a pseudostreamer. Massive structures that rise above the Sun's surface and may be seen from Earth during solar eclipses are known as pseudostreamers.
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 – a dramatic change from the busy barrage of particles the spacecraft usually encounters in the solar wind.
For the first time, the spacecraft encountered an area where the magnetic fields were powerful enough to control particle movement. These characteristics proved the spacecraft had passed the Alfvén critical surface and entered the solar atmosphere, where magnetic forces control the movement of everything in the area.
The mission's first corona crossing, which lasted barely a few hours, is the first of several planned. Parker will continue to spiral closer to the Sun, ultimately approaching the surface at a distance of 8.86 solar radii (3.83 million miles). Parker Solar Probe will most certainly pass through the corona again during upcoming flybys, the next of which is scheduled for January 2022.
“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.”
The size of the corona is also driven by solar activity. As the Sun’s 11-year activity cycle – the solar 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.
“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
Some unexpected physics surfaced even before the first passes into the corona. Parker Solar Probe obtained data on the genesis of zig-zag-shaped structures in the solar wind known as switchbacks during previous solar encounters. The photosphere, which is visible on the Sun's surface, is one place where switchbacks begin, according to the data.
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.
In 2019, at 34 solar radii from the Sun, Parker discovered that switchbacks were not rare, but common in the solar wind. This renewed interest in the features and raised new questions: Where were they coming from? Were they forged at the surface of the Sun, or shaped by some process kinking magnetic fields in the solar atmosphere?
The new findings, in press at the Astrophysical Journal, finally confirm one origin point is near the solar surface.
The clues came as Parker orbited closer to the Sun on its sixth flyby, less than 25 solar radii out. Data showed switchbacks occur in patches and have a higher percentage of helium – known to come from the photosphere – 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.
The magnetic funnels are thought to be the genesis of one component of the solar wind, in well as being the birthplace of switchbacks. The solar wind is divided into two types: fast and slow, and the funnels might be the source of certain particles in the rapid solar wind.
"The shape of the switchback areas lines up with a tiny magnetic funnel structure at the base of the corona," said Stuart Bale, lead author of the new switchbacks research and professor at the University of California, Berkeley. "This is what some theories predict, and it identifies a source for the solar wind itself."
Understanding where and how the components of the fast solar wind emerge, and if they’re linked to switchbacks, could help scientists answer a longstanding solar mystery: 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 can’t yet confirm how they’re formed. One theory suggests they might be created by waves of plasma that roll through the region like ocean surf. Another contends they’re made by an explosive process known as magnetic reconnection, which is thought to occur at the boundaries where the magnetic funnels come together.
“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 visits may yield more more hints about switchbacks and other solar phenomena now that experts know what to look for. Scientists will be able to see into an area that is crucial for superheating the corona and propelling the solar wind to supersonic speeds thanks to the data that will be released soon. Extreme space weather events, which may interrupt telecommunications and destroy satellites orbiting Earth, will require readings from the corona to comprehend and anticipate.
“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.”
Reference: “I Enters the Magnetically Dominated Solar Corona” by J. C. Kasper et al., 14 December 2021, Physical Review Letters.
The Parker Solar Probe is part of NASA's Living with a Star program, which aims to learn more about the Sun-Earth system's effects on life and civilization. NASA's Science Mission Directorate in Washington oversees the Living with a Star program, which is managed by the agency's Goddard Space Flight Center in Greenbelt, Maryland. The Parker Solar Probe mission is managed by NASA's Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, which also planned, constructed, and controls the spacecraft.