The CUSP as a source of plasma for the magnetosphere Original Research Article

For the magnetosphere as a whole, the cusp, or even the dayside is relatively insignificant as a plasma source, but for the magnetosphere near the Earth the situation is different. Most important, quantitatively, is that the cusp acts as a gateway for magnetosheath plasma. Evidence is found in the data for different entry mechanisms; sub-solar point reconnection, merging in anti-parallel fields at different locations on the dayside magnetopause, and more direct entry at high latitudes through the turbulent boundary layer. Much remains to be understood regarding the relative importance of different source regions and mechanisms. Plasma also enters the cusp from the ionosphere at the bottom of the cusp. This region is very active, but due to its small size, the overall importance of the ionosphere as a cusp plasma source is not very big. It is, however, very interesting from both a plasma physics and a magnetospheric physics point of view. Via magnetospheric plasma circulation the cusp and the rest of the magnetosphere exchange particles. Interball data can be used to study many of the processes in both the high- and mid-altitude cusp and offer a solar cycle minimum complement to the Cluster solar cycle maximum data.