2.8 Degenerate stars
If the magnetic flux is conserved during stellar evolution, white dwarfs should be expected to have
magnetic fields of 
. Yet, isolated magnetic white dwarfs are quite rare, comprising about 5% of
all white dwarfs (Wickramasinghe and Ferrario, 2000). Observed spectral variations of magnetic white
dwarfs on a timescale of hours or days suggest a complex magnetic field distribution on their surfaces. In
some cases, spot-like magnetic field enhancements superimposed on a weaker dipole magnetic field can be
identified (Landi Degl’Innocenti, 1976; Maxted et al., 2000). Similar structures are most probably present
on the surfaces of neutron stars as well. There is a growing evidence, based on X-ray and radio
observations, that besides the large-scale dipolar magnetic field, isolated neutron stars possess
small-scale magnetic field enhancements (Geppert et al., 2003). Current theory predicts that such
structures can be generated from strong subsurface toroidal fields on both white dwarfs and neutron
stars.
