  We present experimental measurements of the femtosecond time-scale generation
of strong magnetic-field fluctuations during the interaction of ultrashort,
moderately relativistic laser pulses with solid targets. These fields were
probed using low-emittance, highly relativistic electron bunches from a laser
wakefield accelerator, and a line-integrated $B$-field of $2.70 \pm 0.39\,\rm
kT\,\mu m$ was measured. Three-dimensional, fully relativistic particle-in-cell
simulations indicate that such fluctuations originate from a Weibel-type
current filamentation instability developing at submicron scales around the
irradiated target surface, and that they grow to amplitudes strong enough to
broaden the angular distribution of the probe electron bunch a few tens of
femtoseconds after the laser pulse maximum. Our results highlight the potential
of wakefield-accelerated electron beams for ultrafast probing of relativistic
laser-driven phenomena.
