Ion flux modulations is ultralow-frequency radially polarized geomagnetic pulsations are examined theoretically based on the gyrokinetic analysis of Chen and Hasegawa (1991). The theoretical results thus contain important effects such as plasma anisotropy and inhomogeneities, finite Larmor radii, realistic magnetic field, magnetic trapping, and wave mode structures. The predicted properties are consistent with the satellite observations (Takahashi et al. , 1990) and further support the drift-Alfv¿n ballooning mode as a primary instability candidate. Our analysis, furthermore, demonstrates that, in the case of highly energetic ions, it is crucial to include the finite-Larmor-radius effects self-consistently in order to properly analyze and compare with the satellite observations. ¿ American Geophysical Union 1994