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the Creative Commons Attribution 4.0 License.
| 21 Apr 2026
First Energetic Neutral Atom images of the Earth’s magnetosphere from the JENI detector on board the JUICE mission during the Lunar and Earth Gravity Assist
Abstract. The Jovian Energetic Neutrals and Ions (JENI), part of the Particle Environment Package (PEP), on the Jupiter Icy Moons Explorer (JUICE) spacecraft is a state-of-the-art detector, capable of analyzing the energy and direction of incident Energetic Neutral Atoms (ENAs) from planetary magnetospheres. Our analysis focuses on 2.86–85.5 keV Hydrogen ENA measurements between 21 to 24 August 2024 when JUICE performed the Lunar and Earth Gravity Assist (LEGA). During the LEGA, JENI had a unique opportunity to obtain continuous, 7.5-minute accumulation time global images of the terrestrial magnetosphere as the spacecraft moved outward from ~17 to 150 Earth Radii. Although there were no indications of substantial geomagnetic storms developing during that time, substorm activity was observed based on eight distinct dips of the SuperMAG AL (SML) index down to ~-250 nT for the four last ones and even down to ~-400 nT for the four first ones. Our results indicate increased ENA emissions associated with the onsets of these events, suggesting direct responses to the increased energetic ion activity during those substorm events. Unlike the periods of intense geomagnetic storms, during which proton intensities build up the ring current evolving from an asymmetric distribution during the main phase to a roughly symmetric distribution during the recovery phase, the images from JENI during this interval show non-uniform ENA emissions in the plasma sheet, suggesting transient processes, such as a series of repeated nightside injections. The nightside ENA intensities are generally brighter than those from the dayside, possibly reflecting magnetotail processes associated with energetic ions being injected.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Annales Geophysicae.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2026-2062', Anonymous Referee #1, 20 May 2026
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RC2: 'Comment on egusphere-2026-2062', Anonymous Referee #2, 20 May 2026
This paper reports hydrogen ENA observations of the Earth’s magnetosphere by JUICE/JENI during the Lunar–Earth Gravity Assist. The dataset is valuable, and the observational opportunity is unique: JENI obtained continuous 7.5-minute ENA images while JUICE moved outward from approximately 17 to 150 RE. The observations are potentially useful both as a demonstration of JENI performance and as a case study of ENA variability during non-storm-time substorm activity.
However, in its present form, I do not think that the main physical interpretation is demonstrated quantitatively enough. The manuscript argues that the observed ENA enhancements are direct responses to substorm-related energetic ion activity and that the morphology is consistent with repeated nightside injections. These interpretations are plausible, but the analysis remains largely descriptive. In particular, the effects of viewing geometry, low-altitude emissions, background/foreground contamination, and energy-dependent transport require further clarification.
I therefore recommend major revision. This recommendation reflects the need for a more quantitative and reproducible analysis, rather than a lack of interest in the dataset. I believe the paper could become publishable after substantial improvement of the analysis, figures, and discussion.
- The link between SML dips and ENA enhancements needs to be demonstrated quantitatively. What time delay is expected between the SML onset/minimum and the ENA response, considering ENA travel time, ion injection/drift timescales, and the 7.5-minute accumulation time? The authors should clarify whether the observed timing is physically consistent with substorm-related injections.
- The manuscript identifies eight ENA enhancements, E1–E8, that “roughly coincide” with minima in the SML index. However, the paper then concludes that these are direct responses to substorm-related energetic ion activity. This conclusion seems too strong without a more quantitative analysis.
- The authors should provide an objective definition of E1–E8, timing of ENA enhancement onset, peak, and decay relative to SML onset/minimum, uncertainty estimates on ENA counts or fluxes, residuals after removal of the expected distance trend, a statistical measure of the ENA–SML relationship.
- The total ENA counts decrease approximately as spacecraft distance increases, and the authors compare this with R⁻² reference lines. However, JENI is observing an extended and structured source, not a point source. The spacecraft distance, boresight direction, angular resolution, source filling factor, and selected pixels all evolve during the flyby.
- The authors state that low-altitude emissions can substantially dominate ENA emissions relative to the plasma sheet and that plasma-sheet/ring-current studies require excluding LAE-contaminated pixels. However, the procedure used to exclude LAEs is not described in enough detail. Please elaborate more.
- The paper argues that nightside ENA emissions broaden and intensify during substorm events, whereas dayside emissions do not show comparable variability. This is potentially interesting, but the interpretation seems not sufficiently developed. If the authors want to argue for nightside injections and subsequent drift/circulation, then corresponding energy channels for enhancement should be discussed if they are consistent with previous observations. This might help interpreting this event either plasma-sheet injection, partial ring-current development, and LAE contributions.
- The manuscript refers several times to oxygen ENA behavior during storm-time conditions, but the presented analysis is based on hydrogen ENAs. The summary also mentions associated O⁺ enhancements in the ring current region. Since oxygen ENA measurements are not shown, the discussion should be rewritten?
- Please describe more explicitly how background and possible foreground ion contamination were treated, especially given the deflection-plate voltage used during this period. The authors state that selected pixels away from Earth were used to avoid LAEs, but the selected pixel regions and exclusion criteria are not shown. Since the main interpretation relies on distinguishing nightside plasma-sheet/ring-current ENAs from LAEs, the authors should provide a more explicit description of the LAE removal procedure?
I believe that the figures require substantial improvement.
The figures are currently not adequate for a paper whose main evidence is image-based ENA morphology.
Figure 1 should be remade with clearer labels, visible reference lines (for example no x-y axis for middle and right figures, Earth and magnetic field lines are difficult to see), and marked spacecraft positions corresponding to the example images. The caption also contains an apparent error text, “1.1.2 Equations,” which must be removed.
Figure 2 is central to the manuscript but is too difficult to read all the numbers and small legends on the images. The timing of the “pre” and “during” images should be explicitly given (or if there is, then make it larger please).
Figure 3 should show the pixel-selection regions used for dayside and nightside averaging. Smoothing may be useful for visualization, but the quantitative conclusions should be based on unsmoothed data. I suggest that the authors either show the unsmoothed version in the main text or supplementary material, or explicitly demonstrate that the smoothing does not change the identified enhancements.
Figure 4: The use of 82.5-minute accumulation images should also be justified since this is long compared with substorm timescales?
The description of the dayside emission in Figure 4 should be reconciled with the spectrogram shown in Figure 3. In Figure 4 the dayside ENA emission appears nearly constant and weak, whereas Figure 3 suggests some energy-dependent temporal structure. It would be better if the authors could clarify whether this difference results from different energy ranges, different accumulation times, different pixel selections, or smoothing.
Minor comments
- The title should not emphasize “First” unless the authors explain why this is scientifically relevant.
- SMU should be defined when first introduced.
- IMF and solar-wind conditions are mentioned but not shown. Either include the relevant data or cite the source and summarize the values.
- Acronyms such as LAE should be defined at first use.
- The manuscript contains typographical and grammatical errors, please correct them.
- Line 217 should refer to Figure 2, not Figure 1.
- The introduction should more clearly define the scientific gap. At present, it reads mostly as a literature list and instrument context section.
Citation: https://doi.org/10.5194/egusphere-2026-2062-RC2
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- 1
This manuscript reports a nice ENA observation of the Juice/PEP/JENI instrument during the Earth gravity assist. The observations show a clear decreasing intensity while getting farer to the Earth, furthermore the evolution of ENA observations shows some repeated intensifications possible signature of non-storm substorm activities. The main value of the manuscript is the demonstration of good performances of JENI sensor; however, the observations are interesting and the interpretation seems correct, nevertheless there are number of unclear points in the data analysis and in the figures. For this reason, I suggest a moderate revision of the manuscript. I think that, after considering my suggestions the paper deserves publication.
Detailed comments:
Title: I don’t think that “first” is correct. Are other observations expected? Anyway, is this a relevant point for the article? Please, delete the word.
Line 40: I would write “ and of magnetospheres of other planets”, in fact, the references refer to many planets from Mercury, Venus, Mars, Jupiter and Saturn, not only outer planets. Then the authors refer to exosphere without any explanation. I suggest expanding or delete that.
Figure 1, 2 and 4, the lines in the images are almost invisible in the printed version. So it is hard/impossible to understand the spatial reference. Which is the MLT indication?
Figure 1: the blue line in black background is not recommended. It would be useful to show in the plot on the left where the image on the right has been taken. What is “1.1.2 Equations” at the end of the caption?
Figure 2: labels of left plot are too small. Plots on the left are important for the paper. They should be enlarged. In the images on the right pre and during the events, the timing of the observations are required. The caption refers to a smoothing of the images: is it really needed? I think that the real observations must be shown.
Line 137: please, define also SMU.
Lines 140-142: IMF data are not shown in the manuscript. It would be useful to see them or at least the authors should include a reference to them.
Line 149 and 153: acronym should be defined at first reference. Also add a reference for the LAE definition.
149-159: excluding LAE would require excluding high latitudes signal close to the planet. I am not sure that the performed analysis goes in that direction. In the figure 3 caption the authors refer to the exclusion of the pixels close to Earth, but it is not clear which pixels are excluded. Some more specification on this analysis is required. Furthermore, smoothing the spectrogram “to enhance visibility” is something that should be avoided.
I agree with the authors that the different timing of the night and day ENA associated to E1 seems to confirm a process stating from the night side (higher intensity and earlier decrease) and circulating and decaying toward the dayside. It is compatible with stretching and dipolarization in the plasma sheet, and ion circulation after injection. Is it the same for E2 and E3? The authors refer also to Oxygen while the images are not showing any mass discrimination. Most of the caption of Figure 3 should be moved in the main text. Generally, a more detailed description is needed.
Figure 4 and discussion: the dayside ENA emission seems to contradict the spectrogram in figure 3. Here the dayside ENA remain almost constantly at low intensity. The colorbar labels are too small.
Lines 195-204 and figure 4 right: the low intensity of high-energy ENA during the E1 seems to indicate an opening of ion trajectories at these energies, as it happens during the main phase of a storm, when ring current particles, having open patterns, can populate only partial ring current in the night / dusk side. Even if there is no evidence of storm signatures during the period, the ENA spectra seem to point to the development of a partial ring current. Regarding the low energy ENA, another point to be noted here is that the ions increase the energy while approaching lower L-shells, so it could be possible that the ENA spectra of the whole night side (or dayside) mix outer and inner magnetospheric ions. The decrease of post event ENA at low energies could be due to the movement of ions from outer to inner regions.
Line 217: it should be Figure 2 not Figure 1
Lines 215- 218 “although the 2.86-6.5 keV ENA emissions increase after each nightside injection episode, they do so with a slight time lag”: This sentence contradicts the spectra shown in figure 4. While in figure 4 the intensity at 3-6 keV at 15 UT is lower than the previous ones, in figure 2 the black curve shows a gradual increase (with respect to the expected trend) after E1. The time delay of the increase of low energies ENA seems visible in the plot of figure 2 only for E1. It is hard to see it for other events. This point needs some more analysis and justification.
Lines 218-220 “between E5 and E6 we see ENA enhancements which may correspond to a very small change in the SML index”: this point has not been discussed in the previous section. Some more analysis is needed. In fact, the spatial resolution is not enough at those distances to a detailed analysis, but a general increase of the integral ENA emission (exceeding the expectation after the R^-2 law) is clear and requires some discussion.
Line 248: As mentioned above, the Oxygen ENA are not shown in the manuscript.