Data from the synchronous altitude satellite ATS 1 and near-conjugate measurements of bremsstrahlung X rays and ground magnetic variations have been used to analyze an event of modulated auroral zone electron precipitation and magnetic pulsations in the Pc 3 range. Transverse, azimuthal, nearly linearly polarized waves, strongly peaked at ~25-s period were observed at ATS 1 from 0600 to 1000 LT on August 18, 1967, diminishing in intensity thereafter. Weak pulsations with periods ≲18 s and wave properties similar to the above followed in the interval from 1300 to 1700 LT. Ground magnetic pulsations at College, Alaska, were observed mainly in the transverse H and D components with maximum power spectral density in the period range 25--40 s prior to 1300 LT and from 20 to 30 s thererafter. Four main intervals of modulated electron precipitation, centered roughly on local magnetic noon, were noted in the X ray data obtained near Fort Yukon, Alaska. The intervals, each lasting for ~30 min, were separated by ~90 min. Fluctuations of the 50- to 150-keV trapped electrons at ATS 1 during the precipitation event remained typically within 1 or 2&sgr; of the average counting rates and showed no clear association with pulsations in either the X ray or the magnetic data. The origin of the Pc 3 waves is attributed to local field line resonances induced by Kelvin-Helmholtz instability at the magnetopause. The observed wave periods can be accounted for by a field line distribution of plasma density of the form r-4 with equatorial values at r=6.6Re of order 1 cm-3. The wave resonance model can satisfactorily explain observed differences in the pulsation activity at the ground, balloon, and satellite if account is taken of the spatial sensitivities of the different techniques and the location of observing sites with respect to the probable location of resonant field lines. Although the event pertains to a disturbed period, there is insufficient evidence to associate gross temporal changes in the intensity of pulsation activity with the occurrence of specific substorms. However, the data suggest that electron precipitation pulsations will be found to correlate with Pc 3 magnetic pulsations when substorm injections coupled with azimuthal drift provide enhanced energetic particle fluxes within dayside resonance regions.