Ultrarelativistic electrons in the Van Allen belts: RPS observations and Geant4 simulations

Abstract:

The Relativistic Proton Spectrometer (RPS) aboard the Van Allen Probes spacecraft is designed to measure protons from about 60 MeV to multiple GeV, but it is also sensitive to electrons above several MeV. Its Cherenkov subsystem provides energy resolution for protons above a few hundred MeV, and electrons at extremely high energies, around 50 MeV and above, can also produce high levels of Cherenkov light.

While mapping protons in the inner Van Allen Belt with RPS, Mazur et al. (Fall 2014 AGU meeting, paper SM22A-02) observed a concentration of particle events around L = 2 with Cherenkov light corresponding to protons at energies well above the limit for stable trapping there. We present a preliminary analysis that shows that the patterns of the Cherenkov light distribution are consistent with these particle events instead being caused by electrons at energies of at least several tens of MeV. This energy range is well above that expected from magnetospheric energization, even by a violent event like the March 1991 shock, which injected electrons peaked around 15 MeV (Looper et al., GRL 1994, doi:10.1029/94GL01586). We discuss the possibility that these electrons are instead due to the decay of pions and muons produced by cosmic-ray interactions with the atmosphere, with a characteristic energy set by the pion rest mass of 140 MeV.