The study investigates the long-term evolution of a relativistic, collisionless electron-positron shock propagating into an unmagnetized ambient medium using 2D particle-in-cell (PIC) simulations. The simulations, which are of unprecedented duration and size, reveal the generation of intermittent magnetic structures that grow in size over time. By the end of the simulation, at around 26,000 plasma times, the magnetic coherence scale approaches λ ≈ 100 plasma skin depths both ahead and behind the shock front. The post-shock field becomes concentrated in localized patches, maintaining a local magnetic energy fraction εB ≈ 0.1. Particles spending most of their time in low field regions (εB ≪ 0.1) emit a significant fraction of the synchrotron power in the localized patches with strong fields (εB ≈ 0.1). These findings have important implications for models of gamma-ray burst (GRB) afterglows, suggesting that a significant fraction of the synchrotron power is emitted from high-field regions, which occupy only about 1% of the total volume but contain about half of the magnetic energy. The results also indicate that the shock physics observed in the simulations may be relevant to GRB afterglows, particularly in explaining the large scatter in modeling parameters observed in different afterglow observations.The study investigates the long-term evolution of a relativistic, collisionless electron-positron shock propagating into an unmagnetized ambient medium using 2D particle-in-cell (PIC) simulations. The simulations, which are of unprecedented duration and size, reveal the generation of intermittent magnetic structures that grow in size over time. By the end of the simulation, at around 26,000 plasma times, the magnetic coherence scale approaches λ ≈ 100 plasma skin depths both ahead and behind the shock front. The post-shock field becomes concentrated in localized patches, maintaining a local magnetic energy fraction εB ≈ 0.1. Particles spending most of their time in low field regions (εB ≪ 0.1) emit a significant fraction of the synchrotron power in the localized patches with strong fields (εB ≈ 0.1). These findings have important implications for models of gamma-ray burst (GRB) afterglows, suggesting that a significant fraction of the synchrotron power is emitted from high-field regions, which occupy only about 1% of the total volume but contain about half of the magnetic energy. The results also indicate that the shock physics observed in the simulations may be relevant to GRB afterglows, particularly in explaining the large scatter in modeling parameters observed in different afterglow observations.