  Conservation of magnetic helicity by the Hall drift does not prevent Hall
instability of helical fields. This conclusion follows from stability analysis
of a force-free spatially-periodic Hall equilibrium. The growth rates of the
instability scale as $\sigma \propto B^{3/4}\eta^{1/4}$ with the field strength
$B$ and magnetic diffusivity $\eta$ and can be large compared to the rate of
resistive decay of the background field. The instability deviates the magnetic
field from the force-free configuration. The unstable eigenmodes include a fine
spatial structure which evolves into current sheets at the nonlinear stage of
the instability. The instability catalyses the resistive release of magnetic
energy. The energy is released in a sequence of spikes, every spike emits
several percent of the total energy. A numerically defined scaling for the
energy released in a single spike permits an extrapolation to astrophysically
relevant values of the Hall number. The instability can be relevant to magnetic
energy release in a neutron star crust and, possibly, in stellar coronae.
