# Energetic Charged Particle Measurements During
Juno's Two Close Io Flybys

C. Paranicas, B H Mauk, G. Clark, P. Kollmann, Q. Nénon, R W Ebert, J R

Szalay, A H Sulaiman, J E P Connerney, S. Bolton

► To cite this version:

C. Paranicas, B H Mauk, G. Clark, P. Kollmann, Q. Nénon, et al. Energetic Charged Particle Measurements During Juno's Two Close Io Flybys. Geophysical Research Letters, 2024, 51 (13), 10.1029/2024gl09495. [hal-04770902](https://hal.science/hal-04770902v1)

HAL Id: hal-04770902

https://hal.science/hal-04770902v1

Submitted on 7 Nov 2024

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
# Geophysical Research Letters®

## RESEARCH LETTER

10.1029/2024GL109495

**Key Points:**

* Deep flux decreases over Io and in its immediate wake caused by losses to the moon's surface were observed in Juno JEDI data

* Juno JEDI detected narrow field-aligned beams of electrons and protons directed toward Io's north pole

* Moon absorption and wave-particle interactions may explain the loss of >1 MeV electrons that extend both inward and outward of Io's orbit

**Correspondence to:**

C. Paranicas,
chris.paranicas@jhuapl.edu

**Citation:**

Paranicas, C., Mauk, B. H., Clark, G., Kollmann, P., Nénon, Q., Ebert, R. W., et al. (2024). Energetic charged particle measurements during Juno's two close Io flybys. *Geophysical Research Letters*, *51*, e2024GL109495. https://doi.org/10.1029/2024GL109495

Received 26 MAR 2024

Accepted 21 MAY 2024

**Author Contributions:**

**Writing – original draft:** G. Clark,
Q. Nénon

**Writing – review & editing:** B. H. Mauk,
G. Clark, P. Kollmann, Q. Nénon,
R. W. Ebert, J. R. Szalay, A. H. Sulaiman

© 2024 Applied Physics Laboratory.
This is an open access article under the
terms of the Creative Commons
Attribution-NonCommercial-NoDerivs
License, which permits use and
distribution in any medium, provided the
original work is properly cited, the use is
non-commercial and no modifications or
adaptations are made.

<figure><img src="image_2.png" /></figure>

## Energetic Charged Particle Measurements During Juno's Two Close Io Flybys

C. Paranicas¹ ID, B. H. Mauk¹ ID, G. Clark¹ ID, P. Kollmann¹ ID, Q. Nénon² ID, R. W. Ebert³ ID, J. R. Szalay⁴ ID, A. H. Sulaiman⁵ ID, J. E. P. Connerney⁶ ID, and S. Bolton⁷ ID

¹APL, Laurel, MD, USA, ²IRAP, Toulouse, France, ³Southwest Research Institute/University of Texas at San Antonio, San Anotnio, TX, USA, ⁴Department of Astrophysical Sciences, Princeton University, Princeton, NJ, USA, ⁵University of Minnesota, Minneapolis, MN, USA, ⁶Space Research Corporation, Annapolis, MD, USA, ⁷Southwest Research Institute, San Anotnio, TX, USA

**Abstract** On days 2023-364 and 2024-034, the Juno spacecraft made close passages of Jupiter's moon Io, at altitudes of about 1,500 km. Data obtained from the first flyby, when the spacecraft was on magnetic field lines connected to both Jupiter and Io, revealed deep flux decreases. In addition, Juno's energetic particle detectors observed tens to hundreds of keV electron and proton beams. Such beams could be generated near Jupiter on field lines associated with Io. The second encounter occurred in the plasma wake and a more modest flux decrease was observed. Furthermore, data from both encounters suggest a spatially extensive decrease in >1 MeV electrons that includes regions inward of Io's orbit. In the immediate vicinity of Io, signatures of absorption likely dominate the data whereas diffusion and wave-particle interactions are expected to be needed to understand MeV electron data in the wider spatial region around Io.

**Plain Language Summary** Jupiter's magnetospheric plasma overtakes Io in its orbit. This causes a cavity to be formed over the hemisphere of Io that leads its orbital motion. Within this cavity subtle details of the plasma and energetic charged particle environment can be extracted from the signal, usually dominated by radiation at these distances. Examples include beams traveling in the direction from Jupiter to the moon, with little evidence of reflection at Io. Another major finding of this work is that there is a large region both along Io's orbit and even radially inward of it with a lower level of MeV electron flux. Guided by previous modeling, the most likely candidates for shaping the data are absorption by the moon, wave-particle interactions that can scatter particles into Jupiter's atmosphere, and diffusion.

## 1. Introduction

The Juno spacecraft had close encounters with Jupiter's moon Io on days 2023-364 and 2024-034. During the first of the two flybys, Juno transited magnetic field lines connected to both Jupiter and Io's leading hemisphere. The second flyby also occurred over the leading hemisphere but in the plasma wake, on Jovian field lines. In both cases, the spacecraft was moving from the anti-Jovian toward the sub-Jovian hemisphere and upstream in the plasma (Figure 1).

Some of the salient features of the JEDI data during these two flybys are as follows. While approaching Io on day 2023-364, JEDI detected bi-directional electron beams, followed by a strong flux decrease across many species and energies when it accessed field lines connected to the moon. While on field lines connected to Io, JEDI observed narrow, field-aligned beams of particles moving in the direction away from Jupiter toward the northern hemisphere of the moon. No significant reflected beams, that is, from Io toward Jupiter, were detected by JEDI when Juno occupied field lines connected to both bodies. A weaker, localized flux decrease was observed on day 2024-034, likely from the recent absorption of electrons by Io.

Field-aligned beams of both electrons and ions, based on the Energetic Particles Detector (EPD) data obtained during Galileo's Io flybys, have been reported in the past (e.g., Williams & Thorne, 2003). But JEDI has different capabilities than EPD so there are important differences. For example, EPD's lowest energy pure proton channel was nominally 80–220 keV (e.g., Lagg et al., 2003), whereas JEDI can separately detect protons to much lower energies. Below, we consider the energy ranges and sources of both electron and proton beams. Io's electro-magnetic interaction with the magnetosphere is not localized to the vicinity of the moon so beams can be generated far from Io itself.
<figure><img src="image_3.png" /><figcaption>**Figure 1.** Juno flybys of Io on days 2023-364 (LHS) and 2024-034 (RHS). The z-axis is oriented to be parallel to the spin axis of the planet, the y-axis is the portion of the vector from Io to Jupiter that is perpendicular to it, and the x-axis completes the right-hand system. The illumination of Io was similar on both flybys. The radius of Io (or $R_{Io}$) is taken to be 1,821.6 km for this figure. Closest approach (C/A) to Io is indicated in several places.</figcaption></figure>

Lastly, a much wider spatial decrease in flux, in what we interpret as dominated by > 1 MeV energy electrons, is apparent on both flybys. This flux decrease occurs even inward of Io's orbit. Nénon et al. (2017) modeled MeV electron fluxes in this region and compared their results with spacecraft data obtained before Juno's arrival. We consider the JEDI data in the context of their work below. They have also provided a detailed reference list for measurements made in this region by Pioneer 10 and 11, Voyager 1, and Galileo (GLL).

## 2. JEDI Measurements Within a High Radiation Region

Juno's Jupiter Energetic particle Detector Instrument (JEDI) can detect tens of keV to approximately 1 MeV electrons, several keV to 10 MeV protons, and hundreds of keV to well over 10 MeV heavy ions, with high time and energy resolution. Pitch angles can be determined using the vector magnetic field measured by the Magnetometer Investigation (Connerney et al., 2017), as archived within the Planetary Data System (Connerney, 2017). Detailed information about the JEDI instrument can be found in Mauk et al. (2017) and the supporting information in Mauk et al. (2023).

Some of Jupiter's inner moons are deep within the planetary radiation belts. In these environments, it can be difficult to interpret the JEDI observations. This is because detection of both ions via the technique of time-of-flight and deposited energy (TOFxE) and electrons rely on the use of solid-state detectors (SSDs). These
detectors operate by associating the amount of energy lost by a particle passing through them with the total energy of the particle. A limitation of JEDI's SSDs in the intense radiation belts of Jupiter is that a fraction of electrons with energies >400 keV can fully penetrate the detectors. In other words, these electrons do not leave all of their energy in the detector. Protons and heavy ions can also fully penetrate the JEDI SSDs but this phenomenon only begins to occur at much higher energies for those particles. An MeV electron leaving part of its kinetic energy in the detector mimics the signal of 100–400 keV electrons that would otherwise be considered true foreground (e.g., Mauk et al., 2023). The blockage of radiation by Io creates regions where what are typically less prominent features of the signal at these distances can be examined.

In Figure 2, we show data from day 2023-364 (top three panels) and 2024-034 (bottom three panels). For each flyby, the panels from top to bottom are: (a) detector singles rates near Io closest approach, (b) TOFxE proton intensity, and (c) electron intensity. Singles rates are summed count rates over a wide energy range, usually dominated by the count rates of the lower energy particles above the detector threshold because of the falling energy spectrum. As described elsewhere (see, for example, the supplement to Mauk et al., 2023), the JEDI-A180 “ion” (but dominated here by electrons) detectors SSD1 and SSD3 used for the singles rates (among the line plots in Figure 2) are shielded. For this reason, they should count fewer particles than neighboring detectors, especially in the absence of intense, narrow beams. But the ratio of SSD1 or SSD3 counts per s (cps) divided by SSD2 cps is usually no less than 0.08 because of scattering into SSD1 and SSD3. For the data in this plot, we interpret the near convergence of the SSD count rates in the line plots as a metric for the presence of >1 MeV electrons in large numbers, that can penetrate the foreground shields on SSD1 and SSD3. The line plots in Figure 2 also provide a measure of the reduction in radiation, from one to several orders of magnitude depending on position, at altitudes around closest approach to Io.

In addition, we interpret the horizontal band in the electron data (prominent between 100 and 400 keV) as likely due to the fraction of >0.4 MeV electrons that have fully penetrated the SSDs. This feature is called the “minimum ionizing” peak. A peak occurs because the amount of energy lost by electrons that fully penetrate the SSD is very insensitive to the energy of the penetrating electrons. This finding of a prominent feature of penetrating electrons is furthermore consistent with expectations from the line plots. Finally, the band between 100 and 900 in the proton data results from two processes: the generation of a minimum ionizing feature within the ion SSDs and the triggering of nearly simultaneous start and stop signals in the time-of-flight system.

Comparing the two time periods in Figure 2, the minimum in the singles rates is at a much higher level in the immediate wake than on field lines connected to Io. This could indicate that the flux decrease from the action of the moon becomes less significant moving away from Io. If so, the absorption alone may not be enough to create a significant and persistent decrease along Io's orbit.

### 3. Loss of MeV Electrons

In addition to their deep decrease on field lines connected to Io and in the immediate plasma wake, the singles rates for both time periods in Figure 2 show a wide decrease around Io, whose shape is somewhat different radially inward and outward of the moon. A similar reduction near Io in what is likely penetrating ~10 MeV electron flux was first identified by the Juno Advanced Steller Compass (ASC) team. A careful study of the spectrograms in Figure 2 shows a narrowing and reduction of the horizontal band associated with a decrease in the singles rates in the top panels. These effects can be observed on day 2023-364, for example, when the singles rates fall below about 4.0–4.5 × 10⁴ cps from about 08:25 UTC (Juno at about ~11 Io radii) until about 08:55 UTC (~19 Io radii). The proton spectrogram for day 2024-034 clearly illustrates this point. Therefore, the bands due to penetrating particles in the spectrograms are their own metric of the MeV electrons that can be observed in the singles rates. The data below the horizontal bands (i.e., the lowest energies in the electron and proton spectrograms) could also have background contributions at some level but that subject needs further analysis. A deep, narrow decrease in electrons within a wider flux decrease along Io's orbit was also reported in Paranicas et al. (2019).

The region around Io can be depleted of energetic electrons for a number of reasons. The moon can strongly absorb these particles, particularly on field lines currently in contact with Io, because the plasma traveling over Io's polar cap can be slowed by the local interaction. Williams and Thorne (2003) made a coarse estimate of flow speed from GLL EPD data, finding speeds of 3–7 km/s. Based on discussion in that paper, this speed seems to be with respect to Io. As such, magnetic field lines would be hung up on Io and this can cause electrons that travel
<figure><img src="image_4.png" /><figcaption>**Figure 2.** The top three panels are data from 2023-364 08:15-09:00 and the lower three panels are data from 2024-034 17:30-18:15. Each time period has three panels with: the singles rates for JEDI A-180 "ion" detectors SSD1-SSD5 as line plots, TOFxE proton intensity (color) from JEDI-270 versus time and energy, and electron intensity (color) versus time and energy.</figcaption></figure>

over Io to have enhanced losses at the moon. Electrons move in the prograde direction when their net longitudinal motion due to the rotational electric field is larger than drifts associated with the magnetic field only. When all factors are considered, electrons will overtake Io in its orbital motion when their energy is below about 36 MeV.

The loss of charged particles to Io's surface can be estimated by the bounce times and longitudinal drift distances per half bounce time. A 1 MeV charged particle with an equatorial pitch angle of 45° will have a half bounce time near Io of ~2.7 s (electrons) and ~54 s (protons) and during a half bounce time (between magnetic equator crossings) will move a longitudinal distance of ~147 km (electrons) and ~3,295 km (protons), using a dipole field model. Io's diameter is about 3,643 km. So energetic electrons on field lines connected to Io will likely hit the moon.

## 4. Electron Beams

Partly due to the reduction of radiation belt particles very close to Io, it is possible to observe energetic beams in the same region. For instance, within the blocked region between about 08:33 and 08:40 in Figure 2, a sawtooth pattern is seen in the SSD singles rates (top panel). This pattern likely corresponds to bi-direction beams just after 08:35:30 and then mostly mono-directional beams from 08:36 to 08:38 that may be in the tens of keV with an energetic tail that reaches above 0.4 MeV. The electron spectrogram for the same time shows the bi-directional beam as a narrow, vertical trace separated from the main radiation, see also, the various panels of Figure 3 below. A feature similar to the bi-directional beam on day 2023-364 occurs at about 17:47:30 on day 2024-034 that is so far unexplained.

Figure 3 shows a zoomed-in view of JEDI data around Io closest approach. The top panel again shows singles rates and the middle two panels show electron energy spectra, divided by particles traveling away from Jupiter toward Io (second panel) and away from Io toward Jupiter (third panel). The bottom panel of Figure 3 shows the electron data plotted by local pitch angle. The gray color in the plots in this paper indicate that JEDI did not sample that pitch angle at that time in the data product being displayed. It is clear from the middle two panels that the particle flux toward Io is much greater than the flux away from Io, for example, see the data around 40 keV. There is signal above background at the energy of the penetrators (~160 keV) in the third panel from the top but it is possible this is caused by the downward-moving (toward Io) electrons. The mono- and bi-directional nature of the beams can be seen in the pitch angle display but also in the energy panels, since they are plotted separately by local pitch angle. For reference, the planetary loss cone computed for a dipole at 5.9 $R_J$ is just under 3° wide.

The second panel of Figure 3 suggests the beams are in the tens of keV, but the band around 160 keV hints at the presence of electrons in the beam extending above 400 keV or so. Another clue that the electrons in the beam have a high energy tail is the similarity of the singles count rates in the blocked and unblocked detectors (Paranicas et al., 2018). The alternation in flux in the middle of the second panel from the top correlates with how many very field-aligned particles were being measured at each time. The bottom panel gives an idea of how close the JEDI sensors come to the field line. On the second flyby, the radiation was not as well blocked by the moon and beams were not obvious in the data as of this analysis.
<figure><img src="image_5.png" /><figcaption>**Figure 3.** Electron data obtained from 2023-364 08:35-08:38. Top: singles rates, as in Figure 2, electron intensity versus energy for local pitch angles 0-30° (second from top) and 150-180° (third from top), intensity versus local pitch angle (bottom).</figcaption></figure>

**5. Ion Beams**

Like the electron beams observed over Io on day 2023-364, the energetic proton data displays similar beams. In Figure 4, we show time-of-flight by pulse height (TOFxPH) proton data as energy spectra (top) and pitch angle with energies above about 21 keV combined (second panel from top). The TOFxPH data product does not use an SSD to make a determination of the particle species and energy. Below that we show in two panels another data product from JEDI that covers a higher energy range. TOFxE proton energy spectra (third from top) and pitch angle (bottom), again combining energies above about 40 keV. These beams were not observed by the Galileo spacecraft. EPD could not discriminate among ion species below 100 keV in energy. And the lowest energy pure proton channel on EPD (80–220 keV) had many “out-of-passband” counts.

The beams appear to be traveling from Jupiter toward Io's northern hemisphere, viewed when the fan of the JEDI-270 detectors looks up the magnetic field line only. Like the electron beams, the proton beams are very narrow. These beams are observed in the tens to few hundreds of keV. While this presentation focuses exclusively on charged particles, it should be kept in mind that energetic neutral atoms (ENAs) can be detected by JEDI in the
<figure><img src="image_6.png" /><figcaption>**Figure 4.** Proton data from 2023-364 08:34-08:39. Top: TOFxPH intensity versus energy and intensity versus pitch angle (second from top), TOFxE intensity versus energy (third from top) and intensity versus pitch angle (bottom).</figcaption></figure>

same manner as ions. When ENA flux is visible in time versus pitch angle plots it usually appears in a different area of the plot than ions do. We point this out because the JEDI proton data can sometimes have features not organized by the magnetic field direction. Finally, our initial assessment of the data suggests that the proton beams are not better explained as signal created by electron beams.

**6. Further Interpretation**

The main JEDI signature from both Io flybys in late 2023/early 2024 is a spatially confined decrease in energetic electron flux, likely caused by a recent absorption of particles at Io's surface. The drop-out in flux is not completely aligned with the geometric wake on day 2024-034. But perfect alignment would not be expected in this coordinate system because of perturbations to the electric and magnetic fields close to Io.

Earlier studies tried to make sense of energetic electron data obtained by other spacecraft near Io and its orbital path. For example, Sicard and Bourdarie (2004) proposed that for some MeV electrons, absorption by an inert Io would reduce electron fluxes well beyond what is observed. They instead favored a picture by Thorne et al. (1999) that suggested energetic electron flux is diverted around Io for some energies. If plasma flow speeds over Io's polar caps are as low as those cited above, it would imply plasma flow diversion around the body. A significant slowing of plasma over Io would also reduce the loss of particles to the moon from what would be estimated from full plasma corotation on an inert, solid-body absorber the same size as the moon.
In addition to the radially narrow and deep flux decreases apparent in Figure 2, there is simultaneously a shallower decrease over a much larger spatial area. A similarly structured decrease in MeV electrons near Io's orbit was reported in Paranicas et al. (2019). Two mechanisms that can cause decreases in MeV electron flux are absorption by Io and wave-particle interactions. Io loses neutrals at the rate of about 250–3,000 kg/s with various processes leading to the local production of cold ions (e.g., Bagenal & Dols, 2020). Asymmetric ion distributions with a reservoir of warm electrons can lead to wave growth (e.g., Remya et al., 2023, and references therein). Waves, both whistler-mode and electromagnetic ion-cyclotron (EMIC), can act on electrons, for example, scattering them into the planetary loss cone.

Nénon et al. (2017) used modeling to establish a hierarchy of loss processes of ~MeV electrons inward of Europa's orbit. They concluded that energy loss and scattering in the neutral and plasma environment would be less significant than moon absorption and interactions with low frequency whistler-mode or EMIC waves. For example, figure 11 of Nénon et al. (2017) shows that resonant interaction with whistler-mode waves in the extended region of *M* shells between six and nine was necessary (in their three-dimensional model) to reproduce the overall amplitude and spatial distribution of MeV electron fluxes observed by Pioneer 10, Pioneer 11, and Voyager 1 in the radiation belts. However, close to Io's orbit (M ~ 6), the fluxes simulated with and without the effect of waves (in red and purple on their figure 11) exhibit a flux decrease by a factor of two at *M* = 6. The loss of MeV electrons at *M* = 6 in the no-wave simulation is created by Io's absorption where Io is treated as an inert obstacle and no local perturbation of the fields are considered. In other words, at Io's orbit only, it was ambiguous if waves were needed to reproduce the data. Here *M* is a parameter expressed as a number of planetary radii that identifies a particle's magnetic shell or approximate orbital distance in the magnetic equator, typically computed from a non-dipolar magnetic field model.

The wide spatial decrease in MeV electron flux around Io's orbit, observed on both of the flybys here and in an earlier study (Paranicas et al., 2019), indicates that loss and diffusion rates are competitive. On the other hand, the two Io flybys suggest that absorption signatures may fill in quickly so that losses associated with wave-particle interactions might be required to reproduce the data.

Next, we turn to the subject of the beams that are observed on day 2023-364. Galileo EPD made extensive measurements of energetic beams over Io. The detailed geometry of the GLL Io passes for I31 (over Io's north pole on an Io or “I” encounter during planetary orbit number 31) and I32 (over Io's south pole) have been provided by Chust et al. (2005). Among the EPD discoveries were 29–42 keV electron beams over Io's north and south polar caps and in Io's wake. We assume Williams and Thorne (2003) were publishing the “pitch angle” of the detector look directions and not that of the particles being detected, as seems to be the case with some published EPD data. With that assumption, the 29–42 keV electron beams they reported would have been toward Io in both hemispheres. The validity of that assumption about directionality is even clearer in other citations, for example, Williams et al. (1998). In Figure 4 of that work, the electron loss over the northern pole of Ganymede during G2 must be around local pitch angles of 180° not zero. Other beam data recorded by EPD included some evidence of bi-directional beams and some ion beams. For example, they described proton beams from 0.5 to 1 MeV or so on I31 over the polar caps and also ~450 keV oxygen beams.

The source of both electron and proton beams on field lines connected to both Jupiter and Io is a subject of active investigation. Clark et al. (2023), see their figure 10, have summarized processes from various studies related to these field lines. Sulaiman et al. (2020), for example, proposed that keV electrons could be accelerated along field lines near Jupiter associated with Io. Furthermore, the plasma instrument on Juno observed > tens of keV electrons within both the upward and downward loss cone during Juno's twelfth perijove (PJ12) Main Alfvén Wing (MAW) encounter providing observational evidence of beams at high latitude (Szalay et al., 2020).

Clark et al. (2023), analyzing energetic proton observations associated with Io's footprint tail (FPT)—the low-altitude, high-latitude region where Io's auroral emissions are observed—found persistent features of anti-planetward proton acceleration. The protons were accelerated to tens to hundreds of keV energies depending on the longitudinal separation between the spacecraft and Io's MAW. They put forward the idea that EMIC waves, reported by Sulaiman et al. (2020), were significantly heating the cold ionospheric protons and producing energetic proton conics near the planet. Conservation of the first adiabatic invariant suggests these distributions would become field-aligned beams near Io due to the significant decrease in the local magnetic field strength as compared to their source region close to the planet. The observations by Clark et al. (2020, 2023) show similar
energies associated with the observations of proton beams made here, establishing a link between these beams and that earlier work.

In summary, Juno's close flybys of Io reached altitudes low enough to detect the level of radiation blockage by the moon. This has also made it possible for JEDI to observe fine details of the local environment, such as field-aligned beams in electrons and protons.

## Data Availability Statement

Juno JEDI data can be accessed at NASA's Planetary Data System (PDS), for example, https://pds-ppi.igpp.ucla.edu/collection/JNO-J-JED-3-CDR-V1.0, and also through the link provided in Mauk (2022), see the reference list in this paper.

## Acknowledgments

This work was primarily funded by the NASA New Frontiers Program's Juno project through subcontracts between JHU/APL and the Southwest Research Institute. We appreciate the assistance of the Juno magnetometer team that makes it possible for JEDI flux to be associated with local pitch angle.

## References

Bagenal, F., & Dols, V. (2020). The space environment of Io and Europa. *J. Geophys. Res., Space Phys.*, 125(5), 2019JA027485. https://doi.org/10.1029/2019ja027485

Chust, T., Roux, A., Kurth, W., Gurnett, D., Kivelson, M., & Khurana, K. (2005). Are Io's alfven wings filamented? Galileo observations. *Planet. Space Sci.*, 53(4), 395-412. https://doi.org/10.1016/j.pss.2004.09.021

Clark, G., Mauk, B. H., Kollmann, P., Szalay, J. R., Sulaiman, A. H., Gershman, D. J., et al. (2020). Energetic proton acceleration associated with Io's footprint tail. *Geophys. Res. Lett.*, 47(24), e2020GL090839. https://doi.org/10.1029/2020gl090839

Clark, G., Szalay, J. R., Sulaiman, A. H., Saur, J., Kollmann, P., Mauk, B. H., et al. (2023). Energetic proton acceleration by EMIC waves in Io's footprint tail. *Fsps. Front. Astron. Space Sci.*, 2023, 1016345. https://doi.org/10.3389/fspas.2023.1016345

Connerney, J. E. P. (2017). Juno fluxgate magnetometer calibrated data V1.0. [Dataset]. https://doi.org/10.17189/1519711. NASA Planetary Data System.

Connerney, J. E. P., Benn, M., Bjarno, J. B., Denver, T., Espley, J., Jorgensen, J. L., et. al. (2017). The Juno magnetic field investigation. *Space Science Reviews*, 213(1-4), 39-138. https://doi.org/10.1007/s11214-017-0334-z

Lagg, A., Krupp, N., Woch, J., & Williams, D. J. (2003). In-situ observations of a neutral gas torus at Europa. *Geophys. Res. Lett.*, 30(11), 2003GL017214. https://doi.org/10.1029/2003gl017214

Mauk, B. H. (2022). Juno JEDI data, JNOJED3CDR V1.0. *Planetary Data System/Planetary Plasma Interactions*. https://doi.org/10.17189/1519713

Mauk, B. H., Haggerty, D. K., Jaskulek, S. E., Schlemm, C. E., Brown, L. E., Cooper, S. A., et al. (2017). The jupiter energetic particle detector instrument (JEDI) investigation for the Juno mission. *Space Sci. Rev.*, 213(1-4), 289-346. https://doi.org/10.1007/s11214-013-0025-3

Mauk, B. H., Szalay, J. R., Allegrini, F., Bagenal, F., Bolton, S. J., Clark, G., et al. (2023). How bi-modal are Jupiter's main aurora zones. *J. Geophys. Res., Space Phys.*, 128(4), e2022JA031237. https://doi.org/10.1029/2022ja031237

Nénon, Q., Sicard, A., & Bourdarie, S. (2017). A new physical model of the electron radiation belts of Jupiter inside Europa's orbit. *J. Geophys. Res., Space Res.*, 122(5), 5148-5167. 2017JA023893. https://doi.org/10.1002/2017ja023893

Paranicas, C., Mauk, B. H., Haggerty, D. K., Clark, G., Kollmann, P., Rymer, A. M., et al. (2018). Intervals of intense energetic electron beams over Jupiter's poles. *J. Geophys. Res., Space Phys.*, 123(3), 2017JA025106-1999. https://doi.org/10.1002/2017ja025106

Paranicas, C., Mauk, B. H., Haggerty, D. K., Clark, G., Kollmann, P., Rymer, A. M., et al. (2019). Io's effect on energetic charged particles as seen in Juno data. *Geophys. Res. Lett.*, 46(23), 2019GL085393-13620. https://doi.org/10.1029/2019gl085393

Remya, B., Halford, A. J., Sibeck, D. G., Murphy, K. R., & Fok, M.-C. (2023). Understanding quiet and storm time EMIC waves -- Van Allen Probe results. *J. Geophys. Res., Space Phys.*, 128(8), e2023JA031712. https://doi.org/10.1029/2023ja031712

Sicard, A., & Bourdarie, S. (2004). Physical electron belt model from Jupiter's surface to the orbit of Europa. *Journal of Geophysical Research*, 109(A2), 2003JA010203. A02216. https://doi.org/10.1029/2003ja010203

Sulaiman, A. H., Hospodarsky, G. B., Elliott, S. S., Kurth, W. S., Gurnett, D. A., Imai, M., et al. (2020). Wave-particle interactions associated with Io's auroral footprint: Evidence of Alfven, ion cyclotron, and whistler modes. *Geophys. Res. Lett.*, 47(22), e2020GL088432. https://doi.org/10.1029/2020gl088432

Szalay, J. R., Allegrini, F., Bagenal, F., Bolton, S. J., Bonfond, B., Clark, G., et al. (2020). A new framework to explain changes in Io's footprint tail electron fluxes. *Geophys. Res. Lett.*, 47(18). https://doi.org/10.1029/2020GL089267

Thorne, R. M., Williams, D. J., Zhang, L. D., & Stone, S. (1999). Energetic electron butterfly distributions near Io. *Journal of Geophysical Research*, 104(A7), 14755-14766. https://doi.org/10.1029/1999ja000132

Williams, D. J., Mauk, B. H., & McEntire, R. W. (1998). Properties of Ganymede's magnetosphere as revealed by energetic particle observations. *Journal of Geophysical Research*, 103(A8), 17523-17534. https://doi.org/10.1029/98ja01370

Williams, D. J., & Thorne, R. M. (2003). Energetic particles over Io's polar caps. *Journal of Geophysical Research*, 108(A11), 2003JA009980. https://doi.org/10.1029/2003ja009980