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

Abstract

[1] 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, quasiperpendicular, 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.

On nonstationarity and rippling of the quasiperpendicular zone of the Earth bow shock: Cluster observations

“…The first convincing experimental evidence in favor of shock front reformation was presented by Lobzin et al (2007). Moullard et al (2006) were the first who probably observed another aspect of shock front nonstationarity -rippling. They analyzed a single event when during ∼1 h time interval Cluster spacecraft "touched" the bow shock and than crossed it twice during ∼10 min.…”

“…The first unambiguous evidence of the shock wave nonstationarity was obtained by Morse et al (1972) in laboratory experiments with a plasma-wind-tunnel device. They revealed that the high-Mach-number shock wave oscillates with a frequency comparable to the upstream ion gyrofrequency.…”

“…This shock is almost perpendicular, high-beta, and high-Mach-number, with the fast mode Mach number M f =11. Moullard et al (2006) argue that the observed oscillations of the magnetic field and plasma density within the front can be interpreted as a wave moving along the shock surface. The velocity of this wave seems to make an acute angle (<40 • ) with the upstream magnetic field and varies within the range from 2 to 4 V Ad Up to the present time, the information concerning different aspects of shock front nonstationarity was obtained first of all in numerical modeling, which allows one to follow the temporal evolution and spatial structure of the shock wave with an arbitrary resolution.…”

The Dynamics of Very High Alfvén Mach Number Shocks in Space Plasmas

“…Non-stationarity in the form of rippling of the surface or steepened whistler waves (Moullard et al 2006;Lobzin et al 2007) is an intrinsic feature of the shock, but this is generally manifest as minor perturbations on top of an otherwise stationary shock ramp. Simulations have predicted that if the fraction of ions reflected by the shock front becomes sufficiently high, the quasi-perpendicular shock can become periodically reforming on timescales of the ion gyroperiod.…”

The Solar Wind as a Possible Source of Fast Temporal Variations of the Heliospheric Ribbon

“…These authors point out that the shock was reforming. Shock reformation is a known feature that was predicted by numerical simulations (Burgess et al 1989) and confirmed by Cluster observations (Moullard et al 2006) for the perpendicular Earth's bow shock. Two years before the Voyager spacecraft crossed the TS, they reported energetic Sun-ward streaming ions.…”