Onset of Plasmoid Reconnection during Magnetorotational Instability

TL;DR

This paper investigates how magnetic reconnection and plasmoid formation occur in accretion disks undergoing magnetorotational instability, revealing conditions that lead to spontaneous reconnection and turbulence-driven current sheet instability.

Contribution

It demonstrates the onset of plasmoid reconnection in MRI-driven current sheets through 2D and 3D numerical simulations, highlighting the role of Lundquist number and non-axisymmetric modes.

Findings

Reconnection occurs when Lundquist number exceeds 300.

High Lundquist numbers lead to plasmoid instability in MRI current sheets.

3D turbulence enhances current sheet instability and magnetic island formation.

Abstract

The evolution of current sheets in accretion flows undergoing magnetorotational instability (MRI) is examined through two and three dimensional numerical modelling of the resistive MHD equations in global cylindrical geometry. With an initial uniform magnetic field aligned in the vertical ($z$) direction, MRI produces radially extended toroidal (azimuthal) current sheets. In both 2D and 3D when axisymmetric modes dominate, these current sheets attract each other and merge in the poloidal ($rz$) plane, driving magnetic reconnection when the Lundquist number $S > 3 \times 10^2$, making it a possible source of plasmoids (closed magnetic loops) in accretion disks. At high Lundquist numbers in the 2D regime, starting at $S = 5 \times 10^3$, self-consistent MRI-generated current sheets become thin and subject to plasmoid instability, and therefore spontaneous magnetic reconnection. When non-axisymmetric 3D modes dominate, turbulence makes the azimuthal current sheets further unstable, and stretch vertically. Toroidally extended vertical current sheets in the inner region, as well as larger 3D magnetic islands in the outer regions of the disks are also formed. These findings have strong ramifications for astrophysical disks as potential sources of plasmoids that could cause local heating, particle acceleration, and high energy EM radiation.