Multiple-scale magnetic reconnection
Determine how magnetic reconnection couples global fluid or magnetohydrodynamic scales to local kinetic dissipation scales, whether plasmoid instability exists in magnetohydrodynamic or kinetic plasmas, the critical Lundquist number and normalized system size for that instability, and how multiple X-lines and reconnection rates scale with controlling parameters, upstream or downstream asymmetry, and finite guide fields.
References
Finally, we conclude this facility paper by providing an initial list of research topics for FLARE based on the major scientific challenges in magnetic reconnection, as summarized in recent community whitepapers~\citep{ji20,ji23c} submitted to several decadal surveys. Each of the ten major problems contains a set of sub-problems which are listed below as questions for further reference: Multiple scale problem: How does reconnection couple global fluid (magnetohydrodynamic or MHD) scales to local dissipation (kinetic) scales? Does plasmoid instability exist in MHD or kinetic plasmas? If so, what are the critical Lundquist number and normalized system size? How do the resultant multiple X-lines and the corresponding reconnection rate scale with controlling parameters in each regime? What are effects due to asymmetry in upstream and/or downstream and a finite guide field?
3D problem: How does reconnection take place in 3D on global and local scales? What are possible forms of plasmoid instability in 3D, and do they evolve into interacting flux ropes? How does multiple X-line reconnection scale with the third dimension, and can it become turbulent? Is it possible to externally drive tearing mode reconnection? How do multiple tearing modes interact, and can they lead to magnetic stochasticity?
Energy problem: How are particles heated and accelerated? How do particle heating and acceleration scale with the Lundquist number and normalized system size? What are the apportionments between electrons and ions which can have multiple components? What are effects due to a finite guide field?
Boundary problem: How do boundary conditions affect the reconnection process? How does downstream magnetic and/or plasma pressure affect multiple X-line reconnection? What are the effects due to line-tied boundaries in the third direction?
Onset problem: How does reconnection start? Is reconnection onset local or global? Does reconnection start simultaneously in 2D or essentially in 3D, and how does it depend on multiple X-lines?
Partial ionization problem: How does partial ionization affect reconnection? Do neutrals promote or suppress multiple-scale reconnection? How, and to what extent, are neutral particles energized during reconnection?
Although fast reconnection is widely inferred in these systems, its physical origin remains an open question, particularly the mechanisms responsible for reconnection rates of order 0.1 across different plasma conditions and magnetic-field configurations.