  It is shown that an unmagnetized nonrelativistic thermal electron-proton
plasma spontaneously emits aperiodic turbulent magnetic field fluctuations of
strength $|\delta B|=9\beta_eg^{1/3}W_e^{1/2}$ G, where $\beta_e$ is the
normalized thermal electron temperature, $W_e$ the thermal plasma energy
density and $g$ the plasma parameter. Aperiodic modes fluctuate only in space,
but are not propagating. For the unmagnetized intergalactic medium, immediately
after the reionization onset, the field strength from this mechanism is about
$4.7\cdot 10^{-16}$ G, too weak to affect the dynamics of the plasma. The shear
and/or compression of the intergalactic medium exerted by the first supernova
explosions amplify these seed fields and make them anisotropic, until the
magnetic restoring forces affect the gas dynamics at ordered plasma betas near
unity.
