NASA’s Parker Solar probe, which touches the sun, flew close enough to our star to pick out the finer details of the solar wind, including its origin, the “coronal holes” in the solar atmosphere.
Armed with this information, scientists may now be able to predict better solar storms which can overload dawns on our planet, but it can also disrupt communications and energy infrastructure and pose a threat to satellites, spacecraft and even astronauts.
THE Parker solar probe followed the solar wind, a stream of charged particles flowing continuously by the sun – go back to where it spawns, report a new study. This allowed the researchers to see the characteristics of the solar wind that are lost as it leaves the sun’s outer atmosphere, or crownand before it reaches Earth as a relatively uniform stream.
Related: Parker Solar Probe: the first spacecraft to “touch” the sun
The spacecraft saw that streams of high-energy particles that make up the solar wind correspond to the so-called “supergranulation flows” within the coronal holes. This discovery pointed to these regions as the source of the “fast” solar wind, which is seen above the sun’s poles and can reach speeds of up to 1.7 million mph (2.7 million km/h), about 1,000 times faster than the top speed of a fighter jet.
Coronal holes they are believed to form in areas where magnetic field lines emerge from the sun’s surface but do not return back. This causes open field lines that stretch to fill the space around the sun.
During quiet periods of our star’s 11-year activity cycle, coronal holes are usually found at the sun’s poles. This means that the solar wind emerging from coronal holes is not usually directed towards the Earth. But as the sun becomes more active and its magnetic field “reverses,” the reversed poles, coronal holes become more widespread, and these powerful streams of charged particles can be directed toward our planet. This knowledge, and these new findings, could help predict potentially disruptive solar storms, study team members said.
“Winds carry a lot of information from the sun to Earth, so understanding the mechanism behind the sun’s wind is important for practical reasons on Earth,” team co-leader and University of Maryland-College Park professor said in a statement. James Drake. “This will affect our ability to understand how the sun releases energy and drives geomagnetic storms, which pose a threat to our communications networks.”
A solar shower
The coronal holes work like a shower, spraying out jets of charged particles from evenly spaced “bright spots” where magnetic fields extend from the sun’s surface, team members said. This gives rise to funnels that can be about 18,000 miles (29,000 kilometers) wide, seen on Earth as bright “jets” within coronal holes.
The team thinks that when magnetic fields with opposite directions cross in these funnels, the magnetic field lines break and then reconnect. It’s this process, called magnetic reconnection, that is responsible for ejecting the charged particles we see as solar wind.
Scientists determined this because the speed of some of the observed particles is up to 10 times faster than the average solar wind, which is possible only with a phenomenon as powerful as magnetic reconnection. Such speeds are not possible for particles simply cruising on the plasma, team members said.
‘The photosphere is covered in cells of convection, like in a boiling pot of water, and the larger-scale convection flow is called supergranulation,’ said research co-lead Stuart Bale, professor of physics at the University of California, Berkeley, in the same statement. (The photosphere is the surface of the sun.)
“Where these supergranulation cells meet and come down, they pull the magnetic field along their path in this kind of downward funnel,” added Bale. “The magnetic field intensifies a lot there because it’s just jammed. It’s sort of like a magnetic field scoop going down a drain.”
Bale added that it’s the spatial separation of these small discharges or funnels that the team saw when they examined data collected as the Parker Solar Probe approached the sun.
“The big takeaway is that it’s the magnetic reconnection within these funnel-like structures that provides the energy source for the fast solar wind,” Bale said. “It’s not just coming from all over a coronal hole; it’s substructured within holes coronal to these supergranulation cells. It’s coming from these little bundles of magnetic energy that are associated with convection flows. Our results, we think, are a strong evidence that reconnection does it.”
Related: Facts about the age, size and history of the sun
Get up close and personal to find the origin of the fast solar wind
Studying the minute details of the solar wind is not possible from Earth because, when it traveled 93 million miles (150 million km) to reach our planet and strike its magnetic field, the flux became a smooth flow of magnetic fields and charged particles such as protons, electrons and helium nuclei.
The Parker solar probe launched on August 12, 2018. As of March 17, 2023, the spacecraft had made 15 close approaches to the sun, getting as close as 3.8 million miles (6.1 million km) and passing the star at speeds of up to 365,000 mph (587,000 km/h). Therefore, Parker gets close enough to see the details of the solar wind before they are lost.
“Once you get below that altitude, 25 or 30 solar radii [around 11 million to 13 million miles] or thereabouts, there’s a lot less solar wind evolution, and it’s more structured — you see more footprints of what was on the sun,” Bale said.
In 2021, the spacecraft passed about 5.2 million miles (8.4 million km) from the solar surface and experienced jets of material rather than simple turbulence. The team traced those jets to clustered magnetic fields and supergranulation cells on the photosphere.
What the team wasn’t sure of then, however, was whether those charged particles had been accelerated by the slingshot-like action of magnetic reconnection or if they were riding on waves of hot plasma from the sun. The high-energy state of the particles told the team that the first mechanism was responsible for accelerating the charged particles, which also received a boost from turbulence in the plasma called Alfvén waves.
“Our interpretation is that these reconnecting outflow jets excite Alfvén waves as they propagate,” Bale said. “This is a well-known observation from Earth’s magnetic tail as well, where similar types of processes occur.”
Additional data from the Parker Solar Probe, as it is within about 4 million miles (6.4 million km) of the sun during future close approaches, could help the team confirm their theory. But that could be complicated by the fact that the sun is about to enter solar maximum, a period of chaotic and intense activity.
“There was some consternation early in the solar probe mission that we’re going to launch this thing right into the quietest, most boring part of the solar cycle,” Bale said. “But I think without that, we never would have figured it out. It would just have been too messy. I think we’re lucky to have launched it in the solar minimum.”
The team’s research is detailed in a paper published online today (June 7) in Nature magazine.
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