1. Introduction
Energy spectrum distributions of energetic particles are an essential
indicator for understanding the underlying physics of magnetospheric
particle dynamics. In general, it is well recognized that both electron
and proton energy spectra in space plasmas follow a Kappa or Kappa-like
profile having fluxes steeply falling with increasing energy (e.g.,
Freden, 1965; Pizzella et al., 1962; Summers et al., 2009). In contrast,
a number of previous studies reported ‘reversed’ energy spectra of
energetic particles that do not follow monotonically decrease profiles
of fluxes with increasing energy but represent flux minima at lower
energies and flux maxima at higher energies. Based on the Molniya 1
measurements, Vakulov et al. (1975) observed a flux maximum of the outer
belt electron energy spectra at energy ~1- 2 MeV. West
et al. (1981), using 60 keV to 3 MeV electron data from Ogo5, showed a
flux maximum at ~1.5 MeV and a minimum at
~500 keV. By analyzing the ring current proton spectra
(1 - 300 keV) observed by the Explorer 45, Williams et al. (1973)
reported the flux minima of proton energy spectra at energies
~20 - 100 keV over L ~ 3.2 - 4.0, which
they attributed to the ion-cyclotron instability. Based on the AMPET CCE
measurements, Krimigis et al. (1985) observed the flux maxima at
energies ~100 - 300 keV during both the main and
recovery phases of a geomagnetic storm.
Along with the launch of the twin Van Allen Probes mission in August
2012 (Mauk et al., 2012), the long-term accumulation of unprecedented,
high quality particle datasets with fine energy resolutions has fueled a
resurgence in magnetospheric particle dynamics studies. By combining the
MagEIS and REPT measurements, Zhao et al. (2019a) reported a reversed
energy spectrum of radiation belt electrons, which is featured by the
flux maximum ~1 – 2 MeV and flux minimum
~ 100’s keV and is reasonably accounted for
energy-dependent electron losses induced by hiss wave (e.g., Ni et al.,
2019; Fu et al., 2020). A following statistical analysis of Zhao et al.
(2019b), on basis of the detailed analysis of reversed electron energy
spectra, found that reversed energy spectra dominate inside the
plasmasphere at L < 2.5.
Comparatively, there remains lacking the systematic information about
the global distribution of the reversed energy spectra of ring current
protons. The availability of Van Allen Probes RBSPICE data enables a
comprehensive study in this regard. Therefore, this letter is dedicated
to investigate the global distribution of reversed proton energy spectra
based upon multi-year RBSPICE data from Van Allen Probes. By
establishing an automatic identification criterion for the reversed
proton energy spectra, we intend to perform a statistical analysis to
explore the global distribution features of the reversed proton energy
spectrum, its occurrence pattern, its dependence on geomagnetic
activity, and its profile characteristics in terms of the energy range
of local flux minimum and the ratio of local flux maximum to minimum.