The plasmapause is not a boundary for the radiation belts

The radiation belts consist of the most energetic, magnetically trapped, charged particles in the Earths magnetosphere. The plasmasphere consists of the least energetic. The two populations are traditionally considered to occupy two distinct spatial domains with the inner edge of the outer radiation belt electrons co-located with the outer edge of the plasmasphere. This long-standing framework is misleading. Typically, much of the outer radiation belt can be found inside the plasmasphere where important radiation belt - plasmasphere interactions take place. Under enhanced geomagnetic activity the outer regions of the plasmasphere are eroded and the plasmapause retreats earthward. At the same time radiation belt electron fluxes are enhanced through both local wave-particle acceleration and inward radial diffusion. There is, however, no single location for the inner edge of the radiation belts. While the boundary is sharp it is highly energy-dependent with lower energies penetrating to lower L-shells than higher energies. Whether the radiation belts are inside, outside, or near the plasmapause depends on energy. Similarly, in the recovery phase the Alfven layer moves far outward over the outer belt. Once the larger plasmasphere is of sufficient density to support whistler-mode hiss wave-particle interactions begin to scatter radiation belt electrons into the loss cone. This process is also energy (and L-shell) dependent. Therefore, even after long periods, the relationship between plasmapause location and the outer radiation belts is energy-dependent and, particularly at the highest energies, the radiation belts linger inside the plasmasphere.