Wind blowing from close to the sun’s surface has now been returned to its source by a daring solar probe that rivals Icarus in its audacity.
In November 2021, the Parker solar probe creamed in a more-than-hair-scorch 8.5 million kilometers (5.3 million miles) from the sun, a feat that allowed it to detect the fine structure of the solar wind as it blasted tons of charged particles into the solar system through a hole in the sun’s corona, or atmosphere.
The probe’s measurements give us the best picture yet of how the fast solar wind is generated, suggesting that a specific type of magnetic reconnection is driving 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 a lot of information from the sun to Earth, so understanding the mechanism behind winds from the sun is important on Earth for practical reasons,” Drake explains.
“That will affect our ability to understand how the sun releases energy and creates geomagnetic storms, which threaten our communication networks.”
Coronal holes sound a bit concerning, but it’s a normal solar phenomenon. The sun is one magnetic mess, as you can see in the visualization below, and many of the changes in the magnetic field manifest as phenomena in the corona. A coronal hole forms when, instead of forming closed loops, the magnetic field lines open up and expand outward.
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The result is a slightly cooler, less dense plasma in the corona. You won’t be able to see it just by looking at the sun (not that you should look at the sun without eye protection, don’t), but it will be much darker in the sun. extreme ultraviolet wavelengths.
Powerful solar winds arise from these regions. Unfettered by the usual magnetic fields, these winds blow into space at speeds of up to about 800 kilometers (500 miles) per second, twice as fast as the mean 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 disrupt satellite operations and radio communications.
Coronal holes can appear at any time during the sun’s 11-year activity cycle. When the sun is on the verge of minimum activity or solar minimum, they tend to hang around the poles. But at solar maximum, when the Sun’s magnetic poles switch places, and as activity declines thereafter, coronal holes become more numerous and appear nicely at all latitudes, and faster solar winds are directed in our direction.
As Parker approached the sun in November 2021, one of these coronal holes happened to be located in such a way that the probe was able to collect the closest observations of any of these regions obtained so far.
The resulting data showed, the team says, that the coronal hole looks a bit like a shower head. Roughly evenly spaced jets emerge from places where magnetic field lines “funnel” in and out of the sun’s surface.
“The photosphere is covered with convection cells, like in a boiling pot of water, and the larger-scale convection current is called supergranulation,” Bale explains.
“Where these supergranulation cells meet and go down, they drag the magnetic field in their path into this kind of downward funnel. The magnetic field gets very intensified there because it just gets stuck.
“It’s kind of like a ball of magnetic field going into a drain. And the spatial separation of those little drains, those funnels, is what we’re seeing now with data from solar probes.”
Things can get a little wild in magnetically complicated places on the sun. Magnetic field lines get confused, 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 from electromagnetic waves in coronal holes, called Alfvén waves, which are generated by the interaction between convection currents and magnetic fields.
Parker, the researchers found, clocked particles traveling at incredibly fast speeds, between 10 and 100 times the speed of the average solar wind. This, they say, is more consistent with reconnecting exchanges than Alfvén wave acceleration and consistent with other recent findings based on Parker data.
“The big takeaway is that it is the magnetic reconnection within these funnel structures that is the energy source of the fast solar wind,” Bale says.
“It’s not just coming from all over a coronal hole, it’s substructured in coronal holes to these supergranulation cells. It’s coming from these little beams of magnetic energy associated with the convection currents. Our results, we think, are strong evidence that it’s the reconnection is the one who does.”
The findings have been published in Nature.
