Cluster observations of the Earth's quasi-parallel bow shock

摘要

Cluster observations of the Earth's quasi-parallel shock are used to investigate properties of the transition and the role of magnetic pulsations in the shock process. We use crossings at small spacecraft separations to show that pulsations grow in only a few seconds. We then use an example when the spacecraft were only a few thousand kilometers apart, and yet at times the shock was located within the tetrahedron formation, to demonstrate that for a period of 10 min the thickness of the shock transition was less than 2500 km and that during several shorter intervals the shock thickness was less than 1000 km. In the context of evidence for the extent of SLAMS exceeding 1000 km, this suggests that the thickness of the shock layer over which the bulk of plasma thermalization occurs can be narrow, containing one or at most a few SLAMS. The small-scale spatial properties of structures within the shock are difficult to extract independently of their time evolution, but we present a crossing at which two pairs of spacecraft observed the same magnetic signatures simultaneously. We show that signatures of these pulsations are consistent with their refraction as they are convected antisunward, as predicted by simulation work, and that they are coherent over a distance of at least 1300 km parallel to the expected shock surface.