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  • 1
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    A statistical study of the inner edge of the electron plasma sheet and the net convection potential as a function of geomagnetic activity (2011)
    Wiley
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    Publication Date: 2011-06-25
    Description: A widely accepted explanation of the location of the inner edge of the electron plasma sheet and its dependence on electron energy is based on drift motions of individual particles. The boundary is identified as the separatrix between drift trajectories linking the tail to the dayside magnetopause (open paths) and trajectories closed around the Earth. A statistical study of the inner edge of the electron plasma sheet using THEMIS Electrostatic Analyzer plasma data from November 2007 to April 2009 enabled us to examine this model. Using a dipole magnetic field and a Volland-Stern electric field with shielding, we find that a steady state drift boundary model represents the average location of the electron plasma sheet boundary and reflects its variation with the solar wind electric field in the local time region between 21:00 and 06:00, except at high activity levels. However, the model does not reproduce the observed energy dispersion of the boundaries. We have also used the location of the inner edge of the electron plasma sheet to parameterize the potential drop of the tail convection electric field as a function of solar wind electric field (Esw) and geomagnetic activity. The range of Esw examined is small because the data were acquired near solar minimum. For the range of values tested (meaningful statistics only for Esw 〈 2 mV/m), reasonably good agreement is found between the potential drop of the tail convection electric field inferred from the location of the inner edge and the polar cap potential drop calculated from the model of Boyle et al. (1997).
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 2
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    Generation and properties of in vivo flux transfer events (2012)
    Wiley
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    Publication Date: 2012-05-16
    Description: Of the 3701 flux transfer event signatures that we identified in THEMIS data between May and October of 2007 and 2008 at low-latitudes on the magnetopause, 41 were distinctive in that the north-south flow components reversed direction during the ∼1 min required for THEMIS spacecraft to traverse the structure. We have ruled out the possibility that these 41 “flow reversal events” (FREs) were single X-line structures in motion, and confirmed from their field and plasma properties that they indeed were flux ropes. We have interpreted the plasma flow reversal as evidence that we observed the flux ropes while they were being generated by a pair of X-lines that developed in sequence through component merging, a process that seems to play a significant role in forming flux ropes. Our analysis, which applies only to low latitude flux ropes, provides evidence to modify the updated multiple X-line reconnection scenario with component merging as the dominant associated reconnection process.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 3
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    Periodic motion of Saturn's nightside plasma sheet (2011)
    Wiley
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    Publication Date: 2011-11-05
    Description: Saturn's magnetosphere is replete with magnetospheric periodicities; magnetic fields, plasma parameters, energetic particle fluxes, and radio emissions have all been observed to vary at a period close to that of Saturn's assumed sidereal rotation rate. In particular, periodicities in Saturn's magnetotail can be interpreted in terms of periodic vertical motion of Saturn's outer magnetospheric plasma sheet. The phase relationships between periodicities in different measurable quantities are a key piece of information in validating the various published models that attempt to relate periodicities in different quantities at different locations. It is important to empirically extract these phase relationships from the data in order to distinguish between these models, and to provide further data on which to base new conceptual models. In this paper a simple structural model of the flapping of Saturn's plasma sheet is developed and fitted to plasma densities in the outer magnetosphere, measured by the Cassini electron spectrometer. This model is used to establish the phase relationships between magnetic field periodicities in the cam region of the magnetosphere and the flapping of the plasma sheet. We find that the plasma sheet flaps in phase with Br and B$\theta$ and in quadrature with the B$\varphi$ component in the core/cam region. The plasma sheet phase also has a strong local time asymmetry. These results support some conceptual periodicity models but are in apparent contradiction with others, suggesting that future work is required to either modify the models or study additional phase relationships that are important for these models.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 4
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    Joule heating of the south polar terrain on Enceladus (2011)
    Wiley
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    Publication Date: 2011-04-27
    Description: We report that Joule heating in Enceladus, resulting from the interaction of Enceladus with Saturn's magnetic field, may account for 150 kW to 52 MW of power through Enceladus. Electric currents passing through subsurface channels of low salinity and just a few kilometers in depth could supply a source of power to the south polar terrain, providing a small but previously unaccounted for contribution to the observed heat flux and plume activity. Studies of the electrical heating of Jupiter's moon Europa have concluded that electricity is a negligible heating source since no connection between the conductive subsurface and Alfvén currents has been observed. Here we show that, contrary to results for the Jupiter system, electrical heating may be a source of internal energy for Enceladus, contributing to localized heating, production of water vapor, and the persistence of the “tiger stripes.” This contribution is of order 0.001–0.25% of the total observed heat flux, and thus, Joule heating cannot explain the total south polar terrain heat anomaly. The exclusion of salt ions during refreezing serves to enhance volumetric Joule heating and could extend the lifetime of liquid water fractures in the south polar terrain.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 5
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    Lunar dayside current in the terrestrial lobe: ARTEMIS observations (2014)
    Wiley
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    Publication Date: 2014-04-25
    Description: We report ARTEMIS dual-probe observations of two events in the terrestrial magnetotail lobe, both characterized by upward-moving heavy ions of lunar origin at one of the probes that is magnetically connected with the dayside lunar surface. By treating magnetic measurements at the other probe as the unperturbed lobe fields, we obtain background-subtracted magnetic perturbations (most significantly in B z ) when the first probe moved in the dawn-dusk direction across flux tubes magnetically connected to the Moon. These magnetic perturbations indicate the presence of field-aligned current above the lunar surface. By examining possible carriers of field-aligned current, we find that lunar heavy ions and accompanying electrons both contribute considerably to the current. Observations of the field-aligned current also suggest that the charging process at the dayside lunar surface and the associated lobe plasma environment, which have traditionally been viewed as a one-dimensional current balance problem, are actually more complicated. These observations give the first insights into how heavy ions affect the lunar dayside environment in terms of multi-species plasma dynamics.
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    Topics: Geosciences , Physics
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  • 6
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    Cassini magnetometer observations over the Enceladus poles (2011)
    Wiley
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    Publication Date: 2011-10-14
    Description: In 2009 and 2010, the Cassini spacecraft made seven targeted Enceladus flybys along trajectories parallel to the equatorial plane. The magnetometer on board Cassini made a complete set of vector field distribution measurements on these tracks and observed significant perturbations that further constrained the location of the momentum-exchange region. These observations were made not only above and below the moon Enceladus, but also above and below the momentum-loading center. The observed field perturbations are consistent with previous interpretations of the interaction. Southernly-biased Io-type, Alfvén-wing signatures are penetrated by Cassini. In addition to the draping due to the slowing down, the magnetic field drapes away from Saturn, indicating that the dust in the plume is predominantly negatively charged. No Saturnward or anti-Saturnward tilt of the plume is identified.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
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  • 7
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    Surface Current Balance and Thermoelectric Whistler Wings at Airless Astrophysical Bodies: Cassini at Rhea (2014)
    Wiley
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    Publication Date: 2014-10-08
    Description: Sharp magnetic perturbations found by the Cassini spacecraft at the edge of the Rhea flux tube are consistent with field-aligned flux tube currents. The current system results from the difference of ion and electron gyroradii, and the requirement to balance currents on the sharp Rhea surface. Differential type hybrid codes that solve for ion velocity and magnetic field have an intrinsic difficulty modeling the plasma absorber's sharp surface. We overcome this problem by instead using integral equations to solve for ion and electron currents, and obtain agreement with the magnetic perturbations at Rhea's flux tube edge. An analysis of the plasma dispersion relations and Cassini data reveals that field-guided whistler waves initiated by (1) the electron velocity anisotropy in the flux tube and (2) interaction with surface sheath electrostatic waves on topographic scales, may facilitate propagation of the current system to large distances from Rhea. Current systems like those at Rhea should occur generally, for plasma absorbers of any size such as spacecraft or planetary bodies, in a wide range of space plasma environments. Motion through the plasma is not essential since the current system is thermodynamic in origin; excited by heat flow into the object. The requirements are a difference of ion and electron gyroradii and a sharp surface, i.e., without a significant thick atmosphere.
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    Topics: Geosciences , Physics
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  • 8
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    Ion Composition in Interchange Injection Events in Saturn's Magnetosphere (2014)
    Wiley
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    Publication Date: 2014-11-10
    Description: Interchange injections events are commonly observed by the Cassini spacecraft in the region between about 6 and 12 Rs (1Rs = 60268 km) and even frequently beyond. In this study, thirteen examples of interchange injection events are identified in Cassini/CAPS data under special conditions such that time-of-flight (TOF) mass spectra could be obtained from entirely within the events. Using the TOF data to separate the main ion species H + , H 2 + , and W + , approximate densities of each species are calculated under the assumption that all distributions were isotropic. The light-ion density ratios, H 2 + /H + , in the injection events are not discernibly different from those ratios in control intervals from the ambient plasma. However, the water-group ratio, W + /H + , is significantly lower than ambient. Comparison of the measured density ratios with the range of values observed throughout Saturn's magnetosphere indicates that values of W + /H + that are as low as those observed within the injection events are found primarily beyond L ~ 14 (where L is the equatorial crossing distance, in Rs, of a dipole field line), indicating that the injection events are delivering plasma from the outer magnetosphere, at times traveling at least 6 Rs.
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    Topics: Geosciences , Physics
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  • 9
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    Detection of a strongly negative surface potential at Saturn's moon Hyperion (2014)
    Wiley
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    Publication Date: 2014-09-12
    Description: On September 26 th , 2005, Cassini conducted its only close targeted flyby of Saturn's small, irregularly shaped moon Hyperion. Approximately 6 minutes before the closest approach, the Electron spectrometer (ELS), part of the Cassini Plasma Spectrometer (CAPS) detected a field-aligned electron population originating from the direction of the moon's surface. Plasma wave activity detected by the Radio and Plasma Wave (RPWS) instrument suggests electron beam activity. A dropout in energetic electrons was observed by both CAPS-ELS and the Magnetospheric Imaging Instrument Low Energy Magnetospheric Measurement System (MIMI-LEMMS), indicating that the moon and the spacecraft were magnetically connected when the field-aligned electron population was observed. We show that this constitutes a remote detection of a strongly negative (~ − 200 V) surface potential on Hyperion, consistent with the predicted surface potential in regions near the solar terminator.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
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  • 10
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    Flow vortices associated with flux transfer events moving along the magnetopause: Observations and an MHD simulation (2011)
    Wiley
    Details
    Publication Date: 2011-08-02
    Description: Between May and October in 2007 and 2008, the five THEMIS spacecraft recorded a total of 3701 instances of bipolar magnetic variations in the magnetopause normal direction associated with enhancements of field magnitude that are interpreted as flux transfer events (FTEs) on the magnetopause and/or associated perturbations in the background magnetosphere and magnetosheath. When spacecraft traversed the FTE structures, the velocity components tangential to the magnetopause were generally antisunward, consistent with the sheath flow direction. On the other hand, when the spacecraft were located within the low-latitude boundary layer (LLBL) in the magnetosphere and remotely sensed the perturbations related to FTEs on the magnetopause, the velocity tangential to the magnetopause was found to be antisunward near the magnetopause but sunward further in from the magnetopause. The normal component variations for both groups had the same bipolar structure with inward flows followed by outward flows. This pattern has the form of a flow vortex just inside the magnetopause associated with an FTE moving in an antisunward direction at or outside of the magnetopause. A 2-dimensional magnetohydrodynamic (MHD) simulation code has been developed to understand the flow perturbations outside an FTE. Our simulation starts from an explicit solution, in which it is assumed that the plasma is inviscid and incompressible and no flow vortex is present. Only when we impose finite viscosity near the FTEs do flow vortices develop. However, the origin of this viscosity remains unknown.
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    Topics: Geosciences , Physics
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