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.