Anisotropic electron distributions are often found in the earth’s magnetotail, in the energy range from about 10eV to 10keV [1-3]. Theoretical models are proposed to account for these distributions, based on parallel (Fermi) and perpendicular (betatron) electron heating due to deformation of the magnetic field lines [1]. Recently, Wang et al. (2014) made a statistical analysis of the variation of the electron pitch-angle anisotropies during geomagnetic dipolarization events using THEMIS data [3]. They found that, after the dipolarization, pancake type (perp temperature > parallel) anisotropy increases when the electron energy E < ~1keV, while cigar type (perp < parallel) anisotropy dominates for E>~1keV. When typical parameters are used for the magnetic field and the electron energy, the first adiabatic invariant is always conserved while the second invariant (action associated with the mirroring motion) can be violated depending on the electron energy. When both the first and the second invariants are conserved, types of anisotropy produced by compression of the field depend only on the shape of the magnetic field configuration. If the field remains dipolar, pancake type will be produced. If the magnetic field lines are stretched like those in the magnetotail, cigar-type anisotropy will be produced, since compression (earthward motion) of the field reduces the field line length without much changing the field strength. In this presentation we show our results of test particle simulations to discuss what types of pitch-angle anisotropies would emerge for various field line configurations. In particular, we show that the observed dependence of the types of anisotropies on the electron energy can roughly be explained in the present model.
