Structure of the plasmapause from ISEE 1 low‐energy ion and plasma wave observations

Low‐energy ion pitch angle distributions are compared with plasma density profiles in the near‐earth magnetosphere using ISEE 1 observations. The classical plasmapause determined by the sharp density gradient is not always observed in the dayside region, whereas there almost always exists the ion pitch angle distribution transition from cold, isotropic to warm, bidirectional, field‐aligned distributions. In the nightside region the plasmapause density gradient is typically found, and it normally coincides with the ion pitch angle distribution transition. The sunward motion of the plasma is found in the outer part of the “plasmaspheric” plasma in the dusk bulge region.