# Injections of Multi-MeV Electrons in the Magnetosphere of Jupiter

Quentin Nénon, Ulysse Laffitau, Jonas Rabia

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Quentin Nénon, Ulysse Laffitau, Jonas Rabia. Injections of Multi-MeV Electrons in the Magnetosphere of Jupiter. Journal of Geophysical Research Space Physics, 2025, 130 (9), 10.1029/2025JA034199. hal-05238619

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Distributed under a Creative Commons Attribution 4.0 International License
RESEARCH ARTICLE

10.1029/2025JA034199

**Key Points:**

* The flux of MeV electrons remains unchanged during most of the sub-MeV electron injections

* We report the discovery of strong enhancements of ultra-relativistic electron fluxes during electron injections

* Injections may play an important role for the radial transport and acceleration of MeV electrons

**Supporting Information:**

Supporting Information may be found in the online version of this article.

**Correspondence to:**

Q. Nénon, q.nénon@gmail.com

**Citation:**

Nénon, Q., Laffitau, U., & Rabia, J. (2025). Injections of multi-MeV electrons in the magnetosphere of Jupiter. *Journal of Geophysical Research: Space Physics*, 130, e2025JA034199. https://doi.org/10.1029/2025JA034199

Received 15 MAY 2025
Accepted 19 AUG 2025

©2025. The Author(s).
This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

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# Injections of Multi-MeV Electrons in the Magnetosphere of Jupiter

Q. Nénon¹, U. Laffitau², and J. Rabia²

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¹LATMOS, CNRS—UVSQ—Sorbonne Université—CNES, Paris, France, ²IRAP, CNRS—UPS—CNES, Toulouse, France

**Abstract** Energetic particle injections provide a means for fast inward radial transport and adiabatic acceleration of electrons in the magnetosphere of Jupiter. Furthermore, enhanced whistler-mode wave activity during injections can further accelerate electrons to MeV energy. All in all, injections may contribute to the creation of the most extreme radiation belts of the solar system. In this article, we present the first survey of the >1 and >11 MeV electron fluxes observed by the Galileo mission during <1 MeV electron injections. We find that the flux of MeV electrons remains nearly unchanged during most of the sub-MeV electron injections, possibly because the MeV particles quickly drift out of the injection corridor. Nevertheless, we report the discovery of six time periods at *M* = 11–20 when the flux of MeV electrons is increased by a factor of more than 3 during injection events. The Galileo observations therefore indicate that magnetospheric injections may play a dominant role for the radial transport and acceleration of MeV electrons at Jupiter, at least transiently.

**Plain Language Summary** The giant planet Jupiter generates a powerful magnetic field which traps energetic charged particles in so-called “radiation belts.” The study of these particles is important to understand how planetary magnetospheres work. It is also important because these particles represent a threat for the artificial satellites that explore the Jovian system. In this article, we focus on one mechanism called “injections” during which energetic particles are quickly radially transported and accelerated around Jupiter. Previous work has shown that the electrons (holding a negative charge) are accelerated by injections to a velocity close to the speed of light, which corresponds to a kinetic energy of 1 MeV (MeV). However, do injections also accelerate electrons to kinetic energies higher than 1 MeV? We present a survey of the electron measurements collected by NASA's Galileo mission (1995–2003 at Jupiter) at energies higher than 1 MeV and we reveal that some injections accelerate electrons up to more than 10 MeV. This finding highlights that injections can be an important mechanism for the physics of the most energetic electrons trapped around Jupiter.

# 1. Introduction

## 1.1. Adiabatic Radial Transport of Energetic Electrons at Jupiter

The magnetosphere of Jupiter is the most powerful accelerator of electrons in the solar system (Mauk & Fox, 2010). Understanding how Jupiter accelerates and transports electrons in such an efficient manner despite many sink processes (e.g., moon absorption, interactions with dust, scattering in the loss cone) is important to develop a universal understanding of planetary radiation belts. In particular, the electrons have typical kinetic energies of a few eV when they originate at the orbit of Io and reach energies greater than 100 MeV in the radiation belts (e.g., Kollmann et al., 2018). Furthermore, the characterization and understanding of the Jovian energetic electrons are critical for the assessment of radiation risk for artificial satellites (e.g., Carlton et al., 2018; de Soria-Santacruz et al., 2016; Garrett & Jun, 2021; Sicard-Piet et al., 2011), and for the study of moon surface weathering (e.g., Nordheim et al., 2018; Paranicas et al., 2021).

The adiabatic radial transport of energetic electrons with conservation of the two first adiabatic invariants is likely a dominant mechanism for the transport and acceleration of electrons at Jupiter. Indeed, at *M* > 20, Kollmann et al. (2018) have shown with Galileo data that the slope and energy cutoff of the energy spectrum of electrons are on average consistent with adiabatic radial transport. At *M* < 20, inward adiabatic radial transport may provide the seed population for further acceleration by plasma waves (e.g., Ma et al., 2024; Woodfield et al., 2014). However, the electron Phase Space Densities (PSD) observed by Galileo and Juno show no direct evidence of plasma-wave acceleration to date (Garrett & Jun, 2021; Ma et al., 2021). Table 1 presents the four types of phenomena that may adiabatically transport energetic electrons across drift shells. On the one hand, slow radial diffusion would shape
<table><caption>**Table 1**
The Four Types of Phenomena That Can Produce Inward Adiabatic Radial Transport of Energetic Electrons in the Magnetosphere of Jupiter</caption><thead><tr><th>Phenomenon</th><th>Interchange events</th><th>Injection events</th><th>Large-scale events</th><th>Radial diffusion</th></tr></thead><tbody><tr><td>Timescale</td><td>Tens of seconds to minutes</td><td>Tens of minutes to hour</td><td>Several hours to days</td><td>No time dynamics</td></tr><tr><td>Physical mechanism</td><td>Flux tube interchange?</td><td>Tail collapse?</td><td>Dawn-to-dusk electric field?</td><td>Neutral winds in the Jovian ionosphere?</td></tr><tr><td>References</td><td>Bolton et al. (1997), Kivelson et al. (1997), Thorne et al. (1997), Kurth et al. (2023), Daly et al. (2023), Ma et al. (2024)</td><td>Mauk et al. (1999), Louarn et al. (2014), Clark et al. (2016), Haggerty et al. (2019), Nichols et al. (2023), Brice and Mcdonough (1973)</td><td>Yuan et al. (2021)</td><td>Brice and Mcdonough (1973)</td></tr></tbody></table>

the overall spatial distribution of electrons but may not exhibit any specific time dynamics that could be easily identified with a spacecraft. On the other hand, the three other mechanisms of adiabatic transport (interchange events, injections, and large-scale events) manifest as seconds-to-days enhancements of energetic electron fluxes. The study of the time dynamics of energetic electrons can therefore inform on the three last mechanisms and on their importance for the radial transport and acceleration of energetic electrons.

Magnetospheric «injections» were discovered by Mauk et al. (1997) with Galileo data and manifest as transient increases in the energetic electron flux on a timescale of minutes to hours, as seen by the spacecraft. Injections sometimes have a UV counterpart in the atmosphere of the giant planet (Dumont et al., 2014; Mauk et al., 2002; Nichols et al., 2023) but not systematically (Haggerty et al., 2019). «Interchange events» are observed with a typical duration shorter than injections in the order of seconds to minutes. From the observational point of view, the main difference between interchange events and injections is the perturbation of the magnetic field and depletion of heavy ions observed during interchange events but not seen for injections (for example, Mauk et al., 1999; Nichols et al., 2023). However, the difference between the interchange and injection events and how they may be physically interconnected remain an open question (see for instance the discussion in Section 5.2 of Mauk et al. (1999)). Finally, “large-scale episodic events” of >1 and >11 MeV electron flux increase have been reported by Yuan et al. (2021) based on Galileo measurements. The selection criteria used by Yuan et al. (2021) to identify these events is that a significant increase of electron flux should be seen across a range of radial distance larger than $\Delta R = 1 R_J$ ($1 R_J = 71,492$ km). Yuan et al. (2021) proposed that these events can be caused by a pulse of a large-scale magnetospheric electric field which would perturb the drift trajectories of the energetic electrons.

In this paper, we will refer to the interchange process as “interchange” (often called “plasma injection”) and we will refer to the process that results in minutes-to-hour enhancements of electron fluxes as “injection”, as previously studied by Mauk et al. (1997, 1999), Haggerty et al. (2019), and Clark et al. (2016) at Jupiter. These injections may be associated with tail collapse.

## 1.2. Phenomenology and Origin of Magnetospheric Injections

Mauk et al. (1997) proposed that the increase in energetic electron flux seen during injections results from a “parcel” of longitudinally confined hot plasma, which quickly moves inward from a start M-shell “M_start” to a more-inward M-shell “MObservation” (for a definition of the M-shell parameter, see Section 2). The fast inward transport can preserve the phase space density (PSD) of the energetic electrons. If the “unperturbed” magnetospheric PSD of electrons at MObservation is lower than at M_start, then the fast injection of the higher PSD is seen at MObservation as a flux increase. After the initial phase of radial transport, the injected electrons drift around the planet with energy-dependent azimuthal velocity, so the flux increase can be detected with energy-time dispersion by a spacecraft. This chain of events is summarized in the cartoon of Figure 4 of Mauk et al. (1997).

Energetic electron injections have been previously observed in situ by Galileo and Juno, and remotely with the Hubble Space Telescope, yet the mechanisms that trigger and control this phenomena remain largely unknown. Louarn et al. (2014) proposed to use kilometric and hectometric radio emissions as a proxy for reconnection events in the distant tail. Then, the authors have shown that injections in the middle magnetosphere occur concurrently with the radio emissions, meaning that reconnection events at M > 40 and injections at M = 9–20 may be linked. This does not mean that there is a causal link between the two phenomena. Overall, large-scale disturbance of the magnetosphere, including reconnection events or current sheet collapse (Mitchell et al., 2015), may trigger injections or intensify their occurrence rate, including the creation of “substorm-like” series of clustered injections (Mauk et al., 1999).

Given that injections have been routinely observed by Galileo and Juno at M = 9–20, they may play an important role for the radial transport and acceleration of electrons in this region. Furthermore, Ma et al. (2024) have reported that the intensity of whistler-mode waves can be enhanced during injection events (due to the high anisotropy of >1 keV electrons during the injection), and have shown that resonant interaction with the waves can provide an efficient path for electron acceleration to MeV energies.

All previous work on energetic electron injections at Jupiter focused on the <1 MeV energy range. In this article, we present the first survey of >1 MeV electron fluxes during injections at the giant planet. We reveal that most of the time, injections do not increase the flux of MeV electrons. Still, we identify six time periods of the Galileo
mission when the flux of >1 and >11 MeV electrons is clearly increased during injections at $M = 11-20$. Magnetospheric injections may therefore play a major role for the radial transport and acceleration of MeV electrons at Jupiter, at least transiently.

## 2. Methods: Electron Data From the Galileo Energetic Particle Detector and Identification of Injections

The Energetic Particle Detector onboard Galileo (EPD) was designed to observe 20 keV to >11 MeV electrons (Williams et al., 1992). Here, we use the data set of Kollmann (2022), which is available on the NASA Planetary Data System and includes the latest calibration and correction of the EPD data. Magnetospheric injections are studied with the standard data product of omnidirectional particle fluxes with a time resolution of 3–10 min (the so-called “real-time” mode of EPD). Electron fluxes are available in eight differential channels from 20 to 700 keV (channels *E* and *F* of the instrument), while three integral channels inform on the intensity of >1 and >11 MeV electrons (channels B1, DC2, and DC3).

More specifically, the response of the integral channels B1, DC2, and DC3 to energetic electrons and protons (geometric factor as a function of particle energy) was estimated by Jun et al. (2002) using particle transport simulations. The convolution of the geometric factors with particle energy spectra conducted by Jun et al. (2002), Garrett et al. (2012), and Kollmann (2022) indicate that the B1 and DC2 count rates are dominated by >1 MeV electrons, noting that the B1 channel may respond to a wider energy range than DC2 and its response to electrons cannot be described with a single geometric factor but by an energy-dependent geometric factor that extends to infinite energy. The DC3 channel responds mostly to >11 MeV electrons and does not respond at all to <5 MeV electrons (Garrett et al., 2012; Jun et al., 2002). While DC3 informs on the integral flux of >11 MeV electrons on average, the observed count rates can be dominated by ~5 MeV electrons for specific events. In any case, DC3 responds only to multi-MeV electrons and effectively extends the energy range of EPD well beyond the 700 keV of the E–F differential channels. In this article, we also present the flux of protons recorded by the TP1, TP2, and TP3 channels of EPD (80–500 keV protons) to facilitate the identification of injections and the study of their time dispersion.

The location of injections is documented with the M-shell parameter, which is the distance to the center of Jupiter of the position where the magnetic field amplitude is minimum along the field line, normalized by the radius of the planet (1 Jovian radius is taken equal to $1 R_J = 71,492$ km). In this article, we use the M-shell parameter of Galileo computed by Rabia et al. (2024), who relied on the JRM33 + CON2020 magnetic field model (Connerney et al., 2020, 2022) which includes the contributions of the internal field of the planet and magnetospheric current sheet.

Mauk et al. (1999) conducted the first statistical study of injections at Jupiter and based their analysis on the visual inspection of the time variability of energetic particle fluxes (time series and energy-time spectrograms) with no automatic tool. The authors defined “injections” as “localized (in time) rate increases with respect to surrounding regions observed in multiple particle rate channels.” However, Mauk et al. (1999) highlighted that this approach has a sense of subjectivity. A similar visual approach has been employed by Clark et al. (2016), Louarn et al. (2014), Haggerty et al. (2019), Nichols et al. (2023), and Ma et al. (2024) to coarsely identify injections at Jupiter. A similar method has been applied to the study of injections at Earth (e.g., Turner et al., 2015) and Saturn (e.g., Mauk et al., 2005; Paranicas et al., 2020). In this article, we use the same somehow-subjective method as in previous work. We note that the visual identification and counting of injections during substorm-like series of clustered injections is challenging when the repetition of injections is faster than the time resolution of the data. Furthermore, we call here “injections” the phenomenon of particle flux increase, while radial transport associated with injections can occur without any electron flux increase if the radial gradient of the Phase Space Density is small (see discussion in Section 4). Finally, Mauk et al. (1999) focused their work on energy-time dispersed injections, while other studies have considered both dispersed and dispersionless injections, also called “dispersionless bursty spikes” (e.g., Louarn et al., 2014; Ma et al., 2024; Turner et al., 2015). In the present study, both types of injections are considered.

As already shown in Mauk et al. (1999) and Louarn et al. (2014), three main types of injection events are observed at Jupiter: no injection at all (also labeled “quiet conditions” by Mauk et al. (2020)), isolated injection, and clustered “storm-like” injections. In this study, we identify electron injections in the <1 MeV energy range and
<figure><img src="image_5.png" /><figcaption>**Figure 1.** Electron and proton fluxes recorded by Galileo-EPD during DOY 363 of 2000. Previously, Mauk et al. (2002) identified sub-MeV electron injections between 13:00 UTC and 18:00 UTC (see panels b, d).</figcaption></figure>

inspect the flux of MeV electrons observed by EPD in order to investigate if MeV electron fluxes are enhanced
during magnetospheric injection events.

**3. Results**

**3.1. Survey of MeV Electron Fluxes During Injections**

Figure 1 presents the electron and proton fluxes observed by Galileo-EPD on Day of Year (DOY) 363 of 2000 at M = 19 (00:00 UTC) to M = 9 (~22:00 UTC). Panel a shows the flux of >1 and >11 MeV electrons observed by the B1, DC2, and DC3 channels of EPD. Panel b gives the energy-time spectrogram of 20–700 keV electron flux and panel c shows the energy-time spectrogram for protons. Panels d and e provide the time series of sub-MeV particle fluxes corresponding to panels b and c, respectively.

This specific day has already been studied by Mauk et al. (2002) who reported multiple injection events for
<1 MeV electrons between 12:00 UTC and 18:00 UTC, as can be seen in Figure 1b. Furthermore, the Hubble
Space Telescope concurrently observed a UV auroral spot at the footprint of the magnetic field line connected to
Galileo (Mauk et al., 2002). During these injections, Figure 1a shows that the flux of MeV electrons is not
appreciably increased or modified. Specifically, the injections are clearly seen up to ~300–500 keV (see line plots
of Figure 1d) but not to higher energy of ~700 keV (brown line of Figure 1d) or >1 MeV (Figure 1a). Figure 1
therefore presents an example of electron injections seen at energies of several hundreds of keV but not at higher energy.

Using the same format as Figure 1, the Supporting Information S1 presents the flux of electrons and protons observed by Galileo-EPD during the 96 days when Galileo collected reliable EPD data at $M < 30$. We confirm and extend the findings of Mauk et al. (1999) that injections are routinely observed at $M \sim 9-20$ for $<1$ MeV electrons and less frequently for protons. Table S1 in Supporting Information S1 presents a tentative list of electron injections that we have identified in the EPD data for the entire Galileo mission. However, it must be kept in mind that this list is subjective and that the number of injections is dominated by the storm-like series of injections during which the identification of unique injections is challenging (see Table S1 in Supporting Information S1). Still, our analysis provides an order of magnitude estimate of $\sim 151$ electron injections observed by Galileo in the $<1$ MeV energy regime.

The visual inspection of MeV electron fluxes (Figure 1 and Supporting Information S1) during injection events indicates that the flux of ultra-relativistic electrons is not appreciably increased during 139 of the 151 of the injections, or $\sim 90\%$ of the events. A first hypothesis to explain this observation is that the MeV particles may quickly drift out of the injection as a result of fast azimuthal drift compared to the longitudinal extent of the injection corridor. In other words, injections would not effectively transport the relativistic particles. A second possibility is that injections do radially transport MeV electrons but do not manifest as an increase in electron flux due to a relatively flat radial gradient of the electron PSD at high energy.

## 3.2. Discovery of >11 MeV Electron Injections

We find six time periods when the flux of MeV electrons is clearly increased during magnetospheric injections. Figure 2 presents an «isolated» injection observed by Galileo at approximately 18:30 UTC on 1997–178 at $M \sim 12$. This specific injection has been previously documented by Mauk et al. (1999) for $<1$ MeV electrons and by Clark et al. (2016) for protons and heavy ions. During the event, the flux of $>1$ and $>11$ MeV electrons exhibits a peak-to-peak increase of a factor of $\sim 3$, with a duration of approximately 20 min as seen by the spacecraft (duration of full width at half maximum).

We have attempted to better constrain the energy range of the electrons observed by the B1, DC2, and DC3 channels during the injection event by convolving prescribed analytical energy spectra (with free parameters such as power law exponent and cutoff energy) with the energy-dependent geometric factors of the channels. The free parameters of the energy spectrum were optimized until the “fitted” count rates equal the observed count rates. However, the spectrum observed by EPD for $Ek < 1$ MeV is not a simple power-law, double-power-law, or double-kappa function and contains a bump in the energy spectrum, which makes any extrapolation to $Ek > 1$ MeV difficult. Therefore, we did not find any satisfactory fit for the electron energy spectrum at $Ek > 1$ MeV. We also note that the PDS archive of the EPD data provides such fit parameters, computed by Kollmann et al. (2018), but the fit parameters are set to NaN in the PDS archive during the reported injection, showing that the fit performed by the previous authors was also not successful. Still, the B1, DC2, and DC3 channels all together show that the flux of multi-MeV electrons was increased during the event.

The blue and red dots on Figure 3 give the time of maximum flux enhancement as a function of electron and proton energy in order to study the time dispersion of the event. The sequence of particle arrival at Galileo, from high-energy ions first to low-energy particles and finally high-energy electrons, is the expected result of energy- and-species-dependent gradient-curvature drift. The red and blue solid lines show the theoretical expectation of energy-time dispersion for protons and electrons computed with the method detailed in Mauk et al. (1999) and Haggerty et al. (2019). In this method, the injection is assumed to start its azimuthal drift phase at a given position in the magnetosphere. Then, we use the three energy-time points of protons to constrain the age of the injection and the azimuthal separation between Galileo and the injection site. Using the proton time dispersion (red dots on Figure 3), we find that the injection is relatively young, with an age of only $\sim 1.3$ hr, and was injected approximately $46^\circ$ upstream of Galileo.

For $<1$ and $\sim 1$ MeV electrons (E, F, B1, DC2 channels), a reasonable agreement is found between the timing of observation (blue dots) and the theoretical prediction (solid blue line), with a plateau at low energies already reported by Haggerty et al. (2019) with Juno measurements. However, the $>11$ MeV electrons observed by the «DC3» channel reach a maximum flux enhancement at the same time as $\sim 700$ keV electrons and $>1$ MeV
<figure><img src="image_6.png" /><figcaption>**Figure 2.** Electron and proton fluxes observed by Galileo-EPD in 1997–178. The powerful injection at ~18:30 UTC was previously documented by Mauk et al. (1999) for <1 MeV electrons (panels b, d) and by Clark et al. (2016) for protons (panels c, e). Here, we reveal an associated increase of MeV electron fluxes (panel a).</figcaption></figure>

electrons. The DC3 observation time does not match at all the expected arrival time of either 5 MeV or 11 MeV electrons, considering that DC3 can respond to >5 MeV or >11 MeV electrons (Garrett et al., 2012; Jun et al., 2002). Specifically, the DC3 electrons reach the Galileo spacecraft with an advance of at least 25 min compared to theory. The >5 MeV or >11 MeV electrons observed by EPD-DC3 during the injection have therefore likely not been injected at the same site as lower energy electrons. Instead, these electrons may have been produced locally by the acceleration of sub-MeV electrons to several MeV energy. Ma et al. (2024) have shown that whistler-mode waves can be enhanced during an injection and may generate such an acceleration in a timescale of days, while we find that the injection is only 1.3 hr old. Still, local electron acceleration would explain why the several MeV electrons reach Galileo at the same time as 700 keV–1 MeV electrons.

Figure 4 shows the five other time periods when we find electron injections with MeV electron flux enhancement. We note that the Galileo-EPD data collected during DOY 1998–201 (Figure 4c) have never been studied in the literature (Mauk et al., 1999; Louarn et al., 2014), while it is a day of strong magnetospheric activity. The MeV electron enhancements are observed at $M = 11–20$ and are always concurrent with injections seen in both <1 MeV electrons and protons. Table 2 documents the radial distance, M-shell, and local time of the Galileo spacecraft during the six time intervals when “MeV electron injections” were observed. The injections presented in Figure 4 exhibit almost no time dispersion, so we do not analyze their energy-time dispersion.
<figure><img src="image_7.png" /><figcaption>**Figure 3.** Time of maximum flux enhancement for the 1997–178 18:30 injection event. The Galileo observations are shown with the dots, blue for electrons and red for protons. The vertical error bars represent a ±3 min uncertainty associated with the time resolution of the data set. The blue and red solid lines show the theoretical expectation of detection time based on the fit to the proton time dispersion. Panel (a) is a zoom of panel (b).</figcaption></figure>

Finally, Figure 5 presents the discovered injections of >11 MeV electrons (or >5 MeV, see Section 2) observed by the DC3 channel as a function of radial distance and M-shell (colored curves) overplotted over the DC3 electron fluxes seen by Galileo-EPD during the entire mission (gray dots). In Figure 5b, the injections of ultra-relativistic electrons manifest as increases over a small radial distance due to their short duration of a few tens of minutes compared to the radial velocity of the Galileo spacecraft. It can also be seen that the “MeV electron injections” are at the origin of the highest fluxes of multi-MeV electrons ever observed by Galileo at $R \sim 11 R_J$.

In Figure 5, we also indicate with blue and green colors the large-scale episodic events identified by Yuan et al. (2021). The time interval shown in black at $R = 17–18 R_J$ corresponds to DOY 1998–268 from 05:00 to 06:30. During this period, Yuan et al. (2021) reported a large-scale episodic event, while we find a series of MeV electron injections (Figure 4c). We therefore propose that this specific large-scale event of >11 MeV electrons can be linked, at least in part, to the magnetospheric injection activity seen for <1 MeV electrons. For the other large-scale events identified by Yuan et al. (2021), we do not see evidence of magnetospheric injections. These non-injection-related large-scale events may therefore be generated by a large-scale magnetospheric electric field, as proposed by Yuan et al. (2021).

## 4. Conclusions and Discussion

Magnetospheric injections constitute an efficient means for the inward radial transport and adiabatic acceleration of energetic electrons in the magnetosphere of Jupiter. Furthermore, enhanced whistler-mode wave activity during injections can contribute to the acceleration of electrons to MeV energies (Ma et al., 2024). Previous work based on Galileo and Juno data has investigated the signatures of injections in <1 MeV electron measurements (for example, Haggerty et al., 2019; Louarn et al., 2014; Ma et al., 2024; Mauk et al., 1999). In the present article, we take advantage of the >1 and >11 MeV electron data accumulated by the Galileo Energetic Particle Detector (EPD) to explore the influence of injections on these ultra-relativistic particles.

Our visual inspection of daily plots of electron fluxes indicates that approximately 151 injections have been observed by Galileo-EPD for <1 MeV electrons, mostly at $M = 9–20$. However, this number and the list of injections provided in Table S1 of Supporting Information S1 have to be taken with a grain of salt given that the identification of injections remains subjective and is biased by substorm-like series of clustered events. Still, during approximately 90% of the <1 MeV injections, we find that the flux of >1 and >11 MeV electrons does not change significantly. This finding may show that the MeV electrons are able to quickly drift out of the injection corridor before significant radial transport, as a result of fast azimuthal drift and/or narrow longitudinal extent of the injection (Kollmann et al., 2018). We therefore tend to confirm the assumption of Kollmann et al. (2018) who stated that injections “are not seen at MeV energy at Jupiter” when studying the radial transport and acceleration of energetic electrons.
<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /><figcaption>**Figure 4.** Electron and proton fluxes observed by Galileo-EPD during 3 days. We identify five time periods when MeV electron fluxes are increased during sub-MeV electron injections.</figcaption></figure>
<caption>**Table 2**</caption>

The Six Time Intervals When Galileo-EPD Observed MeV Electron Injections

<table><thead><tr><td>Time interval of MeV electron injections</td><td>Radial distance (R<sub>j</sub>)</td><td>M-shell</td><td>Local time</td></tr></thead><tbody><tr><td>1996–311 00:00–04:00</td><td>11.0–12.2</td><td>10.9–13.0</td><td>11–12 hr</td></tr><tr><td>1997–178 18:30</td><td>~11.5</td><td>~12.1</td><td>~14 hr</td></tr><tr><td>1998–201 08:00–10:00</td><td>12.3–13.0</td><td>12.9–14.0</td><td>6.9–7.5 hr</td></tr><tr><td>1998–201 12:30–13:30</td><td>11.0–11.6</td><td>11.8–12.0</td><td>7.7–8.5 hr</td></tr><tr><td>1998–268 05:00–06:30</td><td>17.5–18.2</td><td>18.0–21.0</td><td>5.1–5.6 hr</td></tr><tr><td>1998–268 08:30–09:30</td><td>16.1–16.6</td><td>17.0–18.5</td><td>~5.7 hr</td></tr></tbody></table>

Our survey reveals six time periods of the Galileo mission when EPD observed a clear increase in the flux of MeV electrons during magnetospheric injections, with peak-to-peak enhancements larger than a factor of 3 (Figures 2, 4 and 5). The six time intervals include 14 injections of >11 MeV electrons, which represents approximately 10% of all sub-MeV injections. Only one of the 14 MeV electron injections exhibits an energy-time dispersion that can be studied (in 1997–178, Figures 2 and 3), as the other 13 events appear dispersionless in time. The analysis of the time dispersion of the 1997–178 injection indicates that it occurred approximately 1.3 hr before its observation by Galileo. All in all, the lack of strong time dispersion for the MeV electron injections indicate that they were all detected just after or during their phase of inward transport, meaning that Galileo was close to or inside the injection.

The analysis of the time dispersion of the injection presented in Figure 3 shows that the protons and electrons with an energy lower or equal to 1 MeV reach the Galileo spacecraft with a timing consistent with the expected gradient-curvature drift. However, the >11 MeV electrons observed by EPD are observed nearly simultaneously with ~1 MeV electrons, while we expect them to reach Galileo only 25 min later. This may indicate that the >11 MeV electrons are accelerated locally, possibly by resonant interaction with whistler-mode waves on a timescale as short as the age of the injection of 1.3 hr.

The injections of MeV electrons reported in this article are observed at $M = 11–18$, which is consistent with the location of sub-MeV electron injections mostly seen at $M = 9–20$ (Mauk et al., 1999). The inward boundary of $M \sim 9$ may be a bias of the orbit of Galileo as the periapsis of the mission was usually close to $M = 9$. This may also be due to instrumental issues which prevent from collecting reliable EPD data inward of $M = 9$ (Kollmann et al., 2018).

The outward boundary of $M = 20$ may result from the relatively flat radial profile of the electron phase space density outward of this limit (for example, Ma et al., 2021). This would mean that inward injections may occur at $M > 20$ but may not have a sufficiently high contrast with the quiet magnetosphere population to be observed. Similarly, our observation of MeV electron injections at $M = 11–18$ does not show that injections do not contribute to the radial transport of relativistic electrons outward of this M-shell range, we may just not see it. An analogy could be to move a parcel of hot water in a hot bath tub: there is no heat modification anywhere and still the transport occurs. Recently, Hao et al. (2024) discovered with Juno data the existence of a slot region where the flux of 100–800 keV electrons is reduced by 2–6 orders of magnitude at $M = 9–16$, compared to the surrounding magnetosphere. If this slot region existed during the Galileo era, it would exacerbate the PSD radial gradient and would increase the injection contrast. The fact that Galileo-EPD observed MeV electron injections in this region may therefore be linked to the slot region, which may itself be the result of resonant interaction with whistler-mode waves and electron precipitation in the planetary loss cone (Hao et al., 2024).

Proton injections are observed less often than their <1 MeV electron counterparts possibly because of the radial gradient of the particle PSD (Clark et al., 2016; Mauk et al., 1999). Here, we find that all the “MeV electron injections” are accompanied with clear proton injections, which again tends to show that the conditions were met to enable high injection contrast during the reported events.

The present study highlights a new impact of injections on the magnetosphere of Jupiter but provides little constrains on their physical origin. Future work can focus on expanding the statistics of MeV electron injections using Juno, JUICE, and Europa-Clipper data in order to further our understanding of the role of injections in producing the most energetic electron radiation belts of the solar system (Clark et al., 2021; Roussos et al., 2021).
<figure><img src="image_11.png" /><figcaption>**Figure 5.** Integral flux of >11 MeV electrons observed by the DC3 channel of Galileo-EPD from 1996 to 2002 as a function of radial distance to the center of Jupiter (panels a, b) and M-shell (panel c). DC3 likely responds to >11 MeV electrons but may also respond to >5 MeV electrons (see Section 2). On panels b and c, the blue and cyan periods correspond to large-scale episodic events identified by Yuan et al. (2021). The black time period corresponds to both a large-scale event and MeV electron injections. Red, brown, magenta and yellow depict time periods of MeV electron injections. The two time periods of 1998–201 (see Table 2) are merged into one in this Figure.</figcaption></figure>
The Juno mission does not have a particle detector designed to observe >1 MeV electrons. However, the background noise let by the penetrating particles in the science and engineering payload can be used to indirectly sense >5–10 MeV electrons (for example, Becker et al., 2017; Herceg et al., 2025; Paranicas et al., 2017). Then, the Juno MeV electron observations could be compared to the Juno-JEDI measurements of <1 MeV electron injections (Haggerty et al., 2019; Mauk et al., 2024). In the next decade, the JUICE and Europa-Clipper missions will also be able to observe the MeV electron injections reported here thanks to the RADEM and Radiation Monitor experiments, respectively (Hajdas et al., 2025; Meitzler et al., 2023).

## Conflict of Interest

The authors declare no conflicts of interest relevant to this study.

## Data Availability Statement

The Galileo-EPD data presented in this article are available on the NASA Planetary Data System (Kollmann, 2022). The M-shell of Galileo was computed by Rabia et al. (2024) and is publicly available on the AMDA database (https://amda.cdpp.eu, Génot et al., 2021) provided by the French national data center for natural plasma of the solar system CDPP (https://www.cdpp.eu/).

## Acknowledgments

We thank Marie Devinat (IRAP) for fruitful discussions on the difference between interchange events and injections.

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