Discrete field line resonances and the Alfven continuum in the outer magnetosphere

Using Pc5 pulsation events observed with the IMAGE magnetometer array, we d emonstrate that mHz frequency field line resonances (FLRs) represent local enhancements in the background Alfven continuum of field line eigenfrequenc ies. By comparing resonance profiles for a typical event with the continuum frequency profile determined using cross-phase techniques, we show that pu lsation frequencies as low as 1-2 mHz can couple to FLRs on high latitude c losed field lines. We also suggest that the transition to open field lines on days of higher geomagnetic activity may lead to a breakdown in pulsation characteristics at the highest latitudes of the IMAGE array. In addition, we show evidence of the U-shaped diurnal variation in field line eigenfrequ ency and suggest that this is primarily due to field line stretching, espec ially on the magnetosphere flanks.