The early time (t RBa+, where RBa+ is the Ba+ gyroradius. As a consequence, any momentum coupling force F between the Ba+ cloud and the solar wind in their relative streaming direction will produce an F¿B˙ acceleration drift in a direction consistent with the observations. The drift will cease when the cloud expands to the extent that the magnetic field inside the cloud is reduced to satisfy L<RBa+. The implications of the model to observables of the interaction are discussed.