Data used in an earlier paper [Ver¿ and Holl¿, 1978> are here reexamined to clarify problems which emerged concerning the previously found relationship between the interplanetary magnetic field and Pc3 geomagnetic pulsations. We show that Pc3 pulsation spectra are strongly affected by wave amplification in the vicinity of the local field line resonance period. Possibilities are presented for the suppression of this effect. The influence of the variability of the interplanetary magnetic field is analyzed, and it is shown that this is a major factor determining momentary pulsation activity. The possibility of exciting field line resonances by impulses imbedded into noise of the interplanetary magnetic field is considered. ¿ 1999 American Geophysical Union