The pitch angle distributions (PADs) of electrons have shown variations due to the changes in field configurations during dipolarization via satellite observations at Earth’s tailside. A specific variation of energy-dependent pattern for the associated field-aligned anisotropies η was also often found from tens of eV to hundreds of eV or from hundreds of eV to thousands of eV during dipolarization by measurements of the instrument ESA (ElectroStatic Analyzer) for < 30keV electrons onboard of THEMIS Mission. In the beginning of dipolarization, a distinct peak of η occurred at a lower energy level; during dipolarization, a peak with smaller value occurred at a higher energy level. Two representative events at 10 Earth radii will be presented. One event showed an increase in differential energy fluxes above ~ 2keV while the other event showed a decrease in these fluxes above a similar energy level. Fermi acceleration of electrons driven by electric fields coupled to curvature drifts is examined by adopting the values of observed electric fields and magnetic fields. Interestingly, deceleration of electrons is found possible to occur through inputting the real observed values of electric/magnetic fields, though this mechanism was used to theoretically explain electrons accelerated to high energies at the presence of a dawn-to-dusk electric field. Therefore, this Fermi acceleration/deceleration is found to yield larger η at higher energy levels closer to observed range as well as produce the observed increase/decrease in changes of differential energy flux above ~ 2keV. This can be considered as the scientific benefit from observations besides theoretical works.
