The Mother's Day Solar Storm of 11 May 2024 and Its Effect on Earth's Radiation Belts

Citation: Pierrard, V.; Winant, A.; Botek, E.; Péters de Bonhome, M. The Mother’s Day Solar Storm of 11 May 2024 and Its Effect on Earth’s Radiation Belts. Universe 2024,10, 391. https://doi.org/10.3390/universe 10100391 Academic Editor: Ezio Caroli Received: 4 September 2024 Revised: 27 September 2024 Accepted: 5 October 2024 Published: 10 October 2024 Copyright: © 2024 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). universe Article The Mother’s Day Solar Storm of 11 May 2024 and Its Effect on Earth’s Radiation Belts Viviane Pierrard 1,2,*,† , Alexandre Winant 1,2,†, Edith Botek 1and Maximilien Péters de Bonhome 1,2 1Solar Wind, Space Physics and Solar-Terrestrial Center of Excellence, Royal Belgian Institute for Space Aeronomy (BIRA-IASB), Avenue Circulaire 3, B-1180 Brussels, Belgium; [email protected] (A.W.); [email protected] (E.B.); [email protected] (M.P.d.B.) 2Center for Space Radiations (CSR), Earth and Life Institute—Climate Sciences ELI-C, Université Catholique de Louvain (UCLouvain), B-1348 Louvain-la-Neuve, Belgium *Correspondence: viviane.pierrar[email protected] †These authors contributed equally to this work. Abstract: The month of May 2024 was characterized by solar energetic particles events directed towards the Earth, especially the big event causing a strong terrestrial geomagnetic storm during the night from 10 to 11 May 2024, with auroras observed everywhere in Europe. This was the strongest storm for the last 20 years with a Disturbed Storm Time index Dst < − 400 nT. In the present work, we show with observations of GOES, PROBA-V/EPT and MetOP/MEPED that this exceptional event was associated with the injection of energetic protons in the proton radiation belt, with important consequences for the South part of the South Atlantic Anomaly (SAA). In addition, the geomagnetic storm caused by the solar eruption has had tremendous impacts on the electron radiation belts. Indeed, we show that for 0.3 to 1 MeV electrons, the storm led to a long lasting four belts configuration which was not observed before with EPT launched in 2013, until a smaller geomagnetic storm took place at the end of June 2024. Moreover, for the first time since its launch, observations of the EPT show that ultra-relativistic electrons with E> 2 MeV have been injected into the inner belt down to McIlwain parameter L = 2.4, violating the impenetrable barrier previously estimated to be located at L = 2.8. Keywords: geomagnetic storm; solar energetic particles; solar event; radiation belts; energetic protons; relativistic electrons 1. Introduction: The 11 May 2024 Event in the Context of Previous Storms Throughout the month of May 2024, several active regions rotated across the solar disc. One of them, called NOAA 13664, especially grew in size and magnetic complexity, and gave rise to numerous large flares, including several X-class flares [ 1 ]. Associated with the activity around this active region were a number of moderate to large fast Coronal Mass Ejections (CMEs), many of which were Earth directed. Active region NOAA 13664 was the largest sunspot group in more than 10 years. It produced the strongest solar flare so far this solar cycle, and was the most flare productive region in more than 3 decades. It is the source of the coronal mass ejection that led to the strongest geomagnetic storm in two decades. On 8 and 9 May, the Sun produced no less than 28 Mand X-class flares, 20 of them of NOAA 13664. NOAA 13664 now occupies the 4th spot in terms of highest Mand X-class flare production since the start of the GOES measurements in 1976. The sunspot group is obviously the largest of the solar cycle so far (2400 millionths of a solar hemisphere, or 14 times the total surface of the Earth). Due to the quick succession and varying speeds of the CMEs, several of them merged and interacted as they travelled through the interplanetary medium leading to enhanced effects in the Earth’s space environment. These subsequent complex magnetic structures traversing the interplanetary medium increased the geo-effectiveness of the events at Earth. Universe 2024,10, 391. https://doi.org/10.3390/universe10100391 https://www.mdpi.com/journal/universe Universe 2024,10, 391 2 of 18 The characteristics of the solar wind measured by OMNI when it reached 1 AU are illustrated on Figure 1. The arrival time of event at 1 AU with regard to solar wind density, velocity and pressure is 10 May 2024 at 17:07 UT. A clear peak of solar wind density, bulk velocity (top panel), temperature and pressure (second panel) is observed on 11 May 2024. The bulk velocity peak (that reached almost 1000 km/s) and the temperature perturbations are longer in time than the density shock. The pressure combines the density and velocity effects, and determines the position of the magnetopause. The inverted negative peak of the Z component of the Interplanetary Magnetic Field Bz (third panel) showing a southward direction implies a strong answer of the geomagnetic field. The solar event indeed gave rise to the largest geomagnetic storm observed in 20 years in the night of 11 May. This storm has been called “Mother’s Day” event, because the date corresponds to Mother’s Day in many countries around the world. Figure 1. Parameters of the solar wind at 1 AU and geomagnetic indices from OMNI from 1 May to 30 June 2024. Top panel: solar wind density (blue) and solar wind speed (red). Second panel: solar wind pressure (blue) and solar wind temperature (red). Third panel: Southward component of the interplanetary magnetic field Bz .Bottom panel: Dst index (blue) and Kp index multiplied by 10 (red). The geomagnetic activity indices of Bartels (Kp) and Disturbed storm time (Dst in nanoTesla) are illustrated in the bottom panel of Figure 1. The initial phase of the storm started at the arrival of the solar wind perturbation, i.e., on 10 May 2024 at 17:07 UT. At this point in time, a strong sudden commencement is observed in the Dst index. During the main phase, Dst drastically dropped to − 412 nT (and a higher resolution minimum SYM-H Universe 2024,10, 391 3 of 18 of − 512 nT not shown here) on 11 May 2024 at 02:14. It is the largest geomagnetic storm (lowest value of Dst) since the November 2003 Halloween events. It is only in OctoberNovember 2003 and November 2004 that storms of comparable strength are found. The 11 May 2024 storm is the 7th day with the most negative Dst value since the measurements started in 1957 (WDC Kyoto—https://wdc.kugi.kyoto-u.ac.jp/ (accessed on 2 September 2024)). It ranks just behind 20 November 2003 but before other major storms such as on 26 May 1967, 31 March 2001 and 30 October 2003. Only the famous 13–14 March 1989 storm ( − 589 nT) and 3 days in the late 1950s have a more negative Dst. As Dst values are only available since 1957, one should not forget there were also other big storms in the past, such as the Carrington event of September 1859, the 4 February 1872 storm, and the 13–15 May 1921 storm. For these events, much stronger individual Dst values of respectively − 949 nT, −834 nT [2,3] and −907 nT [4] have been estimated. The 11 May 2024 storm persisted through the weekend giving rise to auroras at low magnetic latitudes (below 45°) in both hemispheres [ 5 ]. The space weather at this time also resulted in strong radio blackouts and solar energetic particle events with radiation-belt enhancements throughout the days following. This big event had strong effects on the terrestrial radiation belts that will be explained in the present work. Section 2presents the instruments and data used to measure the fluxes in the radiation belts. Section 3presents the injection of protons during the event, while Section 4shows the modifications of the electron fluxes. The new results are discussed in Section 5and conclusions are presented in the Section 6. 2. Instruments and Data 2.1. PROBA-V/EPT The Energetic Particle Telescope (EPT) measures the fluxes of high energy particles in the radiation belts. This instrument was developed by the Center for Space Radiation (CSR) at UCLouvain in Belgium, with the collaboration of the Royal Belgian Institute for Space Aeronomy and QinetiQ Space (presently Redwire Space). This instrument was launched on 7 May 2013 onboard the ESA (European Space Agency) satellite PROBA-V. The spacecraft was sent to a sun-synchronous polar Low Earth Orbit (LEO) at an altitude of 820 km, with an inclination of 98.73° and a descending node at 10:30 am local time [ 6 ]. The concept of the EPT is based on the Bethe-Block formula giving the relationship between the stopping power of a material and the energy of incident charged particles [ 7 ]. The EPT was designed for real-time and contamination-free measurements of charged particle spectra in the space environment and is able to discriminate between electrons, protons, alpha particles and heavier ions while performing direct measurements of their energy spectra [ 8 ]. The EPT features two energy sections. The Low Energy Section (LES) only measures lower energy electron fluxes, while the High Energy Section (HES) measures electron, proton and heavier particle fluxes of higher energy. The EPT measures differential flux of electrons above 500 keV in 6 energy channels, and of protons above 9.5 MeV in 10 energy channels. 2.2. SEM-2/MEPED The Space Environment Monitor 2 (SEM-2) [ 9 ] consists of a suite of instruments that monitor the flux of protons and electrons in near-Earth space. One of the two main instruments of the SEM-2 suite is the Medium Energy Proton and Electron Detector (MEPED) which focuses on measuring fluxes of energetic protons and electrons coming directly from the Sun and trapped in the Earth’s geomagnetic field, i.e., forming the radiation belts. The SEM-2 suite was first fitted on the NOAA-15 (National Oceanic and Atmospheric Administration) POES (Polar Orbiting Environmental Satellites) in 1998 and then on all later versions NOAA-16 to -19. In 2006, SEM-2 was also accommodated on the ESA MetOp-A and also on the later versions MetOp-B and MetOp-C. All the satellites on which SEM-2 is fitted on operate on heliosynchronous LEO at an average altitude of 850 km for NOAA/POES and 820 km for MetOp. Universe 2024,10, 391 4 of 18 The MEPED instrument is composed of two pairs of directional detectors. The first pair is dedicated to the measurement of protons with energies ranging from 30 keV to 200 MeV in 5 differential channels and one integral channel. The second pair of detectors measures the fluxes of electrons of energies between 30 keV to 2500 keV in 3 integral channels. For a given type of particles, the two telescopes are arranged perpendicular to one another and are referred to as the 0° telescope and the 90° telescope. On MetOp, the 0° telescope points directly to the zenith and the 90° telescope points to the antiram direction (i.e., opposite to the velocity vector of the spacecraft). The particular arrangement of the MEPED telescopes allows to characterize both trapped and precipitating particles. Indeed, the 90° telescope mostly measures particles that are trapped in the geomagnetic field whereas the 0° telescope mainly measures particles in the Bounce Loss Cone (BLC) and thus precipitating into the atmosphere. 3. The Effects of the 11 May Storm on the Proton Radiation Belt 3.1. Protons Observed by GOES at Geostationary Orbit The Geostationary Operational Environmental Satellites (GOES), operated by NOAA, also provide observations of solar proton fluxes at the geostationary orbit [ 10 ]. Energetic protons detected by GOES during the event are illustrated on Figure 2. The solar event is associated to the arrival of solar energetic protons: the flux > 10 MeV (in blue) starts increasing on May 8 at 08h00 and reaches on 10 May at 11h00 the threshold of 10 cm−2s−1sr−1 , generally used to qualify an eruption as a solar energetic particle (SEP) event. Peak flux for > 10 MeV protons is first reached on 10 May at 17h00 (corresponding to the Dst storm), quickly followed by a peak on 11 May at 09h00, visible also for higher energy protons, even for E>100 MeV for instance (in green). Figure 2. GOES observations of proton fluxes with energy > 10 MeV (blue), > 50 MeV (orange) and >100 MeV (green) at the geostationary orbit from 1 May 2024 to 30 June 2024. On 13 May, a second peak of the > 10 MeV flux appears above the threshold. An increase is then also visible for >50 MeV, but not for >100 MeV. Note that after a full solar rotation of 27 days, the sunspot still present at the Sun’s surface was responsible of a new solar energetic particle event appearing on 8 June 2024 together for all energies. Since the protons originate from the same active region, the variation of the proton flux in time is very similar to that of 11 May, with a second peak a few days later for 10 and 50 MeV. It is worth mentioning that the first maximum fluxes reached at all energies are higher during the later event. Nevertheless, this second SEP event did not create a geomagnetic storm, which is not so unusual since only 75% of SEP events are followed by Dst storms <−50 nT [11]. 3.2. Proton Injections Observed at LEO Energetic protons detected by PROBA-V/EPT and MetOp/MEPED on a low polar orbit are illustrated in Figures 3and 4. Because those two instruments are located on a low Earth orbit, they are able to measure the flux of protons across the entire range of the Universe 2024,10, 391 5 of 18 radiation belts, i.e., at different values of the McIlwain [ 12 ] parameter L, as opposed to GOES which perform measurements at a fixed location in the outer belt (L∼6.6). Figure 3. Proton differential fluxes observed by PROBA-V/EPT from 1 May to 30 June 2024 as a function of the McIlwain parameter L (vertical axis) and time (horizontal axis) in the first 5 EPT proton energy channels. Fluxes are averaged in bins which are 6 h long in time and 0.25 in L. From top to bottom, the energy of each channel increases and they all share the same colorbar. Figure 4. Proton differential fluxes observed by MEPED from 1 May to 30 June 2024 as a function of the McIlwain parameter L (vertical axis) and time (horizontal axis) in 5th proton energy channel of MEPED. Fluxes are averaged in bins which are 6 h long in time and 0.25 in L. Top panel: 0° telescope, Bottom panel: 90° telescope. In Figure 3, the first 5 proton energy channels of EPT are displayed, with increasing energy from the top panel to the bottom one. Proton fluxes are averaged in L and time bins (L bins: 0.25, time bins: 6 h). Similar to Figure 3, Figure 4presents the differential proton fluxes measured by the 0° and 90° telescopes of MEPED in their highest energy channel (2.5–6.9 MeV). MEPED fluxes have been averaged in the same bins in time and L as for the EPT. Universe 2024,10, 391 6 of 18 It is important to note that even though EPT and MEPED are both observing particle fluxes at a similar altitude, their observations cannot be directly compared to one another, at least quantitatively. There are two main reasons for this: 1. The energy ranges of the measured protons are not the same for the first channel of the EPT and the fifth channel of MEPED. It is thus expected that the flux of protons observed by MEPED is higher than those observed by the EPT, since it measures fluxes at lower energy. 2. The pitch angles of the observed particles differ between the instruments. Indeed, the MEPED 90° telescope can be considered to measure fluxes of trapped particles and the 0° telescope observes the precipitating particles (at least at high latitudes and thus high L values), while EPT measures particle fluxes with a pitch angles between 60° and −60°. Thus, the EPT measures a combination of trapped and precipitating particles. At high L values, especially around L = 6.6, the variations of the proton flux at various energies in time are very consistent with the observations from GOES. Observations from EPT, providing the dependence in energy of the flux, confirm that lower energy protons ( ∼ 10 MeV) are injected first at high L, rapidly followed by the higher energy protons. EPT actually shows that there are already no more double peaked injection for protons with energies >61 MeV, as confirmed by the >100 MeV measured by GOES integral channels. At lower energies ( ∼ 2 MeV), MEPED observations are similar to those of EPT in the first channel. Nonetheless, the different energy ranges of EPT show the different behaviours depending on the energy. The first peak flux of lower energy protons is much larger than above 10 MeV. Moreover, the increased proton fluxes remain longer (1 more day) than at higher energies. During the event of 11 May, protons were injected deep into the radiation belt. Proton flux sharply rose, reaching L ∼ 2.5 in all energy channels displayed in Figure 3, and flux increases were also observed up to channel 8 (182–205 MeV). For protons between 9.5 MeV and 13 MeV, observations of EPT show that protons were injected in the inner belt. This is the first time since the launch of EPT that direct injection of proton in the proton trapped belt is observed [ 11 ]. In channel 1 of EPT, the injection of protons in the inner belt appears between L = 1.9 and 2.1 and these protons form an additional inner belt remaining trapped during more than one month, with a flux slightly decreasing with time. This is also confirmed by the observations from the MEPED 90° telescope. Indeed, channel 5 of MEPED (shown in Figure 4) was proven to be free of electron contamination in [ 13 ] by simulations with GEANT-4. Even though they cannot be directly compared to one another, the first EPT channel and the fifth MEPED channel are complementary, and clearly show that low energy protons (i.e., 2.5 MeV to 13 MeV) were injected in the inner belt. In channels 2 to 4, starting on 13 May, protons are also observed at higher L (between L = 2.5 and L = 4), forming an additional temporary belt which remains only for several days. Those observations may actually come from contamination of these proton channels by electrons. Indeed, those additional belts are appearing at the same time and at the same L shells as the electron fluxes appear after the strong geomagnetic storm associated with this event. 3.3. Effects in the South Atlantic Anomaly Figure 5shows the map of 9.5–13 MeV protons observed by the EPT first channel at different characteristic phases of the event described above. At high latitudes (corresponding also to high L), fluxes are observed only in panels (b,d) corresponding to the periods of proton injections in May and June. Universe 2024,10, 391 7 of 18 Figure 5. Proton fluxes observed in the first EPT proton channel averaged in longitude and latitude bins during four different periods covering most of the period considered in previous figures. The averaging bins here have a width of 10° in longitude and 5° in latitude. Each panel corresponds to a different period: (a) quiet conditions from 1 May to 9 May, (b) storm time and beginning of the recovery from 10 May to 20 May, (c) recovery period from 21 May to 31 May, (d) second proton injection from 1 June to 15 June. Figure 5also clearly illustrates the South Atlantic Anomaly (SAA), area where the fluxes are always very high above South America and South Atlantic. The SAA is a region where inner radiation belt particles can mirror at lower altitudes, increasing the local particle flux. This is due to the fact that the Earth’s magnetic field is particularly low in the SAA, caused by the tilt of approximately 11° of the Earth’s magnetic axis with respect to the Earth’s rotation axis and the location of the magnetic idealized dipole some 400 km away from the Earth’s center. EPT observations indicate that protons at this energy are injected in the SAA corresponding to the inner radiation belt at low altitudes. One can see that it is only the southernmost part of the SAA that is filled after this event in the channel 9.5–13 MeV. Such injections were never directly observed by EPT in the past, even if they could explain the different characteristics observed between the south and north part of the SAA noted in [ 11 , 14 ]. This allows us to exclude the effects of electron contamination in this energy range, since the regions filled by the electrons are located at different L values (see Section 4). 3.4. Neutron Monitors Observations Two main processes responsible for the presence of trapped protons at low L values have been identified: the Cosmic Ray Albedo Neutron Decay (CRAND) and the radial diffusion of injected solar protons [ 15 , 16 ]. While CRAND mainly contributes to the protons flux peaking at L ∼ 1.5 for higher energy, inward diffusion of injected solar protons is the principal source of the main proton belt at lower energies [ 17 ]. In this later work, focusing on proton observation between 1998 and 2006, solar proton trapping were shown to mainly take place at L = 2.3, whereas during the event of 11 May 2024, solar protons were directly injected in the secondary protons belt at L = 2, corresponding to the southernmost part of the SAA. In case of solar energetic particle event like on 11 May and 8 June 2024, protons with enough energy and rigidity can reach the upper atmosphere and interact with neutral atoms to produce neutrons. Earth-bound energetic neutrons are measured by ground neutron monitors (NM). Backscattered neutrons entering the magnetosphere are a possible Universe 2024,10, 391 8 of 18 source of geomagnetic trapped protons and electrons through Beta-decay (Solar Neutron Decay), it is therefore interesting to analyze observations of NM as well. The event of 11 May has been measured by many NM, even at latitudes as low as 49°, as illustrated in Figure 6. The characteristics of the stations where the neutron monitors have measured are summarized in Table 1. Figure 6. Observations of neutron monitors at different stations specified in Table 1, located at all latitudes like Dourbes (Belgium, 50° lat) and SOPO (South Pole latitude − 90°). The perturbation during the night of 10 to 11 May 2024 is well visible. The neutron decrease during the storm (Forbush decrease) is immediately followed by a Ground Level Enhancement. Table 1. Characteristics of the ground-based neutron monitor stations whose data are shown in Figure 6: Full name, short name, altitude, Cutoff Rigidity Rc, latitude and longitude. Full Name Short Name Alt (m) Rc (GV) Coordinates (Lat, Longitude) in Degree Lomnicky LMKS 2634 3.84 49.2000, 20.2200 Dourbes DRBS 225 3.18 50.0971, 4.59003 Kiel KIEL 54 2.36 54.3399, 10.1199 Oulu OULU 15 0.81 65.0544, 25.4681 Apatity APTY 181 0.65 67.5704, 33.3935 South Pole SOPO 2820 0.1 −90.000, 0.00000 Neutron monitors show a clear disturbance during the geomagnetic storm. Figure 6 shows a drop of neutrons, called a “Forbush decrease” that started in the evening of 10 May. It is the result of the arrival of the energetic particles of the CME at the Earth. During the storm, more solar particles were injected in the atmosphere, but less Galactic cosmic rays, because the solar particles create a stronger magnetic shield against the cosmic ray particles originating from outside our solar system. That is why the number of detected neutrons decreases during the storm. It is quite unusual that this effect is observed even at low latitudes (such as in the Dourbes station in Belgium). The small bump in the neutron count, which happens just after the decrease, right during the geomagnetic storm of 10–11 May, is a Ground Level Enhancement (GLE). It is only the second GLE so far during this solar cycle and number 74 since the measurements started back in the 1940s (Oulu, https://gle.oulu.fi (accessed on 2 September 2024)). It is Universe 2024,10, 391 9 of 18 associated with the arrival of proton fluxes with energy higher than 100 MeV (see Figure 2). This GLE 74 is not particularly strong, but observed due to the Forbush decrease. After the event, the level of neutrons remains lower than before the storm even after one month. Note that nothing special was visible on 8 June 2024 in the neutron monitor measurements at the arrival of the second SEP event. 4. Effects on the Electron Radiation Belts 4.1. Observations of New Electron Belts by EPT Solar Proton Albedo Neutron Decay (SPAND) can also produce electrons in the slot region during GLEs, but only at L > 2 because the lower energy of SEP’s compared to cosmic rays limits their access to smaller L’s [ 15 ]. For lower L, CRAND effect makes a small but continuous contribution to the electrons in the inner belt and slot region. Figure 7shows the temporal evolution of the EPT electron differential fluxes averaged on 6 h and 0.25 L bins from 1 May to 30 June 2024. Each panel corresponds to an energy channel of the EPT, increasing in energy as panels are located lower on the figure. Figure 7. Electron differential fluxes observed from 1 May to 30 June 2024 as a function of the McIlwain parameter L (vertical axis) and time (horizontal axis) in the 6 EPT electron energy channels. Fluxes are averaged in bins which are 6 h long in time and 0.25 in L. From top to bottom, the energy of the channels increases and they all share the same color bar. Previous studies have shown that geomagnetic storms have a strong impact on the radiation belts, especially the outer electron belt (e.g., [ 18 ]). For most storms, the perturbations are relatively short lived and follow the same pattern, as also observed during this superstorm. At low altitudes corresponding to the EPT orbit, the main phase of the storm (i.e., when the Dst decreases) causes a sudden drop in the electron flux at high L values called a dropout event [ 19 ]. Droupout events are rapid erosions of the outer edge down to lower L values. The minimum L values reached by the dropout depends on the intensity of Universe 2024,10, 391 16 of 18 with different wave-particle interactions that depend on the energy, the position of the particles and the geomagnetic conditions. 6. Conclusions The mother’s day solar storm of 11 May led to strong injection of energetic particles in the radiation belts, together with a geomagnetic storm of exceptional magnitude. Following those events, the particle populations of the radiation belts were greatly disturbed providing us with some rare observations of the dynamics of the belts under such extreme forcing. During the period of observation presented in this work (May and June 2024), different instruments observed two separate energetic proton events: The first event occurred on 11 May 2024 and injected protons in the trapped proton belt, for the first time since the launch of EPT, while the second took place on the 8 of June, which puts them 27 days apart corresponding to a rotation of the Sun on itself. The temporal evolution of the flux at L = 6.6 is similar for the two events, with a double peak for both > 10 MeV and > 50 MeV protons while at higher energies only one peak is observed. Those two characteristics indicate that the two proton injections that were observed during the period of interest originate from the same solar active region. The discrepancies between the two events can be associated to the strong difference on the geo-effectiveness of the two events (see Figure 1). Indeed, the first event was associated with the strongest geomagnetic storm observed in two decades, while during the second SEP event no storm was observed. The much deeper penetration of protons in the trapped belt during the 11 May event can be explained by the large sudden commencement (SC) of the geomagnetic storm leading to the rapid compression of the magnetosphere which in turn can cause a pulse in the electric field, accelerating and injecting particles to lower L values [ 32 ]. The inner radiation belt is known to be more stable in time than the outer belt, with a strong response to solar cycle [11]. The geomagnetic storm of 11 May caused extreme electron flux variation in both the inner and outer belt, but also led to the formation of complex belt structures several weeks after the storm. One important feature shown in Figure 9is that the injection of electrons following the storm initially takes place at lower L for low energy. Observations from MEPED > 300 keV electrons also confirms that injection at those low energies takes place at low L due to the extremely low Dst value. Rapidly after the injections, the electron profiles are modified by losses appearing at very specific L that are different as a function of energy (see for instance Figures 7and 9). This suggests that the acceleration mechanisms for electrons between 300 keV and several MeV, and even more the losses are extremely dependent on energy. Those gradual losses lead to the formation of multiple belts depending on the energy. For 300 keV to 800 keV electrons, two distinct belts are formed above the main slot and two in the inner belts. In the highest energy range of the EPT, ultra-relativistic electrons are directly injected in the inner belt, violating the impenetrable barrier observed by the Van Allen Probes [21], or at least decreasing its limits to L = 2.4. Author Contributions: Conceptualization, V.P. and A.W.; methodology, V.P.; software, A.W.; validation, V.P., A.W. and E.B.; formal analysis, A.W.; investigation, V.P. and A.W.; resources, V.P.; data curation, A.W.; writing—original draft preparation, V.P.; writing—review and editing, A.W., E.B. and M.P.d.B.; visualization, V.P., A.W. and M.P.d.B.; supervision, V.P.; project administration, V.P.; funding acquisition, V.P. All authors have read and agreed to the published version of the manuscript. Funding: The project 21GRD02 BIOSPHERE has received funding from the European Partnership on Metrology, co-financed by the European Union’s Horizon Europe Research and Innovation Programme and by the Participating States, and from Horizon 2020 PITHIA-NRF project with Grant Agreement 101007599. Universe 2024,10, 391 17 of 18 Data Availability Statement: The data presented in this study are available in a repository at DOI reference number 10.5281/zenodo.13626553. These data were derived from the following resources available in the public domain: PROBA-V/EPT data are publicly available on the Space Situational Awareness website of ESA https://swe.ssa.esa.int/space-radiation (accessed on 2 September 2024). POES data are available on https://satdat.ngdc.noaa.gov/sem/poes/data (accessed on 2 September 2024). GOES on https://www.swpc.noaa.gov/products/goes-proton-flux (accessed on 14 August 2024) and OMNI on https://omniweb.gsfc.nasa.gov/html/ow_data.html (accessed on 1 August 2024). Acknowledgments: The project 21GRD02 BIOSPHERE has received funding from the European Partnership on Metrology, co-financed by the European Union’s Horizon Europe Research and Innovation Programme and by the Participating States. The authors acknowledge the Horizon 2020 PITHIA-NRF project with Grant Agreement 101007599. All authors are grateful to the PROBAV/EPT teams, at B.USOC and ESA/Redu for their involvement in the data acquisition process, to the members of CSR for data validation, and to Belgian Science Policy-Space Research and Applications (BELSPO) for support in data exploitation. The results presented in this document rely on data provided by the Community Coordinated Modeling Center at Goddard Space Flight Center through their integrated Space Weather Analysis (iSWA) system’s HAPI server (https://iswa.gsfc. nasa.gov/IswaSystemWebApp/hapi (accessed on 2 September 2024)). The CCMC is a multi-agency partnership between NASA, AFMC, AFOSR, AFRL, AFWA, NOAA, NSF and ONR. These data were accessed via the University of Colorado’s Space Weather Technology, Research, and Education Center’s (https://colorado.edu/spaceweather (accessed on 2 September 2024)) Space Weather Data Portal (https://lasp.colorado.edu/space-weather-portal). We acknowledge the NMDB database (www.nmdb.eu) founded under the European Union’s FP7 programme (contract no. 213 007), and the PIs of individual neutron monitors, including at: Oulu (Sodankyla Geophysical Observatory of the University of Oulu, Finland), Thule (University of Delaware Department of Physics and Astronomy and the Bartol Research Institute, USA), South Pole (University of Wisconsin, River Falls, USA), Terre Adelie (Observatoire de Paris and the French Polar Institute IPEV, France), Apatity (Russia), Lomnicky (Slovakia), Kiel (Germany) and Dourbes (Belgium). Conflicts of Interest: The authors declare no conflicts of interest. References 1. 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