Charged nanograins in the Enceladus plume Hill, T. W.; Thomsen, M. F.; Tokar, R. L. ...
Journal of Geophysical Research,
20/May , Volume:
117, Issue:
A5
Journal Article
Peer reviewed
Open access
There have been three Cassini encounters with the south‐pole eruptive plume of Enceladus for which the Cassini Plasma Spectrometer (CAPS) had viewing in the spacecraft ram direction. In each case, ...CAPS detected a cold dense population of heavy charged particles having mass‐to‐charge (m/q) ratios up to the maximum detectable by CAPS (∼104 amu/e). These particles are interpreted as singly charged nanometer‐sized water‐ice grains. Although they are detected with both negative and positive net charges, the former greatly outnumber the latter, at least in the m/q range accessible to CAPS. On the most distant available encounter (E3, March 2008) we derive a net (negative) charge density of up to ∼2600 e/cm3 for nanograins, far exceeding the ambient plasma number density, but less than the net (positive) charge density inferred from the RPWS Langmuir probe data during the same plume encounter. Comparison of the CAPS data from the three available encounters is consistent with the idea that the nanograins leave the surface vents largely uncharged, but become increasingly negatively charged by plasma electron impact as they move farther from the satellite. These nanograins provide a potentially potent source of magnetospheric plasma and E‐ring material.
Key Points
Charged nanograins in the Enceladus plume have been analyzed
They dominate the charge density of the plume
They interact strongly with the plume plasma
By traversing the plume erupting from high southern latitudes on Saturn's moon Enceladus, Cassini orbiter instruments can directly sample the material therein. Cassini Plasma Spectrometer, CAPS, data ...show that a major plume component comprises previously‐undetected particles of nanometer scales and larger that bridge the mass gap between previously observed gaseous species and solid icy grains. This population is electrically charged both negative and positive, indicating that subsurface triboelectric charging, i.e., contact electrification of condensed plume material may occur through mutual collisions within vents. The electric field of Saturn's magnetosphere controls the jets' morphologies, separating particles according to mass and charge. Fine‐scale structuring of these particles' spatial distribution correlates with discrete plume jets' sources, and reveals locations of other possible active regions. The observed plume population likely forms a major component of high velocity nanometer particle streams detected outside Saturn's magnetosphere.
During the 14 July 2005 encounter of Cassini with Enceladus, the Cassini Plasma Spectrometer measured strong deflections in the corotating ion flow, commencing at least 27 Enceladus radii (27 x 252.1 ...kilometers) from Enceladus. The Cassini Radio and Plasma Wave Science instrument inferred little plasma density increase near Enceladus. These data are consistent with ion formation via charge exchange and pickup by Saturn's magnetic field. The charge exchange occurs between neutrals in the Enceladus atmosphere and corotating ions in Saturn's inner magnetosphere. Pickup ions are observed near Enceladus, and a total mass loading rate of about 100 kilograms per second ($3 \times 10^{27} H_20$molecules per second) is inferred.
This study reports direct detection by the Cassini plasma spectrometer of freshly‐produced water‐group ions (O+, OH+, H2O+, H3O+) and heavier water dimer ions (HxO2)+ very close to Enceladus where ...the plasma begins to emerge from the plume. The data were obtained during two close (52 and 25 km) flybys of Enceladus in 2008 and are similar to ion data in cometary comas. The ions are observed in detectors looking in the Cassini ram direction exhibiting energies consistent with the Cassini speed, indicative of a nearly stagnant plasma flow in the plume. North of Enceladus the plasma slowing commences about 4 to 6 Enceladus radii away, while south of Enceladus signatures of the plasma interaction with the plume are detected 22 Enceladus radii away.
Electron and ion drift dispersion events are often observed by the Cassini Plasma Spectrometer (CAPS) in the inner magnetosphere of Saturn (5 to 10 RS). These events appear to result from ...azimuthally‐limited injections of plasma and persist for at least several hours. During this time, the events can be analyzed to obtain information on the time and azimuthal location of the injections. The CAPS data show evidence of both remote and local injections. In this paper a conceptual model of Saturnian centrifugal interchange is developed based on the characteristics of the local injections.
We present observations of significant dynamics within two UV auroral storms observed on Saturn using the Hubble Space Telescope in April/May 2013. Specifically, we discuss bursts of auroral emission ...observed at the poleward boundary of a solar wind‐induced auroral storm, propagating at ∼330% rigid corotation from near ∼01 h LT toward ∼08 h LT. We suggest that these are indicative of ongoing, bursty reconnection of lobe flux in the magnetotail, providing strong evidence that Saturn's auroral storms are caused by large‐scale flux closure. We also discuss the later evolution of a similar storm and show that the emission maps to the trailing region of an energetic neutral atom enhancement. We thus identify the auroral form with the upward field‐aligned continuity currents flowing into the associated partial ring current.
Key Points
Saturn's auroral storms exhibit fast‐propagating bursts on the poleward boundary
These are similar to terrestrial PBIs, indicating ongoing closure of lobe flux
Subsequent emission maps to the trailing Region 2 current of an ENA enhancement
Multiple Titan encounters by the Cassini spacecraft have shown that ion chemistry in Titan's upper atmosphere is much more complex than previously thought. As well as showing a great variety of ...species present below 100
amu, they also include the detection of negative ions and of large abundances of ions above 100
amu. Here, we use data from two Cassini instruments, the Cassini plasma spectrometer's ion beam sensor (CAPS/IBS) and the ion and neutral mass spectrometer (INMS) during fourteen Cassini encounters with Titan's upper atmosphere. By simultaneous analysis of the combined data, we are able to determine the ion temperature, one component of the wind speed and spacecraft potential. Using these derived quantities, we are also able to extend the analysis of CAPS/IBS data to quantify the abundance of ions above 100
amu and to statistically estimate their composition.
The Ion and Neutral Camera (INCA), one of three sensors comprising the Magnetospheric Imaging Instrument (MIMI) on the Cassini spacecraft, measures intensities of hydrogen and oxygen ions and neutral ...atoms in the Saturnian magnetosphere. The measured intensity spectrum and anisotropy of hot hydrogen and oxygen ions may be used to deduce the spectral parameters and the velocity of the ion population. The anisotropies are frequently convective in nature, allowing for the determination of a bulk velocity. Under the “frozen in” assumption, this is also the velocity of the cold plasma of magnetospheric ions. Initial analysis of selected measurements of nightside ion populations with strong anisotropies indicates nearly rigid corotation of magnetospheric plasma interior to Titan's orbit. Beyond this distance, these measurements infer that the plasma maintains at best a constant rotation velocity, falling farther behind the rigid corotation rate at increasing distance.
Protein kinase M zeta (PKM zeta) is a newly described form of PKC that is necessary and sufficient for the maintenance of hippocampal long term potentiation (LTP) and the persistence of memory in ...Drosophila. PKM zeta is the independent catalytic domain of the atypical PKC zeta isoform and produces long term effects at synapses because it is persistently active, lacking autoinhibition from the regulatory domain of PKC zeta. PKM has been thought of as a proteolytic fragment of PKC. Here we report that brain PKM zeta is a new PKC isoform, synthesized from a PKM zeta mRNA encoding a PKC zeta catalytic domain without a regulatory domain. Multiple zeta-specific antisera show that PKM zeta is expressed in rat forebrain as the major form of zeta in the near absence of full-length PKC zeta. A PKC zeta knockout mouse, in which the regulatory domain was disrupted and catalytic domain spared, still expresses brain PKM zeta, indicating that this form of PKM is not a PKC zeta proteolytic fragment. Furthermore, the distribution of brain PKM zeta does not correlate with PKC zeta mRNA but instead with an alternate zeta RNA transcript thought incapable of producing protein. In vitro translation of this RNA, however, generates PKM zeta of the same molecular weight as that in brain. Metabolic labeling of hippocampal slices shows increased de novo synthesis of PKM zeta in LTP. Because PKM zeta is a kinase synthesized in an autonomously active form and is necessary and sufficient for maintaining LTP, it serves as an example of a link coupling gene expression directly to synaptic plasticity.
Long-term potentiation (LTP), a persistent synaptic enhancement thought to be a substrate for memory, can be divided into two phases: induction, triggering potentiation, and maintenance, sustaining ...it over time. Many postsynaptic events are implicated in induction, including N-methyl-D-aspartate receptor (NMDAR) activation, calcium increases and stimulation of several protein kinases; in contrast, the mechanism maintaining LTP is not yet characterized. Here we show the constitutively active form of an atypical protein kinase C (PKC) isozyme, protein kinase M zeta (PKMζ), is necessary and sufficient for LTP maintenance.
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