The Pioneer and Voyager spacecraft made close-up measurements of Saturn’s ionosphere and upper atmosphere in the 1970s and 1980s that suggested a chemical interaction between the rings and atmosphere. Exploring this interaction provides information on ring composition and the influence on Saturn’s atmosphere from infalling material. The Cassini Ion Neutral Mass Spectrometer sampled in situ the region between the D ring and Saturn during the spacecraft’s Grand Finale phase. We used these measurements to characterize the atmospheric structure and material influx from the rings. The atmospheric He/H2 ratio is 10 to 16%. Volatile compounds from the rings (methane; carbon monoxide and/or molecular nitrogen), as well as larger organic-bearing grains, are flowing inward at a rate of 4800 to 45,000 kilograms per second.

Estimation of the energy cascade rate in the inertial range of solar wind turbulence has been done so far mostly within the incompressible magnetohydrodynamics (MHD) theory. Here, we go beyond that approximation to include plasma compressibility using a reduced form of a recently derived exact law for compressible, isothermal MHD turbulence. Using in-situ data from the THEMIS/ARTEMIS spacecraft in the fast and slow solar wind, we investigate in detail the role of the compressible fluctuations in modifying the energy cascade rate with respect to the prediction of the incompressible MHD model. In particular, we found that the energy cascade rate: i) is amplified particularly in the slow solar wind; ii) exhibits weaker fluctuations in spatial scales, which leads to a broader inertial range than the previous reported ones; iii) has a power law scaling with the turbulent Mach number; iv) has a lower level of spatial anisotropy. Other features of solar wind turbulence are discussed along with their comparison with previous studies that used incompressible or heuristic (non exact) compressible MHD models.

In the solar wind, power spectral density (PSD) of the magnetic field fluctuations generally follow the so-called Kolmogorov spectrum f shown that only a fraction (35%) of the observed Kolmogorov spectra were populated by shear Alfvénic fluctuations, whereas the majority of the events (65%) was found to be dominated by compressible magnetosonic-like fluctuations, which contrasts with well-known turbulence properties in the solar wind. This study gives a first comprehensive view of the origin of the f -1 and the transition to the Kolmogorov inertial range; both questions remain controversial in solar wind turbulence.

The first estimation of the energy cascade rate | C | of magnetosheath turbulence is obtained using the CLUSTER and THEMIS spacecraft data and an exact law of compressible isothermal magnetohydrodynamics turbulence. | C | is found to be of the order of 10 −13 J.m −3 .s −1 , at least two orders of magnitude larger than its value in the solar wind (order of 10 −16 J.m −3 .s −1 in the fast wind). Two types of turbulence are evidenced and shown to be dominated either by incompressible Alfvénic or compressible magnetosonic-like fluctuations. Density fluctuations are shown to amplify the cascade rate and its spatial anisotropy in comparison with incompressible Alfvénic turbulence. Furthermore, for compressible magnetosonic fluctuations, large cascade rates are found to lie mostly near the linear kinetic instability of the mirror mode. New empirical power-laws relating | C | to the turbulent Mach number and to the internal energy are evidenced. These new finding have potential applications in distant astrophysical plasmas that are not accessible to in situ measurements.Turbulence is a ubiquitous non-linear phenomenon in hydrodynamic and plasmas flows that transfers dynamically energy between different scales. In astrophysical plasmas, turbulence is thought to play a major role in various processes such as accretion disks, star formation, acceleration of cosmic rays, solar corona and solar wind heating, and energy transport in planetary magnetospheres [1][2][3][4]. Thanks to the availability of in situ measurements recorded on board various orbiting spacecraft, the solar wind and the Earth's magnetosheath (i.e., the region of the solar wind downstream of the bow shock) provide a unique laboratory for the observational studies of plasma turbulence. An important feature of magnetosheath turbulence is the high level of density fluctuations in it, which can reach ∼ 50% − 100% [5][6][7][8] in comparison with ∼ 5% − 20% in the solar wind [9,10]. This makes the magnetosheath a key region of the near-Earth space where significant progress can be made in understanding compressible plasma turbulence, which is poorly understood although it is thought to be important in various astrophysical plasmas, such as supernovae remnants or the interstellar medium (ISM) [11][12][13][14][15].In the solar wind the magnetohydrodynamics (MHD) approximation has been successfully used to study turbulence cascade at scales larger than the ion inertial length (or Larmor radius) [16,17]. As in neutral fluid turbulence, an inertial range of MHD turbulence is generally evidenced by the observation of a power spectral density (PSD) exhibiting a power-law over a wide range of scales. This power-law is a manifestation of a turbulent cascade of energy from large scales, where the energy is injected, to the smaller ones where the energy is dissipated. The energy transfer over scales is assumed to occur at a constant rate, which is equal to the rate at which energy is injected and dissipated into the system. This quantity carries therefore a major importance...

Low frequency turbulence in Saturn's magnetosheath is investigated using in-situ measurements of the Cassini spacecraft. Focus is put on the magnetic energy spectra computed in the frequency range ∼ [10 −4 , 1]Hz. A set of 42 time intervals in the magnetosheath were analyzed and three main results that contrast with known features of solar wind turbulence are reported: 1) The magnetic energy spectra showed a ∼ f −1 scaling at MHD scales followed by an ∼ f −2.6 scaling at the sub-ion scales without forming the so-called inertial range; 2) The magnetic compressibility and the crosscorrelation between the parallel component of the magnetic field and density fluctuations C(δn, δB || ) indicates the dominance of the compressible magnetosonic slow-like modes at MHD scales rather than the Alfvén mode; 3) Higher order statistics revealed a monofractal (resp. multifractal) behaviour of the turbulent flow downstream of a quasi-perpendicular (resp. quasi-parallel) shock at the sub-ion scales. Implications of these results on theoretical modeling of space plasma turbulence are discussed.

The role of compressible fluctuations in the energy cascade of fast solar wind turbulence is studied using a reduced form of an exact law derived recently (Banerjee & Galtier 2013) for compressible isothermal magnetohydrodynamics and in-situ observations from the THEMIS B/ARTEMIS P1 spacecraft. A statistical survey of the data revealed a turbulent energy cascade over two decades of scales, which is broader than the previous estimates made from an exact incompressible law. A term-by-term analysis of the compressible model reveals new insight into the role played by the compressible fluctuations in the energy cascade. The compressible fluctuations are shown to amplify (2 to 4 times) the turbulent cascade rate with respect to the incompressible model in ∼ 10% of the analyzed samples. This new estimated cascade rate is shown to provide the adequate energy dissipation required to account for the local heating of the non-adiabatic solar wind.

The ionized upper layer of Saturn's atmosphere, its ionosphere, provides a closure of currents mediated by the magnetic field to other electrically charged regions (for example, rings) and hosts ion-molecule chemistry. In 2017, the Cassini spacecraft passed inside the planet's rings, allowing in situ measurements of the ionosphere. The Radio and Plasma Wave Science instrument detected a cold, dense, and dynamic ionosphere at Saturn that interacts with the rings. Plasma densities reached up to 1000 cubic centimeters, and electron temperatures were below 1160 kelvin near closest approach. The density varied between orbits by up to two orders of magnitude. Saturn's A- and B-rings cast a shadow on the planet that reduced ionization in the upper atmosphere, causing a north-south asymmetry.

Between 26 April and 15 September 2017, Cassini executed 23 highly inclined Grand Finale orbits through a new frontier for space exploration, the narrow region between Saturn and the D Ring, providing the first opportunity for obtaining in situ ionospheric measurements. During the Grand Finale orbits, the Radio and Plasma Wave Science instrument observed broadband whistler mode emissions and narrowband upper hybrid frequency emissions. Using known wave propagation characteristics of these two plasma wave modes, the electron density is derived over a broad range of ionospheric latitudes and altitudes. A two‐part exponential scale height model is fitted to the electron density measurements. The model yields a double‐layered ionosphere with plasma scale heights of 545/575 km for the northern/southern hemispheres below 4,500 km and plasma scale heights of 4,780/2,360 km for the northern/southern hemispheres above 4,500 km. The interpretation of these layers involves the interaction between the rings and the ionosphere.