NASA’s Parker Solar Probe plunges into fast solar wind and discovers its mysterious source

By | June 8, 2023

Parker solar probe touches the sun

NASA’s Parker Solar Probe (PSP) has detected the origin and structure of the solar wind near the sun’s surface by observing high-energy particles aligned with fluxes in the coronal holes. This discovery, which indicates magnetic reconnection within these regions, improves our understanding and prediction of solar storms hitting Earth. Credit: NASA GSFC/CIL/Brian Monroe

NASA’s Parker Solar Probe got close enough to the sun’s surface to see the hidden granular features.

NASA’s Parker Solar Probe (PSP) flew close enough to the sun to detect the fine structure of the solar wind near where it is generated 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 published on June 7 in the magazine Naturea team of scientists led by Stuart D. Bale, professor of physics at the University of California, Berkeley, and James Drake of the University of Maryland-College Park, report that PSP has detected streams of high-energy particles that match the supergranulation it flows inside the coronal holes, which suggests 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.

The Parker Solar Probe spacecraft approaches the 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. Credit: NASA/Johns Hopkins APL/Steve Gribben

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.

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 kind of like a little ball of magnetic field 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 PSP 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. PSP 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[{” attribute=””>plasma or Alfvn waves.

The big conclusion is that its magnetic reconnection within these funnel structures thats providing the energy source of the fast solar wind, Bale said. It doesnt just come from everywhere in a coronal hole, its 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 its reconnection thats 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 designed, built, manages, and operates the spacecraft.

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 Earths 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 Earths atmosphere. Predicting the most intense winds, called solar storms, and their near-Earth consequences is one mission of NASAs Living With a Star program, which funded PSP.

The probe was designed to determine what this turbulent wind looks like where its generated near the suns surface, or photosphere, and how the winds charged particles protons, electrons, and heavier ions, primarily helium nuclei are accelerated to escape the suns gravity.

To do this, PSP 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, theres a lot less evolution of the solar wind, and its more structured you see more of the imprints of what was on the sun, Bale said.

In 2021, PSPs instruments recorded magnetic field switchbacks in the Alfvn 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 theyre surfing a wave?

The fact that PSP 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 Alfvn waves, which likely give the particles an extra boost.

Our interpretation is that these jets of reconnection outflow excite Alfvn waves as they propagate out, Bale said. Thats an observation thats well known from Earths magnetotail, as well, where you have similar kinds of processes. I dont 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 PSP wont 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 teams 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 were 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 were lucky that we launched it in the solar minimum.

Reference: Interchange reconnection as the source of the fast solar wind within coronal holes by S. D. Bale, J. F. Drake, M. D. McManus, M. I. Desai, S. T. Badman, D. E. Larson, M. Swisdak, T. S. Horbury, N. E. Raouafi, T. Phan, M. Velli, D. J. McComas, C. M. S. Cohen, D. Mitchell, O. Panasenco and J. C. Kasper, 7 June 2023, Nature.
DOI: 10.1038/s41586-023-05955-3

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