Where does the solar wind come from? That’s a question solar physicists have wanted an answer to for decades. Now, the Parker Solar Probe is showing them exactly where this stream of particles exits our star on a journey out through interplanetary space.
Parker follows an orbit that takes it very close to the Sun. For any other spacecraft, such a pass would be the kiss of death. But, this probe was built for close approaches so that it can gather in-situ data about the outer atmosphere (called the corona) and strange “breaks” in the corona called coronal holes. That’s how it was able to peer directly at features on the Sun that create so-called “fast” solar winds.
The Secret of Fast Solar Winds
A team of scientists led by Stuart D. Bale (University of California, Berkeley, and James Drake of the University of Maryland-College Park, said Parker detected streams of high-energy particles that match so-called “supergranulation” flows inside coronal holes.
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In particular, the probe recorded an activity called “magnetic reconnection” that creates that fast wind component. Think of coronal holes like solar shower heads. Instead of water, jets of energized particles emerge out of the Sun and travel along magnetic field lines. When magnetic fields of two different polarities encounter each other in these showerhead “funnels”, the magnetic fields can break. Then they rapidly reconnect, and that energetic activity blasts charged particles out into space as part of the fast solar wind.
“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 the magnetic field going down into a drain. And the spatial separation of those little drains, those funnels, is what we’re seeing now with the solar probe.”
Interestingly, Bale said that while the reconnection in the funnels is providing the energy for the solar wind, it only seems to happen in specific areas of the coronal hole. “It comes from these little bundles of magnetic energy that are associated with the convection flows,” he said. “Our results, we think, are strong evidence that it’s reconnection that’s doing that.”
About the Solar Wind
Astronomers have known about the solar wind ever since astronomers Richard C. Harrington and Richard Hodgson first observed solar flares in 1859. Carrington then made the connection between that outburst and a geomagnetic storm that impacted Earth a day later. Other scientists such as Arthur Eddington, Kristian Birkeland, and Ludwig Biermann, continued to study the phenomenon. Still others noticed that this solar activity seemed to affect comet plasma tails. That phenomenon is quite well-understood thanks to decades of comet observations and correlations with data from such probes as the Ulysses spacecraft, the Halley Armada, and others.
The solar wind streams out from the Sun in all directions. It varies in its density (that is, the amount of particles it carries), temperature, and speed. These variations show up across all solar latitudes and longitudes. They also change over time.
Generally, this wind exists in a fast (or high-speed) component and a slow one. Both these regimes affect not just comets, but planets in the solar system. For example, it causes “space weather” on Earth, aurorae on Jupiter and Saturn, and has eroded the Martian atmosphere.
The fast solar wind generally speeds along at around 750 kilometers per second, while the slower component moves at around 300-500 kilometers per second. Both of these components seem to have slightly different origins. The slow-speed wind seems to come from the Sun’s “streamer” belt, which is roughly near the equator. The fast solar wind comes from those coronal holes that Parker has probed in great detail.
Coronal Holes Redux
These aren’t really “holes” in the sense of a physical “hole in the Sun.” They’re actually areas where magnetic field lines emerge from the photosphere of the Sun without looping back inward. Instead, they remain as open field lines that expand outward and fill most of the space around the Sun. Coronal holes usually camp out at the poles during the Sun’s quiet periods. That means the fast solar wind they generate usually doesn’t encounter Earth. However, every 11 years, the Sun’s activity levels ramp up as its magnetic field flips. During those times of heightened activity, coronal holes can show up all over the surface. During this period of “solar maximum”, the bursts of high-speed solar wind can end up aimed directly at Earth.
For a long time, solar physicists didn’t know exactly how the process of solar wind generation in coronal holes works. That’s because the solar wind has to pass through the Sun’s corona. By the time it reaches Earth and other solar observatories, that stream is just a blur of charged particles.
Tracking the Birthplaces of the Fast Solar Wind
On its recent close flyby, Parker came within 25 solar radii (21 million km) of the Sun. During that pass, the probe specifically zeroed in on coronal holes. That’s when it was able to see the fine “funnel structures” that generate the high-speed flow. Nour Raouafi, the Parker Solar Probe project scientist at the Applied Physics Laboratory says those funnel structures probably are related to bright jetlets that can be seen from Earth within coronal holes.
“Solving the mystery of the solar wind has been a six-decade dream of many generations of scientists,” said Raouafi. “Now, we are grasping at the physical phenomenon that drives the solar wind at its source — the corona.”
Peering into the birthplaces of the fast solar wind isn’t just an exercise in solar physics. The data from Parker (along with other solar observatories in space and on the ground) is invaluable when it comes to predicting solar storms. The solar wind plays a huge role in those geomagnetic disturbances that can wreak havoc with satellites, communications systems, and electrical power grids on Earth.
“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 said. “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.”
Parker’s Future in the Solar Wind
The Parker Solar Probe’s mission began in 2018. It will make 24 orbits around the Sun before mid-2025. The closest it will come to the Sun is around 8.8 solar radii above the surface. That’s a distance of about 6.5 million kilometers. If it gets any closer, the heat and tremendous radiation will fry the delicate instruments it uses to study the Sun.
As it is, Parker is doing most of its work now that the Sun is in its period of solar maximum. While that’s an exciting time, heightened solar activity could threaten the spacecraft or obscure some of these finely detailed processes that the team is trying to study.
For More Information
Parker Solar Probe Flies into the Fast Solar Wind and Finds its Source
Interchange Reconnection as the Source of the Fast Solar Wind Within Coronal Holes
Parker Solar Probe
The ULYSSES Comet Watch Network
International Halley Watch