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In Paranicas et al. (2020), https://doi.org/10.1029/2020ja028299, we reported on a method to estimate the inflow speed of interchange injections in Saturn's magnetosphere. The procedure relies on phase space density conservation and mapping and an estimate of the size of the flux decrease along one edge of the injection. Here we describe modifications to the method. We have applied our new technique to an existing list of injections, presenting only those injections with inflow speeds greater than 20 km/s, defined as “fast” injections here. We find at least 20% of the events from the list can be considered fast. Faster injections are more effective in energizing charged particles as the injection moves planetward. This is because shorter transit time limits the number of particles that can drift longitudinally out of the injection.

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 Saturn has a sufficiently strong dipole magnetic field to trap high-energy charged particles and form radiation belts, which have been observed outside its rings. Whether stable radiation belts exist near the planet and inward of the rings was previously unknown. The Cassini spacecraft’s Magnetosphere Imaging Instrument obtained measurements of a radiation belt that lies just above Saturn’s dense atmosphere and is decoupled from the rest of the magnetosphere by the planet’s A- to C-rings. The belt extends across the D-ring and comprises protons produced through cosmic ray albedo neutron decay and multiple charge-exchange reactions. These protons are lost to atmospheric neutrals and D-ring dust. Strong proton depletions that map onto features on the D-ring indicate a highly structured and diverse dust environment near Saturn.

We present energetic ion observations made by Juno during its close Europa flyby on 29 September 2022. These data show significant reductions in the ion intensities occurring in the moon's geometric wake region. The most significant losses occur for locally mirroring ions with gyrophases on the anti‐Jovian facing side of the wake. Ions with pitch angles >30° from the local perpendicular direction have guiding center trajectories that allow them to skip over the moon as they azimuthally drift with velocities ∼100 km s−1. In general, short bounce times and large gyroradii can act as good indicators when to expect significant losses; however, we show asymmetries in energy, pitch angle, and gyrophase that we cannot fully explain and require detailed particle tracing with realistic electromagnetic fields. Finally, we compute ion integral fluxes along Juno's trajectory and show ∼85% of the near equatorially mirroring, >50 keV, ions are absorbed by Europa.

The shape and location of a planetary magnetopause can be determined by balancing the solar wind dynamic pressure with the magnetic and thermal pressures found inside the boundary. Previous studies have found the kronian magnetosphere to show rigidity (like that of Earth) as well as compressibility (like that of Jupiter) in terms of its dynamics. In this paper we expand on previous work and present a new model of Saturn's magnetopause. Using a Newtonian form of the pressure balance equation, we estimate the solar wind dynamic pressure at each magnetopause crossing by the Cassini spacecraft between Saturn Orbit Insertion in June 2004 and January 2006. We build on previous findings by including an improved estimate for the solar wind thermal pressure and include low‐energy particle pressures from the Cassini plasma spectrometer's electron spectrometer and high‐energy particle pressures from the Cassini magnetospheric imaging instrument. Our improved model has a size‐pressure dependence described by a power law DP−1/5.0 ± 0.8. This exponent is consistent with that derived from numerical magnetohydrodynamic simulations.

We present Cassini observations of a plasma vortex in Saturn's dayside outer magnetosphere. The vortex encounter took place on 13 December 2004 as Cassini was travelling toward the planet. The spacecraft crossed the magnetopause 3 times, before being immersed in the low‐latitude boundary layer. During the transition between the boundary layer and the magnetosphere proper, the spacecraft observed deflected boundary layer plasma, a twisted magnetic field topology, and high‐energy (>20 keV) directional electron fluxes. These observations are consistent with an encounter with a vortex on the inner edge of the boundary layer, an interface that is expected to be susceptible to the growth of the Kelvin‐Helmholtz (K‐H) instability due to its low magnetic shear. The size of the vortex is determined to be at least 0.55 RS, and a simple model of the current system resulting from the formation of the vortex is proposed. The possible acceleration mechanisms responsible for the high‐energy electrons are discussed. The identification of the structure provides compelling evidence of the operation of the nonlinear K‐H instability at Saturn's morning magnetospheric boundaries and has implications for our understanding of the transfer of energy and momentum between the solar wind and Saturn's magnetosphere.

Characterizing Europa’s subsurface ocean is essential for assessing Europa’s habitability. The suite of instruments on the Europa Clipper spacecraft will, among others, magnetically sound Europa’s interior by measuring the ocean’s induced magnetic field. This magnetic field is generated in response to the Jovian time-varying magnetic environment in which Europa is immersed. However, the dynamic magnetized plasma flow of the Jovian magnetosphere creates electrical currents that give rise to magnetic perturbations near Europa. These perturbations complicate the interpretation of the induction signal, and hence the characterization and inferences on potential habitability. Thus, characterization of the ocean by magnetic sounding requires an accurate characterization of the plasma as it flows across Europa. We present the Plasma Instrument for Magnetic Sounding (PIMS), the instrument for the Europa Clipper mission that will measure the plasma contribution to the magnetic field perturbations sensed by the Europa Clipper Magnetometer. PIMS is composed of four Faraday Cup plasma spectrometers that use voltage-biased gridded apertures to dissect the space plasmas that they encounter. The instrument uses sensitive preamplifiers and processing electronics to measure the current that results when charged particles strike the instrument’s metal collector plates, thus enabling a measure of the plasma characteristics near Europa to produce a more accurate magnetic sounding of Europa’s subsurface ocean. PIMS consists of two sensors: one placed near the top of the Europa Clipper spacecraft and one near the bottom. Each sensor contains two Faraday Cups with a 90° full-width field-of-view. The sensors were specifically designed to withstand the Europa environment, measure both ions and electrons, and have two separate voltage ranges intended to analyze the magnetospheric and ionospheric environments, respectively. In this paper, we describe the scientific motivation for this experiment, the design considerations for the PIMS instrument, the details of the ground calibration, and other details pertinent to understanding the scientific data retrieved by PIMS.

During the final stages of the Cassini mission, the spacecraft flew between the planet and its rings, providing a new view on this spectacular system (see the Perspective by Ida). Setting the scene, Spilker reviews the numerous discoveries made using Cassini during the 13 years it spent orbiting Saturn. Iess et al. measured the gravitational pull on Cassini, separating the contributions from the planet and the rings. This allowed them to determine the interior structure of Saturn and the mass of its rings. Buratti et al. present observations of five small moons located in and around the rings. The moons each have distinctive shapes and compositions, owing to accretion of ring material. Tiscareno et al. observed the rings directly at close range, finding complex features sculpted by the gravitational interactions between moons and ring particles. Together, these results show that Saturn's rings are substantially younger than the planet itself and constrain models of their origin. Science , this issue p. 1046 , p. eaat2965 , p. eaat2349 , p. eaau1017 ; see also p. 1028 Five small moons located close to Saturn’s rings have unusual morphologies, contain water ice, and have accreted ring material. Saturn’s main ring system is associated with a set of small moons that either are embedded within it or interact with the rings to alter their shape and composition. Five close flybys of the moons Pan, Daphnis, Atlas, Pandora, and Epimetheus were performed between December 2016 and April 2017 during the ring-grazing orbits of the Cassini mission. Data on the moons’ morphology, structure, particle environment, and composition were returned, along with images in the ultraviolet and thermal infrared. We find that the optical properties of the moons’ surfaces are determined by two competing processes: contamination by a red material formed in Saturn’s main ring system and accretion of bright icy particles or water vapor from volcanic plumes originating on the moon Enceladus.

Respiratory syncytial virus (RSV) infection is a leading cause of severe lower respiratory tract illness in young infants, the elderly and immunocompromised individuals. We demonstrate here that the co-inhibitory molecule programmed cell death 1 (PD-1) is selectively upregulated on T cells within the respiratory tract during both murine and human RSV infection. Importantly, the interaction of PD-1 with its ligand PD-L1 is vital to restrict the pro-inflammatory activities of lung effector T cells in situ, thereby inhibiting the development of excessive pulmonary inflammation and injury during RSV infection. We further identify that PD-L1 expression on lung inflammatory dendritic cells is critical to suppress inflammatory T-cell activities, and an interferon–STAT1–IRF1 axis is responsible for increased PD-L1 expression on lung inflammatory dendritic cells. Our findings suggest a potentially critical role of PD-L1 and PD-1 interactions in the lung for controlling host inflammatory responses and disease progression in clinical RSV infection.

Flares and X-ray jets on the Sun arise in active regions where magnetic flux emerges from the solar interior amd interacts with the ambient magnetic field 1 , 2 . The interactions are believed to occur in electric current sheets separating regions of opposite magnetic polarity. The current sheets located in the corona or upper chromosphere have long been thought to act as an important source of coronal heating 3 , 4 , 5 , 6 , requiring their location in the corona or upper chromosphere. The dynamics and energetics of these sheets are governed by a complex magnetic field structure that, until now, has been difficult to measure. Here we report the determination of the full magnetic vector in an interaction region near the base of the solar corona. The observations reveal two magnetic features that characterize young active regions on the Sun: a set of rising magnetic loops and a tangential discontinuity of the magnetic field direction, the latter being the observational signature of an electric current sheet. This provides strong support for coronal heating models based on the dissipation of magnetic energy at current sheets.

We report sample results on Saturn magnetospheric energetic ion spectral shapes using measurements obtained from the Magnetospheric Imaging Instrument (MIMI) suite onboard Cassini. The ion intensities are measured by the Charge Energy Mass Spectrometer (CHEMS) that covers the energy range of 3 to 236 keV/e, the Low Energy Magnetospheric Measurements System (LEMMS) covering the energy range of 0.024 < E < 18 MeV, and the Ion Neutral Camera (INCA) that provides ion measurements in the ion mode at the energy range ∼5.5 to >220 keV for protons. The data used cover several passes from the period 1 July 2004 to 10 April 2007, at various latitudes over the dipole L range 5 < L < 20 RS. The spectra generally show a power law in energy form at larger L values but display a flattening/relative peak at lower (L < 10) values centered at ∼50 to ∼100 keV and can be fit by a κ distribution function with characteristic kT ranging from ∼10 to ∼100 keV. The results are consistent with the assumption that energetic protons are heated adiabatically as they move inward to stronger magnetic fields, in contrast to the singly ionized oxygen that seems to be heated locally at each L shell. The lack of any trend of the O+ temperature versus L shell implies that nonadiabatic energization mechanisms and charge exchange with Saturn's neutral gas cloud play an important role for ion energetics.