  Rossby wave instability (RWI) at dead zone boundaries may play an important
role in planet formation. Viscous hydrodynamics results suggest RWI is excited
only when the viscosity changes over a radial distance less than two density
scale heights. However in the disks around Solar-mass T Tauri stars, it is not
viscosity but magnetic forces that provide the accretion stress beyond about 10
AU, where surface densities are low enough so stellar X-rays and interstellar
cosmic rays can penetrate. Here we aim to explore the conditions for RWI in the
smooth transition with increasing distance, from resistive and
magnetically-dead to conducting and magnetically-active. We perform 3D
unstratified MHD simulations with the Pencil Code, using static resistivity
profiles. As a result, we find that in MHD, contrary to viscous models, the RWI
is triggered even with a gradual change in resistivity extending from 10 to 40
AU (i.e., spanning 15 scale heights for aspect ratio 0.1). This is because
magneto-rotational turbulence sets in abruptly when the resistivity reaches a
threshold level. At higher resistivities the longest unstable wavelength is
quenched, resulting in a sharp decline of the Maxwell stress towards the star.
The sharp gradient in the magnetic forces leads to a localized density bump,
that is in turn Rossby wave unstable. We conclude that even weak gradients in
the resistivity can lead to sharp transitions in the Maxwell stress. The upshot
is that the RWI is more easily activated in the outer disk than previously
thought. Rossby vortices at the outer dead zone boundary thus could underlie
the dust asymmetries seen in the outer reaches of transition disks.
