Magnetic bipoles in rotating turbulence with coronal envelope

¹ Nordita, KTH Royal Institute of Technology and Stockholm University, 10691 Stockholm, Sweden
e-mail: illa.rivero.losada@gmail.com
² Department of Astronomy, AlbaNova University Center, Stockholm University, 10691 Stockholm, Sweden
³ Nordic Optical Telescope, La Palma, Canary Islands, Spain
⁴ Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
⁵ JILA and Department of Astrophysical and Planetary Sciences, University of Colorado, Box 440, Boulder, CO 80303, USA
⁶ Laboratory for Atmospheric and Space Physics, University of Colorado, Box 590, Boulder, CO 80303, USA
⁷ Department of Mechanical Engineering, Ben-Gurion University of the Negev, POB 653, Beer-Sheva 84105, Israel

Received: 13 March 2018
Accepted: 16 September 2018

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.