NASA's sun-touching Parker Solar probe has flown close enough to our star to spot the fine details of the solar wind — including its origin, "coronal holes" in the sun's atmosphere.
Armed with this information, scientists may now be able to better predict solar storms that can supercharge auroras over our planet but can also disrupt communication and power infrastructure and pose a threat to satellites, spacecraft and even astronauts.
The Parker Solar Probe tracked the solar wind — a stream of charged particles flowing continuously from the sun — back to where it is generated, a new study reports. This allowed researchers to see characteristics of the solar wind that are lost as it exits the sun's outer atmosphere, or corona, and before it reaches Earth as a relatively uniform stream.
The spacecraft saw that the streams of high-energy particles that make up the solar wind match so-called "supergranulation flows" within coronal holes. This discovery pointed to these regions as the source of the "fast" solar wind, which is seen over the poles of the sun and can reach speeds as great as 1.7 million mph (2.7 million kph), around 1,000 times faster than the top speed of a jet fighter.
Coronal holes are believed to form in areas where magnetic field lines emerge from the sun's surface but do not loop back there. This causes open field lines that spread 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 poles of the sun. This means that the solar wind that emerges from coronal holes isn't usually directed toward Earth. But when the sun becomes more active and its magnetic field "flips," switching poles, coronal holes become more widespread, and these powerful streams of charged particles can be directed at our planet. That knowledge, and these new results, could aid the prediction of potentially disruptive solar storms, study team members said.
"Winds carry lots 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 James Drake said in a statement. "That's going to affect our ability to understand how the sun releases energy and drives geomagnetic storms, which are a threat to our communication networks."
A sun shower
The coronal holes operate like a showerhead, spraying jets of charged particles from evenly spaced "bright spots" where magnetic fields extend out from the sun's surface, team members said. This gives rise to funnels that can be around 18,000 miles (29,000 kilometers) wide, seen on Earth as bright "jetlets" within coronal holes.
The team thinks that when magnetic fields with opposite directions pass each other in these funnels, magnetic field lines break and then reconnect. It is this process, called magnetic reconnection, that is responsible for flinging out the charged particles that we see as solar wind.
The scientists determined this because the speed of some of the observed particles is up to 10 times greater than the average for the solar wind — something only possible with a powerful phenomenon like magnetic reconnection. Such speeds aren't possible for particles simply surfing along on plasma, team members said.
"The photosphere is covered by convection cells, like in a boiling pot of water, and the larger-scale convection flow is called supergranulation," research co-leader Stuart Bale, a physics professor at the University of California, Berkeley, said in the same statement. (The photosphere is the sun's surface.)
"Where these supergranulation cells meet and go downward, they drag the magnetic field in their path into this downward kind of funnel," Bale added. "The magnetic field becomes very intensified there because it's just jammed. It's kind of a scoop of the magnetic field going down into a drain."
Bale added that it is the spatial separation of these little drains or funnels that the team saw when they looked at data gathered as the Parker Solar Probe made its close approaches to the sun.
"The big conclusion is that it's magnetic reconnection within these funnel structures that's providing the energy source of the fast solar wind," Bale said. "It doesn't just come from everywhere in a coronal hole; it's substructured within coronal holes to these supergranulation cells. It comes from these little bundles of magnetic energy that are associated with the convection flows. Our results, we think, are strong evidence that it's reconnection that's doing that."
Getting up close and personal to find the origin of the fast solar wind
Studying the fine detail of the solar wind isn't possible from Earth because, by the time it has traveled 93 million miles (150 million km) to reach our planet and strike its magnetic field, the stream has become a homogenous flow of magnetic fields and charged particles like protons, electrons and helium nuclei.
The Parker Solar Probe launched on Aug. 12, 2018. As of March 17, 2023, the spacecraft had made 15 close approaches to the sun, coming as close as 3.8 million miles (6.1 million km) and racing past the star at speeds as great as 365,000 mph (587,000 kph). Thus, Parker gets close enough to see solar wind details before they are lost.
"Once you get below that altitude, 25 or 30 solar radii [around 11 million to 13 million miles] or so, there's a lot less evolution of the solar wind, and it's more structured — you see more of the imprints of what was on the sun," Bale said.
In 2021, the spacecraft passed within about 5.2 million miles (8.4 million km) of the solar surface and raced through jets of material rather than mere turbulence. The team traced those jets back to bunched-up magnetic fields and supergranulation cells on the photosphere.
What the team wasn't sure of back then, however, was if those charged particles were being accelerated by the slingshot-like action of magnetic reconnection or if they were surfing on waves of hot plasma from the sun. The high energy status of the particles told the team that the former mechanism was responsible for accelerating the charged particles, which also got a boost from turbulence in the plasma called Alfvén waves.
"Our interpretation is that these jets of reconnection outflow excite Alfvén waves as they propagate out," Bale said. "That's an observation that's well known from Earth's magnetotail, as well, where you have similar kinds of processes."
Further data from the Parker Solar Probe, as it comes to within around 4 million miles (6.4 million km) of the sun during future close approaches, could help the team confirm their theory. But this 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 at the beginning of the solar probe mission that we're going to launch this thing right into the quietest, most dull part of the solar cycle," Bale said. "But I think without that, we would never have understood this. It would have been just too messy. I think we're lucky that we launched it in the solar minimum."