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The magnetorotational instability (MRI) plays a crucial role in regulatingthe accretion efficiency in astrophysical accretion disks. In low-luminositydisks around black holes, such as Sgr A* and M87, Coulomb collisions areinfrequent, making the MRI physics effectively collisionless. The collisionlessMRI gives rise to kinetic plasma effects that can potentially affect itsdynamic and thermodynamic properties. We present 2D and 3D particle-in-cell(PIC) plasma simulations of the collisionless MRI in stratified disks usingshearing boxes with net vertical field. We use pair plasmas, with initial$\beta=100$ and concentrate on sub-relativistic plasma temperatures ($k_BT\lesssim mc^2$). Our 2D and 3D runs show disk expansion, particle and magneticfield outflows, and a dynamo-like process. They also produce magnetic pressuredominated disks with (Maxwell stress dominated) viscosity parameter $\alpha\sim 0.5-1$. By the end of the simulations, the dynamo-like magnetic fieldtends to dominate the magnetic energy and the viscosity in the disks. Our 2Dand 3D runs produce fairly similar results, and are also consistent withprevious 3D MHD simulations. Our simulations also show nonthermal particleacceleration, approximately characterized by power-law tails with temperaturedependent spectral indices $-p$. For temperatures $k_BT \sim 0.05-0.3\, mc^2$,we find $p\approx 2.2-1.9$. The maximum accelerated particle energy depends onthe scale separation between MHD and Larmor-scale plasma phenomena in a wayconsistent with previous PIC results of magnetic reconnection-drivenacceleration. Our study constitutes a first step towards modeling from firstprinciples potentially observable stratified MRI effects in low-luminosityaccretion disks around black holes.

Particle-in-cell Simulations of the Magnetorotational Instability in Stratified Shearing Boxes