Quasiperpendicular High Mach Number Shocks

Shock waves exist throughout the Universe and are fundamental to understanding the nature of collisionless plasmas. Reformation is a process, driven by microphysics, which typically occurs at high Mach number supercritical shocks. While ongoing studies have investigated this process extensively both theoretically and via simulations, their observations remain few and far between. In this Letter we present a study of very high Mach number shocks in a parameter space that has been poorly explored and we identify reformation using in situ magnetic field observations from the Cassini spacecraft at 10 AU. This has given us an insight into quasiperpendicular shocks across 2 orders of magnitude in Alfven Mach number (M-A) which could potentially bridge the gap between modest terrestrial shocks and more exotic astrophysical shocks. For the first time, we show evidence for cyclic reformation controlled by specular ion reflection occurring at the predicted time scale of similar to 0.3 tau(c), where tau(c) is the ion gyroperiod. In addition, we experimentally reveal the relationship between reformation and M-A and focus on the magnetic structure of such shocks to further show that for the same M-A, a reforming shock exhibits stronger magnetic field amplification than a shock that is not reforming.