Plasma Physics Seminar
Monday, October 28
1610 Engineering Hall
Speaker: Dr. Geoffroy Lesur, Centre National de la Recherche Scientifique, Institute of Planetology and Astrophysics of Grenoble
"Non-Ideal MHD Effects in Protoplanetary Discs"
Abstract: The magnetorotational instability (MRI) is the most promising mechanism by which angular momentum could be efficiently transported outwards in astrophysical discs. However, its application to protoplanetary discs remains problematic. These discs are so weakly ionised that they may not support magnetorotational turbulence in regions referred to as `dead zones'. The plasma dynamics inside these dead zones is highly sensitive "non ideal" MHD effects such Ohmic resistivity, Hall effect and ambipolar diffusion. Despite being largely dominant in protoplanetary discs, Hall and ambipolar diffusions have been largely ignored in numerical studies of the MRI until recently.
In this seminar, I will present several key results on how these effects are likely to modify the behaviour of the gas in protoplanetary discs. After presenting the main physical properties of astrophysical discs, I will revisit the MRI problem including Hall and ambipolar diffusion. I will then introduce the first local, three-dimensional, resistive Hall-MHD simulations of the MRI in situations where the Hall effect dominates over Ohmic dissipation. Confirming linear stability analysis, I will demonstrate that the Hall effect allows the MRI to exist in plasmas with very low ionisation fractions. However, instead of vigorous and sustained magnetorotational turbulence, I will show that the MRI saturates quiescently by producing large-scale, long-lived, axisymmetric structures in the magnetic and velocity fields. Implications for protoplanetary disc structure and evolution, as well as for theories of planet formation, will be briefly surmised.