Particle and magnetic field data from the ATS 6 spacecraft have been combined to study a Pc 4 pulsation event. The event occurred as the satellite passed the dawn meridian during the recovery phase of a moderate magnetic storm. Plasma drifts associated with the wave are determined from the measured modulation of low-energy protons. Using the observed ambient and perturbation magnetic fields in conjunction with the plasma drift velocity, we calculate the Poynting vector of the wave, assuming that the electric field is given by the frozen field approximation. The Poynting vector gives convincing evidence that the wave is a standing hydromagnetic wave along the magnetic field. In addition, it shows that the wave is propagating predominantly azimuthally from noon toward the dawn meridian. The phase relationships between the magnetic and plasma drift velocity oscillations are consistent with the occurrence of an odd harmonic of the standing wave. Comparison of observed particle fluxes with model predictions demonstrates that the fundamental mode is the most reasonable interpretation of the data. Ambient ion number densities, predicted by the wave data on the basis of a simple model of standing hydromagnetic waves, are found to be somewhat higher than observed values, on the basis of the assumption that the ambient plasma consists of protons and electrons only. Predicted and observed values of the ion number densities can be reconciled if it is assumed that a significant fraction of the ambient plasma consisted of He+ and/or O+. By using the fact that the wave is a standing wave the particle flow velocity is predicted from the observed magnetic variation and a model-dependent theory. These predictions are found to agree very well with the flow values determined from the data.