Magnetic reconnection plays a key role in energetic events across the solar system, from aurorae to solar flares. Reconnection describes the process in which magnetic field lines in a plasma break and reconnect, converting magnetic energy to heating and acceleration of particles.
Magnetic reconnection contributes to events beyond the solar system. Yet many of the details of reconnection are poorly understood, in part because no one had actually seen it in nature until a few years ago. Most of what physicists know about it comes from modeling and simulations.
One problem is understanding how magnetic reconnection accelerates electrons and ions. “Reconnection is very complex—a lot of energy is released all at once, and it can go into all sorts of different channels, like shocks or generating turbulence or flows or even heating the plasma,” said Joel Dahlin, a NASA postdoctoral researcher at the Goddard Space Flight Center in Maryland. “Trying to untangle all that complexity and say, ‘Oh, this one thing is accelerating particles,’ is difficult.”
Reconnection has drawn increasing interest in recent years as astrophysicists try to account for the electron acceleration detected in stellar flares and jets, magnetars, active galactic nuclei, and other objects and phenomena.
Dahlin has tackled the problem in a study published in Physics of Plasmas. The study reports that the scale of reconnection events plays a key role in acceleration, as do guide fields generated during such events. And Dahlin notes that a constellation of spacecraft currently studying Earth’s magnetosphere provides the best chance to test the results of simulations.
Finding a Middle Ground
Reconnection can release magnetic energy and accelerate electrons through several mechanisms, which Dahlin distilled into three main categories: parallel electric fields, betatron acceleration, and Fermi reflection. The most efficient of the three appears to be Fermi reflection, which operates in plasmoids—a key middle ground in the scale of magnetic reconnection events.
Magnetic energy builds up on macroscales (tens of thousands of kilometers for solar flares, e.g.) yet discharges over microscales (kilometers or even meters).
“When you’re trying to understand how particles get accelerated, you have a problem with trying to attach very large scale processes to very small scale processes,” Dahlin said. The macroscale “is just a big old X—particles come in, and they are ejected outward.” The particles have only one encounter with an acceleration region before they leave the system in large outflows.
The microscale, on the other hand, presents “a diffusion region where you have magnetic field lines snipped and respliced onto each other.” That small-scale reconfiguration allows the system to release energy. The reconfiguration is so small, however, that “it’s hard to think of particles being directly accelerated by the snipping and resplicing process,” Dahlin said.
A more appealing idea is that acceleration takes place on a mesoscale, in plasmoids—plasma bubbles that range from the size of the diffusion region to the size of the entire system. These bubbles trap charged particles. They also move and contract, with particles reflecting inside the bubbles (Fermi reflection), gaining energy as they do.
Although the classical picture of reconnection depicts two magnetic fields flowing past each other in opposite directions, Dahlin notes that in most environments, the fields aren’t so precisely aligned. They reconnect at the interface where they do align, producing a guide field.
“This guide field has a profound impact on the particle acceleration,” Dahlin said. “A guide field of about the same strength as the reconnecting field—a 90° rotation—may yield the most efficient acceleration.”
The fields in Earth’s magnetotail—the portion of the magnetosphere downwind of the planet—are highly symmetrical, so the guide field is weak. In solar flares, however, the fields are less symmetrically aligned, so the guide field is expected to be strong. Simulations (and some tentative evidence) show that the guide field weakens as reconnection proceeds, however, suggesting that it plays a significant role in electron acceleration.
“The results of the role of the guide field…are particularly important and potentially field changing,” said Bin Chen, an associate professor of physics at New Jersey Institute of Technology who was not involved in Dahlin’s study. “They have profound implications in addressing a major puzzle in solar physics and astrophysics: How are electrons accelerated to high energies so efficiently in solar flares and other explosive energy release events?”
On Station in the Magnetosphere
Magnetic reconnection was first observed directly by the Magnetospheric Multiscale (MMS) mission in 2016. The ongoing mission consists of four spacecraft flying in a tight formation. The craft study two main regions of the magnetosphere: one where the solar wind slams into the magnetosphere and one in the magnetotail. They observe magnetic fields and events in far greater detail than any other space- or ground-based instruments.
“Largely, what they’re finding is consistent with some of the simulation results I and others have had, but a big challenge is that everything is different,” Dahlin said. “An important thing to look for in the future is trying to populate our understanding against many different situations…. Each reconnection event that MMS observes will be different, and that can have a profound impact on how the dynamics play out.”
Although magnetic reconnection models are improving, he added, “there’s a lot of work to be done…. The big next challenge is to incorporate what we’ve learned from the basic reconnection, basic physics studies, to the large-scale complexity of solar flares. With flares especially, that’s where the coupling of the macro and micro scales is a big challenge.”
—Damond Benningfield ([email protected]), Science Writer