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Magnetic reconnection in the era of exascale computing and multiscale experiments

Nature Reviews Physics volume 4, pages 263–282 (2022)Cite this article

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Abstract

Astrophysical plasmas have the remarkable ability to preserve magnetic topology, which inevitably gives rise to the accumulation of magnetic energy within stressed regions including current sheets. This stored energy is often released explosively through the process of magnetic reconnection, which produces a reconfiguration of the magnetic field, along with high-speed flows, thermal heating and nonthermal particle acceleration. Either collisional or kinetic dissipation mechanisms are required to overcome the topological constraints, both of which have been predicted by theory and validated with in situ spacecraft observations or laboratory experiments. However, major challenges remain in understanding magnetic reconnection in large systems, such as the solar corona, where the collisionality is weak and the kinetic scales are vanishingly small in comparison with macroscopic scales. The plasmoid instability or formation of multiple plasmoids in long, reconnecting current sheets is one possible multiscale solution for bridging this vast range of scales, and new laboratory experiments are poised to study these regimes. In conjunction with these efforts, we anticipate that the coming era of exascale computing, together with the next generation of observational capabilities, will enable new progress on a range of challenging problems, including the energy build-up and onset of reconnection, partially ionized regimes, the influence of magnetic turbulence and particle acceleration.

Key points

  • Major challenges remain in understanding magnetic reconnection in large astrophysical systems where dissipation scales are extremely small compared with macroscopic scales.

  • The plasmoid instability of reconnecting current sheets is a natural mechanism to bridge this vast range of scales in both fully and partially ionized plasmas.

  • Upcoming multiscale laboratory experiments are poised to provide the first validation tests of the plasmoid instability, whereas exascale simulations will allow researchers to evaluate competing hypotheses regarding the influence of turbulence.

  • These simulations and experiments can also shed new light on the mechanisms of reconnection onset and how the reconnection layers couple with the macroscale systems that supply the magnetic flux.

  • Rapid progress is being made towards understanding the acceleration of highly energetic particles produced by magnetic reconnection, which may have broad relevance to energetic phenomena across the Universe.

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Fig. 1: Examples of large-scale electric current sheets in space, solar atmosphere, astrophysics and laboratory fusion plasmas.
Fig. 2: Magnetic reconnection within a current sheet.
Fig. 3: Phase diagram of magnetic reconnection.
Fig. 4: Two reconnection scenarios with different onset/trigger mechanisms and macroscale coupling.
Fig. 5: Illustration of particle acceleration mechanisms.

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Acknowledgements

H.J., J.J.-A. and J.Y. acknowledge support by the US Department of Energy (DOE) via contract no. DE-AC0209CH11466, and W.D., A.L. and A.S. acknowledge support by the DOE Frontier Plasma Science Program. The authors thank S. Dorfman for providing the top-right panel of Fig. 2 and M. Yamada, A. Bhattacharjee, J. Egedal and F. Guo for their constructive comments on an initial draft of this Roadmap. W.D. acknowledges helpful discussions from the SolFER DRIVE Science Center collaboration.

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Authors and Affiliations

  1. Department of Astrophysical Sciences, Princeton University, Princeton, NJ, USA

    Hantao Ji

  2. Princeton Plasma Physics Laboratory, Princeton University, Princeton, NJ, USA

    Hantao Ji, Jonathan Jara-Almonte & Jongsoo Yoo

  3. Los Alamos National Laboratory, Los Alamos, NM, USA

    William Daughton, Ari Le & Adam Stanier

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All authors contributed to the literature review, drafting of the text, figure production, discussing, editing and revising all aspects of this Roadmap.

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Correspondence to Hantao Ji.

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Glossary

Collisional plasma

(Or collisionless) plasmas in which Coulomb collisions are important (unimportant) for the subject of interest, which, in this Roadmap, is magnetic reconnection.

Magneto-rotational instability

A plasma instability believed to generate turbulence to explain the observed fast accretion in magnetized astrophysical discs where angular speed increases, whereas specific angular momentum decreases radially.

Kelvin–Helmholtz instability

A linear instability driven by velocity shear in a fluid or plasma.

Magnetic Prandtl number

A dimensionless parameter in magnetic hydrodynamic fluids or plasmas to quantify the momentum diffusion relative to magnetic diffusion.

Sawtooth oscillations

Quasi-periodic, sawtooth-like oscillations in soft X-ray measurements of the core tokamak plasmas from rapid loss and gradual recovery of hot electron temperature due to an internal magnetic hydrodynamic instability.

Kink instability

A plasma instability that produces helical kinking of a current channel and is driven by excessively large electric currents for a given magnetic flux in the same direction.

Ballooning instability

A plasma instability that causes the magnetic field to balloon outwards towards the weak field direction due to excessively large plasma pressure gradient.

Torus instability

An expansion magnetic hydrodynamic instability of current-carrying torus in solar and laboratory plasmas due to rapid decrease of the required equilibrium transverse magnetic field in the expansion direction.

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Ji, H., Daughton, W., Jara-Almonte, J. et al. Magnetic reconnection in the era of exascale computing and multiscale experiments. Nat Rev Phys 4, 263–282 (2022). https://doi.org/10.1038/s42254-021-00419-x

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