The Sun flings charged particles and accompanying magnetic fields into the solar system, but how? NASA’s Parker Solar Probe dives in to find out.

Coronal hole
The dark area across the top of the sun in this image is a coronal hole, a region on the Sun where the magnetic field is open to interplanetary space. (The hole appears dark in this ultraviolet image because the plasma is sparse and thus cooler.) Plasma in these regions accelerates into a fast solar wind, but how this happens has remained an open question.

The solar wind — the flow of charged particles, or plasma, from the Sun — radiates out over all the bodies in the solar system, forming a vast, Sun-centered bubble in the dark void of space. It’s essential to life on Earth, but it also stirs up geomagnetic storms that produce power cuts, aurorae, and other earthly disturbances. The mechanism by which the solar wind arises is still not fully understood, so a team of researchers led by Stuart Bale (University of California, Berkeley) set out to tackle the quandary.

The team, whose findings are published in Nature, drew on data collected by NASA’s Parker Solar Probe. Since it launched in 2018, Parker has been circling the Sun in a series of gradually shrinking orbits that are bringing it closer to the solar surface than any previous human-made object.

Indeed, Parker has even dipped into the Sun’s atmosphere, or corona, flying as close as 5.3 million miles to the Sun’s visible surface, or photosphere, which is covered in boiling “bubbles” called granules as well as larger-scale patterns called supergranules.

yellow and orange image in the shape of rocks or bubbles
On the Sun's visible surface, or photosphere, heating plasma rises in the bright, convective “bubbles” (granules) then cools and falls into the dark lanes between the granules.
Image Credit: NSF/AURA/NSO Image Processing: Friedrich Wöger(NSO), Catherine Fischer (NSO)
Spotted Sun
Supergranules are similar to granules but on much larger scales (about 35,000 km across). They're best seen in measurements of the "Doppler shift" where light from material moving toward us is shifted to the blue while light from material moving away from us is shifted to the red. These bulk, boiling-type motions occur over the entire Sun and hold the key to understanding the fast solar wind.
NASA Marshall Space Flight Center / D. Hathaway

Parker was deployed to answer what accelerates the plasma and accompanying magnetic fields that launch off the photosphere and into the corona. This solar wind travels at different speeds, fast and slow. While both speeds of wind are linked to magnetism in the Sun’s atmosphere, each has its own source. There’s a broad consensus that the fast solar wind originates in coronal holes, regions where the Sun’s magnetic field lines are effectively “open,” extending far into interplanetary space before looping back in.

Magnetic field lines on the Sun
Over a coronal hole (here, the dark region near the center of the image) are open to interplanetary space, allowing accelerated to flow fast and furious, at speeds up to 800 km/s (2 million mph).
SDO/ AIA / Veronig A. & Temmer M. (University of Graz, Austria)

But what happens in these open fields to drive the solar wind? There are two competing theories. It could be either magnetic waves known as Alfvén waves, which pass through plasma, or magnetic reconnection. Both processes are known to take place on the Sun.

The researchers analyzed a range of measurements taken by Parker in November 2021, during its 10th perihelion, when it passed close to the Sun. The probe measured the solar wind’s plasma density and energy, as well as its magnetic field strength. The team combined these data with surface magnetic field measurements from the Solar Dynamics Observatory, a separate surveyor that launched in 2010.

The team saw that Parker was buffeted during its encounter by microstreams — short, jet-like bursts of high-speed solar wind. With the help of computer simulations, the team established that these microstreams are likely driven by reconnection.

According to the reconnection hypothesis, plasma is accelerated off the surface of the Sun by magnetic reconnection within the coronal holes, specifically in the regions between the supergranules.

“Where these supergranulation cells meet and go downward, they drag the magnetic field in their path into this downward kind of funnel,” Bale says. “The magnetic field becomes very intensified there because it's just jammed.”

Magnetic reconnection
This diagram shows the reorganization of magnetic fields by a process called magnetic reconnection. Here, two opposing magnetic field lines meet (A), connecting at the point where they would cross (B), and then sending a burst of particles accelerating outward accompanied by an S-shape twist in the magnetic field (C). Parker sees the S-shape twist as a magnetic switchback.
Gregg Dinderman / S&T; source: Justin Kasper / Levi Hutmacher / University of Michigan Engineering

As the funnels pull in neighboring magnetic fields, the fields reconnect, releasing magnetic energy. Microstreams of sped-up plasma are spewed out into space, along with associated sudden changes in magnetic field direction known as switchbacks. These bursts of solar wind are what hit the spacecraft. 

The researchers showed this by linking the bursts and switchbacks to their footprints in two coronal holes. They also estimated the rate at which magnetic energy was released in the reconnection events and found that it was equivalent to that required to power the fast solar wind flows. In addition, they measured unusually high-energy particles in the jets, which point to the involvement of magnetic reconnection.

Coronal window
As the Parker Solar Probe flies over a coronal hole (shown here in white on the Sun's surface), it encounters switchback magnetic fields as well as small, speedy bursts of solar wind.
University of California, Berkeley; spacecraft image: NASA / Johns Hopkins APL

Alfvén waves, under this interpretation, are an effect of reconnection. As jets of reconnection-accelerated plasma gush out of the coronal holes, they may produce Alfvén waves. These waves may in turn boost the solar wind, but they are not the primary driver of it.

Michael Hahn (Columbia University), who wasn’t involved in the study, finds the evidence “convincing”. But understanding precisely how reconnecting magnetic fields heat and accelerate plasma remains an open question, he points out.

“The authors favor an explanation that the reconnection directly heats the plasma close to the Sun,” Hahn says. But he explains that another option mentioned in the study, indirect transfer of energy, is also possible. First, the energy goes into magnetic waves in the plasma or into turbulence, and only later, farther away from the Sun, does that energy transfer to the plasma.

The myriad energy-generating processes taking place in the corona makes it complex to identify which are the most significant, Hahn adds. This complexity presents a challenge for solar physicists to overcome.

By broadening the evidence base and bolstering the case for magnetic reconnection fueling the fast solar wind, the Nature study opens up future avenues of inquiry, such as an investigation into how much of the energy generated by reconnection is transferred into turbulence. It also paves the way for improved prediction of geomagnetic storms. Far from being the end of the matter, it represents but the latest development in an ongoing discussion.


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