The role of photon polarization modes in the magnetization and instability of the vacuum in a supercritical field

The response of the QED vacuum in an asymptotically large electromagnetic field is studied. In this regime the vacuum energy is strongly influenced by the vacuum polarization effect. The possible interaction between the virtual electromagnetic radiation and a superstrong magnetic field suggests that a background of virtual photons is a source of magnetization to the whole vacuum. The corresponding contribution to the vacuum magnetization density is determined by considering the individual contribution of each vacuum polarization eigenmode in the Euler-Heisenberg Lagrangian. Additional issues concerning the transverse pressures are analyzed. We also study the case in which the vacuum is occupied by a superstrong electric field. It is discussed that, in addition to the electron-positron pairs, the vacuum could create photons with different propagation modes. The possible relation between the emission of photons and the birefringent character of the vacuum is shown as well.