@article{b40bd3b637624ddf990cb64d4cad0e9c,
title = "Magnetic bipoles in rotating turbulence with coronal envelope",
abstract = "Context. The formation mechanism of sunspots and starspots is not yet fully understood. It is a major open problem in astrophysics. Aims. Magnetic flux concentrations can be produced by the negative effective magnetic pressure instability (NEMPI). This instability is strongly suppressed by rotation. However, the presence of an outer coronal envelope was previously found to strengthen the flux concentrations and make them more prominent. It also allows for the formation of bipolar regions (BRs). We aim to understand the important issue of whether the presence of an outer coronal envelope also changes the excitation conditions and the rotational dependence of NEMPI. Methods. We have used direct numerical simulations and mean-field simulations. We adopted a simple two-layer model of turbulence that mimics the jump between the convective turbulent and coronal layers below and above the surface of a star, respectively. The computational domain is Cartesian and located at a certain latitude of a rotating sphere. We investigated the effects of rotation on NEMPI by changing the Coriolis number, the latitude, the strengths of the imposed magnetic field, and the box resolution. Results. Rotation has a strong impact on the process of BR formation. Even rather slow rotation is found to suppress BR formation. However, increasing the imposed magnetic field strength also makes the structures stronger and alleviates the rotational suppression somewhat. The presence of a coronal layer itself does not significantly reduce the effects of rotational suppression.",
keywords = "Dynamo, Magnetohydrodynamics (MHD), Sun: activity, Sun: magnetic fields, Sun: rotation, Turbulence",
author = "Losada, {I. R.} and J. Warnecke and A. Brandenburg and N. Kleeorin and I. Rogachevskii",
note = "Funding Information: Acknowledgements. J.W. acknowledges funding by the Max-Planck/Princeton Center for Plasma Physics and funding from the People Program (Marie Curie Actions) of the European Union{\textquoteright}s Seventh Framework Programmed (FP7/2007-2013) under REA grant agreement No. 623609. This work has been supported in part by the NSF Astronomy and Astrophysics Grants Program (grant 1615100), the Research Council of Norway under the FRINATEK (grant 231444), the Swedish Research Council (grant 2012-5797), and the University of Colorado through its support of the George Ellery Hale visiting faculty appointment. I.R. acknowledges the hospitality of NORDITA and Max Planck Institute for Solar System Research in G{\"o}ttingen. We acknowledge the allocation of computing resources provided by the Swedish National Allocations Committee at the Center for Parallel Computers at the Royal Institute of Technology in Stockholm. This work utilized the Janus supercomputer, which is supported by the National Science Foundation (award number CNS-0821794), the University of Colorado Boulder, the University of Colorado Denver, and the National Center for Atmospheric Research. The Janus supercomputer is operated by the University of Colorado Boulder. Additional simulations have been carried out on supercomputers at GWDG, on the Max Planck supercomputer at RZG in Garching, in the facilities hosted by the CSC – IT Center for Science in Espoo, Finland, which are financed by the Finnish ministry of education. Funding Information: J.W. acknowledges funding by the Max-Planck/Princeton Center for Plasma Physics and funding from the People Program (Marie Curie Actions) of the European Union's Seventh Framework Programmed (FP7/2007- 2013) under REA grant agreement No. 623609. This work has been supported in part by the NSF Astronomy and Astrophysics Grants Program (grant 1615100), the Research Council of Norway under the FRINATEK (grant 231444), the Swedish Research Council (grant 2012-5797), and the University of Colorado through its support of the George Ellery Hale visiting faculty appointment. I.R. acknowledges the hospitality of NORDITA and Max Planck Institute for Solar System Research in Gottingen. We acknowledge the allocation of computing resources provided by the Swedish National Allocations Committee at the Center for Parallel Computers at the Royal Institute of Technology in Stockholm. This work utilized the Janus supercomputer, which is supported by the National Science Foundation (award number CNS-0821794), the University of Colorado Boulder, the University of Colorado Denver, and the National Center for Atmospheric Research. The Janus supercomputer is operated by the University of Colorado Boulder. Additional simulations have been carried out on supercomputers at GWDG, on the Max Planck supercomputer at RZG in Garching, in the facilities hosted by the CSC - IT Center for Science in Espoo, Finland, which are financed by the Finnish ministry of education Publisher Copyright: {\textcopyright} ESO 2019.",
year = "2019",
month = jan,
day = "1",
doi = "10.1051/0004-6361/201833018",
language = "English",
volume = "621",
journal = "Astronomy and Astrophysics",
issn = "0004-6361",
publisher = "EDP Sciences",
}