Dynamics of Local Isolated Magnetic Flux Tubes in a Rapidly Rotating Stellar Atmosphere

Abstract

The dynamics of magnetic flux tubes in a rapidly rotating stellar atmosphere is considered. We focus on the effects and signatures of the instability of the flux tube emergence influenced by the Coriolis force. We present the results from a linear stability analysis and discuss its possible signatures during the course of the evolution of G-type and M-type stars. We carried out three-dimensional magnetohydrodynamical simulations of local isolated magnetic flux tubes under a magnetic buoyancy instability in co-rotating Cartesian coordinates. We have found that the combination of the buoyancy instability and the Coriolis effect gives rise to a mechanism to twist the emerging magnetic flux tube into a helical structure. The tilt angle, east-west asymmetry, and magnetic helicity of the twisted flux tubes in the simulations have been studied in detail. The linear and nonlinear analyses provide hints as to what kind of pattern of large spots in young M-type main-sequence stars might be observed. We have found that young and old G-type stars may have different distributions of spots, while M-type stars may always have low-latitude spots. The size of stellar spots may decrease when a star becomes older, due to a decrease in the magnetic field. A qualitative comparison with solar observations is also presented.