  The dynamics of magnetic field decay with Hall drift is investigated.
Assuming that axisymmetric magnetic fields are located in a spherical crust
with uniform conductivity and electron number density, long-term evolution is
calculated up to Ohmic dissipation. The nonlinear coupling between poloidal and
toroidal components is explored in terms of their energies and helicity.
Nonlinear oscillation by the drift in strongly magnetized regimes is clear only
around the equipartition between two components. Significant energy is
transferred to the poloidal component when the toroidal component initially
dominates. However, the reverse is not true. Once the toroidal field is less
dominant, it quickly decouples due to a larger damping rate. The polar field at
the surface is highly distorted from the initial dipole during the Hall drift
timescale, but returns to the initial dipole in a longer dissipation timescale,
since it is the least damped one.
