The perturbed velocity ΔV and ΔB of mirror waves are usually coplanar with the wave vector k and background magnetic field B0. However, recent satellite observations and hybrid simulations show the presence of extra non-coplanar components in ΔV and ΔB associated with mirror waves. In this paper, we show from hybrid simulations that in the nonlinear stage of mirror instability, the noncoplanar perturbations in ΔV and ΔB of high wavenumber modes can be decoupled from mirror waves and generate a pair of kinetic Alfv¿n waves propagating in opposite direction. The Hall effect can explain the excitation of noncoplanar components in mirror waves. These kinetic Alfv¿n waves can be generated in the dayside magnetosheath, propagate tailward along the magnetic field lines draped around the magnetopause, and contribute to the plasma transport into magnetosphere. ¿ 2001 American Geophysical Unio