Dawn‐Dusk Asymmetry in Energetic (>20 keV) Particles Adjacent to Saturn's Magnetopause

Energetic particles (>∼25 keV) have been observed routinely in the terrestrial magnetosheath, but have not been well studied at the magnetosheaths of the outer planets. Here we analyze energetic electrons and ions (mostly protons) in the vicinity (±1 RS) of Saturn's magnetopause, using particle data acquired with the low‐energy magnetosphere measurements system, one of the three sensors of the magnetosphere imaging instrument on board the Cassini spacecraft, during a period of ∼14 years (2004–2017). It is found that energetic particles, especially ions, are also common in Saturn's magnetosheath. A clear inward (toward Saturn) gradient in the electron differential flux is identified, suggestive of magnetospheric sources. Such an inward gradient does not appear in some of the ion channels. We conclude that Saturn's magnetopause acts as a porous barrier for energetic electrons and, to a lesser extent, for energetic ions. A dawn‐dusk asymmetry in the gradient of particle flux across the magnetopause is also identified, with a gradual decrease at the dawn and a sharp decrease at the dusk magnetopause. It is also found that magnetic reconnection enhanced flux levels just outside of the magnetopause, with evidence suggesting that these particles are from magnetospheric sources. These findings strongly suggest that Saturn's magnetosphere is most likely the main source of energetic particles in Saturn's magnetosheath and magnetosphere leakage is an important process responsible for the presence of the energetic particles in Saturn's magnetosheath

Resonant behavior of a single plasmonic helix

Chiral plasmonic nanostructures will be of increasing importance for future applications in the field of nano optics and metamaterials. Their sensitivity to incident circularly polarized light in combination with the ability of extreme electromagnetic field localization renders them ideal candidates for chiral sensing and for all-optical information processing. Here, the resonant modes of single plasmonic helices are investigated. We find that a single plasmonic helix can be efficiently excited with circularly polarized light of both equal and opposite handedness relative to that of the helix. An analytic model provides resonance conditions matching the results of full-field modeling. The underlying geometric considerations explain the mechanism of excitation and deliver quantitative design rules for plasmonic helices being resonant in a desired wavelength range. Based on the developed analytical design tool, single silver helices were fabricated and optically characterized. They show the expected strong chiroptical response to both handednesses in the targeted visible range. With a value of 0.45 the experimentally realized dissymmetry factor is the largest obtained for single plasmonic helices in the visible range up to now.Comment: main: typo in the author's name corrected, SI: update

Pattern recognition receptor-mediated cytokine response in infants across 4 continents⋆

Background Susceptibility to infection as well as response to vaccination varies among populations. To date, the underlying mechanisms responsible for these clinical observations have not been fully delineated. Because innate immunity instructs adaptive immunity, we hypothesized that differences between populations in innate immune responses may represent a mechanistic link to variation in susceptibility to infection or response to vaccination. Objective Determine whether differences in innate immune responses exist among infants from different continents of the world. Methods We determined the innate cytokine response following pattern recognition receptor (PRR) stimulation of whole blood from 2-year-old infants across 4 continents (Africa, North America, South America, and Europe). Results We found that despite the many possible genetic and environmental exposure differences in infants across 4 continents, innate cytokine responses were similar for infants from North America, South America, and Europe. However, cells from South African infants secreted significantly lower levels of cytokines than did cells from infants from the 3 other sites, and did so following stimulation of extracellular and endosomal but not cytosolic PRRs. Conclusions Substantial differences in innate cytokine responses to PRR stimulation exist among different populations of infants that could not have been predicted. Delineating the underlying mechanism(s) for these differences will not only aid in improving vaccine-mediated protection but possibly also provide clues for the susceptibility to infection in different regions of the world

Single-File Diffusion of Atomic and Colloidal Systems: Asymptotic Laws

In this work we present a general derivation of the non-Fickian behavior for the self-diffusion of identically interacting particle systems with excluded mutual passage. We show that the conditional probability distribution of finding a particle at position $x_{t}$ after time $t$, when the particle was located at $x_{0}$ at $t=0$, follows a Gaussian distribution in the long-time limit, with variance $2W(t)\sim t^{1/2}$ for overdamped systems and with variance $2W(t)\sim t$ for classical systems. The asymptotic behavior of the mean-squared displacement, $W(t)$, is shown to be independent of the nature of interactions for homogeneous systems in the fluid state. Moreover, the long-time behavior of self-diffusion is determined by short-time and large scale collective density fluctuations.Comment: 4 page

Effects of radial motion on interchange injections at Saturn

Charged particle injections are regularly observed in Saturn's inner magnetosphere by Cassini. They are attributed to an ongoing process of flux-tube interchange driven by the strong centrifugal force associated with Saturn's rapid rotation. Numerical simulations suggest that these interchange injections can be associated with inward flow channels, in which plasma confined to a narrow range of longitudes moves radially toward the planet, gaining energy, while ambient plasma in the adjacent regions moves more slowly outward. Most previous analyses of these events have neglected this radial motion and inferred properties of the events under the assumption that they appear instantaneously at the spacecraft's L-shell and thereafter drift azimuthally. This paper describes features of injections that can be related to their radial motion prior to observation. We use a combination of phase space density profiles and an updated version of a test-particle model to quantify properties of the injection. We are able to infer the longitudinal width of the injection, the radial travel time from its point of origin, and the starting L shell of the injection. We can also predict which energies can remain inside the channel during the radial transport. To highlight the effects of radial propagation at a finite speed, we focus on those interchange injections without extensive features of azimuthal dispersion. Injections that have traveled radially for one or more hours prior to observation would have been initiated at a different local time than that of the observation. Finally, we describe an injection where particles have drifted azimuthally into a flow channel prior to observation by Cassini. (C) 2015 Elsevier Inc. All rights reserved

Sources, sinks and transport of energetic electrons near Saturn’s main rings

The inner boundary of Saturn's electron radiation belts, near the planet's A‐ring (∼2.27 Rs), is studied using Cassini's Proximal orbit measurements. We find that variable convective flows transport energetic electrons to the A‐ring, which absorbs them instantaneously, forming the inner belt boundary. These flows are also responsible for a variable and longitudinally asymmetric boundary configuration. Pre‐noon, the boundary oscillates towards and away from the A‐ring with a two‐week period. Post‐noon, it maps persistently near the F‐ring (∼2.32 Rs) and coexists with localized MeV electron intensity enhancements (microbelts). We propose that the microbelts contain electrons in drift resonance with corotation, trapped in local‐time confined trajectories which result from the aforementioned convective flows. The microbelts' collocation with the F‐ring implies either a local, secondary electron production due to Galactic Cosmic Ray collisions with F‐ring dust, or an enhanced resonant electron trapping due to an electrodynamic interaction between the F‐ring and Saturn's magnetosphere

Callisto's Atmosphere and Its Space Environment: Prospects for the Particle Environment Package on Board JUICE

The JUpiter ICy moons Explorer (JUICE) of the European Space Agency will investigate Jupiter and its icy moons Europa, Ganymede, and Callisto, with the aim to better understand the origin and evolution of our Solar System and the emergence of habitable worlds around gas giants. The Particle Environment Package (PEP) on board JUICE is designed to measure neutrals and ions and electrons at thermal, suprathermal, and radiation belt energies (eV to MeV). In the vicinity of Callisto, PEP will characterize the plasma environment, the outer parts of Callisto's atmosphere and ionosphere and their interaction with Jupiter's dynamic magnetosphere. Roughly 20 Callisto flybys with closest approaches between 200 and 5,000 km altitude are planned over the course of the JUICE mission. In this article, we review the state of the art regarding Callisto's ambient environment and magnetospheric interaction with recent modeling efforts for Callisto's atmosphere and ionosphere. Based on this review, we identify science opportunities for the PEP observations to optimize scientific insight gained from the foreseen JUICE flybys. These considerations will inform both science operation planning of PEP and JUICE and they will guide future model development for Callisto's atmosphere, ionosphere, and their interaction with the plasma environment