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.