Review of Thanatology in Protoplanetary Discs:
The combined influence of Ohmic, Hall, and ambipolar diffusion on dead zones (Lesur, Kunz & Fromang 2014)
Introduction
Protoplanetary discs are cool and dense, their gas is composed of neutrals, ions and electron, and in the presence of a background magnetic field this becomes magnetised plasma. Due to the fact that this is normally poorly ionised, the plasma in protoplanetary discs, is therefore subject to three non-ideal magnetohydrodynamic (MHD) effects; Ohmic dissipation, ambipolar diffusion and Hall effect. Following the discovery of the magnetorotional instability (MRI) (Balbus & Hawley 1991,1998) the possibility of turbulence magnetically driven in protoplanetary disc as a process to transport angular momentum has been an important question to answer. Ohmic dissipation is caused by collisions between the electrons and neutrals, ambipolar diffusion is due to the collisions between electrons and neutrals and the Hall effect by the velocity difference between the positively and negatively charged species. All of these act to decouple the gas from the magnetic field and therefore reduced the effect MRI. To properly understand the effect MRI has in formation and evolution of protoplanetary it is important to understand the non-ideal MHD effects in the plasma and the effect on MRI, this is the aim of the paper discussed here.
The paper discussed in this essay focuses on the combined influence of the Ohmic, Hall and ambipolar diffusion on the magnetically “dead zones” in the midplane of the disc, this is the first time that all three of these non-ideal MHD effects have been accounted in a numerical simulation of the MRI. The “dead zone” is the region where the disc is not su...
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...the Hall effect and its ability to generate an azimuthal field in the “dead” zone. This leads to important consequences for a description of angular momentum transfer in protoplanetary discs and ultimately the formation of planetesimals and planets as now all three of the non-ideal MHD effects must be continued in future work to ensure accurate results because of the new finding of the this Hall generated azimuthal field and effects this has on the midplane of the disc.
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