Van Allen Probes show that the inner radiation zone contains no MeV electrons: ECT/MagEIS data

Abstract

We present Van Allen Probe observations of electrons in the inner radiation zone. The measurements were made by the Energetic Particle, Composition, and Thermal Plasma/Magnetic Electron Ion Spectrometer (MagEIS) sensors that were designed to measure electrons with the ability to remove unwanted signals from penetrating protons, providing clean measurements. No electrons >900 keV were observed with equatorial fluxes above background (i.e., >0.1 el/(cm 2 s sr keV)) in the inner zone. The observed fluxes are compared to the AE9 model and CRRES observations. Electron fluxes <200 keV exceeded the AE9 model 50% fluxes and were lower than the higher-energy model fluxes. Phase space density radial profiles for 1.3 ≤ L* < 2.5 had mostly positive gradients except near L*~2.1, where the profiles for μ = 20-30 MeV/G were flat or slightly peaked. The major result is that MagEIS data do not show the presence of significant fluxes of MeV electrons in the inner zone while current radiation belt models and previous publications do.

Upper limit on the inner radiation belt MeV electron intensity

“…REPT represents a state‐of‐art instrument for accurately measuring the energetic particles in the magnetosphere—both REPT instruments on the twin Van Allen Probes show identical features throughout their entire orbits, including the outer belt, slot region, and the inner belt. This, supported by the high‐quality measurements of the MagEIS instruments that show no measurable >800 keV electrons in the inner belt [ Fennell et al , ], leaves little doubt that the upper limits of MeV electrons bounded by REPT measurements should be the standard for future model improvement, at least for periods like the last few years when no extreme solar wind conditions were observed.…”

“…Fennell et al [] have shown that the energy spectrum of relativistic electrons in the inner belt is very steep and the flux of ≥ 800 keV electrons are already at the background noise level of the MagEIS, while the flux of < 400 keV electrons is even higher than the AE9 mean value as shown in Figure .…”

High‐energy radiation belt electrons from CRAND

“…For E > Q the spectra approximate a power law ∼ E −4 . Also shown for comparison are exponential energy spectra that approximate electron intensity measured at L = 1.5, from the DEMETER low‐altitude satellite during 2009 for E < 800 keV [ Selesnick , ] and from Van Allen Probes near the magnetic equatorial plane during 2014 for E < 900 keV [ Fennell et al , ]. As there are no reliable measurements of higher‐energy electrons, the measured spectra are extrapolated to higher energies assuming the same exponential shapes.…”

“…Until recently it had been thought that the inner radiation belt electron intensity at any energy was much too high for a CRAND source [ Lyons and Thorne , ; Walt and Farley , ]. However, it is now apparent that electron measurements for MeV were contaminated by the abundant high‐energy protons, and the actual high‐energy electron intensity is much lower than previously thought [ Li et al , ; Fennell et al , ], except following a rare injection of higher‐energy electrons [ Blake et al , ]. Therefore, it is now of interest to reexamine the CRAND source of electrons, with particular attention to the high‐energy component that was not computed previously.…”

A new method to study the time correlation between Van Allen Belt electrons and earthquakes

“…Nevertheless, a good rule-of-thumb is that below about 700 keV, electron data in the inner belt is relatively safe to use. Latest results showed that there is an impenetrable barrier to ultrarelativistic electrons in the Van Allen radiation belts and that the inner radiation zone contains no MeV electrons by analyzing ECT-MagEIS data (Fennell et al 2015). Given the above facts, our study therefore focuses on hundreds of keV energetic electrons (∼350-450 keV) in the inner radiation belt with data from "Pixel" 3 and 4 of unit MED75-A,B (see Table 1 for details) from 7 September 2012 to 29 January 2015.…”