Flux tube dynamics in active stars

We propose a model for the storage, instability and eruption of magnetic flux tubes in active cool stars. Isolated toroidal flux tubes are stored in mechanical force equilibrium within the overshoot layer below the (outer) convective envelope. The equilibrium is susceptible to a non-axisymmetric instability once a critical field strength is exceeded. This leads to the eruption of magnetic flux loops which rise towards the stellar surface and form pairs of starspots. If the stellar rotation is sufficiently fast, the Coriolis force due to the conservation of angular momentum deflects the rising loops towards high latitudes. This, together with the poleward motion of the anchored parts of the flux tubes, may lead to the polar features observed on many active spotted stars. The critical field strength necessary for instability and eruption of the flux tubes depends on the latitudinal position of the equilibrium. The required field strength is particularly large in middle latitudes, so that the resulting flux loops rise nearly radially due to their strong buoyancy. This may explain the simultaneous existence of polar spots and starspots in middle latitudes as is observed on some stars.