Uri Keshet, Kuan-Chou Hou

Galaxy-cluster virial (structure-formation accretion) shock observations are shown to imply $\gtrsim1\%$ magnetization of a layer extending $\gtrsim10^{16}$ Debye lengths downstream, challenging the modelling of high Alfv\'en-Mach collisionless shocks. Unlike similar shocks in supernova remnants or relativistic shocks in $\gamma$-ray burst afterglows, where macroscopic magnetized layers were detected but purportedly attributed to preexisting or non-resonant cosmic-ray streaming-seeded substructure, the upstream of strong virial shocks is both weakly magnetized and pristine. Hence, some mechanism must generate large-scale and possibly self-similar magnetic sub-structure out of the accreted primordial plasma; such a mechanism may dominate other high-Mach shock systems, too.