  Electron-positron pair production in space- and time-dependent
electromagnetic fields is investigated. Especially, the influence of a
time-dependent, inhomogeneous magnetic field on the particle momenta and the
total particle yield is analyzed for the first time. The role of the Lorentz
invariant $\mathbf{E}^2 - \mathbf{B}^2$, including its sign and local values,
in the pair creation process is emphasized.
