Cluster observations of the magnetopause boundary layer: transition parameter.

The transition parameter (TP, Hapgood and Bryant, 1992) re-sorts the data through the magnetopause/magnetosheath traversal to reveal underlying order in the magnetopause layer, not readily seen in the time series data because of rapid boundary motion. In addition, within a boundary layer induced during magnetic field merging, the TP sorts the data according to time elapsed since reconnection. The TP therefore works irrespective of the presence of mixed populations, accelerated flows, wave motions or FTE signatures. We investigate boundary layer structure and features of the magnetospheric and magnetosheath regions adjacent to the separatrix, via anisotropies in the electron and ion distribution. Previous application of the TP to AMPTE-UKS and -IRM data identified the total LLBL thickness to be of order 1500 km. The form of the TP variation in boundary aligned co-ordinates (and specifically with normal distance) is likely to be different in each case, however, and how the TP is related to spatial boundary-normal distance is not well understood. Cluster data allows this question to be probed further: since knowledge of boundary orientation, thickness and motion can independently scale the plasma and field signatures across the boundary layer, the four point Cluster data allow us to compare the TP ordering in the context of well defined boundary behaviour and also allows us to look at the effects of reconnection site motions.