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The magnetorotational instability (MRI) is a fundamental mechanismdetermining the macroscopic dynamics of astrophysical accretion disks. Incollisionless accretion flows around supermassive black holes, MRI-drivenplasma turbulence cascading to microscopic (i.e. kinetic) scales can result inenhanced angular-momentum transport and redistribution, nonthermal particleacceleration, and a two-temperature state where electrons and ions are heatedunequally. However, this microscopic physics cannot be captured with standardmagnetohydrodynamic (MHD) approaches typically employed to study the MRI. Inthis work, we explore the nonlinear development of MRI turbulence in a pairplasma, employing fully kinetic Particle-in-Cell (PIC) simulations in two andthree dimensions. First, we thoroughly study the axisymmetric MRI with 2Dsimulations, explaining how and why the 2D geometry produces results thatdiffer substantially from MHD expectations. We then perform the largest (todate) 3D simulations, for which we employ a novel shearing-box approach,demonstrating that 3D PIC models can reproduce the mesoscale (i.e. MHD) MRIdynamics in sufficiently large runs. With our fully kinetic simulations, we areable to describe the nonthermal particle acceleration and angular-momentumtransport driven by the collisionless MRI. Since these microscopic processesultimately lead to the emission of potentially measurable radiation inaccreting plasmas, our work is of prime importance to understand current andfuture observations from first principles, beyond the limitations imposed byfluid (MHD) models. While in this first study we focus on pair plasmas forsimplicity, our results represent an essential step toward designing morerealistic electron-ion simulations, on which we will focus in future work.

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