  The quantum phenomenon of spectral flow which has been observed in laboratory
superfluids, such as 3He-B, controls the drift velocity of proton type II
superconductor vortices in the liquid core of a neutron star and so determines
the rate at which magnetic flux can be expelled from the core to the crust. In
the earliest and most active phases of the anomalous X-ray pulsars and
soft-gamma repeaters, the rates are low and consistent with a large fraction of
the active crustal flux not linking the core. If normal neutrons are present in
an appreciable core matter-density interval, the spectral flow force limits
flux expulsion in cases of rapid spin-down, such as in the Crab pulsar or in
the propeller phase of binary systems.
