Spot-like Structures of Neutron Star Surface Magnetic Fields

There is growing evidence, based on both X-ray and radio observations of isolated neutron stars, that besides the large–scale (dipolar) magnetic field, which determines the pulsar spin–down behaviour, small–scale poloidal field components are present, which have surface strengths one to two orders of magnitude larger than the dipolar component. We argue in this paper that the Hall–effect can be an efficient process in producing such small–scale field structures just above the neutron star surface. It is shown that due to a Hall–drift induced instability, poloidal magnetic field structures can be generated from strong subsurface toroidal fields, which are the result of either a dynamo or a thermoelectric instability acting at early times of a neutron star’s life. The geometrical structure of these small–scale surface anomalies of the magnetic field resembles that of some types of ``star–spots’'. The magnetic field strength and the length–scales are comparable with values that can be derived from various observations.