  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.
