  We study the type III migration of a Saturn mass planet in low viscosity
discs. The planet is found to experience cyclic episodes of rapid decay in
orbital radius, each amounting to a few Hill radii. We find this to be due to
the scattering of large- scale vortices present in the disc. The origin and
role of vortices in the context of type III migration is explored. It is shown
through numerical simulations and semi- analytical modelling that spiral shocks
induced by a sufficiently massive planet will extend close to the planet
orbital radius. The production of vortensity across shock tips results in thin
high vortensity rings with a characteristic width of the local scale height.
For planets with masses equal to and above that of Saturn, the rings are
co-orbital features extending the entire azimuth. Linear stability analysis
show there exists unstable modes that are localised about local vortensity
minima which coincide with gap edges. Simulations show that vortices are
non-linear a outcome. We used hydrodynamic simulations to examine vortex-planet
interactions. Their effect is present in discs with kinematic viscosity less
than about an order of magnitude smaller than the typically adopted value of
\nu = 10^{-5}\Omega_pr_p(0)^2, where r_p(0) and \Omega_p are the initial
orbital radius and angular velocity of the planet respectively. We find that
the magnitude of viscosity affects the nature of type III migration but not the
extent of the orbital decay. The role of vortices as a function of initial disc
mass is also explored and it is found that the amount of orbital decay during
one episode of vortex-planet interaction is independent of initial disc mass.
We incorporate the concept of the co-orbital mass deficit in the analysis of
our results and link it to the presence of vortices at gap edges.
