@article{10.1038/s41467-020-18053-z,
  author   = {Allison, Hayley J. and Shprits, Yuri Y.},
  title    = {Local heating of radiation belt electrons to ultra-relativistic energies},
  journal  = {Nature Communications},
  year     = {2020},
  doi      = {10.1038/s41467-020-18053-z},
  volume   = {11},
  number   = {1},
  abstract = {Electrically charged particles are trapped by the Earth’s magnetic field, forming the Van Allen radiation belts. Observations show that electrons in this region can have energies in excess of 7 MeV. However, whether electrons at these ultra-relativistic energies are locally accelerated, arise from betatron and Fermi acceleration due to transport across the magnetic field, or if a combination of both mechanisms is required, has remained an unanswered question in radiation belt physics. Here, we present a unique way of analyzing satellite observations which demonstrates that local acceleration is capable of heating electrons up to 7 MeV. By considering the evolution of phase space density peaks in magnetic coordinate space, we observe distinct signatures of local acceleration and the subsequent outward radial diffusion of ultra-relativistic electron populations. The results have important implications for understanding the origin of ultra-relativistic electrons in Earth’s radiation belts, as well as in magnetized plasmas throughout the solar system.}
}