Name/Affiliation: Rakesh K. Yadav (Max-Planck Institute for solar system research)
Spontaneous formation of cool polar-spots in global numerical simulations
Sunspots are one of the most remarkable manifestations of the Sun's magnetic field. Their properties have played an instrumental role in shaping our understanding of the solar dynamo. Observational techniques have revealed that other cool stars also harbor cool spot-like features on their surface. But, unlike the sunspots, coolspots on other stars appear on a wide range of latitudes. Some cool stars, especially the rapidly rotating ones, exhibit coolspots very close to (or at) the rotational poles, sometimes forming the so called "polar-caps". Flux-tube models, which have been developed to explain the sunspot properties, have been extended to explain such high-latitude coolspots. Although such models show some agreement with the observations, several underlying simplifications do not allow a self-consistent treatment of dynamo mechanism and coolspot formation. Many have argued that the dynamo mechanism in rapidly rotating stars might be fundamentally different from the solar case. A distributed dynamo, operating throughout the stellar convection zone, has been proposed for such stars. To explore the plausibility of such dynamo mechanism we simulate magnetic field generation in a density-stratified rapidly rotating spherical shell. Due to rapid rotation helical convection develops in the interior and maintains a strong large-scale magnetic field. Granular-type convection in the outer layers, promoted by the strong density contrast, collects the magnetic flux in convergent downwellings. In some regions where the magnetic flux concentration is large enough, convection is highly quenched, leading to the formation of coolspots. Sizable coolspots form at high-latitudes due to the distribution of the magnetic field. Our simulations demonstrate that a distributed dynamo can spontaneously generate coolspots at high latitudes.