  Stars born on near-circular orbits in spiral galaxies can subsequently
migrate to different orbits due to interactions with non-axisymmetric
disturbances within the disc such as bars or spiral arms. This paper extends
the study of migration to examine the role of external influences using the
example of the interaction of the Sagittarius dwarf galaxy (Sgr) with the Milky
Way (MW). We first make impulse approximation estimates to characterize the
influence of Sgr disc passages. The tidal forcing from Sgr can produce changes
in both guiding radius ($\Delta R_g$) and orbital eccentricity, as quantified
by the maximum radial excursion, $\Delta R_ {\rm max} $. These changes follow a
quadrupole-like pattern across the face of the disc, with amplitude increasing
with Galactocentric radius. We next examine a collisionless N-body simulation
of a Sgr-like satellite interacting with a MW-like galaxy and find that Sgr's
influence in the outer disc dominates over the secular evolution of orbits
between disc passages. Finally, we use the same simulation to explore possible
observable signatures of Sgr-induced migration by painting the simulation with
different age stellar populations. We find that following Sgr disc passages,
the migration it induces manifests within an annulus as an approximate
quadrupole in azimuthal metallicity variations ($\delta_ {\rm [Fe/H]} $), along
with systematic variations in orbital eccentricity, $\Delta R_ {\rm max} $.
These systematic variations can persist for several rotational periods. We
conclude that this combination of signatures may be used to distinguish between
the different migration mechanisms shaping the chemical abundance patterns of
the Milky Way's thin disc.
