Particle measurements from the University of Minnesota detector aboard the geostationary ATS 6 satellite reveal striking equatorial pitch angle anisotropies. A study of 7 days of data shows a diurnal variation in anisotropy with J(&agr;=40¿) >J(&agr;=90¿) during times of low ‖B‖ on the nightside and J(&agr;=90¿) >J(&agr;=40¿) during times of high ‖B‖ on the dayside. Six representative anisotropy events are studied in finer detail. The 32- to 51-keV electron anisotropies increase and decrease with the total magnetic field intensity. The proton and higher energy electron anisotropies do not show as much variation. The particle anisotropies are studied in light of adiabatic and cyclotron resonance theory; the results indicate that adiabatic effects are the dominant modulation mechanism of particle pitch angle distributions in the outer radiation belt.