  Following an earlier proposal by Choudhuri (2003) for the origin of twist in
the magnetic fields of solar active regions, we model the penetration of a
wrapped up background poloidal field into a toroidal magnetic flux tube rising
through the solar convective zone. The rise of the straight, cylindrical flux
tube is followed by numerically solving the induction equation in a comoving
Lagrangian frame, while an external poloidal magnetic field is assumed to be
radially advected onto the tube with a speed corresponding to the rise
velocity. One prediction of our model is the existence of a ring of reverse
current helicity on the periphery of active regions. On the other hand, the
amplitude of the resulting twist depends sensitively on the assumed structure
(diffuse vs. concentrated/intermittent) of the active region magnetic field
right before its emergence, and on the assumed vertical profile of the poloidal
field. Nevertheless, in the model with the most plausible choice of assumptions
a mean twist comparable to the observations can result rather naturally. Our
results indicate that the contribution of this mechanism to the twist can be
quite significant, and under favourable circumstances it can potentially
account for most of the current helicity observed in active regions.
