  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.
