The Rise of a Magnetic Flux Tube in a Background Field: Solar Helicity Selection Rules

Abstract

Abstract The buoyant transport of magnetic fields from the solar interior toward the surface plays an important role in the emergence of active regions, the formation of sunspots, and the overall solar dynamo. Observations suggest that toroidal flux concentrations, often referred to as “flux tubes,” rise from their region of initiation likely in the solar tachocline toward the solar surface due to magnetic buoyancy. Many studies have assumed the existence of such magnetic structures and studied the buoyant rise of an isolated flux tube in a quiescent, field-free environment. Here, motivated by the mechanisms of flux tube formation, we relax the latter assumption and study the rise of a toroidal flux tube embedded in a large-scale poloidal background magnetic field. We find that the presence of the large-scale background field severely affects the dynamics of the rising tube. A relatively weak background field, as low as 6% of the tube strength, can destroy the rise of a tube that would otherwise rise in the absence of the background field. Surprisingly, the rise of tubes with one sign of the twist is suppressed by a significantly weaker background field than the other. This demonstrates a potential mechanism for the selection of the preferred helicity of rising and emerging tubes for the solar case that is commensurate with many features of the hemispherical rule.