  The microscopic structure within a two-dimensional shock was studied using
data from a dusty plasma experiment. A single layer of charged microparticles,
levitated in a glow-discharge plasma, was perturbed by an electrically floating
wire that was moved at a steady supersonic speed to excite a compressional
shock. A rearrangement of particles was observed, from a hexagonal lattice in
the preshock into a quadrilateral microstructure within the shock. This
quadrilateral structure would not be stable in a monolayer of identical
repulsive particles, under equilibrium conditions. Glaser-Clark polygon
analysis of the microstructure helped in identifying quadrilaterals. Voronoi
analysis was used to characterize the defect fraction behind the shock, as an
indication of shock-induced melting.
