  We present the highest resolution study to date of super-Earths migrating in
inviscid and low-viscosity discs, motivated by the connection to laminar,
wind-driven models of protoplanetary discs. Our models unveil the critical role
of vortices in determining the migration behaviour for partial gap-opening
planets. Vortices form in pressure maxima at gap edges, and prevent the
disc-feedback stopping of migration for intermediate planets in low-viscosity
and inviscid discs, contrary to the concept of the `inertial limit' or `disc
feedback' halting predicted from analytical models. Vortices may also form in
the corotation region, and can dramatically modify migration behaviour through
direct gravitational interaction with the planet. These features become
apparent at high resolution, and for all but the highest viscosities there
exist significant difficulties in obtaining numerically converged results. The
migration of partial gap-opening planets, however, clearly becomes chaotic for
sufficiently low viscosities. At moderate viscosity, a smooth disc-feedback
regime is found in which migration can slow substantially, and the migration
time-scale observed corresponds to migration being driven by diffusive
relaxation of the gap edges. At high viscosity classical Type I migration is
recovered. For Jupiter-analogue planets in inviscid discs, a wide, deep gap is
formed. Transient Type II migration occurs over radial length-scales
corresponding to the gap width, beyond which migration can stall. Finally, we
examine the particle trapping driven by structures left in inviscid discs by a
migrating planet, and find that particle traps in the form of multiple rings
and vortices can persist long after the planet has passed. In this case, the
observation of particle traps by submillimetre interferometers such as ALMA
cannot be used to infer the current presence of an adjacent planet.
