Evolution and slow decay of an unusual narrow ring of relativistic electrons near L similar to 3.2 following the September 2012 magnetic storm

A quantitative analysis is performed on the decay of an unusual ring of relativistic electrons between 3 and 3.5 R-E, which was observed by the Relativistic Electron Proton Telescope instrument on the Van Allen probes. The ring formed on 3 September 2012 during the main phase of a magnetic storm due to the partial depletion of the outer radiation belt for L>3.5, and this remnant belt of relativistic electrons persisted at energies above 2MeV, exhibiting only slow decay, until it was finally destroyed during another magnetic storm on 1 October. This long-term stability of the relativistic electron ring was associated with the rapid outward migration and maintenance of the plasmapause to distances greater than L=4. The remnant ring was thus immune from the dynamic process, which caused rapid rebuilding of the outer radiation belt at L>4, and was only subject to slow decay due to pitch angle scattering by plasmaspheric hiss on timescales exceeding 10-20days for electron energies above 3MeV. At lower energies, the decay is much more rapid, consistent with the absence of a long-duration electron ring at energies below 2MeV.