Wind that blows from close to the surface of the Sun has now been traced back to its source by a daredevil solar probe rivaling Icarus in its audacity.
In November 2021, the Parker Solar Probe skimmed within a more-than-hair-singeing 8.5 million kilometers (5.3 million miles) of the Sun, a feat enabling it to detect the fine structure of the solar wind as it gusted tons of charged particles out into the Solar System through a hole in the Sun's corona, or atmosphere.
The probe's readings give us the closest look yet at how the fast solar wind is generated, suggesting that a specific type of magnetic reconnection is what drives this powerful force of nature, according to a team of physicists led by Stuart Bale of the University of California, Berkeley, and James Drake of the University of Maryland, College Park.
"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," Drake explains.
"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."
Coronal holes sound a bit worrying, but they're a normal solar phenomenon. The Sun is a magnetic mess, as you can see in the visualization below, and a lot of the changes in its magnetic field manifest as phenomena in the corona. A coronal hole happens when, rather than forming closed loops, the magnetic field lines open and expand outward.
The result is a patch of cooler, less dense plasma in the corona. You won't be able to see it just by looking at the Sun (not that you ought to look at the Sun without eye protection, don't do that), but it will show up much darker in extreme ultraviolet wavelengths.
From these regions emerge powerful solar winds. Untrammeled by the usual magnetic fields, these winds gust into space at up to around 800 kilometers (500 miles) per second, twice as fast as the average solar wind.
It blows charged particles far into the Solar System; here on Earth, they can interact with our upper atmosphere to generate spectacular auroras and interfere with satellite operations and radio communications.
During the Sun's 11-year activity cycle, coronal holes can appear at anytime. When the Sun is at the point of minimal activity, or solar minimum, they tend to hang around the poles. But at solar maximum, when the Sun's magnetic poles swap places, and as activity declines afterward, coronal holes become more numerous and appear pretty at all latitudes, and more fast solar winds are pointed in our direction.
When Parker made its close approach to the Sun in November 2021, one of these coronal holes was fortuitously situated so that the probe could collect the closest observations of one of these regions obtained yet.
The resulting data showed, the team says, that the coronal hole is a bit like a shower head. Roughly evenly spaced jets emerge from places where magnetic field lines "funnel" into and out of the surface 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 explains.
"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."
In magnetically complicated spots on the Sun, things can get a bit wild. Magnetic field lines tangle, snap, and reconnect. This magnetic reconnection is a violent process that releases a lot of energy.
One of the possible ways the solar wind is generated is when open and closed magnetic fields reconnect in a process called interchange reconnection. Another possible explanation is particle acceleration by electromagnetic waves in coronal holes called Alfvén waves, generated by the interaction between convective flows and magnetic fields.
Parker, the researchers found, clocked particles traveling at incredibly high speeds, between 10 and 100 times the speed of the average solar wind. This, they say, is more consistent with interchange reconnection than Alfvén wave acceleration and consistent with other recent findings based on Parker data.
"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 says.
"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 findings have been published in Nature.