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We investigate the long-term evolution of black hole accretion disks formedin neutron star mergers. These disks expel matter that contributes to an$r$-process kilonova, and can produce relativistic jets powering shortgamma-ray bursts. Here we report the results of a three-dimensional,general-relativistic magnetohydrodynamic (GRMHD) simulation of such a diskwhich is evolved for long enough ($\sim 9$s, or $\sim 6\times 10^5 r_{\rmg}/c$) to achieve completion of mass ejection far from the disk. Our modelstarts with a poloidal field, and fully resolves the most unstable mode of themagnetorotational instability. We parameterize the dominant microphysics andneutrino cooling effects, and compare with axisymmetric hydrodynamic modelswith shear viscosity. The GRMHD model ejects mass in two ways: a promptMHD-mediated outflow and a late-time, thermally-driven wind once the diskbecomes advective. The total amount of unbound mass ejected ($0.013M_\odot$, or$\simeq 40\%$ of the initial torus mass) is twice as much as in hydrodynamicmodels, with higher average velocity ($0.1c$) and a broad electron fractiondistribution with a lower average value ($0.16$). Scaling the ejected fractionsto a disk mass of $\sim 0.1M_\odot$ can account for the red kilonova fromGW170817 but underpredicts the blue component. About $\sim 10^{-3}M_\odot$ ofmaterial should undergo neutron freezout and could produce a bright kilonovaprecursor in the first few hours after the merger. With our idealized initialmagnetic field configuration, we obtain a robust jet and sufficient ejecta withLorentz factor $\sim 1-10$ to (over)produce the non-thermal emission fromGW1708107.

Long-term GRMHD Simulations of Neutron Star Merger Accretion Disks: Implications for Electromagnetic Counterparts