Suppression of magnetic turbulence by electron heating in protoplanetary disks
Turbulence in protoplanetary disks hinders planet formation by preventing dust settling and by inducing collisional disruption of solid particles. One mechanism of generating disk turbulence is the magnetorotational instability (MRI). The MRI needs sufficient ionization fraction to grow and to generate vigorous magnetic turbulence. It has recently been shown that the electric field induced by the MRI can heat up electrons and thereby affect the ionization balance in the gas. In particular, in a disk where abundant dust grains are present, the electron heating causes a reduction of the electron abundance. Thus, the electron heating might quench the magnetic turbulence (Mori & Okuzumi 2016).
To examine this possibility, we perform magnetohydrodynamical simulations in which the effect of electron heating on the resistivity is mimicked by a simple analytic model. Our simulations confirm that electron heating suppresses magnetic turbulence. When the effect of electron heating is significant, turbulence completely dies away, leaving a steady laminar flow where the accretion stress is dominated by ordered magnetic fields. Based on the simulation results and the scaling relation between the Maxwell stress and current density, we obtain an analytic formula that successfully predicts the accretion stress in the presence of electron heating. Finally, we will show the turbulence strength distribution and discuss the planet formation theory that electron heating would affect.