19.06.2023
A new theory describes how particle interactions fuel fast magnetic reconnection, the process behind solar flares and other astrophysical jets.
Two simulations of an accreting black hole. On the left, the plasma is modeled as a fluid. On the right, it’s treated like an ensemble of particles, which produces dramatic differences in plasma density (purple) and magnetic field lines (white).
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 satellitesidentified 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% of the speed at which magnetic fields propagate — orders of magnitude faster than Sweet and Parker’s theory predicts.
The 10% reconnection rate is observed so universally that many scientists consider it “God’s given number,” said Alisa Galishnikova, a researcher at Princeton. But invoking the divine does little to explain what’s making reconnection so fast.
God’s Number
In the 1990s, physicists turned away from treating plasmas as fluids, which had turned out to be too simplistic. Zoomed in, a magnetized soup is really made up of individual particles. And how those particles interact with one another makes a crucial difference.
“When you get to the microscales, the fluid description starts breaking down,” said Amitava Bhattacharjee, a plasma physicist at Princeton. “The [microphysical] picture has things in it that the fluid picture can never capture.”
For the past two decades, physicists have suspected that an electromagnetic phenomenon known as the Hall effect might hold the secret to speedy reconnection: Negatively charged electrons and positively charged ions have different masses, so they travel along magnetic field lines at different speeds. That speed differential generates a voltage between the separated charges.
In 2001, Bhattacharjee and his colleagues showed that only models that included the Hall effect yielded appropriately fast reconnection rates. But precisely how that voltage produced the magical 10% remained a mystery. “It did not show us the ‘how’ and ‘why,’” said Yi-Hsin Liu, a plasma physicist at Dartmouth College.
Now, in two recently published theoretical papers, Liu and colleagues have attempted to fill in the details.
The first paper, published in Communications Physics, describes how the voltage induces a magnetic field that draws electrons away from the center of the two colliding magnetic regions. That diversion produces a vacuum that sucks in new field lines and pinches them in the center, allowing the magnetic slingshot to form more quickly.
“That picture was missed … [but] it was staring at us in the face,” said Jim Drake, a plasma physicist at the University of Maryland. “This is the first convincing argument I’ve ever seen.”
In the second paper, published in Physical Review Letters, Liu and his undergraduate research assistant Matthew Goodbred describe how the same vacuum effect emerges in extreme plasmas containing different ingredients. Around black holes, for example, plasmas are thought to consist of electrons and equally massive positrons, so the Hall effect no longer applies. Yet, “magically, reconnection is still working in a similar way,” Liu said. The researchers propose that within these stronger magnetic fields, most of the energy is spent accelerating particles rather than heating them — again creating a pressure depletion that yields the divine 10% rate.
“It’s a major milestone theoretically,” said Lorenzo Sironi, a theoretical astrophysicist at Columbia University who works on computer simulations of high-energy plasma jets. “This gives us confidence … that what we’re seeing in our simulations is not crazy.”
Picking Particles
Scientists can’t model each individual particle in large-scale plasma simulations. Doing so would produce billions of terabytes of data and take hundreds of years to complete, even using the most advanced supercomputers. But researchers recently figured out how to treat such an unwieldy system as a smaller, more manageable set of particles.
To investigate the importance of considering individual particles, Galishnikova and colleagues compared two simulations of an accreting black hole — one treating the plasma as a homogeneous fluid, and the other tossing roughly a billion particles into the mix. Their results, published in March in Physical Review Letters, show that incorporating the microphysics leads to distinctly different pictures of a black hole’s flares, particle accelerations and variations in brightness.
Now, scientists hope theoretical advances such as Liu’s will lead to models of magnetic reconnection that more accurately reflect nature. But while his theory aims to settle the reconnection-rate problem, it does not explain why some field lines collide and trigger reconnection but not others. It also doesn’t describe how the outflowing energy is divvied up into jets, heat and cosmic rays — or how any of this works in three dimensions and on larger scales. Still, Liu’s work shows how, under the right circumstances, magnetic reconnection can be efficient enough to drive ephemeral but violent celestial outbursts.
“You have to answer the question ‘why’ — that’s a crucial part of moving forward with science,” Drake said. “Having the confidence that we understand the mechanism gives us a much better ability to try to figure out what’s going on.”
Quelle: Quantamagazine