Transitional behavior of different energy protons based on Van Allen Probes observations

Abstract

Understanding the dynamical behavior of ~1 eV to 50 keV ions and identifying the energies at which the morphologies transit are important in that they involve the relative intensities and distributions of the large-scale electric and magnetic fields, the outflow, and recombination rates. However, there have been only few direct observational investigations of the transition in drift behaviors of different energy ions before the Van Allen Probes era. Here we statistically analyze ~1 eV to 50 keV hydrogen (H+) differential flux distributions near geomagnetic equator by using Van Allen Probes observations to investigate the H+ dynamics under the regulation of large-scale electric and magnetic fields. Our survey clearly indicates three types of H+ behaviors within different energy ranges, which is consistent with previous theory predictions. Using simple electric and magnetic field models in UBK coordinates, we have further constrained the source regions of different energy ions and their drift directions.