  We identify a mechanism for magnetic field generation in the interaction of
intense electromagnetic waves and underdense plasmas. We show that Raman
scattered plasma waves trap and heat the electrons preferentially in their
propagation direction, resulting in a temperature anisotropy. In the trail of
the laser pulse, we observe magnetic field growth which matches the Weibel
mechanism due to the temperature anisotropy. We discuss the role of the initial
electron temperature in our results. The predictions are confirmed with
multi-dimensional particle-in-cell simulations. We show how this configuration
is an experimental platform to study the long-time evolution of the Weibel
instability.
