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Transport of magnetic flux in astrophysical accretion discs

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If you have a question about this talk, please contact Dr William B├ęthune.

The evolution of a large-scale poloidal magnetic field is in an accretion disc is an important problem because it determines the launching of winds and the feasibility of the magnetorotational instability to generate turbulence or channel flows. Recent studies, both semi-analytic and numerical simulations, have highlighted the crucial role non-ideal MHD effects (Ohmic resistivity, Hall drift and ambipolar diffusion), relevant in the protoplanetary disc context, might play in magnetic flux evolution in the disc. In some cases these magnetic effects led to the formation of large scale structures (rings and gaps), which may be relevant for planet formation theory. We investigated the flux transport in discs through one-dimensional semi-analytic models in the vertical direction, exploring regimes where different physical effects dominate. Flux transport rates and vertical structure profiles are calculated for a range of diffusivities and disc magnetisations. We found similar results to previous studies in how Ohmic and ambipolar diffusivities drive radially outward flux transport with an inclined field, while a wind would drive inward transport. The Hall effect offers a correctional contribution to the flux transport given a background Ohmic and/or ambipolar diffusivity, and drives no flux transport when it is the only non-ideal effect present. We report the surprise finding of a non-zero laminar $\alpha$ in the vertical structures of our models in the absence of wind and viscosity, suggesting that diffusivities have a role in disc accretion as well. Future plans for further semi-analytic work and shearing box simulations will be presented.

This talk is part of the DAMTP Astro Lunch series.

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