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Magnetic field decay in isolated neutron stars

Goldreich, Peter and Reisenegger, Andreas (1992) Magnetic field decay in isolated neutron stars. Astrophysical Journal, 395 (1). pp. 250-258. ISSN 0004-637X. doi:10.1086/171646.

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We investigate three mechanisms that promote the loss of magnetic flux from an isolated neutron star. Ohmic decay produces a diffusion of the magnetic field with respect to the charged particles. It proceeds at a rate that is inversely proportional to the electric conductivity and independent of the magnetic field strength. Ohmic decay occurs in both the fluid core and solid crust of a neutron star, but it is too slow to directly affect magnetic fields of stellar scale. Ambipolar diffusion involves a drift of the magnetic field and charged particles relative to the neutrons. The drift speed is proportional to the second power of the magnetic field strength if the protons form a normal fluid. Variants of ambipolar diffusion include both the buoyant rise and the dragging by superfluid neutron vortices of magnetic flux tubes. Ambipolar diffusion operates in the outer part of the fluid core where the charged particle composition is homogeneous, protons and electrons being the only species. The charged particle flux associated with ambipolar diffusion decomposes into a solenoidal and an irrotational component. Both components are opposed by frictional drag. The irrotational component perturbs the chemical equilibrium between neutrons, protons, and electrons, thus generating pressure gradients that effectively choke it. The solenoidal component is capable of transporting magnetic flux from the outer core to the crust on a short time scale. Magnetic flux that threads the inner core, where the charged particle composition is inhomogeneous, would be permanently trapped unless particle interactions could rapidly smooth departures from chemical equilibrium. Magnetic fields undergo a Hall drift related to the Hall component of the electric field. The drift speed is proportional to the magnetic field strength. Hall drift occurs throughout a neutron star. Unlike ohmic decay and ambipolar diffusion which are dissipative, Hall drift conserves magnetic energy. Thus, it cannot by itself be responsible for magnetic field decay. However, it can enhance the rate of ohmic dissipation. In the solid crust, only the electrons are mobile and the tangent of the Hall angle is large. There, the evolution of the magnetic field resembles that of vorticity in an incompressible fluid at large Reynolds number. This leads us to speculate that the magnetic field undergoes a turbulent cascade terminated by ohmic dissipation at small scales. The small-scale components of the magnetic field are also transported by Hall drift waves from the inner crust where ohmic dissipation is slow to the outer crust where it is rapid. The diffusion of magnetic flux through the crust takes ~ 5 x 10^8/B_(12) yr, where B_(12) is the crustal magnetic field strength measured in units of 10^(12) G.

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Additional Information:© 1992 American Astronomical Society. Received 1991 September 23; accepted 1992 February 14. The authors thank R. Blandford, S. Kulkarni and C. Thompson for enlightening discussions. This research was supported by NSF grant AST 89-13664 and NASA grant NAGW 2372. Part of it was performed while P. G. was visiting the School of Natural Sciences at the Institute for Advanced Study at Princeton and the Department of Astronomy at the University of California at Berkeley. He is grateful for the award of a visiting fellowship at the IAS and a Miller Professorship at UCB.
Group:UNSPECIFIED, Division of Geological and Planetary Sciences
Funding AgencyGrant Number
NSFAST 89-13664
Princeton Institute for Advanced Study Visiting FellowshipUNSPECIFIED
UC Berkeley Miller ProfessorshipUNSPECIFIED
Subject Keywords:stars: magnetic, stars: neutron
Issue or Number:1
Record Number:CaltechAUTHORS:20130313-154541025
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Official Citation:Magnetic field decay in isolated neutron stars Goldreich, Peter; Reisenegger, Andreas Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 395, no. 1, p. 250-258.
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:37512
Deposited By: Ruth Sustaita
Deposited On:14 Mar 2013 15:57
Last Modified:09 Nov 2021 23:29

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