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The molecular dication CO₂⁺⁺ has, as previously reported, been detected in the Martian ionosphere by the Neutral Gas and Ion Mass Spectrometer on the Mars Atmosphere and Volatile Evolution (MAVEN) mission. Photochemical models have also been developed to reproduce the CO₂⁺⁺ density in the Martian dayside ionosphere but underestimate significantly the observations. In this study, we examine the influence of the CO₂⁺⁺ natural lifetime against spontaneous dissociation on its modeled density. We show that extending the assumed CO₂⁺⁺ lifetime significantly reduces the discrepancy between the photochemical model predictions and MAVEN observations. Specifically, when treating CO₂⁺⁺ as stable against natural dissociation, instead of invoking a lifetime of 4 s as done in previous studies, the data-to-model ratio comes close to unity throughout the altitude range 160–220 km. We argue that stability of CO₂⁺⁺ against natural dissociation does not necessarily conflict with results from a frequently cited experimental investigation. Our study provides new insights for advancing photochemical modeling of the Martian ionosphere and underscores the need for further laboratory measurements targeting fundamental properties of doubly charged ions.

Atmospheric ion escape plays a crucial role in the evolution of planetary climate and habitability. While Mars has been the focus of extensive in-situ spacecraft observations, our understanding of ion escape at Mars has been constrained by single-point spacecraft measurements, which fail to distinguish spatial and temporal variability. Observations from NASA's Mars Atmosphere and Volatile EvolutioN (MAVEN) mission and China's Tianwen-1 mission provide complementary observations the Martian space environment and a unique opportunity to study the variability of ion escape. Here, we report that ion escape at Mars exhibits unexpected spatial-temporal variability under steady and weak external solar wind conditions. In the hemisphere where the solar wind electric field is directed toward the planet, a condition that usually hinders ion escape into space, we instead observe the transient appearance of escaping planetary ions with high energies and strong escape fluxes. This finding underscores that planetary ion escape can be unsteady and dynamic, even under stable external conditions.

BepiColombo, the joint ESA/JAXA mission to Mercury, was launched in October 2018 and is scheduled to arrive at Mercury in November 2026 after an 8-year cruise. Like other planetary missions, its scientific objectives focus mostly on the nominal, orbiting phase of the mission. However, due to the long duration of the cruise phase covering distances between 1.2 and 0.3 AU, the BepiColombo mission has been able to outstandingly contribute to characterise the solar wind and transient events encountered by the spacecraft, as well as planetary environments during the flybys of Earth, Venus, and Mercury, and contribute to the characterisation of the space radiation environment in the inner Solar System and its evolution with solar activity. In this paper, we provide an overview of the cruise observations of BepiColombo, highlighting the most relevant science cases, with the aim of demonstrating the importance of planetary missions to perform cruise observations, to contribute to a broader understanding of Space Weather in the Solar System, and in turn, increase the scientific return of the mission.

Context: The interaction of the solar wind (SW) with the coupled magnetosphere-exosphere-surface of Mercury is complex. Charged particles released by the SW can precipitate along planetary magnetic field lines on specific areas of the surface of the planet. The processes responsible for the particle precipitation strongly depend on the orientation of the interplanetary magnetic field (IMF) upstream of Mercury.

Aims: During the third Mercury flyby (MFB3) by BepiColombo, the properties of the SW inferred from BepiColombo observations of a highly compressed magnetosphere corresponded to those of a very dense plasma embedded in a slow SW. The Mercury Electron Analyzer (MEA) measured continuous high-energy electron fluxes in the nightside dawn sector of the compressed magnetosphere. In order to constrain further studies related to the origin of these populations, we aim to firmly confirm the initial inferences and detail the SW properties throughout MFB3.

Methods: We took advantage of a close radial alignment between Parker Solar Probe (PSP) and Mercury. We monitored the activity of the Sun using SOHO coronagraphs and we used a potential field source surface model to estimate the location of the magnetic footpoints of PSP and BepiColombo on the photosphere of the Sun. We propagated the plasma parameters and the IMF measured by PSP at BepiColombo, to check if the plasma impacted Mercury.

Results: We show that during MFB3, PSP and BepiColombo connected magnetically to the same region at the solar surface. The slow SW perturbation first measured at PSP propagated to Mercury and BepiColombo, as was confirmed by similarly elevated plasma densities measured at PSP and BepiColombo. The IMF orientation stayed southward during the whole MFB3.

Conclusions: Our results provide strong constraints for future studies of the magnetospheric structure and dynamics during MFB3, including tail reconnection, electron and ion energization, and subsequent plasma precipitation onto the surface of Mercury.

Although Venus appears to present a predominantly ionospheric obstacle to the solar wind, the magnetic connectivity between the solar wind and the Venus ionosphere, or magnetic topology, is important for characterizing the Venus space environment. In particular, magnetic connectivity is relevant to the magnetization state of the ionosphere, particle precipitation into the atmosphere causing ionization and auroral emissions, and planetary ion escape at Venus. The spatial distributions of different magnetic topologies were statistically analyzed, with some unexpected results. Here, we build on those results by investigating how the external factors of solar cycle phase and upstream conditions affect the occurrence rates of the three magnetic topologies and consider their implications regarding the state of Venus's induced magnetosphere. We find that both the solar cycle phase and upstream dynamic pressure variations control its expansion or contraction. Under solar minimum conditions, the interplanetary magnetic field (IMF) more deeply penetrates into the collisional atmosphere, increasing the occurrence rates of open and closed topologies at low altitudes and in Venus's wake. We also find hemispheric differences in the occurrences of dayside-connected and nightside-connected open fields, likely related to mass loading of the near-Venus plasma environment by planetary pickup ions.

Venus's induced magnetosphere is characterized by regions with different plasma and magnetic field properties, which are separated by plasma boundaries. These boundaries' locations and shapes vary with upstream solar wind conditions, and these variations have been characterized by several previous studies. In this study, we developed quantitative parametric models of the bow shock and ion composition boundary (ICB), which allow us to determine the location and shape of the boundaries given a set of upstream conditions. To quantitatively model these boundaries, we used a database of boundary crossings derived from plasma and magnetic field measurements by Venus Express. We modeled the bow shock as a conic section curve, which depends on the interplanetary magnetic field (IMF) magnitude and the solar wind proton flux. Furthermore, we considered the shock normal angle, the angle between the IMF and the local shock normal vector, to describe a quasi-perpendicular/quasi-parallel shock asymmetry. We modeled the dayside ICB as a half sphere that depends solely on the solar EUV flux and the solar wind proton flux. These parametric models are compared with models that average over upstream conditions; our bow shock parametric model improves the prediction accuracy by 16% and the ICB parametric model by 6%.

The Cassini observations of Saturn's ionosphere during the proximal orbits 288-293 in the altitude range 1450-4000 km (above 1-bar level) are revisited. A thorough re-analysis is made of all 159 available Langmuir probe sweeps of the Radio & Plasma Wave Science (RPWS) measurements. We relate them to the RPWS plasma wave inferred electron number densities and compare them with the available Ion Neutral Mass Spectrometer (INMS) measurements of the H+ and H-3(+) number densities. Different analysis methods are used by RPWS to provide consistent electron number density values for the whole measured altitude interval. Consistent RPWS electron number density (n(e)) and INMS positively charged ion number density (n(i+)) profiles are derived for altitudes above similar to 2200 km. Below this altitude the inability of INMS to measure ions above 8 amu at the 34 km/s flyby speed lead us to infer the presence of heavy ions (> 8 amu) and a negatively charged ion component, presumably related to infalling material from the D-ring of Saturn with its associated local ion-molecule-aerosol chemistry. This lower altitude region shows a highly time variable layered structure. The Langmuir probe data in this region are strongly affected by secondaries emitted from the spacecraft and sensor surfaces when traversing a molecule-rich atmosphere at 34 km/s. There are clear signatures of secondary electron and ion emissions from the spacecraft and sensor surfaces in the data. In the Langmuir probe sweep analysis, we correct for the effect of such impact-generated products. This gives corrected total ion number densities that can be compared to the INMS ion number densities and the electron number densities. From this analysis the number of negative ions and/or nm-sized aerosol/dust particles can be constrained. A clear ionospheric peak is not identified, not even at the lowest observed altitude of approximately 1450 km. There are clear latitudinal variations and temporal evolving structures, which we infer are representative of the difference in infalling material from different regions of the D-ring. In addition, there are indications of a strong heating source for the ambient electrons that are well above expected thermal equilibrium levels (up to 4000 K). The cause of this heating is unknown but may be linked to collisional deacceleration of infalling ring material. The observational profiles presented here can be used for ionosphere theory/model comparisons in the future.

The Martian dayside ionosphere has been widely modeled using photochemical equilibrium calculations. These efforts have mostly focused on dominant ion species in order to make comparisons with orbital observations and on displaying non-negligible model-observation discrepancies. In this study, we investigate Ar⁺ions in the Martian dayside ionosphere, an ion species with a relatively simple chemistry, and perform both case-by-case orbital comparisons and a statistical comparison over five years of observations by the Neutral Gas and Ion Mass Spectrometer (NGIMS) on the Mars Atmosphere and Volatile Evolution (MAVEN) mission. Statistically, the ratio of modeled to observed Ar⁺densities increases from ∼1 near 130 km to ∼4 at 220 km, with notable variations as a function of the solar zenith angle. Pressure-dependent discrepancies show a weaker correlation with the solar zenith angle. Model performance improves when incorporating (i) a higher reaction rate coefficient for the charge transfer between Ar⁺and CO₂ and/or (ii) reduced solar irradiance. At altitudes above 200 km, Ar⁺loss via reactions with H₂ becomes increasingly important. However, we find that model-observation agreement varies between orbits: Some show strong consistency, particularly during Deep Dip campaigns, while others exhibit systematic deviations or significant discrepancies. We suggest that while systematic adjustments to reaction rate coefficients, ionization cross sections, solar irradiance, or background neutral densities may improve model fidelity for certain orbits, capturing the dynamic and time-varying nature of the Martian ionosphere requires further comprehensive investigations.

Measurements of concentrations of neutral and ion species in the upper atmosphere of Mars by the Neutral Gas and Ion Mass Spectrometer (NGIMS) on board the Mars Atmosphere and Volatile EvolutioN mission have served as model input and/or for comparison with model output in numerous earlier studies of the Martian dayside ionosphere. While many models reproduce the altitudinal density profiles of key ion species within a factor of a few, it has proven challenging to achieve a level of agreement within tens of percent for multiple ion species over a wide range of altitudes. We explore means to overcome this issue while keeping with a reduced chemical model and utilizing the assumptions of photochemical equilibrium and that the NGIMS data are devoid of any measurement errors. We entertain, for instance, the idea that the rate coefficient for the charge-transfer reaction between CO₂⁺ and O may vary with altitude as a result of a pressure-controlled internal energy distribution of the CO₂⁺ population.

Venus, lacking an intrinsic global dipole magnetic field, serves as a textbook example of an induced magnetosphere, formed by interplanetary magnetic fields (IMF) enveloping the planet. Yet, various aspects of its magnetospheric dynamics and planetary ion outflows are complex and not well understood. Here we analyze plasma and magnetic field data acquired during the fourth Venus flyby of the Parker Solar Probe (PSP) mission and show evidence for closed topology in the nightside and downstream portion of the Venus magnetosphere (i.e., the magnetotail). The formation of the closed topology involves magnetic reconnection-a process rarely observed at non-magnetized planets. In addition, our study provides an evidence linking the cold Venusian ion flow in the magnetotail directly to magnetic connectivity to the ionosphere, akin to observations at Mars. These findings not only help the understanding of the complex ion flow patterns at Venus but also suggest that magnetic topology is one piece of key information for resolving ion escape mechanisms and thus the atmospheric evolution across various planetary environments and exoplanets. Magnetic reconnection dynamics in Venus' magnetosphere are not well-known due to limited observations. Here, the authors show direct evidence for closed magnetic topology in Venus' magnetotail and a link between the cold ion flow in the magnetotail and its direct magnetic connectivity to the ionosphere.