The cusp as a source of magnetospheric particles

Observations made by a number of satellites have shown that plasma of solar wind/magnetosheath origin enters the high altitude polar cusp creating diamagnetic cavities of large dimensions. The Earth's dipole field can be excluded from this region in a turbulent manner with the magnitude of the field strength reaching close to 0 nT at times. In these regions of strongly varying magnetic fields, energetic particles are produced with intensities, which exceed those measured in the trapping regions of the magnetosphere for L>6 at a given magnetic moment. These particles can then flow back out of the cusp along field lines that form the magnetopause. A fraction of these particles enter the magnetosphere through the magnetopause on the dusk and dawn flanks. Due to existing gradients in the geomagnetic field, cusp accelerated ions can enter the magnetosphere along the dawn flank and electrons along the dusk flank. For those particles entering near the geomagnetic equatorial plane with pitch angles close to 90°, they will drift along contours of constant magnetic field strength reaching deep into the nightside inner magnetosphere. From observations made by the Polar, Cluster, and ISEE satellites it is shown that this cusp source appears to be capable of providing energetic ions and electrons to the magnetosphere, which form the source population of the subsequent radial diffusion and formation of the radiation belts.

► Plasmas of solar wind and of ionospheric origin are both found in the exterior cusp.
► Such plasmas undergo efficient acceleration in the cusp independent of their origin.
► Main energization occurs from 10 to 200 keV, the energy of the ring current.
► Cusp energetic particles (CEP) have access to the equatorial magnetosphere for L>5.
► CEPs contribute to the ring current and to the radiation belt source particle fluxes.