Abstract
This paper reviews recent advances and current debates in modeling the solar cycle as a hydromagnetic dynamo process. Emphasis is placed on (relatively) simple dynamo models that are nonetheless detailed enough to be comparable to solar cycle observations. After a brief overview of the dynamo problem and of key observational constraints, we begin by reviewing the various magnetic field regeneration mechanisms that have been proposed in the solar context. We move on to a presentation and critical discussion of extant solar cycle models based on these mechanisms. We then turn to the origin of fluctuations in these models, including amplitude and parity modulation, chaotic behavior, and intermittency. The paper concludes with a discussion of our current state of ignorance regarding various key questions, the most pressing perhaps being the identification of the physical mechanism(s) responsible for the generation of the Sun’s poloidal magnetic field component.
Highlights
This paper reviews recent advances and current debates in modeling the solar cycle as a hydromagnetic dynamo process
In the solar cycle context, the dynamo problem is reformulated towards identifying the circumstances under which the flow fields observed and/or inferred in the Sun can sustain the cyclic regeneration of the magnetic field associated with the observed solar cycle
The introduction of diffusivity quenching reduces the diffusivity in the shear region, “naturally” turning the model into a bona fide interface dynamo, supporting once again oscillatory solutions in the form of dynamo waves travelling in the “latitudinal” x-direction. This basic model was later generalized by various authors (Tobias, 1997; Phillips et al, 2002) to include the nonlinear backreaction of the dynamo-generated magnetic field on the differential rotation; further discussion of such nonlinear models is deferred to Section 5.3.1
Summary
The cyclic regeneration of the Sun’s large-scale magnetic field is at the root of all phenomena collectively known as “solar activity”. A near-consensus exists to the effect that this magnetic cycle is to be ascribed to the inductive action of fluid motions pervading the solar interior. At this writing nothing resembling consensus exists regarding the detailed nature and relative importance of various possible inductive flow contributions. To review “dynamo models of the solar cycle”, is daunting. This review focuses on the cyclic regeneration of the large-scale solar magnetic field through the inductive action of fluid flows, as described by various approximations and simplifications of the partial differential equations of magnetohydrodynamics. The focus is on models of the solar cycle, i.e., constructs seeking primarily to describe the observed spatio-temporal variations of the Sun’s large-scale magnetic field
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