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Mercury lacks an (significant) ionosphere, leading to the hypothesis that its large-scale field-aligned currents (FACs) are instead closed through its highly conductive core. Mercury's interior is usually characterized by a two-layer model: a resistive outer layer (the crust and mantle) and a conductive inner layer (the iron-rich core). While this model is widely used, the effects of Mercury's conductivity and core on FACs have not been extensively explored. Therefore, we conducted analytical calculations combined with hybrid-kinetic simulations to study these effects. We found that the total currents of FACs are enhanced by ∼2 times when a conducting core is included, and the current density is directly proportional to the outer layer's conductivity. Combining our analysis with previous observations, the conductivity of the outer layer is estimated to be ∼5.4×10−7−1.1×10−6S/m. Our study suggests that future observations of Mercury's FACs will better constrain Mercury's conductivity profile.

The interaction between lunar magnetic anomalies and the solar wind plasma creates unique structures known as “lunar mini-magnetospheres,” which reflect and partially shield the lunar surface from impinging solar wind protons. Using data from the Sub-KeV Atom Reflecting Analyzer onboard Chandrayaan-1, we produce new surface maps of energetic neutral atom (ENA) and reflected proton emissions. We show that solar wind proton precipitation can be reduced by up to 80% inside magnetic anomalies and increased by up to 50% on scales larger than 1,000 km around magnetic anomalies. The morphology of these proton precipitation enhancement and depletion regions varies differently as a function of upstream solar wind dynamic pressure for small, isolated anomalies compared to the large South Pole-Aitken (SPA) magnetic cluster. In contrast to small magnetic anomalies, which are compressed and less effective at shielding the surface from the solar wind at high dynamic pressures, the SPA magnetic cluster creates a large “mini-magnetosphere” that alters proton precipitation patterns on global-scales (>1,000 km) inside and around the cluster. We show that this behavior may result from the interaction between protons reflected by the SPA magnetic anomaly cluster and the solar wind.

Because Ganymede lacks a substantial atmosphere, its surface is exposed to the harsh space environment in the Jovian magnetosphere. Generally speaking, the space environment interacts with and alters the surface of any airless icy moon via four processes:

1. Sputtering and radiolysis by particle irradiation

2. Thermal sublimation

3. Micrometeoroid impacts

4. Photo-stimulated desorption

These interaction processes are important both for the surface itself and as sources for the neutral atmosphere and the ionosphere of Ganymede. The latter two will be covered in detailin chapters 16 (atmosphere) and 17 (ionosphere). The focus of this chapter lies on the surface interaction processes themselves.

Context: The solar wind affects the plasma environment around all Solar System bodies. A strong solar wind dynamic pressure pushes plasma boundaries closer to these objects. For small objects, kinetic effects on scales smaller than an ion gyroradius play an important role and species with various mass-per-charge may act differently. In this case, the solar wind composition can be important.

Aims: Protons are the dominant ion species in the solar wind, however, the density of alpha particles can sometimes increase significantly. We analyse the effect of different solar wind alpha-to-proton ratios on the plasma boundaries of a weakly outgassing comet (Q ≈ 1 − 2 × 1026 s−1). In addition, we investigate the energy transfer between the solar wind ions, the cometary ions, and the electromagnetic fields.

Methods: Using the hybrid model Amitis, we simulated two different alpha-to-proton ratios and analysed the resulting plasma structures. We calculated the power density (E ∙ J) of all three ion species (solar wind protons, alphas, and cometary ions) to identify load and generator regions. The integrated 1D power density shows the evolution of the power density from the upstream solar wind to downstream of the nucleus.

Results: A higher alpha-to-proton ratio leads to a larger comet magnetosphere, but weaker magnetic field pile-up. The protons transfer energy to the fields and the cometary ions in the entire upstream region and the pile-up layer. Upstream of the nucleus, alphas are inefficient in transferring energy and can act as a load, especially for low alpha-to-proton ratios. The transfer of energy from alphas to cometary ions happens further downstream due to their larger inertia.

Conclusions: For a multi-species solar wind, the mass loading and energy transfer upstream of the pile-up layer will be most efficient for the species with the lowest inertia. Protons represent such a species, and different gyroradii of the ions result in distinct flow patterns for each individual species.

Previous studies suggested that Mercury's magnetosphere could possess Earth-like field-aligned currents (FACs) despite the absence of an ionosphere. However, due to the limited coverage of spacecraft observations, our understanding of Mercury's FACs is scarce. Here, we employed Amitis, a hybrid-kinetic plasma model, to investigate the establishment and global pattern of Mercury's FACs. The responses of Mercury's FACs to various interior conductivity profiles and different orientations of the upstream interplanetary magnetic field (IMF) were simulated. It has been shown that the profile of a less resistive upper layer and a conducting core favors the establishment of FACs. Three types of large-scale FACs (Region 1-like, Region 2-like and NBZ-like FACs) are shown in simulations. Comparison with previous observations suggests that Mercury's effective conductance for closing R1-like FACs is ∼2.4–3.4 S. The influence of IMF orientation on FACs is similar to that observed in Earth's magnetosphere, but the response of the R2-like FACs to the IMF orientation is different.

Magnetosheath jets, plasma structures with enhanced dynamic pressure, are frequently observed in the terrestrial magnetosheath. However, their mass, momentum, and energy content are still unknown. We utilize Amitis, a 3D hybrid-kinetic plasma simulation, to study the mass, momentum, and energy content of jets in the subsolar magnetosheath. We also analyze the kinetic, thermal, and electromagnetic energy flux associated with jets. Jets comprise up to 21% of the quasi-parallel magnetosheath and can carry up to half of the kinetic energy. Furthermore, jets convert kinetic energy to thermal energy. Our hybrid simulations also suggest that while jets can form downstream of the quasi-perpendicular shock, their volume and energy content are much small compared to jets downstream of the quasi-parallel bow shock. We conclude that magnetosheath jets play a vital role in heating up the magnetosheath and significantly influence the dynamics of the quasi-parallel magnetosheath.

The bow shock current (BSC) plays an important role in supplying the magnetosphere with solar wind energy, in particular during times of low solar wind magnetosonic Mach numbers. Since the magnetic pile-up in the magnetosheath has to be maintained, the BSC cannot close locally, but must instead connect to magnetospheric current systems. However, the details of this closure remain poorly understood. For east–west interplanetary magnetic field (IMF) it has been hypothesized that the BSC partly closes to the high-latitude ionosphere, as field-aligned currents (FACs) on open field lines, but there is still no statistical evidence of this. In order to investigate this hypothesis, we use 9 years of Defense Meteorological Satellite Program (DMSP) data to construct normalized FAC maps of the northern hemisphere polar cap. We sort them according to different IMF clock angles, IMF magnitudes and magnetosonic Mach numbers. By separating opposite polarity FACs, we show that, on average, a unipolar FAC exists in the dayside polar cap when the IMF (Formula presented.), regardless of the sign of the IMF (Formula presented.). This current flows out of (into) the ionosphere in the northern hemisphere for IMF (Formula presented.) (Formula presented.) and is thus of the correct polarity to connect to the north–south component of the BSC. Moreover, it is strongest when the BSC flows predominantly in the north–south direction. These results constitute the first statistical evidence in support of at least a partial closure of the BSC to the ionosphere during non-zero IMF (Formula presented.).

Ion impacts on airless bodies such as Mercury alter their surfaces and contribute to their exospheres via sputtering. Their exact contribution in comparison to other effects is still uncertain, but observations by the MESSENGER spacecraft largely indicated influences from micrometeoroids. In this paper, we present an updated modeling of sputtering at Mercury to help estimate the role of sputtering at average solar wind conditions. To achieve this, we account for ion precipitation due to the planet's magnetosphere and for the presence of a porous regolith: We combine H+ and He++ fluxes to the surface from the Amitis hybrid model with sputter yields derived from a regolith simulation in SDTrimSP-3D. We find that H+ and He++ show similar precipitation patterns, but H+ energies are much more reduced and variable than those of He++. Globally, H+ and He++ contribute about equal amounts of sputtering. Our laboratory-calibrated sputter yields are significantly lower than estimates used in previous studies, resulting in a global sputtering source of around 1023 atoms s−1. Specifically for Ca and Mg exospheres we find source rates from sputtering that are largely unaffected by Mercury's seasonal orientation and too small by up to around two orders of magnitudes to explain MESSENGER observations. This supports a micrometeoroid-impact-dominated source of refractory elements. We find, however, that this is an effect of the reduced magnetospheric precipitation at Mercury. At other bodies such as the Moon, a different regime should be prevalent and sputtering should contribute at least similarly to the exospheres of refractory elements.

We present new model results for H2O, O2, H2, O, and H in the atmosphere of Ganymede. The results are obtained from a collision-less 3D Monte-Carlo model that includes sublimation, ion and electron sputtering, and ion and electron radiolysis. Because Ganymede has its own magnetic field, its immediate plasma environment is particularly complex. The interaction between Ganymede's and Jupiter's magnetospheres makes it highly variable in both space and time. The recent Juno Ganymede flyby provided us with new data on the electron local environment. Based on the electron measurements recorded by the Jovian Auroral Distributions Experiment (JADE), we implement two electron populations, one for the moon's polar regions and one for the moon's auroral regions. Comparing the atmospheric contribution of these newly defined electron populations to the overall source and loss processes is one of the main goals of this work. Our analysis shows that for H2O, sublimation remains the most important source process even after accounting for the new electron populations, delivering more than three orders of magnitude more H2O molecules to the atmosphere than all other source processes combined. The source fluxes for O2 and H2, on the other hand, are dominated by radiolysis induced by the auroral electrons, assuming that the electron fluxes JADE measured during Juno's transit of Ganymede's magnetopause current layer are representative of auroral electrons. Atomic O and H are mainly added to the atmosphere through the dissociation of O2 and H2, which is primarily induced by auroral electrons. Our understanding of Ganymede's atmosphere today is mainly based on spectroscopic observations. The interpretation of spectroscopic data strongly depends on assumptions taken, though. Our analysis shows that for a holistic understanding of Ganymede's atmosphere, simultaneous observations of the moon's surface, atmosphere, and full plasma environment (thermal and energetic ions and electrons) at different times and locations (both with respect to Ganymede and with respect to Jupiter) are particularly important. Such measurements are planned by ESA's Jupiter ICy moons Explorer (JUICE), in particular by the Particle Environment Package (PEP), which will greatly advance our understanding of Ganymede and its atmosphere and plasma environment.