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Register a new userDoes nonaxisymmetric dynamo operate in the Sun?
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We explore effects of random non-axisymmetric perturbations of kinetic helicity (the $alpha$ effect) and diffusive decay of bipolar magnetic regions on generation and evolution of large-scale non-axisymmetric magnetic fields on the Sun. Using a reduced 2D nonlinear mean-field dynamo model and assuming that bipolar regions emerge due to magnetic buoyancy in situ of the large-scale dynamo action, we show that fluctuations of the $alpha$ effect can maintain the non-axisymmetric magnetic fields through a solar-type $alpha^{2}Omega$ dynamo process. It is found that diffusive decay of bipolar active regions is likely to be the primary source of the non-axisymmetric magnetic fields observed on the Sun. Our results show that the non-axisymmetric dynamo model with stochastic perturbations of the $alpha$ effect can explain periods of extremely high activity (`super-cycle events) as well as periods of deep decline of magnetic activity. We compare the models with synoptic observations of solar magnetic fields for the last four activity cycles, and discuss implications of our results for interpretation of observations of stellar magnetic activity.
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In the quiet Sun, magnetic fields are usually observed as small-scale magnetic elements, `salt and pepper, covering the entire solar surface. By using 3D radiative MHD numerical simulations we demonstrate that these fields are a result of local dynamo action in the top layers of the convection zone, where extremely weak `seed magnetic fields can locally grow above the mean equipartition field (e.g., from a $10^{-6}$ G `seed field to more than 1000 G magnetic structures). We find that the local dynamo action takes place only in a shallow, about 500 km deep, subsurface layer, from which the generated field is transported into deeper layers by convection downdrafts. We demonstrate that the observed dominance of vertical magnetic fields at the photosphere and the horizontal fields above the photosphere can be explained by multi-scale magnetic loops produced by the dynamo.
We seek to understand the transition from nearly axisymmetric configurations at solar rotation rates to nonaxisymmetric configurations for rapid rotation using 3D numerical simulations of turbulent convection and considering rotation rates between 1 and 30 times the solar value. We find a transition from axi- to nonaxisymmetric solutions at around 1.8 times the solar rotation rate. This transition coincides with a change in the rotation profile from antisolar- to solar-like differential rotation with a faster equator and slow poles. In the solar-like rotation regime, the field configuration consists of an axisymmetric oscillatory field accompanied by an m=1 azimuthal mode (two active longitudes), which also shows temporal variability. At slow (rapid) rotation, the axisymmetric (nonaxisymmetric) mode dominates. The axisymmetric mode produces latitudinal dynamo waves with polarity reversals, while the nonaxisymmetric mode often exhibits a drift in the rotating reference frame and the strength of the active longitudes changes cyclically over time between the different hemispheres. Most of the obtained dynamo solutions exhibit cyclic variability either caused by latitudinal or azimuthal dynamo waves. In an activity-period diagram, the cycle lengths normalized by the rotation period form two different populations as a function of rotation rate or magnetic activity level. The slowly rotating axisymmetric population lies close to what is called the inactive branch in observations, while the rapidly rotating models are close to the superactive branch with a declining cycle to rotation frequency ratio with increasing rotation rate. We can successfully reproduce the transition from axi- to nonaxisymmetric dynamo solutions for high rotation rates, but high-resolution simulations are required to limit the effect of rotational quenching of convection at rotation rates above 20 times the solar value.
Magnetic fields are usually observed in the quiet Sun as small-scale elements that cover the entire solar surface (the `salt and pepper patterns in line-of-sight magnetograms). By using 3D radiative MHD numerical simulations we find that these fields result from a local dynamo action in the top layers of the convection zone, where extremely weak seed magnetic fields (e.g., from a $10^{-6}$ G) can locally grow above the mean equipartition field, to a stronger than 2000~G field localized in magnetic structures. Our results reveal that the magnetic flux is predominantly generated in regions of small-scale helical downflows. We find that the local dynamo action takes place mostly in a shallow, about 500~km deep, subsurface layer, from which the generated field is transported into the deeper layers by convective downdrafts. We demonstrate that the observed dominance of vertical magnetic fields at the photosphere and horizontal fields above the photosphere can be explained by small-scale magnetic loops produced by the dynamo. Such small-scale loops play an important role in the structure and dynamics of the solar atmosphere and that their detection in observations is critical for understanding the local dynamo action on the Sun.
Sunspots are cool areas caused by strong surface magnetic fields inhibiting convection. Moreover, strong magnetic fields can alter the average atmospheric structure, degrading our ability to measure stellar masses and ages. Stars more active than the Sun have more and stronger dark spots than in the solar case, including on the rotational pole itself. Doppler imaging, which has so far produced the most detailed images of surface structures on other stars than the Sun, cannot always distinguish the hemisphere in which the starspots are located, especially in the equatorial region and if the data quality is not optimal. This leads to problems in investigating the north-south distribution of starspot active latitudes (those latitudes with more spot activity), which are crucial constraints of dynamo theory. Polar spots, inferred only from Doppler tomography, could plausibly be observational artifacts, casting some doubt on their very existence. Here we report imaging of the old, magnetically-active star $zeta$ Andromedae using long-baseline infrared interferometry. In our data, a dark polar spot is seen in each of two epochs, while lower-latitude spot structures in both hemispheres do not persist between observations revealing global starspot asymmetries. The north-south symmetry of active latitudes observed on the Sun is absent on $zeta$ And, which hosts global spot patterns that cannot be produced by solar-type dynamos.
The Stellar Imager mission concept is a space-based UV/Optical interferometer designed to resolve surface magnetic activity and subsurface structure and flows of a population of Sun-like stars, in order to accelerate the development and validation of a predictive dynamo model for the Sun and enable accurate long-term forecasting of solar/stellar magnetic activity.
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