Generation of neutron star magnetic fields by thermomagnetic effects

Abstract

This paper considers a linear stage of the magnetic field generation in outer (ϱ≲1011 g cm−3) layers of rather hot rotating neutron stars by thermomagnetic phenomena which accompany the heat transport from the stellar core. For a neutron star with surface gravity |$g=10^{14}\enspace \text {cm s}^{-2}$| whose envelope consists of degenerate electrons and 56Fe ions in the fluid phase the thermomagnetic instability is developed at surface temperatures Te≳3×106 K and rotation periods P≲1 s. Unstable are axially-symmetric toroidal modes of magnetic field. In particular, at |$T_e=5\times 10^6$| K the maximal instability increment equals |$1/t_0=1/70^\text d$| and corresponds to the magnetic configuration with wavelength |$\lambda \approx80$| m in the meridional direction and maximal field at a depth |$x \approx40$| m. At this Te the magnetic field modes with 30 m≲λ≲1 km are generated. If Te falls down to |$3\times 10^6$| K due to the neutron star cooling, the instability increment decreases to zero, while the spectrum of wavelengths tightens to |$\lambda \approx 120$| m. It is possible that at the non-linear stage the proposed mechanism leads to the generation of strong magnetic fields |$\sim 10^{10}-10^{13}$| G present in the surface layers of neutron stars.