Magnetospheric filter effect for Pc 3 Alfvén mode waves

We present a ray‐tracing study of the propagation of Pc 3 Alfvén mode waves originating at the dayside magnetopause. This study reveals interesting features of a magnetospheric filter effect for these waves. Pc 3 Alfvén mode waves cannot penetrate to low Earth altitudes unless the wave frequency is below approximately 30 mHz. Configurations of the dispersion curves and the refractive index show that the gyroresonance and pseudo‐cutoff introduced by the heavy ion O + block the waves. When the O + concentration is removed from the plasma composition, the barriers caused by the O + no longer exist, and waves with much higher frequencies than 30 mHz can penetrate to low altitudes. The result that the 30‐mHz or lower frequency Alfvén waves can be guided to low altitudes agrees with ground‐based power spectrum observations at high latitudes.