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MHD turbulence plays a crucial role in the dust dynamics of protoplanetarydiscs. It affects planet formation, vertical settling and is one possibleorigin of the large scale axisymmetric structures, such as rings, recentlyimaged by ALMA and SPHERE. Among the variety of MHD processes, themagnetorotational instability (MRI) has raised particular interest since itprovides a source of turbulence and potentially organizes the flow into radialstructures. However, the weak ionization of discs prevents the MRI from beingexcited beyond 1 AU. The strong sedimentation of millimetre dust measured inT-Tauri discs is also in contradiction with predictions based on ideal MRIturbulence. In this paper, we study the effects of non-ideal MHD and winds onthe dynamics and sedimentation of dust grains. We consider a weakly ionizedplasma subject to ambipolar diffusion characterizing the disc outer regions (>1AU). For that, we perform numerical MHD simulations in the stratified shearingbox, using the PLUTO code. Our simulations show that the mm-cm dust iscontained vertically in a very thin layer, with typical heightscale ~0.4 AU at30 AU, compatible with recent ALMA observations. Horizontally, the grains aretrapped within pressure maxima induced by ambipolar diffusion, leading to theformation of dust rings. For micrometer grains, dust and gas scaleheights aresimilar. In this regime, the settling cannot be explained by a simple 1Ddiffusion theory but results from a large scale 2D circulation induced by bothMHD winds and zonal flows. Overall, our results show that non-ideal MHD effectsand their related winds play a major role in shaping the radial and verticaldistribution of dust in protoplanetary discs. Leading to substantial accretionefficiency, non-ideal effects also a promising avenue to reconcile the lowturbulent activity measured in discs with their relatively high accretionrates.

Dust settling and rings in the outer regions of protoplanetary discs subject to ambipolar diffusion