IMF control of the high-altitude cusp dynamics

Interconnection of the interplanetary magnetic field with the geomagnetic field is thought to be the dominant process for mass, energy, and momentum transfer from the magnetosheath into the magnetosphere. Downward precipitation from the reconnection site is observed in the cusp by the spacecraft orbiting in this region. Using observations of two closely separated Interball spacecraft, we show the importance of the IMF orientation for a formation of the cusp and surrounding magnetosheath region as well as some peculiarities of the cusp–magnetosheath transition. A short statistical study of the high-altitude cusp location under different Earth’s dipole tilts and IMF BY and BZ conditions is presented as a basis for case studies that address (1) formation of the cusp under northward and southward IMF; (2) cusp bifurcation due to large duskward/dawnward IMF; (3) differences between spatial and temporal dispersion patterns in the cusp precipitation; and (4) reformation of the cusp after a change of the IMF orientation. The paper focuses on a description of the high-altitude cusp as seen by the Interball project, the first multi-point experiment in this region.