the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 15 Apr 2026
First results from the Juice magnetometer during the Lunar-Earth Gravity Assist
Abstract. Before reaching the Jovian system in 2031, the ESA Jupiter Icy Moons Explorer (Juice) spacecraft will perform three Earth flybys. As well as providing gravitational assistance, these flybys are prime opportunities to assess performance and undertake in-flight calibration of the science payload in a well-understood environment. The first such interval occurred in August 2024, dubbed the Lunar-Earth gravity assist (LEGA) as it included a pass by the Moon in advance of the approach to Earth. The unique spacecraft trajectory allowed for sampling of various regions of the magnetosphere before exiting into the solar wind. In this paper, we report on the performance of the Juice magnetometer instrument (J-MAG) during the LEGA. J-MAG comprises three sensors—two fluxgate vector sensors and one Coupled Dark State Magnetometer (CDSM) scalar sensor—all of which were operational during the LEGA period. This represents the first time in the mission that the scalar sensor has operated in its nominal state. Here, we analyse J-MAG observations during key periods of the flyby. As well as inter-sensor comparison, we assess J-MAG data against geomagnetic field models during the approach to Earth, and compare with measurements from other spacecraft at the Moon and in the solar wind, which allow us to make suggestions for future calibration activities. Overall, the three sensors showed excellent performance at this early stage in the mission and confirmed that the scalar sensor meets the requirements for in-flight calibration of the fluxgate sensors at Ganymede. The LEGA also demonstrates the potential value of using cruise phase measurements for scientific exploration and solar wind monitoring.
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Status: open (until 18 Jun 2026)
- RC1: 'Comment on egusphere-2026-2054', Anonymous Referee #1, 28 May 2026 reply
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The paper presents some initial results from the J-MAG experiment during the JUICE Earth Flyby.
What is shown is mainly that a J-MAG measurements are largely in-line with expectations during the passage through the generally well-understood terrestrial magnetosphere - no new understanding is developed beyond this, with the exception of some initial indications as to the instrument behavior. This is to be seen within context of the other papers of this LEGA special issue. The bigger loss is the lack of deep-tail measurements that would otherwise have been possible given the JUICE trajectory, but were not operationally feasible, as such measurements are very rare and could have lead to genuinely new science.
The discussion regarding discrepancies between the constituent sensors is cursory, reflecting the early stage of the analysis. However, data quality should not be judged in this paper, as the data are not yet subject to conclusive calibration. The analysis shown demonstrates the performance of the instrument tentatively, without the data having undergone "the final calibration process" (whatever that may mean in practice - we are not told). The text makes this point consistently.
Annales Geo. publication policy is clear that the data comprising the figures should also be available. The JUICE project presented a possible repository for such un-calibrated data; others also exist. The current statement given in the paper is however not compatible with Annales' policy as it stands in my interpretation.
More detail would be interesting surrounding the discrepancies seen between the IBS and OBS sensor:
It is stated that "These axes were determined to be roughly perpendicular to the axis of the magnetometer boom during this period. It is therefore believed that a current source along the boom harness may be responsible for the signal seen by IBS, which would also explain why it was not seen by OBS, which is mounted further along the boom."
"Roughly perpendicular" could presumably be easily quantified? Because, what would be the alternative? A current through the plasma would presumably be insignificant and not manifest in this way in the J-MAG sensors, and larger currents flowing elsewhere on the S/C bus would also not have such an appearance?
Many readers will not be familiar with the scalar magnetometer technique. A brief elaboration would be a courtesy to the reader.
Line 143: "By performing a minimisation, we determined that a small rotation of less than 1º in each axis can reduce the maximum difference between the measured and modelled field at any point to 20 nT, which we consider very good agreement in this high field environment."
In my reading, this is ambiguous when stated so. Have the data shown in the figure (and subsequently) had this rotation applied or not?
A potential temperature sensitivity is mentioned, but without reference to any similar effects known to affect previous missions in the same environment. Is there something here specific to JUICE/J-MAG in terms of susceptibility to this in this design?
OMNI data are used, but a statement is made that "No timing adjustments have been made to either dataset". OMNI data are as a matter of routine, time shifted to make them relevant to the sub-solar bow shock. This is the "algorithmic propagation" referred to earlier. It would also be very easy, for example, to calculate and display the GSE-x displacement of JUICE relative to this point. One could also argue that, for the specific purpose of comparing individual instruments and sensors, it might be relevant to know which of the OMNI spacecraft are being used at this specific time in order to produce their B-field product.
Around Line 240, the usefulness of these data for space weather are discussed. This is a little far fetched, given the data are not calibrated often (as in exactly this case). The bigger loss, however, is that the relevant instruments including most importantly J-MAG where not operated continuously during the long passage through the Earth's magnetotail, which is a very poorly explored region of the magnetosphere, particularly at large distances, and on such quasi-radial trajectories. This argument could also be made.