Why doesn't the ring current injection rate saturate?

Instead, we argue that during storms a quasi-steady reconnection region forms in the tail near the Earth. This reconnection region moves closer to the Earth for higher values of solar wind Bs, although the polar cap potential, the dayside merging and nightside reconnection rates, and the amount of open flux do not change much as a function of Bs once the polar cap potential has become saturated. As the neutral line moves closer, the volume per unit magnetic flux in the closed field line region is less.

Flux tubes leaving the reconnection region in general have lower PVγ as Bs increases, and lower PVy flux tubes can penetrate deeper into the inner magnetosphere, leading to a corresponding greater injection of particles into the inner magnetosphere. Thus a reconnection region that is closer to Earth is more effective in creating a strong ring current. This leads to a continued dependence of the ring current injection rate on VBs, although the polar cap potential has saturated.