We have constructed a model of plasmaspheric composition and density in the region bounded by the 500-km altitude level and by the dipole field line L=5. We first discuss the collisionless kinetic theory model of quasi-neutral plasma distributions along a dipole field line under the gravitation field, the ambipolar diffusion electric field and the inhomogeneous magnetic field. A simpler fluid-equation model of the plasma distribution under the influence of the above force fields is shown to be able to incorporate effects of plasmasphere thermal structure. With O+, H+, and He+ densities at 500 km as input boundary values and with a phenomenological thermal structure of the plasmasphere abstracted from Ogo 5, Prognoz, and S3-3 satellite data, we show that the plasma density density distributions in the fluid-equation model are consistent with the densities derived from in situ satellite measurements and from whistler observations, both in the topside ionosphere and in the outer plasmasphere.