Sebastian Cervantes

We investigate losses of energetic electrons, protons, sulfur ions and oxygen ions, measured by Galileo's Energetic Particle Detector (EPD) during the E12 Europa flyby. We use a particle tracing code and previously published magnetohydrodynamic simulations of the interaction of plasma with Europa's atmosphere and plume. We find that localized dropouts in energetic protons (~100 keV) and electrons (~42 keV), whose gyroradii are smaller than Galileo’ s altitude, are not reproducible under nominal (plume-less) conditions. These losses are located near the central axis of the plume proposed by Jia et al., 2018. Particle tracing simulations show that these localized dropouts may be created by perturbed electromagnetic fields associated with a plume (protons and electrons) and charge exchange with the plume neutrals (protons). Interpretation of higher energy protons (>250 keV) and oxygen (>500 keV) dropouts is more complicated, as dropouts can be caused by surface impact, and, therefore, are harder to isolate from other loss processes. However, our simulations show that dropouts of large-gyroradius ions near the central axis of the plume should also be modified by the plume-perturbed fields. A single model setup fully reproducing all species and energies is lacking. Accounting for the magnitude of smaller scale magnetic field perturbations, presumably related to the plume, improves the fit to the measurements. We conclude that a plume may have played a role in producing the measured dropouts of energetic particles, but that magnetic field models which reproduce perturbations at smaller scales are required.

Auroral emissions on Ganymede provide critical insights into its magnetospheric dynamics and atmospheric composition. Using a physics‐based atmospheric model with recent atmospheric observations and laboratory measurements of electron‐impact cross‐sections, this study revisits and significantly refines the relationship between auroral emission intensities and impacting electron energies. The improvements in predicted auroral brightness include: the peak intensity at 135.6 nm is significantly higher than previous estimates, and the threshold electron energies necessary for intense auroral brightness (>300 Rayleighs) decrease to ∼50–60 eV, resolving previous theoretical challenges. Our refined model also demonstrates the spatial dependence of the emission‐energy relation. In particular, we highlight how incorporating these updated inputs reshapes the energy dependence relationship, enhancing consistency with recent high‐resolution UV observations. These refined constraints not only advance our understanding of Ganymede's aurora and its interaction with Jupiter's magnetosphere, but also improve the interpretive framework for future in situ missions, by spacecraft such as JUICE.

The Energetic Particle Detector (EPD) onboard the Galileo spacecraft observed pronounced depletions of energetic protons during the I27 flyby of Io, providing a diagnostic of neutral-particle interactions in the moon’s environment. We analyze proton fluxes within 10 RIo across the 115–1155 keV energy range and two pitch angle cases (near field-aligned and near perpendicular). In Io’s intense radiation environment, energetic particles can penetrate the detector and cause false proton detections. We develop a method to quantify the proton fraction of measured counts to correct for contamination and select high-quality intervals. The reconstructed flux profiles show that ∼100 keV protons experience stronger depletions than ∼1 MeV ones, consistent with an energy-dependent loss process such as charge exchange with Io’s neutral environment. Two distinct loss regimes appear at ∼100 keV: gradual reductions in the extended corona, and sharper losses in the dense atmosphere. Energy- and pitch angle-dependent Monte Carlo (MC) simulations of charge exchange probabilities reproduce the largest depletions for near perpendicular protons and indicate that the corona affects protons out to at least ∼1.7 RIo. Comparisons between simulated and corrected fluxes reveal where the neutral model requires refinement or additional processes might contribute. We provide upper limits on the column densities of Io’s neutral environment along the magnetic field lines intersected by Galileo’s trajectory, offering a constraint for future modeling, as actual column densities might exceed those assumed. Combining proton flux reconstructions with charge exchange simulations provides a tool for interpreting energetic particle measurements near Io and constraining its neutral environment.

Electrons within Earth's radiation belts exhibit large variability in space and time during geomagnetic storms, which could potentially damage satellites and harm astronauts in space. Physics‐based models describe the evolution of energetic electrons in the radiation belts, but they may suffer from uncertainties and errors, particularly in the initial and boundary conditions. Data assimilation can overcome these limitations by combining models with satellite observations, incorporating all available information to create a more reliable reconstruction. This study validates the data‐assimilative three‐dimensional Versatile Electron Radiation Belt code (VERB‐3D) model using data from three independent satellite missions: Arase and GOES for assimilation, and Van Allen Probes for validation. The data sets were cleaned and normalized to ensure their compatibility. The validation shows that the model effectively reproduces the radiation belt dynamics, where the reanalysis remains within a factor of 2 for 88.3% of the time, 98.4% within a factor of 5, and 99.4% within one order of magnitude for 1 MeV electrons. The results highlight the potential of data assimilation for space weather forecasting and as an input for specification models.

We present observations of a rare configuration of Mercury's magnetosphere in response to sub‐Alfvénic upstream conditions, driven by an interplanetary coronal mass ejection (ICME) that impacted the planet on 1 May 2013. Using data from the Mercury Surface, Space Environment, Geochemistry, and Ranging (MESSENGER) spacecraft, supported by a global three‐dimensional magnetohydrodynamic (MHD) simulation of the event, we demonstrate that Mercury's magnetospheric response during this interval was distinct from the typical super‐Alfvénic state. During the sub‐Alfvénic upstream conditions, MESSENGER measured a distorted magnetotail with a depleted southern magnetotail lobe. An MHD simulation closely reproduces these observations, providing a plausible global context for the reconfiguration of Mercury's magnetosphere under sub‐Alfvénic conditions. The simulation predicts that a pair of Alfvén wings formed during this event, redirecting magnetic flux and plasma within the magnetosphere. The interplanetary magnetic field orientation during this event was primarily sunward/dawnward, generating asymmetric Alfvén wings with respect to the flow direction, in contrast to previously observed north–south wing configurations at the planet. Using Solar Orbiter observations in the inner heliosphere, we estimate that the solar wind is sub‐Alfvénic approximately 2.5 times per Earth year near solar maximum, with intervals lasting between 10 s and 12 hr. Studies of these rare, sub‐Alfvénic solar wind‐magnetospheric interactions provide valuable insights into exoplanet–stellar wind interactions under similarly sub‐Alfvénic conditions where in situ observations are not available.

During Juno's only flyby of Europa, the Jupiter Energetic Particle Detector Instrument (JEDI) measured complex dropouts in the energetic ion flux in Europa's wake. We investigate the causes of these dropouts, focusing specifically on energetic protons of keV and MeV, using back‐tracking particle simulations, a prescribed description of Europa's atmosphere and a three‐dimensional single fluid magnetohydrodynamics (MHD) model of the plasma‐atmosphere interaction. We investigate the role of magnetic field perturbations resulting from the interaction between Jupiter's magnetospheric plasma and Europa's atmosphere and the presence of field‐aligned electron beams in Europa's wake. We compare the simulated effect of the perturbed fields on the pitch angle distributions of the ion losses to Juno‐JEDI measurements. We find that at keV, field perturbations are the dominant factor controlling the distribution of the losses along the flyby, while at MeV a combination of field perturbations and absorption by the surface due to short half bounce periods is required to explain the measured losses. We also find that the effect of charge‐exchange with Europa's tenuous atmosphere is weak and absorption by dust in Europa's environment is negligible. Furthermore, we find that the perturbed magnetic fields which best represent the measurements are those that account for the plasma interaction with a sub‐/anti‐Jovian asymmetric atmosphere, non‐uniform ionization of the atmosphere, and electron beams. This sensitivity to the specific field perturbation demonstrates that combining observations and modeling of proton depletions constitute an important tool to probe the electromagnetic field and atmospheric configurations of Europa.

The Galilean satellite Europa is a primary candidate for the study of ocean worlds. In anticipation of NASA's Europa Clipper and ESA's JUICE missions, we revisit the magnetic field measurements of the Galileo spacecraft. The ocean's induction response is governed by its induction amplitude and phase shift, which are a function of the ocean properties, i.e., its depth, thickness, and electrical conductivity. While prior work focused on deriving values for the former two by a qualitative comparison with measurements, this work emphasizes the characterization of Europa's ocean properties directly, including their uncertainties. For that, we perform an inversion of magnetic field measurements. Our model includes magnetic field contributions due to Europa's plasma interaction and the inhomogeneous field of Jupiter's magnetospheric plasma. We estimate the uncertainties of our plasma interaction model, our background field model, and the inducing field. Employing a chi‐squared analysis, we utilize these uncertainties to quantify the ocean properties and their uncertainties. We apply this method to the E14 flyby, which is currently the best flyby for such an inversion. Using existing depth constraints from the literature, we find a minimum conductivity of 1.6 S/m at 20 km depth and 2.8 S/m at 35 km depth. At the derived maximum depth, 49 km, the required conductivity exceeds current estimates. Our work is a first step for the development of methods which quantitatively constrain the various magnetic field contributions around Europa in future measurements, including their uncertainties, and highlights the challenges of a quantitative characterization using magnetic sounding.

We investigate the transient nature of water plume deposits on Europa’s surface, focusing on how radiolysis and ion sputtering (Europa’s two most effective erosion processes) affect their lifetime. Our study is motivated by the lack of observed surface changes in imaging data, despite tentative plume detections, and aims to inform future missions, such as JUICE and Europa Clipper, about expected timescales of deposit survival to optimise their observational strategies. We present two models that simulate a first order estimate of the erosion time of deposits formed by water plumes considering a wide range of mass flux and eruption duration. Model 1 considers a confined (10 km) deposit near the source, while Model 2 evaluates the erosion material deposited 25 km from the plume source to match observation limitations in other studies. Our results show that plume deposits are short lived relative to Europa’s average surface age for a wide range of mass fluxes and eruption times. Even intense plumes with $10{,}000$~kg/s mass fluxes produce deposits that erode within $\sim 10$~Ma. For plumes such as the one observed by HST in 2012, deposits may be removed within years to decades. Erosion rates vary significantly with location, with polar regions offering the best conditions for long-term preservation. These findings support the hypothesis that the absence of visible deposits in current data may be due to rapid erosion rather than a lack of activity. They highlight the importance of time sensitive and geographically targeted observations for maximising the scientific output of upcoming missions.

In September 2022, the Juno mission performed its only close flyby of Europa and traversed the moon's wake at a minimum distance of 350 km. Among other findings, the Jovian Auroral Distributions Experiment (JADE) detector onboard the spacecraft discovered intense field‐aligned electron beams (30–300 eV) downstream of the moon. In this study, we apply a three‐dimensional magnetohydrodynamic model to simulate the plasma interaction of Jupiter's magnetosphere with Europa and its atmosphere for the conditions of this flyby, and we specifically focus on the influence of the electron beams on the plasma density and the magnetic field in the moon's space environment. We include these beams in our simulations as sheets of locally enhanced ionization, and we use electron impact ionization rates of derived from JADE electron measurements to characterize the sheets. We compare our results with the magnetic field and the total ion number density measurements from Juno's magnetometer and JADE detector, respectively. Our results show that the beams fill the wake downstream of Europa with newly ionized plasma, and that they generate large variations in the magnetic field which contribute partially to the observed magnetic field. Our study demonstrates that the electron beams are critical factors in shaping Europa's magnetic field and plasma environment, and thus they need to be accounted for in the data analysis of the upcoming JUICE and Europa Clipper missions.

Electrons in Earth’s radiation belts exhibit significant variability in both space and time during geomagnetic storms, posing potential risks to satellites and astronauts. Physics-based models aim to describe the behavior of energetic electrons in the radiation belts but often face challenges due to uncertainties and inaccuracies, especially in the initial and boundary conditions. Data assimilation addresses these limitations by integrating satellite observations with model predictions, incorporating all available information to produce a more reliable reconstruction. This study evaluates the performance of the data-assimilative 3D Versatile Electron Radiation Belt code (VERB-3D) using data from three independent satellite missions: Arase and GOES for assimilation and Van Allen Probes for validation. The datasets were carefully cleaned and normalized to ensure compatibility. The results confirm that the model accurately reproduces radiation belt dynamics, highlighting the effectiveness of data assimilation techniques for space weather research and improving our understanding of the radiation belt environment.