Presentation #101.01 in the session Universality of Mesoscale Processes in Space and Solar Physics.
Plasmoid-mediated magnetic reconnection in plasma systems with a high Lundquist number (S) and large normalized size (λ) has been proposed as a promising multiscale mechanism coupling the global fluid macroscale and the local microscale which is dissipative and often kinetic. On the microscale, the physics of fast kinetic reconnection has been established between multiple in-situ spacecraft measurements, well-controlled and diagnosed laboratory experiments, and kinetic simulations with artificial but proper boundary and initial conditions. On the macroscale, the physics of storage and release of magnetic free energy to drive impulsive fast reconnection has been also established between multiple wavelength remote-sensing measurements, fluid simulation with realistic boundary and initial conditions, and the relevant laboratory experiments in collisional regimes. However, there is a missing link to couple macroscale and microscale via a viable multiscale mechanism allowing energy on one scale to simultaneously serve as a driver for dynamics on smaller scales and also as a product or participator of dynamics on larger scales. Now, the magnetic reconnection research has entered a new and final stage to establish this missing link. One promising multiscale concept is based on the plasmoid instability of Sweet-Parker type long collisional current sheets in a self-similar fashion, organized in the reconnection phase diagram in the (S, λ) parameter space for different multiscale mechanisms [H. Ji & W. Daughton, Phys. Plasmas 18, 111207 (2011)]. In this paradigm, plasmoids serve as a vital mesoscale structure driving dynamics on smaller scales while also crucially contributing to larger scales. In this talk, we use a few examples from Earth’s magnetosphere, the solar atmosphere, and laboratory experiments to illustrate plasmoids as a crucial mesoscale structure during multiscale magnetic reconnection. Two outstanding and practical questions will be highlighted as a consequence of mesoscale dynamics: The onset of large-scale reconnection and non-thermal particle acceleration. We will discuss specific near-term opportunities to meet these challenges in remote-sensing and in-situ observations, numerical simulation, and laboratory experiments [H. Ji, W. Daughton, J. Jara-Almonte, A. Le, A. Stanier & J. Yoo, Nat. Rev. Phys. 4, 263 (2022)].