The first evidence is presented of associated magnetic field, electric field, and electron precipitation fluctuations from ground-based and balloon-based measurements. The magnetic field fluctuations are produced by magnetohydrodynamic waves of approximately 5 min period in the earth's magnetosphere. The electric, magnetic, and particle variations are observed to be in close phase relationship when the measurements are made near the resonance region of the magnetospheric waves. When the resonance region is not close to the location of the electric field observations no clear wave-associated electric field or particle variations are observed. The observed wave frequency is ~3 times lower than the fundamental Alfv¿n resonance frequency. This may arise from a Doppler shift of the resonant frequency by a diamagnetic plasma drift. The height-integrated ionospheric Pedersen conductivity deduced from the simultaneous electric and magnetic field variations corresponds to approximately 30 mho.