An interval of long-period hydromagnetic wave activity occurring in the decay phase of a magnetic storn has been studied using ground-based magnetic field and cosmic noise absorption data recorded at L≲4.2. During an ~90-min interval of decreasing ring current following the storm maximum, a hydromagnetic pulsation train in the Pc 5--6 range emerged (frequencies decreasing monotonically from ~3 mHz to ~1 mHz), accompanied by similar oscillations of the cosmic radio noise signal. There is some evidence in the data for the wave frequencies to be ~0.2 mHz lower at L~3.0--3.2 than at L~3.8--4.2. The low wave frequencies are inconsistent with resonant oscillations of the field lines within the L shell range and probable magnetosphere plasma density conditions of observation. The waves were observed to propagate westward over at least 3 hours of local time, near dawn. An azimuthal wave number m≈9 and a westward phase velocity varying from ~2.5 to ~7.5 km s-1 at 55¿ magnetic latitude characterized the event. The low azimuthal phase velocities make it unlikely that the waves were generated by the Kelvin-Helmholtz instability on the magnetopause. These Pc 5--6 waves appear to be odd mode, locally driven nonresonant oscillations. The ULF wave generation and propagation were most likely located on geomagnetic field lines in the range 3.5≲L≲6 in the region of hot storm time plasma outside the plasmapause where a gradient in particle precipitation flux was also observed. Neither the drift mirror instability nor the compressional drift wave instability appears to be a likely source for these waves. Modulation by the ULF waves of the pitch angle diffusion of energetic electrons injected into the outer radiation zone during the magnetospheric storm is suggested to account for the cosmic noise absorption pulsations observed outside the plasmapause. Irregular plasma density gradients in this region, inferred from the propagation characteristics of whistler mode VLF signals, may have caused the observed H-D plane wave polarization variations. ¿ American Geophysical Union 1988