On kinetic structure of quasi-perpendicular collisionless shocks

We discuss one-dimensional electromagnetic full particle simulations of quasi-perpendicular collisionless shocks with physical mass ratio in the context of recent Cluster bow shock measurements. The density ramp scale in units of the ion inertial length obtained from simulations of high ion beta shocks (β_i ≈ 1) does not increase with Mach number, contrary to what has been reported from Earth's bow shock measurements. Several possibilities are offered which could explain this discrepancy. In low beta shocks the modified two-stream instability is important in the foot of the shock and can lead to large density and electric field spikes. It is suggested that recent Cluster observations of short scale electric field spikes at the quasi-perpendicular Earth's bow shock can be explained in terms of the modified two-stream instability.