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Abstract: In this paper we reconsider the problem of magnetic field diffusion inneutron star cores. We model the star as consisting of a mixture of neutrons,protons and electrons, and allow for particle reactions and binary collisionsbetween species. Our analysis is in much the same spirit as that of Goldreich andReisenegger 1992, and we content ourselves with rough estimates of magneticdiffusion timescales, rather than solving accurately for some particular fieldgeometry. However, our work improves upon previous treatments in one crucialrespect: we allow for superfluidity in the neutron star matter. We find thatthe consequent mutual friction force, coupling the neutrons and chargedparticles, together with the suppression of particles collisions and reactions,drastically affect the ambipolar magnetic field diffusion timescale. Inparticular, the addition of superfluidity means that it is unlikely that thereis ambipolar diffusion in magnetar cores on the timescale of the lifetimes ofthese objects, contradicting an assumption often made in the modelling of theflaring activity commonly observed in magnetars. Our work suggests that if adecaying magnetic field is indeed the cause of magnetar activity, the fieldevolution is likely to take place outside of the core, and might representHall-Ohmic diffusion in the stellar crust, or else that a mechanism other thanstandard ambipolar diffusion is active, e.g. flux expulsion due to theinteraction between neutron vortices and magnetic fluxtubes.

Author: K. Glampedakis, D. I. Jones, L. Samuelsson


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