Magnetic reconnection in the era of exascale computing and multiscale experiments

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. Magnetic reconnection explosively releases stored magnetic energy in astrophysical plasmas. Thanks to advances in observations, exascale computing and multiscale experiments, it will be possible to solve outstanding physics problems, including the immense separation between global and dissipation scales, reconnection onset and particle acceleration. 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. 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.