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: open (until 28 Jun 2026)
- RC1: 'Comment on egusphere-2026-2055', Anonymous Referee #1, 07 Jun 2026 reply
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RC2: 'Comment on egusphere-2026-2055', Anonymous Referee #2, 12 Jun 2026
reply
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
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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