Artist’s concept of the Parker Solar Probe spacecraft approaching the sun. Launched in 2018, the probe is increasing our ability to forecast major space-weather events that impact life on Earth.
Based on the team’s analysis, the coronal holes are like showerheads, with roughly evenly spaced jets emerging from bright spots where magnetic field lines funnel into and out of the surface of the sun. The scientists argue that when oppositely directed magnetic fields pass one another in these funnels, which can be 18,000 miles across, the fields often break and reconnect, slinging charged particles out of the sun.
"The photosphere is covered by convection cells, like in a boiling pot of water, and the larger scale convection flow is called supergranulation,” Bale said. "Where these supergranulation cells meet and go downward, they drag the magnetic field in their path into this downward kind of funnel. The magnetic field becomes very intensified there because it’s just jammed. It’s kind of a scoop of magnetic field going down into a drain. And the spatial separation of those little drains, those funnels, is what we’re seeing now with solar probe data.”
Based on the presence of some extremely high-energy particles that the Parker Solar Probe has detected - particles traveling 10 to 100 times faster than the solar wind average - the researchers conclude that the wind could only be made by this process, which is called magnetic reconnection. The probe was launched in 2018 primarily to resolve two conflicting explanations for the origin of the high-energy particles that comprise the solar wind: magnetic reconnection or acceleration by plasma or Alfvén waves.
"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.”
The funnel structures likely correspond to the bright jetlets that can be seen from Earth within coronal holes, as reported recently by Nour Raouafi, a co-author of the study and the Parker Solar Probe project scientist at the Applied Physics Laboratory at Johns Hopkins University. APL, located in Laurel, Maryland, designed, built, manages and operates the spacecraft.
"Solving the mystery of the solar wind has been a six-decade dream of many generations of scientists," said Raouafi. "Now, we are grasping at the physical phenomenon that drives the solar wind at its source - the corona."
Plunging into the sun
By the time the solar wind reaches Earth, 93 million miles from the sun, it has evolved into a homogeneous, turbulent flow of roiling magnetic fields intertwined with charged particles that interact with Earth’s own magnetic field and dump electrical energy into the upper atmosphere. This excites atoms, producing colorful auroras at the poles, but has effects that trickle down into Earth’s atmosphere. Predicting the most intense winds, called solar storms, and their near-Earth consequences is one mission of NASA’s Living With a Star program, which funded Parker.
The previous image marked with colored lines that indicate the boundaries of the open field lines (outward-pointing is red, inward-pointing is blue) as predicted by a computer model. These regions correspond well to the coronal holes in the EUV map. The white boxes show the points of origin of the magnetic field lines that the Parker Solar Probe passed through as it traveled across the sun’s surface.
The probe was designed to determine what this turbulent wind looks like where it’s generated near the sun’s surface, or photosphere, and how the wind’s charged particles - protons, electrons and heavier ions, primarily helium nuclei - are accelerated to escape the sun’s gravity.
To do this, Parker had to get closer than 25 to 30 solar radii, that is, closer than about 13 million miles.
"Once you get below that altitude, 25 or 30 solar radii 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, Parker’s instruments recorded magnetic field switchbacks in the Alfvén waves that seemed to be associated with the regions where the solar wind is generated. By the time the probe reached about 12 solar radii from the surface of the sun - 5.2 million miles - the data were clear that the probe was passing through jets of material, rather than mere turbulence. Bale, Drake and their colleagues traced these jets back to the supergranulation cells in the photosphere, where magnetic fields bunch up and funnel into the sun.
But were the charged particles being accelerated in these funnels by magnetic reconnection, which would slingshot particles outward, or by waves of hot plasma - ionized particles and magnetic field - streaming out of the sun, as if they’re surfing a wave?
The fact that Parker detected extremely high-energy particles in these jets - tens to hundreds of kiloelectron volts (keV), versus a few keV for most solar wind particles - told Bale that it has to be magnetic reconnection that accelerates the particles and generates the Alfvén waves, which likely give the particles an extra boost.
"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 kind of processes. I don’t understand how wave damping can produce these hot particles up to hundreds of keV, whereas it comes naturally out of the reconnection process. And we see it in our simulations, too. ”
The Parker Solar Probe won’t be able to get any closer to the sun than about 8.8 solar radii above the surface - about 4 million miles - without frying its instruments. Bale expects to solidify the team’s conclusions with data from that altitude, though the sun is now entering solar maximum, when activity becomes much more chaotic and may obscure the processes the scientists are trying to view.
"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.
RELATED INFORMATION
Interchange reconnection as the source of the fast solar wind within coronal holes (Nature)
By Robert Sanders
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