Plain language summary
The energy spectra of ring current protons are significant for us to understand the ring current dynamics. Several previous studies reported the reversed energy spectra of ring current protons that represent flux minima at lower energies and flux maxima at higher energies. In the literature, there is rather limited statistical investigation on the global distribution features of such spectra. The availability of high quality Van Allen Probes RBSPICE data enables a comprehensive study in this regard. Based on RBSPICE measurements, we report the reversed energy spectra of ring current protons particularly featured by the distinct flux minima around 50 - 100 keV and flux maxima around 200 - 400 keV. We further adopt 4-year RBSPICE proton datasets from 2013 to 2016 to investigate the occurrence rate, corresponding energies of flux maximum and minimum, and the ratio of flux maximum to minimum as a function of L shell, MLT and geomagnetic activity level. Our results demonstrate that these reversed proton energy spectra are prevalent inside the plasmasphere, with the occurrence rates > 90% at L ~ 2 – 4 during geomagnetically quiet periods. The strong L and geomagnetic activity dependence imply the physical processes in association with proton substorm injections.