Production of Axions by Cosmic Magnetic Helicity
  We investigate the effects of an external magnetic helicity production on the evolution of the cosmic axion field. It is shown that a helicity larger than (few \times 10^{-15} G)^2 Mpc, if produced at temperatures above a few GeV, is in contradiction with the existence of the axion, since it would produce too much of an axion relic abundance. 
L. Campanelli and M. Giannotti