  The outer parts of the Milky Way's disc are significantly out of equilibrium.
Using only distances and proper motions of stars from Gaia's Early Data Release
3, in the range |b|<10{\deg}, 130{\deg}<l<230{\deg}, we show that for stars in
the disc between around 10 and 14 kpc from the Galactic centre, vertical
velocity is strongly dependent on the angular momentum, azimuth, and position
above or below the Galactic plane. We further show how this behaviour
translates into a bimodality in the velocity distribution of stars in the outer
Milky Way disc. We use an N-body model of an impulse-like interaction of the
Milky Way disc with a perturber similar to the Sagittarius dwarf to demonstrate
that this mechanism can generate a similar disturbance. It has already been
shown that this interaction can produce a phase spiral similar to that seen in
the Solar neighbourhood. We argue that the details of this substructure in the
outer galaxy will be highly sensitive to the timing of the perturbation or the
gravitational potential of the Galaxy, and therefore may be key to
disentangling the history and structure of the Milky Way.
