Fully kinetic shearing-box simulations of magnetorotational turbulence in 2D and 3D. I. Pair plasmas

TL;DR

This paper uses fully kinetic simulations to explore how the magnetorotational instability drives turbulence and particle acceleration in collisionless pair plasmas, revealing differences from traditional MHD models and advancing understanding of accretion disk physics.

Contribution

It introduces the first large-scale 3D fully kinetic PIC simulations of MRI turbulence, demonstrating the feasibility of capturing mesoscale dynamics beyond MHD limitations.

Findings

2D simulations show significant differences from MHD expectations.

3D PIC simulations can reproduce mesoscale MRI dynamics.

The study reveals nonthermal particle acceleration and angular-momentum transport mechanisms.

Abstract

The magnetorotational instability (MRI) is a fundamental mechanism determining the macroscopic dynamics of astrophysical accretion disks. In collisionless accretion flows around supermassive black holes, MRI-driven plasma turbulence cascading to microscopic (i.e. kinetic) scales can result in enhanced angular-momentum transport and redistribution, nonthermal particle acceleration, and a two-temperature state where electrons and ions are heated unequally. However, this microscopic physics cannot be captured with standard magnetohydrodynamic (MHD) approaches typically employed to study the MRI. In this work, we explore the nonlinear development of MRI turbulence in a pair plasma, employing fully kinetic Particle-in-Cell (PIC) simulations in two and three dimensions. First, we thoroughly study the axisymmetric MRI with 2D simulations, explaining how and why the 2D geometry produces results that differ substantially from MHD expectations. We then perform the largest (to date) 3D simulations, for which we employ a novel shearing-box approach, demonstrating that 3D PIC models can reproduce the mesoscale (i.e. MHD) MRI dynamics in sufficiently large runs. With our fully kinetic simulations, we are able to describe the nonthermal particle acceleration and angular-momentum transport driven by the collisionless MRI. Since these microscopic processes ultimately lead to the emission of potentially measurable radiation in accreting plasmas, our work is of prime importance to understand current and future observations from first principles, beyond the limitations imposed by fluid (MHD) models. While in this first study we focus on pair plasmas for simplicity, our results represent an essential step toward designing more realistic electron-ion simulations, on which we will focus in future work.