  The fully kinetic simulations of magnetic nozzle acceleration are conducted
to investigate the axial momentum gains of ions and electrons with the
electrostatic and Lorentz forces. Axial momentum gains per ion and electron are
directly calculated from the kinetics of charged particles, indicating that
electrons in the magnetic nozzle obtain the net axial momentum by the Lorentz
force even though they are decelerated by the electrostatic force. Whereas ions
are also accelerated by the electrostatic force, the axial momentum gain of
electrons increases significantly with increasing the magnetic field strength
and becomes dominant in the magnetic nozzle. In addition, it is clearly shown
that the axial momentum gain of electrons is due to the electron momentum
conversion from the radial to axial direction, resulting in the significant
increase in the thrust and the exhaust velocity.
