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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.

The interaction of Titan's ionosphere with Saturn's magnetosphere leads to a mix of perturbed electromagnetic fields and accelerated and thermalized plasma in the induced magnetosphere. The complexity of this region has been noted in previous studies. However, many local structures and processes have not been studied and addressed in detail before. In this case study, we examine the origin of quasi-periodic plasma structures in Titan's induced magnetosphere observed during the T36 flyby. We use data from the electron and ion spectrometers CAPS/ELS and IMS, the RPWS Langmuir probe and electric antenna, and the fluxgate magnetometer (MAG) to analyze plasma parameters, for example, density and temperature and magnetic field fluctuations, to characterize the processes involved. The observed plasma structures are quasi-periodic on a scale of about 20 s (or local ion gyroperiod) and possess acceleration signatures from a few eV up to 700 eV. A burst of low-frequency (around the ion-cyclotron and lower-hybrid frequency) and low-amplitude (B_bg ≈ 7 nT, δB/B_bg ≈ 0.14) waves are observed in the proximity of the plasma structures. We discuss possible mechanisms leading to the development of the observed plasma structures, for example, magnetohydrodynamics instabilities and the contribution of the local electric fields.

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.

The Radio & Plasma Wave Investigation (RPWI) onboard the ESA JUpiter ICy moons Explorer (JUICE) is described in detail. The RPWI provides an elaborate set of state-of-the-art electromagnetic fields and cold plasma instrumentation, including active sounding with the mutual impedance and Langmuir probe sweep techniques, where several different types of sensors will sample the thermal plasma properties, including electron and ion densities, electron temperature, plasma drift speed, the near DC electric fields, and electric and magnetic signals from various types of phenomena, e.g., radio and plasma waves, electrostatic acceleration structures, induction fields etc. A full wave vector, waveform, polarization, and Poynting flux determination will be achieved. RPWI will enable characterization of the Jovian radio emissions (including goniopolarimetry) up to 45 MHz, has the capability to carry out passive radio sounding of the ionospheric densities of icy moons and employ passive sub-surface radar measurements of the icy crust of these moons. RPWI can also detect micrometeorite impacts, estimate dust charging, monitor the spacecraft potential as well as the integrated EUV flux. The sensors consist of four 10 cm diameter Langmuir probes each mounted on the tip of 3 m long booms, a triaxial search coil magnetometer and a triaxial radio antenna system both mounted on the 10.6 m long MAG boom, each with radiation resistant pre-amplifiers near the sensors. There are three receiver boards, two Digital Processing Units (DPU) and two Low Voltage Power Supply (LVPS) boards in a box within a radiation vault at the centre of the JUICE spacecraft. Together, the integrated RPWI system can carry out an ambitious planetary science investigation in and around the Galilean icy moons and the Jovian space environment. Some of the most important science objectives and instrument capabilities are described here. RPWI focuses, apart from cold plasma studies, on the understanding of how, through electrodynamic and electromagnetic coupling, the momentum and energy transfer occur with the icy Galilean moons, their surfaces and salty conductive sub-surface oceans. The RPWI instrument is planned to be operational during most of the JUICE mission, during the cruise phase, in the Jovian magnetosphere, during the icy moon flybys, and in particular Ganymede orbit, and may deliver data from the near surface during the final crash orbit.

In this paper, we study Titan's magnetotail using Cassini data from the T122-T126 flybys. These consecutive flybys had a similar flyby geometry and occurred at similar Saturn magnetospheric conditions, enabling an analysis of the magnetotail's structure. Using measurements from Cassini's magnetometer (MAG) and Radio and Plasma Wave System/Langmuir probe (RPWS/LP) we identify several features consistent with reported findings from earlier flybys, for example, T9, T63 and T75. We find that the so-called ’split’ signature of the magnetotail becomes more prominent at distances of at least 3,260 km (1.3 R_T) downstream of Titan. We also identify a specific signature of the sub-alfvenic interaction of Titan with Saturn, the Alfvén wings, which are observed during the T123 and T124 flyby. A coordinate transformation is applied to mitigate variations in the upstream magnetic field, and all the flybys are projected into a new reference frame—aligned to the background magnetic field reference frame (BFA). We show that Titan's magnetotail is confined to a narrow region of around ∼4 R_T Y_BFA. Finally, we analyze the general draping pattern in Titan's magnetotail throughout the TA to T126 flybys.

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.

Context. In past decades, several spacecraft have visited comets to investigate their plasma environments. In the coming years, Comet Interceptor will make yet another attempt. This time, the target comet and its outgassing activity are unknown and may not be known before the spacecraft has been launched into its parking orbit, where it will await a possible interception. If the approximate outgassing rate can be estimated remotely when a target has been identified, it is desirable to also be able to estimate the scale size of the plasma environment, defined here as the region bound by the bow shock.

Aims. This study aims to combine previous measurements and simulations of cometary bow shock locations to gain a better understanding of how the scale size of cometary plasma environments varies. We compare these data with models of the bow shock size, and we furthermore provide an outgassing rate-dependent shape model of the bow shock. We then use this to predict a range of times and cometocentric distances for the crossing of the bow shock by Comet Interceptor, together with expected plasma density measurements along the spacecraft track.

Methods. We used data of the location of cometary bow shocks from previous spacecraft missions, together with simulation results from previously published studies. We compared these results with an existing model of the bow shock stand-off distance and expand on this to provide a shape model of cometary bow shocks. The model in particular includes the cometary outgassing rate, but also upstream solar wind conditions, ionisation rates, and the neutral flow velocity.

Results. The agreement between the gas-dynamic model and the data and simulation results is good in terms of the stand-off distance of the bow shock as a function of the outgassing rate. For outgassing rates in the range of 10²⁷–10³¹–s^-1, the scale size of cometary bow shocks can vary by four orders of magnitude, from about 10² km to 10⁶ km, for an ionisation rate, flow velocity, and upstream solar wind conditions typical of those at 1 AU. The proposed bow shock shape model shows that a comet plasma environment can range in scale size from the plasma environment of Mars to about half of that of Saturn.

Conclusions. The model-data agreement allows for the planning of upcoming spacecraft comet encounters, such as that of Comet Interceptor, when a target has been identified and its outgassing rate is determined. We conclude that the time a spacecraft can spend within the plasma environment during a flyby can range from minutes to days, depending on the comet that is visited and on the flyby speed. However, to capture most of the comet plasma environment, including pick-up ions and upstream plasma waves, and to ensure the highest possible scientific return, measurements should still start well upstream of the expected bow shock location. From the plasma perspective, the selected target should preferably be an active comet with the lowest possible flyby velocity.

The upcoming Comet Interceptor mission involves a parking phase around the Sun-Earth L2 point before transferring to intercept the orbit of a long period comet, interstellar object or a back-up target in the form of a short-period comet. The target is not certain to be known before the launch in 2029. During the parking phase there may thus arise a scenario wherein a decision needs to be taken of whether to go for a particular comet or whether to discard that option in the hope that a better target will appear within a reasonable time frame later on. We present an expectation value-based formalism that could aid in the associated decision making provided that outlined requirements for its implementation exist.