  We explore the hypothesis that a passing satellite or dark matter subhalo has
excited coherent oscillations of the Milky Way's stellar disk in the direction
perpendicular to the Galactic midplane. This work is motivated by recent
observations of spatially dependent bulk vertical motions within ~ kpc of the
Sun. A satellite can transfer a fraction of its orbital energy to the disk
stars as it plunges through the Galactic midplane thereby heating and
thickening the disk. Bulk motions arise during the early stages of such an
event when the disk is still in an unrelaxed state. We present simple toy-model
calculations and simulations of disk-satellite interactions, which show that
the response of the disk depends on the relative velocity of the satellite.
When the component of the satellite's velocity perpendicular to the disk is
small compared with that of the stars, the perturbation is predominantly a
bending mode. Conversely, breathing and higher order modes are excited when the
vertical velocity of the satellite is larger than that of the stars. We argue
that the compression and rarefaction motions seen in three different surveys
are in fact breathing mode perturbations of the Galactic disk.
