the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 15 Apr 2026
In-Situ and Remote Observations of the Ring Current and Radiation Belt during JUICE Lunar Earth Gravity Assist
Abstract. This report presents an overview of the observations and performance of the Jovian Energetic Neutrals and Ions (JENI) and the Jovian Energetic Electrons (JoEE) belonging to the Particle Environment Package (PEP) onboard the Jupiter Icy Moon Explorer (JUICE) during its Lunar-Earth Gravity Assist (LEGA) 19–23 August 2024. Observations from the Lunar and Earth flybys are presented and discussed. While the environment around the Moon was very quiet, the Earth flyby presented a characteristic, nearly symmetric recovery phase ring current with adiabatically increasing energies with decreasing distance to Earth, and butterfly pitch-angle distributions (PADs) at high L-shells transitioning to distributions rounded around 90° pitch angle at lower L-shells. The electron radiational belt showed a double-structured outer zone, possibly from a fresh intensification, and the characteristic slot region created by loss through interactions with plasmaspheric hiss. On the outbound, JENI was operated as an Energetic Neutral Atom (ENA) camera and observed the energetic ions of the inner magnetosphere from 17.4 RE to 150 RE. During this period multiple intensifications were observed in the night side magnetosphere consistent with substorm injections. The LEGA observations provided useful data for further calibrating the sensors that overall showed a nominal performance.
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-2026-2050', Anonymous Referee #1, 07 May 2026
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AC1: 'Reply on RC1', Pontus Brandt, 14 Jul 2026
The authors would like to thank the referee for supportive and constructive review. Below find first the reply to comments and questions.
1) The abstract says « The LEGA observations provided useful data for further calibrating the sensors » → Do you want to add a sentence or two in the main text on this topic? For instance, what strategy will be employed to calibrate the sensors from the LEGA observations (also including the future 2026 Earth fly-by data) ?
Response: We will add sentence on the calibration strategy. Foremost this revolves around comparing overlapping energy ranges with the other PEP sensors and RADEM, and pitch angle distributions to aide in flat-fielding improvements.
2) Table 1: would it be possible to remind why the energy range of electrons observed by JENI is mentioned, as well as the energy range of ions for JoEE? Do JENI and JoEE have an electron and an ion mode, respectively?
Response: Good catch! Yes, both JENI and JoEE have capabilities to measure electrons and total ions. We will describe this in the text.
3) Caption of Figure 3: the end of the caption sentence says « (d) same as (c ), but for He+ », I guess it should be « (c ) same as (b), but for He+ » ?
Response: Correct! The figure will also be corrected and have (a), (b) and (c) properly annotated.
4) The text mentions the observation of He+ ions, is it possible to say with JENI if those are He+ or He2+ ions (alpha particles)? My guess is no, so maybe He^{n+} could be more accurate.
Response: Correct guess. It will be difficult to resolve He+ from alphas using the TOFxE system. We will make the clarification in the text,
5) Caption of Figure 6 « mismatches in detector efficiencies » → do you want to add a reference to the paper of Clark et al. if they further discuss this issue ? « (see Clark et al. 2026) »
Response: The paper by G. Clark is still in progress. We will add a reference. In case it is not submitted in time, we will have to remove it before publication of this paper.
6) Figure 6 : was JoEE also able to observe the pitch angle distribution of energetic electrons ?
Response: Yes! A JoEE PAD spectrogram will be added below the energy spectrogram of Figure 6.
7) The articles refers to Dialynas et al. 2026 but this paper is not listed in the reference section.
Response: thanks for good catch. This reference will be added properly. The paper is under review and making good progress towards publication.
Many thanks again!
Citation: https://doi.org/10.5194/egusphere-2026-2050-AC1
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AC1: 'Reply on RC1', Pontus Brandt, 14 Jul 2026
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RC2: 'Comment on egusphere-2026-2050', Anonymous Referee #2, 11 May 2026
The manuscript presents a timely and interesting overview of the JENI and JoEE observations obtained during the JUICE Lunar-Earth Gravity Assist. I think the paper is suitable for publication after minor revisions. The topic is relevant, the dataset is valuable, and the manuscript provides a useful overview of both the in-situ and remote observations. My comments below are mainly intended to improve clarity, consistency, and presentation.
- Title: I suggest adding “PEP” or “PEP-Hi” to the title, as this would make the instrument context clearer and help readers identify the paper more easily.
- Introduction / early mission description:
- Line 30: Please consider adding references for PEP and for the JUICE mission.
- Line 33: I suggest revising the wording to read more smoothly, for example: “...ENA camera, and JoEE...”.
- Line 34: The launch date of JUICE should be corrected. The manuscript currently gives 23 April 2023, whereas JUICE was launched on 14 April 2023.
- Line 35: I recommend adding a reference to the LEGA trajectory / operations paper: Dietz, A., Boutonnet, A., and Budnik, F.: The JUICE Lunar-Earth gravity assist from trajectory design, navigation and spacecraft operations perspective, EGUsphere [preprint], https://doi.org/10.5194/egusphere-2026-1015, 2026.
- Table 1: Table 1 is not referenced in the main text and should be explicitly cited.
- Line 60: Please spell out IMAGE at first use.
- Sections 2.1 and 2.2: It would help the reader if a simple schematic of the JENI and JoEE detector concepts were added.
- Trajectory / flyby details:
- Line 86: The quoted lunar and Earth closest-approach values should be checked against the LEGA operations paper. The manuscript gives 755 km and 6807 km, while the operations paper reports 752 km above the Moon and 6839 km above Earth.
- Line: 90, I suggest revising the wording to “the Moon and Earth”.
- Line 92: Since the Venus gravity assist has already taken place, I suggest adding the date for clarity. ESA reports the Venus flyby on 31 August 2025.
- Minor wording / style corrections:
- Line 93: “ENA mode to attempt ENA imaging image” should be corrected.
- Line 100: “The SYMH index was therefore only ~10 nT” reads better than the current phrasing.
- Lines 111–113: Could be rephrased, as the current sentence structure sounds somewhat awkward even though the meaning is understandable.
- Figure 2: I suggest expanding the caption to describe the different figure elements more explicitly. This would make the figure easier to interpret for readers outside the immediate subfield.
- Figure 3:
- The caption appears to misidentify the subplots: it refers to “(a), (b), (d)” even though the text discusses panels (a), (b), and (c).
- The spectrogram does not look entirely symmetric to me. Although the general structure is similar inbound and outbound, the outbound portion seems somewhat more elongated. A short comment on this asymmetry would strengthen the interpretation.
- Figure 4: I suggest highlighting the proton and He+ bands more clearly for readability.
- Section 3.2: It may help to add a brief sentence noting whether JoEE and other relevant JUICE and/or other mission measurements show comparable structures, if such a comparison is appropriate.
- Conclusions: The manuscript would benefit from a short Conclusions section. At present, it ends with Figure 7.
Citation: https://doi.org/10.5194/egusphere-2026-2050-RC2 -
AC2: 'Reply on RC2', Pontus Brandt, 26 Jul 2026
The authors would like to thank the reviewer for constructive and helpful comments to further improve the manuscript. Please find responses to each comment below.
Title: I suggest adding “PEP” or “PEP-Hi” to the title, as this would make the instrument context clearer and help readers identify the paper more easily.
Response: Good suggestion and will be added.
Introduction / early mission
Line 30: Please consider adding references for PEP and for the JUICE mission.
Response: Added.
Line 33: I suggest revising the wording to read more smoothly, for example: “...ENA camera, and JoEE...”.Response: Will be done.
Line 34: The launch date of JUICE should be corrected. The manuscript currently gives 23 April 2023, whereas JUICE was launched on 14 April 2023.
Response: Good catch and will be corrected!
Line 35: I recommend adding a reference to the LEGA trajectory / operations paper: Dietz, A., Boutonnet, A., and Budnik, F.: The JUICE Lunar-Earth gravity assist from trajectory design, navigation and spacecraft operations perspective, EGUsphere [preprint], https://doi.org/10.5194/egusphere-2026-1015, 2026.Response: thanks for reference and will be added.
Table 1: Table 1 is not referenced in the main text and should be explicitly cited.Response: Good catch and will be added.
Line 60: Please spell out IMAGE at first use.Response: will do.
Sections 2.1 and 2.2: It would help the reader if a simple schematic of the JENI and JoEE detector concepts were added.Response: Simple figures will be added.
Trajectory / flyby details:
Line 86: The quoted lunar and Earth closest-approach values should be checked against the LEGA operations paper. The manuscript gives 755 km and 6807 km, while the operations paper reports 752 km above the Moon and 6839 km above Earth.Response: We will double check and confirm the correct number.
Line: 90, I suggest revising the wording to “the Moon and Earth”.Response: Will do.
Line 92: Since the Venus gravity assist has already taken place, I suggest adding the date for clarity. ESA reports the Venus flyby on 31 August 2025.Response: Will do.
Minor wording / style corrections:Response: Thanks for good edits. All edits will be included.
Line 93: “ENA mode to attempt ENA imaging image” should be corrected.
Line 100: “The SYMH index was therefore only ~10 nT” reads better than the current phrasing.
Lines 111–113: Could be rephrased, as the current sentence structure sounds somewhat awkward even though the meaning is understandable.
Figure 2: I suggest expanding the caption to describe the different figure elements more explicitly. This would make the figure easier to interpret for readers outside the immediate subfield.
Figure 3:
The caption appears to misidentify the subplots: it refers to “(a), (b), (d)” even though the text discusses panels (a), (b), and (c).
The spectrogram does not look entirely symmetric to me. Although the general structure is similar inbound and outbound, the outbound portion seems somewhat more elongated. A short comment on this asymmetry would strengthen the interpretation.
Figure 4: I suggest highlighting the proton and He+ bands more clearly for readability.
Section 3.2: It may help to add a brief sentence noting whether JoEE and other relevant JUICE and/or other mission measurements show comparable structures, if such a comparison is appropriate.Conclusions: The manuscript would benefit from a short Conclusions section. At present, it ends with Figure 7.
Response: Good suggestion and will be done.
Many thanks!
Citation: https://doi.org/10.5194/egusphere-2026-2050-AC2
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- 1
The submitted article provides an overview of the JENI and JoEE observations conducted during the historical LEGA flyby of JUICE. The paper presents the observations for the entire LEGA period and serves as a global introduction to other papers which will focus on only one time period of the fly-by and/or one instrument.
The paper is very well written and the Figures have a nice layout. All of this makes the article easy to read and increases its potential impact to the community. I recommend the paper for publication in ANGEO and I list below some minor comments that the authors can consider.
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Specific comments
1) The abstract says « The LEGA observations provided useful data for further calibrating the sensors » → Do you want to add a sentence or two in the main text on this topic? For instance, what strategy will be employed to calibrate the sensors from the LEGA observations (also including the future 2026 Earth fly-by data) ?
2) Table 1: would it be possible to remind why the energy range of electrons observed by JENI is mentioned, as well as the energy range of ions for JoEE? Do JENI and JoEE have an electron and an ion mode, respectively?
3) Caption of Figure 3: the end of the caption sentence says « (d) same as (c ), but for He+ », I guess it should be « (c ) same as (b), but for He+ » ?
4) The text mentions the observation of He+ ions, is it possible to say with JENI if those are He+ or He2+ ions (alpha particles)? My guess is no, so maybe He^{n+} could be more accurate.
5) Caption of Figure 6 « mismatches in detector efficiencies » → do you want to add a reference to the paper of Clark et al. if they further discuss this issue ? « (see Clark et al. 2026) »
6) Figure 6 : was JoEE also able to observe the pitch angle distribution of energetic electrons ?
7) The articles refers to Dialynas et al. 2026 but this paper is not listed in the reference section.