Interball contribution to the high-altitude cusp observations

The polar cusps have traditionally been described as narrow funnel-shaped regions of magnetospheric magnetic field lines directly connected to magnetosheath, allowing the magnetosheath plasma to precipitate into the ionosphere. However, recent observations and theoretical considerations revealed that the formation of the cusp cannot be treated separately from the processes along the whole dayside magnetopause and that the plasma in regions like cleft or low-latitude boundary layer is of the same origin. Our review of statistical results as well as numerous case studies identified the anti-parallel merging at the magnetopause as the principal source of the magnetosheath plasma in all altitudes. Since effective merging requires a low plasma speed at the reconnection spot, we have found that the magnetopause shape and especially its indentation at the outer cusp is a very important part of the whole process. The plasma is slowed down in this indentation and arising multiscale turbulent processes enhance the reconnection rate.