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Balbus \& Hawley (1991) identified a rapidly growing instability of weakly magnetized accretion disks, which seems a promising candidate for a fundamental source of turbulence in such systems. However, the detailed dependence of the instability growth rate on various physical quantities, as well as the critical value of the magnetic field strength above which stabilization occurs, $B_{z,crit}$, cannot be determined in their purely local theory. With this in mind, we present the results of a global (in the radial direction) linear stability analysis. Free boundary conditions are applied at the inner and outer fluid boundaries, and these are found to increase the instability growth rate (over that of rigid boundaries) in all cases. The growth rates of unstable global modes are always less than, but comparable to, the local growth rate. We find that $B_{z,crit}$ is large (the corresponding Alfv\'en speed is comparable to the sound speed) for realistic disk sizes, implying that the instability is always present. Finally, it is explicitly shown how the local limit is obtained in the global picture.
