  When a relativistic intensity laser interacts with the surface of a solid
density target, suprathermal electron currents are subject to Weibel
instability filamentation when propagating through the thermal population of
the bulk target. We present time resolved shadowgraphy of radial ionization
front expansion and Weibel instability filamentation within a thin, sub-micron,
sheet initiated by irradiation with a short pulse, high intensity laser. High
temporal (100 fs) and spatial (1 $\mu$m) resolution shadowgraphy of the
interaction reveals a relativistic expansion of the ionization front within a
120 $\mu$m diameter region surrounding the laser-target interaction,
corroborated by simulations to expand at $0.77c$, where $c$ is the speed of
light. Filamentation within the patch persists for several picoseconds and
seeds the eventual recombination and heating dynamics on the nanosecond
timescale. Particle-in-cell simulations were conducted to elucidate the
electron dynamics leading to the radial expansion of the critical surface.
Computational results report the ionization expansion is due to field
ionization of the expanding hot electron population. Filamentation within the
expansion is due to the Weibel instability which is supported by the magnetic
fields present.
