  We investigate the possibility of dc soliton motion sustained by pure ac
driving in the sine-Gordon model. We show by means of the collective coordinate
formalism that ac driving induces a net dc velocity whose modulus and direction
depend on the driving phase. Numerical simulations of the full sine-Gordon
equation confirm the correctness and accuracy of this prediction. Non trivial
cases when dc soliton motion is transformed into oscillatory as well as the
effects of damping are analyzed. Our results settle a long standing issue about
the existence and characteristics of this phenomenon, whose possible appearance
in other systems is also discussed.
