The effects of geomagnetic tail fields on drift mirror instability are studied for a magnetic field model that contains a magnetic gradient and a tail field perpendicular to an ambient field. The magnetospheric plasma is assumed to consist of a cold component and a high &bgr; anisotropic inhomogeneous component. The gyrokentic equations are used to derive the dispersion equation including the effects of magnetic gradient and curvature drifts. The dispersion equation is solved locally for wave frequencies less than the ion gyrofrequency. The numerical results indicate that the instability growth rate decreases when the tail field strength increases. The drift mirror instability is found to be stabilized when larger tail fields require a smaller pressure anisotropy and a smaller plasma &bgr;⊥ to maintain equilibrium. The theoretical results are used to explain the GOES satellite observations that storm time Pc 5 waves occur less frequently during periods of large tail fields.