Density-Transition Scale at Quasiperpendicular Collisionless Shocks

Abstract

Measurements of a spacecraft floating potential, on the four Cluster spacecraft, are used as a proxy for electron plasma density to study, for the first time, the macroscopic density transition scale at 98crossings of the quasiperpendicular terrestrial bow shock. A timing analysis gives shock speeds and normals; the shock speed is used to convert the temporal measurement to a spatial one. A hyperbolic tangent function is fitted to each density transition, which captures the main shock transition, but not overshoot or undershoot nor foot features. We find that, at a low Mach number M, the density transition is consistent with both ion inertial scales c/ω_pi and convected gyroradii v_sh,n/Ω_ci,2, while at M≥4 5 only the convected gyroradius is the preferred scale for the shock density transition and takes the value L≈0.4v_sh,n/Ω_ci,2.