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Paradigm shifts of the Solar Dynamo

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If you have a question about this talk, please contact Dr Geoffroy Lesur.

Solar dynamo theory has experienced arguably three major paradigm shifts since its broad initial acceptance during the 1970s. Inevitably, these paradigm shifts have brought the modelling further away from the original ideas that were based on dynamo theory. At the same time solar dynamo theory has lost much of its initial rigor that dynamo theory used to be based on. It its therefore important that the motivation for such departures from the original theory are well justified. In the following we comment briefly on each of the three paradigm shift.

In the Sun an oscillatory magnetic field is generated, but new research now shows that at large magnetic Reynolds numbers this can only happen if the Sun sheds small-scale magnetic twist through the surface while regenerating an interlinked assembly of large-scale poloidal and toroidal magnetic fields. The Sun is believed to accomplish this through coronal mass ejections, which are known to shed approximately the required amount of magnetic twist or helicity.

The inclusion of the effects of coronal mass ejections into the model is believed to be one of the key factors of future solar dynamo models. Other factors include the recently discovered near-surface shear layer of the Sun, where the shear has the opposite radial gradient than in the bulk of the convection zone, and can lead to equatorward migration of sunspot activity, which is a major problem in understanding the solar dynamo. Finally, the discovery that convection pumps magnetic fields downward and thus opposes magnetic buoyancy losses is another factor that makes so-called distributed dynamo model viable. Here, the magnetic field resides in the entire convection zone, and is thus not confined to the thin layer just beneath the convection zone, which is still assumed in many models.

Regardless of the nature and the location of the dynamo, the effect of magnetic helicity fluxes is crucial for allowing the dynamo to reach significant saturation levels. This is most dramatically demonstrated with two examples of convective dynamo action in the presence of either vertical or horizontal shear. In both cases high saturation levels are reached provided the contours of constant shear cross a boundary that transmits magnetic helicity fluxes. Shear also allows for cyclic dynamo action with a frequency that scales with the quenched value of the magnetic diffusivity. This quantity is now determined with the test field method which will also be discussed in my talk.

This talk is part of the DAMTP Astro Mondays series.

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