Anisotropy observation of diffuse ions (>30 keV/e) upstream of the Earth's bow shock

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.