During fleeting fits, the sun occasionally hurls a colossal amount of energy into space. Called solar flares, these eruptions last for mere minutes, and they can trigger catastrophic blackouts and dazzling auroras on Earth. But our leading mathematical theories of how these flares work fail to predict the strength and speed of what we observe.
At the heart of these outbursts is a mechanism that converts magnetic energy into powerful blasts of light and particles. This transformation is catalyzed by a process called magnetic reconnection, in which colliding magnetic fields break and instantly realign, slingshotting material into the cosmos. In addition to powering solar flares, reconnection may power the speedy, high-energy particles ejected by exploding stars, the glow of jets from feasting black holes, and the constant wind blown by the sun.
Despite the phenomenon’s ubiquity, scientists have struggled to understand how it works so efficiently. A recent theory proposes that when it comes to solving the mysteries of magnetic reconnection, tiny physics plays a big role. In particular, it explains why some reconnection events are so stupefyingly fast—and why the strongest seem to occur at a characteristic speed. Understanding the microphysical details of reconnection could help researchers build better models of these energetic eruptions and make sense of cosmic tantrums.
“So far, this is the best theory I can see,” said Hantao Ji, a plasma physicist at Princeton University who was not involved in the study. “It’s a big achievement.”
Fumbling With Fluids
Nearly all known matter in the universe exists in the form of plasma, a fiery soup of gas where infernal temperatures have stripped down atoms into charged particles. As they zip around, those particles generate magnetic fields, which then guide the particles’ movements. This chaotic interaction knits a scrambled mess of magnetic field lines that, like rubber bands, store more and more energy as they’re stretched and twisted.
In the 1950s, scientists proposed an explanation for how plasmas eject their pent-up energy, a process that came to be called magnetic reconnection. When magnetic field lines pointing in opposite directions collide, they can snap and cross-connect, launching particles like a double-sided slingshot.
But this idea was closer to an abstract painting than a complete mathematical model. Scientists wanted to understand the details of how the process works—the events that influence the snapping, the reason why so much energy is unleashed. But the messy interplay of hot gas, charged particles and magnetic fields is tricky to tame mathematically.
The first quantitative theory, described in 1957 by the astrophysicists Peter Sweet and Eugene Parker, treats plasmas as magnetized fluids. It suggests that collisions of oppositely charged particles draw in magnetic field lines and set off a runaway chain of reconnection events. Their theory also predicts that this process occurs at a particular rate. The reconnection rates observed in relatively weak, laboratory-forged plasmas match their prediction, as do the rates for smaller jets in the lower layers of the sun’s atmosphere.
But solar flares release energy much more quickly than Sweet and Parker’s theory can account for. By their calculations, those flares should unfurl over months rather than minutes.
More recently, observations from NASA’s magnetospheric satellites identified this speedier reconnection happening even closer to home, in Earth’s own magnetic field. Those observations, along with evidence from decades of computer simulations, confirm this “fast” reconnection rate: In more energetic plasmas, reconnection occurs at roughly 10 percent of the speed at which magnetic fields propagate—orders of magnitude faster than Sweet and Parker’s theory predicts.