Compression-Driven Kinetic Instabilities in Magnetically Arrested Disks

Abstract

Event horizon-scale observations of low-luminosity black hole accretion flows favor magnetically arrested disks, characterized by dynamically important magnetic fields ($\beta\lesssim1$, where $\beta$ is the ratio of plasma thermal pressure to magnetic pressure) and a two-temperature transrelativistic plasma. Motivated by plasma conditions in the synchrotron-emitting regions of these models, we perform 2D particle-in-cell simulations of electron-ion plasmas with a realistic mass ratio, subject to continuous compression perpendicular to the mean magnetic field $\boldsymbol{B}_0$. Conservation of particle magnetic moments drives pressure anisotropy $P_{\perp}>P_{\parallel}$, triggering anisotropy-driven instabilities. For ion plasma beta $\beta_{i0}=0.5$ and ion temperature $k_{\text{B}}T_{i0}/m_i c^2=0.05$, the ion pressure anisotropy is regulated by the ion cyclotron instability, while the mirror mode influences the late-time electron anisotropy. Both species develop nonthermal components at high energies, consistent with stochastic acceleration by cyclotron-scale fluctuations. We characterize how the onset and time evolution of the plasma instabilities, as well as the resulting ion and electron anisotropies and energy spectra, vary with $\beta_{i0}$, $k_{\text{B}}T_{i0}/m_i c^2$, electron-to-ion temperature ratio $T_{e0}/T_{i0}$, and the compression rate. Increasing the thermal energy toward relativistic values raises the anisotropy thresholds for all instabilities observed in our simulations, allowing larger anisotropies to develop. For $T_{e0}/T_{i0}<1$, as expected in collisionless two-temperature accretion flows, the growth of mirror and whistler instabilities is delayed or suppressed, leading to increasingly adiabatic evolution of the electrons. Our findings can be used to inform global fluid models of black hole accretion.