  We investigate the chiral properties of QCD in presence of a magnetic
background field and in the low temperature regime, by lattice numerical
simulations of N_f = 2 QCD. We adopt a standard staggered discretization, with
a pion mass around 200 MeV, and explore a range of magnetic fields (180 MeV)^2
\leq|e|B \leq (700 MeV)^2, in which we study magnetic catalysis, i.e. the
increase of chiral symmetry breaking induced by the background field. We
determine the dependence of the chiral condensate on the external field,
compare our results with existing model predictions and show that a substantial
contribution to magnetic catalysis comes from the modified distribution of
non-Abelian gauge fields, induced by the magnetic field via dynamical quark
loop effects.
