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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 generated by the incident sunlight on Cassini’s surfaces, that are difficult to calibrate for on the ground and then observe and characterise in the LP data.We present algorithms for identifying when Cassini is in the shadow of Saturn and its rings, and when the LP is in the shadow of Saturn, its rings or Cassini itself. The LP data inside and outside the eclipses are compared using the algorithms developed. In this presentation we will first 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. Then, using that knowledge, we attempt to define the optical depth of the rings in the wavelengths the LP operates in.

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].

Langmuir waves (electrostatic waves near the electron plasma frequency) are often observed in the solar wind, playing an important role in the energy dissipation of electrons. The largest amplitude waves are typically associated with type II and III solar radio bursts and planetary foreshocks. However, Langmuir waves not connected with radio bursts are also found in the solar wind. The causes of these Langmuir waves are not well understood. Langmuir waves are also found around magnetic holes, a localised depression of the magnetic field strength. This study aims to investigate the relationship between Langmuir waves and magnetic holes in the solar wind using electric and magnetic field measurements performed by the Solar Orbiter’s RPW and MAG instruments during 2020 and 2021. We identified a large set of Langmuir wave events from the RPW/TDS (Time Domain Sampler) waveform data using the plasma density estimated from the spacecraft’s potential obtained by RPW, showing that ~7% of them have been spotted inside magnetic holes. We will compare these events with local plasma conditions analysing the electron distribution functions, and discuss the mechanisms that may lead to the generation of Langmuir waves associated with magnetic holes in the solar wind.

The ESA-JAXA BepiColombo mission will provide simultaneous measurements from two spacecraft, offering an unprecedented opportunity to investigate magnetospheric and exospheric dynamics at Mercury as well as their interactions with the solar wind, radiation, and interplanetary dust. Many scientific instruments onboard the two spacecraft will be completely, or partially devoted to study the near-space environment of Mercury as well as the complex processes that govern it. Many issues remain unsolved even after the MESSENGER mission that ended in 2015. The specific orbits of the two spacecraft, MPO and Mio, and the comprehensive scientific payload allow a wider range of scientific questions to be addressed than those that could be achieved by the individual instruments acting alone, or by previous missions. These joint observations are of key importance because many phenomena in Mercury's environment are highly temporally and spatially variable. Examples of possible coordinated observations are described in this article, analysing the required geometrical conditions, pointing, resolutions and operation timing of different BepiColombo instruments sensors., Comment: 78 pages, 14 figures, published

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.

During the Grand Finale of the Cassini mission, the southern hemisphere of Saturn was shadowed by its rings and the substructures within, whose more intense shadows can be mapped to specific ionospheric altitudes. We successfully connect small-scale variations (dips) in the ionospheric H2+ density below 2500 km, measured by the Ion and Neutral Mass Spectrometer (INMS) during orbits 288 and 292, to the shadows of individual ringlets and plateaus in the C Ring. From the H2+ density signatures we estimate lower limits of the associated ringlet or plateau opacities. These will be compared with results obtained from stellar occultations. Potential implications/constraints on the ionospheric dynamics will be discussed. The ringlet and plateau shadows are not associated with obvious dips in the electron density.

Measurements of electrons and ions in Saturn's ionosphere down to 1,500-km altitudes as well as the ring crossing region above the ionosphere obtained by the Langmuir probe onboard the Cassini spac ...

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.

A large-amplitude impact-induced like electric field signal is often observed by the Radio and Plasma Wave (RPW) Instrument onboard Solar Orbiter. The signal has a sharp increase followed by an exponential decay, typically observed when spacecraft experiences a dust impact. The amplitude can reach several V/m. The impact dust size can be estimated from the electric field amplitude and is similar to the characteristic dust size near the sun expected from the zodiacal-light observations. On the other hand, the signal's decay time is the order of second, unusually long compared to the dust impact signals previously reported by the other spacecraft. We will show the characteristics of these signals and discuss the origin.

International audience; Planetary rings are a multifaceted player in the system. Recent advances about Saturn’s rings argue that ring science at the ice giants, with magnetospheric and atmospheric sciences, are essential in advancing our knowledge about solar system evolution, the evolution of the moons and Ocean Worlds, and phenomena observed in the ice giant systems.

Cassini’s Grand Finale orbits brought us historical first in-situ measurements of Saturn’s ionosphere, showing that it contains dusty plasma in the equatorial region. We present the Pedersen and Hall conductivities of the top ionosphere (10:50 – 12:17 Saturn Local Time, 10N – 20S planetocentric latitude), derived from particle and magnetometer data. We constrain the Pedersen conductivities to be at least 10-5 – 10-4 S/m at ionospheric peak, a factor 10-100 higher than estimated previously by remote measurements, while the Hall conductivities are very close to 0 or in fact negative. We show that this is an effect of dusty plasma. Another effect is that ionospheric dynamo region thickness is increased to 300-800 km. Furthermore, our results suggest a temporal variation (decrease) of the plasma densities, mean ion masses and consequently the conductivities over the period of one month.

It is well known that in the magnetosphere of the outer planets (eg. Saturn, Jupiter, Neptune), even in the absence of an electric current, a polarization electric field develops as a consequence of charge separation in a plasma, providing a restoring force to maintain charge neutrality. It is also well established that certain regions of these planetary systems (ionosphere, icy moons, rings) are populated by significant amount of charged dust that play an important role in the physical and chemical processes in the surrounding plasma environment.In the present work, we study the effect of the charged dust grains on the polarization electric field using Saturn’s F-ring region as a case study. We derive a general expression for E parallel to the magnetic field (E_para) and then using the Cassini RPWS/LP measurements we estimate for the first time in situ E_para close to Janus/Epimetheus ring during the F ring grazing orbits. We further demonstrate that the presence of charged dust, as small as nanometers in size, can significantly influence the plasma transport processes, in particular the ambipolar diffusion along the magnetic field lines. We show that, close to the ring plane (Z

We present a comprehensive statistical study of magnetic holes, defined as localized decreases of the magnetic field strength of at least 50%, in the solar wind near Mercury, using MESSENGER orbital data. We investigate the distributions of several properties of the magnetic holes, such as scale size, depth, and associated magnetic field rotation. We show that the distributions are very similar for linear magnetic holes (with a magnetic field rotation across the magnetic holes of less than 25 degrees) and rotational holes (rotations >25 degrees), except for magnetic holes with very large rotations (greater than or similar to 140 degrees). Solar wind magnetic hole scale sizes follow a log-normal distribution, which we discuss in terms of multiplicative growth. We also investigate the background magnetic field strength of the solar wind surrounding the magnetic holes, and conclude that it is lower than the average solar wind magnetic field strength. This is consistent with finding solar wind magnetic holes in high-beta regions, as expected if magnetic holes have a connection to magnetic mirror mode structures. We also present, for the first time, comprehensive statistics of isolated magnetic holes in a planetary magnetosheath. The properties of the magnetosheath magnetic holes are very similar to the solar wind counterparts, and we argue that the most likely interpretation is that the magnetosheath magnetic holes have a solar wind origin, rather than being generated locally in the magnetosheath.

We have used Spacecraft Plasma Interaction Software (SPIS) simulations to study the characteristics (i.e., dimensions, ion depletion, and evolution with the changing spacecraft attitude) of the Cassini ion wake. We focus on two regions, the plasma disk at 4.5-€“4.7 RS, where the most prominent wake structure will be formed, and at 7.6 RS, close to the maximum distance at which a wake structure can be detected in the Cassini Langmuir probe (LP) data. This study also reveals how the ion wake and the spacecraft plasma interaction have impacted the Cassini LP measurements in the studied environments, for example, with a strong decrease in the measured ion density but with minor interference from the photoelectrons and secondary electrons originating from the spacecraft. The simulated ion densities and spacecraft potentials are in very good agreement with the LP measurements. This shows that SPIS is an excellent tool to use for analyses of LP data, when spacecraft material properties and environmental parameters are known and used correctly. The simulation results are also used to put constraints on the ion temperature estimates in the inner magnetosphere of Saturn. The best agreement between the simulated and measured ion density is obtained using an ion temperature of 8 eV at ∼4.6 RS. This study also shows that SPIS simulations can be used in order to better constrain plasma parameters in regions where accurate measurements are not available.

The present study combines Radio and Plasma Wave Science/Langmuir Probe and Ion and Neutral Mass Spectrometer data from Cassini’s last four orbits into Saturn’s lower ionosphere to constrain the effective recombination coefficient α 300 from measured number densities and electron temperatures at a reference electron temperature of 300 K. Previous studies have shown an influx of ring material causes a state of electron depletion due to grain charging, which will subsequently affect the ionospheric chemistry. The requirement to take grain charging into account limits the derivation of α 300 to upper limits. Assuming photochemical equilibrium and using an established method to calculate the electron production rate, we derive upper limits for α 300 of ≲ 3 × 10−7 cm3 s−1 for altitudes below 2000 km. This suggests that Saturn’s ionospheric positive ions are dominated by species with low recombination rate coefficients like HCO+. 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.

Cassini plasma wave and charged particle observations are combined with magnetometer measurements to investigate Titan's induced magnetosphere. Electric field emissions close to Titan are identifie...

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 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). The detailed analysis of the complete Cassini LP dataset below 1200 km (57 flybys) shows the systematic detection of 2 to 4 electron populations, with reproducible characteristics depending on altitude and solar illumination. Their densities and temperatures are deduced from the Orbital Motion Limited theory. Statistical correlations with other quantities measured by Cassini are investigated. We finally discuss the origins of the detected populations, one being possibly emitted by aerosols. 1- Introduction The Cassini mission discovered that Titan’s ionosphere is the birthplace of the solid orange aerosols surrounding Titan [1]. It is an ionized environment (plasma) hosting a complex ion chemistry [2]. The aerosols are formed and certainly eroded [3] in this environment. They are likely to interact strongly with the plasma species (i.e. electrons, ions, radicals and excited species): the ionosphere of Titan is a ‘dusty plasma’ below ~1100 km [4], [5]. Besides, models of the ionosphere do not match well measurements below 1100 km [6]. Recent works [7]–[9] showed the necessity to take negative ions and aerosols into account in the models. The present work aims to finely analyse the behaviour of electrons in the aerosols-containing region of the ionosphere (~900-1200 km) sounded at 57 occasions by the Cassini Langmuir Probe (LP), part of the Radio and Plasma Wave Science (RPWS) package. 2- Re-analysis using 2 to 4 electron contributions The current collected by the Langmuir probe in the conditions of the ionosphere of Titan can be modelled according to three hypotheses: (1) the electron velocity distributions are considered Maxwellians; (2) in the case where the probe repels electrons, the current collected can be given by the Orbital Motion Limited (OML) theory [10], [11]; (3) in the case where the probe attracts electrons, the current is fitted using the Sheath Limited theory [12]. To obtain a satisfactory fitting, we show the necessity of using 2 to 4 electron populations (named P1, P2, P3 and P4) at different potentials [13]. We observe (see Figure 1) that the second derivative of the current is useful to deduce the number of electron populations detected on a measurement and gives an idea of their relative proportions. Indeed, d²I/dU² is physically related to the electron energy distribution functions (EEDF) through the Druyvesteyn method [14]. 3- Populations: strong dependence with SZA and altitude The visualisation of electron populations with d²I/dU² is used in Figure 2 to show the variations of the populations with 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 [10] or secondary electrons emitted on the probe stick. P3 electrons are only absent on the far nightside, while P4 electrons are detected only on dayside, and below the altitude of 1200 km. For this reason, they are suspected to be related to aerosols or heavy negative ions that appear below 1200 km. 4- Densities and temperatures for all the electron populations Figures 3 and 4 give statistics on electron densities and temperatures measured for all the populations for the 57 Cassini flybys reaching at least 1200 km. They show that electron temperatures do not vary much with altitude between 1200 and 950 km, except for P4. P3 and P4 have increasing densities with pressure on dayside. 5- Statistics The large dataset enables to do statistics and search for correlations. In particular, we observe that P3 and P4 densities are correlated with the extreme UV flux. This enforces the idea that these populations are formed by processes involving solar photons (certainly UV). We also observe a strong correlation between the density and the temperature of the P4 population, as illustrated in Figure 5. 6- Conclusion: origins of the detected electron populations From the above 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 supposed to be background thermalized electrons, possibly formed through collisions of gas species with magnetospheric suprathermal electrons. -P3 electrons are denser with stronger solar illumination and higher pressure (up to 3000 cm-3). They are hotter than P2 electrons (~0.06-0.07 eV). Therefore, they could be formed by the photo-chemistry occurring in Titan’s ionosphere. -P4 electrons are only observed on dayside and below 1200 km, in the place where heavy negative ions and aerosols are present. 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 [5], [15]; (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 [16]. References [1] J. H. Waite et al. Science 316 (2007) [2] V. Vuitton et al. Icarus 324 (2019) [3] A. Chatain et al. Icarus 345 (2020) [4] P. Lavvas et al. PNAS 110 (2013) [5] O. Shebanits et al. J. Geophys. Res. Sp. Phys. 121 (2016) [6] M. Galand et al. in Titan, Cambridge University Press (2014) pp. 296–361. [7] O. Shebanits et al. Astrophys. J. 850 (2017) [8] R. T. Desai et al. Astrophys. J. 844 (2017) [9] A. Wellbrock et al. Mon. Not. R. Astron. Soc. 490 (2019) [10] J. E. Wahlund et al. Planet. Space Sci. 57 (2009) [11] E. C. J. Whipple, PhD thesis, Georg. Washingt. Univ. (1965) [12] R. T. Bettinger and E. H. Walker, Phys. Fluids 748 (1965) [13] A. Chatain et al., “Electron temperature(s) in Titan’s ionosphere” in EPSC-DPS Joint Meeting (2019) [14] M. J. Druyvesteyn, Zeitschrift für Phys. 64 (1930) [15] S. Tigrine et al. Astrophys. J. 867 (2018) [16] A. Woodard et al. J. Vac. Sci. Technol. A 38 (2020)

The present study combines RPWS/LP and INMS data from Cassini's orbit 292, which reached an altitude of 1685 km at the lowest point, to constrain the effective recombination coefficient α300 from measured densities and electron temperatures at a reference electron temperature of 300 K. Assuming photochemical equilibrium at these low altitudes and linking established methods to calculate the electron production rate and the dissociative recombination rate results in a formula to calculate an upper limit for α300. This is then compared against rate constants of individual recombination reactions as measured in the laboratory.We derive upper limits for α300 of ∼ 2.5∗10-7cm3 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 constants > 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.

This paper describes the design of MEFISTO (Mercury Electric Field In-Situ Tool) and WPT (Wire Probe Antenna) electric field sensors for Plasma Wave Investigation (PWI) on the BepiColombo Mio spacecraft (Mercury Magnetospheric Orbiter, MMO). The two sensors will enable the first observations of electric fields, plasma waves and radio waves in and around the Hermean magnetosphere and exosphere. MEFISTO and WPT are dipole antennas with 31.6 m tip-to-tip length. Each antenna element has a spherical probe at each end of the wire (15 m length). They are extended orthogonally in the spin plane of the spacecraft and enable measurements of the electric field in the frequency range of DC to 10 MHz by the connection to two sets of receivers, EWO for a lower frequency range and SORBET for higher frequencies. In the initial operations after the launch (20 Oct. 2018), we succeeded to confirm the health of both antennas and to release the launch lock of the WPT. After Mercury orbit insertion planned at the end of 2025, both sensors will be fully deployed and activate full operations of the PWI electric field measurements.

Cassini’s Grand Finale orbits provided for the first time in-situ measurements of Saturn’s topside ionosphere. We present the Pedersen and Hall conductivities of the top near-equatorial dayside ionosphere, derived from the in-situ measurements by the Cassini Radio and Wave Plasma Science Langmuir Probe, the Ion and Neutral Mass Spectrometer and the fluxgate magnetometer. The Pedersen and Hall conductivities are constrained to at least 10−5–10−4 S/m at (or close to) the ionospheric peak, a factor 10–100 higher than estimated previously. We show that this is due to the presence of dusty plasma in the near-equatorial ionosphere. We also show the conductive ionospheric region to be extensive, with thickness of 300–800 km. Furthermore, our results suggest a temporal variation (decrease) of the plasma densities, mean ion masses and consequently the conductivities from orbit 288 to 292.

We present new models of Saturn's equatorial ionosphere based on the first in situ measurements of its upper atmosphere. The neutral spectrum measured by Cassini's Ion and Neutral Mass Spectrometer, which includes substantial methane, ammonia, and organics in addition to the anticipated molecular hydrogen, helium, and water, serves as input for unexpectedly complex ionospheric chemistry. Heavy molecular ions are found to dominate Saturn's equatorial low‐altitude ionosphere, with a mean ion mass of 11 Da. Key molecular ions include H3O+ and HCO+; other abundant heavy ions depend upon the makeup of the mass 28 neutral species, which cannot be uniquely determined. Ion and Neutral Mass Spectrometer neutral species lead to generally good agreement between modeled and observed plasma densities, though poor reproduction of measured H+ and H3+ variability and an overabundance of modeled H3+ potentially hint at missing physical processes in the model, including a loss process that affects H3+ but not H+.

The role of Z-mode emission in the diffusive scattering and resonant acceleration of electrons is believed to be important at Saturn. A survey of the "5 kHz" component of this emission at Saturn earlier reported strong intensity in the lower density regions where the ratio of plasma frequency to cyclotron frequency, fp/fc < 1. At Saturn this occurs along the inner edge of the Enceladus torus near the equator and at higher latitudes. Using the Cassini radio and plasma wave science (RPWS) instrument observations during the Cassini proximal orbits we have now identified these emissions extending down to and within the ionosphere. Wave polarization measurements and unique frequency cutoffs are used to positively identify the wave mode. Analogous to the role of whistler mode chorus at Earth, Saturn Z-mode emissions may interact with electrons contributing to the filling or depleting of Saturn's inner radiation belts.

We report on the electron, ion, and dust number densities and the electron temperatures obtained by the Radio and Plasma Wave Science instruments onboard Cassini during the Ring-Grazing orbits. The ...

During the Ring Grazing orbits near the end of Cassini mission, the spacecraft crossed the equatorial plane near the orbit of Janus/Epimetheus (similar to 2.5 Rs). This region is populated with dust particles that can be detected by the Radio and Plasma Wave Science (RPWS) instrument via an electric field antenna signal. Analysis of the voltage waveforms recorded on the RPWS antennas provides estimations of the density and size distribution of the dust particles. Measured RPWS profiles, fitted with Lorentzian functions, are shown to be mostly consistent with the Cosmic Dust Analyzer, the dedicated dust instrument on board Cassini. The thickness of the dusty ring varies between 600 and 1,000 km. The peak location shifts north and south within 100 km of the ring plane, likely a function of the precession phase of Janus orbit.

International audience; Effects of solar EUV on positive ions and heavy negative charge carriers (molecular ions, aerosol, and/or dust) in Titan's ionosphere are studied over the course of almost 12 years, including 78 flybys below 1400 km altitude between TA (October 2004) and T120 (June 2016). The Radio and Plasma Wave Science/Langmuir Probe-measured ion charge densities (normalized by the solar zenith angle) show statistically significant variations with respect to the solar EUV flux. Dayside charge densities increase by a factor of ≈2 from solar minimum to maximum, while nightside charge densities are found to anticorrelate with the EUV flux and decrease by a factor of ≈3-4. The overall EUV dependence of the ion charge densities suggest inapplicability of the idealized Chapman theory below 1200 km in Titan's ionosphere. Nightside charge densities are also found to vary along Titan's orbit, with higher values in the sunward magnetosphere of Saturn compared to the magnetotail.

Using a particle-in-cell electrostatic simulation, we examine the conditions that allow low energy ions, like those produced in the Enceladus plume, to be attracted and trapped within the sheaths of negatively-charged dust grains. The conventional wisdom is that all new ions produced in the Enceladus plume are free to get picked up (i.e., accelerated by the local E-field to then undergo vB acceleration). However, we suggest herein that the presence of submicron charged dust in the plume impedes this pickup process since the local grain electric field greatly exceeds the co-rotation E-fields. The simulations demonstrate that cold ions will tend to accelerate toward the negatively charged grains and become part of the ion plasma sheath. These trapped ions will move with the grains, exiting the plume region at the dust speed. We suggest that Cassini's Langmuir probe is measuring the entire ion population (free and trapped ions), while the Cassini magnetometer detects the magnetic perturbations associated with pickup currents from the smaller population of free ions, with this distinction possibly reconciling the ongoing debate in the literature on the ion density in the plume.

Cassini enters Saturn's ionosphere The upper reaches of most planetary atmospheres contain a layer that is ionized by incoming solar radiation—the ionosphere. As it went through its final orbits around Saturn, the Cassini spacecraft dipped close enough to the planet to pass directly through the ionosphere. Wahlund et al. examined the plasma data collected in situ and found that Saturn's ionosphere is highly variable and interacts with the planet's inner ring. They also observed decreases in ionization within regions shaded from the Sun by the rings. Science , this issue p. 66

We present the electron density (n(e)) altitude profiles of Saturn's ionosphere at near-equatorial latitudes from all 23 orbits of Cassini's Grand Finale. The data are collected by the Langmuir pro ...

In 2017 the Cassini Orbiter made the first in situ measurements of the upper atmosphere and ionosphere of Saturn. The Ion and Neutral Mass Spectrometer in its ion mode measured densities of light i ...

Saturn's magnetospheric magnetic field, planetary radio emissions, plasma populations and magnetospheric structure are all known to be modulated at periods close to the assumed rotation period of the planetary interior. These oscillations are readily apparent despite the high degree of axi-symmetry in the internally produced magnetic field of the planet, and have different rotation periods in the northern and southern hemispheres. In this paper we study the spatial structure of (near-) planetary period magnetic field oscillations in Saturn's equatorial magnetosphere. Extending previous analyses of these phenomena, we include all suitable data from the entire Cassini mission during its orbital tour of the planet, so as to be able to quantify both the amplitude and phase of these field oscillations throughout Saturn's equatorial plane, to distances of 30 planetary radii. We study the structure of these field oscillations in view of both independently rotating northern and southern systems, finding spatial variations in both magnetic fields and inferred currents flowing north-south that are common to both systems. With the greatly expanded coverage of the equatorial plane achieved during the latter years of the mission, we are able to present a complete survey of dawn-dusk and day-night asymmetries in the structure of the oscillating fields and currents. We show that the general structure of the rotating currents is simpler than previously reported, and that the relatively enhanced nightside equatorial fields and currents are due in part to related periodic vertical motion of Saturn's magnetotail current sheet.

Recent results from the MAVEN Langmuir Probe and Waves (LPW) instrument suggest higher than predicted electron temperatures (T sub e) in Mars dayside ionosphere above approx. 180 km in altitude. Correspondingly, measurements from Neutral Gas and Ion Mass Spectrometer (NGIMS) indicate significant abundances of O2+ up to approx. 500 km in altitude, suggesting that O2+ may be a principal ion loss mechanism of oxygen. In this article, we investigate the effects of the higher T(sub e) (which results from electron heating) and ion heating on ion outflow and loss. Numerical solutions show that plasma processes including ion heating and higher T(sub e) may greatly increase O2+ loss at Mars. In particular, enhanced T(sub e) in Mars ionosphere just above the exobase creates a substantial ambipolar electric field with a potential (e) of several k(sub b)T(sub e), which draws ions out of the region allowing for enhanced escape. With active solar wind, electron and ion heating, direct O2+ loss could match or exceed loss via dissociative recombination of O2+. These results suggest that direct loss of O2+ may have played a significant role in the loss of oxygen at Mars over time.

International audience; We present new results obtained by the Radio and Plasma Wave Science Langmuir probe on board Cassini during the Grand Finale. The total direct current sampled by the Langmuir probe at negative bias voltage is used to study the effect of the ring shadows on the structure of the Kronian topside ionosphere. The D and C rings and the Cassini Division are confirmed to be optically thin to extreme ultraviolet solar radiation. However, different responses from the opaque A and B rings are observed. The edges of the A ring shadow are shown to match the A ring boundaries, unlike the B ring, which indicates variable responses to the B ring shadow. We show that the variable responses are due to the ionospheric plasma, more precisely to the longer chemical lifetime of H+ compared to H2+ and H3+ suggesting that the plasma is transported from the sunlit region to the shadowed one in the ionosphere.

Plain Language Summary Cassini flew through the gap between Saturn and its rings for 22 times before plunging into the atmosphere of Saturn, ending its 20-year mission. The radio and plasma waves instrument on board Cassini helped quantify the dust hazard in this previously unexplored region. The measured density of large dust particles was much lower than expected, allowing high-value science observations during the subsequent Grand Finale orbits.

We report on a set of clear and abrupt decreases in the high-frequency boundary of whistler mode emissions detected by Cassini at high latitudes (about +/- 40 degrees) during the low-altitude proximal flybys of Saturn. These abrupt decreases or dropouts have start and stop locations that correspond to L shells at the edges of the A and B rings. Langmuir probe measurements can confirm, in some cases, that the abrupt decrease in the high-frequency whistler mode boundary is associated with a corresponding abrupt electron density dropout over evacuated field lines connected to the A and B rings. Wideband data also reveal electron plasma oscillations and whistler mode cutoffs consistent with a low-density plasma in the region. The observation of the electron density dropout along ring-connecting field lines suggests that strong ambipolar forces are operating, drawing cold ionospheric ions outward to fill the flux tubes. There is an analog with the refilling of flux tubes in the terrestrial plasmasphere. We suggest that the ring-connected electron density dropouts observed between 1.1 and 1.3 R-s are connected to the low-density ring plasma cavity observed overtop the A and B rings during the 2004 Saturn orbital insertion pass. Plain Language Summary We present Cassini observations during the close passes by the planet Saturn indicating that plasma on magnetic field lines that pass through the A and B rings is of anomalously low density. These observations are consistent with the Saturn orbit insertion observations of a plasma cavity located at equatorial regions overtop the dense B ring. Using a terrestrial analogy, we suggest that the low-density conditions overtop the rings create an electrical force, called an ambipolar electric field that draws plasma out of the ionosphere in an attempt to replenish the plasma void found at equatorial regions.

International audience; In its Grand Finale phase, Cassini traversed a region of the Saturnian system not explored in the preceding 12 years in orbit. These high inclination, highly eccentric orbits took Cassini through the source regions of Saturn kilometric radiation (SKR) on auroral magnetic eld lines, across eld lines that tie the planet to its magnetosphere, the ring system, and through the topside ionosphere near the sub- solar equator just below the ring system. The Radio and Plasma Wave Science (RPWS) instrument studied the conditions in the SKR source region that had only been traversed twice in the entire preceding mission. Near 5 kHz intense narrowband emissions are observed in the Z mode at latitudes above about 10°. Plasma wave phenomena known as VLF saucers were observed on eld lines threading both Enceladus and the ring system, providing evidence of electron beams and quite possibly currents connecting these members of the Saturnian system to the planet. The RPWS found only very small numbers of micronsized dust grains in the region between the rings and the atmosphere. Perhaps some of the most important measurements were of plasma densities and temperatures in Saturn’s equatorial topside ionosphere, providing important information for understanding how the ring system and the ionosphere interact. These observations revealed small- scale structures in the ionospheric densities and large-scale asymmetries associated with ring shadowing. The ionosphere revealed a new plasma wave phenomenon apparently driven by a lower hybrid instability. This paper will summarize evidence of coupling between Saturn, the rings, and the more distant magnetosphere a orded by Cassini’s Grand Finale orbits.

Cassini's final phase of exploration The Cassini spacecraft spent 13 years orbiting Saturn; as it ran low on fuel, the trajectory was changed to sample regions it had not yet visited. A series of orbits close to the rings was followed by a Grand Finale orbit, which took the spacecraft through the gap between Saturn and its rings before the spacecraft was destroyed when it entered the planet's upper atmosphere. Six papers in this issue report results from these final phases of the Cassini mission. Dougherty et al. measured the magnetic field close to Saturn, which implies a complex multilayer dynamo process inside the planet. Roussos et al. detected an additional radiation belt trapped within the rings, sustained by the radioactive decay of free neutrons. Lamy et al. present plasma measurements taken as Cassini flew through regions emitting kilometric radiation, connected to the planet's aurorae. Hsu et al. determined the composition of large, solid dust particles falling from the rings into the planet, whereas Mitchell et al. investigated the smaller dust nanograins and show how they interact with the planet's upper atmosphere. Finally, Waite et al. identified molecules in the infalling material and directly measured the composition of Saturn's atmosphere. Science , this issue p. eaat5434 , p. eaat1962 , p. eaat2027 , p. eaat3185 , p. eaat2236 , p. eaat2382

INTRODUCTION Planetary auroral radio emissions are powerful nonthermal cyclotron radiation produced by magnetized planets. Their remote observation provides information on planetary auroral processes and magnetospheric dynamics. Understanding how they are generated requires in situ measurements from within their source region. During its early 2008 high-inclination orbits, the Cassini spacecraft unexpectedly sampled two local sources of Saturn’s kilometric radiation (SKR) at 10-kHz frequencies, corresponding to the low-frequency (LF) portion of its 1- to 1000-kHz typical spectrum (hence only observable from space). These provided insights into the underlying physical excitation mechanism. The combined analysis of radio, plasma, and magnetic field in situ measurements demonstrated that the cyclotron maser instability (CMI), which generates auroral kilometric radiation (AKR) at Earth, is a universal generation mechanism able to operate in widely different planetary plasma environments. The CMI requires accelerated (out-of-equilibrium) electrons and low-density magnetized plasma, such as auroral regions where the ratio of the electron plasma frequency fpe to the electron cyclotron frequency fce is much lower than unity. RATIONALE Intensifications of the SKR spectrum in general, and of its LF part in particular, have been widely used as a diagnostic of Saturn’s large-scale magnetospheric dynamics, such as the SKR rotational modulation or major auroral storms driven by the solar wind. However, the limited set of events encountered in 2008 did not provide a comprehensive picture of the source of SKR LF emissions and of the conditions in which the CMI can trigger them. During the 20 ring-grazing high-inclination orbits and the preceding 7 orbits, which together spanned late 2016 to early 2017, the Cassini spacecraft repeatedly sampled the top of the SKR emission region, at distances of a few planetary radii (RS), corresponding to the lowest-frequency part of the SKR spectrum. SKR emission frequency is close to fce, itself proportional to the magnetic field amplitude, and so decreases with increasing distance from the planet. We conducted a survey of these orbits to extract the average properties of the radio sources encountered and assess the ambient magnetospheric plasma parameters that control them. RESULTS Throughout this set of trajectories, we were able to identify only three SKR sources. They covered the 10- to 20-kHz range (3.5 to 4.5 RS distances from the planet’s center) and were solely found on the northern dawn-side sector. The source regions hosted narrow-banded emission, propagating in the extraordinary wave mode, and radiated quasi-perpendicularly to the magnetic field lines. Their emission frequency, measured in situ, was fully consistent with the CMI mechanism driven by 6- to 12-keV electron beams with shell-type velocity distribution functions, as for AKR at Earth. The SKR sources were embedded within larger regions of upward currents, themselves coincident with the ultraviolet (UV) auroral oval, which was observed simultaneously with the Hubble Space Telescope. However, unlike the terrestrial case, the spacecraft exited the radio source region before exiting the main oval, itself strictly coincident with the upward current layer, when the ratio fpe/fce exceeded the typical CMI threshold of 0.1. This occurred at times of sudden local increases of the magnetospheric electron density. CONCLUSION The generation conditions of SKR LF emission appear to be strongly time-variable. The SKR spectrum additionally displays a significant local time dependence, with the lowest frequencies (the highest altitudes) reached for dawn-side radio sources. The characteristics of CMI-unstable electrons at 3.5 to 4.5 RS imply that downward electron acceleration took place at farther distances along the auroral magnetic field lines and brings new constraints to particle acceleration models. Finally, the LF SKR component is mainly controlled by local plasma conditions, namely time-variable magnetospheric electron densities, which can quench the CMI mechanism even if accelerated electrons are present. This explains why the magnetic field lines hosting SKR LF sources can map to a restricted portion of the UV auroral oval and associated upward current region.

We present Mars' electron temperature (Te) and density (ne) altitude profiles derived from the MAVEN (Mars Atmosphere and Volatile EvolutioN) mission deep dip orbits in April 2015, as measured by the Langmuir probe instrument. These orbits had periapsides below 130 km in altitude at low solar zenith angles. The periapsides were above the peak in ne during this period. Using a Chapman function fit, we find that scale height and projected altitude of the ne peak are consistent with models and previous measurements. The peak electron density is slightly higher than earlier works. For the first time, we present in situ measurements of Te altitude profiles in Mars' dayside in the altitude range from ~130 km to 500 km and provide a functional fit. Importantly, Te rises rapidly with altitude from ~180 km to ~300 km. These results and functional fit are important for modeling Mars' ionosphere and understanding atmospheric escape.

The first in situ nightside electron density and temperature profiles at Mars are presented as functions of altitude and local time (LT) from the Langmuir Probe and Waves (LPW) instrument on board the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission spacecraft. LPW is able to measure densities as low as similar to 100 cm(-3), a factor of up to 10 or greater improvement over previous measurements. Above 200 km, near-vertical density profiles of a few hundred cubic centimeters were observed for almost all nightside LT, with the lowest densities and highest temperatures observed postmidnight. Density peaks of a few thousand cubic centimeters were observed below 200 km at all nightside LT. The lowest temperatures were observed below 180 km and approach the neutral atmospheric temperature. One-dimensional modeling demonstrates that precipitating electrons were able to sustain the observed nightside ionospheric densities below 200 km.

We use data from several instruments on board Cassini to determine the characteristics of the plasma and dust regions around Saturn׳s moon Enceladus. For this we utilize the Langmuir probe and the electric antenna connected to the wideband receiver of the radio and plasma wave science (RPWS) instrument package as well as the magnetometer (MAG). We show that there are several distinct plasma and dust regions around Enceladus. Specifically they are the plume filled with neutral gas, plasma, and charged dust, with a distinct edge boundary region. Here we present observations of a new distinct plasma region, being a dust trail on the downstream side. This is seen both as a difference in ion and electron densities, indicating the presence of charged dust, and directly from the signals created on RPWS antennas by the dust impacts on the spacecraft. Furthermore, we show a very good scaling of these two independent dust density measurement methods over four orders of magnitude in dust density, thereby for the first time cross-validating them. To establish equilibrium with the surrounding plasma the dust becomes negatively charged by attracting free electrons. The dust distribution follows a simple power law and the smallest dust particles in the dust trail region are found to be 10 nm in size as well as in the edge region around the plume. Inside the plume the presence of even smaller particles of about 1 nm is inferred. From the magnetic field measurements we infer strong field-aligned currents at the geometrical edge of Enceladus.

This article reviews the different sources of plasma for Saturn’s magnetosphere, as they are known essentially from the scientific results of the Cassini-Huygens mission to Saturn and Titan. At low and medium energies, the main plasma source is the \(\mathrm{H}_{2}\mathrm{O}\) cloud produced by the “geyser” activity of the small satellite Enceladus. Impact ionization of this cloud occurs to produce on the order of 100 kg/s of fresh plasma, a source which dominates all the other ones: Titan (which produces much less plasma than anticipated before the Cassini mission), the rings, the solar wind (a poorly known source due to the lack of quantitative knowledge of the degree of coupling between the solar wind and Saturn’s magnetosphere), and the ionosphere. At higher energies, energetic particles are produced by energy diffusion and acceleration of lower energy plasma produced by the interchange instabilities induced by the rapid rotation of Saturn, and possibly, for the highest energy range, by contributions from the CRAND process acting inside Saturn’s magnetosphere. Discussion of the transport and acceleration processes acting on these plasma sources shows the importance of rotation-induced radial transport and energization of the plasma, and also shows how much the unexpected planetary modulation of essentially all plasma parameters of Saturn’s magnetosphere remains an unexplained mystery.

Enceladus' southern plume is one of the major discoveries of the Cassini mission. The water neutrals and water ice particles (dust) ejected by the cryovolcanic activity populate Saturn's E ring and the neutral torus, and they interact with the plasma environment of Saturn's magnetosphere. The plasma neutrality inside Enceladus' plume has been shown by the Langmuir probe measurement to be modified by the presence of the dust particles. We present an independent method of determining the electron density inside the plume. Sometimes, after dust impacts, plasma oscillations (ringing) were detected by the Cassini Radio and Plasma Wave Science instrument. The frequencies of these oscillations have been shown to be consistent with the local plasma frequency, thus providing a measurement of the local electron density.

We present statistical results from the Cassini Radio and Plasma Wave Science (RPWS) Langmuir probe measurements recorded during the time interval from orbit 3 (1 February 2005) to 237 (29 June 2016). A new and improved data analysis method to obtain ion density from the Cassini LP measurements is used to study the asymmetries and modulations found in the inner plasma disk of Saturn, between 2.5 and 12 Saturn radii (1 R S = 60, 268 km). The structure of Saturn's plasma disk is mapped, and the plasma density peak, n max , is shown to be located at ∼4.6 R S and not at the main neutral source region at 3.95 R S. The shift in the location of n max is due to that the hot electron impact ionization rate peaks at ∼4.6 R S. Cassini RPWS plasma disk measurements show a solar cycle modulation. However, estimates of the change in ion density due to varying EUV flux is not large enough to describe the detected dependency, which implies that an additional mechanism, still unknown, is also affecting the plasma density in the studied region. We also present a dayside/nightside ion density asymmetry, with nightside densities up to a factor of 2 larger than on the dayside. The largest density difference is found in the radial region 4 to 5 R S. The dynamic variation in ion density increases toward Saturn, indicating an internal origin of the large density variability in the plasma disk rather than being caused by an external source origin in the outer magnetosphere.

The ion bulk speeds in the equatorial region of Saturn’s inner magnetosphere, according to data from the Langmuir Probe (LP) on board the Cassini spacecraft, are about 60% of the ideal co-rotation speed; the ion speeds are between the co-rotation and Keplerian speeds (Holmberg et al., Ion densities and velocities in the inner plasma torus of Saturn, Planetary and Space Science). These findings suggest that sub-micrometer negatively charged E ring dust contributes to the plasma dynamics in the plasma disk. We calculated the ion speeds by using a multi-species fluid model, taking into account dust interactions to investigate the effects of ion–dust coulomb collision, mass loading, as well as taking into account magnetosphere–ionosphere coupling to investigate the effect of the magnetospheric electric field. The results show that the ion speeds can be significantly reduced by the electric fields generated by the collisions between ions and dusts when the dust density is high and the thickness of dust distribution is large. We also show that the ion speeds from our model are consistent with the LP observations when the maximum density of dust is larger than ∼10 5 m −3 .

We present plasma data from the Cassini Radio and Plasma Wave Science (RPWS) Langmuir probe (LP), mapping the ion density and velocity of Saturn's inner plasma torus. Data from 129 orbits, recorded during the period from the 1st of February 2005 to the 27th of June 2010, are used to map the extension of the inner plasma torus. The dominant part of the plasma torus is shown to be located in between 2.5 and 8 Saturn radii (1 RS=60,268 km) from the planet, with a north-southward extension of ± 2 R S . The plasma disk ion density shows a broad maximum in between the orbits of Enceladus and Tethys. Ion density values vary between 20 and 125 cm−3 at the location of the density maximum, indicating considerable dynamics of the plasma disk. The equatorial density structure, | z | 0.5 R S , shows a slower decrease away from the planet than towards. The outward decrease, from 5 RS, is well described by the relation n eq = 2.2 × 10 4 ( 1 / R ) 3.63 . The plume of the moon Enceladus is clearly visible as an ion density maximum of 105 cm−3, only present at the south side of the ring plane. A less prominent density peak, of 115 cm−3, is also detected at the orbit of Tethys, at ∼ 4.9 R S . No density peaks are recorded at the orbits of the moons Mimas, Dione, and Rhea. The presented ion velocity v i , θ shows a clear general trend in the region between 3 and 7 RS, described by v i , θ = 1.5 R 2 − 8.7 R + 39 . The average v i , θ starts to deviate from corotation at around 3 RS, reaching ∼ 68 % of corotation close to 5 RS.

This article reviews the different sources of plasma for Saturn’s magnetosphere, as they are known essentially from the scientific results of the Cassini-Huygens mission to Saturn and Titan. At low and medium energies, the main plasma source is the \(\mathrm{H}_{2}\mathrm{O}\) cloud produced by the “geyser” activity of the small satellite Enceladus. Impact ionization of this cloud occurs to produce on the order of 100 kg/s of fresh plasma, a source which dominates all the other ones: Titan (which produces much less plasma than anticipated before the Cassini mission), the rings, the solar wind (a poorly known source due to the lack of quantitative knowledge of the degree of coupling between the solar wind and Saturn’s magnetosphere), and the ionosphere. At higher energies, energetic particles are produced by energy diffusion and acceleration of lower energy plasma produced by the interchange instabilities induced by the rapid rotation of Saturn, and possibly, for the highest energy range, by contributions from the CRAND process acting inside Saturn’s magnetosphere. Discussion of the transport and acceleration processes acting on these plasma sources shows the importance of rotation-induced radial transport and energization of the plasma, and also shows how much the unexpected planetary modulation of essentially all plasma parameters of Saturn’s magnetosphere remains an unexplained mystery.