Cyclotron Emissivity in the Early Universe
  Primordial magnetic fields present between the eras of neutrino decoupling and of positron-electron annihilation can lead to the emission of neutrino-anti neutrino pairs or axions by electrons in excited Landau levels, thereby changing the electron energy spectrum. The rate is calculated and shown to not affect significantly the standard Big Bang picture. 
Gino Segre