Solar flares and coronal mass ejections on the sun are caused by “magnetic reconnection” —when magnetic field lines of opposite directions merge, rejoin, and snap apart, creating explosions that release massive amounts of energy. Credit: NASA Conceptual Image Laboratory
Researchers identify the physics that enables rapid magnetic explosions in space.
When magnetic field lines of opposite directions merge, they create explosions that can release tremendous amounts of energy. The merging of opposing field lines on the sun creates solar flares and coronal mass ejections, which are massive blasts of energy that can travel to Earth in less than a day.
While the general mechanics of magnetic reconnection are well understood, researchers have struggled for over a half-century to explain the precise physics behind the rapid energy release that occurs.
A new Dartmouth research study published yesterday (April 28, 2022) in the journal Communications Physics provides the first theoretical description of how a phenomenon known as the “Hall effect” determines the efficiency of magnetic reconnection.
Magnetic reconnection occurs when magnetic field lines of opposite directions merge, rejoin, and snap apart, releasing massive amounts of energy to heat plasmas and drive high-speed outflows. Credit: Yi-Hsin Liu/Dartmouth College
“The rate at which magnetic field lines reconnect is of extreme importance for processes in space that can impact Earth,” said Yi-Hsin Liu, an assistant professor of physics and astronomy at Dartmouth. “After decades of effort, we now have a full theory to address this long-standing problem.”
Magnetic reconnection exists throughout nature in plasmas, the fourth state of matter that fills most of the visible universe. Reconnection takes place when magnetic field lines of opposite directions are drawn to each other, break apart, rejoin, and then violently snap away.
In the case of magnetic reconnection, the snapping of magnetic lines forces out magnetized 
Around the region where reconnection occurs, the departure of the ion motion (blue streamlines in (a)) from the electron motion (red streamlines in (a)) gives rise to the “Hall effect,” which results in the electromagnetic energy transport pattern illustrated by yellow streamlines in (b). This transport pattern limits the energy conversion at the center, enabling fast reconnection. Credit: Yi-Hsin Liu/Dartmouth College
The Dartmouth research focused on the reconnection rate problem, the key component of magnetic reconnection that describes the speed of the action in which magnetic lines converge and pull apart.
Previous research found that the Hall Effect — the interaction between electric currents and the magnetic fields that surround them — creates the conditions for fast magnetic reconnection. But until now researchers were unable to explain the details of how exactly the Hall effect enhances the reconnection rate.
The Dartmouth theoretical study demonstrates that the Hall effect suppresses the conversion of energy from the magnetic field to plasma particles. This limits the amount of pressure at the point where they merge, forcing the magnetic field lines to curve and pinch, resulting in open outflow geometry needed to speed up the reconnection process.
Dartmouth’s Xiaocan Li, postdoctoral researcher (left); Yi-Hsin Liu, Assistant Professor of Physics and Astronomy (center); Shan-Chang Lin, PhD candidate (right). Credit: Robert Gill/Dartmouth College
“This theory addresses the important puzzle of why and how the Hall effect makes reconnection so fast,” said Liu, who serves as deputy lead of the theory and modeling team for 
Yi-Hsin Liu, Assistant Professor of Physics and Astronomy, Dartmouth College. Credit: Robert Gill/Dartmouth College