  Isolated neutron stars show a diversity in timing and spectral properties,
which has historically led to a classification in different sub-classes. The
magnetic field plays a key role in many aspects of the neutron star
phenomenology: it regulates the braking torque responsible for their timing
properties and, for magnetars, it provides the energy budget for the outburst
activity and high quiescent luminosities (usually well above the rotational
energy budget). We aim at unifying this observational variety by linking the
results of the state-of-the-art 2D magneto-thermal simulations with
observational data. The comparison between theory and observations allows to
place two strong constraints on the physical properties of the inner crust.
First, strong electrical currents must circulate in the crust, rather than in
the star core. Second, the innermost part of the crust must be highly
resistive, which is in principle in agreement with the presence of a novel
phase of matter so-called nuclear pasta phase.
