Axisymmetric magnetic modes of neutron stars having mixed poloidal and toroidal magnetic fields

Abstract

We calculate axisymmetric magnetic modes of a neutron star possessing a mixed poloidal and toroidal magnetic field, where the toroidal field is assumed to be proportional to a dimensionless parameter $\zeta_0$. Here, we assume an isentropic structure for the neutron star and consider no effects of rotation. Ignoring the equilibrium deformation due to the magnetic field, we employ a polytrope of the index $n=1$ as the background model for our modal analyses. For the mixed poloidal and toroidal magnetic field with $\zeta_0\not=0$, axisymmetric spheroidal and toroidal modes are coupled. We compute axisymmetric spheroidal and toroidal magnetic modes as a function of the parameter $\zeta_0$ from $0$ to $\sim 1$ for the surface field strengths $B_S=10^{14}$G and $10^{15}$G. We find that the frequency $\omega$ of the magnetic modes decreases with increasing $\zeta_0$. We also find that the frequency of the spheroidal magnetic modes is almost exactly proportional to $B_S$ for $\zeta_0\lesssim 1$ but that this proportionality holds only when $\zeta_0\ll 1$ for the toroidal magnetic modes. The wave patterns of the spheroidal magnetic modes and toroidal magnetic modes are not strongly affected by the coupling so long as $\zeta_0\lesssim 1$. We find no unstable modes having $\omega^2<0$.