Explaining the "Impenetrable Barrier" to Ultra-relativistic Electrons in the Van Allen Belts

In recent observations, Baker et al. (Nature, 2014) reported the observation of an "impenetrable barrier" to the inner edge of the ultra-relativistic electron radiation belt. These authors demonstrated that this barrier location was not co-incident with the location of the plasmapause nor any other identifiable magnetospheric boundary; nor could it be explained by spatial structure resulting from the impacts of ground-based VLF transmitters. Here we show how the location of the "impenetrable barrier" can be explained as the location where there is a balance between inwards ULF wave radial diffusion and loss from lower band chorus and/or plasmaspheric hiss. Using recently derived data-driven ULF wave radial diffusion rates based on observations of ground-based ULF wave power characterised upto Kp=9 we can estimate the inward transport rates across a wide range of extreme activity levels. Contrary to the suggestion by Baker et al., there does not appear to be any need for active local wave particle acceleration between the plasmapause and the edge of the barrier at L*~2.8 since the radial diffusion rates appear to be sufficient to transport particles there during the most active times. The "impenetrable barrier" is explained as being defining as the locus where the rate of drift averaged loss matches the rate of inwards ULF wave radial diffusion. During more active times the location where loss is dominant moves inwards but reaches a limit of closest approach at L*~2.8 during the most active geomagnetic conditions (except during very short exceptional periods where interplanetary shocks and other shorter timescale processes can cause temporary filling of the slot). Overall, the "impenetrable barrier" is explained as a simple and natural consequence of the activity-dependent rates of ULF wave transport balanced by slow wave-particle scattering losses to the atmosphere closer to the Earth.