Cassini Langmuir Probe Research Articles

A simple spacecraft – vector intersection methodology and applications

Abstract Observations with spacecraft-mounted instruments are usually limited by their field-of-view and are often affected by the spacecraft's shadow or wake. Their extent though can be derived from the spacecraft's geometry. In this work we present a robust method for calculating the field-of-view as well as the extent of a spacecraft shadow and wake from readily available spacecraft CAD models. We demonstrate these principles on Cassini, where we give examples of vector-spacecraft intersection for the Cassini Langmuir Probe, as well the field-of-view of the Langmuir Probe and the Cassini Plasma Spectrometer.

Implications from secondary emission from neutral impact on Cassini plasma and dust measurements

ABSTRACT We investigate the role of secondary electron and ion emission from impact of gas molecules on the Cassini Langmuir probe (RPWS-LP or LP) measurements in the ionosphere of Saturn. We add a model of the emission currents, based on laboratory measurements and data from comet 1P/Halley, to the equations used to derive plasma parameters from LP bias voltage sweeps. Reanalysing several hundred sweeps from the Cassini Grand Finale orbits, we find reasonable explanations for three open conundrums from previous LP studies of the Saturn ionosphere. We find an explanation for the observed positive charging of the Cassini spacecraft, the possibly overestimated ionospheric electron temperatures, and the excess ion current reported. For the sweeps analysed in detail, we do not find (indirect or direct) evidence of dust having a significant charge-carrying role in Saturn’s ionosphere. We also produce an estimate of H2O number density from the last six revolutions of Cassini through Saturn’s ionosphere in greater detail than reported by the Ion and Neutral Mass Spectrometer. Our analysis reveals an ionosphere that is highly structured in latitude across all six final revolutions, with mixing ratios varying with two orders of magnitude in latitude and one order of magnitude between revolutions and altitude. The result is generally consistent with an empirical photochemistry model balancing the production of H+ ions with the H+ loss through charge transfer with e.g. H2O, CH4, and CO2, for which water vapour appears as the likeliest dominant source of the signal in terms of yield and concentration.

Re‐Analysis of the Cassini RPWS/LP Data in Titan's Ionosphere: 1. Detection of Several Electron Populations

AbstractCurrent models of Titan's ionosphere have difficulties in explaining the observed electron density and/or temperature. In order to get new insights, we re‐analyzed the data taken in the ionosphere of Titan by the Cassini Langmuir probe (LP), part of the Radio and Plasma Wave Science (RPWS) instrument. This is the first of two papers that present the new analysis method (current paper) and statistics on the whole data set. We suggest that between two and four electron populations are necessary to fit the data. Each population is defined by a potential, an electron density and an electron temperature and is easily visualized by a distinct peak in the second derivative of the electron current, which is physically related to the electron energy distribution function (Druyvesteyn method). The detected populations vary with solar illumination and altitude. We suggest that the four electron populations are due to photo‐ionization, magnetospheric particles, dusty plasma and electron emission from the probe boom, respectively.

Cassini‐Plasma Interaction Simulations Revealing the Cassini Ion Wake Characteristics: Implications for In‐Situ Data Analyses and Ion Temperature Estimates

AbstractWe 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.

Re‐Analysis of the Cassini RPWS/LP Data in Titan's Ionosphere: 2. Statistics on 57 Flybys

AbstractThe ionosphere of Titan hosts a complex ion chemistry leading to the formation of organic dust below 1,200 km. Current models cannot fully explain the observed electron temperature in this dusty environment. To achieve new insight, we have re‐analyzed the data taken in the ionosphere of Titan by the Cassini Langmuir probe (LP), part of the Radio and Plasma Wave Science package. A first paper (Chatain et al., 2021) introduces the new analysis method and discusses the identification of four electron populations produced by different ionization mechanisms. In this second paper, we present a statistical study of the whole LP dataset below 1,200 km which gives clues on the origin of the four populations. One small population is attributed to photo‐ or secondary electrons emitted from the surface of the probe boom. A second population is systematically observed, at a constant density (∼500 cm−3), and is attributed to background thermalized electrons from the ionization process of precipitating particles from the surrounding magnetosphere. The two last populations increase in density with pressure, solar illumination and Extreme ultraviolet flux. The third population is observed with varying densities at all altitudes and solar zenith angles (SZA) except on the far nightside (SZA > ∼140°), with a maximum density of 2,700 cm−3. It is therefore certainly related to the photo‐ionization of the atmospheric molecules. Finally, a fourth population detected only on the dayside and below 1,200 km reaching up to 2000 cm−3 could be photo‐ or thermo‐emitted from dust grains.

Ion Trapping by Dust Grains: Simulation Applications to the Enceladus Plume.

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.

Ion energy distributions and densities in the plume of Enceladus

Enceladus has a dynamic plume that is emitting gas, including water vapor, and dust. The gas is ionized by solar EUV radiation, charge exchange, and electron impact and extends throughout the inner magnetosphere of Saturn. The charge exchange collisions alter the plasma composition. Ice grains (dust) escape from the vicinity of Enceladus and form the E ring, including a portion that is negatively charged by the local plasma. The inner magnetosphere within 10RS (Saturn radii) contains a complex mixture of plasma, neutral gas, and dust that links back to Enceladus. In this paper we investigate the energy distributions, ion species and densities of water group ions in the plume of Enceladus using test particle and Monte Carlo methods that include collisional processes such as charge exchange and ion-neutral chemical reactions. Ion observations from the Cassini Ion and Neutral Mass Spectrometer (INMS) for E07 are presented for the first time. We use the modeling results to interpret observations made by the Cassini Plasma Spectrometer (CAPS) and the INMS. The low energy ions, as observed by CAPS, appear to be affected by a vertical electric field (EZ=−10µV/m) in the plume. The EZ field may be associated with the charged dust and/or the pressure gradient of plasma. The model results, along with the results of earlier models, show that H3O+ ions created by chemistry are predominant in the plume, which agrees with INMS and CAPS data, but the INMS count rate in the plume for the model is several times greater than the data, which we do not fully understand. This composition and the total ion count found in the plume agree with INMS and CAPS data. On the other hand, the Cassini Langmuir Probe measured a maximum plume ion density more than 30,000cm−3, which is far larger than the maximum ion density from our model, 900cm−3. The model results also demonstrate that most of the ions in the plume are from the external magnetospheric flow and are not generated by local ionization. The origin of the ions in the plume was investigated using two different velocity models. Most ions were created by the interaction with background magnetospheric plasma and by photoionization. INMS and CAPS also detected water cluster ions. We will interpret these observations as a result of ion collisions with neutral water clusters, (H2O)n, originating in the tiger stripe vents as suggested by Tokar et al. (2009). We also estimated the process of generating cluster ions based on the INMS observations. We suggest that the most likely source is reaction of H3O+ with neutral water clusters or dimers such as (H2O)2 formed in the plume vents.

Dusty plasma of the Enceladus plume

Geological active south-pole fractures of the icy moon Enceladus produce a specific region, a so-called plume that extends up to 1000 km into space. The main constituents of the plume are electrons, ions, water molecules, and charged nanograins, which are responsible for the unusual properties of this kind of low-temperature dusty plasma. The examples discussed represent our results in this field within the last few years, and mostly such cases were chosen, in which measurements of different Cassini instruments could be compared with theory or where the phenomenon has a diagnostic application. Dust charging, role of dust size distributions and dust charge fluctuations are discussed. The findings are used for interpretations of the Cassini Plasma Spectrometer data and improve constraints on the dust characteristics. Then the main forces and dust dynamics are discussed in conditions relevant for the near-Enceladus plasma. It is also examined how the charged dust can affect the plasma shielding length which is of importance for the reliable Cassini Langmuir probe measurements. Considering the dust grains as heavy negative ion species, the electric conductivity tensor is modified, whose elements are the key quantities for understanding the magnetic field perturbations registered by the Cassini Magnetometer during Enceladus flybys.

Particle-in-cell simulation of spacecraft/plasma interactions in the vicinity of Enceladus

The Cassini Langmuir Probe of the Radio and Plasma Wave Science instrument has measured an electron depletion in a region extending at least 50 satellite radii away from Saturn’s small but geologically active icy moon Enceladus. The maximal imbalance between the electron and ion densities was observed in the dust loaded plume and to date is attributed to the electron attachment to abundant dust grains. We report the results from a three dimensional particle-in-cell simulation of a plasma structure formed around a charged spacecraft in the conditions relevant inside the Enceladus torus and in the moon’s plume. In addition to the plasma population the plume simulation includes singly charged nanograins detected by the Cassini Plasma Spectrometer. The accompanied spacecraft plasma perturbations can significantly modify an ambient plasma at the Cassini Langmuir probe positions and thus impact the plasma measurements. Our modeling reveals a domination of water group ions over the electron population due to the formation of a conventional plasma sheath at the ram-oriented probe positions in the Enceladus torus and in the plume regions with low dust density (nd0<ne0). In the dust-dominated plume (nd0>ne0) the plasma perturbations are strongly reduced in the ram direction but can significantly compromise the probe measurements in the orbiter wake. Simulation results can qualitatively explain the long profiles of the electron–ion imbalance registered by the Cassini Langmuir probe during the flybys E2, E3 and E5. In either case, the plasma perturbations associated with the moving Cassini orbiter appear to be important for reliable interpretations of the Langmuir probe measurements.

Identification of when a Langmuir probe is in the sheath of a spacecraft: The effects of secondary electron emission from the probe

AbstractLangmuir probes on spacecraft have been used for characterizing the ambient plasma parameters in space. When their boom is short compared to the Debye length, the probes remain immersed in the spacecraft sheath, causing the current‐voltage (I‐V) characteristics to deviate from that of a probe far away from the spacecraft. We present identification of when a Langmuir probe is in a sheath, based on the secondary electron (SE) emission from the probe itself. The I‐V characteristics of a spherical probe are investigated in a plasma sheath above a conducting plate. Plasmas with cold and hot electrons (1 eV and 10 eV), as well as monoenergetic electrons (50–100 eV), are created. The derivative (dI/dV) of the probe I‐V curves shows that in addition to a “knee” at a potential more positive than the plasma potential, an additional knee appears at a sheath potential at the probe location. This additional knee is created due to the SE emission from the probe and is identified as an indication of the probe being immersed in the sheath. Our experimental results reproduced the aspects of the Cassini Langmuir probe I‐V characteristics, suggesting that at times, the probe may have been immersed in the sheath of the spacecraft in Saturn's magnetosphere, and SE emission from the probe itself may have significantly altered its I‐V characteristics.

Deriving the characteristics of warm electrons (100–500 eV) in the magnetosphere of Saturn with the Cassini Langmuir probe

Though Langmuir probes (LP) are designed to investigate cold plasma regions (e.g. ionospheres), a recent analysis revealed a strong sensitivity of the Cassini LP measurements to hundreds of eV electrons. These warm electrons impact the surface of the probe and generate a significant current of secondary electrons, that impacts both the DC level and the slope of the current–voltage curve of the LP (for negative potentials) through energetic contributions that may be modeled with a reasonable precision.We show here how to derive information about the incident warm electrons from the analysis of these energetic contributions, in the regions where the cold plasma component is small with an average temperature in the range ~[100–500]eV. First, modeling the energetic contributions (based on the incident electron flux given by a single anode of the CAPS spectrometer) allows us to provide information about the pitch angle anisotropies of the incident hundreds of eV electrons. The modeling reveals indeed sometimes a large variability of the estimated maximum secondary electron yield (which is a constant for a surface material) needed to reproduce the observations. Such dispersions give evidence for strong pitch angle anisotropies of the incident electrons, and using a functional form of the pitch angle distribution even allows us to derive the real peak angle of the distribution.Second, rough estimates of the total electron temperature may be derived in the regions where the warm electrons are dominant and thus strongly influence the LP observations, i.e. when the average electron temperature is in the range ~[100–500]eV. These regions may be identified from the LP observations through large positive values of the current–voltage slope at negative potentials. The estimated temperature may then be used to derive the electron density in the same region, with estimated densities between ~0.1 and a few particles/cm3 (cc). The derived densities are in better agreement with the CAPS measurements than the values derived from the proxy technique (Morooka et al., 2009) based on the floating potential of the LP. Both the electron temperature and the density estimates lie outside the classical capabilities of the LP, which are essentially ne>5cc and Te<5eV at Saturn. This approximate derivation technique may be used in the regions where the cold plasma component is small with an average temperature in the range ~[100–500]eV, which occurs often in the L range 6.4–9.4 RS when Cassini is off the equator, but may occur anywhere in the magnetosphere. This technique may be all the more interesting since the CAPS instrument was shut down, and, though it cannot replace the CAPS instrument, the technique can provide useful information about the electron moments, with probably even better estimates than CAPS in some cases (when the plasma is strongly anisotropic).Finally, a simple modeling approach allows us to predict the impact of the energetic contributions on LP measurements in any plasma environment whose characteristics (density, temperature, etc.) are known. LP observations may thus be influenced by warm electrons in several planetary plasma regions in the solar system, and ambient magnetospheric electron density and temperature could be estimated in some of them (e.g. around several galilean satellites) through the use of Langmuir probes.

Dust charging in the Enceladus torus

Employing the parameter space, consistent with available Cassini data, dust charging is studied in the near‐Enceladus plasma with special focus on the region close to the moon's plume. The emphasis is on the role of dust size distribution and grain density, the electron temperature, fraction of the hot electrons, and multi‐ion composition including the effect of newborn, nonthermalized ions. The sensitivity of the grain potential to variations in the ambient plasma parameters and in the radiative conditions is examined. It is shown that a significant reduction of the electrical potential and the charging time in the dust‐loaded regions occurs. This can explain the predominance of nanograins with small negative charges ~e in the plume as detected by the Cassini plasma spectrometer during close encounters with Enceladus. The theoretical findings can be of importance for reliable interpretations of the Cassini Langmuir probe measurements.

The influence of the secondary electrons induced by energetic electrons impacting the Cassini Langmuir probe at Saturn

The Cassini Langmuir Probe (LP) onboard the Radio and Plasma Wave Science experiment has provided much information about the Saturnian cold plasma environment since the Saturn Orbit Insertion in 2004. A recent analysis revealed that the LP is also sensitive to the energetic electrons (250–450 eV) for negative potentials. These electrons impact the surface of the probe and generate a current of secondary electrons, inducing an energetic contribution to the DC level of the current‐voltage (I‐V) curve measured by the LP. In this paper, we further investigated this influence of the energetic electrons and (1) showed how the secondary electrons impact not only the DC level but also the slope of the (I‐V) curve with unexpected positive values of the slope, (2) explained how the slope of the (I‐V) curve can be used to identify where the influence of the energetic electrons is strong, (3) showed that this influence may be interpreted in terms of the critical and anticritical temperatures concept detailed by Lai and Tautz (2008), thus providing the first observational evidence for the existence of the anticritical temperature, (4) derived estimations of the maximum secondary yield value for the LP surface without using laboratory measurements, and (5) showed how to model the energetic contributions to the DC level and slope of the (I‐V) curve via several methods (empirically and theoretically). This work will allow, for the whole Cassini mission, to clean the measurements influenced by such electrons. Furthermore, the understanding of this influence may be used for other missions using Langmuir probes, such as the future missions Jupiter Icy Moons Explorer at Jupiter, BepiColombo at Mercury, Rosetta at the comet Churyumov‐Gerasimenko, and even the probes onboard spacecrafts in the Earth magnetosphere.

Extreme densities in Titan's ionosphere during the T85 magnetosheath encounter

AbstractWe present Cassini Langmuir probe measurements of the highest electron number densities ever reported from the ionosphere of Titan. The measured density reached 4310 cm−3 during the T85 Titan flyby. This is at least 500 cm−3 higher than ever observed before and at least 50% above the average density for similar solar zenith angles. The peak of the ionospheric density is not reached on this flyby, making the maximum measured density a lower limit. During this flyby, we also report that an impacting coronal mass ejection (CME) leaves Titan in the magnetosheath of Saturn, where it is exposed to shocked solar wind plasma for at least 2 h 45 min. We suggest that the solar wind plasma in the magnetosheath during the CME conditions significantly modifies Titan's ionosphere by an addition of particle impact ionization by precipitating protons.

The detection of energetic electrons with the Cassini Langmuir probe at Saturn

The Cassini Langmuir probe, part of the Radio and Plasma Wave Science (RPWS) instrument, has provided a wealth of information about the cold and dense plasma in the Saturnian system. The analysis of the ion side current (current for negative potentials) measured by the probe from 2005 to 2008 reveals also a strong sensitivity to energetic electrons (250–450 eV). These electrons impact the surface of the probe, and generate a detectable current of secondary electrons. A broad secondary electrons current region is inferred from the observations in the dipole L Shell range of ∼6–10, with a peak full width at half maximum (FWHM) at L = 6.4–9.4 (near the Dione and Rhea magnetic dipole LShell values). This magnetospheric flux tube region, which displays a large day/night asymmetry, is related to the similar structure in the energetic electron fluxes as the one measured by the onboard Electron Spectrometer (ELS) of the Cassini Plasma Spectrometer (CAPS). It corresponds spatially to both the outer electron radiation belt observed by the Magnetosphere Imaging Instrument (MIMI) at high energies and to the low‐energy peak which has been observed since the Voyager era. Finally, a case study suggests that the mapping of the current measured by the Langmuir probe for negative potentials can allow to identify the plasmapause‐like boundary recently identified at Saturn, and thus potentially identify the separation between the closed and open magnetic field lines regions.

Spacecraft charging near Enceladus

In the vicinity of Enceladus, a geologically active moon of Saturn, the modeled spacecraft potential is significantly more negative than indicated by the Cassini Langmuir probe measurements. To understand this potential difference, we introduce two new dust–related charging currents: a) the dust ram current; and b) the dust impact–plasma current, in addition to the customary collection of electrons and ions, and photoemission. Our results show that these dust currents are important at high relative speeds between the spacecraft and the dust, or in regions with a low plasma‐to‐dust ratio, and can lead to reduced spacecraft charging.

Electron energetics in the Enceladus torus

[1] The inner magnetosphere of Saturn contains both neutral gas and plasma whose composition is dominated by water group ion species. This gas is distributed in a torus-shaped region located near the orbit of Enceladus. The source of this gas is the predominately water vapor plume located in the south polar region of Saturn's icy satellite Enceladus. The electron distribution in the torus comprises two populations: a relatively cold ( 10 eV) suprathermal tail. This paper describes model calculations of the electron energy distribution and energetics in the torus in order to explore how the observed electron distributions can be explained. A thermal electron energy equation is numerically solved, and electron temperatures are calculated. A separate electron energy deposition model determines suprathermal electron fluxes as functions of energy. The main suprathermal electron population is due to photoelectron production from the absorption of solar radiation by water and other neutral species, and this is demonstrated by a comparison of the model results with recently published Cassini data. The model includes heating of thermalized electrons by fresh photoelectrons and by hot pickup ions and also includes cooling by collisions with water and other neutral species. Calculated electron temperatures for the thermal population are about 104 − 2 × 104 K (i.e., 1–2 eV), values which are somewhat lower than recently published Cassini Langmuir probe electron temperature values of 2–3 eV.

Potential structure around the Cassini spacecraft near the orbit of Enceladus

We present the results of numerical simulations of the potential structure around an object in a streaming plasma with parameters relevant for the Cassini spacecraft passing through Saturn’s plasma disk near the orbit of Enceladus. Two- and three-dimensional particle-in-cell codes have been used allowing the potential of the simulated spacecraft body to develop self-consistently through the collection of charge by its surface. The dependence of the density and potential profiles on ambient plasma density, electron temperature, and ion drift speed is discussed. The spacecraft floating potential values, found in the simulations, are compared to those deduced from the analysis of Cassini Langmuir probe characteristics.

Cassini Langmuir probe measurements in the inner magnetosphere of Saturn

In the inner magnetosphere of Saturn, the plasma density and drift velocity are high enough, and the photoelectron current low enough, for a Langmuir probe to produce useful data on ion parameters. Plasma density and velocity are found by analyzing the current due to collected ions and emitted photoelectrons for a negative probe potential. In order to correctly analyze the data, the current caused by photoelectrons emitted from the probe must be known. For a spherical probe at negative bias this should be a constant current, but for Cassini's probe it varies with attitude. A likely cause of this is a leakage current from the stub to the probe. The plasma drift velocities derived from Langmuir probe measurements did not agree with those found by the Cassini plasma spectrometer in the inner magnetosphere, but did so elsewhere. A possible solution to this is a two-component plasma where the components have different drift velocities.