Reply to Comment on “An Active Plume Eruption on Europa During Galileo Flyby E26 as Indicated by Energetic Proton Depletions”

• Published in: Geophysical research letters 2021-09, Vol.48 (18), p.n/a
• Main Authors: Huybrighs, H. L. F., Roussos, E., Blöcker, A., Krupp, N., Futaana, Y., Barabash, S., Hadid, L. Z., Holmberg, M. K. G., Witasse, O.
• Format: Article
• Language: English
• Subjects: Astrophysics; Charge exchange; Depletion; Electromagnetic fields; Electromagnetism; energetic protons; EPD; Eruptions; Europa; Fields; Flyby missions; Galileo; Identification; Ions; Magnetospheres; Magnetospheric plasma; particle tracing; Physics; Plumes; Protons; Water plumes
• ISSN: 0094-8276; 1944-8007; 1944-8007
• DOI: 10.1029/2021GL095240

In Huybrighs et al. (2020, https://doi.org/10.1029/2020gl087806) we investigated energetic proton depletions along Galileo's Europa flyby E26. Based on a particle tracing analysis, we proposed that depletions are caused by perturbed electromagnetic fields combined with atmospheric charge exchange and possible plumes. One depletion feature identified as a plume signature was shown to be an artifact (Jia et al., 2021, https://doi.org/10.1029/2020gl091550). Despite that, here we emphasize that Huybrighs et al. (2020, https://doi.org/10.1029/2020gl087806) demonstrates that plumes can cause proton depletions and that these features should be sought after. Furthermore, the conclusions on the importance of perturbed electromagnetic fields and atmospheric charge exchange on the depletions are unaffected. We suggest that the artifact's cause is a mistagging of protons as heavier ions by EPD. The artifact prevents us from confirming or excluding that there is a plume‐associated depletion. We also address comments on the MHD simulations and demonstrate that 540–1,040 keV losses are not necessarily inconsistent with 115–244 keV losses by plume‐associated charge exchange. Plain Language Summary In Huybrighs et al. (2020, https://doi.org/10.1029/2020gl087806) we identified why fast protons were disappearing during Europa flyby E26 by Galileo. Beyond impacting on the surface, we identified several contributing factors: First, perturbed electromagnetic fields resulting from the interaction of Europa's atmosphere with the magnetospheric plasma, which deflect the protons. Second, atmospheric charge exchange. We also showed that a water plume eruption could cause a region in which disappearances occur due to a combination of charge exchange and magnetic deflections. We identified a 20s decrease of protons as evidence of such a plume. However, an artifact in the data reported by Jia et al. (2021, https://doi.org/10.1029/2020gl091550) coincides with this 20s moment and prevents us from reaching a conclusion on the occurrence of a plume‐associated depletion. We emphasize that our conclusions on the importance of perturbed fields and charge exchange are unaffected, as the artifact only affects a short segment of the data we analyzed. Furthermore, our results demonstrate that plumes can cause proton depletions and that these features should be sought after in the data. Key Points EPD artifact does not affect the conclusion that perturbed fields and atmospheric charge exchange dri