Parker Solar Probe flies in fast solar wind and finds its source

a satellite hovering above an angry red sun

a satellite hovering above an angry red sun

Artists’ concept of the Parker Solar Probe spacecraft approaching the sun. Launched in 2018, the probe is increasing our ability to predict key space weather events that impact life on Earth. (Image credit: NASA)

NASA’s Parker Solar Probe flew close enough to the sun to detect the fine structure of the solar wind near where it generates on the sun’s surface, revealing details that are lost as the wind exits the corona as a uniform burst of charged particles.

It’s like seeing jets of water coming out of a shower head through the jet of water hitting you in the face.

In an article to be published this week in the magazine Naturea team of scientists led by Stuart D. Bale, a physics professor at the University of California, Berkeley, and James Drake of the University of Maryland-College Park, report that the Parker Solar Probe has detected streams of high-energy particles that match to supergranulation flows within the coronal holes, suggesting that these are the regions where the so-called “fast” solar wind originates.

Coronal holes are areas where magnetic field lines emerge from the surface without flowing back inward, thus forming open field lines that expand outward and fill most of the space around the sun. These holes are usually at the poles during quiet periods of the sun, so the fast solar wind they generate doesn’t impact Earth. But when the sun becomes active every 11 years as its magnetic field flips, these holes appear all over the surface, generating blasts of solar wind aimed directly at Earth.

Understanding how and where the solar wind originates will help predict solar storms which, by producing beautiful auroras on Earth, can also wreak havoc on satellites and the power grid.

Winds carry a lot of information from the sun to Earth, so understanding the mechanism behind the solar wind is important for practical reasons on Earth, Drake said. This will affect our ability to understand how the sun releases energy and drives geomagnetic storms, which pose a threat to our communication networks.

amber map of the sun's surface, with dark and bright regions

A flattened map of the sun’s entire surface, or corona, taken in extreme ultraviolet wavelengths by NASA’s Solar Dynamics Observatory (SDO) satellite. The two dark regions below the center of the image are coronal holes sampled by the Parker Solar Probe. Inside these coronal holes, flows in the solar atmosphere create intense and complex magnetic fields that annihilate and produce the pressure and energy to overcome the solar gravity and send high-energy particles outward in the fast solar wind. . Not visible within the coronal holes are the intense magnetic field funnels where the fast solar wind actually originates large convection cells called supergranulations. (Image courtesy of NASA)

Based on the team’s analysis, the coronal holes are like dandelions, with roughly evenly spaced jets emerging from bright spots where magnetic field lines funnel in and out of the sun’s surface. Scientists argue that when oppositely directed magnetic fields cross in these funnels, which can be 18,000 miles wide, the fields often break up and reconnect, hurling charged particles out of the sun.

“The photosphere is covered in cells of convection, like in a boiling pot of water, and the larger-scale convection flow is called supergranulation,” Bale said. “Where these supergranulation cells meet and descend downwards, they drag the magnetic field along their path in this type of descending funnel. The magnetic field there intensifies a lot because it’s just blocked. It’s sort of like a magnetic field scoop going down a drain. And the spatial separation of those little outlets, those funnels, is what we’re seeing now with the solar probe data.”

Based on the presence of some very high-energy particles that the Parker Solar Probe has detected particles traveling 10 to 100 times faster than the average solar wind, the researchers conclude that the wind could only be produced by this process, which is called magnetic reconnection. The spacecraft was launched in 2018 primarily to resolve two conflicting explanations for the origin of the high-energy particles that make up the solar wind: magnetic reconnection or acceleration of plasma or Alfvn waves.

“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 doesn’t just come from all over in a coronal hole, it’s substructured within coronal holes to these supergranulation cells. It comes from these small beams of magnetic energy associated with convection flows. Our findings, we think, are strong evidence that it’s reconnection that’s doing it.

The funnel-like structures likely correspond to the bright jets that can be seen from Earth inside the coronal holes, as recently reported by Nour Raouafi, study co-author and Parker Solar Probe project scientist at Johns Hopkins University’s Applied Physics Laboratory . APL, headquartered in Laurel, Maryland, designed, built, operates and operates the spacecraft.

Solving the mystery of the solar wind has been a six-decade dream of many generations of scientists, Raouafi said. Now, we’re holding on to the physical phenomenon that pushes the solar wind back to its source, the corona.

Soak up the sun

By the time the solar wind reaches Earth, 93 million miles from the sun, it has evolved into a smooth, turbulent flow of turbulent magnetic fields entangled with charged particles that interact with Earth’s magnetic field and discharge electrical energy into the upper atmosphere. This excites the atoms, producing colorful auroras at the poles, but has effects that spill over into Earth’s atmosphere. Predicting the most intense winds, called solar storms, and their aftermath near Earth is a mission of NASA’s Living With a Star program, which is funded by Parker.

same image of the sun with strokes marked by red and blue lines and white diamonds

The above image marked with colored lines indicating 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 origin points of the magnetic field lines the Parker Solar Probe passed through as it traversed the surface of the sun.

The probe was designed to determine what this turbulent wind looks like where it is generated near the sun’s surface, or photosphere, and how the wind’s charged particles, protons, electrons and heavier ions, mainly nuclei of helium, are accelerated to escape the sun’s gravity.

To do this, Parker had to get 25-30 solar radii closer, that is, closer than about 13 million miles.

“Once you go below that altitude, about 25 or 30 solar radii, there’s a lot less solar wind evolution, and it’s more structured that you see more footprints of what was on the sun,” he said. Bale.

In 2021, Parker’s instruments recorded magnetic field shifts in Alfvn waves that appeared to be associated with regions where the solar wind is generated. When the probe reached about 12 solar radii from the sun’s surface 5.2 million miles, the data was clear that the probe was traveling through jets of material, rather than simple turbulence. Bale, Drake and their colleagues traced these jets to supergranulation cells in the photosphere, where magnetic fields build up and funnel towards the sun.

But were the charged particles accelerated in these funnels by magnetic reconnection, which would hurl particles outward, or by waves of ionized particles in the hot plasma and magnetic field escaping from the sun, as if they were riding a wave?

The fact that Parker detected very high-energy particles in these jets of tens to hundreds of kiloelectron volts (keV), versus a few keV for most solar wind particles, told Bale it must be magnetic reconnection accelerating the particles. and generates Alfvn waves, which probably give the particles an extra boost.

“Our interpretation is that these reconnecting outflow jets excite Alfvn waves as they propagate,” Bale said. “This is a well-known observation also from the magnetic tail of the Earth, where similar processes take place. I don’t understand how wave damping can produce these hot particles up to hundreds of keV, as it comes naturally out of the reconnection process. And we also see it in our simulations. “

The Parker Solar Probe won’t be able to get close to the sun about 8.8 solar radii above the surface at about 4 million miles without frying its instruments. Bale expects to solidify the team’s conclusions with data from that altitude, even as the sun is now entering solar maximum, when activity becomes much more chaotic and could obscure processes scientists are trying to see.

“There was some consternation early in 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 figured it out. It would have just been too messy. I think we’re lucky to have launched it in the solar minimum.

The work was funded by NASA (Contract NNN06AA01C).

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