Re‐ordered electron data in the low‐latitude boundary layer

We have made a complete survey of all electron data recorded during low‐latitude boundary layer crossings of the AMPTE‐UKS spacecraft. In 27 of the 31 cases there is a clear anticorrelation between the density and the mean energy of the electrons during the transition across the boundary layer. The simplest interpretation of this anticorrelation is that, in the majority of cases, there is a smooth change of plasma state between the bounding solar and terrestrial plasmas, and that the abrupt changes frequently seen in time series data arise from boundary layer motions. Our data indicate that there are always two distinct sections to the anticorrelation. We suggest that these represent two physically distinct regions within the boundary layer and indicate how electron density and energy might vary across them. We briefly discuss an alternative interpretation in which filaments of magnetosheath plasma penetrate the boundary layer. We show that our results provide a constraint on such a model.