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.
The 10 percent 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 percent remained a mystery. “It did not show us the ‘how’ and ‘why,’” said Yi-Hsin Liu, a plasma physicist at Dartmouth College.
Electrons (red) and ions (white) travel at different speeds along magnetic field lines in astrophysical plasmas, generating a voltage that makes magnetic reconnection more efficient.Video: NASA’s Scientific Visualization Studio
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.
