Henry T. Smith

Discrete classification and electron energy spectra of Titan's varied magnetospheric environment

We analyse combined electron spectra across the dynamic range of both Cassini electron sensors in order to characterise the background plasma environment near Titan for 54 Cassini-Titan encounters as of May 2009. We characterise the encounters into four broad types: Plasma sheet, Lobe-like, Magnetosheath and Bimodal. Despite many encounters occurring close to the magnetopause only two encounters to date were predominantly in the magnetosheath (T32 and T42). Bimodal encounters contain two distinct electron populations, the low energy component of the bi-modal populations is apparently associated with local water group products. Additionally, a hot lobe-like environment is also occasionally observed and is suggestively linked to increased local pick-up. We find that 34 of 54 encounters analysed are associated with one of these groups while the remaining encounters exhibit a combination of these environments. We provide typical electron properties and spectra for each plasma regime and list the encounters appropriate to each. Citation: Rymer, A.M., H. T. Smith, A. Wellbrock, A.J. Coates, and D.T. Young (2009), Discrete classification and electron energy spectra of Titans varied magnetospheric environment, Geophys. Res. Lett., 36, L15109, doi: 10.1029/2009GL039427.

Charge states of energetic oxygen and sulfur ions in Jupiter's magnetosphere

Pitch angle distributions of proton and energetic heavy ion fluxes near Europas orbit have been measured by the Galileo Energetic Particles Detector (EPD). At similar energies, these distributions have important differences. If their source and transport processes are similar, as we hypothesize here, then it is difficult to reconcile their different pitch angle distributions. By looking at the same question, other researchers have proposed that the heavies are multiply charged, leading to differences in how the particles are lost. This could not be confirmed directly with EPD because that detector does not separate heavy ion measurements by charge state. However, indirect analyses of the data have extracted the charge state of a few sulfur events. We present here a complete list of ion injections observed with EPD over the whole mission. Energetic sulfur and oxygen charge states can be inferred through a dispersion analysis of dynamic injections that makes use of the charge-dependent nature of the gradient-curvature azimuthal drift. We find that sulfur is predominantly multiply charged, whereas oxygen is more evenly distributed between singly and doubly charged states. In addition to current theories on energetic heavy ion transport near the Europa region, we propose that charge gain for the oxygen ions (electron stripping) may play an important role in the character of energetic particles in that region.

A radiation belt of energetic protons located between Saturn and its rings

Cassinis final phase of exploration 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 planets 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 planets 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 planets upper atmosphere. Finally, Waite et al. identified molecules in the infalling material and directly measured the composition of Saturns atmosphere. Science, this issue p. eaat5434, p. eaat1962, p. eaat2027, p. eaat3185, p. eaat2236, p. eaat2382 INTRODUCTION Most magnetized planets are known to possess radiation belts, where high-energy charged particles are trapped in large numbers. The possibility that a radiation belt could exist also in the confined region between Saturn and its main rings has been proposed on the basis of remote sensing observations and simulations. It was not until the final 5 months of the Cassini mission that in situ measurements were obtained from this region with the Magnetosphere Imaging Instrument (MIMI). This paper provides an overview of these measurements and their interpretation. RATIONALE Saturn’s main rings prevent the inward transport of trapped charged particles in the magnetosphere. Material from the outer radiation belts cannot directly access the low-altitude region within the rings. The isolation of this region allows the study of energetic particle source and loss processes because it is only indirectly coupled to the dynamics of the rest of the magnetosphere. Potential sources include cosmic ray albedo neutron decay (CRAND) and multiple-charge exchange, whereas losses are likely dominated by energy deposition and scattering of trapped particles by dust and atmospheric neutrals. All of these mechanisms involve charged particle interactions with materials in space, meaning that MIMI measurements can provide information to probe the material itself—particularly the tenuous D-ring, the innermost component of Saturn’s main rings, which is difficult to constrain by remote sensing observations. RESULTS We observed an inner radiation belt extending between 1.03 and 1.22 Saturn radii (1 RS = 60,268 km) at the equatorial plane, dominated by protons with energies from 25 MeV up to the giga–electron volt range. This belt is limited by the atmosphere at its inner edge and by the D73 ringlet (at 1.22 RS), a component of the D-ring, at its outer boundary. Another ringlet (D68 at 1.12 RS) splits the trapped particle population in two. The outer sector overlaps with the extended D-ring, and its intensity is reduced compared with that of the inner sector, owing to proton losses on ring dust. The proton angular distributions are highly anisotropic with fluxes that are orders of magnitude higher near the magnetic equator compared with fluxes of particles that can reach high latitudes. No time variability could be discerned in the >25-MeV proton population over the 5-month period of the observations. Trapping of lower-energy (tens of kilo–electron volt) protons was clearly observed in at least one case by imaging the emission of energetic neutral atoms (ENAs) coming from below ~1.06 RS (altitude < 3800 km). Energetic electrons (18 keV to several mega–electron volts) and heavy ions (27 keV per nucleon to hundreds of mega–electron volts per nucleon), if present, have fluxes close to or lower than the detection limit of the MIMI sensors. CONCLUSION The radial profile, the stability of the >25-MeV proton fluxes, and the lack of heavy ions are features consistent with a radiation belt originating from CRAND. The strong anisotropy of the proton distributions is primarily the result of proton losses in collisions with atmospheric neutrals, though an anisotropy in the production of CRAND protons from Saturn’s rings may also contribute. The low-altitude, kilo–electron volt proton population is transient and derives from charge stripping of planetward ENAs, which are generated at the variable magnetospheric ring current. Saturn’s proton radiation belts. Saturn’s permanent proton radiation belt extends outward to the orbit of the moon Tethys but is segmented because of proton absorption by moons and rings. The innermost radiation belt (inset) threads through Saturn’s D-ring and contains protons with energies up to several giga–electron volts, much higher than observed outside the main rings. These protons are among the β-decay products of neutrons, which are released through galactic cosmic ray collisions with Saturn’s rings (CRAND process). 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.

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

Cassinis final phase of exploration 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 planets 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 planets 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 planets upper atmosphere. Finally, Waite et al. identified molecules in the infalling material and directly measured the composition of Saturns atmosphere. Science, this issue p. eaat5434, p. eaat1962, p. eaat2027, p. eaat3185, p. eaat2236, p. eaat2382 INTRODUCTION Ring material has long been thought to enter Saturn’s atmosphere, modifying its atmospheric and ionospheric chemistry. This phenomenon, dubbed “ring rain,” involves the transport of charged dust particles from the main rings along the planetary magnetic field. RATIONALE At the end of the Cassini mission, measurements by onboard instruments tested this hypothesis as well as whether ring material falls directly into the equatorial atmosphere. The final 22 orbits of the Cassini mission sent the spacecraft through the gap between the atmosphere and the innermost of the broad ring system, the D-ring. RESULTS The Magnetospheric Imaging Instrument—designed to measure energetic neutral atoms, ions, and electrons—recorded very small dust grains [8000 to 40,000 unified atomic mass units (u), or roughly 1- to 3-nm radius] in two sensors. At 3000-km altitude, a peak rate of ~300,000 counts s–1 was detected by one sensor as Cassini crossed the equatorial plane. At lower altitude (1700 to 2000 km), a second sensor recorded positively charged dust in the upper atmosphere and ionosphere over a size range of ~8000 to 40,000 u (~1 to 2 nm, assuming the density of ice). Consistent with this observation, larger dust in the 0.1- to 1-µm range was detected by the Cassini Dust Analyzer and the Radio and Plasma Wave Science instrument. CONCLUSION We modeled the interaction of dust with the H and H2 exospheric populations known to populate the gap. Collisions between small dust grains and H atoms provide sufficient drag to de-orbit the dust, causing it to plunge into the atmosphere over ~4 hours. The analysis indicates that at least ~5 kg s−1 of dust is continuously precipitating into the atmosphere. At 3000-km altitude, the dust is distributed symmetrically about the equator, mostly between ±2° latitude with a peak density of ~0.1 cm−3. On the wings of the distribution, consistent with ring rain transport along the magnetic field, almost all of the dust was observed to be charged. At the 2000 to 1700 km altitude, the dust has reached a diffusive terminal velocity and, although showing some bias toward the equator, is ordered mostly by a scale height of ~180 km in altitude. The most probable source for this dust population is the innermost bright ringlet of the D-ring, known as the D68 ringlet. We predict that this kinetic process generates a highly anisotropic neutral hydrogen population, concentrated near the equatorial plane with periapses between ~4000 and 7000 km, and apoapses ranging to as high as 10 Saturn radii, with a small fraction on escape trajectories. Ring dust. (Top left) Data and model fits for the equatorial dust population near 3000-km altitude for three orbits through the D-ring gap. HV, plate detector high voltage. Red line uses left scale (percent). (Bottom left) Dust counts (blue) from ~2000 to 1700 km (modulated by sensor energy/charge steps, green), ordered by altitude and latitude, consistent with diffusive transport. (Right) Model trajectory of a dust particle in three frames of reference, as collisions with exospheric hydrogen degrade its velocity. Saturn has been shrunk to expand the gap for clarity. 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.