  The ohmic decay of magnetic fields in the crusts of neutron stars is
generally believed to be governed by Hall drift which leads to what is known as
a Hall cascade. Here we show that helical and fractionally helical magnetic
fields undergo strong inverse cascading like in magnetohydrodynamics (MHD), but
the magnetic energy decays more slowly with time $t$: $\propto t^{-2/5}$
instead of $\propto t^{-2/3}$ in MHD. Even for a nonhelical magnetic field
there is a certain degree of inverse cascading for sufficiently strong magnetic
fields. The inertial range scaling with wavenumber $k$ is compatible with
earlier findings for the forced Hall cascade, i.e., proportional to $k^{-7/3}$,
but in the decaying cases, the subinertial range spectrum steepens to a novel
$k^5$ slope instead of the $k^4$ slope in MHD. The energy of the large-scale
magnetic field can increase quadratically in time through inverse cascading.
For helical fields, the energy dissipation is found to be inversely
proportional to the large-scale magnetic field and proportional to the fifth
power of the root-mean square (rms) magnetic field. For neutron star conditions
with an rms magnetic field of a few times $10^{14}\,$G, the large-scale
magnetic field might only be $10^{11}\,$G, while still producing magnetic
dissipation of $10^{33}\,$erg$\,$s$^{-1}$ for thousands of years, which could
manifest itself through X-ray emission. Finally, it is shown that the
conclusions from local unstratified models agree rather well with those from
stratified models with boundaries.
