An explanation of anomalous velocities in pulsar scintillation patterns

Velocities of the interstellar medium deduced from the scintillation of pulsar signals frequently exhibit surprising features. They are sometimes very large and can reverse direction in the course of an hour or so. In cases where the pulsar is known to have a high intrinsic velocity the observed pattern velocities are often strangely low. These features are explained in terms of a model of the interstellar medium containing a shear in transverse velocities. Relatively small changes in the medium are required to reverse the high observed velocities and no intrinsic modulation of the pulsar is required.