Electron distributions in the ion foreshock

Measurements of the electron distribution function in the ion foreshock by AMPTE UKS display distributions which vary on a short timescale between anisotropic (T-parallel to > T-perpendicular to) and isotropic (T-parallel to approximate to T-perpendicular to). The anisotropy appears to change quasi-periodically in a way that correlates well with the changes in the magnetic field magnitude; high field regions matching with the more isotropic distributions. It is shown how these changes in anisotropy can be largely explained by the conservation of the magnetic moment of the electrons moving through different magnetic field regions. Consequently, the variations in the electron distribution functions may be mainly due to the adiabatic response of the electrons to the field variations with wave-particle scattering effects being less important.