Charles F. Bowers

Magnetic flux ropes are twisted magnetic field lines that are commonly observed in space and astrophysical plasmas. At Mars, these structures are especially interesting because the planet does not have a global magnetic field like Earth, but instead has remanent magnetic fields preserved in its crust. In this study, we report a close conjunction observation by MAVEN and Tianwen-1 of a magnetic flux rope within the Martian ionosphere. The two spacecraft observed closely related magnetic signatures at near-identical locations and altitudes, allowing us to examine the spatial structure of the event. Our analysis suggests that the observed flux rope may have a loop-like geometry, with magnetic-field lines connected to the Martian dayside ionosphere at both ends. This structure may have formed or evolved through ionospheric instabilities under unusually low solar-wind dynamic pressure or internal reconnection between neighboring crustal magnetic fields. This event shows that Mars may produce a loop-like magnetic flux rope that resembles, in geometry, twisted magnetic loops seen in the solar atmosphere, providing a useful connection between planetary and solar plasma processes.

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.

Mercury's proximity to the Sun results in the lowest average Alfvénic Mach number in the upstream solar wind compared to any other planet. Under extreme conditions, the upstream Alfvén speed can exceed the solar wind speed, resulting in a sub‐Alfvénic interaction in which standing Alfvén wings form. Previous studies suggest strong Alfvén wing formation requires a large obstacle conductance, yet estimates of Mercury's interior conductance remain comparatively small, raising questions about how strong Alfvénic interactions occur. Here, we investigate Mercury's Alfvén wing current system using global magnetohydrodynamic simulations with coupled planetary interior under varied sub‐Alfvénic solar wind conditions and interior conductances. Based on magnetic perturbations within the modeled Alfvén wings, we estimate that an effective magnetospheric conductance exceeds the relatively weak conductance associated with the planetary interior, increasing overall Alfvénic interaction strength. Additional current‐closure pathways provided by Mercury's intrinsic magnetosphere enhance the planet's effective conductance, strengthening Alfvén wing formation.

Mercury experiences the most intense and variable solar wind (SW) conditions in the solar system due to its close, eccentric orbit about the Sun. In addition to variation driven by coronal source and solar cycle, the SW arriving at Mercury varies periodically as the planet's heliocentric distance changes by over 50% per orbit. We use 30 years of SW measurements near Earth and scale them to Mercury's orbital distance to determine the importance of this “seasonal” change in the SW. We find the seasonal change to be substantial, systematic, and significant, and that it dominates over other sources of variability (e.g., fast vs. slow SW) and even exceeds the relative change at Earth from ICMEs compared to the ambient SW. As a result, Mercury's magnetosphere is expected to exhibit substantial seasonal effects.

We present a new list of bow shock and magnetopause crossings based on automated region classification for the MESSENGER (Mercury Surface, Space Environment, Geochemistry and Ranging) mission. We fit a random forest model to magnetometer and ephemeris data to classify the solar wind, magnetosheath, and magnetosphere regions surrounding Mercury. The random forest was highly accurate when predicting the testing data set, with an accuracy of . We apply this model to the orbital phase (March 2011 to April 2015) of the MESSENGER mission, and determine crossings automatically where changes in region classification occur. We apply several post‐processing steps to this list of crossings and determine 19,989 bow shock crossings and 11,414 magnetopause crossings for the MESSENGER mission. We find good visual comparison with the spatial distribution of these new crossings and existing data sets of boundary crossing intervals.

We report in situ evidence for Enceladus' Alfvén wing system and its coupling with Saturn's ionosphere, based on multi‐instrument observations from the Cassini spacecraft. Analysis of 36 events, including 13 from non‐flyby paths, confirms the existence of a Main Alfvén Wing (MAW) current system generated at Enceladus, and associated Reflected Alfvén Wings (RAWs) occurring both at Saturn's ionosphere and on the density gradient of Enceladus' plasma torus, extending longitudinally to at least (2,000 moon radii) downstream of the moon. Additionally, the observations reveal the systematic existence of a filamentation process of these large‐scale Alfvénic perturbations (MAW and RAWs) during their propagation at any distance from their source. These findings demonstrate a more extensive electrodynamic coupling than previously reported for Enceladus and more generally for any moon‐magnetosphere interaction. Moreover, the observation of energetic electron depletions and water‐group ion signatures at longitudes even further from the moon supports the interpretation of an extended and persistent interaction region. These results highlight Enceladus' role in shaping Saturn's magnetospheric environment and underscore the importance of future missions to exhaustively analyze this type of complex interaction between a moon and a planet.

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.