title: Upper limit on the inner radiation belt MeV electron intensity.
abstract: No instruments in the inner radiation belt are immune from the unforgiving penetration of the highly energetic protons (tens of MeV to GeV). The inner belt proton flux level, however, is relatively stable; thus, for any given instrument, the proton contamination often leads to a certain background noise. Measurements from the Relativistic Electron and Proton Telescope integrated little experiment on board Colorado Student Space Weather Experiment CubeSat, in a low Earth orbit, clearly demonstrate that there exist sub-MeV electrons in the inner belt because their flux level is orders of magnitude higher than the background, while higher-energy electron (>1.6 MeV) measurements cannot be distinguished from the background. Detailed analysis of high-quality measurements from the Relativistic Electron and Proton Telescope on board Van Allen Probes, in a geo-transfer-like orbit, provides, for the first time, quantified upper limits on MeV electron fluxes in various energy ranges in the inner belt. These upper limits are rather different from flux levels in the AE8 and AE9 models, which were developed based on older data sources. For 1.7, 2.5, and 3.3 MeV electrons, the upper limits are about 1 order of magnitude lower than predicted model fluxes. The implication of this difference is profound in that unless there are extreme solar wind conditions, which have not happened yet since the launch of Van Allen Probes, significant enhancements of MeV electrons do not occur in the inner belt even though such enhancements are commonly seen in the outer belt.
  
  
  CONCLUSIONS
  Quantified upper limit of MeV electrons in the inner beltActual MeV electron intensity likely much lower than the upper limitMore detailed understanding of relativistic electrons in the magnetosphere.
authors.affiliation: Laboratory for Atmospheric and Space Physics, University of Colorado Boulder Boulder, Colorado, USA ; Department of Aerospace Engineering Sciences, University of Colorado Boulder Boulder, Colorado, USA.
authors.forename: X
authors.initials: X
authors.lastname: Li
authors.affiliation: Space Vehicles Directorate, Air Force Research Laboratory Kirtland AFB, New Mexico, USA.
authors.forename: R S
authors.initials: RS
authors.lastname: Selesnick
authors.affiliation: Laboratory for Atmospheric and Space Physics, University of Colorado Boulder Boulder, Colorado, USA.
authors.forename: D N
authors.initials: DN
authors.lastname: Baker
authors.affiliation: Laboratory for Atmospheric and Space Physics, University of Colorado Boulder Boulder, Colorado, USA.
authors.forename: A N
authors.initials: AN
authors.lastname: Jaynes
authors.affiliation: NASA/Goddard Space Flight Center Greenbelt, Maryland, USA.
authors.forename: S G
authors.initials: SG
authors.lastname: Kanekal
authors.affiliation: Laboratory for Atmospheric and Space Physics, University of Colorado Boulder Boulder, Colorado, USA ; Department of Aerospace Engineering Sciences, University of Colorado Boulder Boulder, Colorado, USA.
authors.forename: Q
authors.initials: Q
authors.lastname: Schiller
authors.affiliation: Laboratory for Atmospheric and Space Physics, University of Colorado Boulder Boulder, Colorado, USA ; Department of Aerospace Engineering Sciences, University of Colorado Boulder Boulder, Colorado, USA ; Now at Space Sciences Laboratory, University of California Berkeley, California, USA.
authors.forename: L
authors.initials: L
authors.lastname: Blum
authors.affiliation: Space Science Applications Laboratory, Aerospace Corporation El Segundo, California, USA.
authors.forename: J
authors.initials: J
authors.lastname: Fennell
authors.affiliation: Space Science Applications Laboratory, Aerospace Corporation El Segundo, California, USA.
authors.forename: J B
authors.initials: JB
authors.lastname: Blake
keywords: CSSWE; CubeSat; Earth's magnetosphere; MeV electron; Van Allen Probes; inner radiation belt