One Reconnection Rate Theory, Three Generations of Dartmouth Research

Magnetic reconnection happens when magnetic field lines break away, rejoin, and then explosively snap apart. In space, this process flings away nearby plasma particles at high speeds and can release massive amounts of energy. Solar flares and coronal mass ejections result from this process, and are some of the most energetic events in our solar system. Professor Yi-Hsin Liu studies the properties of magnetic reconnection that allow for such fast and powerful particle output. His latest research marks the third iteration of what is turning out to be quite a Dartmouth "family business" in magnetic reconnection work. It seems fitting that Liu eventually found his way to Dartmouth after a career of overlapping research with our faculty and alumni.

Liu has been researching magnetic reconnection since he was a PhD student under the mentorship of Professor Jim Drake and Dr. Marc Swisdak at the University of Maryland, who, along with Dartmouth faculty Barrett Rogers and Richard Denton, and Liu's academic brothers Professor Mike Shay (U. Delaware) and Professor Paul Cassak (West Virginia U.), proposed and studied the importance of the Dispersive Wave Property for fast magnetic reconnection nearly 25 years ago. The Dispersive Wave Property predicts that the speed at which plasma particles, specifically the electrons, are flung outwards will increase at smaller scales of magnetic reconnection. Liu studied, graduated, and became a Dartmouth physics professor alongside Rogers and Denton, all while the role of the Dispersive Wave Property in reconnection rate determination remained unclear.

Before Liu joined Dartmouth, he worked at NASA on a project that shaped his current research focus. In 2015, NASA launched the Magnetospheric MultiScale mission to study magnetic reconnection in Earth's magnetosphere at higher spatial and temporal resolution than had ever been previously attained. Using the high cadency MMS data, Denton and emeritus Professor Bengt Sonnerup of Dartmouth's Thayer School of Engineering created schemes to reconstruct the three-dimensional structure of magnetic reconnection so that the process could be visualized in space. 

In 2018, after recording the physics associated with standard reconnection, the spacecraft of the MMS mission uncovered a novel type of magnetic reconnection outside of the magnetosphere that only involved electrons in the plasma. Electron-only reconnection occurs at a higher reconnection rate and in systems smaller than the larger, heavier ions can interact with. This first electron-only event was discovered by Dartmouth alumni Dr. Tai Phan, who was Sonnerup's PhD student and now a renown experimental space physicist at the Space Sciences Laboratory of UC. Berkeley. Later, Denton and Sonnerup reconstructed the initiation of electron-only reconnection.

Liu joined Dartmouth in between NASA MMS missions, in 2017, and still remains on the MMS research team. In 2022 professor Liu led a team of researchers to develop the very first theoretical description of standard reconnection rates. Liu was beginning to fit many of his colleagues' disparate discoveries, predictions, and properties into a single synthesis, but he still couldn't explain -and test- his model at different scales of reconnection. 

Galvanized by Phan's discovery of electron-only reconnection years earlier, Liu and his collaborators now re-evaluated key components of his standard model to develop a new analysis, expanding his theory work to explain this smaller, faster, and more transient variant of reconnection. They managed to incorporate the Dispersive Wave Property into his reconnection rate framework by modeling the Hall electric field, whose importance was originally emphasized by Sonnerup in 1979. Liu realized that the Hall effect not only determines reconnection geometry but also causes the Dispersive Wave Property. This connection proved critical for modeling the transition from standard reconnection to the electron-only limit.

Liu's new models worked. They accurately explained existing numerical simulations of electron-only reconnection. His model demonstrates that system size at sub-ion-scales indeed plays a critical role in determining the outflow speed and reconnection rate. With his new theoretical development, Liu successfully unites his predecessor's Dispersive Wave Property with his own approach to modeling magnetic reconnection rates. Perhaps most exciting for the college community, Liu's new paper completes the bridge between three generations of Dartmouth professors, and lays a solid theoretical foundation for significant observational findings by Dartmouth alumni.   

You can read Yi-Hsin Liu's full publication here. Liu and his collaborators would like to express their gratitude to the continuous support from federal agencies, including  NASA, NSF and DOE.