MAGNETOSPHERIC FILTER EFFECT FOR PC-3 ALFVEN MODE WAVES

We present a ray-tracing study of the propagation of Pc 3 Alfven mode waves originating at the dayside magnetopause, This study reveals inte resting features of a magnetospheric filter effect for these waves. Pc 3 Alfven mode waves cannot penetrate to low Earth altitudes unless th e 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. Whe n the O+ concentration is removed from the plasma composition, the bar riers caused by the O+ no longer exist, and waves with much higher fre quencies than 30 mHz can penetrate to low altitudes. The result that t he 30-mHz or lower frequency Alfven waves can be guided to low altitud es agrees with ground-based power spectrum observations at high latitu des.