Cyril Simon Wedlund

In the cometary plasma environment, the solar wind interacts with the expanding coma of heavy cometary ions. At high gas production rates, it can have plasma environment characteristics similar to the environment of Mars, with a bow shock and a cometosheath. Magnetosheath jets are transient enhancements in the dynamic pressure of the magnetosheath plasma. They can be generated at a disturbed bow shock surface and have been confirmed at Mars, Earth, and possibly Jupiter and Mercury. We have adapted commonly used methods to find magnetosheath jets to the Rosetta data at 67P/Churyumov–Gerasimenko and find candidate events throughout the postperihelion mission. We show that the identified density enhancements have similarities to magnetosheath jets. The events are of typical duration and density enhancement, and occur at a time when the bow shock is thought to be close to the spacecraft. We found a cometary ion velocity enhancement, although no conclusion on the solar wind velocity can be reached due to lack of data. These jet-like features are embedded in the heavy-ion population. We hypothesize that jets are formed in the solar wind ion population in the cometosheath, and then, as the two plasmas interact, are transferred to the cometary ion population. While we cannot rule out other phenomena that could cause these jet-like enhancements, they bear a remarkable resemblance to magnetosheath jets at Mars and Earth. If they are indeed jets, then this is the first time jets have been shown to exist in a plasma environment dominated by heavy ions.

The interaction between the supermagnetosonic solar wind and the atmosphere of the weakly magnetized planet Mars forms an induced magnetosphere preceded by a magnetic pileup boundary (MPB) and bow shock (BS). In situ measurements have shown that these boundaries are permanent and well-defined structures that exhibit significant variability driven by external (Sun, solar wind) and internal (planetary) factors. In this work, we used MAVEN observations from 2014 to 2019 to analyze the coupled behavior of the BS and MPB, focusing on their spatial correlation and variability under varying conditions. We examine contiguous BS-MPB spacecraft crossings and the magnetosheath region between them, and we explore the effects of various plasma parameters on the relative position of the boundaries and the thickness of the magnetosheath. We demonstrate a statistical correlation between the BS and MPB positions across multiple timescales, including sustained local solar wind and planetary conditions, seasonal changes, and solar cycle variations. These results support the interpretation that the MPB plays a key role as an electromagnetic obstacle to the solar wind, effectively controlling the formation of the bow shock. We also confirm the limited size of the Martian magnetosheath, both in absolute terms and relative to fundamental plasma scales, which has important implications for understanding the microscopic plasma processes responsible for solar wind thermalization. While acknowledging the limitations of single-spacecraft measurements, this study provides a foundation for future multipoint probing of the Martian environment and the magnetosphere as a system.

Helium-4 (He) in the Venusian atmosphere has previously been identified through remote spectroscopic observations as well as in-situ plasma and mass spectrometer measurements from earlier spacecraft missions (e.g., Mariner 10, Venera 11 and 12, and PVO).In this study, we analyze magnetic field and plasma observations from the Venus Express (VEX) mission to investigate ion cyclotron waves (ICWs) generated by the pickup of exospheric He-ions by the solar wind. Through the detection and analysis of these wave signatures, we identify for the first time a nonthermal exospheric He population extending from approximately 1 Venus radius up to 11 Venus radii. Average kinetic temperatures of around 2700 K and an inferred exobase density of about 200 cm-3, characterize the derived population.Our results indicate that there are likely two energization mechanisms for this nonthermal helium component. The first mechanism is related to collisions between thermal He atoms and suprathermal oxygen atoms near or slightly above the exobase. This process produces a hotter nonthermal helium population, consistent with mechanisms previously proposed for both Venus and Mars. The second possible mechanism is the dissociation of HeH+ in the exobase region.Using the helium escape flux derived in this study, approximately 4 × 104 cm-2 s-1, we further investigate the long-term evolution of helium in the Venusian atmosphere. The comparatively low escape rate suggests that the present atmospheric He abundance could have accumulated since the last major resurfacing event, or potentially over even longer geological timescales

In the terrestrial foreshock, sunward propagating ion beam of several keV collimated along the interplanetary field lines (Field-Aligned Beams) are usually observed within a region upstream from the quasi-perpendicular shock. Numerous observations indicate that these beams are not seen along IMF lines that make a angle larger than 70o with the local shock normal, thereby marking the edge of the ion foreshock. The ion foreshock boundary reflects the maximum level of energization that solar wind ions can reach via coherent interaction with a planetary shock. In the present study, the Martian ion foreshock boundary is investigated for the first time using MAVEN particle and magnetic field data. We found that no FAB is observed for a shock-θBn larger than 50o-55o. Our results indicate that the Martian ion foreshock boundary is located downstream of its terrestrial counterpart. This finding is in good agreement with a recent report showing that FABs observed in the Martian foreshock have noticeably lower speeds than those observed at Earth.

Electromagnetic waves at the local proton cyclotron frequency are frequently observed upstream from the Martian bow shock. They are excited by unstable velocity distributions of newborn protons continuously produced locally by ionization of exospheric hydrogen atoms (pickup protons). The analysis of MAVEN magnetic field data demonstrates for the first time that the amplitude of these waves undergo a sharp change when crossing the electron foreshock boundary. Moreover, a decrease of the amplitude with the increasing distance from the shock along the ambient average magnetic field is observed inside the foreshock. Both signatures are correlated with the variations of the energetic electron fluxes. These two properties connecting the wave growth to electron physics raise an issue since the waves are excited purely through an ion-ion instability. We propose that the extra free energy necessary to increase the wave amplitude is due to additional ionization of hydrogen atoms by electron impact ionization inside the foreshock. These results imply that extreme caution is needed when directly deriving the exospheric density at Mars and other similar environments from the local pickup ion wave amplitude, especially in the foreshock region.

Planetary magnetosheaths are plasma regions between the solar wind and planetary magnetospheres where temperature anisotropies act as a source of free energy driving plasma instabilities. While these instabilities have been extensively studied at Earth, their properties at Mercury remain poorly investigated. To support the science goals of the ongoing BepiColombo mission, we investigate the development of ion temperature anisotropy throughout Mercury's magnetosheath using the first global hybrid particle‐in‐cell simulations of the Hermean plasma environment under turbulent solar wind conditions. We employ the 3D hybrid PIC code Menura to model Mercury's magnetosphere for both laminar and turbulent upstream states and classify magnetosheath regions by local bow‐shock geometry (quasi‐parallel, quasi‐perpendicular, and intermediate). The simulations reveal strong spatial variations in plasma stability, with quasi‐perpendicular sectors showing enhanced occurrence of mirror‐mode and ion‐cyclotron unstable plasma. Turbulent solar wind produces a dayside magnetosheath that is, on average, more unstable than in laminar case, while the nightside becomes comparatively more stable. These results demonstrate that upstream solar wind turbulence plays a role in regulating anisotropy‐driven instabilities in Mercury's magnetosheath and should be accounted for in the interpretation of in situ BepiColombo observations.

Context . Io, the innermost Galilean moon of Jupiter, is the main source of plasma in the Jovian magnetosphere. The neutral gas coming from the moon will get ionized through ultra-violet radiation and electron impacts. The newly created ions then get picked up by the Jovian magnetic field and start gyrating, thereby creating a ring-beam distribution in velocity space. This type of distribution is unstable with respect to the generation of ion cyclotron waves. Aims . The aim of this study is to characterize the escaping gas from Io’s atmosphere into the Jovian magnetosphere. Methods . The Galileo magnetometer data have been investigated for the five Io flybys that have magnetometer data available. The ion cyclotron waves can be measured with magnetometers and through spectral analysis the specific pick-up ions can be determined. Assuming that the energy of the ions in the ring-beam distribution is fully transferred to the cyclotron waves, the pick-up ion densities can be estimated for all these species. Results . We found evidence of sulfur-bearing ions SO ₃ ⁺ , SO ₂ ⁺ , SO ⁺ , and S ⁺ , as well as either H ₂ S ⁺ or ³⁴ S ⁺ (which have the same mass-to-charge ratio and cannot be discerned), and for non-sulfur-bearing ions: ³⁵ Cl ⁺ , ³⁷ Cl ⁺ , K ⁺ , and Si ⁺ . We also present a first plausible detection of Io-genic phosphorous through the detection of P ⁺ cyclotron waves. Conclusions . The main pick-up densities are related to SO ₂ ⁺ and SO ⁺ , varying with distance from Io between ∼10 ⁸ and ∼10 ⁶ m ⁻³ , with the other ions exhibiting a similar variation, but their pick-up densities are lower by an order of magnitude.

Magnetic holes (MHs) are localized depressions in the magnetic field commonly observed in space plasmas such as the solar wind, planetary magnetosheaths, and cometary environments. Despite the abundance of spacecraft observations, the mechanisms governing the generation of these structures are not fully understood. In this study, we investigate the stability of magnetic depressions in a controlled plasma environment via two-dimensional hybrid particle-in-cell simulations using the Menura code. Initializing the system with preexisting magnetic field depressions embedded in a mirror-stable plasma allows us to isolate the fundamental physical mechanisms responsible for stabilization and equilibrium. We analyze the roles of the initial depth, characteristic width, and magnetic field geometry of the depression. Our results demonstrate that narrow depressions (of the order of ion kinetic scales) are unstable, whereas broader structures can reach stable equilibria through ion trapping, resulting in localized density enhancements and temperature anisotropy consistent with in situ observations of large-scale MHs. The findings highlight the importance of ion trapping for stabilization and provide a controlled framework to investigate MH dynamics.

Context. Rosetta followed comet 67P at heliocentric distances from 1.25 to 3.6 au. Close to perihelion, Rosetta was located in the solar wind ion cavity, with only sporadic observations of solar wind ions. Just outside the solar wind ion cavity, the solar wind ion-flow direction was mainly sunward. Aims. We aim to study the evolution of solar wind interaction with a comet as its scale size increases from sub-ion gyroradius to above the proton gyroradius. The former was observed at 67P during most of the Rosetta mission. The latter was observed during sporadic solar wind encounters around perihelion. Methods. We analysed particle data from the mass-resolving ion spectrometer ICA, searching for the weak and sporadic occurrences of solar wind ions close to perihelion. All Rosetta plasma data were used to understand the plasma parameter regime of the observed interaction. Results. At comet 67P, we observe the transition from solar wind interaction during the first partial gyration of the solar wind ions in the slowed-down plasma around the comet to multiple gyrations in an interaction region larger than the proton gyroradius. During the multiple-gyration stage, the protons show a consistent pattern of motion ordered by the direction of the solar wind electric field. In the hemisphere where the electric field points away from the nucleus, the protons move sunward. They move anti-sunward in the opposite hemisphere. This can be explained by simple trochoid trajectories and a strong density gradient. Under the same conditions, the water ions also show a consistent flow pattern relative to the electric field determined by the point in their gyration at which they pass closest to the nucleus. Protons are slowed down and heated, which is consistent with a shock upstream. The plasma beta is close to one, and the magnetosonic Mach number was typically less than unity.

Energy transfer, cross‐scale coupling, and dissipation in astrophysical plasmas remain fundamental unresolved problems. The velocity‐shear–driven Kelvin–Helmholtz instability (KHI), ubiquitous in plasmas, is a key multiscale mechanism enabling plasma mixing, particle energization and the solar wind–magnetosphere coupling, making it a critical process for understanding how plasma flow energy is converted from large to kinetic scales. Although in situ observations and simulations have linked KHI to turbulence onset and plasma heating, the precise pathways of energy transfer remain unexplored. By leveraging high‐resolution, multi‐point Magnetospheric Multiscale (MMS) observations, we quantify, for the first time, KHI energy conversion channels via pressure–strain and diagnostics. Inside vortices, thermal‐to‐flow energy transfer dominates, coinciding with local non‐thermal features. Notably, energy pathways seem influenced by vortex evolution: rolled‐up vortices tend to convert flow energy into thermal energy, whereas early‐stage vortices mostly convert thermal energy back into flow. This study provides the first energy‐balance‐based description of KHI dynamics and further reveals the complexity of velocity shear energy dissipation into heat.