1. Investigating the Cusp at Multiple Altitudes by W. Keith 1, M. Goldstein 1, D. Winningham 2, M. Wilber 3, A. Fazakerley 4, H. Reme 5, T. Fritz 6, A. Balogh 7 and N. Cornilleau-Wehrlin 8 1 NASA Goddard Space Flight Center, Code 692, Greenbelt, MD 20771, USA 2 Southwest Research Institute, P. O. Drawer 28510, San Antonio, TX 78228, USA 3 University of California, Space Science Laboratory 7450, Berkeley, CA 94720, USA 4 Mullard Space Science Lab, Holmbury St Mary, Dorking, Surrey, RH5 6NT, UK 5 CESR BP 4346, 9 Ave Colonel Roche, Cedex, Toulouse, 31029, France 6 Boston University, Ctr. Space Physics, 725 Commonwealth Ave, Boston, MA 02215, USA 7 Imperial College Space and Atmospheric Physics group, The Blackett Laboratory, London, UK 8 CETP, 10-12 Ave de Europe, Velizy, 78140, France

2. Crooker Antiparallel merging model results in a wedge-shaped Cusp that rotates with IMF B y Figure depicts northern polar cap (shaded) for pure (a) B y and (b) - B y Edges map to neutral lines in opposite hemispheres Most energetic particles towards the edges, least energetic in the center Green satellite tracks will see a “double” or “V” cusp Red satellite tracks will see normal cusp dispersion pattern

3. Low-altitude DMSP “V” Cusp Crossing

4. High-altitude Cluster-3 “V” Cusp Crossing

5. High-altitude Cluster-1 turbulent “V” Cusp Crossing

6. Cluster 1-4, RAPID/FGM/PEACE/STAFF

7. OVT plot of Cluster positions in x-z GSE plane, time-shifted to cusp crossing times Tracks represent position from 9:30 to 10:30 UT Crossing times indicated next to markers Spacecraft motion is northward

8. Savin outer cusp region with possible satellite tracks for Cluster-1 and Cluster-3

9. Conclusions/Future Work Small-scale cusp feature consistently present for large IMF B y Consistent with Crooker wedge cusp and turbulent exterior cusp Feature present over large altitude range with consistent scaling Cluster differences may be spatial (altitude) rather than temporal Confirm temporal stability of the mid-altitude features Include remaining Cluster instruments in study (especially CODIF) Find additional mid-altitude crossings for study Attempt simultaneous multi-altitude (Cluster/DMSP) observations