Characteristics of the magnetospheric boundary layer and magnetopause layer as observed by Imp 6

Imp 6 observations of the low‐latitude magnetospheric boundary layer indicate that the plasma within it is supplied primarily by direct entry of magnetosheath plasma across the magnetopause layer. We define the magnetopause layer as the current layer (separating the magnetosheath from the boundary layer) through which the magnetic field shifts in direction. High temporal resolution (3‐s average) data reveal that in a majority of Imp 6 magnetopause crossings, no distinct changes in electron density or energy spectra are observed at the magnetopause layer. In all Imp 6 crossings, some magnetosheathlike plasma is observed earthward of the magnetopause layer, implying the existence of a boundary layer. Boundary layer electron energy spectra are often virtually indistinguishable from the adjacent magnetosheath spectra. Low‐latitude boundary layer bulk plasma flow as observed by Imp 6 almost always has an antisunward component and often has a significant cross‐field component. The boundary layer thickness is highly variable and is generally much larger than the magnetopause layer thickness. Energetic electron pitch angle distributions indicate that the low‐latitude boundary layer is normally on closed field lines. We conclude that diffusive as well as nondiffusive processes probably contribute to the entry of magnetosheath plasma into the boundary layer.