  The mean value of the normalized current helicity in solar active regions is
on the order of 1e-8 1/m, negative in the northern hemisphere, positive in the
southern hemisphere. Observations indicate that this helicity has a subsurface
origin. Possible mechanisms leading to a twist of this amplitude in magnetic
flux tubes include the solar dynamo, convective buffeting of rising flux tubes,
and the accretion of weak external poloidal flux by a rising toroidal flux
tube. After briefly reviewing the observational and theoretical constraints on
the origin of helicity, we present a recently developed detailed model for
poloidal flux accretion.
