This study presents 2D particle-in-cell simulations of a relativistic electron-positron shock propagating into an unmagnetized medium, revealing the long-term evolution of the shock and its magnetic structures. The simulations show that the shock generates increasingly larger magnetic structures over time, with magnetic coherence scales reaching up to 100 plasma skin depths. The post-shock magnetic field is concentrated in localized patches, maintaining a magnetic energy fraction of ~0.1. Particles spend most of their time in low field regions but emit a large fraction of their synchrotron power in strong field regions. These results have important implications for models of gamma-ray burst afterglows. The shock evolves through a series of cavity cycles, which generate magnetic structures that grow in size and contribute to the magnetic field's broadband spectrum. The magnetic field's transverse coherence scale increases over time, reaching ~100 plasma skin depths. The magnetic energy fraction distribution shows a hard power-law tail, indicating intermittent magnetic structures. These structures contribute significantly to synchrotron emission, with a large fraction of the total magnetic energy and synchrotron power coming from regions with magnetic energy fractions ~0.1. The simulations also show that particle acceleration at the shock increases over time, with the maximum Lorentz factor growing faster than the canonical scaling. The particle energy spectrum develops a suprathermal component, which connects the Maxwellian peak to a high-energy power law tail. The suprathermal component grows at the expense of the thermal component, and the high-energy tail shifts to higher Lorentz factors over time. The results suggest that the shock's magnetic field evolves to larger scales over time, with the magnetic coherence scale increasing linearly with time. This growth is consistent with the shock accelerating particles to higher energies. The simulations also show that the magnetic field's properties are important for particle acceleration, with the ratio of the magnetic to particle energy density affecting the shock's behavior. The study highlights the importance of magnetic structures in the shock's evolution and their role in particle acceleration and emission. The results provide insights into the long-term evolution of relativistic shocks and their implications for gamma-ray burst afterglows. The simulations demonstrate that the shock's magnetic field can reach large scales, which may help explain the observed properties of gamma-ray burst afterglows. The study also shows that the magnetic field's properties are crucial for understanding the acceleration and emission processes in relativistic shocks.This study presents 2D particle-in-cell simulations of a relativistic electron-positron shock propagating into an unmagnetized medium, revealing the long-term evolution of the shock and its magnetic structures. The simulations show that the shock generates increasingly larger magnetic structures over time, with magnetic coherence scales reaching up to 100 plasma skin depths. The post-shock magnetic field is concentrated in localized patches, maintaining a magnetic energy fraction of ~0.1. Particles spend most of their time in low field regions but emit a large fraction of their synchrotron power in strong field regions. These results have important implications for models of gamma-ray burst afterglows. The shock evolves through a series of cavity cycles, which generate magnetic structures that grow in size and contribute to the magnetic field's broadband spectrum. The magnetic field's transverse coherence scale increases over time, reaching ~100 plasma skin depths. The magnetic energy fraction distribution shows a hard power-law tail, indicating intermittent magnetic structures. These structures contribute significantly to synchrotron emission, with a large fraction of the total magnetic energy and synchrotron power coming from regions with magnetic energy fractions ~0.1. The simulations also show that particle acceleration at the shock increases over time, with the maximum Lorentz factor growing faster than the canonical scaling. The particle energy spectrum develops a suprathermal component, which connects the Maxwellian peak to a high-energy power law tail. The suprathermal component grows at the expense of the thermal component, and the high-energy tail shifts to higher Lorentz factors over time. The results suggest that the shock's magnetic field evolves to larger scales over time, with the magnetic coherence scale increasing linearly with time. This growth is consistent with the shock accelerating particles to higher energies. The simulations also show that the magnetic field's properties are important for particle acceleration, with the ratio of the magnetic to particle energy density affecting the shock's behavior. The study highlights the importance of magnetic structures in the shock's evolution and their role in particle acceleration and emission. The results provide insights into the long-term evolution of relativistic shocks and their implications for gamma-ray burst afterglows. The simulations demonstrate that the shock's magnetic field can reach large scales, which may help explain the observed properties of gamma-ray burst afterglows. The study also shows that the magnetic field's properties are crucial for understanding the acceleration and emission processes in relativistic shocks.