Pc3-4 ULF waves observed by Cluster prenoon in outer magnetosphere

Pc3-4 pulsations generally result from field line resonance in the inner magnetosphere or guided Alfven waves in the outer magnetosphere. Regular Pc3-4 pulsations were observed over 0700-1100UT on 13 April 2004 by the Cluster constellation of four satellites on an outbound leg from L~5.9 to L~11.1 near 10 MLT in the high latitude cusp region. The waves were mainly transverse with toroidal and poloidal components, and frequency decreasing with increasing L-value, from ~25 mHz to ~10mHz. The magnitude of toroidal mode waves significantly increased with L-value, reaching a maximum close to the magnetopause. The poloidal mode did not show the same increase. The waves propagated earthward and westward, perpendicular to the geomagnetic field direction. Furthermore, the sense of polarization was variable, clearly changed from left-hand to right-hand at a crossing point identified by the interval where the main wave packet occurred near the magnetopause. The Poynting flux was in range of 0.04~0.07mW/m² nearly perpendicular to the magnetic field vector. The wave properties were also correlated with the solar wind parameters. We conclude that the Pc3-4 waves observed adjacent to the magnetopause are field line resonances, driven by the upstream fast waves.