The stability of a multispecies plasma under magnetospheric storm conditions is investigated. The dynamics of the impurity ions are incorporated in the hot plasma component response, since the response (to perturbations) of the cold component is totally dominated by the electrons. The coupling between drift compressional and shear Alfven modes is studied in weak and strong coupling limits, the coupling being weakened when a finite fraction of the hot ions belongs to the heavy species. The characteristic growth rates of the mirror instablity, modified by the presence of heavy impurity ions, are determined. For the case of strong coupling, the dominant unstable mode is a compressional Alfven oscillation, driven by the finite density gradient of the plasma. The main effect of the impurity ions, in this case, is to stabilize the mode through their inertia. The instability is found to be self-limiting with increasing plasma pressure, and the asymptotic normalized growth rate is determined.