Logo: to the web site of Uppsala University

The Neutral Gas and Ion Mass Spectrometer (NGIMS) onboard the Mars Atmosphere and Volatile EvolutioN mission has provided critical insights into the composition and isotopic structure of Mars' ionosphere. However, significant model-observation discrepancies of densities of some ion species suggest the presence of crosstalk, that is, transmission between adjacent mass-to-charge (m/z) channels due to non-ideal ion optics and mass filtering, that can introduce anomalously high signals in certain channels. In this study, we conduct a comprehensive investigation of crosstalk in NGIMS ion mass spectra using a decade of data (2014-2024). We investigate crosstalk behavior across the ion mass spectrum, identifying significant transmission from major source channels at m/z = 32 (O₂⁺) and m/z = 44 (CO₂⁺). The observed variability of crosstalk ratios suggests temporal changes in the effective crosstalk window, potentially driven by spacecraft maneuvers such as those occurring immediately before and after Deep Dip campaigns. Crosstalk exists in both the inbound and outbound measurements in the ion mode and is systematically higher than in the neutral mode. We perform synthetic tests using different crosstalk patterns to generate contaminated spectra and then reconstruct the true ion spectra using prescribed crosstalk ratios, which may be obtained from ground-based measurements to apply to in-flight data. Although the identified crosstalk mainly influences the identification of minor species and has minimal impact on studies of major ions, our results improve the interpretation of NGIMS ion composition measurements and provide a more robust foundation for both scientific analyses and instrumental characterization in mass spectrometer investigations of planetary atmospheres.

The Cassini Radio & Plasma Wave Science (RPWS) and Ion and Neutral Mass Spectrometer (INMS) observations of Saturn's ionosphere during the proximal orbits (Rev) 288-293 in the altitude range 1450-2500 km (above the 1-bar pressure level) are modelled with the inclusion of a dust component. Previous reports have revealed that large amounts of nm- to mu m-sized dust grains precipitate from the D-ring into the atmosphere of Saturn's equatorial region. We find that the processed charged dust (< 2 nm radius) has a profound effect on the ionospheric structure, enhancing the ion number density well above photochemical equilibrium levels, while the free electrons tend to become attached to the dust population as a result of a low photo-electron detachment rate by EUV light from the Sun at the distance of Saturn. The charged dust layers are influenced by the strong near horizontal magnetic field inhibiting vertical ambipolar diffusion as most charged components are magnetized above about 1800 km. Our dust-ionosphere model calculations reveal that layers of mostly singly negatively charged dust can explain much of the observed ionospheric densities. Modelling uncertainties include sticking coefficients, recombination rates, dust distributions and the presence of negative ions as well as variations thereof along the spacecraft trajectory. Our models generate overall acceptable fits to the observed RPWS electron and ion densities. The observations are therefore compatible with charged grains (and/or cluster ions) being present and playing a dominant role in the Kronian ionosphere structure and chemistry. Similar processes may be active at gas giants with associated ring systems.

Ions with mass-to-charge ratios from 2 to 150 have been measured in the Martian ionosphere by the Neutral Gas and Ion Mass Spectrometer (NGIMS) onboard the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission. Among these, protonated species at 31, 33, and 41 atomic mass units (amu) are observed at densities significantly higher than those predicted by photochemical models. In this study, we combine photochemical modelling with NGIMS measurements to investigate these discrepancies in the dayside ionosphere of Mars. For 41 amu ions, the photochemical model predicts ArH+ densities more than one order of magnitude lower than NGIMS measurements. For 31 and 33 amu ions, modelled densities of HNO+ and HO2+ are three orders of magnitude lower than observed values. We find that the vertical profiles reveal striking similarities among (a) the 31, 32, and 33 amu channels and (b) the 41, 42, 43, and 44 amu channels, which cannot be fully explained by interlinked chemical pathways and contributions from oxygen isotopes. Instead, we suggest that instrumental effects, specifically mass channel cross-talk from the strong 32 and 44 amu, provide a plausible explanation for the anomalously high densities reported at 31, 33, and 41 amu.

Measurements by the dual Langmuir Probe (LAP) on board Rosetta in the inner coma of comet 67P/Churyumov-Gerasimenko revealed an unexpected similar to 50% level of attenuation of the impinging solar extreme-ultraviolet (EUV) radiation while the comet was near perihelion. It was argued that this possibly could be a sign of upstream grain fragmentation. Here the effect of grain fragmentation on the EUV transmission fraction in a cometary coma is looked at through two simple analytical models: a continuous fragmentation model and an abrupt fragmentation model. We assume in both models radially expanding grains that readily reach their respective size-dependent terminal velocity. Without specifying driving mechanisms, we assume in the continuous fragmentation model that any grain splits into the same number of equal-sized fragments at a fixed size- and position-independent rate, Gamma , as long as said grain has not yet reached a prescribed minimal size from which point it cannot fragment further. In the abrupt fragmentation model, we instead assume that grains undergo complete fragmentation into minimal-sized subunits at a fixed cometocentric distance. In order for the models to produce transmission fraction profiles compatible with the aforementioned LAP observations, specific conditions appear necessary. A particularly puzzling aspect is the apparent requirement that a significant fraction of the dayside dust population disintegrate gradually or abruptly down to sizes of several tens of nanometers already by a distance of several 1000 km.

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.