Interpreting multipoint observations of substructure at the quasi-perpendicular bow shock: Simulations

Abstract

Two-dimensional hybrid simulations of high Mach number quasi-perpendicular collisionless shocks, as relevant to the Earth's bow shock, show strong fluctuations of density and magnetic field within the average shock profile. In particular, there are rapid variations moving across the shock surface, in terms of local changes in its position and the shock gradient normal direction on scales of order of the ion inertial length. Using virtual probes, we simulate the magnetic field time series that would be observed by a multipoint observation at a high Mach number, moderate plasma beta, quasi-perpendicular shock as a function of shock crossing speed. For crossing speeds greater than twice the upstream Alfvén speed, the time series profiles are similar to the spatially averaged profile. As the crossing speed decreases, more structure becomes visible in the time series so that oscillations and major differences of amplitude and gradient are evident even between closely spaced probes. These signatures are directly linked to the spatiotemporal variability of the shock front. Even at slow crossing speeds sharp gradients can sometimes be seen in the time series owing to propagation of fluctuations across the shock surface. On the basis of the simulations, oscillatory substructure becomes increasingly significant when the crossing speed becomes less than the Alfvén speed. We suggest some signatures that may then be used observationally to infer the presence of the associated surface fluctuations.