Magnetorotational Instability in electron-ion plasma: Shearing-box simulations

Accretion disks are widely appearing in the universe around massive compact objects such

as black holes (BHs), and often consist of relativistically hot, radiative electron-ion plasma

in a turbulent state. Global general relativistic magnetohydrodynamics (GRMHD) has been

widely and successfully employed to study the global structure of accretion disks, as well

as to interpret recent observational data obtained by Event Horizon Telescope

(EHT) collaboration [1]. However, it is commonly accepted that low-luminosity targets of

EHT are essentially collisionless, with the lack of Coulomb interactions between ions and

electrons potentially leading to a two-temperature state of the plasma. The interpretation of

observations via GRMHD simulations, which is dependent on the temperature ratio

between ions and electrons determined by kinetic physics, is therefore fundamentally

limited [2]. It is therefore critical to more accurately determine the ion-to-electron heating

ratio. To improve current existing heating ratio prescriptions and account for a

collisionless regime, the kinetic particle-in-cell (PIC) approach is essential to model

plasma around BHs. To keep first-principles simulations computationally accessible, we

propose a novel shearing-box [2,3] approach, in which only a small region within the

accretion disk is modelled. The primary driver for turbulence in this case is the

magnetorotational instability (MRI) [4], which significantly amplifies magnetic fields and

injects energy at large scales, which can cascade and energize particles. Using this

approach, we aim to provide a more accurate prescription for ion-electron heating ratios

for more precise global simulations in future.