6 Open Questions and Final Remarks

The study of flux emergence has received a great push forward from recent advances in observational capability, theory, and numerical modeling. The large number of flux emergence models has provided invaluable insight into the physical mechanisms that are central to the myriad of observable phenomena driven by the passage of magnetic flux from the solar convection zone all the way into the corona. The role of magnetic buoyancy instabilities to bring flux from the photosphere into the corona is well-studied. Furthermore, it is well established that granular convective flows play an important role in the morphology of emerging flux at the photosphere. The formation of sea serpentine field-lines (whether formed from interaction with granular flows or from the Parker instability) is understood to be an important step for removing the mass burden from field lines rising into the corona. Simulations dealing with the emergence of twisted flux ropes show how the Lorentz force drives rotational and shearing motions at the photosphere. The Poynting flux of energy and helicity associated with such motion is shown to be important even beyond the initial emergence phase (i.e., when the vertical unsigned flux at the photosphere is increasing) and are likely key for eruptive phenomena. Models of emerging flux interacting with pre-existing ambient field show how the former can sometimes destablize the pre-existing field to launch jets and CMEs.

A number of challanges and open questions remain. For instance, models of the full life-cyle of active regions from birth to decay remain to be developed. Such models may settle long standing questions about the depth from which active region fields are spawned which, in turn, tells us something about how the dynamo-generated magnetic field is structured in the convection zone. The decay of ARs is also an important topic that needs to be addresssed since the turbulent diffusion of AR-flux over the solar surface and the subduction of flux back into the convection zone likely determine how much remnant flux can be replenished for the large-scale dynamo field.

Helioseismic analysis of pre-emerging flux regions is still a nascent field. Numerical models are needed in order to validate helioseismic techniques which, in turn, are used on real observations to constrain numerical models. Continued work in this regard is likely essential for answering the question of whether we can robustly infer subsurface properties of emerging ARs well before their appearance at the surface.

In the near future, we anticipate substantial developments in data-driven models of emerging flux. Such models will use observed magnetogram sequences for boundary conditions of 3D MHD models. With such models, synthetic diagnostics can be directly compared with observations. Success in reproducing observables that quantatively match observations is crucial for a critical assessment of the maturity of MHD models. Furthermore, advances in this realm are essential for novel approaches to space weather forecasting that go beyond the use of heuristics and probabilistic models.

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