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

The shock ramp is traditionally defined as the narrow region over which the magnetic field primarily jumps from upstream to downstream conditions. Alt hough narrow in comparison to other features in the shock profile, the ramp plays the most important role in providing the necessary dissipation of th e incident solar wind flow. However, its features are not well understood, particularly for shocks observed when the upstream solar wind has a high Ma ch number and high plasma beta. Using the ISEE 1 and 2 spacecraft to measur e the spatial scales in supercritical, quasi-perpendicular bow shock profil es, we examine the scale size of the ramp and pay particular attention to f eatures found within the ramp. It is shown that the ramp can usually be cha racterized 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 cha nge in magnetic field at the shock. It is shown that both scales are charac teristic of the quasi-stationary shock profile (and are stationary within a n ion gyroperiod), which allows for a reliable conversion from measured tem poral durations to spatial lengths in the shock profile. In most shocks the global ramp width is 0.4-1 ion inertial lengths (c/omega(pi)), and the sub ramp scale is about 0.1-0.2 c/omega(pi). We argue that presence of these sm all-scale, large-amplitude, quasi-stationary structures in the ramp may be important for ion dynamics. An oscillatory behavior of the ramp is also obs erved in some shocks. Also, the global ramp width and subramp scales show l ittle dependence on upstream parameters: The global ramp scale thins as the ta(Bn), approaches 90 degrees, but not as much as predicted, and there is l ittle overall correlation between ramp scales and either Mach number or bet a. Future multispacecraft observations of the bow shock will require high-t emporal-resolution measurements and close spatial separations to address th e problem of shock structure. Present plans for the Cluster mission will pr ovide little data at the close separations needled for such a study.