The ramp widths of high‐Mach‐number, quasi‐perpendicular collisionless shocks

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Abstract
The shock ramp is traditionally defined as the narrow region over which the magnetic field primarily jumps from upstream to downstream conditions. Although narrow in comparison to other features in the shock profile, the ramp plays the most important role in providing the necessary dissipation of the incident solar wind flow. However, its features are not well understood, particularly for shocks observed when the upstream solar wind has a high Mach number and high plasma β. Using the ISEE 1 and 2 spacecraft to measure the spatial scales in supercritical, quasi‐perpendicular bow shock profiles, we examine the scale size of the ramp and pay particular attention to features found within the ramp. It is shown that the ramp can usually be characterized by two different scales: (1) a large scale (or global ramp width) within which the main transition from the upstream to downstream magnetic field occurs and (2) a thinner subramp scale which contains steep jumps in the magnetic field magnitude with amplitudes comparable to the overall change in magnetic field at the shock. It is shown that both scales are characteristic of the quasi‐stationary shock profile (and are stationary within an ion gyroperiod), which allows for a reliable conversion from measured temporal durations to spatial lengths in the shock profile. In most shocks the global ramp width is 0.4–1 ion inertial lengths (c/ωpi), and the subramp scale is about 0.1–0.2 c/ωpi We argue that presence of these small‐scale, large‐amplitude, quasi‐stationary structures in the ramp may be important for ion dynamics. An oscillatory behavior of the ramp is also observed in some shocks. Also, the global ramp width and subramp scales show little dependence on upstream parameters: The global ramp scale thins as θBn approaches 90°, but not as much as predicted, and there is little overall correlation between ramp scales and either Mach number or β. Future multispacecraft observations of the bow shock will require high‐temporal‐resolution measurements and close spatial separations to address the problem of shock structure. Present plans for the Cluster mission will provide little data at the close separations needed for such a study.

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