We have performed a detailed study of the temporal development of field-aligned anisotropies and density gradient anisotropies during several diffuse ion events observed upstream of the earth's bow shock. A clear anticorrelation between the field-aligned anisotropy and the particle intensity is found; in the solar wind frame this anisotropy, which is directed away from the bow shock, decreases as the particle flux increases. Furthermore, the field-aligned anisotropy increases with increasing particle energy. In the strong scattering limit the one-dimensional diffusion-convection formalism predicts a decrease of the anisotropy with increasing energy. It is concluded that upstream escape is significant for higher energy particles and leads to the deviation from the prediction of the diffusion-convection formalism. Quantitatively, the diffusion-convection formalism predicts a field-aligned anisotropy which is larger than measured for the lowest energy particles (~30 keV/e). Along with the variation of the field-aligned anisotropy, the appearance of a density gradient anisotropy was also observed. The observations are discussed in terms of a first-order Fermi model for the upstream particles and in terms of a magnetospheric origin. It is concluded that field-aligned anisotropy measurements obtained upstream of the bow shock cannot help to differentiate between the two possible models for the upstream population.