Saturn's magnetosphere is different from most other planetary environments due to the number and complexity of the internal plasma sources to be found there. In addition to the solar wind and the planet's own ionosphere, the ice moons, Mimas, Enceladus, Tethys, Dione and Rhea, the rings and Titan represent abundant, even major sources of magnetospheric plasma, and largely determine its composition. The different magnetospheric plasma populations can be used as tracers and can provide original information on the mechanisms of production, loss and transport of plasma within Saturn's magnetosphere.
Among the many results obtained at LATMOS, LPP and IRAP based on the analysis of the data from the CAPS* experiment, we may note in particular:
*The works carried out at LATMOS, LPP and IRAP on the CAPS experiment were supported by the CNES, particularly through the financing of two postdoctoral bursaries, paid by CNES from 2004 to 2006.
Detection and modelling of the ionospheric halo around the inner rings of Saturn
Model (top half of figure) of the ionospheric halo surrounding Saturn's rings, and comparison of the model with the observations (bottom half of the figure) obtained by the CAPS experiment on board Cassini (orbit insertion, July 2004). © Bouhram et al. (2006), André et al. (2008).
Numerous works have suggested the existence of a thin atmosphere in the vicinity of Saturn's inner rings, and suggested photolysis or the screening of ice particles by energetic particles. However, until the Cassini probe entered into orbit around Saturn on 1 July 2004, the environment around the rings remained largely unknown and the models proposed had never been tested by observations.
The first measurements made by Cassini during the insertion manoeuvre, in particular the plasma measurements due to the CAPS experiment, provided completely new data, which made it possible to advance our understanding of this region of Saturn's magnetosphere. They revealed the existence, around inner rings A and B, of an ionosphere composed mainly of O+ and O2+ ions.
These observations can be explained by the presence of a molecular O2 oxygen atmosphere around the rings, created by the photolytic decomposition of the ice that makes up the grains. The existence of this atmosphere is made possible by the property of oxygen molecules enabling them not to stick to the ice under the temperature conditions of the grains, and consequently not to be lost in their collisions with the grains.
A two-dimensional model describing the formation of the ionospheric halo surrounding the rings was recently developed at CETP. This model is based on a specific approach that makes it possible to take into account the principal physico-chemical mechanisms governing the formation of ions, as well as their transport into the vicinity of Saturn's rings. The results of this model accord with the observations made by the CAPS IMS spectrometer.
Identification of the electron populations present in Saturn's magnetosphere
Identification of the frontier at 9 Saturnian radii, using data from the CAPS ELS and RPWS instruments on board Cassini. From top: time-energy spectrograph from CAPS ELS, angle of attack distribution (PAD, in degrees) of electrons with energy 217 eV, frequency-time spectrograph from RPWS. The appearance of an electron population of higher energy beyond this frontier appears very clearly, as does the evolution of the angle of attack distribution, which is no longer aligned to the magnetic field (at 0 and 180 degrees) and the intensification of low frequency waves within this frontier. © Schippers et al. (2008).
In the magnetospheres of giant planets, there are many and varied sources of plasma, all basically originating within the system (atmospheres, planetary and lunar ionospheres, lunar surfaces, rings), unlike the case of Earth where the importance of external (solar wind) and internal (planetary ionosphere) sources is comparable. The magnetospheres of giant planets are thus structured into different regions and composed of plasmas with different characteristics and origins. The linkage between the different regions, particularly where they interface and there are significant exchanges of mass, quantity of motion and energy, creates the dynamics of these magnetospheres.
The electron populations of the magnetospheric plasma constitute an essential element in understanding many key magnetospheric processes; they act as tracers for the characterisation of plasma diffusion and transport processes, wave-particle interactions and linkages between the magnetospheric and auroral regions, within Saturn's atmosphere, via the precipitation of particles along the lines of the magnetic field. The continual acquisition by Cassini of electron measurements across a wide range of energies in Saturn's magnetosphere makes it possible to study all these physical processes, in particular their spatial and temporal location and their intensity and energy distribution.
For example, a strongly marked magnetospheric frontier was observed at a distance of 7-9 planetary radii. This frontier appears like a signature in various magnetospheric parameters, coinciding with the presence of a transition in angle of attack of the electrons (whose distributions are aligned to the magnetic field beyond this region), the intensification of low-frequency electrostatic waves and the inner edge of the annular current (where the contribution to the magnetospheric magnetic field of the currents linked to the movement of charged particles becomes significant). Similar observations have been reported in the Jovian magnetosphere, in the regions where a strong linkage (via a system of currents aligned along the lines of the magnetic field) between the magnetospheric regions and the regions of Jupiter's upper atmosphere result in intense and permanent auroral emissions. The future analysis of the observations collected during Cassini's high-latitude orbits around Saturn should make it possible to clarify and validate this comparison.
Inter-calibration of CAPS ELS and MIMI LEMMS electron plasma instruments on board Cassini
Radial distance-energy spectrograph of the observations of the CAPS ELS instrument on board Cassini (top: before correction for the effects of the radiation belts contaminating the measurements within 5 planetary radii; bottom: after correction). The photoelectric potential is determined based on identification of the energy (in black) of the photoelectrons observed beyond 7 planetary radii.
Composite flux at 15 min resolution of the instruments CAP ELS and MIMI LEMMS on Cassini (in blue: fluxes calculated on the basis of the geometric factors after inter-calibration, in green: fluxes calculated on the basis of the initial geometric factors). © Schippers (2009).
The CAPS ELS and MIMI LEMMS instruments use different technologies for measuring the electron populations of Saturn's magnetosphere, at low (1 eV - 26 keV) and high energies (20 keV - >1 MeV) respectively. These instruments have different bandwidths and their observations must be combined in order to provide complete coverage of the electrons, from energies in the order of 1 eV up to tens of MeVs, without any bandwidth interruptions. However, the spectral study extended across the whole bandwidth covered by the two instruments requires the data sets to be harmonised on the same basis of physical units, in order for them to be subsequently used in a quantitative manner.
In order to construct composite spectra for electrons over an energy range extended across both instruments, the raw data acquired by the instruments is transformed into common physical units independent of the instrument geometry, the differential number flux (DNF) of the particles or the intensities, which are expressed in keV-1s-1sr-1cm-2. Using the raw data in counts per second, the differential fluxes of the electrons are calculated using the effective geometric factors of each instrument. The geometric factors were calibrated using ground-based prototypes, Monte-Carlo simulation codes, the overflight of the terrestrial magnetosphere in 2000, and more recently, thanks to the intercalibration of the fluxes of the CAPS ELS and MIMI LEMMS instruments. Once the CAPS ELS and MIMI LEMMS data sets were harmonised in fluxes, the first composite spectra were set up. An augmentation of the high energy flux from the CAPS ELS instrument (20-26 keV) of around 10 times that of the lower energy channel of the MIMI LEMMS instrument became clear. Recalibrations of the instruments were therefore necessary in order to converge on similar fluxes.
The CAPS ELS electron measurements are moreover subject to several sources of contamination, due to the interaction of the Cassini probe and the instruments with the magnetospheric environment, which may have significant effects. Two of these are described below:
Effect of penetrating particles: the presence of a background noise independent of the energy is always measured on the CAPS ELS instrument. The origin of this noise is attributed to the source of energetic radiation emitted by radioisotope thermoelectric generators and radioisotope heater units (RTGs and RHUs) on board Cassini. The source of this radiation is anisotropic and greater in certain actuator positions and certain detector anodes. It is low background noise, not affecting measurements in areas where the signal to noise ratio is high (for example in Saturn's inner magnetosphere). In regions where the flux measured is close to the instrument's detection threshold (for example in the lobes and in Saturn's external magnetosphere), the signal to noise ratio is low and the contamination source significantly affects the measurements. In the innermost region of the magnetosphere (< 6 planetary radii), the energetic particles trapped in Saturn's radiation belts penetrate into the CAPS ELS instrument and drastically affect the quality of its measurements. This effect can however be corrected by defining the value of the penetrating radiation as the minimum counting rate in the high energy channels of the CAPS ELS instrument.
The satellite electric potential effect: artificial photoelectrons. The solar radiation produces a photoelectric effect on the heat cover of the Cassini probe, causing the emission of photoelectrons of a few eV. The probe then becomes positively charged, accelerating the electrons around it. The consequence of this is that part of the electron spectrum is contaminated by the presence of photoelectrons and, on the other hand, the measured energy of the electrons by the CAPS ELS instrument is overestimated, as is the flux. This effect may be corrected using photoelectric current data determined by identifying the peak energy of the photoelectrons in each spectrum measured by the instrument.
Finally, after several iterations, a new set of effective geometric factors for the CAPS ELS instrument was defined, derived from the comparison of all the electron measurements by the different instruments on board Cassini - which made it possible to correct the effects of satellite electric potential and contamination by the radiation belts - and from the comparison of the fluxes from CAPS ELS and MIMI LEMMS in their common energy band - which made it possible to establish composite spectra in which the MIMI LEMMS fluxes appear in continuity with the CAPS ELS fluxes.
Plasma sources, transport and loss within Saturn's magnetosphere
Internal region of Saturn's magnetosphere observed (top to bottom) using MAG (magnetic field intensity), RPWS (time-frequency spectrograph), CAPS ELS (time-energy spectrograph), CAPS IMS (time-energy spectrograph) and CAPS ELS (electronic density) instruments on board Cassini (orbit A, October 2004). Overall view (left) and zoom (right) on the signatures of the sub-dense flux tubes (empty of plasma content), which exchange positions with their denser neighbours and are thus transported radially towards the interior of the magnetosphere. © André et al. (2007).
Planetary magnetospheres are simultaneously subjected to the influence of planetary rotation and solar wind. The resulting dynamic can be mostly dominated by the rapid rotation of the planet, or controlled by the solar wind. Within the magnetospheres of giant planets, locally and continuously created plasma is trapped along the lines of the magnetic field and pulled around the planet as the latter rotates. The plasma created by internal sources cannot accumulate indefinitely, and a circulation system, with or without changes to the magnetic topology, is set up, allowing the plasma to be evacuated radially towards the outer regions of the system and to be liberated into the interplanetary medium, or to be lost through ion-electron recombination to reform neutral molecules.
Thus one of the key problems with the magnetospheric physics of Saturn is to understand how the plasmas created in the solar wind, in Saturn's upper atmosphere, and in particular by the icy moons of Saturn and by Titan, especially Enceladus, which has proved to be the main source of internal plasma, diffuse radially, are accelerated and eventually recombine. It is a question of retracing the history of the evolution and mutual interaction of the different plasmas in a magnetosphere that is subjected at the same time to interaction with the solar wind and with planetary rotation.
In the past, several theoretical works have suggested that the radial transport of plasma from the internal regions of Saturn's magnetosphere, whence it is injected into the external regions from where it can escape into the interplanetary medium, results from a centrifugal instability - that is, a Rayleigh-Taylor type instability in which the centrifugal acceleration plays the role of gravity. The exchange instability is in fact the equivalent of convective instability, of the Rayleigh-Taylor type, which influences the mixture of the adjacent layers in planetary atmospheres in the presence of gravity. Under the effect of the centrifugal force produced by the rapid planetary rotation, which tends to pull the plasma outwards, it likewise causes the mixture of adjacent magnetic shells, fed by different sources of plasma at different radial distances. When it develops, this instability gives rise to exchange movements between magnetic flux tubes, such that the flux tubes loaded with plasma move outwards, while tubes emptied of their content return inwards.
Using, among others, the observations of the CAPS instrument on board Cassini, researchers from IRAP showed the signatures of this mechanism in Saturn's internal magnetosphere (region located between 5 and 10 planetary radii), where the ice moons orbit. These signatures were observed, moreover, on each of Cassini's orbits. These observations also underline the importance of the magnetic flux tube exchange mechanism in a magnetosphere under rapid rotation, such as those of Saturn and Jupiter.