  The magnetic field associated with neutron stars is generally believed to be
threaded inside the star. In the presence of a magnetic field, the plasma
present in the interior of the star goes through several processes that lead to
magnetic field evolution. It is thought that magnetar activities are mainly due
to field decay. The most important process of field decay inside the core of
the star is the ambipolar diffusion of the charged particles present in the
interior plasma. The decay rate due to ambipolar diffusion is directly
connected to the ambipolar velocity of the charged particles under the
influence of the present magnetic field. The ambipolar velocity of the charged
particles depends on the internal dynamics of the particles. We outline a
general method to solve the particle dynamics in the presence of a magnetic
field using a magnetohydrodynamic equation for ambipolar velocity. The equation
is general and applies to all possible surrounding conditions \eg temperature,
and matter states like normal or superfluid.
