title: An impenetrable barrier to ultrarelativistic electrons in the Van Allen radiation belts.
abstract: Early observations indicated that the Earth's Van Allen radiation belts could be separated into an inner zone dominated by high-energy protons and an outer zone dominated by high-energy electrons. Subsequent studies showed that electrons of moderate energy (less than about one megaelectronvolt) often populate both zones, with a deep 'slot' region largely devoid of particles between them. There is a region of dense cold plasma around the Earth known as the plasmasphere, the outer boundary of which is called the plasmapause. The two-belt radiation structure was explained as arising from strong electron interactions with plasmaspheric hiss just inside the plasmapause boundary, with the inner edge of the outer radiation zone corresponding to the minimum plasmapause location. Recent observations have revealed unexpected radiation belt morphology, especially at ultrarelativistic kinetic energies (more than five megaelectronvolts). Here we analyse an extended data set that reveals an exceedingly sharp inner boundary for the ultrarelativistic electrons. Additional, concurrently measured data reveal that this barrier to inward electron radial transport does not arise because of a physical boundary within the Earth's intrinsic magnetic field, and that inward radial diffusion is unlikely to be inhibited by scattering by electromagnetic transmitter wave fields. Rather, we suggest that exceptionally slow natural inward radial diffusion combined with weak, but persistent, wave-particle pitch angle scattering deep inside the Earth's plasmasphere can combine to create an almost impenetrable barrier through which the most energetic Van Allen belt electrons cannot migrate.
authors.affiliation: Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, Colorado 80303, USA.
authors.forename: D N
authors.initials: DN
authors.lastname: Baker
authors.affiliation: Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, Colorado 80303, USA.
authors.forename: A N
authors.initials: AN
authors.lastname: Jaynes
authors.affiliation: Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, Colorado 80303, USA.
authors.forename: V C
authors.initials: VC
authors.lastname: Hoxie
authors.affiliation: Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles, California 90095, USA.
authors.forename: R M
authors.initials: RM
authors.lastname: Thorne
authors.affiliation: Massachusetts Institute of Technology, Haystack Observatory, Westford, Massachusetts 01886, USA.
authors.forename: J C
authors.initials: JC
authors.lastname: Foster
authors.affiliation: Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, Colorado 80303, USA.
authors.forename: X
authors.initials: X
authors.lastname: Li
authors.affiliation: Aerospace Corporation Space Sciences Lab, Los Angeles, California 90009, USA.
authors.forename: J F
authors.initials: JF
authors.lastname: Fennell
authors.affiliation: School of Physics and Astronomy, University of Minnesota, Minneapolis, Minnesota 55455, USA.
authors.forename: J R
authors.initials: JR
authors.lastname: Wygant
authors.affiliation: NASA Goddard Space Flight Center, Greenbelt, Maryland 20771, USA.
authors.forename: S G
authors.initials: SG
authors.lastname: Kanekal
authors.affiliation: Massachusetts Institute of Technology, Haystack Observatory, Westford, Massachusetts 01886, USA.
authors.forename: P J
authors.initials: PJ
authors.lastname: Erickson
authors.affiliation: Department of Physics, University of Iowa, Iowa City, Iowa 52242, USA.
authors.forename: W
authors.initials: W
authors.lastname: Kurth
authors.affiliation: Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles, California 90095, USA.
authors.forename: W
authors.initials: W
authors.lastname: Li
authors.affiliation: Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles, California 90095, USA.
authors.forename: Q
authors.initials: Q
authors.lastname: Ma
authors.affiliation: Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, Colorado 80303, USA.
authors.forename: Q
authors.initials: Q
authors.lastname: Schiller
authors.affiliation: Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, Colorado 80303, USA.
authors.forename: L
authors.initials: L
authors.lastname: Blum
authors.affiliation: Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, Colorado 80303, USA.
authors.forename: D M
authors.initials: DM
authors.lastname: Malaspina
authors.affiliation: Center for Solar-Terrestrial Research, New Jersey Institute of Technology, Newark, New Jersey 07102, USA.
authors.forename: A
authors.initials: A
authors.lastname: Gerrard
authors.affiliation: Center for Solar-Terrestrial Research, New Jersey Institute of Technology, Newark, New Jersey 07102, USA.
authors.forename: L J
authors.initials: LJ
authors.lastname: Lanzerotti