Magnetic field and particle flux observations on board ATS 6 at synchronous altitude are used to examine the dawn-dusk asymmetry of characteristics of Pc 4--5 waves and associated particle flux modulation. Most waves at synchronous orbit having ground correlations are polarized in the azimuthal direction (A class) and are usually detected in the dawn sector. Waves with a radially oriented polarization ellipse (R-class) are almost never observed near the subsatellite point on the ground, except for the regular pulsations known as giant pulsation Pg, observed in the early morning. R class Pc 4 waves occur at all local times and have an occurrence peak in the afternoon.

It is found that A class Pc 4--5 waves are observed during moderately disturbed conditions, associated with a solar wind velocity of about 600 km/s or more. R class Pc 4 waves tend to predominate in the afternoon sector under quiet magnetic condition. The modulation of proton fluxes in the energy range 25--500 keV depends on the mode of the associated magnetic waves. The phase relationship between magnetic field and proton flux oscillations is simpler for the A-class waves as compared with the R class waves. The oscillation of the azimuthal component is out of phase with proton flux oscillations in the energy below about 120 keV. Proton flux modulation is larger at pitch angle near 90¿ than it is at smaller angles. In contrast, the phase relationship is not so simple for R class Pc 4 waves which are the dominant waves in the afternoon sector. For these waves the phase depends on both energy and pitch angle. Two thirds of the R class Pc 4 events examined show a large modulation at small pitch angle of 30¿--40¿.

A large-amplitude pulsation of R class, storm-time PC 5, observed in the afternoon is associated with proton flux oscillations around 90¿ pitch angle. For these Pc 5 waves the proton flux variations are out of phase with the compressional component of magnetic field, and their phase is almost independent of energy. Implications of the observed dawn-dusk asymmetry in characteristics of the azimuthal and radial wave modes and their associated proton flux modulations are discussed. ¿ American Geophysical Union 1989