Magnetic bipoles in rotating turbulence with coronal envelope

Abstract

Context. The formation mechanism of sunspots and starspots is not 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). It is important to understand whether the presence of an outer coronal envelope also changes the excitation conditions and the rotational dependence of NEMPI. Methods. We use direct numerical simulations and mean-field simulations. We adopt 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 investigate 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 their 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.

Non-Fourier description of heat flux evolution in 3D MHD simulations of the solar corona

“…However, Chatterjee ( 2018) developed a 2D model, where the near-surface convection is included with a realistic treatment of the solar corona. Simplified two-layer simulations of the convection zone and the corona of the Sun and stars using the Pencil Code have been successfully used to investigate the dynamo-corona interplay Brandenburg 2014, Warnecke et al 2016a), to selfconsistently drive current helicity ejection into the corona (Warnecke and Brandenburg 2010, Warnecke et al 2011, 2012a, 2013a and the formation of sunspot-like flux concentrations (Warnecke et al 2013b, 2016b, Losada et al 2019.…”