Description: [1]  The plume of Enceladus is a remarkable plasma environment containing several charged particle species. These include cold magnetospheric electrons, negative and positive water clusters, charged nanograins and ‘magnetospheric photoelectrons’ produced from ionization of neutrals throughout the magnetosphere near Enceladus. Here we discuss observations of a population newly identified by the CAPS ELS instrument – photoelectrons produced in the plume ionosphere itself. These were found during the E19 encounter, in the energetic particle shadow where penetrating particles are absent. Throughout E19, CAPS was oriented away from the ram direction where the clusters and nanograins are observed during other encounters. Plume photoelectrons are also clearly observed during the E9 encounter, and are also seen at all other Enceladus encounters where electron spectra are available. This new population, warmer than the ambient plasma population, is distinct from, but adds to, the magnetospheric photoelectrons. Here we discuss the observations and examine the implications, including the ionization source these electrons provide.

Description: 123 I-CLINDE is a radiotracer developed for SPECT and targets the 18-kDa translocator protein (TSPO). TSPO is upregulated in glial cells and used as a measure of neuroinflammation in a variety of central nervous system diseases. The aim of this study was to examine the test–retest variability of 123 I-CLINDE binding in healthy subjects. Methods: SPECT scans were acquired over 90 min in 16 healthy controls (9 women, 8 mixed-affinity binders [MABs] and 8 high-affinity binders [HABs] twice with an interval of 35 ± 15 d). Arterial input functions were based on individual blood measurements in 8 subjects and a population-based approach in combination with individual whole-blood time–activity curves in the other 8 subjects. Seven brain volumes of interest were extracted and quantified by SUVs and by 2-tissue-compartment modeling for calculation of distribution volumes ( V T ). Test–retest variability was measured by percentage difference (PD), the absolute PD, intraclass correlation coefficient (ICC), and coefficient of variation. Results: The absolute PD of brain SUV and the V T had similar values. The ICC values were higher for V T s than for brain SUVs, which were both moderate to high; however, lower ICC values were observed when calculated separately for HABs and MABs. Test–retest reproducibility was higher in subjects with immediate centrifugation of blood samples. The population-based method efficiently recovered data with delayed centrifugation. The V T of a 49-y-old male HAB was 7.5 ± 1.4 mL/cm 3 compared with 4.6 ± 1.4 mL/cm 3 of a sex- and age-matched MAB. The SUVs of a 49-y-old male HAB and MAB were 1.03 ± 0.14 and 0.88 ± 0.15 g/mL, respectively. Conclusion: The test–retest reproducibility of 123 I-CLINDE is comparable or better than that reported for commonly used PET TSPO tracers. Because of the binding of 123 I-CLINDE to blood cells and peripheral tissues, SUV is not a sufficient surrogate of V T from 2-tissue-compartment modeling. The population-adjusted method has the potential to reduce the complexity of blood analyses of TSPO tracers.

Description: The plasma properties, especially the flow parameters, obtained from numerical integration of Cassini/CAPS Ion Mass Spectrometer measurements during intervals when the CAPS field of view encompassed both inward and outward flow directions relative to corotation are examined for nightside data (18–06 local time) during 2006, 2009, and 2010. The results show good agreement with previously-reported values derived using different selection criteria and a different analysis technique. Nightside flows are dominantly in or near the corotation direction, indicating continuing influence of connection to the ionosphere. There is no evidence for a quasi-steady reconnection x-line within the surveyed region of the tail, although dynamic events attributable to transient reconnection have been observed. There is a net radial mass outflow, leading to an estimated net mass loss between 18 and 03 local time of ~34 kg/s. Part of this mass loss occurs as a “planetary wind” along the dusk flank. The remainder probably occurs ultimately much deeper in the tail and along the distant dawn magnetopause, when the mass disconnects from the weakened planetary magnetic field.

Description: The degree to which solar wind driving may affect Saturn's magnetosphere is not yet fully understood. We present observations that suggest that under some conditions the solar wind does govern the character of the plasma sheet in Saturn's outer magnetosphere. On 16 September 2006, the Cassini spacecraft, at a radial distance of 37 Rs near local midnight, observed a sunward-flowing ion population for ~5 hours, which was accompanied by enhanced Saturn Kilometric Radiation emissions. We interpret this beam as the outflow from a long-lasting episode of Dungey-type reconnection, i.e., reconnection of previously-open flux containing magnetosheath material. The beam occurred in the middle of a several-day interval of SKR activity and enhanced lobe magnetic field strength, apparently caused by the arrival of a solar-wind compression region with significantly higher than average dynamic pressure. The arrival of the high-pressure solar wind also marked a change in the composition of the plasma-sheet plasma, from water-group-dominated material clearly of inner-magnetosphere origin to material dominated by light-ion composition, consistent with captured magnetosheath plasma. This event suggests that under the influence of prolonged high solar wind dynamic pressure, the tail plasma sheet, which normally consists of inner-magnetospheric plasma, is eroded away by ongoing reconnection that then involves open lobe field lines. This process removes open magnetic flux from the lobes and creates a more Earth-like, Dungey-style outer plasma sheet dominantly of solar wind origin. This behavior is potentially a recurrent phenomenon driven by repeating high-pressure streams (corotating interaction regions) in the solar wind, which also drive geomagnetic storms at Earth.

Description: During certain portions of the Cassini mission to Saturn, Cassini made repeated and periodic crossings of the magnetospheric current sheet that lies near the magnetic equator and extends well down the magnetospheric tail. These repeated crossings are part of the puzzling set of planetary period variations in numerous magnetospheric properties that have been discovered at Saturn. During 2010 these periodic crossings often display asymmetries such that the northbound crossing occurs faster than the southbound crossing or vice versa, while at other times the crossings are more symmetric. The character of the crossings is well organized by the relative phase of the northern vs. southern perturbation currents inferred in earlier analyses of the magnetic field observations. Further, the dependence of the character of the crossings on the relative phase is consistent with similar asymmetries predicted both by the dual rotating current systems inferred from magnetic field observations and by global MHD models that incorporate the effects of hypothesized atmospheric vortices. The two models are themselves in generally good agreement on those predictions. In both models the asymmetries are attributable to a periodic thickening and thinning of the magnetospheric current sheet, combined with a periodic vertical flapping of the sheet. The Cassini observations thus provide additional observational support to such current systems as a likely explanation for many of the known magnetospheric planetary period variations.

Description: Magnetic reconnection is an important process that occurs at the magnetopause boundary of Earth's magnetosphere because it leads to transport of solar wind energy into the system, driving magnetospheric dynamics. However, the nature of magnetopause reconnection in the case of Saturn's magnetosphere is unclear. Based on a combination of Cassini spacecraft observations and simulations we propose that plasma β conditions adjacent to Saturn's magnetopause largely restrict reconnection to regions of the boundary where the adjacent magnetic fields are close to anti-parallel, severely limiting the fraction of the magnetopause surface that can become open. Under relatively low magnetosheath β conditions we suggest that this restriction becomes less severe. Our results imply that the nature of solar wind-magnetosphere coupling via reconnection can vary between planets, and we should not assume that the nature of this coupling is always Earth-like. Studies of reconnection signatures at Saturn's magnetopause will test this hypothesis.