  The buoyant transport of magnetic fields from the solar interior towards 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 towards 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 their formation (Cline et
al. 2003; Brummell et al. 2002), 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.
