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We examine the migration of low mass planets in laminar protoplanetary discs,threaded by large scale magnetic fields in the dead zone that drive radial gasflows. As shown in Paper I, a dynamical corotation torque arises due to theflow-induced asymmetric distortion of the corotation region and the evolvingvortensity contrast between the librating horseshoe material and backgrounddisc flow. Using simulations of laminar torqued discs containing migratingplanets, we demonstrate the existence of the four distinct migration regimespredicted in Paper I. In two regimes, the migration is approximately locked tothe inward or outward radial gas flow, and in the other regimes the planetundergoes outward runaway migration that eventually settles to fast steadymigration. In addition, we demonstrate torque and migration reversals inducedby midplane magnetic stresses, with a bifurcation dependent on the disc surfacedensity. We develop a model for fast migration, and show why the outwardrunaway saturates to a steady speed, and examine phenomenologically itstermination due to changing local disc conditions. We also develop ananalytical model for the corotation torque at late times that includesviscosity, for application to discs that sustain modest turbulence. Finally, weuse the simulation results to develop torque prescriptions for inclusion inpopulation synthesis models of planet formation.

Low mass planet migration in magnetically torqued dead zones - II. Flow-locked and runaway migration, and a torque prescription