Magnetic bipoles in rotating turbulence with coronal envelope

TL;DR

This study investigates how rotation and a coronal envelope influence magnetic flux concentration formation via NEMPI in stellar environments, revealing rotation suppresses bipolar region formation despite the coronal layer's presence.

Contribution

It demonstrates that rotation strongly suppresses bipolar region formation in NEMPI, and the coronal envelope does not significantly mitigate this rotational suppression.

Findings

Rotation suppresses bipolar region formation even at slow speeds.

Increasing magnetic field strength enhances flux structures and reduces rotational suppression.

The coronal layer does not significantly alleviate rotational effects on NEMPI.

Abstract

The formation of sunspots and starspots is not yet fully understood and is therefore one of the major open problems in solar and stellar physics. 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 want to know whether the presence of an outer coronal envelope also changes the excitation conditions and the rotational dependence of NEMPI. 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, and the box resolution. 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 alleviate the effects of rotational suppression.