Learning about solar/stellar dynamo physics from the variability
Abstract
I will highlight three reasons where current simulations fail to reproduce the Sun, but I will also highlight three robust results, and finally, I will highlight three striking differences between simulations and observations. (A) Realistic solar/stellar convectively driven dynamo simulations suffer from three principle difficulties: (i) the tremendous stratification, the range of time and length scales, and the lack of realistic surface physics (radiation); (ii) a barely resolved near-surface shear layer, especially at higher latitudes; (iii) angular velocity contours that are still not as spoke-like as suggested by helioseismology; (B) Nevertheless, several robust results have emerged from such simulations: (i) at some rotation rate (Rossby number close to the solar value), there is a transition from solar-like to antisolar-like differential rotation for slower rotation; (ii) at a very similar rotation rate, there is another transition from axisymmetric to nonaxisymmetric (m=1) large-scale magnetic fields; (iii) the rotation to cycle period ratio decreases with increasing stellar activity or decreasing Rossby number. (C) Conflicts between observations and simulations include: (i) the presence of an intermediate solar-like branch where the period frequency ratio increases with increasing stellar activity. The transition to nonaxisymmetric large-scale magnetic fields would be to the right of this branch (larger activity), and the transition to anti solar differential rotation would be to the left, but in simulations the two transitions are found to appear at the same rotation rate, so this intermediate solar-like branch has disappeared. (ii) Both simulations and observations can show evidence for multiple periods. If this is real, it is unclear how the surface appearance of the magnetic field changes. Also, while some observational work has associated the longer periods with dynamo activity closer to the surface, different models show instead that longer periods may originate from deeper down in the convection zone. (iii) Variability in the sunspot data shows relatively nice cycles with a single grand minimum. Stellar cycles are never that clean, and simulations show surface patterns with additional significant variation away from the sunspot belts. Can these discrepancies simply be explained by sunspots being a threshold phenomenon, or is there more behind this difference.
- Publication:
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Solar Heliospheric and INterplanetary Environment (SHINE 2019)
- Pub Date:
- May 2019
- Bibcode:
- 2019shin.confE.220B