the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 15 Apr 2026
RADEM observations of the Van Allen belts during the JUICE Lunar-Earth Gravity Assist
Abstract. On 19–20 August 2024, the European Space Agency (ESA) Jupiter Icy Moons Explorer (JUICE) mission performed the first ever Lunar–Earth Gravity Assist (LEGA) manoeuvre. The mission was launched on April 14, 2023, and is currently on an 8-year interplanetary cruise to the Jovian system. It is equipped with the RADiation-hard Electron Monitor (RADEM), a facility instrument designed to measure the most energetic particle populations (electrons, protons and ions) in the Jovian environment where JUICE will operate. During LEGA, JUICE crossed the Van Allen belts, providing a unique opportunity to evaluate the in-flight response of RADEM and to optimize its configuration for the Jupiter phase. In this paper, we report RADEM observations of the Van Allen belts, showing clear sensitivity to trapped electrons and protons. We also discuss how the Earth-flyby geometry, including pitch-angle effects, influenced the measurements and the implications for future operations. The observations also demonstrate that while RADEM is a facility instrument, it has the potential to enhance the scientific return of the JUICE mission by monitoring a key energy range in Jupiter's radiation belts that no other instrument on JUICE is covering.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Annales Geophysicae.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.- Preprint
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Status: final response (author comments only)
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RC1: 'Comment on egusphere-2026-2055', Anonymous Referee #1, 07 Jun 2026
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AC1: 'Reply on RC1', Marco Pinto, 26 Jul 2026
General comment
This is a good comprehensive paper describing the observations and the performance of two of the RADEM detectors (EDH and PDH) during the first JUICE Lunar-Earth Gravity assist. It gives all the necessary details to demonstrate that in the future these detectors will be able to characterize the electron and proton population in the Jupiter Icy Moons environment. With this, the paper contains new data and is clearly presented.
A: We thank the reviewer for the positive assessment of the manuscript and for the constructive suggestions. We address each comment individually below. Two supplementary figures are included in support of our response to Comment 2.
Specific comments
The following information would be welcome to enhance the paper:
- On Figure 3 it would be an asset to also show the evolution of the magnetic latitude during the flyby. The relevance of this parameter is apparent as it intervenes several times during interpretation of observations (line 130, line 188 and following)
A: We agree with the reviewer. We will add the spacecraft magnetic latitude as a new panel in Fig. 3 and update the figure caption and corresponding discussion accordingly.
- Question and comment when reading discussion on Figure3 and Figure 4:
Radiation belt models are not perfect, but it would have been interesting to see what they estimate as general shapes of the radiation belts for the given orbit of the flyby (for fluxes of representative energies for protons and electrons). Did you ever do such analysis, also to have an estimate if the absolute flux values you observe are realistic?
I also think about this latter as there is no comment on the absolute flux scales in the text when introducing Figure 4. Even though (also when looking at Pinto 2026) I understand that there will still be evolution in the flux determination, a small general statement on the here deduced flux levels would be welcome.
A: We thank the reviewer for this helpful suggestion. We have compared the RADEM differential electron and proton fluxes with AE9 and AP9 predictions calculated along the JUICE flyby trajectory using OMERE. We selected model energies close to the RADEM bow-tie effective energies: 0.75, 1.5, 2.0, and 2.5 MeV for electrons, and 8, 11, 18, and 30 MeV for protons. The resulting comparison figures are included with this response.
In regions where the RADEM signal is clearly above the background, AE9 and AP9 reproduce the broad locations and overall magnitudes of the electron and proton radiation belts. The measured and modelled peak fluxes are generally within approximately one order of magnitude. The models do not reproduce the observed multi-peak structures, the secondary proton enhancement, or all inbound–outbound differences. This is expected because AP9/AE9 describe a statistical radiation environment and do not represent event-specific geomagnetic conditions, the detector’s restricted pitch-angle coverage, or all local-time and longitudinal effects.
We will add the two comparison figures to the Supplement and include a brief discussion of the absolute flux levels in the manuscript.
- Line 186-190. To explain the difference in outbound and inbound proton fluxes, argumentation is done with respect to magnetic latitude in the context of anisotropic pitch angle distributions. That’s OK but in Figure 3 it can be seen that during the outbound segment the local pitch angle is farer away (up to ~135°) from the 90° line than during the inbound segment, hence this may also contribute to the fact that inbound fluxes are higher than the outbound fluxes (even if magnetic latitude has not changed much). Can’t this also contribute to the discrepancy at L=2.8 in addition to explanation in line 194-199? Please comment.
A: Thank you for this comment. We agree that the outbound PDH viewing direction is farther from , which may contribute to the lower outbound flux near L=2.8. However, the available measurements do not allow us to quantify the pitch-angle dependence, and therefore we cannot determine whether this effect is sufficient to explain the full observed variation. We will revise the text to acknowledge pitch-angle sampling as one contributing factor, in addition to the spatial or temporal effects discussed in the manuscript.
- Line 207. What is actually the time resolution for the measurements? Please add the value. Suggestion: replace “the time resolution is also too slow to capture” by “the time resolution (xxx seconds) is not sufficient to capture…”. The term "too slow" is not clear.
A: Thank you for the suggestion. The time resolution of the measurements is 60 s. We agree that this value should be stated explicitly in the manuscript. We will add it to Sect. 2 and revise the sentence at line 207 accordingly.
- Line 216 -220: Please revise sentence starting with “RADEM also captured….” , were conclusion on electron and proton observations are mixed. Suggestion: separate electron and proton conclusion, and maybe mention more directly that you measured inner belt protons (line 220-224).
A: We agree with the reviewer. We propose to replace the current text with the following:
In the outer electron radiation belt, RADEM observed a dual-peaked electron structure consistent with the recovery phase of the intense geomagnetic storm that occurred several days before the flyby. Differences between the inbound and outbound electron profiles likely reflect a combination of magnetic-local-time-dependent magnetospheric geometry, detector pitch-angle coverage, and temporal or longitudinal structure. The enhanced sub-MeV electron flux in the slot region may be associated with sudden particle enhancements at low L-shells.
RADEM also measured trapped protons in the inner radiation belt at energies from 7.4 to 28.9 MeV. The proton flux displayed a pronounced maximum near , together with a secondary lower-energy enhancement near . The inbound–outbound differences are likely influenced by both magnetic latitude and the pitch angle sampled by the PDH field of view. Additional magnetic-local-time, longitudinal, or transient proton structure may also contribute, particularly near the outer edge of the inner belt.
These observations demonstrate RADEM’s ability to distinguish and characterize energetic electron and proton populations in a trapped-radiation environment and provide an important in-flight validation data set before operations at Jupiter.
Technical correction
Line 26: (Oldham and McLean, 2003) is the reference for cumulative ionizing dose and (Dodd and Massengill, 2003) is the reference for single event effects. Please change.
For all the figures, please increase the length of the ticks on the graphs so that they are clearly visible without extreme zoom. They are not really visble when the page is at 100%. Visible ticks make it easier to locate points on the graphs and follow the argumentation.
Line 163. Point missing at end of sentence/paragraph.
A: We thank the reviewer for identifying these issues. We will correct the references at line 26, increase the tick lengths in all figures, and add the missing full stop at line 163.
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AC1: 'Reply on RC1', Marco Pinto, 26 Jul 2026
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RC2: 'Comment on egusphere-2026-2055', Anonymous Referee #2, 12 Jun 2026
The manuscript describes electron and proton measurements of JUICE-RADEM during the Earth-Moon flyby of JUICE in 2024. The authors demonstrate RADEM’s capability of measuring radiation belt electrons and protons in the Earth’s magnetosphere. I believe this manuscript is an important contribution to the special issue "The first-ever lunar–Earth flyby: a unique test environment for JUICE", given RADEM's expected scientific return and its capability to characterize Jupiter's radiation belts.
Before the manuscript can be published I ask the authors to take care of the following comments:
General science questions and comments
Magnetospheric waves:
Several of the observed radiation-belt structures could potentially be influenced by wave-particle interactions (e.g. Thorne et al., 2010 https://doi.org/10.1029/2010GL044990). I appreciate that a detailed wave analysis may be beyond the scope of this RADEM-focused paper. Nevertheless, I encourage the authors to briefly discuss whether wave activity could contribute to the observed features and to highlight the complementary measurements available from other JUICE instruments that could be used in future investigations.Moon signature
The discussion of the expected lunar signature would benefit from additional detail. Please clarify:
• Line 205: How the simulated lunar signature was generated (e.g. based solely on the apparent angular size of the Moon, or including detector field-of-view and viewing geometry).
• Line 207: What is meant by the detector "angular response". Do the authors mean to say that the detectors were looking in the wrong direction and could therefore not see the shadow of the moon? If so, could the authors plot or state the angle between the detectors and the moon?
• Line 207: The temporal resolution of the measurements and the expected duration of the lunar shadowing signature.Other RADEM detectors
On line 74-75 it is mentioned that HIDH and DDH are not used in this manuscript. Could the authors briefly comment on why that is?Radiation belt science
The manuscript discusses several radiation-belt structures and possible physical interpretations. I encourage the authors to include a brief summary statement, for example in the Conclusions section, indicating whether the observed features are broadly consistent with established radiation-belt morphology and dynamics or whether any features appear unusual and merit further investigation.Minor comments
Line 22: ...2013)-extremely... -> the hyphen seems misplaced hereLine 24: ... those on earth and jupiter -> those of Earth and Jupiter
Line 57: the authors highlight the scientific value of RADEM given its capability to study the radiation belts. I suggest that the authors also briefly mention that RADEM could also contribute to scientific investigations of the icy moons, for example concerning the interaction of radiation belt particles with the planetary surfaces (e.g. Nordheim et al., 2018 https://doi.org/10.1038/s41550-018-0499-8).
Figure 1:
- The authors distinguish between electrons and protons using diamond and circle markers on the respective lines. However, these markers are quite small and difficult to distinguish without substantial zooming. In addition, they contribute to an already visually busy figure. I encourage the authors to consider simplifying the figure presentation. For example, the diamond and circle markers could be removed and electrons and protons instead distinguished using different colours, as in Figure 3.
- Some of the lines appear to be dashed or dot-dashed. However, based on the figure caption, these line styles do not appear to convey any additional information. I suggest using a single line style throughout to reduce visual clutter.Line 78: ... these particles ... -> please clarify which are ‘these particles’
Line 79: Since the separation of the two particle populations appears to be an important assumption underlying the analysis, please provide a reference supporting this statement.
Line 130: I ask the authors to show the magnetic latitude in the figure that is being described here
Line 133-134 and Figure 3: just before the closest approach there seems to be a correlation between Electrons 2 and the protons with a peak in both. Can the authors briefly comment on this feature?
Line 183: should it not be more correct to say near L=2.3. There appear to be no measurements at L=2.2 in Figure 6.Line 190: please state explicitly the expected physical relationship between Bt and the expected flux
Figure 7:
- please define a.u. (presumably arbitrary units?)
- please show a vertical line for the closest approach
- please clarify the time labels. e.g. MM-DD HR?Data availability: Please provide a link to the RADEM data archive.
Citation: https://doi.org/10.5194/egusphere-2026-2055-RC2 -
AC2: 'Reply on RC2', Marco Pinto, 26 Jul 2026
The manuscript describes electron and proton measurements of JUICE-RADEM during the Earth-Moon flyby of JUICE in 2024. The authors demonstrate RADEM’s capability of measuring radiation belt electrons and protons in the Earth’s magnetosphere. I believe this manuscript is an important contribution to the special issue "The first-ever lunar–Earth flyby: a unique test environment for JUICE", given RADEM's expected scientific return and its capability to characterize Jupiter's radiation belts.
A: We thank the reviewer for the positive assessment of the manuscript and for the constructive comments. We address each point individually below.
Before the manuscript can be published I ask the authors to take care of the following comments:
General science questions and comments
Magnetospheric waves:
Several of the observed radiation-belt structures could potentially be influenced by wave-particle interactions (e.g. Thorne et al., 2010 https://doi.org/10.1029/2010GL044990). I appreciate that a detailed wave analysis may be beyond the scope of this RADEM-focused paper. Nevertheless, I encourage the authors to briefly discuss whether wave activity could contribute to the observed features and to highlight the complementary measurements available from other JUICE instruments that could be used in future investigations.A: We agree that wave–particle interactions may contribute to some of the observed electron structures. However, assessing their role would require simultaneous wave measurements and a dedicated phase-space-density analysis, which are beyond the scope of this instrument-focused paper. We will add the following brief statement acknowledging this possibility and the complementary role of RPWI:
“Wave–particle interactions may also have contributed to the observed electron structures through acceleration or pitch-angle scattering (Thorne et al., 2010). Investigating their role would require simultaneous wave measurements, such as those provided by RPWI (Wahlund et al., 2025), together with particle phase-space-density analysis, and is beyond the scope of the present study.”
Moon signature:
The discussion of the expected lunar signature would benefit from additional detail. Please clarify:
• Line 205: How the simulated lunar signature was generated (e.g. based solely on the apparent angular size of the Moon, or including detector field-of-view and viewing geometry).
• Line 207: What is meant by the detector "angular response". Do the authors mean to say that the detectors were looking in the wrong direction and could therefore not see the shadow of the moon? If so, could the authors plot or state the angle between the detectors and the moon?
• Line 207: The temporal resolution of the measurements and the expected duration of the lunar shadowing signature.A: We thank the reviewer for these questions. The curve in Fig. 7 is a simplified geometric estimate based on the apparent angular size of the Moon and does not include the direction-dependent response of the RADEM detectors. At GCR energies, the response is nearly omnidirectional because sufficiently energetic ions can penetrate the collimator stacks and reach the sensors from outside the nominal apertures. However, the response still depends on the orientation of the sensor plane and the amount of intervening material. We will clarify this point, provide the angle between the Moon direction and the sensor-plane normals, and add the 60s accumulation time and the predicted duration of the shadowing signature.
Other RADEM detectors
On line 74-75 it is mentioned that HIDH and DDH are not used in this manuscript. Could the authors briefly comment on why that is?A: The paper focuses on the electron and proton measurements made with EDH and PDH, which were the primary RADEM data sets used to characterize the terrestrial radiation belts during LEGA. HIDH was not included because only one of its two sensors was operational during the flyby, preventing the coincidence measurements required for quantitative heavy-ion identification. The DDH data require further pixel-to-pixel cross-calibration before its 28 viewing directions can be compared quantitatively. This directional analysis is ongoing and will be presented separately. We will add a brief explanation in Sect. 2.
Radiation belt science
The manuscript discusses several radiation-belt structures and possible physical interpretations. I encourage the authors to include a brief summary statement, for example in the Conclusions section, indicating whether the observed features are broadly consistent with established radiation-belt morphology and dynamics or whether any features appear unusual and merit further investigation.A: We agree with the reviewer. We will add a summary statement to the Conclusions distinguishing the large-scale radiation-belt morphology from the finer structures that require further investigation.
The large-scale observations are broadly consistent with the expected terrestrial radiation-belt structure: RADEM measured energetic electrons primarily in the outer belt, a depletion of electrons above approximately 1 MeV in the slot region, and MeV protons concentrated in the inner belt. Our comparison with AP9/AE9 also reproduces the broad belt locations and gives flux levels of generally comparable magnitude (see supplementary figures in RC1).
The finer structures—including the dual-peaked outer-belt electron profiles, the sub-MeV enhancement and inbound–outbound difference in the slot region, and the secondary lower-energy proton enhancement near —are not necessarily inconsistent with established radiation-belt dynamics, but they cannot be explained uniquely with the available RADEM observations. Further investigation would require particle phase-space-density calculations and combined analysis with magnetic-field, wave, and lower-energy particle measurements.
Minor comments
Line 22: ...2013)-extremely... -> the hyphen seems misplaced hereA: We will correct this.
Line 24: ... those on earth and jupiter -> those of Earth and Jupiter
A: We will correct this.
Line 57: the authors highlight the scientific value of RADEM given its capability to study the radiation belts. I suggest that the authors also briefly mention that RADEM could also contribute to scientific investigations of the icy moons, for example concerning the interaction of radiation belt particles with the planetary surfaces (e.g. Nordheim et al., 2018 https://doi.org/10.1038/s41550-018-0499-8).
A: We agree with this suggestion. The contribution of RADEM to characterizing the energetic-particle environment and space weathering of the icy moons is discussed in the RADEM instrument paper (Hajdas et al., 2025). We will add a brief statement to the manuscript noting that RADEM measurements may also support studies of radiation-induced surface alteration, and cite Hajdas et al. (2025) and Nordheim et al. (2018).
Figure 1:
- The authors distinguish between electrons and protons using diamond and circle markers on the respective lines. However, these markers are quite small and difficult to distinguish without substantial zooming. In addition, they contribute to an already visually busy figure. I encourage the authors to consider simplifying the figure presentation. For example, the diamond and circle markers could be removed and electrons and protons instead distinguished using different colours, as in Figure 3.- Some of the lines appear to be dashed or dot-dashed. However, based on the figure caption, these line styles do not appear to convey any additional information. I suggest using a single line style throughout to reduce visual clutter.
A: Thank you for the suggestion. We agree that Fig. 1 is visually busy. We will remove the markers and use a single line style throughout. The individual panels and axis labels will continue to distinguish the incident particle species.
Line 78: ... these particles ... -> please clarify which are ‘these particles’
A: We will correct this.
Line 79: Since the separation of the two particle populations appears to be an important assumption underlying the analysis, please provide a reference supporting this statement.
A: We agree. We will clarify that the statement refers to the spatial separation of the dominant particle populations during the flyby, with energetic electrons mainly observed in the outer belt and protons in the inner belt, and will add Kanekal and Miyoshi (2021) as reference.
Line 130: I ask the authors to show the magnetic latitude in the figure that is being described here
A: The magnetic latitude will be added to Figure 3. We will reference it at the end of this sentence.
Line 133-134 and Figure 3: just before the closest approach there seems to be a correlation between Electrons 2 and the protons with a peak in both. Can the authors briefly comment on this feature?
A: We agree that a small enhancement in ELECTRONS 2 coincides with the proton peak. Since this channel is also sensitive to high-energy protons, residual proton contamination is a likely explanation. We will briefly discuss this feature and revise the statement that the profiles are “uncorrelated” to “largely uncorrelated.
Line 183: should it not be more correct to say near L=2.3. There appear to be no measurements at L=2.2 in Figure 6.A: Thank you for noticing. Indeed, the lowest L-shell is 2.29. We will change it to L=2.3 in the text.
Line 190: please state explicitly the expected physical relationship between Bt and the expected flux
A: We agree and will clarify this relationship. We propose to modify the pagraph to:
During the inbound leg, JUICE crossed the inner belt closer to the magnetic equator, where the local magnetic field was weaker. Previous observations indicate that energetic trapped-proton pitch-angle distributions in this region can be enhanced near at the magnetic equator (Selesnick et al., 2014). Such particles mirror near the equator and may not reach the higher-magnetic-latitude, higher- outbound location, which is consistent with a lower outbound flux. However, RADEM did not measure the complete proton pitch-angle distribution, so this contribution cannot be quantified.
Figure 7:
- please define a.u. (presumably arbitrary units?)
A: Yes, these are arbitrary units. We will change the label to “Normalized Count Rate”.
- please show a vertical line for the closest approach
A: We will implement a vertical line for closest approach.
- please clarify the time labels. e.g. MM-DD HR?A: That is correct. We will change the time label to be in accordance with Figure 3.
Data availability: Please provide a link to the RADEM data archive.
A: We will add a direct link to the RADEM data collection in ESA’s Planetary Science Archive in the Data Availability section.
Citation: https://doi.org/10.5194/egusphere-2026-2055-AC2
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AC2: 'Reply on RC2', Marco Pinto, 26 Jul 2026
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General comment
This is a good comprehensive paper describing the observations and the performance of two of the RADEM detectors (EDH and PDH) during the first JUICE Lunar-Earth Gravity assist. It gives all the necessary details to demonstrate that in the future these detectors will be able to characterize the electron and proton population in the Jupiter Icy Moons environment. With this, the paper contains new data and is clearly presented.
Specific comments
The following information would be welcome to enhance the paper:
Radiation belt models are not perfect, but it would have been interesting to see what they estimate as general shapes of the radiation belts for the given orbit of the flyby (for fluxes of representative energies for protons and electrons). Did you ever do such analysis, also to have an estimate if the absolute flux values you observe are realistic?
I also think about this latter as there is no comment on the absolute flux scales in the text when introducing Figure 4. Even though (also when looking at Pinto 2026) I understand that there will still be evolution in the flux determination, a small general statement on the here deduced flux levels would be welcome.
Can’t this also contribute to the discrepancy at L=2.8 in addition to explanation in line 194-199? Please comment.
Suggestion: replace “the time resolution is also too slow to capture” by “the time resolution (xxx seconds) is not sufficient to capture…”. The term "too slow" is not clear.
Suggestion: separate electron and proton conclusion, and maybe mention more directly that you measured inner belt protons (line 220-224).
Technical correction