The AMPTE CCE spacecraft observed a transverse Pc 5 magnetic pulsation (period ~200 s) at 2155-2310 UT on November 20 (day 324), 1985, at a radial distance of 5.7--7.0 RE, at a magnetic latitude of 1.2¿--1.9¿, and near 1300 magnetic local time. The magnetic field perturbation was observed primarily in the radial component with an amplitude of 15 nT peak to peak. Ion fluxes (energy >50 keV) measured by the medium energy particle analyzer (MEPA) on board CCE were also observed to oscillate at the frequency of the magnetic pulsation. The wide range of energy and pitch angle of ions covered by the MEPA allowed us to study the ion flux oscillations in great detail. It is found that (1) regardless of energy the oscillation amplitude tends to maximize near the field-aligned directions while it is essentially zero at 90¿ pitch angle, (2) for a given energy and the given location (east or west) of ion guiding centers flux oscillations at pitch angle &agr; and at its conjugate, 180¿-&agr;, are 180¿ out of phase, (3) for a given look direction, the oscillation phase changes with energy, and (4) for a given pitch angle and energy, the eastside flux oscillation leads to the westside flux oscillation. These observations can be explained by the adiabatic theory of ion flux pulsations with finite Larmor radius effects included (Southwood and Kivelson, 1981; Kivelson and Southwood, 1983), if we assume an antisymmetric standing wave on the field line, westward propagation of the wave, and a large azimuthal wave number ‖m‖~110. These properties of the wave are consistent with a second-harmonic standing Alfv¿n wave excited in the region where the ring current ions have an inward density gradient. ¿ American Geophysical Union 1990