Electron Heating in the Magnetorotational Instability: Implications for Turbulence Strength in Outer Regions of Protoplanetary Disks

TL;DR

This paper investigates how electron heating caused by the MRI influences ionization and turbulence in protoplanetary disks, revealing an extended region where MRI activity is suppressed, which impacts dust grain growth and disk turbulence levels.

Contribution

It introduces a nonlinear Ohm's law model incorporating electron heating to identify the e-heating zone, significantly larger than the traditional dead zone, affecting MRI turbulence and dust evolution.

Findings

The e-heating zone extends up to 80 AU in the minimum-mass solar nebula.

MRI turbulence in this zone has a lower saturation level.

Dust grains become highly negatively charged, hindering collisional growth.

Abstract

The magnetorotational instability (MRI) drives vigorous turbulence in a region of protoplanetary disks where the ionization fraction is sufficiently high. 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, thereby preventing further growth of the MRI. By using the nonlinear Ohm's law that takes into account electron heating, we investigate where in protoplanetary disks this negative feedback between the MRI and ionization chemistry becomes important. We find that the "e-heating zone," the region where the electron heating limits the saturation of the MRI, extends out up to 80 AU in the minimum-mass solar nebula with abundant submicron-sized grains. This region is considerably larger than the conventional dead zone whose radial extent is $\sim20$ AU in the same disk model. Scaling arguments show that the MRI turbulence in the e-heating zone should have a significantly lower saturation level. Submicron-sized grains in the e-heating zone are so negatively charged that their collisional growth is unlikely to occur. Our present model neglects ambipolar and Hall diffusion, but our estimate shows that ambipolar diffusion would also affect the MRI in the e-heating zone.