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Sunday, June 23, 2024

Tiny, Explosive ‘Jetlets’ Might Be Fueling the Solar Wind

Streaming out of the sun at a million miles an hour, the solar wind—a blistering plasma of electrons, protons, and ions flowing through space—is a decades-old enigma. Scientists know it once stripped Mars of its atmosphere, and some think it put ice on the moon. Today, it causes the glimmering Northern Lights displays and messes with satellite communication systems. But researchers haven’t been able to nail down how the solar wind gets made, heats up to millions of degrees, or accelerates to fill the entire solar system. 

Now, a team of researchers think they’ve figured it out: The solar wind, they say, is driven by jetlets—tiny, intermittent explosions at the base of the sun’s upper atmosphere, or corona. The theory, which was just published in The Astrophysical Journal, emerged from data taken by NASA’s Parker Solar Probe, a car-sized satellite that has repeatedly flown by the sun since 2018. It measures properties of the solar wind and traces the flow of heat and energy in the outermost part of the sun’s atmosphere that begins about 1,300 miles above its surface. The team’s idea is strengthened by data from other satellites and ground-based telescopes showing that jetlets could be ubiquitous and powerful enough to account for the mass and energy of the solar wind. Uncovering its origins will help scientists better understand how stars work, and predict how the gusty flow of plasma affects life on Earth.

Higher resolution data is needed to prove this hypothesis, but the evidence so far is tantalizing. “We sensed from early on that we were onto something big,” says Nour Raouafi, an astrophysicist at Johns Hopkins University’s Applied Physics Laboratory who led the study. “We were thinking that we might be solving the 60-year-old puzzle of the solar wind. And I believe we are.” 

The existence of solar wind, first proposed by the late Eugene Parker—namesake of the Parker Solar Probe—was confirmed by NASA in the early 1960s. Since then, scientists have been perplexed by how that plasma can move as far and as fast as it does. The sun’s corona is hot—millions of degrees on any temperature scale—but not hot enough to push the solar wind to those speeds. 

Jetlets, on the other hand, weren’t discovered until 2014, in a study led by Raouafi showing that these mini explosions drive coronal plumes, bright funnels of magnetized plasma near the solar poles. Looking closely at the base of the plumes, he found that jetlets arise when the sun’s churning surface forces two regions of repelling magnetic polarity together until they snap. But after that paper, Raouafi moved on to other projects. “And we basically left it there,” he says. 

Then in 2019, while Raouafi was working as a project scientist on the Parker Solar Probe, the craft saw something weird. As it skimmed the top of the corona, it observed that, quite often, the direction of the magnetic field it was flying through would flip. Then it would flip back. Raouafi assembled a team to hunt down a source of these intermittent “switchbacks” lower in the atmosphere. His mind immediately went to jetlets. If they could be found elsewhere in the corona, and not just in its plumes, he reasoned, they might be numerous enough to generate enough material and power to be the solar wind itself. 

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But the probe can only take samples at the very top of the corona—if it gets too close, it’ll melt. More remote satellites are better at seeing deeper into the sun, closer to the bottom of the corona. So the research team analyzed high-resolution images of the lower corona from NASA’s Solar Dynamics Observatory satellite, and the Solar Ultraviolet Imager instrument aboard a super-high-altitude weather satellite that orbits Earth. “And sure enough, we found what we think is the smoking gun for the origin of the solar wind,” says study co-author Craig DeForest, a solar physicist at the Southwest Research Institute in Boulder, Colorado. 

The data revealed that jetlets were everywhere. They were also present as far back in time as the researchers searched—to data from 2010. Unlike solar flares and coronal mass ejections, which wax and wane in a natural 11-year cycle, the jetlets’ presence didn’t vary. Like the solar wind, they seemed to be a stable feature, persistently hurling plasma into space. 

To prove the jetlets go off with enough power and are prevalent enough to account for the solar wind, the researchers did a rough calculation. Up to 1035 protons can be ejected per jetlet, and the sun loses around 6 x 1035 protons per second to the solar wind. That means it would take six jetlets per second, or about 500,000 per day, to power the wind. 

They compared this number to maps of the sun’s surface that indicate where jetlets might be. These maps were imaged by the Big Bear Solar Observatory in California, and show variations in the magnetic polarity over fine scales, with negative poles in darker patches and positive poles in lighter ones, giving the images a salt-and-pepper appearance.  The team concluded that there were enough sites with neighboring opposite poles to potentially produce the number of jetlets needed to fuel the solar wind. “We haven’t sealed the case beyond a reasonable doubt yet,” DeForest says. “But this is a major step forward.”

Learning about the solar wind is important, DeForest says, because it’s an integral part of our own environment. “Solar physics is the only field of astrophysics that has actual applications on Earth,” he says. The wind perturbs our planet’s magnetic field, which protects us from potentially harmful space radiation. It also causes space weather that can affect the orbits and operations of satellites, including GPS networks. Understanding how the solar wind works can also help scientists figure out how stars slow down as they age, and how that influences the atmospheres of their orbiting planets—which could make them more or less habitable

The idea that intermittent explosions could generate a steady stream of plasma challenges the notion that the solar wind’s driving mechanism must be a single, continuous source. But it’s not inconceivable: Parker did once hypothesize that something like this could fuel the wind—though he called them “nanoflares.” And DeForest points out that many small bursts can collectively act like one smooth flow. “You drive a car down the road, and what you feel is a smooth thrust,” he says. “But really, what’s going on is zillions of little explosions inside the gas engine.” 

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Charles Kankelborg, a solar physicist at Montana State University, finds the theory plausible—but the idea itself surprises him. Tiny explosions, like those created by other kinds of small solar events, have never been shown to meaningfully contribute to the energy of the sun’s atmosphere. “To see this paper suggesting that these could very well be supplying the full solar wind as we know it—my jaw kind of dropped,” says Kankelborg, who was not involved in the work. It’ll take more data for him to believe that jetlets alone can supply the wind’s energy, but he feels it’s an exciting idea worth considering. 

Raouafi and his colleagues are on it. Higher resolution data already shows that they’ve underestimated the speed of the jetlets, meaning they have more energy than originally accounted for. “Which is a very good sign. That’s what we need,” he says. Two follow-up studies are in the works, and Raouafi hopes to publish them this summer. Those will include more observations from the Solar Dynamics Observatory, new data taken by the European Space Agency’s Solar Orbiter, and magnetic field information from the Daniel K. Inouye Solar Telescope in Hawaii, which has three times the magnetic field resolution of Big Bear Solar Observatory. 

In the future, linking this data with direct measurements by the Parker Solar Probe, as well as more global observations of the solar wind from NASA’s upcoming Polarimeter to Unify the Corona and Heliosphere (PUNCH) mission, will help scientists glean even more precise information about its nature. “Bringing these two tools together”—remote imaging and at-the-source measurements—“means we’ll really get a handle on the system as a unified whole,” says DeForest, who is the principal investigator for the PUNCH mission. 

The team is confident that they’re on the brink of a big discovery. “I wish Gene Parker was still with us,” Raouafi says. “I believe he would have been pleased that we are, in a way, confirming his theory.”

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