The tilt and offset of the Earth's magnetic field can significantly affect the longitudinal and seasonal distribution of electron density in the plasmasphere. Here we show that for the solar maximum conditions of 1990–1991, the largest annual variation determined from CRRES measurements of plasmaspheric equatorial electron density in the range L = 2.5–5.0 occurs at American longitudes (−60°E), while no annual variation occurs at Asian longitudes (+100°E). Plasmaspheric electron density is larger in December than in June at most longitudes, from −180°E eastward to +20°E. At all other longitudes the density ratio from December to June is very close to 1.0. The largest December/June density ratio is at L = 3.0 at American longitudes (−60°E). At L = 4.5 and above, the annual variation disappears. The lowest electron density values for a given L-shell occur at American longitudes, in June. Ion densities also show significant annual variations, with similar longitudinal and seasonal characteristics in the case of IMAGE EUV He⁺ measurements. Atomic mass density measurements calculated using the magnetometer cross-phase technique show significant seasonal variations but also imply composition changes with longitude. Using the quasilinear PADIE code we calculate the bounce-averaged diffusion rate of electrons by plasmaspheric hiss with a fixed wave intensity. December to June variations in plasmaspheric density, particularly at American longitudes, drive changes in the wave-particle interactions, increasing diffusion into the loss cone by a factor of ∼3 at 1 MeV at L = 3.0, thus hardening the electron precipitation spectrum during the southern hemisphere winter (in June).