Radio surface fluctuations in radio relics

Recent observations have revealed detailed structures of radio relics in a wide range of frequencies. In this work, we perform three-dimensional magnetohydrodynamical simulations of merger shocks propagating through a turbulent magnetized intracluster medium, and employ on-the-fly Lagrangian particles to explore the physical processes originating radio substructures and their appearances in high and low-frequency observations. We employ two cosmic-ray (CR) electron acceleration models: the fresh injection of electrons from the thermal pool and the re-acceleration of mildly relativistic electrons. We use the relative surface brightness fluctuations, δ S_ν, to define a "degree of patchiness”. We find that: 1) Patchiness is produced if the shock's surface has a distribution of Mach numbers, rather than a single Mach number; 2) Radio relics appear patchier if the Mach number distribution consists of a large percentage of low Mach numbers (ℳ≲2.5); 3) As the frequency increases, the patchiness also becomes larger. Nevertheless, if radio relics are patchy at high frequencies (e.g., 18.6 GHz), they are necessarily also at low frequencies (e.g., 150 MHz); 4) To produce noticeable differences in the patchiness at low and high frequencies, the shock front should have a Mach number spread of σ_ℳ≳0.3-0.4; 5) The amount of the patchiness depends on the Mach number distribution as well as the CR acceleration model. We propose δ S_ν as a potential tool for extracting merger shock properties and information about particle acceleration processes at shocks in radio observations.