The magnetosphere of Mercury is highly dynamic, a consequence of its small size, weak intrinsic magnetic field, and proximity to the Sun. One intriguing phenomenon is the presence of magnetic field fluctuations around 1 Hz. Here, we present a comprehensive statistical survey of these waves using the full span of the MESSENGER magnetometer measurements from 2011 to 2015. We find that ~1 Hz waves are observed during 10-20 % of the time that the spacecraft spent on closed field lines in Mercury’s magnetosphere, as determined from the KT17 magnetic field model. Wave occurrence is increased under magnetospheric conditions that favour an expanded closed field line region. We present the first global characterisation of the ~1 Hz waves at Mercury and demonstrate their dependence on both external drivers, such as upstream IMF conditions, and internal magnetospheric activity, such as the occurrence of identified dipolarization events. These results are discussed in the context of the BepiColombo mission, which will provide new opportunities to identify the nature of these waves and to assess their role in Mercury’s highly dynamic plasma environment.

Abstract. The inner heliosphere hosts a dynamically rich population of dust particles – the Zodiacal Dust Cloud – spanning sizes from nanometers to hundreds of micrometers and originating primarily from comets and asteroids. Over the past two decades, a generation of space missions has transformed our observational grasp of this environment. This review consolidates the advances enabled by the Wind, STEREO, Parker Solar Probe, and Solar Orbiter space missions, which together observe from heliocentric distances of 1 au down to less than 0.05 au. Two complementary observational approaches are addressed in detail. The first is in-situ dust detection via radio and plasma wave instruments, which commonly measure characteristic voltage pulses resulting from hypervelocity dust impacts on spacecraft. We describe the underlying physics, signal interpretation, dust particle mass calibration and machine-learning classification methods for this technique, which constitutes the only in-situ dust measurement capability on current inner-heliosphere missions. The second approach is remote sensing of the Zodiacal Light and F-corona through white-light coronagraph and heliospheric imagers. The combination of these approaches has yielded a consistent picture of the radial brightness profile of the cloud, provided the first observational evidence for a dust depletion zone developing sunward of 35 solar radii and a dust-free zone interior to 5 solar radii. The in-situ measurements established flux densities of dust in hyperbolic trajectories (β-meteoroids) across multiple missions, revealed variable fluxes of nanodust, and revealed solar-cycle modulation of the interstellar dust flux. This review presents these results, discusses their implications, and outlines the prospects offered by near-future missions.

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 ( nT, 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.

Solar wind magnetic holes are small-scale, isolated decreases of the magnetic field strength. They are commonly divided into two types, linear and rotational magnetic holes, based on the rotation of the magnetic field vector from one side of the hole to the other. We present Solar Orbiter, Parker Solar Probem and MESSENGER measurements of magnetic holes from the inner heliosphere (~0.1-1.0 AU) and Cassini measurements from the outer heliosphere (~9 ̶ 10 AU). We compare properties such as rate of occurrence, and distributions of scale size, depth, and amount of magnetic field rotation, and discuss the findings in terms of local generation of magnetic holes, versus transport of magnetic holes generated in the inner heliosphere. We also discuss the relative importance of magnetic holes in interacting with the magnetospheres of planets in the inner and outer heliosphere.

Hypervelocity (>1 km/s) dust grains orbiting in the inner heliosphere can collide with a spacecraft and create a plasma cloud that changes electrical conditions in the surrounding plasma. These changes can be detected by the onboard radio and plasma wave receivers acting as efficient dust impact detectors. Estimated dust impact rates depend on the observation time window and they are commonly extrapolated. Our study presents the RPW/TDS Maximum Amplitudes (MAMP) data that continuously monitors signals from up to four RPW antenna configurations (monopole or dipole, and HF Search Coil) onboard the Solar Orbiter satellite. The signal is sampled in the high cadence (2.091 Msps) and stored in a buffer as the absolute maximum amplitude. MAMP values are then provided with a cadence between 32 and 128 sps, giving us a time resolution between 8 and 31 ms. Individual dust impacts detected by the onboard algorithm evaluating 62ms-long waveform snapshots every second are compared with the MAMP observations and show a very good match. After corrections for the high amplitude plasma waves or non-standard operational modes, and together with the TDS Statistics, the MAMP observations are used for the individual dust impact identification and corrected impact rates during the entire Solar Orbiter mission.

During Cassini's Grand Finale in 2017, the number densities of electron and light ions in Saturn's low-latitude ionosphere were measured in situ. This region is strongly influenced by the influx of ring material from Saturn's D ring.The electron data from the onboard Langmuir probe (LP) and light ion densities from the Ion and Neutral Mass Spectrometer (INMS) correlate very well even on short timescales after correcting the INMS timestamps, whereas prior the correlation was limited to broader scales. We analyze the electron-to-ion ratios for the proximal orbits and identify three distinct regions in Saturn's ionosphere:1) For altitudes above ∼2500 km and latitudes between -20° and 20°, the electron-to-light-ion ratios for the four analysed orbits are generally 8000 km) the ratios increase rapidly. This may indicate the presence of heavier ions, such as O+ and water group species, spiralling in from the C ring.

It is known that Enceladus is one of the main mass sources in Saturn’s inner magnetosphere, with other bodies – e.g. Dione, Tethys, Saturn’s rings and atmosphere – thought to play a smaller role. In this study we analysed electron density and temperature as estimated from Cassini’s Langmuir Probe (LP) for the entire duration of the Cassini mission (August 2004 – September 2017). The main motivation for this work is to understand the impact of Enceladus as a main mass source and search for evidence of additional sources. We also have been investigating the impact of the spacecraft-plasma interaction, and illumination conditions on the LP current-voltage curves and henceforth on the bulk electron parameters.One particular facet of this work is the discrimination of spacecraft-generated photoelectrons in the current-voltage curves so that they can be excluded from further analysis of the ambient magnetospheric electrons. To develop a robust algorithm to identify photoelectrons we have been working with data during transitions in and out of shadow caused by two main events: i) when the LP is shadowed by the spacecraft, or ii) during eclipses by Saturn or its moons.With a cleaned data set we focused on analysing data within 0.5RS of the Kronian in the inner magnetosphere (r<7RS). We compared our data with the hybrid power-law model, as described in Persoon et al. (2013, JGR), that used the electron densities estimated from the upper hybrid resonance frequency. We also looked for local time asymmetries in electrons, as reported in ion plasma in the same region (e.g. Holmberg et al., 2007, GRL).

Ion-acoustic waves (IAWs) commonly occur near interplanetary (IP) shocks. These waves are important because of their potential role in the dissipation required for collisionless shocks to exist. We study IAW occurrence statistically at different heliocentric distances using Solar Orbiter to identify the processes responsible for IAW generation near IP shocks. We show that close to IP shocks the occurrence rate of IAW increases and peaks at the ramp. In the upstream region, the IAW activity is highly variable among different shocks and increases with decreasing distance from the Sun. We show that the observed currents near IP shocks are insufficient to reach the threshold for the current-driven instability. We argue that two-stream proton distributions and suprathermal electrons are likely sources of the waves. Ion-acoustic waves (IAWs) are fluctuations in the electric field that occur at frequencies close to the ion plasma frequency. These waves are commonly found in the solar wind and often cluster around interplanetary (IP) shock waves. In this study, we investigate and quantify how common IAWs are in the vicinity of IP shocks. Our research revealed that IAW activity is enhanced before and after most IP shock passages. Furthermore, IAWs are more likely to be observed preceding IP shocks that are closer to the Sun. We find that the occurrence rate of IAWs shows no clear dependence on the IP shock parameters. We explore the possible mechanisms that could explain the presence of these IAWs. For instance, IAW modes can be excited by electric currents if the associated drift velocity between ions and electrons is above a certain threshold. However, the currents alone are not strong enough to generate the IAWs found near IP shocks. We discuss other potential generation mechanisms, such as velocity distributions of ions and electrons deviating from thermodynamic equilibrium. The occurrence of Ion-acoustic waves (IAWs) is enhanced at interplanetary (IP) shocks, peaking at the shock ramp The occurrence rate of IAWs in the upstream region of IP shocks increases with decreasing radial distance from the Sun IAWs are observed upstream of an IP shock together with two-stream protons and an electron strahl

We combine RPWS/LP and INMS data from Cassini's Grand Finale orbits into Saturn's lower ionosphere to calculate the effective recombination coefficient α300 at a reference electron temperature of 300 K. Assuming photochemical equilibrium at altitudes below 2500 km and using an established method to determine the electron production rate, we derive upper limits for α300 of ∼ 2.5∗10-7 cm3 s-1, which suggest that Saturn's ionospheric positive ions are dominated by species with low recombination rate coefficients.An ionosphere dominated by water group ions or complex hydrocarbons, as previously suggested, is incompatible with this result, as these species have recombination rate coefficients > 5∗10-7 cm3 s-1 at an electron temperature of 300 K. The results do not give constraints on the nature of the negative ions.

The Langmuir Probe (LP) onboard Cassini was one of the three experiments that could measure the cold inner magnetospheric plasma, along with the Radio and Plasma Waves Science (RPWS) and the Cassini Plasma Spectrometer (CAPS). While the century-old LP theory looks quite straight-forward, in reality things are much more complicated.The operation of the LP is quite simple: by applying positive bias voltages, the probe attracts the electrons and repels the ions of the surrounding plasma. From the resulting current-voltage curve characteristics of the ambient electrons can be estimated, i.e. density and temperature. When negative bias voltages are applied to the probe the characteristics of the ambient ions can be estimated, i.e. density, temperature, and mass.Though the LP operation and interpretation are quite simple and straightforward, there are assumptions made and therefore the theoretical models may not always reflect the actual plasma conditions in Saturn’s magnetosphere. For this study we are focused on the effect of the photoelectrons, i.e. electrons that are generated by the incident sunlight on Cassini’s surfaces, which are difficult to be observed and corrected for in a laboratory plasma.We developed a robust algorithm that identifies the transitions of the LP in and out of shadow caused by the Saturn and its rings. The LP data inside and outside the eclipses are compared using the algorithm developed. In this presentation we will discuss the impact of the photoelectron generation from the spacecraft surfaces to the LP current-voltage curves, and understand the variations of the measured plasma density connected with the photoelectrons.

The magnetospheric systems of ice giants, as the ideal and the unique template of a typical class of exoplanets, have not been sufficiently studied in the past decade. The complexity of these asymmetric and extremely dynamic magnetospheres provides us a great chance to systematically investigate the general mechanism of driving the magnetospheres of such common exoplanets in the Universe, and the key factors of influencing the global and local magnetospheric structures of this type of planets. In this paper, we discuss the science return of probing magnetospheric systems of ice giants for the future missions, throughout different magnetospheric regions, across from the interaction with upstream solar wind to the downstream region of the magnetotail. We emphasize the importance of detecting the magnetospheric systems of ice giants in the next decades, which enables us to deeply understand the space enviroNMent and habitability of not only the ice giants themselves but also the analogous exoplanets which are widely distributed in the Universe.

The Radio & Plasma Wave Investigation (RPWI) onboard the ESA JUpiter ICy moons Explorer (JUICE) is here 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 like, e.g., electromagnetic and plasma waves, electrostatic acceleration structures, induction fields etc. A full wave vector, waveform, polarization, and Poynting flux determination is aimed for. In addition, RPWI will enable characterization of 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. RPWI can also detect micrometeorite impacts, estimate dust charging, monitoring the spacecraft potential as well as the integrated EUV flux. The sensors consist of four 10 cm diameter Langmuir probes each mounted at the tip of 3 m long booms, a triaxial search coil magnetometer and a triaxial radio antenna system both mounted on the 10.5 m long MAG boom, each with radiation resistant pre-amplifiers near the sensors. There are three receiver boards, 2x Digital Processing Units (DPU) and 2x Low Voltage Power Supply (LVPS) boards in a central box in a radiation vault at the centre of the JUICE spacecraft. Together, the RPWI system can carry out a powerful and ambitious planetary science investigation in and around the Galilean icy moons and the Jovian space environment. Some of the more important science objectives & capabilities will be described here. RPWI focuses, apart from cold plasma studies, on the understanding of how, through electro-dynamic and electromagnetic coupling, the momentum and energy transfer occur in the surrounding space environments and 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 will hopefully deliver data from the near surface during the final crash orbit.

We discuss a suite of instruments cable of carrying out the next generation of in situ cosmic dust measurements from the heliosphere into interstellar space in support of the Interstellar Probe mission concept.A Dust Analyzer should be considered as the highest priority for its coverage of both compositional and dynamical information of the bulk of interstellar dust (ISD) and interplanetary dust particle (IDP) populations, essential to address the major science questions.A PVDF Dust Counter and Plasma Wave Antenna instrument could additionally provide critical improvement through the detection of larger, rarer dust populations, to constraining the mass density of ISD as well as providing additional directionality coverage.A Neutral Mass Spectrometer bridges the measurement gap between microscopic dust grains and gas species, potentially relevant for understanding the nature and interactions of the very local interstellar medium and our heliosphere.The Dust Analyzer, PVDF Dust Counter, and Plasma Wave Antenna instruments can be calibrated using dust accelerator facilities (University of Colorado, USA, see Shu et al., 2012; and Universität Stuttgart, Germany) with ISD-relevant materials at realistic mass and speed ranges.

Context. Langmuir waves (electrostatic waves near the electron plasma frequency) are often observed in the solar wind and may play a role in the energy dissipation of electrons. The largest amplitude Langmuir waves are typically associated with type II and III solar radio bursts and planetary foreshocks. In addition, Langmuir waves not related to radio bursts occur in the solar wind, but their source is not well understood. Langmuir waves have been observed inside isolated magnetic holes, suggesting that magnetic holes play an important role in the generation of Langmuir waves. Aims. We provide the statistical distribution of Langmuir waves in the solar wind at different heliocentric distances. In particular, we investigate the relationship between magnetic holes and Langmuir waves. We identify possible source regions of Langmuir waves in the solar wind, other than radio bursts, by analyzing the local plasma conditions. Methods. We analyzed data from Solar Orbiter’s Radio and Plasma Waves (RPW) and Magnetometer (MAG) instruments. We used the triggered electric field snapshots and onboard statistical data (STAT) of the Time Domain Sampler (TDS) of RPW to identify Langmuir waves and investigate their properties. The plasma densities were derived from the spacecraft potential estimated by RPW. The MAG data were used to monitor the background magnetic field and detect magnetic holes, which are defined as regions with an isolated decrease in | B | of 50% or more compared to the background level. The statistical analysis was performed on data from 2020 to 2021, comprising heliocentric distances between 0.5 AU and 1 AU. Results. We show that 78% of the Langmuir waves in the solar wind not connected to radio bursts occur in regions of local magnetic field depletions, including the regions classified as isolated magnetic holes. We also show that the Langmuir waves occur more frequently inside magnetic holes than in any other region in the solar wind, which indicates that magnetic holes are important source regions of solar wind Langmuir waves. We find that Langmuir waves associated with magnetic holes in the solar wind typically have lower amplitudes than those associated with radio bursts.

For orbits 288 and 292 of Cassini’s Grand Finale, clear dips (sharp and narrow decreases) are visible in the H 2 + densities measured by the Ion and Neutral Mass Spectrometer (INMS). In 2017, the southern hemisphere of Saturn was shadowed by its rings and the substructures within. Tracing a path of the solar photons through the ring plane to Cassini’s position, we can identify regions in the ionosphere that were shadowed by the individual ringlets and plateaus (with increased optical depths) of Saturn’s C ring. The calculated shadowed altitudes along Cassini’s trajectory line up well with the dips in the H 2 + data when adjusting the latter based on a detected evolving shift in the INMS timestamps since 2013, illustrating the potential for verification of instrument timings. We can further estimate the mean optical depths of the ringlets/plateaus by comparing the dips to inbound H 2 + densities. Our results agree well with values derived from stellar occultation measurements. No clear dips are visible for orbits 283 and 287, whose periapsides were at higher altitudes. This can be attributed to the much longer chemical lifetime of H 2 + at these higher altitudes, which in turn can be further used to estimate a lower limit for the flow speed along Cassini’s trajectory. The resulting estimate of ∼0.3 km s −1 at an altitude of ∼3400 km is in line with prior suggestions. Finally, the ringlet and plateau shadows are not associated with obvious dips in the electron density, which is expected due to their comparatively long chemical (recombination) lifetime.

The Cassini Langmuir Probe (LP) data acquired in the ionosphere of Titan are re-analysed to finely study the electron behaviour in the birthplace of Titan’s aerosols (900-1200 km) [Waite et al 2007]. The detailed analysis of the complete Cassini LP dataset below 1200 km (57 flybys) shows the systematic detection of 2 to 4 electron populations (further named P1, P2, P3, P4), with reproducible characteristics depending on altitude and solar illumination. Populations P1 and P2 are always present, contrarily to P3 and P4. Due to their low density and low potential, P1 electrons are suspected to be photo-electrons [Wahlund et al 2009] or secondary electrons emitted on the probe stick. The electron populations densities and temperatures are deduced from the Orbital Motion Limited theory and the Sheath Limited theory [Wahlund et al 2009, Whipple 1965]. We observe that electron temperatures do not vary much with altitude between 1200 and 950 km, except for P4. Statistical correlations with other quantities measured by Cassini are investigated. In particular, we observe that P3 and P4 densities are correlated with the extreme UV flux. From our results we suggest possible origins for the three populations P2, P3 and P4, coming from the plasma surrounding the probe: -P2 is detected in all cases, at rather low density (~500 cm-3) and temperature (~0.04 eV). These are possibly induced by particle precipitation. -P3 electrons are denser with stronger solar illumination and higher pressure (up to 3000 cm-3). Therefore, they are likely to be related to photo-ionization. They are hotter than P2 electrons (~0.06-0.07 eV). -P4 electrons are only observed on dayside and below 1200 km, in the place where heavy negative ions and aerosols are present. They are then plausibly linked to dusty plasma effects. We suggest two possible formation processes: (1) the photo-emission of electrons from grains could be triggered by photons of a few eV due to the negative charge born by the aerosols [Shebanits et al 2016; Tigrine et al 2018] ; (2) electrons could also be thermo-emitted from the grains, as a result of their heating by diverse processes such as heterogeneous chemistry, sticking of electrons or recombination of radicals [Woodard et al 2020].

We present a semianalytical photochemical model of Saturn’s near-equatorial ionosphere and adapt it to two regions (∼2200 and ∼1700 km above the 1 bar level) probed during the inbound portion of Cassini’s orbit 292 (2017 September 9). The model uses as input the measured concentrations of molecular hydrogen, hydrogen ion species, and free electrons, as well as the measured electron temperature. The output includes upper limits, or constraints, on the mixing ratios of two families of molecules, on ion concentrations, and on the attachment rates of electrons and ions onto dust grains. The model suggests mixing ratios of the two molecular families that, particularly near ∼1700 km, differ notably from what independent measurements by the Ion Neutral Mass Spectrometer suggest. Possibly connected to this, the model suggests an electron-depleted plasma with a level of electron depletion of around 50%. This is in qualitative agreement with interpretations of Radio Plasma Wave Science/Langmuir Probe measurements, but an additional conundrum arises in the fact that a coherent photochemical equilibrium scenario then relies on a dust component with typical grain radii smaller than 3 Å.

We study the effect of negatively charged dust on the magnetic-field-aligned polarisation electrostatic field ($\boldsymbol {E}_{\parallel }$) using Cassini's RPWS/LP in situ measurements during the ‘ring-grazing’ orbits. We derive a general expression for $\boldsymbol {E}_{\parallel }$ and estimate for the first time in situ $\lVert \boldsymbol {E}_{\parallel } \rVert$ (approximately $10^{-5} \, \text {V}\, \text {m}^{-1}$) near the Janus and Epimetheus rings. We further demonstrate that the presence of the negatively charged dust close to the ring plane ($\vert \text {Z} \vert \lesssim 0.11 \, \text {R}_{s}$) amplifies $\lVert \boldsymbol {E}_{\parallel } \rVert$ by at least one order of magnitude and reverses its direction due to the effect of the charged dust gravitational and inertial forces. Such reversal confines the electrons at the magnetic equator within the dusty region, around $0.047 \, \text {R}_{s}$ above the ring plane. Furthermore, we discuss the role of the collision terms, in particular the ion–dust drag force, in amplifying $\boldsymbol {E}_{\parallel }$. These results imply that the charged dust, as small as nanometres in size, can have a significant influence on the plasma transport, in particular ambipolar diffusion along the magnetic field lines, and so their presence must be taken into account when studying such dynamical processes.

Transient electric field perturbations are commonly observed when the interplanetary dust grains impact spacecraft, and their characteristics are well-studied. The signals are interpreted as due to the plasma expansion at the impact site and last typically in the order of micro-to milli-seconds. Radio and Plasma Wave (RPW) Instrument onboard Solar Orbiter can observe grains with a dedicated mode to capture such short-lived signals by the dust in the inner Heliosphere. On the other hand, a large impact can cause electric field disturbance for a longer time in tens of seconds. The long signals are observed in the low-frequency range (<10 kHz) and found more frequently during the inbound of the Solar Orbiter excursion. We will discuss the plasma and spacecraft conditions for the long durational impact signals.

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