On the one hand, the sun provides life-giving heat and light. On the other, it spews an incessant stream of potentially harmful charged particles. These particles form the solar wind, and it is no less formidable than our star’s other products. Without Earth’s magnetic field to shield our planet’s surface, we would constantly face a bombardment of ionizing radiation.
But astronomers have never been completely certain where those particles come from or how they travel into interplanetary space. Now, they’ve found a promising clue. Using ESA’s Solar Orbiter spacecraft, researchers have found miniature jets that seem to channel particles up through holes in the sun’s corona and away from the star. These jets might combine to blow the solar wind, a group of astronomers suggests in a paper published in the journal Science on Thursday.
The corona, a star’s outermost layer, is a sheath of undulating plasma. It is almost always hidden in visible light, although it’s thousands of times hotter than the layers below. We might only see this outer layer during a solar eclipse, when the moon blots out the rest of the sun.
But the corona is not one even layer. Imaging the sun in ultraviolet reveals shifting dark swatches: regions where the corona’s plasma is cooler and less dense. Astronomers call these areas coronal holes.
[Related: Why is space cold if the sun is hot?]
Coronal holes also seem to resculpt the sun’s powerful, endlessly changing magnetic field. In these parts, lines that guide the sun’s magnetic field seem to blow outward. “Usually, magnetic fields loop back to the solar surface, but in these open field regions the lines of force stretch into interplanetary space,” says Lakshmi Pradeep Chitta, an astronomer at the Max Planck Institute for Solar System Research in Göttingen, Germany, and one of the paper’s authors.
It’s also within coronal holes that the sun’s magnetic field lines can knot about themselves. When that happens, the magnetic field realigns and reconnects, creating fierce electrical surges. Those energetic outbursts siphon matter from deeper layers of the sun and toss them away in jets that can stretch more than a thousand miles across. Astronomers had long suspected that these jets fuel the solar wind, but didn’t know if these jets could provide enough particles to fill the solar wind we observe.
Sun-watching spacecraft like Yohkoh and SOHO have been able to see jets since the 1990s. But astronomers say that none have the sightseeing abilities of Solar Orbiter, which launched in 2020. At its closest approach, Solar Orbiter dips closer to the sun than Mercury.
“Solar Orbiter has the advantage of being located close to the sun, so it can detect smaller and fainter jets,” says Yi-Ming Wang, an astronomer at the US Naval Research Laboratory, who was not an author of the paper.
In March 2022, Chitta and his colleagues focused one of Solar Orbiter’s ultraviolet cameras upon a coronal hole situated near the sun’s south pole. When they did, they glimpsed a type of miniature jet never before seen by humans. Each of these tiny jets carried around one-trillionth the energy of a full-size version. The authors dubbed these “picoflare jets,” dipping into SI system prefixes.
These adorable-sounding surges don’t stick around. Each fleeting picoflare jet lasts about a minute. But this is still the sun—a place of immense power. A single solar picojet might create enough energy to power a small city for a year.
The authors scoured only one small part of the sun, but they saw picoflare jets in every corner they looked. It’s likely they cover much of the sun’s surface. Myriads of miniature jets, then, might combine into a large-scale process that transfers charged particles away from the star and out toward the planets.
“We suggest that these tiny picoflare jets could actually be a major source of mass and energy to sustain the solar wind,” Chitta says.
In years past, many astronomers thought of the solar wind as a steady flow, streaming away from the sun at a constant rate. But, if surging picoflare jets drive the solar wind, then the phenomenon might actually be ragged, uneven, and constantly in flux. Picoflare jets may not be the only source of the solar wind, but if Chitta and colleagues are correct, they’re at least a significant contributor.
Fortunately, scientists in a few years’ time will have plenty of additional tools to peer into the sun. Alongside the Solar Orbiter—and future sun-seeing spacecraft, such as the Japanese-led SOLAR-C—they’ll have more powerful solar magnetograms, instruments that allow them to directly measure the sun’s magnetic field from places like Southern California and Maui, able to track the magnetic fluctuations powering the sun’s jets from right here on Earth.