A Nasa probe designed to make observations of the outer corona of the Sun has flown close enough to detect the fine structure of the solar winds, which could provide key data on how to weather future flares which disrupt electronic equipment.
The Parker Solar Probe (PSP) was launched in 2018 and will approach within four million miles (6.5 million kilometres) from the centre of the Sun during its mission. It has already become the closest-ever artificial object to the Sun.
A report from the University of California Berkeley has said the probe detected streams of high-energy particles that match the “supergranulation flows within coronal holes”, which suggests that these are the regions where the so-called “fast” solar winds originate.
Solar winds are streams of charged particles released from the upper atmosphere of the Sun that can disrupt communications, navigation systems, satellites and even cause power outages.
Fast solar winds typically originate from the coronal holes usually at the Sun’s poles during its quiet periods, so they generally do not hit Earth. When the Sun becomes active every 11 years as its magnetic field flips, these holes appear all over the surface, generating bursts of solar wind aimed directly at Earth.
Understanding how and where the solar wind originates will help predict when solar storms will occur and prepare for any disruption.
“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,” said professor James Drake, study leader. “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.”
Based on the team’s analysis, the coronal holes are like showerheads, with roughly evenly spaced jets emerging from bright spots where magnetic field lines funnel into and out of the surface of the Sun. The scientists believe that when oppositely directed magnetic fields pass one another in these funnels, which can be 18,000 miles across, the fields often break and reconnect, slinging charged particles out 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 said. “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.”
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 Earth’s own magnetic field and dump electrical energy into the upper atmosphere.
This excites atoms, producing colourful auroras at the poles, but has effects that trickle down into Earth’s atmosphere. Predicting the most intense winds, called solar storms, and their near-Earth consequences is one mission of Nasa’s ‘Living With a Star’ program, which funded PSP.
The probe was designed to determine what this turbulent wind looks like where it’s generated near the Sun’s surface and how the wind’s charged particles are accelerated to escape the Sun’s gravity.
The PSP won’t be able to get any closer to the Sun than about four million miles without frying its instruments. Bale expects to solidify the team’s 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 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 understood this. It would have been just too messy. I think we’re lucky that we launched it in the solar minimum.”
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