Final Report Summary - HALLDISCS (Hall dominated turbulence in protoplanetary discs)
Protoplanetary discs are poorly ionised due to their low temperatures and high column densities, and are therefore subject to three "non-ideal" magnetohydrodynamic effects: Ohmic dissipation, ambipolar diffusion, and the Hall effect.
The existence of magnetically driven turbulence in these discs has been a central question since the discovery of the magnetorotational instability. Early models considered Ohmic diffusion only and led to a scenario of layered accretion, in which a magnetically "dead" zone in the disc midplane is embedded within magnetically "active" surface layers at distances ~1-10 au from the central protostellar object. Recent work has suggested that a combination of Ohmic dissipation and ambipolar diffusion can render both the midplane and surface layers of the disc inactive and that torques due to magnetically driven outflows are required to explain the observed accretion rates. The HallDiscs project reassess this picture by performing three-dimensional numerical simulations that include, for the first time, all three non-ideal MHD effects. The main finding of this project is that Hall effect can generically "revive" dead zones by producing a dominant azimuthal magnetic field and a large-scale Maxwell stress throughout the midplane. We have moreover demonstrated the possibility of generating self-organising structures such as large scale zonal flows, which possibly have observable counterparts like ring-like structures.
These results have dramatic consequences on our understanding of protoplanetary disc dynamics and planet formation process. In particular, they provide a natural framework explaining the recent observations of ring like structures in protoplanetary discs, without needing planets.
The existence of magnetically driven turbulence in these discs has been a central question since the discovery of the magnetorotational instability. Early models considered Ohmic diffusion only and led to a scenario of layered accretion, in which a magnetically "dead" zone in the disc midplane is embedded within magnetically "active" surface layers at distances ~1-10 au from the central protostellar object. Recent work has suggested that a combination of Ohmic dissipation and ambipolar diffusion can render both the midplane and surface layers of the disc inactive and that torques due to magnetically driven outflows are required to explain the observed accretion rates. The HallDiscs project reassess this picture by performing three-dimensional numerical simulations that include, for the first time, all three non-ideal MHD effects. The main finding of this project is that Hall effect can generically "revive" dead zones by producing a dominant azimuthal magnetic field and a large-scale Maxwell stress throughout the midplane. We have moreover demonstrated the possibility of generating self-organising structures such as large scale zonal flows, which possibly have observable counterparts like ring-like structures.
These results have dramatic consequences on our understanding of protoplanetary disc dynamics and planet formation process. In particular, they provide a natural framework explaining the recent observations of ring like structures in protoplanetary discs, without needing planets.