## 3 Results
### Case on 10 May 2024  
The super storm on 10 May was started with SSC of amplitude ~100 nT at ~17:05 UT, followed by the main phase ~1 h later. The storm peaked with SYM-H \(\sim-\)512 nT at ~2 UT on 11 May (the strongest storm of the last 25 years). Another feature of this powerful storm was a very strong substorm with AE \(\sim\) 3600 nT [53], which occurred between 19 and 20 UT, and the next day there were even two substorms with AE > 3000 nT. It should be noted that such strong substorms are rare events, classified as supersubstorms and intensively studied in recent years [54; 55; 56]. As shown in Table 1, at the beginning of the main phase (19:22 UT), one of five POES satellites (7-19 and 9-21 LT) detected the first FEE enhancement in the pre-noon sector (9 LT). Since intense fluxes of >30 and >100 electrons were observed simultaneously, we assume that the satellite was near the injection region. Note that the time 19:22 UT corresponds to the moment of crossing the equator, and the beginning of the increase in the flux was recorded at 19:15 UT. Figure 5 shows geomagnetic and solar wind conditions during the storm from 17:00 to 22:00 UT on 10 May 2024. The upper panel presents the SMR-12 index and X-components from low-latitude stations BSL (30.3\({}^{\circ}\) N, 89.6\({}^{\circ}\) W) and PPT (17.6\({}^{\circ}\) S, 149.6\({}^{\circ}\) W). At 19 UT local time was ~13 h at BSL and 9 h at PPT. Lower panels present the solar wind parameters observed by the Wind monitor at upstream location ~216 Re. The time delay is estimated at approximately 35 min at solar wind plasma speed of ~650 km/s. In Figure 5, within 1 h of the FEE-time, an abrupt pressure decrease (\(\Delta\)P \(\sim-\)20 nPa) is observed at 18:15 UT (time of Wind). The negative pressure change \(\Delta\)P is accompanied with a northward turn of \(Bz\) IMF. The type of magnetic structure is classified as an interplanetary sheath [57]. According to the low-latitude observations at BSL and PPT at ~19 UT, the geomagnetic field rapidly decreases (for 7 min) by ~60 and ~150 nT in the post-noon and pre-noon, respectively. At the beginning of the storm main phase, the ring current is asymmetrical and its influence on the dayside is still insignificant. Therefore, a change in the SMR-12 index also shows a large decrease in the daytime geomagnetic field. Hence, the observed large negative pressure change \(\Delta\)P led to the magnetosphere expansion and decrease in the X-components at daytime ground stations. [PERSON] et al. [53] reported that in this interval of the northward Bz IMF (see Figure 5) there was no energy input to the magnetosphere (Akasofu parameter was near zero) and the low-energy (tens of keV) particle fluxes in the inner and outer ERB decreased, as observed by the THEMIS probes. They suggested that particles intensively precipitated into the atmosphere over SAA through efficient scattering by wave-particle interaction. On the other hand, observations of the NOAA/POES fleet showed that soon after the sharp pressure decrease, at 19:15 UT, the fluxes of the 30-100 keV quasi-trapped electrons registered by NOAA/POES began suddenly to increase at L-shell of ~1.11 on pre-noon (Table 1). That is, these electrons could be injected below the inner ERB after 19 UT during an absence of the energy input. On the other hand, there were no particle injections at higher L-shells into the inner ERB (L < 2). The injections appeared later, after 21:30 UT, during a gradual increase of the energy input, as observed by THEMIS [53]. Clearly, the FEE-associated injection could not have been due to injections into the inner ERB in this storm. Quite surprisingly, close to this time, the supersubstorm onset was at 19:10 UT, peaked at 19:45 UT, and was recorded as a negative AL excursion at several magnetometer stations in the American sector (pre-noon) according to the INTERMAGNET network (see Supplementary Material).