Andreas Reisenegger: Neutron star magnetic fields: a theoretical perspective

Neutron star matter is stably stratified by a composition gradient, i.e. is not a barotropic fluid, and the magnetic fields are weak (the fluid pressure is about 7 orders of magnitude greater than the magnetic pressure for magnetar strength fields). Stable hydromagnetic equilibria appear to exist, where the stable stratification plays an important role. Erosion of this stratification through beta decays or ambipolar diffusion (relative motion of charged particles and neutrons) allows magnetic field evolution.