The understanding of many observed phenomena occurring in neutron stars (and briefly reviewed in
Section 12, for instance, pulsar glitches or torsional oscillations in Soft Gamma Repeaters) requires
modeling the dynamic evolution of the crust. So far theoretical efforts have been mainly devoted
to modeling the dynamic evolution of the liquid core by considering a mixture of superfluid
neutrons and superconducting protons (see, for instance, the recent review by Andersson &
Comer [15]).
Macroscopic models of neutron star crusts, taking into account the presence of the neutron superfluid at
(see Section 8), have been developed by Carter and collaborators. They have shown how to
extend the two-fluid picture of neutron star cores [93
] to the inner crust layers in the Newtonian
framework [79
, 94
]. They have also discussed how to calculate the microscopic coefficients of this
model [78
, 77]. More elaborate models treating the crust as a neutron superfluid in an elastic medium and
taking into account the effects of a frozen-in magnetic field have been very recently developed both in
general relativity [73
, 85
] and in the Newtonian limit [73
, 72
]. All these models are based on an action
principle that will be briefly reviewed in Section 10.1. We will consider a simple nonrelativistic two-fluid
model of neutron star crusts in Section 10.2 using the fully-4D covariant formulation of Carter &
Chamel [74
, 75
, 76
]. Entrainment effects and superfluidity will be discussed in Sections 10.3 and 10.4,
respectively.
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