## 5 Discussion  
The hit of the ICME gives rise to a compression of the magnetosphere and a backward motion of the plasmasphere, as shown in Figure 6. Indeed, the magnetopause (black line), modeled using the T01 model [39; 40] at the moment of the minimum value of the main phase of the geomagnetic storm (i.e., Dst minimum), steps back from \(\sim\)10\(R_{E}\) before the storm down to \(\sim\)7.7\(R_{E}\) (\(R_{E}\) being the Earth's radius); while the plasmapause, evaluated by the model of [PERSON] and [PERSON] [41], moves from \(\sim\)5\(R_{E}\) down to \(\sim\)3.8\(R_{E}\). On the other hand, the position of the inner boundary of the outer radiation belt (ORB) should reach \(L_{ORB}=3.5\), in accordance with the relation of [PERSON] [50] (\(|SyntH|_{max}=c\cdot L_{ORB}^{-4}\), where \(c=3\times 10^{4}\) nT). A depletion of the particle count rate (Figure 5) during the main phase of the storm is followed by a clear enhancement during recovery, which coincides with large substorm activity (\(>\)1000 nT) as measured by the AL index (red line in Figure 3f). The increase can be spotted at L-shells \(\gtrsim\)3 for energies above 3 MeV (Figure 5, top panel) and, to a lesser extent, at L-shells \(\gtrsim\)4 for energies above 4.5 MeV (Figure 5, second panel). Figure 5: Top three panels: Trigger rates for three different HEPD configurations over the period August-September 2018; from top to bottom, T, T & P1, and T & P1 & P2. Adding more calorimeter planes to the trigger configuration results in increasing the energy threshold for electron detection (\(>\)3 MeV for T, \(>\)4.5 MeV for T & P1, and \(>\)8 MeV for T & P1 & P2). The proton contribution to the trigger rate increase is negligible due to the absence of direct injection from SEPs. The vertical white lines are due to a lack of data. Bottom panel: Time evolution of the Dst index. As a comparison, the \(>\)130 keV electron fluxes measured over the same period by the 90\({}^{\circ}\) Medium Energy Proton and Electron Detector (MEPED) on board the NOAA19 satellite are reported in Figure 7. The azimuthal telescopes of the MEPED class are affected by small \(>\)280 keV proton contamination even at large L-shells in disturbed periods (\(<\)0.6% at 4 \(<\) L \(<\) 7) [35]. The sub-MeV MEPED-90\({}^{\circ}\) observations appear fairly consistent with their MeV counterpart detected by the HEPD, yet reaching flux peaks two orders of magnitude larger than the count rates captured at higher energies. The arrival of the ICME triggered a clear slot-filling event that lasts several days after the impact, with flux enhancements reaching L-shells lower than those occupied by MeV electrons, in accordance with the apparent \"barrier\" revealed by the Van Allen Probes to significant inward transfer of ultrarelativistic electrons below L \(\sim\)2.5 [51]. Prolonged and intense substorm activity during recovery (Figure 3f) shows that auroral processes play a non-negligible role in the analysis of particle acceleration in the  
Figure 6: Magnetopause (black line) and plasmapause (red line) profiles as evaluated using the Tsyganenko T01 and the [PERSON] (2014) models, respectively. The reference frame is the GSM. The SW parameters used in the T01 model are: \(P=8.7\) nPa; \(B_{y,IMF}=4.5\) nT; \(B_{z,IMF}=-16.8\) nT; \(\mathrm{Dst}=-174\) nT. The Dst index value used in the plasmapause model is the same as in T01. Figure 7: Integral electron fluxes (\(>\)130 keV) measured by the MEPED-90\({}^{\circ}\) directional telescope on board the NOAA19 satellite over the period Aug-Sept 2018. As reported in [35], proton contamination remains modest in this class of azimuthal detectors even at large L-shells in disturbed periods (\(<\)0.6% at 4 \(<\) L \(<\) 7). ORB and that electrons can undergo a quick acceleration on typical timescales of auroral substorms. Taking into account that the auroral oval (AO) is mapped into the outer portion of the ring current (see [52] and the references therein) can help us better understand the connection between auroral processes and ORB dynamics.