4. Discussions
Our results show that the reversed proton spectra are preferentially
inside the plasmasphere and show significant losses of lower energy
(~50 - 100 keV) protons after the geomagnetic storm
(Figures 1f-h). It is significant for us to further understand the
mechanisms that produce the reversed proton energy spectra. There are
several possible explanations for the formation of the reversed proton
energy spectra.
Firstly, charge exchange is found to be the main loss process of ring
current protons by capturing electrons from neutral atoms (Ebihara &
Ejiri, 2003; Dessler & Parker, 1959; Smith & Bewtra, 1976). The
lifetime of proton due to charge exchange is shortest at energies around
tens of keV with the value of 0.2 – 1 day (Fok et al., 1991). This
energy range is consistent with our statistical distributions of
Emin in reversed proton spectra. Furthermore, the
densities of neutral hydrogen are higher at lower L shells (Østgaard et
al., 2003). Thus, the loss effect due to charge exchange is stronger at
lower L shells. We also find that the reversed proton energy spectra
show a high occurrence at low L shells (L=2 - 4). These agreements in
spatial distribution suggest that charge exchange plays an important
role in the formation of reversed proton energy spectral.
Another candidate for the loss of protons is Coulomb collision (Fok et
al., 1996; Jordanova et al., 1996, 1999). When particles travel through
the plasma, they will loss energy or change pitch angles due to
collisions with other particles. Similar to charge exchange, Coulomb
collision also likes to occur in lower L shells (Fok et al., 1991).
However, this process is dominant in decreasing the proton fluxes at
low-energies (<10 keV) (Fok et al., 1996) and is not likely to
produce the reversed proton energy spectra with local minima at energies
~50 - 100 keV.
There are also two collisionless scattering mechanisms for the ring
current decay: wave-particle interactions and field line curvature (FLC)
scattering. The electromagnetic ion cyclotron (EMIC) waves can
effectively scatter several keV to hundreds of keV protons into the loss
cone due to pitch angle diffusion with a time scale of a few hours (Cao
et al., 2019, 2020; Cornwall, 1977; Jordanova et al., 1997; Xiao et al.,
2011; Summers, 2005). The field line curvature scattering is of
importance for the ring current decay when the field line configuration
is stretching (Chen et al., 2019; Ebihara et al., 2011; Sergeev et al.,
1993; Yu et al., 2020). Yu et al. (2020) investigated the role of FLC
scattering in ring current decay during the 17 March 2013 storm and
found that the associated proton precipitation mainly occurs at L
> 5 on the nightside. This finding is basically consistent
with our observations that the reversed proton energy spectra are
distributed at L > 5 on two MLT sectors (15-19, 22-05). The
formation mechanism of reversed energy spectra at L>5 may
be different from those at L<5.
Although several loss mechanisms have been proposed to explain the decay
of Earth’s ring current. The relative contributions of difference
mechanisms to the formation of the reversed proton energy spectra still
remain to be fully understood, which however is outside the scope of
this study and is left to a future study.