Using Pc5 pulsation events observed with the IMAGE magnetometer array, we demonstrate that mHz frequency field line resonances (FLRs) represent local enhancements in the background Alfv¿n continuum of field line eigenfrequencies. By comparing resonance profiles for a typical event with the continuum frequency profile determined using cross-phase techniques, we show that pulsation frequencies as low as 1--2 mHz can couple to FLRs on high latitude closed 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 eigenfrequency and suggest that this is primarily due to field line stretching, especially on the magnetosphere flanks. ¿ 1999 American Geophysical Union