Ripples observed on the surface of the Earth's quasi-perpendicular bow shock

Abstract

Multipoint measurements from Cluster of a particularly slow encounter with the Earth's bow shock are presented. The shock is classified as high Mach number, quasi-perpendicular, and high plasma beta. Coherent oscillations of the plasma density and the magnetic field amplitude are seen in the foot and ramp of the shock with period approximately 15 s; the traversal of the shock layer lasts 2 min. The oscillation amplitude decreases upstream from a maximum value at the location of the shock overshoot so that it is confined within the shock layer. Phase differences in the oscillations as seen on the four Cluster spacecraft indicate that the oscillations are propagating, and cannot be explained by a one-dimensional shock profile fluctuating in position. Four-point timing shows that the oscillations correspond to ripples traveling across the surface of the shock, with wavelength 1000-2000 km and propagation direction roughly parallel to the magnetic field. The presence of these oscillations could have major implications for the analysis of crossings observed at higher relative shock-spacecraft speeds or where the interspacecraft spacing is larger and the shock velocity is changing.