Magnetospheric Imaging Instrument Research Articles

Effect of the Relativistic Electron Beam on Propagating Whistler-Mode Wave for Ring Distribution in the Saturn Magnetosphere

Cassini and many investigators reported whistler chorus near Saturn equatorial plane moving outwards. Whistler can propagate when going to high latitude and can alter its characteristics while interacting resonantly with available energetic electrons. Here investigating wave for a relativistic beam of the electron. It is observed and reported by Cassini Magnetospheric Imaging Instrument (MIMI) that inward radial injection of highly energetic particles is most dominant in Saturn intrinsic magnetosphere. Within this paradigm, an empirical energy dispersion relation for propagated whistler-mode oscillations in quasi Saturn magnetospheric plasma from such a non-monotonous ringed distribution function has been established. The kinetic approach and method of characteristics methodologies were used in the computations, which have been shown to be the best for building perturbed plasma states. The perturbed distribution function was estimated using the unperturbed particle routes. The ring distribution function was used to construct an unexpected growth rate expression for relativistic plasma in the inner magnetosphere. The results from the Saturn magnetosphere have been calculated and interpreted using a range of parameters. Temperature heterogeneity was shown to be a significant source of free energy that aided the propagation of a whistler-mode wave. By raising the peak value, the bulk injection of energetic hot electron injection impacts the growth rate. Growth was also demonstrated to be accelerated when the propagation angle increased. The research contributes to a better understanding of the relationship between wave and particle emissions and VLF emissions on a large scale.

Zebra Stripe Patterns in Energetic Ion Spectra at Saturn

AbstractMagnetospheric convective electric fields that could radially transport particles adiabatically, may leave their imprints in the form of energy‐time dispersed patterns termed ”zebra stripes”. Inspired by comprehensive studies on zebra stripes in Earth's electron radiation belts, similar electron signatures were found and explained in Saturn's inner magnetosphere. Using Cassini Magnetosphere Imaging Instrument observations, we demonstrate that the zebra‐stripes in proton and water group ion spectrograms at the energy range from 30 keV to ∼1 MeV, which coexist with electron zebra‐stripes, can be traced to the same electric field, even though the associated radial flows were originally thought to be too slow to disturb the fast azimuthal energetic ion drift. The simultaneous observations of zebra stripes in electrons and ions further establish the importance of the planet's noon‐midnight electric field as a regulator of radiation belt dynamics.

A Complete Data Set of Equatorial Projections of Saturn's Energetic Neutral Atom Emissions Observed by Cassini‐INCA

AbstractObservations of energetic neutral atoms (ENAs) are a useful tool for analyzing ion and neutral abundances in planetary magnetospheres. They are created when hot plasma, originating for example from magnetic reconnection sites, charge‐exchanges with the ambient neutral population surrounding the planet. The motion of ENAs is not governed by the magnetic field, allowing remote imaging. During the Cassini mission, the Ion and Neutral CAmera (INCA) of the Magnetosphere Imaging Instrument (MIMI) collected vast amounts of hydrogen and oxygen ENA observations of Saturn's magnetosphere from a variety of different viewing geometries. To enable investigations of the morphology and dynamics of Saturn's ring current, it is useful to re‐bin and re‐project the camera‐like views from the spacecraft‐based perspective into a common reference frame. We developed an algorithm projecting INCA's ENA observations into a regular grid in Saturn's equatorial plane. With most neutrals and ions being confined into an equatorial rotating disc, this projection is quite accurate in both spatial location and preservation of ENA intensity, provided the spacecraft is located at large enough elevations. Such projections were performed for all INCA ENA data from the Cassini Saturn tour; the data are available for download together with a Python routine flagging contaminated data and returning detailed spacecraft geometry information. The resulting data set is a good foundation for investigating for example the statistical properties of Saturn's ring current and its complicated dynamics in relation to other remote and in situ observations of, for example, auroral emissions and magnetotail reconnection events.

Dawn‐Dusk Asymmetry in Energetic (>20 keV) Particles Adjacent to Saturn's Magnetopause

AbstractEnergetic particles (>∼25 keV) have been observed routinely in the terrestrial magnetosheath, but have not been well studied at the magnetosheaths of the outer planets. Here we analyze energetic electrons and ions (mostly protons) in the vicinity (±1 RS) of Saturn's magnetopause, using particle data acquired with the low‐energy magnetosphere measurements system, one of the three sensors of the magnetosphere imaging instrument on board the Cassini spacecraft, during a period of ∼14 years (2004–2017). It is found that energetic particles, especially ions, are also common in Saturn's magnetosheath. A clear inward (toward Saturn) gradient in the electron differential flux is identified, suggestive of magnetospheric sources. Such an inward gradient does not appear in some of the ion channels. We conclude that Saturn's magnetopause acts as a porous barrier for energetic electrons and, to a lesser extent, for energetic ions. A dawn‐dusk asymmetry in the gradient of particle flux across the magnetopause is also identified, with a gradual decrease at the dawn and a sharp decrease at the dusk magnetopause. It is also found that magnetic reconnection enhanced flux levels just outside of the magnetopause, with evidence suggesting that these particles are from magnetospheric sources. These findings strongly suggest that Saturn's magnetosphere is most likely the main source of energetic particles in Saturn's magnetosheath and magnetosphere leakage is an important process responsible for the presence of the energetic particles in Saturn's magnetosheath.

Magnetospheric Interactions of Saturn's Moon Dione (2005–2015)

AbstractThe moon Dione orbits Saturn at 6.2 Saturn radii RS deep in the Kronian magnetosphere. In situ studies of the moon‐magnetosphere interaction processes near Dione were possible with the Cassini/Huygens mission which flew by close to Dione five times at distances between 99 and 516 km. In addition, Cassini crossed Dione's L‐shell more than 400 times between 2004 and 2017 and documented the variability of Saturn's magnetosphere. Different flyby geometries allowed to study the interaction processes upstream, in the low‐energy wake, and above the north pole of Dione. We describe here the energetic particle measurements from the Low Energy Magnetospheric Measurement System (LEMMS), part of the Magnetosphere Imaging Instrument (MIMI) onboard Cassini. We also use hybrid simulation results from “A.I.K.E.F.” to interpret the signatures in the particle fluxes. This paper is a continuation of Krupp et al. (2013, https://doi.org/10.1016/j.icarus.2013.06.007) and Kotova et al. (2015, https://doi.org/10.1016/j.icarus.2015.06.031). The key results are as follows: (1) Saturn's magnetosphere at Dione's orbit is highly variable with changes in energetic charged particle fluxes by 1–2 orders of magnitude. (2) The dropout signatures near Dione are basically consistent with a fully absorbing obstacle, but some features point to more complex interaction processes than plasma and energetic particle absorption. (3) Absorption signatures are found to be asymmetric with respect to the orientation of the moon, indicative of the presence of radial drift components for electrons. (4) The deepest absorption signatures were observed at the edge of the low‐energy wake pointing to gradient‐B drifts strongest in that part of the interaction region.

Energetic Neutral and Charged Particle Measurements in the Inner Saturnian Magnetosphere During the Grand Finale Orbits of Cassini 2016/2017

AbstractWe give an overview of the charged and neutral particle measurements during the Grand Finale of Cassini performed by the Magnetosphere Imaging Instrument, MIMI, on board. The first in situ measurements of MeV particles between the D‐ring and the atmosphere of the planet showed protons with energies above 300 MeV with higher count rates when magnetically connected to the D‐ring which was less dense than expected, while more material was present at the inner edge. The variability in the count rates was a combination of sensor look direction, pitch angle coverage, changes in the L‐range, and latitude. Enhanced emissions of energetic neutral atoms during ring current activity was detected and confirmed the existence of a low‐energy ion radiation belt discovered by Cassini in 2004. No detection of low‐energy ions was indicative of a source population from a region closer than 1,800 km above the 1 bar cloud level.

Dust grains fall from Saturn's D-ring into its equatorial upper atmosphere.

The sizes of Saturn's ring particles range from meters (boulders) to nanometers (dust). Determination of the rings' ages depends on loss processes, including the transport of dust into Saturn's atmosphere. During the Grand Finale orbits of the Cassini spacecraft, its instruments measured tiny dust grains that compose the innermost D-ring of Saturn. The nanometer-sized dust experiences collisions with exospheric (upper atmosphere) hydrogen and molecular hydrogen, which forces it to fall from the ring into the ionosphere and lower atmosphere. We used the Magnetospheric Imaging Instrument to detect and characterize this dust transport and also found that diffusion dominates above and near the altitude of peak ionospheric density. This mechanism results in a mass deposition into the equatorial atmosphere of ~5 kilograms per second, constraining the age of the D-ring.

Energetic Ion Moments and Polytropic Index in Saturn's Magnetosphere using Cassini/MIMI Measurements: A Simple Model Based on κ‐Distribution Functions

AbstractMoments of the charged particle distribution function provide a compact way of studying the transport, acceleration, and interactions of plasma and energetic particles in the magnetosphere. We employ κ‐distributions to describe the energy spectra of H+ and O+, based on >20 keV measurements by the three detectors of Cassini's Magnetospheric Imaging Instrument, covering the time period from DOY 183/2004 to 016/2016, 5 < L < 20. From the analytical spectra we calculate the equatorial distributions of energetic ion moments inside Saturn's magnetosphere and then focus on the distributions of the characteristic energy (Ec=IE/In), temperature, and κ‐index of these ions. A semiempirical model is utilized to simulate the equatorial ion moments in both local time and L‐shell, allowing the derivation of the polytropic index (Γ) for both H+ and O+. Primary results are as follows: (a) The ∼9 < L < 20 region corresponds to a local equatorial acceleration region, where subadiabatic transport of H+ (Γ∼1.25) and quasi‐isothermal behavior of O+ (Γ∼0.95) dominate the ion energetics; (b) energetic ions are heavily depleted in the inner magnetospheric regions, and their behavior appears to be quasi‐isothermal (Γ<1); (c) the (quasi‐) periodic energetic ion injections in the outer parts of Saturn's magnetosphere (especially beyond 17–18 RS) produce durable signatures in the energetic ion moments; (d) the plasma sheet does not seem to have a ground thermodynamic state, but the extended neutral gas distribution at Saturn provides an effective cooling mechanism that does not allow the plasma sheet to behave adiabatically.

Energetic electron measurements near Enceladus by Cassini during 2005–2015

Enceladus is the main source of neutral and charged particles in the Saturnian magnetosphere. The particles originate at more than 100 active geysers forming a plume above the south pole of the moon and are continuously released into Saturn’s magnetosphere. Therefore the understanding of the interaction of those particles and the local magnetospheric environment of the moon is very important. One technique to study that interaction is to study the typical motion of charged particles in the perturbed plasma flow and the associated magnetic field lines in the vicinity of the moon especially during close flybys. The Cassini spacecraft flew by Enceladus 23 times between 2005 and 2015 at distances between 25 and 5000 km. During some of the flybys Cassini went directly through the south polar plume. Other flybys happened north of the moon or on high-latitude trajectories with respect to the moon.In this paper we present the energetic electron measurements during those flybys obtained by the Low Energy Magnetosphere Measurement System LEMMS, part of the Magnetosphere Imaging Instrument MIMI onboard Cassini (Krimigis et al., 2004). As already shown in Krupp et al. (2012) for the first 14 flybys MIMI/LEMMS typically observes dropouts in the particle intensities in the region of disturbed field lines and in the presence of the moon itself or dense material blocking the bounce and drift motions of the particles. We present in this paper a continuation of the Krupp et al. (2012) results and add a full classification for all 23 flybys using the full data set of energetic electron measurements of MIMI/LEMMS. We distinguish the observed absorption and dust signatures into four different categories: (1) full absorption signatures when all the particles within a fluxtube connecting the spacecraft with the moon are lost onto the moon during one of the particle motions; (2) partial dropouts (ramp-like feature) when not all the particles inside the fluxtube are lost; (3) short dropouts in the fluxes when particles are suddenly lost for a short period in time; and interpret those features as full or partial losses onto the moon or its environment as a result of different plasma and dust regimes in the vicinity of Enceladus. We compare the results with those of Meier et al. (2014) and Engelhardt et al. (2015); (4) In addition we also show dust-related “false electron” measurements for those flybys when Cassini directly went through the dense regions of the south polar plume. Those “dust-peaks” can be interpreted as the result of impacting dust particles inside the LEMMS aperture or nearby creating a plasma cloud.

Energetic particle imaging: The evolution of techniques in imaging high‐energy neutral atom emissions

AbstractEnergetic neutral atom imaging instruments have been flown on a variety of space missions to satisfy a variety of science requirements. In this paper we discuss the most recent developments that lead to improvements in energy range, angular resolution, and background rejection for the high‐energy range, as represented in the past by the Cassini magnetosphere imaging instrument Ion and Neutral Camera, the Imager for Magnetopause‐to‐Aurora Global Exploration (IMAGE) mission High‐Energy Neutral Atom instrument, and to some degree the IMAGE mission Medium‐Energy Neutral Atom instrument. The new approaches discussed here rely on the use of ultrathin foils without UV filters and on very high speed coincidence logic to reduce accidentals from intense background sources. We present laboratory results demonstrating an electrostatic design that meets the coincidence timing requirements, position, and scattering performance consistent with angular resolution (full width at half maximum) of ~2° for hydrogen above 10 keV and a hydrogen energy threshold ≤1 keV.

Statistical analysis and multi-instrument overview of the quasi-periodic 1-hour pulsations in Saturn’s outer magnetosphere

The in-situ exploration of the magnetospheres of Jupiter and Saturn has revealed different periodic processes. In particular, in the Saturnian magnetosphere, several studies have reported pulsations in the outer magnetosphere with a periodicity of about 1 h in the measurements of charged particle fluxes, plasma wave, magnetic field strength and auroral emissions brightness. The Low-Energy Magnetospheric Measurement System detector of the Magnetospheric Imaging Instrument (MIMI/LEMMS) on board Cassini regularly detects 1-hour quasi-periodic enhancements in the intensities of electrons with an energy range from a hundred keV to several MeV. We extend an earlier survey of these relativistic electron injections using 10 years of LEMMS observations in addition to context measurements by several other Cassini magnetospheric experiments. The one-year extension of the data and a different method of detection of the injections do not lead to a discrepancy with the results of the previous survey, indicating an absence of a long-term temporal evolution of this phenomenon. We identified 720 pulsed events in the outer magnetosphere over a wide range of latitudes and local times, revealing that this phenomenon is common and frequent in Saturn’s magnetosphere. However, the distribution of the injection events presents a strong local time asymmetry with ten times more events in the duskside than in the dawnside. In addition to the study of their topology, we present a first statistical analysis of the pulsed events properties. The morphology of the pulsations shows a weak local time dependence which could imply a high-latitude acceleration source. We provide some clues that the electron population associated with this pulsed phenomenon is distinct from the field-aligned electron beams previously observed in Saturn’s magnetosphere, but both populations can be mixed. We have also investigated the signatures of each electron injection event in the observations acquired by the Radio and Plasma Wave Science (RPWS) instrument and the magnetometer (MAG). Correlated pulsed signatures are observed in the plasma wave emissions, especially in the auroral hiss, for 12% of the electron injections identified in the LEMMS data. Additionally, in about 20% of the events, such coincident pulsed signatures have been also observed in the magnetic field measurements, some of them being indicative of field-aligned currents. This analysis combined with the multi-instrument approach sets constraints on the origin and significance of the pulsed events. Hence, our results suggest that the acceleration process providing the quasi-periodic relativistic electrons takes place at high-latitudes.

An empirical approach to modeling ion production rates in Titan's ionosphere II: Ion production rates on the nightside

AbstractIonization of neutrals by precipitating electrons and ions is the main source of Titan's nightside ionosphere. This paper has two goals: (1) characterization of the role of electron impact ionization on the nightside ionosphere for different magnetospheric conditions and (2) presentation of empirical ion production rates determined using densities measured by the Cassini Ion and Neutral Mass Spectrometer on the nightside. The ionosphere between 1000 and 1400 km is emphasized. We adopt electron fluxes measured by the Cassini Plasma Spectrometer‐Electron Spectrometer and the Magnetospheric Imaging Instrument as classified by Rymer et al. (2009). The current paper follows an earlier paper (Paper I), in which we investigated sources of Titan's dayside ionosphere and demonstrated that the photoionization process is well understood. The current paper (Paper II) demonstrates that modeled and empirical ionization rates on the nightside are in agreement with an electron precipitation source above 1100 km. Ion production rate profiles appropriate for different Saturnian magnetospheric conditions, as outlined by Rymer et al., are constructed for various magnetic field topologies. Empirical production rate profiles are generated for deep nightside flybys of Titan. The results also suggest that at lower altitudes (below 1100 km) another source, such as ion precipitation, is probably needed.

Detection of a strongly negative surface potential at Saturn's moon Hyperion.

On 26 September 2005, Cassini conducted its only close targeted flyby of Saturn's small, irregularly shaped moon Hyperion. Approximately 6 min before the closest approach, the electron spectrometer (ELS), part of the Cassini Plasma Spectrometer (CAPS) detected a field-aligned electron population originating from the direction of the moon's surface. Plasma wave activity detected by the Radio and Plasma Wave instrument suggests electron beam activity. A dropout in energetic electrons was observed by both CAPS-ELS and the Magnetospheric Imaging Instrument Low-Energy Magnetospheric Measurement System, indicating that the moon and the spacecraft were magnetically connected when the field-aligned electron population was observed. We show that this constitutes a remote detection of a strongly negative (∼ −200 V) surface potential on Hyperion, consistent with the predicted surface potential in regions near the solar terminator.

Tracking plasma rotation speeds in Saturn's magnetosphere

Researchers are learning more about the dynamics of Saturn's magnetosphere by applying a technique that is commonly used to study Earth's aurora. Using data from the magnetospheric imaging instrument on the Cassini spacecraft, Carbary and Mitchell tracked blobs of energetic neutral hydrogen atoms within Saturn's magnetosphere by creating keograms—time versus latitude plots assembled from multiple images, providing an instantaneous global view of azimuthal speeds of the blobs. The energetic neutral atoms serve as “imagers” of the ion population of a magnetosphere because they originate from charge exchange between neutral atoms and energetic ions.

The lens feature on the inner saturnian satellites

We have modeled an electron precipitation pattern expected on Mimas, Tethys, and Dione, using two different approaches. In the first approach, we adapt a previously developed model to compute an integrated energy flux into the surfaces of Mimas, Tethys, and Dione. This is a guiding-center, bounce-averaged model. In the second approach, we track individual particles in an electromagnetic field for an inert or slightly magnetized satellite. This second approach allows us to include the effects of electron pitch angle and gyrophase on the weathering pattern. Both methods converge on an enhanced dose pattern on each satellite’s leading hemisphere that is lens-shaped. We also present mission-averaged electron energy spectra obtained near these satellites by Cassini’s Magnetosphere Imaging Instrument (MIMI). These data are interpreted using our current understanding of both the environment and the instrument’s response. Fits to the data are integrated to find an energy flux into each satellite’s surface, as a function of longitude and latitude. Using positions on the moon accessible to energetic electrons from the modeling and the integrated energy flux based on data, we find lens patterns that fall off with increasing moon latitude. The predicted patterns are qualitatively consistent with some but not all of the optical observations made of these hemispheres.

Energetic particle measurements in the vicinity of Dione during the three Cassini encounters 2005–2011

The Cassini spacecraft encountered the moon Dione three times between 2005 and 2011. During the flybys particles and fields measurements have been performed and used to study the magnetospheric interaction between the fourth largest moon in the saturnian system and the surrounding magnetospheric plasma. Here we present the results of the Low Energy Magnetospheric Measurement System (LEMMS), part of the Magnetospheric Imaging Instrument MIMI, measuring energetic ions and electrons above 20keV up to several MeV and results from the magnetometer instrument (MAG) onboard the spacecraft. The trapped particles in the vicinity of Dione bounce and drift in Saturn’s magnetosphere and eventually are lost onto the surface of the moon. The location and depth of the absorption signature depends on species, their energy and on the geometry of the flyby. Only for the upstream flyby D1 were energy-dependent ion absorption signatures measured and we conclude that protons present in the upstream region can explain the observed dropout features. The other two encounters D2 and D3 went through the moon’s geometrical wake. During each of the encounters we observed absorption signatures in the fluxes of electrons caused by losses of those particles onto the moon during their drift motion in the saturnian magnetosphere. The profile of these absorption signatures is energy dependent, and becomes asymmetric between the planetward and anti-planetward sectors of the moon’s wake at energies above about 100keV. We compare these electron wake profiles with ones resulting from electron guiding center trajectory tracings in a simulated environment of Dione’s magnetospheric interaction using the “Adaptive hybrid model for space plasma simulations (A.I.K.E.F.)”. Our results indicate that the magnetic and electric field perturbations in Dione’s interaction region, as well as magnetospheric diffusion need to be taken into account in order to explain the features of the energetic electron data at different energies.

The extended Saturnian neutral cloud as revealed by global ENA simulations using Cassini/MIMI measurements

We show that the neutral gas vertical distribution at Saturn must be ~3–4 times more extended than previously thought for the >5 RSregions, while the neutral H distribution is consistent with H densities that reach up to ~150/cm3close to the orbit of Titan. We utilize a technique to retrieve the global neutral gas distribution in Saturn's magnetosphere, using energetic ion and energetic neutral atom (ENA) measurements, obtained by the Magnetospheric Imaging Instrument (MIMI) onboard the Cassini spacecraft. Our ENA measurements are consistent with a neutral cloud that consists of H2O, OH, H, and O, while the overall shapes and densities numbers concerning the neutral gas distributions are constrained according to already existing models as well as recent observations. The neutral gas distribution at Saturn is determined by simulating a 24–55 keV hydrogen image of the Saturnian magnetosphere, measured by the Ion and Neutral Camera (INCA), averaged over the time period from 1 July 2004 to 23 August 2005. The ionic input of the model includes a proton distribution of combined Charge Energy Mass Spectrometer (CHEMS, 3–230 keV/e), Low Energy Magnetospheric Measurements System (LEMMS, 30.7 keV to 2.3 MeV), and INCA (5–300 keV) in situ measurements. These measurements cover several passes from 1 July 2004 to 10 April 2007, at various local times over the dipole L range 5< L <20RS. A parameterized neutral gas distribution is changed until agreement between the simulated and average INCA image is obtained.

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.

Analysis of energetic electron drop-outs in the upper atmosphere of Titan during flybys in the dayside magnetosphere of Saturn

We discuss the high energy electron absorption signatures at Titan during the Cassini dayside magnetospheric encounters. We use the electron measurements of the Low Energy Measurement System of the Magnetospheric Imaging Instrument. We also examine the mass loading boundary based on the ion data of the Ion Mass Spectrometer sensor of the Cassini Plasma Spectrometer. The dynamic motion of the Kronian magnetopause and the periodic charged particle flux and magnetic field variations – associated with the magnetodisk of Saturn – of the subcorotating magnetospheric plasma creates a unique and complex environment at Titan. Most of the analysed flybys (like T25–T33 and T35–T51) cluster at similar Saturn Local Time positions. However the instantaneous direction of the incoming magnetospheric particles may change significantly from flyby to flyby due to the very different magnetospheric field conditions which are found upstream of Titan within the sets of encounters.The energetic magnetospheric electrons gyrate along the magnetic field lines of Saturn, and at the same time bounce between the mirror points of the magnetosphere. This motion is combined with the drift of the magnetic field lines. When these flux tubes interact with the upper atmosphere of Titan, their content is depleted over approximately an electron bounce period. These depletion signatures are observed as sudden drop-outs of the electron fluxes. We examined the altitude distribution of these drop-outs and concluded that these mostly detected in the exo-ionosphere of Titan and sometimes within the ionosphere.However there is a relatively significant scatter in the orbit to orbit data, which can be attributed to the which can be attributed to the variability of the plasma environment and as a consequence, the induced magnetosphere of Titan. A weak trend between the incoming electron fluxes and the measured drop-out altitudes has also been observed.

The Cassini Enceladus encounters 2005–2010 in the view of energetic electron measurements

The moon Enceladus, embedded in Saturn’s radiation belts, is the main internal source of neutral and charged particles in the Kronian magnetosphere. A plume of water ice molecules and dust released through geysers on the south polar region provides enough material to feed the E-ring and also the neutral torus of Saturn and the entire magnetosphere. In the time period 2005–2010 the Cassini spacecraft flew close by the moon 14 times, sometimes as low as 25km above the surface and directly through the plume. For the very first time measurements of plasma and energetic particles inside the plume and its immediate vicinity could be obtained. In this work we summarize the results of energetic electron measurements in the energy range 27keV to 21MeV taken by the Low Energy Magnetospheric Measurement System (LEMMS), part of the Magnetospheric Imaging Instrument (MIMI) onboard Cassini in the vicinity of the moon in combination with measurements of the magnetometer instrument MAG and the Electron Spectrometer ELS of the plasma instrument CAPS onboard the spacecraft. Features in the data can be interpreted as that the spacecraft was connected to the plume material along field lines well before entering the high density region of the plume. Sharp absorption signatures as the result of losses of energetic electrons bouncing along those field lines, through the emitted gas and dust clouds, clearly depend on flyby geometry as well as on measured pitch angle/look direction of the instrument. We found that the depletion signatures during some of the flybys show “ramp-like” features where only a partial depletion has been observed further away from the moon followed by nearly full absorption of electrons closer in. We interpret this as partially/fully connected to the flux tube connecting the moon with Cassini. During at least two of the flybys (with some evidence of one additional encounter) MIMI/LEMMS data are consistent with the presence of dust in energetic electron data when Cassini flew directly through the south polar plume. In addition we found gradients in the magnetic field components which are frequently found to be associated with changes in the MIMI/LEMMS particles intensities. This indicates that complex electron drifts in the vicinity of Enceladus could form forbidden regions for electrons which may appear as intensity drop-outs.