The Evolution of Earth's Outer Radiation Belt Over Geomagnetic Storm Phase in Van Allen Probe Era

Earth's outer radiation belts are highly dynamic during geomagnetic storms. Using electron flux data from 226 keV to 2.6 MeV measured by the Van Allen Probes, we statistically analyzed the peak flux position $\left(L_{\mathrm{peak}}\right)$ and inner boundary position $\left(L_{\min }\right)$ of the outer radiation belt across different storm phases: pre-storm quiet time, main phase, early recovery phase, and later recovery phase. This analysis covered 196 geomagnetic storm events from October 2012 to September 2019. Our results indicate that: (a) During the pre-storm, $L_{\mathrm{peak}}$ decreases with increasing energy. From the pre-storm to the early recovery phase, $L_{\mathrm{peak}}$ shifts inward for energies below 1 MeV and outward for energies above 1 MeV. For all energies, $L_{\mathrm{peak}}$ converges to approximately L = $\sim$4.3–4.6 in the early recovery phase. (b) Below approximately 1 MeV, $L_{\min }$ generally move inward from the main phase to the early recovery phase. Above 1 MeV, $L_{\min }$ remains nearly unchanged across different storm phases. (c) The half-width ($L_{\mathrm{peak}}$−$L_{\min }$) of the outer belt decreases during the main phase for energies below 1 MeV and increases during the recovery phase for energies above 1.5 MeV. (d) In the early recovery phase, $L_{\mathrm{peak}}$ and $L_{\min }$ at 593–742 keV show a moderate correlation with storm intensity ($\mid$CC$\mid \sim$ 0.7–0.8), while $L_{\mathrm{peak}}$ and $L_{\min }$ at energies greater than 1.1 MeV exhibit low correlations ($\mid$CC$\mid \le \sim$0.4) during each phase. These results confirm the complex, energy-dependent morphology of the outer radiation belt throughout geomagnetic storm phases.