We study the generation of magnetic fields during preheating within an
scenario of hybrid inflation at the electroweak (EW) scale. We find that the
non-perturbative and strongly out-of-equilibrium process of magnetic field
production occurs along the lines predicted by Vachaspati many years ago. The
system starts in the false vacuum at the end of inflation, and very quickly the
initial quantum fluctuations of the Higgs field get amplified via long
wavelength spinodal instabilities. The subsequent nucleation of the random
Gaussian Higgs field bubbles (lumps) leads to EW symmetry breaking, and to the
creation of Z-strings, which soon decay, along with longwave magnetic flux
tubes with nontrivial helicity. The intensity and scales in these helical
magnetic fields are consistent with their later development into the microgauss
fields observed in galaxies and clusters of galaxies.Comment: 4 pages, 6 figures, uses revtex