Laser wakefield acceleration experiments were performed using a 30 fs, 1 J laser pulse interacting with an underdense helium plasma. Temporally resolved polarimetry measurements demonstrate the presence of magnetic fields at the ionization front within the plasma which had a peak strength of ~2.8 MG and a radial extent of approximately 200 μm. The field was seen to vary in strength over picosecond time-scales. The field is likely generated by return current generated in the plasma at the interface between plasma and neutral gas and which is caused by hot electrons produced in the wakefield during formation of a plasma 'bubble' and prior to the time of wave-breaking (beam injection). These effects are confirmed using particle-in-cell simulations. Such measurements can be useful as a diagnostic of bubble formation in laser wakefield accelerators.